Up-regulation of the interleukin-6-signal transducing protein (gp130) by interleukin-6 and dexamethasone in HepG2 cells

Recent Advances in Electrochemical and Optical Biosensors Designed for Detection of Interleukin 6

Interleukin 6 (IL-6), being a major component of homeostasis, immunomodulation, and hematopoiesis, manifests multiple pathological conditions when upregulated in response to viral, microbial, carcinogenic, or autoimmune stimuli. High fidelity immunosensors offer real-time monitoring of IL-6 and facilitate early prognosis of life-threatening diseases. Different approaches to augment robustness and enhance overall performance of biosensors have been demonstrated over the past few years. Electrochemical- and fluorescence-based detection methods with integrated electronics have been subjects of intensive research due to their ability to offer a better signal-to-noise ratio, high specificity, ultra-sensitivity, and wide dynamic range. In this review, the pleiotropic role of IL-6 and its clinical significance is discussed in detail, followed by detection schemes devised so far for their quantitative analysis. A critical review on underlying signal amplification strategies and performance of electrochemical and optical biosensors is presented. In conclusion, we discuss the reliability and feasibility of the proposed detection technologies for commercial applications.

Multiplicity of Glucocorticoid Action in Inhibiting Allograft Rejection

Glucocorticoids (GCs) are used as immunosuppressive and antiinflammatory agents in organ transplantation and in treating autoimmune diseases and inflammatory disorders. GCs were shown to exert their antiproliferative effects directly through blockade of certain elements of an early membrane-associated signal transduction pathway, modulation of the expression of select adhesion molecules, and by suppression of cytokine synthesis and action. GCs may act indirectly by inducing lipocortin synthesis, which in turn, inhibits arachidonic acid release from membrane-bound stores, and also by inducing transforming growth factor (TGF)-β expression that subsequently blocks cytokine synthesis and T cell activation. Furthermore, by preferentially inhibiting the production of Th1 cytokines, GCs may enhance Th2 cell activity and, hence, precipitate a long-lasting state of tolerance through a preferential promotion of a Th2 cytokine-secreting profile. In exerting their antiproliferative effects, GCs influence both transcriptional and posttranscriptional events by binding their cytosolic receptor (GR), which subsequently binds the promoter region of cytokine genes on select DNA sites compatible with the GCs responsible elements (GRE) motif. In addition to direct DNA binding, GCs may also directly bind to, and hence antagonize, nuclear factors required for efficient gene expression, thereby markedly reducing transcriptional rate. The pleiotrophy of the GCs action, coupled with the diverse experimental conditions employed in assessing the GCs effects, indicate that GCs may utilize more than one mechanism in inhibiting T cell activation, and warrant careful scrutiny in assigning a mechanism by which GCs exert their antiproliferative effects. © 1998 Elsevier Science Inc.

Characterisation, expression and ontogeny of interleukin-6 and its receptors in zebrafish (Danio rerio)

Interleukin-6 (IL-6) is one of the most pleiotropic cytokines due to its importance in both innate and adaptive immune responses and other physiological processes. In this study, we identified the zebrafish (Danio rerio) IL-6 homologue by investigating the synteny between the human (Homo sapiens), the fugu (Takifugu rubripes) and the zebrafish genome. Although zebrafish IL-6 showed a low sequence homology with other IL-6 sequences in other species, it presented a high structural similarity to human IL-6. We also analysed IL-6 expression in several different tissues, along with analysis of the expression of the genes that form the IL-6 receptor complex, IL-6R and gp130. After treatment with bacterial or viral stimuli, zebrafish IL-6 expression was modulated in a manner similar to that of other proinflammatory molecules, such as IL-1β and TNF-α. The expression of IL-6, IL-6R and gp130 was also studied during the ontogeny of zebrafish larvae using quantitative PCR and in situ hybridisation. Our results indicated that the transcripts were detected very early, increased during the first week of life and were predominantly expressed in the head, epidermis and neuromasts of the anterior and posterior lateral line system, suggesting their involvement in the normal development of these tissues.

IL-6 trans-signaling licenses mouse and human tumor microvascular gateways for trafficking of cytotoxic T cells

Immune cells are key regulators of neoplastic progression, which is often mediated through their release of cytokines. Inflammatory cytokines such as IL-6 exert tumor-promoting activities by driving growth and survival of neoplastic cells. However, whether these cytokines also have a role in recruiting mediators of adaptive anticancer immunity has not been investigated. Here, we report that homeostatic trafficking of tumor-reactive CD8+ T cells across microvascular checkpoints is limited in tumors despite the presence of inflammatory cytokines. Intravital imaging in tumor-bearing mice revealed that systemic thermal therapy (core temperature elevated to 39.5°C ± 0.5°C for 6 hours) activated an IL-6 trans-signaling program in the tumor blood vessels that modified the vasculature such that it could support enhanced trafficking of CD8+ effector/memory T cells (Tems) into tumors. A concomitant decrease in tumor infiltration by Tregs during systemic thermal therapy resulted in substantial enhancement of Tem/Treg ratios. Mechanistically, IL-6 produced by nonhematopoietic stromal cells acted cooperatively with soluble IL-6 receptor-α and thermally induced gp130 to promote E/P-selectin- and ICAM-1-dependent extravasation of cytotoxic T cells in tumors. Parallel increases in vascular adhesion were induced by IL-6/soluble IL-6 receptor-α fusion protein in mouse tumors and patient tumor explants. Finally, a causal link was established between IL-6-dependent licensing of tumor vessels for Tem trafficking and apoptosis of tumor targets. These findings suggest that the unique IL-6-rich tumor microenvironment can be exploited to create a therapeutic window to boost T cell-mediated antitumor immunity and immunotherapy.

Regulation of Cytokine Receptor Signaling by Nuclear Hormone Receptors: A New Paradigm for Receptor Interaction

Cytokine receptors act through a complex signaling network involving Janus kinases (JAKs) and the signal transducers and activators of transcription (STATs) to regulate diverse biological processes controlling growth, development, homeostasis, and immune function. JAK/STAT signaling is terminated by negative regulators including the suppressors of cytokine signaling (SOCSs), protein tyrosine phosphatases, and protein inhibitors of activated STAT. There is a wealth of evidence that nuclear hormone receptors (NHRs) are important regulators of cytokine action. The molecular mechanisms underlying NHR regulation are incompletely understood, but may include control of cytokine production and modulation of the expression and signaling of cytokine receptors. NHRs regulate cytokine receptor signaling by affecting STAT expression and by acting as coregulators of STAT transcriptional action. More recently, NHRs have been shown to exert regulatory effects indirectly through SOCSs, which is a novel mechanism for receptor crosstalk. Better understanding of the regulatory interaction between these two classes of receptors potentially leads to new drug design and/or therapeutic strategies for treatment of cytokine-related diseases.

