The inhibition of interleukin-6-dependent STAT activation by mitogen-activated protein kinases depends on tyrosine 759 in the cytoplasmic tail of glycoprotein 130

Role of SOCS and VHL Proteins in Neuronal Differentiation and Development

The basic helix-loop-helix factors play a central role in neuronal differentiation and nervous system development, which involve the Notch and signal transducer and activator of transcription (STAT)/small mother against decapentaplegic signaling pathways. Neural stem cells differentiate into three nervous system lineages, and the suppressor of cytokine signaling (SOCS) and von Hippel-Lindau (VHL) proteins are involved in this neuronal differentiation. The SOCS and VHL proteins both contain homologous structures comprising the BC-box motif. SOCSs recruit Elongin C, Elongin B, Cullin5(Cul5), and Rbx2, whereas VHL recruits Elongin C, Elongin B, Cul2, and Rbx1. SOCSs form SBC-Cul5/E3 complexes, and VHL forms a VBC-Cul2/E3 complex. These complexes degrade the target protein and suppress its downstream transduction pathway by acting as E3 ligases via the ubiquitin-proteasome system. The Janus kinase (JAK) is the main target protein of the E3 ligase SBC-Cul5, whereas hypoxia-inducible factor is the primary target protein of the E3 ligase VBC-Cul2; nonetheless, VBC-Cul2 also targets the JAK. SOCSs not only act on the ubiquitin-proteasome system but also act directly on JAKs to suppress the Janus kinase-signal transduction and activator of transcription (JAK-STAT) pathway. Both SOCS and VHL are expressed in the nervous system, predominantly in brain neurons in the embryonic stage. Both SOCS and VHL induce neuronal differentiation. SOCS is involved in differentiation into neurons, whereas VHL is involved in differentiation into neurons and oligodendrocytes; both proteins promote neurite outgrowth. It has also been suggested that the inactivation of these proteins may lead to the development of nervous system malignancies and that these proteins may function as tumor suppressors. The mechanism of action of SOCS and VHL involved in neuronal differentiation and nervous system development is thought to be mediated through the inhibition of downstream signaling pathways, JAK-STAT, and hypoxia-inducible factor-vascular endothelial growth factor pathways. In addition, because SOCS and VHL promote nerve regeneration, they are expected to be applied in neuronal regenerative medicine for traumatic brain injury and stroke.

Overlapping and distinct biological effects of IL-6 classic and trans-signaling in vascular endothelial cells

IL-6 affects tissue protective/reparative and inflammatory properties of vascular endothelial cells (ECs). This cytokine can signal to cells through classic and trans-signaling mechanisms, which are differentiated based on the expression of IL-6 receptor (IL-6R) on the surface of target cells. The biological effects of these IL-6-signaling mechanisms are distinct and have implications for vascular pathologies. We have directly compared IL-6 classic and trans-signaling in ECs. Human ECs expressed IL-6R in culture and in situ in coronary arteries from heart transplants. Stimulation of human ECs with IL-6, to model classic signaling, triggered the activation of phosphatidylinositol 3-kinase (PI3K)-Akt and ERK1/2 signaling pathways, whereas stimulation with IL-6 + sIL-6R, to model trans-signaling, triggered activation of STAT3, PI3K-Akt, and ERK1/2 pathways. IL-6 classic signaling reduced persistent injury of ECs in an allograft model of vascular rejection and inhibited cell death induced by growth factor withdrawal. When inflammatory effects were examined, IL-6 classic signaling did not induce ICAM or CCL2 expression but was sufficient to induce secretion of CXCL8 and support transmigration of neutrophil-like cells. IL-6 trans-signaling induced all inflammatory effects studied. Our findings show that IL-6 classic and trans-signaling have overlapping but distinct properties in controlling EC survival and inflammatory activation. This has implications for understanding the effects of IL-6 receptor-blocking therapies as well as for vascular responses in inflammatory and immune conditions.

Biochanin A impedes STAT3 activation by upregulating p38δ MAPK phosphorylation in IL-6-stimulated macrophages

OBJECTIVE

IL-6-induced STAT3 activation is associated with various chronic inflammatory diseases. In this study, we investigated the anti-STAT3 mechanism of the dietary polyphenol, biochanin A (BCA), in IL-6-treated macrophages.

