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

Pleiotropic, Unique and Shared Responses Elicited by IL-6 Family Cytokines in Human Vascular Endothelial Cells

Vascular endothelial cells express glycoprotein 130 (gp130), which is utilized as a signaling receptor by cytokines in the interleukin-6 (IL-6) family. Several IL-6 family cytokines can be found in the circulatory system during physiological or pathological conditions, and may influence endothelial function and response. This study evaluated and compared the cellular and molecular responses induced by IL-6 family cytokines in human endothelial cells. A proteomic analysis showed that IL-6 family cytokines induce the release of a range of proteins from endothelial cells, such as C-C motif chemokine ligand 23, hepatocyte growth factor, and IL-6. Pathway analysis indicated that gp130-signaling in endothelial cells regulates several functions related to angiogenesis and immune cell recruitment. The present investigation also disclosed differences and similarities between different IL-6 family cytokines in their ability to induce protein release and regulate gene expression and intracellular signaling, in regards to which oncostatin M showed the most pronounced effect. Further, this study showed that soluble gp130 preferentially blocks trans-signaling-induced responses, but does not affect responses induced by classic signaling. In conclusion, IL-6 family cytokines induce both specific and overlapping molecular responses in endothelial cells, and regulate genes and proteins involved in angiogenesis and immune cell recruitment.

Interleukin-6/interleukin-6 receptor complex promotes osteogenic differentiation of bone marrow-derived mesenchymal stem cells

BACKGROUND

Interleukin-6 (IL-6) with IL-6 receptor (IL-6R) play an important role in the tissue regeneration in vivo, especially bone metabolism. Bone marrow -derived mesenchymal stem cells (BM-MSCs) are multipotent stromal cells, which are main origin of osteoblasts. However, the roles of IL-6 and IL-6R in the osteogenic differentiation of BM-MSCs are still unclear.

METHODS

The expression of IL-6 and IL-6R was detected in BM-MSCs during osteogenic differentiation. The activation of the STAT3 pathway was assessed and its role in the osteogenic differentiation of BM-MSCs was determined using the specific inhibitor AG490. Exogenous IL-6/soluble IL-6R or antibodies against IL-6/IL-6R were used to confirm the mechanism by which the IL-6/IL-6R complex promotes the osteogenic differentiation.

RESULTS

The levels of IL-6 and IL-6R, especially the level of membranous IL-6R but not that of soluble IL-6R, increased during osteogenic differentiation in BM-MSCs. The levels of IL-6 and IL-6R were positively correlated with the osteogenic potential of BM-MSCs. The STAT3 signaling pathway was activated during the osteogenic differentiation of BM-MSCs. AG490 markedly inhibited the activation of the STAT3 pathway and, subsequently, the osteogenic differentiation potential of BM-MSCs. Additionally, exogenous IL-6 and soluble IL-6R accelerated the osteogenic differentiation of BM-MSCs. In contrast, antibodies against IL-6 or IL-6R suppressed the osteogenic differentiation of BM-MSCs. Moreover, IL-6 and IL-6R were found to stimulate each other's expression in BM-MSCs.

CONCLUSIONS

IL-6 and IL-6R levels increase during the osteogenic differentiation of BM-MSCs. These two molecules form a complex to activate the downstream STAT3 signaling pathway, which promotes osteogenic differentiation in BM-MSCs via an autocrine/paracrine feedback loop.

Alterations in membrane-associated CD14 expression and the simultaneous liberation of soluble CD14 fragment in adherent macrophages mediated by a leukocyte carboxyl/aspartate protease

Investigations sought to discover the biochemical mechanisms in macrophages that mediate the `shedding' of soluble CD14 fragment. Stimulated macrophages display both increased liberation of soluble CD14 fragment and decreases in residual membrane-associated CD14 complexes following exposure to activating agents ( fMLP/A23187). Application of `class-specific' protease inhibitors revealed that a thiol/cysteine was involved in the biochemical production of soluble CD14 fractions and that a metalloprotease enzymatically degraded soluble CD14 fragment. Exposure of macrophages to individual proteases revealed that both cathepsin-D and elastase promoted variable depletion of membrane-associated CD14 complexes. Additionally, cathepsin-D, and to a lesser extent elastase, generated soluble CD14 fragment. Related studies isolated a carboxyl/aspartate protease from activated macrophages using pepstatin-A affinity chromatography. The physical and functional properties of macrophage pepstatin-A binding protein fractions closely corresponded with the known characteristics of cathepsin-D with respect to: (i) cellular origin; (ii) binding-avidity of carboxyl/aspartate proteases for pepstatin-A; (iii) non-specific proteolysis of haemoglobin detected by Hb-PAGE zymography; and (iv) hydrolysis of a synthetic cathepsin-D-specific peptide substrate. Interpretation of these findings collectively implies that activated leukocytes can biochemically alter membrane-associated CD14 complex expression and promote the liberation of soluble CD14 fragment in both activated and non-activated cell populations.

