Microheterogeneity of human interleukin 6 synthesized by transfected NIH/3T3 cells: comparison with human monocytes, fibroblasts and endothelial cells

Interleukin-6 and its receptors: a highly regulated and dynamic system

Interleukin-6 (IL-6) is a multifunctional cytokine with well-defined pro- and anti-inflammatory properties. Although only small amounts in the picogram range can be detected in healthy humans, IL-6 expression is highly and transiently up-regulated in nearly all pathophysiological states. IL-6 induces intracellular signaling pathways after binding to its membrane-bound receptor (IL-6R), which is only expressed on hepatocytes and certain subpopulations of leukocytes (classic signaling). Transduction of the signal is mediated by the membrane-bound β-receptor glycoprotein 130 (gp130). In a second pathway, named trans-signaling, IL-6 binds to soluble forms of the IL-6R (sIL-6R), and this agonistic IL-6/sIL-6R complexes can in principle activate all cells due to the uniform expression of gp130. Importantly, several soluble forms of gp130 (sgp130) are found in the human blood, which are considered to be the natural inhibitors of IL-6 trans-signaling. Most pro-inflammatory roles of IL-6 have been attributed to the trans-signaling pathway, whereas anti-inflammatory and regenerative signaling, including the anti-bacterial acute phase response of the liver, is mediated by IL-6 classic signaling. In this simplistic view, only a minority of cell types expresses the IL-6R and is therefore responsive for IL-6 classic signaling, whereas gp130 is ubiquitously expressed throughout the human body. However, several reports point towards a much more complex situation. A plethora of factors, including proteases, cytokines, chemical drugs, and intracellular signaling pathways, are able to modulate the cellular expression of the membrane-bound and soluble forms of IL-6R and gp130. In this review, we summarize current knowledge of regulatory mechanisms that control and regulate the dynamic expression of IL-6 and its two receptors.

Detection of ligand-induced CNTF receptor dimers in living cells by fluorescence cross correlation spectroscopy

Ciliary neurotrophic factor (CNTF) signals via a receptor complex consisting of the specific CNTF receptor (CNTFR) and two promiscuous signal transducers, gp130 and leukemia inhibitory factor receptor (LIFR). Whereas earlier studies suggested that the signaling complex is a hexamer, more recent analyses strongly support a tetrameric structure. However, all studies so far analyzed the stoichiometry of the CNTF receptor complex in vitro and not in the context of living cells. We generated and expressed in mammalian cells acyl carrier protein-tagged versions of both CNTF and CNTFR. After labeling CNTF and CNTFR with different dyes we analyzed their diffusion behavior at the cell surface. Fluorescence (cross) correlation spectroscopy (FCS/FCCS) measurements reveal that CNTFR diffuses with a diffusion constant of about 2 x 10(-9) cm(2) s(-1) independent of whether CNTF is bound or not. FCS and FCCS measurements detect the formation of receptor complexes containing at least two CNTFs and CNTFRs. In addition, we measured Förster-type fluorescence resonance energy transfer between two differently labeled CNTFs within a receptor complex indicating a distance of 5-7 nm between the two. These findings are not consistent with a tetrameric structure of the CNTFR complex suggesting that either hexamers and or even higher-order structures (e.g. an octamer containing two tetramers) are formed.