METHODS

The effect of BCA on STAT3 and p38 MAPK was analyzed by immunoblot. The localization of both these transcription factors was determined by immunofluorescence and fractionation studies. The impact on DNA-binding activity of STAT3 was studied by luciferase assay. To understand which of the isoforms of p38 MAPK was responsible for BCA-mediated regulation of STAT3, overexpression of the proteins, site-directed mutagenesis, pull-down assays and computational analysis were performed. Finally, adhesion-migration assays and semi-quantitative PCR were employed to understand the biological effects of BCA-mediated regulation of STAT3.

RESULTS

BCA prevented STAT3 phosphorylation (Tyr⁷⁰⁵) and increased p38 MAPK phosphorylation (Thr¹⁸⁰/Tyr¹⁸²) in IL-6-stimulated differentiated macrophages. This opposing modulatory effect of BCA was not observed in cells treated with other stress-inducing stimuli that activate p38 MAPK. BCA abrogated IL-6-induced nuclear translocation of phospho-STAT3 and its transcriptional activity, while increasing the cellular abundance of phospho-p38 MAPK. BCA-induced phosphorylation of p38δ, but not α, β, or γ was responsible for impeding IL-6-induced STAT3 phosphorylation. Interestingly, interaction with phospho-p38δ masked the Tyr⁷⁰⁵ residue of STAT3, preventing its phosphorylation. BCA significantly reduced STAT3-dependent expression of icam-1 and mcp-1 diminishing IL-6-mediated monocyte adhesion and migration.

CONCLUSION

This differential regulation of STAT3 and p38 MAPK in macrophages establishes a novel anti-inflammatory mechanism of BCA which could be important for the prevention of IL-6-associated chronic inflammatory diseases.

A Novel Small-Molecule Inhibitor Targeting the IL-6 Receptor β Subunit, Glycoprotein 130

IL-6 is a major causative factor of inflammatory disease. Although IL-6 and its signaling pathways are promising targets, orally available small-molecule drugs specific for IL-6 have not been developed. To discover IL-6 antagonists, we screened our in-house chemical library and identified LMT-28, a novel synthetic compound, as a candidate IL-6 blocker. The activity, mechanism of action, and direct molecular target of LMT-28 were investigated. A reporter gene assay showed that LMT-28 suppressed activation of STAT3 induced by IL-6, but not activation induced by leukemia inhibitory factor. In addition, LMT-28 downregulated IL-6-stimulated phosphorylation of STAT3, gp130, and JAK2 protein and substantially inhibited IL-6-dependent TF-1 cell proliferation. LMT-28 antagonized IL-6-induced TNF-α production in vivo. In pathologic models, oral administration of LMT-28 alleviated collagen-induced arthritis and acute pancreatitis in mice. Based on the observation of upstream IL-6 signal inhibition by LMT-28, we hypothesized IL-6, IL-6Rα, or gp130 to be putative molecular targets. We subsequently demonstrated direct interaction of LMT-28 with gp130 and specific reduction of IL-6/IL-6Rα complex binding to gp130 in the presence of LMT-28, which was measured by surface plasmon resonance analysis. Taken together, our data suggest that LMT-28 is a novel synthetic IL-6 inhibitor that functions through direct binding to gp130.

Serine-727 phosphorylation activates hypothalamic STAT-3 independently from tyrosine-705 phosphorylation

Transcriptional activity of signal transducer and activator of transcription-3 (STAT-3) is a key element in the central regulation of appetite and energy homeostasis. Activation of hypothalamic STAT-3 has been attributed to cytokine-promoted phosphorylation at tyrosine-705 (Tyr-705). In nonhypothalamic cells, STAT-3 is also phosphorylated at serine-727 (Ser-727), but the functional significance of Ser-727 in the regulation of hypothalamic STAT-3 is not known. We used 2 hypothalamic cell lines and analyzed the effects of various hormones on STAT-3-dependent reporter gene activity and observed that IFN-γ, epidermal growth factor (EGF), and bradykinin (BK) induce similar STAT-3 reporter activation. EGF and BK solely increased Ser-727 and IFN-γ increased Tyr-705 phosphorylation of STAT-3. Specific inhibition of ERK-1/2 activity blocked EGF- and BK-induced STAT-3 activation and Ser-727 phosphorylation. BK-induced ERK-1/2 activation occurred via EGF receptor transactivation. Consequently, the BK-mediated effects on STAT-3 were blocked by a specific EGF receptor antagonist. Next, we analyzed the effects of IFN-γ and EGF on the expression of the STAT-3-dependent genes thyroliberin-releasing hormone and suppressors of cytokine signaling-3. EGF but not IFN-γ enhanced thyroliberin-releasing hormone expression via STAT-3. With regard to suppressors of cytokine signaling-3, we observed prolonged expression induced by IFN-γ and a transient effect of EGF that required coactivation of the activator protein-1. Thus, EGF-promoted Ser-727 phosphorylation by ERK-1/2 is not only sufficient to fully activate hypothalamic STAT-3, but, in terms of targeted genes and required cofactors, entails distinct modes of STAT-3 actions compared with IFN-γ-induced Tyr-705 phosphorylation.