Oral administration of soybean peptide Vglycin normalizes fasting glucose and restores impaired pancreatic function in Type 2 diabetic Wistar rats

Vglycin, a natural 37-residue polypeptide isolated from pea seeds in which six half-cysteine residues are embedded in three pairs of disulfide bonds, is resistant to digestive enzymes and has antidiabetic potential. To investigate the pharmacological activity of Vglycin in vivo and to examine the mechanisms involved, the therapeutic effect of Vglycin in diabetic rats was examined. Diabetes was induced in Wistar rats by high-fat diet and multiple streptozotocin intraperitoneal injections. Diabetic rats were treated daily with Vglycin for 4 weeks. Body weight, food intake, fasting plasma glucose and insulin levels were assayed weekly. Glucose and insulin tolerance tests were conducted on Day 29. Subsequently, levels of p-Akt in the liver and pancreas and cleaved PARP, Pdx-1 and insulin in the pancreas were detected by immunoblotting. The morphology of the pancreas and the insulin expression in the pancreas were analyzed by hematoxylin-eosin staining and immunohistochemistry, respectively. Furthermore, human liver-derived cell lines were used to explore the in vitro effects of Vglycin on insulin sensitivity and glucose uptake. Chronic treatment with Vglycin normalized fasting glucose levels in diabetic rats. The improvement in glucose homeostasis and the increased insulin sensitivity mediated by restored insulin signaling likely contributed to decreased food intake and reduced body weight. Vglycin protected pancreatic cells from damage by streptozotocin. Although insulin synthesis and secretion in impaired β-cell were not significantly elevated, islets morphology was improved in the Vglycin-treated groups. These results suggest that Vglycin could be useful in Type 2 diabetes for restoring impaired insulin signaling, glucose tolerance and pancreatic function.

Plasticity and cross-talk of interleukin 6-type cytokines

Interleukin (IL)-6-type cytokines are critically involved in health and disease. The duration and strength of IL-6-type cytokine-mediated signaling is tightly regulated to avoid overshooting activities. Here, molecular mechanisms of inter-familiar cytokine cross-talk are reviewed which regulate dynamics and strength of IL-6 signal transduction. Both plasticity and cytokine cross-talk are significantly involved in pro- and anti-inflammatory/regenerative properties of IL-6-type cytokines. Furthermore, we focus on IL-6-type cytokine/cytokine receptor plasticity and cross-talk exemplified by the recently identified composite cytokines IL-30/IL-6R and IL-35, the first inter-familiar IL-6/IL-12 family member. The complete understanding of the intra- and extracellular cytokine networks will aid to develop novel tailor-made therapeutic strategies with reduced side effects.

Glucocorticoids increase interleukin-6-dependent gene induction by interfering with the expression of the suppressor of cytokine signaling 3 feedback inhibitor

Glucocorticoids are known to be potent regulators of inflammation and have been used pharmacologically against inflammatory, immune, and lymphoproliferative diseases for more than 50 years. Due to their possible and well-documented side effects, it is crucial to understand the molecular mechanisms and targets of glucocorticoid action in detail. Several modes of action have been discussed; nevertheless, none of them fully explain all the functions of glucocorticoids. Therefore, we analyzed the cross-talk between glucocorticoids and interleukin-6 (IL-6) in the liver. IL-6 exerts pro-inflammatory as well as anti-inflammatory properties and is a main inducer of the acute-phase response. The balance between the proinflammatory and anti-inflammatory activities of IL-6 is tightly regulated by suppressor of cytokine signaling 3 (SOCS3), a well-known feedback inhibitor of IL-6 signaling. Here, it is demonstrated that glucocorticoids enhance IL-6-dependent γ-fibrinogen expression. Studying of the underlying mechanism revealed prolonged activation of signal transducer and activator of transcription 3 (STAT3) caused by down-regulation of SOCS3 protein expression. Consequently, in SOCS3-deficient cells glucocorticoids do not affect IL-6-induced signal transduction. Moreover, in hepatocytes lacking the SOCS3 recruiting motif within gp130, IL-6-dependent γ-fibrinogen expression is not influenced by glucocorticoid treatment.