IL-6 receptor independent stimulation of human gp130 by viral IL-6

The genome of human herpes virus 8, which is associated with Kaposi's sarcoma, encodes proteins with similarities to cytokines and chemokines including a homologue of IL-6. Although the function of these viral proteins is unclear, they might have the potential to modulate the immune system. For viral IL-6 (vIL-6), it has been demonstrated that it stimulates IL-6-dependent cells, indicating that the IL-6R system is used. IL-6 binds to IL-6R, and the IL-6/IL-6R complex associates with gp130 which dimerizes and initiates intracellular signaling. Cells that only express gp130 but no IL-6R cannot be stimulated by IL-6 unless a soluble form of the IL-6R is present. This type of signaling has been shown for hematopoietic progenitor cells, endothelial cells, and smooth muscle cells. In this paper we show that purified recombinant vIL-6 binds to gp130 and stimulates primary human smooth muscle cells. IL-6R fails to bind vIL-6 and is not involved in its signaling. A Fc fusion protein of gp130 turned out to be a potent inhibitor of vIL-6. Our data demonstrate that vIL-6 is the first cytokine which directly binds and activates gp130. This property points to a possible role of this viral cytokine in the pathophysiology of human herpes virus 8.

Role of soluble interleukin-6 receptor in inflamed gingiva for binding of interleukin-6 to gingival fibroblasts

Interleukin-6 (IL-6), frequently detected in periodontitis, is known to mediate important signals in the inflammatory cytokine network. Gingival fibroblasts (GF) secrete cytokines upon stimulation with inflammatory mediators. However, it is not clear if GF respond to IL-6. We examined the IL-6 receptor gene expression in GF. Furthermore, we tested whether GF are target cells for IL-6 by examination of binding of IL-6. GF were found to contain trace amounts of mRNA for IL-6 receptor (IL-6R), but had high levels of mRNA for 130-kDa glycoprotein (gp130), which is a signal transducer for IL-6/IL-6R complex. Based on this observation, we hypothesized that IL-6 could bind GF if exogenous soluble forms of IL-6R (sIL-6R) existed in the gingiva or culture condition. Thus, we investigated the existence of sIL-6R in gingiva using enzyme-linked immunosorbent assay and whether sIL-6R influenced the binding of IL-6 to GF in vitro. In inflamed gingiva, sIL-6R was detected and its concentration ranged from 150 to 700 pg/microgram protein. The sIL-6R enhanced the binding of IL-6 to GF in a dose-dependent manner. This enhancement was inhibited by an antibody against gp130, suggesting that the IL-6/sIL-6R complex bound to the fibroblasts via gp130. These data demonstrated that gingival fibroblasts can be target cells for IL-6 in the presence of appropriate amounts of sIL-6R. This situation may exist during inflammation in periodontal tissue.

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.

Alternative assay procedures for cytokines and soluble receptors of the IL-6 family

Human hepatoma cells (HepG2 cells) were transfected with expression vectors for human IL-6 (hIL-6) and rat IL-6R (rIL-6-R). The cell lines were used for testing the biological activity of different IL-6 species, soluble hIL-6R (shIL-6R) and some members of the IL-6 cytokine family by means of an ELISA procedure. The assay is based on induction of the gene expression of the acute phase protein haptoglobin in hepatoma cells and provides an alternative bioassay taking advantage of the hepatocyte stimulatory activity of IL-6 (as opposed to the B9 proliferative assay). A dose-response experiment with IL-6 showed that half-maximal stimulation was achieved with approx. 5 ng/ml of hIL-6 in HepG2 cells and with 5-10 ng/ml muIL-6 in HepG2-rIL-6R cells after 24 h. The same response was achieved with 10 ng/ml shIL-6R in HepG2-IL6 cells. In conclusion, the assay is fast and reliable and might be adopted for other cytokines and receptors with hepatocyte stimulating activity.