Glucocorticoid regulation of the inflammatory response to injury

During the first half of the 20th century, physiologists were interested in the adrenal glands primarily because adrenalectomized animals failed to survive even mild degrees of systemic stress. It eventually became clear that hormones secreted by the adrenal cortex were critical for survival and, in this context, adrenal cortical hormones were widely considered to support or stimulate important responses to stress or injury. With the purification and manufacture of adrenal cortical hormones in the 1930s and 1940s, clinicians suddenly discovered the potent anti-inflammatory actions of glucocorticoids (GCs). This dramatic, and unexpected, discovery has dominated clinical and laboratory research into GC actions throughout the second half of the 20th century. More recent research is again reporting GC-induced stimulatory effects on a variety of inflammatory response components. These effects are usually observed at low GC concentrations, close to concentrations that are observed in vivo during basal, unstimulated states. For example, GC-mediated stimulation has been reported for the hepatic acute-phase response, for cytokine secretion, expression of cytokine/chemokine receptors, and for the pro-inflammatory mediator, macrophage migration inhibition factor. It seems clear that the long-held clinical view that GCs act solely as anti-inflammatory agents needs to be re-assessed. Varying doses of GCs do not lead simply to varying degrees of inflammation suppression, but rather GCs can exert a full range of effects from permissive to stimulatory to suppressive.

Developmental stage-dependent self-regulation of embryonic cortical precursor cell survival and differentiation by leukemia inhibitory factor

Cortical precursor cells secrete soluble factors for their own survival and self-renewal. We show here that neural precursor cells isolated from embryonic rat cortices abundantly secrete leukemia inhibitory factor (LIF) and express its receptor components, gp130 and LIF receptor. LIF signaling is responsible for cortical precursor cell survival. As described previously, LIF caused astrocytic differentiation of cultured embryonic cortical precursor cells. LIF-mediated survival and astrocytic differentiation of cortical precursor cells were differentially regulated, depending on the developmental ages of embryos from which cortical precursors were isolated. LIF did not enhance the survival of cortical precursor cells isolated from later embryos (embryonic day 16, E16). Moreover, LIF-mediated astrocytic differentiation was not observed in early (E12) cortical precursors. Inhibition studies revealed that Janus-activated kinase/signal transducer and activator of transcription and phosphatidylinositol 3 kinase/Akt pathways participate in both the LIF-mediated effects. However, mitogen-activated protein kinase, another signal pathway activated by LIF, was specifically responsible for astrocytic differentiation. These findings collectively indicate that precursor cells self-regulate the sequential processes of brain development, such as early maintenance of the precursor cell population and later differentiation into astrocytes, via common LIF signaling.

The role of interleukin-6 in nociception and pain

IMPLICATIONS

That IL-6 is an interesting target in the study of pain is underscored by its biomolecular properties, its localization after experimental pain, and its modulating effect on pain after administration.

Heparin acts synergistically with interleukin-11 to induce STAT3 activation and in vitro osteoclast formation

We have previously demonstrated that long-term heparin treatment causes cancellous bone loss in rats due in part to an increase in the number of osteoclasts lining the trabecular bone surface. In the present study, we investigated this phenomenon by examining the ability of heparin to synergistically enhance interleukin-11 (IL-11)-induced osteoclast formation. Treatment of murine calvaria and bone marrow cells with IL-11 was found to induce the formation of tartrate-resistant acid phosphatase-positive (TRAP(+)) multinucleated cells (MNCs) in a dose-dependent fashion. No effect was seen when cocultures were treated with heparin alone. However, when cocultures were treated with both IL-11 and heparin, IL-11's ability to induce TRAP(+) MNC formation was enhanced 6-fold. In an attempt to resolve the mechanism responsible for this effect, we examined the ability of heparin to influence IL-11 signaling using murine calvaria cells. Heparin was found to enhance both IL-11-induced STAT3-DNA complex formation and transactivation without altering either STAT3 (signal transducer and activator of transcription-3) tyrosine or serine phosphorylation. Heparin was also found to enhance IL-11's ability to induce the expression of both receptor activator of nuclear factor-kappaB ligand (RANKL) and glycoprotein (gp) 130. When taken together, these findings suggest a plausible mechanism by which heparin may cause increased osteoclastogenesis and therefore bone loss when administered long-term.

IL-6 up-regulates CNTF mRNA expression and enhances neurite regeneration

Interleukin-6 (IL-6) is a neurotrophic cytokine, however, its direct effect on nerve regeneration has not been well characterized. We therefore examined the effect of IL-6 on neurite regeneration using the rat dorsal root ganglion. IL-6 significantly enhanced neurite regeneration from transected nerve terminals. We also examined the mRNA expression of IL-6 family cytokines and their receptors during the regeneration. The mRNA expressions of IL-6, IL-6 receptor, leukemia inhibitory factor (LIF), ciliary neurotrophic factor (CNTF) receptor alpha, and LIF receptor beta showed no significant differences by the addition of IL-6. In contrast, IL-6 enhanced the mRNA expression of gp130 and CNTF. In addition, CNTF significantly increased neurite regeneration when added exogenously. Our data suggest that IL-6 enhanced regeneration via up-regulating CNTF expression.

Combined interleukin 6 and soluble interleukin 6 receptor accelerates murine liver regeneration

BACKGROUND & AIMS

Liver regeneration after loss of hepatic tissue leads to hepatocyte and nonparenchymal cell proliferation and rapid restoration of liver parenchyma. Interleukin (IL)-6 is a key inducer of transcription factors involved in liver regeneration. Whenever IL-6 activates target cells, it binds to a specific IL-6 receptor (IL-6R). The IL-6/IL-6R complex then associates with the signal transducer gp130, leading to activation of intracellular signaling.

METHODS

We have recently constructed the designer cytokine Hyper-IL-6 consisting of soluble IL-6R covalently linked to IL-6, which directly stimulates gp130 even in the absence of membrane-bound IL-6R. We compared the influence of IL-6 and Hyper-IL-6 on liver regeneration after partial hepatectomy in mice.

RESULTS

The IL-6/soluble IL-6 fusion protein Hyper-IL-6, but not IL-6 alone, led to an earlier onset of hepatocellular proliferation resulting in an acceleration of liver weight restoration. Also, during liver regeneration, soluble IL-6R levels were increased.