CONCLUSION

Glucocorticoids interfere with IL-6-induced expression of the feedback inhibitor SOCS3, thereby leading to enhanced expression of acute-phase genes in hepatocytes. This mechanism contributes to the explanation of how glucocorticoids affect inflammation and acute-phase gene induction.

Interleukin-6 directly influences proliferation and invasion potential of head and neck cancer cells

Interleukin-6 (IL-6) is a multifunctional regulator of immune response and hematopoiesis. Recently, it has been reported that expression of IL-6 is correlated with prognosis in various cancer patients. In this study, we investigated whether the proliferation and invasion potential of head and neck squamous cell carcinomas (HNSCCs) were influenced by IL-6. All HNSCC cell lines, HEp-2, HSC-2, HSC-4, and SAS, were tested by reverse transcription-polymerase chain reaction (RT-PCR) and expressed the IL-6 receptor (IL-6R), and glycoprotein 130, which is responsible for signal transduction. HEp-2, HSC-2, and HSC-4 also produced IL-6. IL-6 inhibited the proliferation of HSC-2 and SAS, but the invasion potential of all the cell lines increased. Moreover, IL-6 down-regulated soluble IL-6R expression. Anti-IL-6R antibody abrogated the inhibited proliferation and increased invasion induced by IL-6. IL-6 stimulation also induced the extracellular regulated protein kinase 1/2 activation and increased vascular endothelial growth factor release. These results suggest that IL-6 can directly influence cell proliferation and the invasion potential as the first step of tumor metastasis.

Febrile-range temperature but not heat shock augments the acute phase response to interleukin-6 in human hepatoma cells

The relationship between the stress protein response and the acute phase response (APPR) was studied in human hepatoma cells to investigate the hierarchy of regulation of these survival responses. Huh-7 cells were subjected to heat treatment (febrile-range temperature 40 degrees C or heat shock 43 degrees C) followed by recovery at 37 degrees C in the presence or absence of IL-6 given either before or after heat treatment. The effects on total, fractional, and acute phase protein synthesis were then analyzed by metabolic labeling, ELISA, real-time PCR, Northern blot analysis, and activation of an alpha(1)-antitrypsin reporter plasmid. Cell energetics were studied under the same conditions using an index of mitochondrial activity and measurement of cellular ATP levels. Febrile-range temperature (40 degrees C) augmented acute phase protein production when cells had been pretreated with IL-6. Pretreatment of cells with IL-6 also prevented heat shock-induced suppression of alpha(1)-antichymotrypsin (ACT) but not transferrin. mRNA expression of ACT and alpha(1)-antitrypsin reporter activation studies was consistent with transcriptional regulation of these proteins. Expression of mRNA transcripts for transferrin was increased despite protein expression being reduced by heat shock. The effects of heat shock on acute phase protein synthesis can be modified by preincubation with IL-6, whereas addition of this ligand after heat treatment has no effect on the suppressive effect of heat on the APPR. The mechanism of this action appears to be transcriptionally regulated in the case of ACT, but in the case of transferrin, it may be mediated by another process such as posttranslational modification.

Effect of hepatitis C serology on C-reactive protein in a cohort of Brazilian hemodialysis patients

Hepatitis C (HCV) is not an uncommon feature in hemodialysis (HD) patients and may be a cause of systemic inflammation. Plasma cytokine interleukin-6 (IL-6) is mainly produced by circulating and peripheral cells and induces the hepatic synthesis of C-reactive protein (CRP), which is the main acute phase reactant. The aim of this study was to investigate the influence of HCV on two markers of systemic inflammation, serum CRP and IL-6, in HD patients. The study included 118 HD patients (47% males, age 47 +/- 13 years, 9% diabetics) who had been treated by standard HD for at least 6 months. The patients were divided into two groups depending on the presence (HCV+) or absence (HCV-) of serum antibodies against HCV. Serum albumin (S-Alb), plasma high sensitivity CRP (hsCRP), IL-6, and alanine aminotransferase (ALT) were measured and the values were compared with those for 22 healthy controls. Median hsCRP and IL-6 values and hsCRP/IL-6 ratio were: 3.5 vs 2.1 mg/l, P < 0.05; 4.3 vs 0.9 pg/ml, P < 0.0001, and 0.8 vs 2.7, P < 0.0001, for patients and controls, respectively. Age, gender, S-Alb, IL-6 and hsCRP did not differ between the HCV+ and HCV- patients. However, HCV+ patients had higher ALT (29 +/- 21 vs 21 +/- 25 IU/l) and had been on HD for a longer time (6.1 +/- 3.0 vs 4.0 +/- 2.0 years, P < 0.0001). Moreover, HCV+ patients had a significantly lower median hsCRP/IL-6 ratio (0.7 vs 0.9, P < 0.05) compared to the HCV- group. The lower hsCRP/IL-6 ratio in HCV+ patients than in HCV- patients suggests that hsCRP may be a less useful marker of inflammation in HCV+ patients and that a different cut-off value for hsCRP for this population of patients on HD may be required to define inflammation.

