Activation of the Jak/Stat signal transduction pathway in GH-treated rat osteoblast-like cells in culture

In this study, activation of the Jak/Stat signaling pathway was followed upon growth hormone (GH) stimulation, using the rat osteosarcoma cell-line UMR-106.01 that expresses high affinity GH receptors. The results show a GH-induced and sustained phosphorylation of Jak2 and Stat5 on tyrosine residues. The tyrosine phosphorylation status of Jak2 was increased in a dose-dependent manner. In contrast to Jak2, tyrosine phosphorylation of Stat5, also elicited at 42 ng/ml GH, remained unchanged when GH concentration was raised up to 4200 ng/ml. DNA binding activity of Stat5 was also observed in response to GH. However, GH was unable to cause transactivation of reporter gene constructs harboring Stat5 binding sites (the GHREII from the rat spi 2.1 gene promoter, and the LHRE from the rat beta-casein gene promoter), except in cells transiently transfected with either Stat5 cDNAs or the rat GHR cDNA. Altogether the results suggest that UMR-106.01 cells exhibit original features of the GH-dependent Jak/Stat signaling pathway.

Hepatocyte-derived cell lines express a functional receptor for cardiotrophin-1

Cardiotrophin-1 (CT-1) is a recently isolated cytokine belonging to the interleukin-6 cytokine family. In the present study, we show that CT-1 binds to hepatocyte-derived cell lines of rat and human origin with high (Kd = 600-800 pM) and low (Kd approximately 3-6 nM) binding affinities. Treatment of HepG2 cells with CT-1 resulted in the induction of tyrosine phosphorylation of both transducing receptor subunits, gp130 and LIF receptor, and this phosphorylation was completely inhibited by a neutralizing anti-gp130 mAb. Addition of CT-1 to HepG2 or H35 cell cultures induced a dose-dependent production of several acute phase proteins (haptoglobin, fibrinogen, alpha1-acid glycoprotein, alpha2-macroglobulin). Moreover, the use of a neutralizing mAb to gp130 in cultures of HepG2 cells grown in the presence of CT-1, inhibited the induction of acute phase protein secretion, indicating an absolute requirement of gp130 in the formation of a functional CT-1 receptor. Altogether, these results suggest that CT-1 could play an important role in the regulation of hepatocyte metabolism in inflammatory responses.

Signaling of type II oncostatin M receptor

Oncostatin M (OSM) mediates its bioactivities through two different heterodimer receptors. They both involve the gp130-transducing receptor, which dimerizes with either leukemia inhibitory receptor beta or with OSM receptor beta (OSMRbeta) to generate, respectively, type I and type II OSM receptors. Co-precipitation of gp130-associated proteins, flow cytometry, polymerase chain reaction, and tyrosine phosphorylation analyses allowed the characterization of both types of OSM receptors expressed on the surface of different cell lines. It also allowed the detection of a large size protein, p250, that specifically associates to the type II OSM receptor components and that is tyrosine-phosphorylated after the activation peak of the gp130.OSMRbeta heterocomplex. The restricted expression of type I OSM receptor by the JAR choriocarcinoma cell line, and type II receptor by the A375 melanoma cell line, permitted the characterization of their signaling machineries. Both type I and type II OSM receptors activated Jak1, Jak2, and Tyk2 receptor-associated tyrosine kinases. The information is next relayed to the nucleus by the STAT3 transcriptional activator, which is recruited by both types of OSM receptors. In addition, STAT5b was specifically activated through the gp130.OSMRbeta type II heterocomplex. The signaling pathway differences observed between the common type I LIF/OSM receptor and the specific type II OSM receptor might explain some of the bioactivities specifically displayed by OSM.

Signaling of the cardiotrophin-1 receptor. Evidence for a third receptor component

Cardiotrophin-1 (CT-1) is a recently isolated cytokine belonging to the interleukin-6 cytokine family. In the present study we show that CT-1 activates its receptor expressed at the surface of a human neural cell line by recruiting gp130 and gp190/leukemia inhibitory factor receptor beta, as shown by analyzing their tyrosine phosphorylation level. Neutralizing antibody directed against gp130 and reconstitution experiments performed in the COS-7 cell line demonstrate that gp130-gp190 heterocomplex formation is essential for CT-1 signaling. Analysis of the subsequent activation events revealed that CT-1 induces and utilizes Jak1-, Jak2-, and Tyk2-associated tyrosine kinases, which are in turn relayed by STAT-3 transcription factor. Cross-linking of iodinated CT-1 to the cell surface led to the identification of a third alpha component in addition to gp130 and gp190, with an apparent molecular mass of 80 kDa. Removal of N-linked carbohydrates from the protein backbone of the alpha component resulted in a protein of 45 kDa. Our results provide evidence that the CT-1 receptor is composed of a tripartite complex, a situation similar to the high affinity receptor for ciliary neurotrophic factor.