CONCLUSIONS

These results emphasize a central role for IL-6 and soluble IL-6R in liver regeneration and indicate a possible therapeutic potential for the designer cytokine Hyper-IL-6 in clinical situations associated with liver regeneration such as acute hepatic failure or resection of chronically damaged liver tissue.

Expression levels of gp130 in bone marrow stromal cells determine the magnitude of osteoclastogenic signals generated by IL-6-type cytokines

Interleukin-6 (IL-6)-type cytokines stimulate osteoclast formation by activating the glycoprotein 130 (gp130) receptor subunit on stromal/osteoblastic cells, which in turn leads to signal transducer and activator of transcription 3 (STAT3)-mediated expression of receptor activator of NF-kappaB ligand (RANKL). Based on evidence that gp130 expression is regulated by a variety of cytokines and hormones, we have determined here whether changes in gp130 levels directly contribute to the magnitude of the osteoclastogenic stimulus delivered by IL-6-type cytokines. To accomplish this, gp130 protein levels were modulated using a tetracycline-regulated expression system in a stromal/osteoblastic cell line, UAMS-32, which supports osteoclast formation. Removal of doxycycline from the culture medium elevated gp130 expression and increased the responsiveness of a STAT-responsive promoter-luciferase construct to IL-6 complexed with its soluble receptor (IL-6+sIL-6R), but diminished the responsiveness to oncostatin M (OSM). IL-6+sIL-6R-stimulated osteoclast formation was greater when osteoclast precursors were cocultured with the cells expressing elevated gp130 levels than when cells expressing low gp130 levels were used. However, increased gp130 levels reduced OSM-stimulated osteoclast formation. These results establish that the level of gp130 in stromal/osteoblastic cells directly modulates the magnitude of the osteoclastogenic response to IL-6-type cytokines such that an increase in gp130 increases the cellular responsiveness to IL-6+sIL-6R but reduces responsiveness to OSM.

Distinct effects of Broncho-Vaxom (OM-85 BV) on gp130 binding cytokines

BACKGROUND

Broncho-Vaxom (OM-85 BV) is known to support respiratory tract resistance to bacterial infections. In vivo and in vitro studies in animals and humans have shown that the action of the drug is based on the modulation of the host immune response, and it has been found to upregulate interferon gamma (IFN-gamma) and interleukin (IL)-2, IL-6, and IL-8. These immunomodulatory effects of the compound may explain its stimulation on T helper cells and natural killer cells. Following earlier findings that OM-85 BV induces the synthesis of IL-6, a study was undertaken to investigate its possible effect on other gp130 binding cytokines including IL-11, IL-12, leukaemia inhibitory factor (LIF), oncostatin M (OSM), and ciliary neutrophil factor (CNTF). Its modulation of the corresponding receptors of the above mentioned cytokines and of the signal transducer gp130 in human pulmonary fibroblasts and peripheral blood lymphocytes was also studied.

METHODS

Transcription of cytokines was assessed by Northern blot analysis. Secretion of cytokines was analysed using commercially available enzyme linked immunosorbent assay kits. Cytokine receptors and gp130 proteins were determined by Western blot analysis.

RESULTS

OM-85 BV increased the expression of IL-11 in human lung fibroblasts, but not in lymphocytes, in a dose and time dependent manner by maximal fivefold within 20 hours. The compound inhibited serum induced IL-12 expression in peripheral blood lymphocytes but did not induce OSM, LIF, or CNTF at any concentration. In lung fibroblasts the expression of the IL-6 receptor was enhanced fourfold at a concentration of 10 microg/ml OM-85 BV while that of the IL-11 receptor was not altered. In peripheral blood lymphocytes LIF receptor alpha expression was downregulated in the presence of 10 microg/ml OM-85 BV. At a concentration of 10 microg/ml OM-85 BV enhanced gp130 gene transcription fivefold and increased gp130 protein accumulation in cell membranes by 2.5 times.

CONCLUSION

In vitro OM-85 BV exerts immunomodulatory action via modulation of the signal transducer gp130 and gp130 binding cytokines. The increase of IL-6 and IL-11 may explain enhanced T and B cell activity, immunoglobulin synthesis, and IgM to IgG switch. Suppression of IL-12 and LIF receptor-alpha further contributes to organ protection. With regard to gp130 mediated signalling of the investigated cytokines, OM-85 BV modifies the host immune response towards an increased sensitisation of cells to gp130 binding proteins.

Stimulation of osteoclast differentiation in vitro by mouse oncostatin M, leukaemia inhibitory factor, cardiotrophin-1 and interleukin 6: synergy with dexamethasone

The role of oncostatin M in bone metabolism is not clearly defined, and the actions of mouse oncostatin M (mOSM) on osteoclast development has not been previously determined. We therefore examined the ability of recombinant mOSM to stimulate osteoclast formation and activity using cocultures of murine calvaria and bone marrow cells, and compared the responses to other members of the interleukin 6 family of cytokines including mouse leukaemia inhibitory factor (LIF), cardiotrophin-1 (CT-1) and IL-6. Mouse OSM, LIF and CT-1 strongly induced the formation of tartrate resistant acid phosphatase positive (TRAP(+)) multinucleated cells (MNC) in a dose-dependent fashion. OSM, LIF or CT-1 also elevated the number and size of resorptive pits when cocultures were added to smooth cortical bone slices, indicating enhancement of osteoclast activity. The activity of OSM was reduced by indomethacin (10(-8)-10(-6) M), whereas addition of dexamethasone (DEX) at 10(-7)-10(-5) M synergistically enhanced OSM-induced numbers of TRAP(+)MNC. DEX (10(-7) M) costimulation also synergistically enhanced TRAP(+)cell numbers of LIF, and CT-1 treated cocultures. IL-6 had no activity alone, but further enhanced TRAP(+)cell formation in mOSM or DEX (10(-7) M) treated cocultures. When added to mouse calvarial osteoblast cultures, mOSM induced secretion of IL-6 protein and elevation of mRNA whereas LIF or CT-1 did not. IL-6 mRNA levels and protein secretion were reduced in osteoblasts by costimulation with DEX. These results show that mouse OSM, LIF and CT-1 induce osteoclast differentiation and activation, that DEX synergizes with each in this activity, and that mouse OSM induces responses in osteoblasts that are not shown by LIF or CT-1. Collectively these data suggest an important role of these cytokines in osteoporosis caused by high levels of corticosteroid.