Azithromycin inhibits interleukin-6 but not fibrinogen production in hepatocytes infected with cytomegalovirus and chlamydia pneumoniae

Chlamydia pneumoniae and cytomegalovirus (CMV) have been associated with the development of atherosclerosis. Inflammatory stimuli initiate the biosynthesis of fibrinogen, interleukin (IL)-6 and plasminogen activator inhibitor (PAI)-1 in the liver. Chronic infection may perpetuate the inflammatory status. We hypothesized that infection of human hepatocytes with the intracellular pathogens C pneumoniae and CMV accelerates biosynthesis of fibrinogen, IL-6, and PAI-1 but that this biosynthesis can be reduced with the use of azithromycin. HepG2 human hepatocytes were infected with C pneumoniae and CMV in vitro in the presence of 0, 0.016, 0.125, or 1 microg/mL azithromycin. We measured IL-6, PAI-1, and fibrinogen after 24, 48, 72, and 96 hours. C pneumoniae-infected hepatocytes produce IL-6 (2667 +/- 309 pg/mL vs 137 +/- 120 pg/mL in uninfected cells after 96 hours. Incubation with 0.016 microg/mL azithromycin decreased IL-6 levels to a mean of 1516 +/- 402 pg/mL, and incubation with 0.125 and 1 microg/mL azithromycin decreased IL-6 to 871 +/- 364 and 752 +/- 403 pg/mL, respectively. C pneumoniae-induced IL-6 production was time- and dose-dependent. The interaction of C pneumoniae with azithromycin treatment was significant, indicating an inhibitory effect of azithromycin on C pneumoniae-induced IL-6 production. CMV infection did not lead to IL-6 production by hepatocytes. C pneumoniae and CMV infection did not induce any changes in PAI-1 production. Fibrinogen production was increased by CMV infection after 72 hours (838 +/- 88 ng/mL; P <.01) and after 96 hours by infection with both C pneumoniae and CMV (765 +/- 100 and 846 +/- 123 ng/mL, respectively; P <.05). Azithromycin did not suppress CMV- or C pneumoniae-induced fibrinogen production. Moreover, we could not confirm an antiinflammatory effect of azithromycin in experiments with cross-titrations of azithromycin against either IL-1 or IL-6 (P >.05). Azithromycin reduces C pneumoniae-induced IL-6 production, but not fibrinogen production, by human hepatocytes. This is a result of the antimicrobial properties of azithromycin and not a direct antiinflammatory effect.

Palmitate, but not unsaturated fatty acids, induces the expression of interleukin-6 in human myotubes through proteasome-dependent activation of nuclear factor-kappaB

Circulating interleukin-6 (IL-6), insulin, and free fatty acid (FFA) concentrations are associated with impaired insulin action in obese and type 2 diabetic individuals. However, a causal relationship between elevated plasma FFAs and IL-6 has not been shown. Because skeletal muscle represents a major target of impaired insulin action, we studied whether FFAs may affect IL-6 expression in human myotubes. We demonstrate that specifically saturated FFAs, e.g. palmitate (0.25 mm), induce IL-6 mRNA expression and protein secretion by a proteasome-dependent mechanism that leads to a rapid and chronic activation of nuclear factor-kappaB. Insulin, high glucose concentrations, or unsaturated FFAs did not activate IL-6 expression. In fact, the unsaturated FFA linoleate inhibited palmitate-induced IL-6 production. Because inhibition of palmitate metabolism by the acyl-CoA synthetase inhibitor triacsin C did not abolish IL-6 expression, it appears that the palmitate molecule per se exerts the observed effects. Furthermore, we show that in human myotubes, IL-6 activates the phosphorylation of signal transducer and activator of transcription 3 in concentrations similar to hepatocytes. However, no inhibitory effect of IL-6 on insulin action, determined as phosphatidylinositol 3-kinase association with insulin receptor substrate-1, Akt phosphorylation, and glycogen synthesis, was detected. We conclude that IL-6 expression may be modulated by the composition of circulating FFA, e.g. by diet, and that skeletal muscle cells could be target cells for IL-6.