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

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

The soluble interleukin-6 receptor is generated by shedding

The ligand-binding subunit (gp80) of the human interleukin-6 receptor (IL-6R) was transiently expressed in COS-7 cells. The metabolically labeled protein was shown to be quantitatively released from the membrane within 20 h. We identified the protein released from the transfected COS-7 cells after purification to homogeneity and N-terminal sequencing as a soluble form of the gp80/IL-6R. Shedding of the gp80 protein was strongly induced by 4 beta-phorbol-12-myristate-13-acetate, indicating that the process was regulated by protein kinase C (PKC). This was further corroborated by the finding that co-transfection of a PKC expression plasmid led to enhanced shedding of the gp80 protein. Since shedding of gp80 could not be prevented by treatment of the cells with inhibitors of all known classes of proteases, a novel protease seems to be involved. As a control, an unrelated membrane protein (vesicular stomatitis virus glycoprotein) was transfected into COS-7 cells and analyzed for shedding. Since the turnover of this protein was not mediated by shedding, we conclude that the release of gp80 from COS-7 cells is a specific process. The shed gp80 protein specifically binds IL-6, and this complex shows biological activity on human hepatoma cells. Human peripheral blood monocytes released a soluble form of the gp80 protein into the culture medium upon PMA treatment indicating that PKC-regulated shedding is the physiological mechanism of generation of the soluble IL-6R.

Protein kinase C activity is rate limiting for shedding of the interleukin-6 receptor

An analysis of the mechanism of generation of the soluble interleukin-6 receptor (IL-6R) has been performed. The membrane-bound receptor is proteolytically cleaved to release a soluble receptor form which retained its ligand binding capacity. Furthermore, the soluble IL-6R is unique in its ability to induce a biological signal in complex with the ligand interleukin-6 (IL-6) on cells which by themselves do not bind IL-6. Shedding of the IL-6R is strongly activated by PMA and can be inhibited by the protein kinase inhibitor staurosporine. The generation of the IL-6R is not dependent on protein synthesis. The inactive PMA analogue 4-alpha-phorbol-12,13-didecanoate fails to induce shedding of the IL-6R. Transfection of a protein kinase C expression plasmid into IL-6R expressing cells leads to enhanced shedding of the receptor. These experiments clearly show that protein kinase C regulates shedding of the IL-6R.

Soluble human interleukin-6-receptor modulates interleukin-6-dependent N-glycosylation of alpha 1-protease inhibitor secreted by HepG2 cells

Interleukin-6 (IL-6) induces changes in gene expression and the N-glycosylation pattern of acute-phase proteins in hepatocytes. IL-6 exerts its action via a cell surface receptor complex consisting of an 80 kDa IL-6 binding protein (gp80) and a 130 kDa glycoprotein (gp130) involved in signal transduction. A genetically engineered gp80-derived soluble human IL-6-receptor (shIL-6-R) significantly enhanced the IL-6 effect on N-glycosylation changes (revealed by reactivity with the lectin-concanavalin A) of a1-protease inhibitor (PI) secreted by human hepatoma cells (HepG2). Stable transfection of IL-6-cDNA into HepG2 cells (HepG2-IL-6) resulting in constitutive secretion of 2 micrograms of IL-6 per 10(6) cells in 24 h led to a down-regulation of surface-bound gp80 and subsequent homologous desensitization of HepG2-IL-6 cells towards IL-6. Soluble human IL-6-R functionally substituted membrane-bound gp80 resulting in a reconstitution of responsiveness of HepG2-IL-6 cells.

Phosphorylation of interleukin-6 at serine54: an early event in the secretory pathway in human fibroblasts

The cytokine interleukin-6 (IL-6) is the major phosphoprotein secreted by human fibroblasts induced with interleukin-1 alpha (IL-1 alpha). We have determined that Ser54 is the predominant site of phosphorylation on the fibroblast-derived IL-6 polypeptide; the amino acid motif surrounding this site is reminiscent of the target site for the Golgi-associated protein (casein) kinase. It has been shown earlier that the IL-6 polypeptide follows the classical secretory pathway where N-linked glycosylation is detectable within the first 15 minutes of labeling with [35S]-methionine and O-linked glycosylation occurs between 15-30 minutes after the start of polypeptide synthesis. Pulse-chase experiments using [32P]-orthophosphate or [35S]-methionine as tracers indicated that phosphorylation of IL-6 occurred prior to its O-glycosylation suggesting that the de novo synthesized IL-6 polypeptide is rapidly, perhaps even cotranslationally, phosphorylated at an intravesicular site (in the endoplasmic reticulum and/or Golgi). When IL-1 alpha-induced fibroblasts were exposed to cycloheximide there was enhancement of the net de novo synthesis and secretion of IL-6 as followed by [35S]-methionine labeling ("superinduction") but the secreted cytokine was no longer phosphorylated as monitored by [32P] labeling. Thus, phosphorylation of the IL-6 polypeptide is not an obligatory requirement for secretion of this cytokine. Furthermore, IL-6 phosphorylation is inhibited by cycloheximide through a mechanism different from the drug's effects on polypeptide synthesis per se.