Factors influencing the effect of the soluble IL-6 receptor on IL-6 responses in HepG2 hepatocytes

The soluble IL-6 receptor (sIL-6R) can increase IL-6-induced signalling by forming a complex with IL-6 and membrane-bound gp130 (the receptor beta chain which transduces signals). The conditions affecting this response to sIL-6R were studied using fibrinogen release from HepG2 hepatocytes. Exogenous sIL-6R had no effect alone or in the presence of a submaximal concentration of IL-6, but increased responses to supramaximal IL-6 concentrations in a concentration-related manner. Dexamethasone increased the expression of the membrane IL-6R and endogenous sIL6R release, and increased responses to supramaximal but not submaximal IL-6 concentrations. The amount of endogenous sIL-6R released is relatively small and is unlikely to influence the effects of the exogenous sIL-6R. The observed concentration-related decrease in sIL-6R production in the presence of IL-6 may indicate internalization of ligand/receptor complexes. This would significantly decrease the amount of IL-6R (soluble or membrane) available for signalling and limit continued functional response later in the cultures. These data indicate that the major factor influencing responses to exogenous sIL-6R is an excess of IL-6 which is necessary to form complexes with the sIL-6R, which can then interact with gp130 to increase signalling.

Soluble interleukin-6 receptor (sIL-6R) makes IL-6R negative T cell line respond to IL-6; it inhibits TNF production

The receptor for interleukin-6 (IL-6) consists of two subunits: a ligand specific IL-6Ralpha and gp130 that is responsible for signal-transduction. A soluble form of the ligand specific chain was described that when complexed to IL-6 is capable of binding to the membrane-bound gp130 subunit and thus can elicit signal-transduction. This soluble receptor can act on cells that express only the gp130 but not the ligand-specific subunit of the IL-6R. This phenomenon, called trans-signaling, introduced a novel aspect of cytokine action. In this study we examined the response of Jurkat cells, that are known not to express IL-6Ralpha, to IL-6, the soluble IL-6 receptor (sIL-6R) and a covalent complex of IL-6 and sIL-6R termed Hyper-IL-6. We studied the expression of tumour necrosis factor (TNF) and interferon-gamma (IFN-gamma). The complex of IL-6+sIL-6R and Hyper-IL-6 inhibited significantly the production of TNF in a gp130-dependent manner, whereas no differences in IFN-gamma expression were found. IL-6 and sIL-6R alone were not effective. Because we did not detect major differences in the TNF mRNA levels upon treatments, we conclude that the inhibition of TNF production should occur at the post-transcriptional level. These results provide another example of trans-signaling and underline the physiological importance of sIL-6R, and in the case of Hyper-IL-6 its possible therapeutic application can also be considered.

Regulation of angiogenin expression in human HepG2 hepatoma cells by mediators of the acute-phase response

Angiogenin is a potent inducer of neovascularization in vivo. However, like other angiogenic molecules, its specific physiologic roles and mechanisms regulating its expression remain to be elucidated. Angiogenin is a liver-derived component of normal serum whose concentration can increase in various disease states. This suggests that it might participate in the acute-phase response. In an initial study we showed that angiogenin protein and mRNA levels transiently increased in mice following an acute inflammatory stimulus. We now report that IL-6, a major inducer of acute-phase proteins, stimulates the synthesis and secretion of angiogenin protein in human HepG2 cells within 24 hr following treatment, an effect enhanced by dexamethasone. IL-6 also increases the amount of angiogenin mRNA without altering its half-life. This increase, suppressible by cycloheximide, peaks at 12 hr following stimulation and returns to basal levels by 48 hr. IL-1 alone slightly decreases the basal production of angiogenin protein and mRNA, but essentially abolishes the response to IL-6 in the absence or presence of dexamethasone. This antagonistic effect by IL-1 on IL-6 activity is not a result of changes in mRNA stability nor is it dependent on new protein synthesis. Thus, the combined effects of IL-6, IL-1, glucocorticoids, and perhaps other related factors may specifically control angiogenin expression. Since angiogenin is regulated in a manner similar to that of acute phase proteins both in vitro and in vivo, it may play a role in the host response to injury.

In Vivo and In Vitro Activities of the gp130-Stimulating Designer Cytokine Hyper-IL-6

IL-6 is a multifactorial cytokine mediating acute inflammatory responses in the liver. When IL-6 binds to a specific receptor (IL-6R), the IL-6/IL-6R complex associates with the signal transducer gp130, initiating intracellular signaling. A soluble form of the IL-6R (sIL-6R) renders target cells sensitive to IL-6 that do not express the IL-6R on their surfaces. A designer cytokine, termed Hyper-IL-6, consisting of IL-6 covalently linked to the sIL-6R was fully active on gp130-expressing cells at 100- to 1000-fold lower concentrations than unlinked IL-6 and IL-6R. Mice were injected i.p. with Hyper-IL-6 or IL-6. Upon injection of Hyper-IL-6 into mice, the acute phase response, as measured by haptoglobin mRNA expression in the liver, was markedly increased and lasted significantly longer compared with that in mice injected with a 10-fold higher dose of IL-6 alone. On human hepatoma cells, Hyper-IL-6 caused similar effects, indicating that the longer lasting response to the fusion protein could not only be explained by the longer plasma half-life of the fusion protein. Experiments using iodinated IL-6 and Hyper-IL-6 revealed that Hyper-IL-6 bound with high affinity to gp130 and was less efficiently internalized. This effect might explain the longer lasting activity of this protein on cells. The highly active IL-6/sIL-6R designer protein might be of significant clinical importance for the stimulation of cells that are more responsive to the IL-6/sIL-6R complex than to IL-6 alone. Such cells include hemopoietic progenitor cells and hepatocytes.

Induction of cytokine receptors by glucocorticoids: functional and pathological significance

Current concepts on the role of glucocorticoid hormones in the regulation of inflammatory and immune responses depict this role as being inhibitory. Over the past decade, however, a large variety of studies have shown that glucocorticoids also exert stimulatory effects on immune function, suggesting that the present concept of the role of glucocorticoids in the immune system in not sufficient and needs to be extended. Here, Jan Wiegers and Hans Reul ask how these apparently paradoxical effects fit together and what their functional and pathological significance might be.