Interleukin-6 induces cellular insulin resistance in hepatocytes

Interleukin (IL)-6 is one of several proinflammatory cytokines that have been associated with insulin resistance and type 2 diabetes. A two- to threefold elevation of circulating IL-6 has been observed in these conditions. Nonetheless, little evidence supports a direct role for IL-6 in mediating insulin resistance. Here, we present data that IL-6 can inhibit insulin receptor (IR) signal transduction and insulin action in both primary mouse hepatocytes and the human hepatocarcinoma cell line, HepG2. This inhibition depends on duration of IL-6 exposure, with a maximum effect at 1-1.5 h of pretreatment with IL-6 in both HepG2 cells and primary hepatocytes. The IL-6 effect is characterized by a decreased tyrosine phosphorylation of IR substrate (IRS)-1 and decreased association of the p85 subunit of phosphatidylinositol 3-kinase with IRS-1 in response to physiologic insulin levels. In addition, insulin-dependent activation of Akt, important in mediating insulin's downstream metabolic actions, is markedly inhibited by IL-6 treatment. Finally, a 1.5-h preincubation of primary hepatocytes with IL-6 inhibits insulin-induced glycogen synthesis by 75%. These data suggest that IL-6 plays a direct role in insulin resistance at the cellular level in both primary hepatocytes and HepG2 cell lines and may contribute to insulin resistance and type 2 diabetes.

The role of transsignalling via the agonistic soluble IL-6 receptor in human diseases

The activation of cells that do not express the membrane bound interleukin-6 6 receptor (IL-6R) by IL-6 and the soluble IL-6 receptor (sIL-6R) is termed transsignalling. Transsignalling may be an pathogenetic factor in human diseases as diverse as multiple myeloma (MM), Castleman's disease, prostate carcinoma, Crohn's disease, systemic sclerosis, Still's disease, osteoporosis and cardiovascular diseases. IL-6 and sIL-6R may directly or indirectly enhance their own production on endothelial or bone marrow stromal cells. Positive feedback autocrine loops thus created in affected organs may either cause or maintain disease progression. In autoimmune or vasculitic disease, the ability of the IL-6/sIL-6R complex to inhibit apoptosis of autoreactive T-cells may be central to the development of tissue specific autoimmunity. The anti-apoptotic effect of the IL-6/sIL-6R complex may be involved in tumour genesis and resistance to chemotherapy. Only in rare cases, where counterregulation has failed, there is a notable systemic effect of IL-6/sIL-6R. Appropriate animal models are necessary to establish the pathogenetic role of the IL-6/sIL-6R complex. A specific treatment option for diseases influenced by the sIL-6R could be based on gp130-Fc, a soluble gp130 (sgp130) linked to the Fc-fragment of IgG1. gp130-Fc has shown efficacy in vivo in animal models of Crohn's disease.

Gp130 activation by soluble interleukin-6 receptor/interleukin-6 enhances osteoblastic differentiation of human bone marrow-derived mesenchymal stem cells

Interleukin-6 (IL-6) promotes osteodifferentiation in bone-located progenitors; however, it is not known whether this cytokine affects the differentiation of bone marrow-located osteoprogenitors. To address this issue, we prepared human bone marrow-derived mesenchymal stem cells (MSCs), which were characterized by a cell surface phenotype and multipotential nature. It was observed that in the presence of IL-6, MSCs were not differentiated into the osteogenic lineage, as evidenced by a failure to induce alkaline phosphatase activity, an earlier marker of osteodifferentiation. The lack of effect of IL-6 correlates with the observation that MSCs do not express a membrane-bound or soluble IL-6 receptor (sIL-6R). The incompetence of IL-6 was not reversed by the addition of sIL-6R alone or the sIL-6R/IL-6 complex, as it occurs in other IL-6R-negative cells. However, after MSC osteocommittment by dexamethasone, sIL-6R or the sIL-6R/IL-6 complex enhanced alkaline phosphatase activity. The effect of sIL-6R or sIL-6R/IL-6 proved to be dependent on gp130 availability, which is expressed by MSCs, and involves stat-3 phosphorylation. These data suggest that IL-6R deficiency may represent for bone marrow-located mesenchymal progenitors a sort of protective mechanism to escape the osteogenic effect of IL-6, which is produced by the MSC itself as well as by other marrow stromal cells.