IL-6 and soluble IL-6 receptors (sIL-6R and sgp130) in human pleural effusions: massive IL-6 production independently of underlying diseases

IL-6, soluble IL-6 receptor (sIL-6R) and soluble gp130 (sgp130) levels were measured in sera and pleural effusions from 42 patients with metastatic carcinoma, non-Hodgkin's lymphoma, tuberculosis, cardiac failure and miscellaneous diseases. Pleural IL-6 levels measured by ELISA were very high in all patient groups (mean 34.8 +/- 15.3 ng/ml) without significant difference according to diseases. IL-6 was shown to be biologically active in a proliferative assay. Serum IL-6 levels were low (0.049 +/- 0.014 ng/ml) and did not correlate with pleural fluid levels. Pleural IL-6 levels correlated with the number of polymorphonuclear cells in pleural fluid (P < 0.03). Pleural sIL-6R levels (76 +/- 8 ng/ml) were always lower than serum levels (196 +/- 12 ng/ml; P < 0.0001) but correlated with them (P < 0.01). Pleural sIL-6R and albumin levels correlated (P < 0.01), suggesting a transudation of sIL-6R from the serum. Pleural sgp130 levels (10.9 +/- 1.0 ng/ml) were lower than serum levels (24.6 +/- 2.8 ng/ml; P < 0.002). After gel filtration of pleural fluid, the bulk of IL-6 (> 90%) was recovered in a 15,000-30,000 fraction, corresponding to the expected mol. wt of free IL-6. These results suggest a production and a sequestration of IL-6 in the pleural cavity in all studied conditions.

Interleukin-6 family of cytokines induced activation of different functional sites expressed by gp130 transducing protein

Gp130 transducing protein was shown to be involved in the formation of the high affinity receptors for interleukin 6 (IL-6), interleukin-11 (IL-11), leukemia inhibitory factor, oncostatin M (OSM), ciliary neurotrophic factor (CNTF), and cardiotrophin-1. In the present study we have characterized the functional properties of antibodies directed against this protein and identified a group of monoclonal antibodies able to antagonize the biological activities of all the cytokines belonging to the IL-6 cytokine family. The B-R3 pan-blocking antibody weakly interfered with the binding of the radiolabeled ligands (with the exception of OSM, whose binding was abrogated in the presence of B-R3 monoclonal antibody) but inhibited the gp130 homodimerization or its association with gp190/leukemia inhibitory factor receptor, as well as the subsequent tyrosine phosphorylation events. In addition we identified antibodies that were able to neutralize only one single cytokine of the IL-6 family. This was the case for the B-K5 antibody, which antagonized the binding of OSM to gp130 but did not interfere with the signals provided by the related cytokines triggering the proliferation of the TF1 erythroleukemia cell line or the induction of haptoglobin synthesis in the HepG2 hepatoma cell line. Similarly, we also characterized two additional antibodies B-P8 and B-P4, which inhibited the TF1 cell proliferation observed in the presence of CNTF and IL-11, respectively. B-P8 antibody only faintly interfered with the binding of the gp130-ligands and might modulate the signal transduction pathways. This study indicates that in addition to functional site(s) required by the whole family of IL-6 type cytokines to transduce the signal insight the cell, specific cognate functional sites were recruited by OSM, CNTF, or IL-11.

gp130 transducing receptor cross-linking is sufficient to induce interleukin-6 type responses

gP130 transducing receptor is involved in the formation of high affinity receptors for the cytokines of the interleukin-6 (IL-6) family. Recruitment of gp130 by IL-6 associated to its receptor leads to the dimerization of the transducing component. In the present study we did characterize the B-S12 monoclonal antibody raised against gp130 and able to elicit IL-6 type biological activities. B-S12 antibody triggered strongly the proliferation of TF1 and XGI hematopoietic cell lines and was able to increase the synthesis of acute phase proteins in HepG2 hepatoma cell line. B-S12 also behaved as a synergistic factor with granulocyte-macrophage colony-stimulating factor for both proliferation and differentiation of CD34-positive hematopoietic cell progenitors. By using a symmetric enzyme-linked immunosorbent assay, allowing the detection of dimeric forms of soluble gp130, we found that addition of B-S12 to gp130 led to its dimerization. Analysis of the tyrosine phosphorylation events in gp130 and Jak kinase family members revealed that B-S12 quickly induced the phosphorylation of gp130 in a neural derived cell line, and that Jak1 and Jak2 were also recruited. In conclusion, we show that gp130 cross-linking with the B-S12 monoclonal antibody was sufficient to generate functional IL-6 type responses in hematopoietic, neural, and hepatic cells.

Involvement of gp130/interleukin-6 receptor transducing component in interleukin-11 receptor

The recently cloned interleukin (IL)-11 displays many biological properties in common with those reported for IL-6. In order to analyze the nature and the functionality of the IL-11 receptor we developed a proliferative assay using the human multifactor-dependent cell line TF1. We showed that a blocking monoclonal antibody GPX7 raised against the gp130/IL-6 receptor transducing subunit was also able to inhibit the IL-11-triggered TF1 line proliferation. In addition, involvement of gp130 in IL-11 signaling was demonstrated by an induction of the transducing protein phosphorylation in response to IL-11, as observed for IL-6. In contrast, the blocking monoclonal antibody B-R6, which recognized the gp80/IL-6 binding subunit failed to interfere with the IL-11 proliferative signal in the TF1 cell line. Similarly, we did not observe any competition between IL-6 and IL-11 for a putative common binding site on the cell surface. These results suggest that the IL-11 binding component is different from the gp80/IL-6 receptor. In conclusion, IL-11, along with IL-6, leukemia inhibitory factor, oncostatin M and ciliary neurotrophic factor, belongs to the same family of cytokines, using gp130 as a transducing protein.