Structural and functional studies on the human hepatic interleukin-6 receptor. Molecular cloning and overexpression in HepG2 cells

cDNAs coding for the human hepatic interleukin-6 receptor (IL-6-R) have been isolated from a library made from poly(A) RNA of dexamethasone-treated human hepatoma cells (HepG2). We found the hepatic IL-6-R to be identical to the one expressed by leucocytes. A polyclonal antiserum was raised in rabbits against the IL-6-R protein expressed in Escherichia coli. Although the entire IL-6-R protein was used for immunization, only antibodies to the cytoplasmic domain of the IL-6-R were obtained. It is demonstrated by affinity cross-linking and subsequent immunoprecipitation with antibodies against the ligand as well as against the receptor that the cloned cDNA codes for the functional IL-6-R on HepG2 cells. When the hepatic IL-6-R cDNA was overexpressed in HepG2 cells, these cells became more sensitive to low concentrations of IL-6 with respect to the induction of gamma-fibrinogen mRNA.

Marked cell-type-specific differences in glycosylation of human interleukin-6

Human interleukin-6 (IL-6) secreted by cytokine- or endotoxin-induced fibroblasts, monocytes, keratinocytes, endometrial stromal cells, and endothelial cells, when analyzed under denaturing and reducing conditions, consists of a set of differentially modified phosphoglycoproteins of molecular mass in the range from 23 to 30 kD (a set of at least three O-glycosylated 23- to 25-kD species and a set of at least three N- and O-glycosylated 28- to 30-kD species). The 23- to 25-kD and 28- to 30-kD fibroblast-derived IL-6 species have been separately purified to homogeneity with the use of a combination of lectin and immunoaffinity chromatography. Glycosidase digestion experiments on such purified preparations confirmed that almost all human fibroblast-derived IL-6 species were O-glycosylated; additionally, the 28- to 30-kD species were N-glycosylated. Amino acid sequencing revealed that the major amino terminus in the fibroblast-derived 23- to 25-kD O-glycosylated IL-6 was at Ala28 whereas the major amino terminus in the 28- to 30-kD N- and O-glycosylated IL-6 was at Val30, suggesting that targeting of newly synthesized IL-6 polypeptides into the two different processing pathways in fibroblasts may be keyed to differences in the signal peptide cleavage site. Unexpectedly, IL-6 "constitutively" secreted by the Epstein-Barr virus (EBV)-infected human and primate (tamarin) B-cell lines designated sfBJAB and sfBT, respectively, consisted of a major apparently unglycosylated 21-kD species and a minor 25-kD N-glycosylated species.(ABSTRACT TRUNCATED AT 250 WORDS)

The expression of interleukin-6 by a rat macrophage-derived cell line

A stable rat macrophage-derived cell line (RMSV1) was established by transformation of primary peritoneal exudate cells with the SV40 virus. The RMSV1 cell line was used as a model to study the regulation of the interleukin-6 (IL6) gene expression in rat macrophages with respect to lipopolysaccharides (LPS), interleukin-1 (IL1) and glucocorticoids. The IL6 mRNA level in RMSV1 cell lines was induced 20-fold within 4 h by LPS, whereas IL1 had no effect. The glucocorticoids were able to inhibit completely the induction of the IL6 mRNA synthesis by LPS, indicating the negative regulation of the IL6 gene expression by glucocorticoids.