Cytokine balance in the lungs of patients with acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) involves an intense inflammatory response in the lungs, with accumulation of both pro- and antiinflammatory cytokines in bronchoalveolar lavage fluid (BALF). Our goal was to determine how the balance between pro- and antiinflammatory mediators in the lungs changes before and after the onset of ARDS. We identified 23 patients at risk for ARDS and 46 with established ARDS and performed serial bronchoalveolar lavage (BAL). We used immunoassays to measure tumor necrosis factor alpha (TNF-alpha) and soluble TNF-alpha receptors I and II; interleukin 1 beta (IL-1 beta), IL-1 beta receptor antagonist, and soluble IL-1 receptor II; IL-6 and soluble IL-6 receptor; and IL-10. We used sensitive bioassays to measure net TNF-alpha, IL-1 beta, and IL-6 activity. Although individual cytokines increased before and after onset of ARDS, greater increases occurred in cognate receptors and/or antagonists, so that molar ratios of agonists/antagonists declined dramatically at the onset of ARDS. The molar ratios remained low for 7 d or longer, limiting the activity of soluble IL-1 beta and TNF-alpha in the lungs at the onset of ARDS. This significant antiinflammatory response early in ARDS may provide a key mechanism for limiting the net inflammatory response in the lungs.

Biochemical alteration of membrane-associated IL-6 RI (80-kDa) in adherent macrophages and vascular endothelium

The potential biochemical mechanisms that mediate the 'shedding' of soluble IL-6 RI (80-kDa) receptor fragments in populations of adherent macrophages were explored. Stimulated macrophages displayed proportional increases in both the expression of membrane-associated IL-6 RI (80-kDa) and the release of soluble receptor fragments. The use of protease inhibitors implicated thiol/cysteine and carboxyl/aspartate proteases in this process. Cathepsin-D depleted membrane-associated IL-6 RI (80-kDa) complexes and generated soluble receptor fragments. A carboxyl/aspartate protease from activated macrophages isolated utilizing pepstatin-A affinity chromatography, was also found to affect membrane-associated IL-6 RI (80-kDa) complexes and generate soluble receptor fragments. Most likely, this fraction corresponded to cathepsin-D based upon its origin, pepstatin-A binding avidity, Hb-PAGE zymography, and hydrolysis of an enzyme-specific substrate. We conclude that cathepsin-D can generate soluble fragments of IL-6 RI (80-kDa) expressed by both macrophages and vascular endothelium.

Coupling of inflammatory cytokine signaling pathways probed by measurements of extracellular acidification rate

There is a growing interest in the mechanisms of how cells integrate the multitude of signals that emanate during inflammatory stimuli, such as the hepatic acute phase response to burn or trauma. We have used measurements of extracellular acidification rate (ECAR) of HepG2 cells cultured on microporous membranes to probe the coupling between signaling pathways for gp130 family cytokines (interleukin-6, oncostatin M) and IL-1, each of which is considered to play a significant role in the hepatic acute phase response. We found that brief (30 min or less) exposure to any of these cytokines desensitized the HepG2 cells to subsequent exposure with the same cytokine. Furthermore, we found that this property serves as a probe of the coupling of signaling pathways: exposure to IL-1 did not desensitize the cells to exposure to OSM and vice versa. However, cells exposed to IL-6 with soluble gp80, which together share with OSM the use of gp130 as a signal transducing receptor, were subsequently unable to respond to OSM, and vice versa. Simultaneous exposure of cells to moderate concentrations (near their respective EC50 values) of both IL-1 and OSM resulted in synergistic effects on the ECAR, but simultaneous exposure to saturating concentrations of IL-1 and OSM resulted in a response that tracked that of OSM alone. These results suggest that the signaling pathways of IL-1 and OSM may be simultaneously activated in HepG2 cells under moderate inflammatory cytokine challenge but that the cells must prioritize their response under extreme cytokine challenges.