Oncostatin M regulates SOCS3 mRNA stability via the MEK-ERK1/2-pathway independent of p38(MAPK)/MK2

The induction of suppressor of cytokine signalling (SOCS)3 expression context dependently involves regulation of SOCS3 transcript stability as previously demonstrated for MAPK activated protein kinase (MK)2-dependent regulation of SOCS3 expression by TNFα (Ehlting et al., 2007). In how far the IL-6-type cytokine OSM, which in contrast to IL-6 is a strong activator of p38(MAPK)/MK2 signalling, also involves regulation of transcript stability and activation of MK2 to induce SOCS3 expression is unclear. In contrast to IL-6, OSM induces SOCS3 expression in murine fibroblasts and in primary human and murine hepatocytes, but not in macrophages because the latter lack the OSM receptor (OSMR)β subunit. Evidence is provided that regulation of OSM-induced expression of SOCS3 involves MEK1- and Erk1/2-mediated stabilization of the SOCS3 transcript. Consistently, OSM-induced stabilization of the SOCS3 transcript is impaired in the presence of inhibitors that specifically block activation of MEK1/2 (U0126) and ERK1/2 (FR180204) or upon knock-down of ERK1/2 expression using specific siRNA. As a potential target site that integrates the stability regulating effect of OSM and OSM-induced activation of MEK1/2 and ERK1/2 a region containing three copies of a pentameric AUUUA motif located within position 2422 and 2541 in closed proximity to the 3' UTR of the SOCS3 transcript has been identified. Unexpectedly, activation of the p38(MAPK)/MK2 pathway, which apart from STAT3 and ERK1/2, is also strongly activated by OSM in human and murine hepatocytes and murine fibroblasts is dispensable for stabilization of the SOCS3 transcript as suggested from inhibitor studies using the p38(MAPK) inhibitor SB203580 or from the analysis of MK2-deficient hepatocytes. However, analysis of MK2-deficient macrophages and hepatocytes revealed that, although MK2 is dispensable for regulation of OSM-induced SOCS3 expression, MK2 is essential for LPS-induced OSM production in macrophages and limits the overall availability of the OSMRβ subunit in hepatocytes. Thus MK2 plays a role for the induction and sensing of OSM-mediated intercellular signalling between macrophages and hepatocytes during LPS-induced inflammation.

The role of suppressors of cytokine signalling in human neoplasms

Suppressors of cytokine signalling 1-7 (SOCS1-7) and cytokine-inducible SH2-containing protein (CIS) are a group of intracellular proteins that are well known as JAK-STAT and several other signalling pathways negative feedback regulators. More recently several members have been identified as tumour suppressors and dysregulation of their biological roles in controlling cytokine and growth factor signalling may contribute to the development of many solid organ and haematological malignancies. This review explores their biological functions and their possible tumour suppressing role in human neoplasms.