Requirement for cooperative interaction of interleukin-6 responsive element type 2 and glucocorticoid responsive element in the synergistic activation of mouse metallothionein-I gene by interleukin-6 and glucocorticoid

Metallothionein (MT)-inducing activity of interleukin (IL)-6 depends on the presence of glucocorticoid in hepatic cells. The synergistic action of IL-6 and glucocorticoid was observed in the transcriptional activation of the mouse MT (mMT)-I gene. We found that a 281-bp promoter was sufficient for IL-6 and glucocorticoid stimulation. Our inspection of this region revealed the putative type 1 and 2 IL-6 responsive elements (REs). Functional analyses of these regions were performed using luciferase reporter constructs, and it was observed that the type 2 IL-6RE exerted the major response to the IL-6 signal. The transcriptional factor binding to type 1 IL-6RE, nuclear factor-IL-6, hardly contributed to the activation of the mMT-I promoter by IL-6 and glucocorticoid. A glucocorticoid responsive element (GRE) was also required for the synergistic activation by IL-6 and glucocorticoid. Interestingly, this synergism was not observed when the type 2 IL-6RE and the GRE were kept apart. Therefore, the synergistic activation of the mMT-I gene by IL-6 and glucocorticoid may require not only that signal transducers and activators 3 (Stat3) and the glucocorticoid receptor (GR) bind to their respective responsive elements, but also that Stat3 and the GR physically interact with one another.

Differential temporal expression of mRNAs for ciliary neurotrophic factor (CNTF), leukemia inhibitory factor (LIF), interleukin-6 (IL-6), and their receptors (CNTFR alpha, LIFR beta, IL-6R alpha and gp130) in injured peripheral nerves

The mRNA expression of the neuropoietic cytokines, ciliary neurotrophic factor (CNTF), leukemia inhibitory factor (LIF), interleukin-6 (IL-6), and their receptor components (CNTFRalpha, LIFRbeta, IL-6Ralpha and gp130) was examined in peripheral nerves after two different types of injury, crush and transection. The CNTF mRNA expression levels decreased after injury and remained low in the transected model, but recovered in 4 weeks in the crushed model. The LIF mRNA rapidly increased after damage and returned gradually to control levels. The IL-6 mRNA expression increased rapidly within 1 day after injury but dramatically decreased soon after. The CNTFRalpha mRNA levels gradually increased after nerve injury. LIFRbeta was expressed in the intact nerve and decreased slightly after injury. The IL-6Ralpha expression was observed faintly in the intact nerve and increased significantly soon after injury. There was also an increase in the expression of gp130. Although the temporal expression of these neuropoietic cytokines and receptors was extremely different, their pattern was similar between the crushed and transected models, except for CNTF. These results suggest that the expression of the ligands and receptors are differentially regulated after peripheral nerve injury, implying that each cytokine and signal transduction system has entirely distinctive functions in neuronal regeneration and repair.

Regulation of the gp80 and gp130 subunits of the IL-6 receptor by sex steroids in the murine bone marrow

Both estrogen and androgen exert their antiosteoporotic effects, at least in part, by inhibiting IL-6 production, thereby suppressing osteoclastogenesis. Several observations, however, suggest that besides increased IL-6 production, sensitivity of the osteoclastogenic process to this cytokine is altered after ovariectomy. Based on this and evidence that the ligand-binding subunit of the IL-6 receptor (gp80) is a limiting factor for the actions of IL-6 on bone, we hypothesized that sex steroids regulate expression of the IL-6 receptor as well. We report that 17beta-estradiol or dihydrotestosterone in vitro decreased the abundance of the gp80 mRNA as well as the mRNA of the signal-transducing subunit of the IL-6 receptor (gp130) in cells of the bone marrow stromal/osteoblastic lineage, and also decreased gp130 protein levels. These effects did not require new protein synthesis. In contrast to sex steroids, parathyroid hormone stimulated gp130 expression; this effect was opposed by sex steroids. Consistent with these findings, ovariectomy in mice caused an increase in expression of gp80, gp130, and IL-6 mRNAs in ex vivo bone marrow cell cultures as determined by quantitative reverse transcription (RT)-PCR, and confirmed on an individual cell basis using in situ RT-PCR. The demonstration of increased expression of the IL-6 receptor after loss of sex steroids provides an explanation for why IL-6 is important for skeletal homeostasis in the sex steroid-deficient, but not replete, state.

Isolation and characterization of the human gp130 promoter. Regulation by STATS

Glycoprotein 130 (gp130), a shared component of all the receptors for the interleukin-6 cytokine family, transduces cytokine signals in part by activating latent cytoplasmic signal transducers and activators of transcription (STATs). STATs subsequently translocate into the nucleus and stimulate gene expression. In the studies reported here, the 5'-flanking region of the human gp130 gene was isolated and the transcription initiation sites were mapped. To demonstrate that the isolated DNA fragment contained a functional promoter, a plasmid construct containing 2433 base pairs of the gp130 5'-flanking region, inserted upstream from the firefly luciferase gene, was transiently transfected into HepG2 hepatoma cells. The construct exhibited constitutive promoter activity. In addition, a 5-h treatment with interleukin-6 or oncostatin M stimulated the activity of this promoter severalfold. Localization of the cytokine response element by 5'-deletion analysis and site-directed mutagenesis revealed a cis-acting binding site for activated STAT complexes. Furthermore, DNA binding analysis demonstrated that this element binds activated STAT1 and STAT3 homo- and heterodimers. This STAT-binding element was sufficient to confer cytokine stimulation to a minimal herpesvirus thymidine kinase promoter. These results establish that the DNA fragment we have isolated contains the human gp130 promoter and that interleukin-6 type cytokines may influence the activity of this promoter via activated STATs.

Cytokines and the hepatic acute phase response

The acute phase response is an orchestrated response to tissue injury, infection or inflammation. A prominent feature of this response is the induction of acute phase proteins, which are involved in the restoration of homeostasis. Cytokines are important mediators of the acute phase response. Uncontrolled and prolonged action of cytokines is potentially harmful, therefore mechanisms exist which limit the activity of cytokines; these include soluble cytokine receptors and receptor antagonists. The cytokine signal is transmitted into the cell via membrane-bound receptors. Different intracellular signalling pathways are activated by different cytokine-receptor interactions. Eventually, cytokine-inducible transcription factors interact with their response elements in the promotor region of acute phase genes and transcription is induced. Systemic inflammation results in a systemic acute phase response. However, local inflammatory or injurious processes in the liver may also induce an acute phase response, for example after partial hepatectomy and during hepatic fibrosis. The acute phase proteins induced in these conditions probably act to limit proteolytic and/or fibrogenic activity and tissue damage. The possible function of the acute phase protein alpha 2-macroglobulin in hepatic fibrosis is discussed in some detail.

Acute phase proteins and transformed cells

Acute phase proteins (APP) are plasma proteins whose concentration and glycosylation alters in response to tissue injury, inflammation, or tumor growth. Significant interspecies and sex differences in APP response exist. APP are produced mainly by hepatocytes, and their synthesis and glycosylation are controlled by a network consisting of cytokines, their soluble receptors, and glucocorticoids. The major cytokines involved in these processes belong to a group of interleukin-6-type cytokines that act through the hematopoietin receptor complex on hepatocytes and JAK-STAT signal transduction pathway. Transformed cells (hepatoma) display significant differences in synthesis of APP, cytokine responsiveness, expression of cytokine-receptor subunits and signal-transduction machinery. The most striking variability relates to the glycosylation alterations induced by cytokines. However, transformed cells (hepatoma) form a basic model for studying and understanding mechanisms controlling the synthesis and glycosylation of APP. Furthermore, APP may be secreted by transformed (tumor) cells of various origins and may display a growth factor-like function in certain cancer types.

Interleukin-6 in the injured patient. Marker of injury or mediator of inflammation?

OBJECTIVE

The effects of interleukin (IL)-6 in the injured patient are examined in an attempt to clarify the potential pathophysiologic role of IL-6 in the response to injury.

SUMMARY BACKGROUND DATA

Interleukin-6 is an integral cytokine mediator of the acute phase response to injury and infection. However, prolonged and excessive elevations of circulating IL-6 levels in patients after trauma, burns, and elective surgery have been associated with complications and mortality. The mechanistic role of IL-6 in mediating these effects is unclear.

METHODS

A review of current literature is performed to summarize the origins, mechanisms of action, and biologic effects of IL-6 and to characterize the IL-6 response to injury.

RESULTS

Interleukin-6 is a multifunctional cytokine expressed by a variety of cells after a multitude of stimuli, under complex regulatory control mechanisms. The IL-6 response to injury is uniquely consistent and related to the magnitude of the insult. Moreover, the early postinjury IL-6 response correlates with complications as well as mortality.

CONCLUSIONS

Interleukin-6 appears to play an active role in the postinjury immune response, making it an attractive therapeutic target in attempts to control hyperinflammatory provoked organ injury.

Cytokine expression by human marrow-derived mesenchymal progenitor cells in vitro: effects of dexamethasone and IL-1 alpha

We previously reported the purification, culture-expansion, and osteogenic differentiation potential of mesenchymal progenitor cells (MPCs) derived from human bone marrow. As a first step to establishing the phenotypic characteristics of MPCs, we reported on the identification of unique cell surface proteins which were detected with monoclonal antibodies. In this study, the phenotypic characterization of human marrow-derived MPCs is further established through the identification of a cytokine expression profile under standardized growth medium conditions and in the presence of regulators of the osteogenic and stromal cell lineages, dexamethasone and interleukin-1 alpha (IL-1 alpha), respectively. Constitutively expressed cytokines in this growth phase include G-CSF, SCF, LIF, M-CSF, IL-6, and IL-11, while GM-CSF, IL-3, TGF-beta 2 and OSM were not detected in the growth medium. Exposure of cells in growth medium to dexamethasone resulted in a decrease in the expression of LIF, IL-6, and IL-11. These cytokines have been reported to exert influence on the differentiation of cells derived from the bone marrow stroma through target cell receptors that utilize gp130-associated signal transduction pathways. Dexamethasone had no effect on the other cytokines expressed under growth medium conditions and was not observed to increase the expression of any of the cytokines measured in this study. In contrast, IL-1 alpha increased the expression of G-CSF, M-CSF, LIF, IL-6 and IL-11 and induced the expression of GM-CSF. IL-1 alpha had no effect on SCF expression and was not observed to decrease the production of any of the cytokines assayed. These data indicate that MPCs exhibit a distinct cytokine expression profile. We interpret this cytokine profile to suggest that MPCs serve specific supportive functions in the microenvironment of bone marrow. MPCs provide inductive and regulatory information which are consistent with the ability to support hematopoiesis, and also supply autocrine, paracrine, and juxtacrine factors that influence the cells of the marrow microenvironment itself. In addition, the cytokine profiles expressed by MPCs, in response to dexamethasone and IL-1 alpha, identify specific cytokines whose levels of expression change as MPCs differentiate or modulate their phenotype during osteogenic or stromagenic lineage entrance/progression.

Glucocorticoid-dependent induction of interleukin-6 receptor expression in human hepatocytes facilitates interleukin-6 stimulation of amino acid transport

OBJECTIVE

The authors studied the effects of interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-alpha) on glutamine and alanine transport in isolated human hepatocytes. They also evaluated the role of dexamethasone in modulating this response and its effects on the expression of the plasma membrane high-affinity IL-6 receptor.

SUMMARY BACKGROUND DATA

Animal studies indicate that cytokines are important mediators of the increased hepatic amino acid uptake that occurs during cancer and sepsis, but studies in human tissues are lacking. The control of transport by cytokines and cytokine receptor expression in the liver may provide a mechanism by which hepatocytes can modulate amino acid availability during catabolic disease states.

METHODS

Human hepatocytes were isolated from wedge biopsy specimens and plated in 24-well trays. Interleukin-6 and TNF-alpha, in combination with the synthetic glucocorticoid dexamethasone, were added to hepatocytes in culture, and the transport of radiolabeled glutamine and alanine was measured. Fluorescent-activated cell sorter (FACS) analysis was used to study the effects of dexamethasone on IL-6 receptor number in the well-differentiated human hepatoma HepG2.

RESULTS

Both IL-6 and TNF-alpha exerted a small stimulatory effect on alanine and glutamine transport. Dexamethasone alone did not alter transport rates, but pretreatment of cells augmented the effects of both cytokines on carrier-mediated amino acid uptake. Dexamethasone pretreatment and a combination of IL-6 and TNF-alpha resulted in a greater than twofold increase in transport activity. Fluorescent-activated cell sorter analysis demonstrated that dexamethasone induced a threefold increase in the expression of high-affinity IL-6 receptors.

CONCLUSIONS

Interleukin-6 and TNF-alpha work coordinately with glucocorticoids to stimulate amino acid uptake in human hepatocytes. Dexamethasone exerts a permissive effect on cytokine-mediated increases in transport by increasing IL-6 receptor expression on the cell surface. It is likely that this upregulation of IL-6 receptors "primes" human liver cells for subsequent stimulation by cytokines. The resulting increase in hepatic amino acid transport provides the liver with substrate to support key metabolic pathways during catabolic states.