FXR ligands protect against hepatocellular inflammation via SOCS3 induction

Because of the anti-inflammatory actions of farnesoid X receptor (FXR) agonists, FXR has received much attention as a potential therapeutic target. However, the molecular mechanisms of actions have not yet been elucidated. In the present study, we reported that in the animal model of LPS-induced liver injury, administration of the FXR natural ligand CDCA could attenuate hepatocyte inflammatory damage, reduce transaminase activities, suppress inflammation mediators (IL-6, TNF-α and ICAM-1) expression and inhibit STAT3 phosphorylation. These protective effects of FXR were accompanied by an increased expression of suppressor of cytokine signaling 3 (SOCS3), which is a negative feedback regulator of cytokine-STAT3 signaling. We then demonstrated that the beneficial effects of FXR agonist in STAT3 activation were weakened by small interfering RNA-mediated SOCS3 knockdown in hepacytes. Moreover we observed both natural ligand CDCA and synthetic ligand GW4064 could upregulate SOCS 3 expression by enhancing the promoter activity in hepatocytes. These results suggest modulation of SOCS3 expression may represent a novel mechanism through which FXR activation could selectively affect cytokine bioactivity in inflammation response. FXR ligands may be potentially therapeutic in the treatment of liver inflammatory diseases via SOCS3 induction.

Signal transducer and activator of transcription-5 mediates neuronal apoptosis induced by inhibition of Rac GTPase activity

In several neuronal cell types, the small GTPase Rac is essential for survival. We have shown previously that the Rho family GTPase inhibitor Clostridium difficile toxin B (ToxB) induces apoptosis in primary rat cerebellar granule neurons (CGNs) principally via inhibition of Rac GTPase function. In the present study, incubation with ToxB activated a proapoptotic Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway, and a pan-JAK inhibitor protected CGNs from Rac inhibition. STAT1 expression was induced by ToxB; however, CGNs from STAT1 knock-out mice succumbed to ToxB-induced apoptosis as readily as wild-type CGNs. STAT3 displayed enhanced tyrosine phosphorylation following treatment with ToxB, and a reputed inhibitor of STAT3, cucurbitacin (JSI-124), reduced CGN apoptosis. Unexpectedly, JSI-124 failed to block STAT3 phosphorylation, and CGNs were not protected from ToxB by other known STAT3 inhibitors. In contrast, STAT5A tyrosine phosphorylation induced by ToxB was suppressed by JSI-124. In addition, roscovitine similarly inhibited STAT5A phosphorylation and protected CGNs from ToxB-induced apoptosis. Consistent with these results, adenoviral infection with a dominant negative STAT5 mutant, but not wild-type STAT5, significantly decreased ToxB-induced apoptosis of CGNs. Finally, chromatin immunoprecipitation with a STAT5 antibody revealed increased STAT5 binding to the promoter region of prosurvival Bcl-xL. STAT5 was recruited to the Bcl-xL promoter region in a ToxB-dependent manner, and this DNA binding preceded Bcl-xL down-regulation, suggesting transcriptional repression. These data indicate that a novel JAK/STAT5 proapoptotic pathway significantly contributes to neuronal apoptosis induced by the inhibition of Rac GTPase.

Phospho-Ser727 of STAT3 regulates STAT3 activity by enhancing dephosphorylation of phospho-Tyr705 largely through TC45

Signal transducer and activator of transcription 3 (STAT3) is a latent cytoplasmic transcription factor. It is activated by cytokines, including interleukin-6 (IL-6) through phosphorylation at Tyr705 (pY705), which is required for its dimerization and nuclear translocation. However, the role of Ser727 phosphorylation, occurring during activation, remains poorly understood. Using a combination of HepG2-stat3-knockdown cells reconstituted with various STAT3 mutants and protein kinase inhibitors, we showed that phospho-S727 has an intrinsic mechanism for shortening the duration of STAT3 activity, in turn shortening the duration of socs3 mRNA expression. Both STAT3WT and STAT3Ser727Asp (S727D) but not STAT3Ser727Ala (S727A) showed rapid dephosphorylation of pY705 after the inhibition of tyrosine kinases. We found that the nuclear TC45 phosphatase is most likely responsible for the phospho-S727-dependent pY705 dephosphorylation because TC45 knockdown caused prolonged pY705 with sustained socs3 mRNA expression in STAT3WT but not in STAT3S727A, and overexpressed TC45 caused rapid dephosphorylation of pY705 in STAT3WT but not in STAT3S727A. We further showed that phospho-S727 did not affect the interaction of TC45 with STAT3, and that a reported methylation at K140 of STAT3 occurring after phospho-S727 was not involved in the pY705 regulation. These findings indicate that phospho-Ser727 determines the duration of STAT3 activity largely through TC45.