Cytokine expression by human marrow-derived mesenchymal progenitor cells in vitro: effects of dexamethasone and IL-1 alpha

We previously reported the purification, culture-expansion, and osteogenic differentiation potential of mesenchymal progenitor cells (MPCs) derived from human bone marrow. As a first step to establishing the phenotypic characteristics of MPCs, we reported on the identification of unique cell surface proteins which were detected with monoclonal antibodies. In this study, the phenotypic characterization of human marrow-derived MPCs is further established through the identification of a cytokine expression profile under standardized growth medium conditions and in the presence of regulators of the osteogenic and stromal cell lineages, dexamethasone and interleukin-1 alpha (IL-1 alpha), respectively. Constitutively expressed cytokines in this growth phase include G-CSF, SCF, LIF, M-CSF, IL-6, and IL-11, while GM-CSF, IL-3, TGF-beta 2 and OSM were not detected in the growth medium. Exposure of cells in growth medium to dexamethasone resulted in a decrease in the expression of LIF, IL-6, and IL-11. These cytokines have been reported to exert influence on the differentiation of cells derived from the bone marrow stroma through target cell receptors that utilize gp130-associated signal transduction pathways. Dexamethasone had no effect on the other cytokines expressed under growth medium conditions and was not observed to increase the expression of any of the cytokines measured in this study. In contrast, IL-1 alpha increased the expression of G-CSF, M-CSF, LIF, IL-6 and IL-11 and induced the expression of GM-CSF. IL-1 alpha had no effect on SCF expression and was not observed to decrease the production of any of the cytokines assayed. These data indicate that MPCs exhibit a distinct cytokine expression profile. We interpret this cytokine profile to suggest that MPCs serve specific supportive functions in the microenvironment of bone marrow. MPCs provide inductive and regulatory information which are consistent with the ability to support hematopoiesis, and also supply autocrine, paracrine, and juxtacrine factors that influence the cells of the marrow microenvironment itself. In addition, the cytokine profiles expressed by MPCs, in response to dexamethasone and IL-1 alpha, identify specific cytokines whose levels of expression change as MPCs differentiate or modulate their phenotype during osteogenic or stromagenic lineage entrance/progression.

Upregulated expression of IL-4 receptors and increased levels of IL-4 in rheumatoid arthritis patients

The level of IL-4R expression on peripheral lymphocyte subsets from rheumatoid arthritis (RA) patients and controls was studied by flow cytometric analysis of the binding of phycoerythrin-labelled IL-4 (PE-IL-4). In normal lymphocytes, IL-4R is mainly expressed on CD19+ cells, although it was also seen, at lower levels, on CD3+, CD4+ and CD8+ cells. In RA patients, a significantly increased spontaneous expression of IL-4R was observed, compared with controls, in the CD3+, CD4+ and CD19+ cell subsets. No significant differences in IL-4R expression were found between patients receiving steroids and those who were not, suggesting that steroids are not involved in upregulating IL-4R levels in vivo. Because IL-4 is a potent upregulator of IL-4R, we considered the possibility that incremented levels of circulating IL-4 in RA accounted for the high surface expression of IL-4R. By ELISA, we found abnormally high levels of immunoreactive IL-4 in 35.13% of patient serum samples, while it was undetectable in control sera. In addition, we examined IL-4 mRNA expression by polymerase chain reaction (PCR) in the PBMC of patients and controls. IL-4 PCR products were observed in four out of 10 patients studied but in none of the controls. No correlation was observed between the seric concentrations of IL-4 and IL-4R, indicating that activator factors other than IL-4 contribute to the upregulation of IL-4R expression in RA. Since the patients' sedimentation rate and CRP values did not correlate with the concentration of circulating IL-4, we conclude that this lymphokine does not contribute to the deleterious effect of the disease. Rather, due to its antiinflammatory properties, the overproduction of IL-4 in RA may be a compensatory mechanism neutralizing the harmful effect of activated macrophages.

Interferon-alpha down-regulates the interleukin-6 receptor in a human multiple myeloma cell line, U266

The effects of interferon-alpha (IFN-alpha) on the interleukin-6 (IL-6) receptor in a multiple myeloma cell line, U266, have been examined. IFN-alpha inhibits [3H]thymidine incorporation in U266 cells in a time- and dose-dependent manner. Furthermore, IFN-alpha inhibits the ability of IL-6 to induce increases in [3H]thymidine incorporation. While IFN-alpha suppresses the ability of 125I-IL-6 to bind to the IL-6 receptor on U266 cells, this effect is not due to competition of IFN-alpha with IL-6 for the IL-6 receptor. Although IFN-alpha induces IL-6 synthesis in the U266 cell, inhibition of IL-6 binding occurs when IL-6 synthesis is minimal. Furthermore, after pretreatment of U266 cells with neutralizing anti-IL-6 antibodies, IFN-alpha still inhibits 125I-IL-6 binding. These data suggest that IFN-alpha inhibition of 125I-IL-6 binding does not involve IL-6 synthesis. IFN-alpha reduces 125I-IL-6 binding without affecting its affinity, suggesting that IFN-alpha inhibits IL-6 receptor expression. Although pretreatment with cycloheximide inhibits 125I-IL-6 binding, IFN-alpha does not cause a selective decrease in the levels of gp130 or IL-6 receptor mRNA at times when 125I-IL-6 binding is inhibited. These observations indicate that IFN-alpha lowers IL-6 receptor density on U266 cells by mechanisms other than competitive binding or lowering IL-6 receptor mRNA production. Receptor down-regulation may be a mechanism of IFN-alpha-induced inhibition of growth in U266 cells.

Glucocorticoids and immune function: physiological relevance and pathogenic potential of hormonal dysfunction

The more knowledge accumulates on the molecular action of glucocorticoids, the less appears to be known about the physiological relevance of these data. Mechanisms that determine bioactivity of glucocorticoids are largely neglected in studies on their molecular actions in immunoregulation. However, alterations of these mechanisms may contribute to the pathogenesis of acute of chronic inflammation, ranging from septic shock to autoimmune disease, or even acquired immuno deficiency syndrome. In this article, Thomas Wilckens attempts to challenge the long-standing dogma of glucocorticoids being only immunosuppressive in their action, and suggests a physiological role in which they are a prerequisite for a coordinated immune response.

Separate regulation of a membrane protein, gp130, present in receptor complex specific for interleukin-6 and other functionally related cytokines

In addition to specific ligand binding elements, receptor assembly for interleukin(IL)-6, oncostatin-M, leukaemia inhibitory factor, ciliary neurotrophic factor and IL-11 includes an additional unit, gp130. This molecule is a transmembrane glycoprotein of 130 kDa. In this paper, reviewing molecular, biochemical and functional data on gp130, we describe the dissimilar action of IL-3 on the expression of the binding unit of the IL-6 receptor and that of gp130. According to FACS studies, resting basophils express only IL-6 receptors and no gp130 molecules on the plasma membranes. After incubation with IL-3, the surface appearance and de novo transcription of gp130 was shown by FACS and mRNA polymerase chain reaction analysis.

Regulation of interleukin-6 receptor expression in rat Kupffer cells: modulation by cytokines, dexamethasone and prostaglandin E2

Interleukin-6 has a variety of biological effects, mainly on the immune system. The regulation of this signal at both the site of production and the site of action is necessary to maintain the organism's homeostasis. In the microenvironment of the hepatic sinusoids, Kupffer cells as resident macrophages are the most potent source of interleukin-6 during inflammation. This cytokine is an important signal to hepatocytes during the early stages of the acute-phase response, leading to the expression of several major plasma proteins. Kupffer cells were found to express interleukin-6 receptor constitutively. Interleukin-6 decreased the level of interleukin-6 receptor mRNA, indicating an autocrine pathway by which Kupffer cells regulate their responsiveness to interleukin-6. Furthermore, lipopolysaccharide, tumor necrosis factor-alpha, interferon-gamma, interleukin-1 beta and phorbol ester induced interleukin-6 production and, at the same time, suppressed the level of interleukin-6 receptor mRNA. The existence of an autocrine loop in rat Kupffer cells may be physiologically relevant, as it would contribute to a regulated interleukin-6 signal chain in the liver. The anti-inflammatory mediators dexamethasone or PGE2 and its second messenger, cyclic AMP, increased interleukin-6 receptor mRNA, whereas prostaglandin D2 or the Ca2+ ionophore, A 23187, were without effect. The changes in interleukin-6 mRNA were paralleled by the number of interleukin-6 receptors present on Kupffer cells as detected by binding of 125I-interleukin-6. These results suggest the existence of control mechanisms involving several soluble mediators that help balance the level of interleukin-6-R mRNA in rat liver macrophages.

Effects of interleukin 6 on megakaryocytes and on canine platelet function

The multifunctional cytokine interleukin 6 (IL-6) is a potent promoter of megakaryocytic maturation in vitro. In vivo, IL-6 has similar effects on the maturation of megakaryocytes, as shown by enhancing size, ploidy and platelet production. IL-6 is capable of augmenting the platelet count in both normal animals and those with reduced megakaryocyte mass; ongoing clinical trials suggest a similar thrombocytopoietic effect in man. Moreover, IL-6 alters platelet function, rendering them more sensitive to activation by thrombin and platelet activating factor. Finally, IL-6 promotes increases in plasma fibrinogen and von Willebrand factor, and a decrease in free protein S concentration. These modifications of the platelet and coagulant phases of the clotting mechanism may result in an overall prohemostatic tendency, which may prove beneficial for the amelioration of bleeding propensity following chemotherapy. However, additional investigation will be required to determine if IL-6-mediated alterations of hemostasis may lead to pathologic thrombosis.

Regulation of interleukin-6 receptor expression by interleukin-6 in human monocytes--a re-examination

We have studied the expression and regulation of the interleukin-6 receptor (gp80) and its signal transducer gp130 in primary human blood monocytes. Here, we show that freshly isolated human monocytes express mRNAs for gp80 and gp130. In contrast to a previous report [(1989) FEBS Lett. 249, 27-30] we find that neither lipopolysaccharide nor interleukin-6 (IL-6) lead to a down-regulation of IL-6 receptor mRNA in monocytes. Also in the human monocytic cell line Mono Mac 6 no effect of IL-6 on receptor mRNA levels was observed. For signal transducer gp130 mRNA in monocytes a small and transient up-regulation by IL-6 was found.

Differential regulation of interleukin-6 receptors by interleukin-6 and interferons in multiple myeloma cell lines

Interleukin-6 (IL-6) mediates pleiotropic functions through specific receptors (IL-6R) composed of an 80-kDa binding protein, associated with a non-ligand binding protein (gp130) which transduces the signal. Because IL-6 is the major tumor growth factor in multiple myeloma, we investigated the regulation of IL-6R in two human multiple myeloma cell lines. Binding experiments with 125I-labeled IL-6 showed that IL-6R were expressed at a high density on RPMI-8226 cells (15 000 receptors/cell), but no specific binding was detected on XG-1 cells, whose growth depends on the presence of exogenous IL-6. However, when IL-6 was removed from the culture medium, high-affinity IL-6R appeared on the surface of XG-1 cells (5300 sites/cell). Treatment of RPMI-8226 cells with IL-6 reduced the number of IL-6R without changing their affinity. This reduction was dose dependent and was not affected by acid treatment which dissociates ligand-receptor complexes. Cross-linking experiments showed that the formation of one IL-6/receptor complex of 160 kDa markedly decreased upon IL-6 treatment, while the other complex of 190 kDa became undetectable. These data provide evidence for ligand-induced down-regulation of membrane IL-6R expression in myeloma cells. Treatment of RPMI-8226 cells with interferon-alpha (IFN-alpha), which inhibits the growth of these cells, stimulated IL-6R expression and increased the formation of the 160-kDa IL-6/receptor complex. This stimulation was specific for IFN-alpha, since IFN-gamma reduced the number of IL-6R. These data indicate that, in myeloma cells, IL-6R are differentially regulated by IL-6 and IFN-alpha.

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.

Ciliary neurotrophic factor induces acute-phase protein expression in hepatocytes

During inflammatory states, hepatocytes are induced to synthesize and secrete a group of proteins called acute-phase proteins. It has recently been shown that besides interleukin-6 (IL-6), related cytokines such as leukemia inhibitory factor, oncostation M and interleukin-11 are also mediators of the hepatic acute-phase response. All these mediators belong to the hematopoietic family of alpha-helical cytokines. Here we show that an additional member of this cytokine family, ciliary neurotrophic factor (CNTF), induces the hepatic acute-phase protein genes haptoglobin, alpha 1-antichymotrypsin, alpha 2-macroglobulin and beta-fibrinogen in human hepatoma cells (HepG2) and in primary rat hepatocytes with a time course and dose-response comparable with that of IL-6. Our next aim was to define the receptor components used by CNTF on hepatic cells. Using a cell-free binding assay we exclude that CNTF binds to the 80 kDa IL-6 receptor, a protein with significant homology to the CNTF receptor which has recently been cloned from neuroblastoma cells. In human hepatoma cells (Hep3B) which lack the leukemia inhibitory factor receptor, CNTF was not able to induce acute-phase protein synthesis, indicating that this receptor protein may be part of the functional CNTF receptor on hepatic cells.