NDRG2-mediated Modulation of SOCS3 and STAT3 Activity Inhibits IL-10 Production

Background

N-myc downstream regulated gene 2 (NDRG2) is a member of the NDRG gene family. Our previous report indicated a possible role for NDRG2 in regulating the cytokine, interleukin-10 (IL-10), which is an important immunosuppressive cytokine. Several pathways, including p38-MAPK, NF-κB, and JAK/STAT, are used for IL-10 production, and the JAK/STAT pathway can be inhibited in a negative feedback loop by the inducible protein, SOCS3. In the present study, we investigated the effect of NDRG2 gene expression on IL-10 signaling pathway that is modulated via SOCS3 and STAT3.

Methods

We generated NDRG2-overexpressing U937 cell line (U937-NDRG2) and treated the cells with PMA to investigate the role of NDRG2 in IL-10 production. U937 cells were also transfected with SOCS3- or NDRG2-specific siRNAs to examine whether the knockdown of SOCS3 or NDRG2 influenced IL-10 expression. Lastly, STAT3 and SOCS3 induction was measured to identify the signaling pathway that was associated with IL-10 production.

Results

RT-PCR and ELISA assays showed that IL-10 was increased in U937-mock cells upon stimulation with PMA, but IL-10 was inhibited by overexpression NDRG2. After PMA treatment, STAT3 phosphorylation was decreased in a time-dependent manner in U937-mock cells, whereas it was maintained in U937-NDRG2 cells. SOCS3 was markedly reduced in U937-NDRG2 cells compared with U937-mock cells. IL-10 production after PMA stimulation was reduced in U937 cells when SOCS3 was inhibited, but this effect was less severe when NDRG2 was inhibited.

Conclusion

NDRG2 expression modulates SOCS3 and STAT3 activity, eventually leading to the inhibition of IL-10 production.

Protein kinase Cbeta mediates hepatic induction of sterol-regulatory element binding protein-1c by insulin

Sterol-regulatory element binding protein-1c (SREBP-1c) is a transcription factor that controls lipogenesis in the liver. Hepatic SREBP-1c is nutritionally regulated, and its sustained activation causes hepatic steatosis and insulin resistance. Although regulation of SREBP-1c is known to occur at the transcriptional level, the precise mechanism by which insulin signaling activates SREBP-1c promoter remains to be elucidated. Here we show that protein kinase C beta (PKCbeta) is a key mediator of insulin-mediated activation of hepatic SREBP-1c and its target lipogenic genes. Activation of SREBP-1c in the liver of refed mice was suppressed by either adenoviral RNAi-mediated knockdown or dietary administration of a specific inhibitor of protein kinase C beta. The effect of PKCbeta inhibition was cancelled in insulin depletion by streptozotocin (STZ) treatment of mice. Promoter analysis indicated that PKCbeta activates SREBP-1c promoter through replacement of Sp3 by Sp1 for binding to the GC box in the sterol regulatory element (SRE) complex, a key cis-element of SREBP-1c promoter. Knockdown of Sp proteins demonstrated that Sp3 and Sp1 play reciprocally negative and positive roles in nutritional regulation of SREBP-1c, respectively. This new understanding of PKCbeta involvement in nutritional regulation of SREBP-1c activation provides a new aspect of PKCbeta inhibition as a potential therapeutic target for diabetic complications.

Signal transduction pathways involved in brain death-induced renal injury

Kidneys derived from brain death organ donors show an inferior survival when compared to kidneys derived from living donors. Brain death is known to induce organ injury by evoking an inflammatory response in the donor. Neuronal injury triggers an inflammatory response in the brain, leading to endothelial dysfunction and the release of cytokines in the circulation. Serum levels of interleukin-6, -8, -10, and monocyte chemoattractant protein-1 (MCP-1) are increased after brain death. Binding with cytokine-receptors in kidneys stimulates activation of nuclear factor-kappa B (NF-kappaB), selectins, adhesion molecules and production of chemokines leading to cellular influx. Mitogen-activated protein kinases (MAP-kinases) mediate inflammatory responses and together with NF-kappaB they seem to play an important role in brain death induced renal injury. Altering the activation state of MAP-kinases could be a promising drug target for early intervention to reduce cerebral injury related donor kidney damage and improve outcome after transplantation.

Intracellular signaling mechanisms and activities of human herpesvirus 8 interleukin-6

Human herpesvirus 8 (HHV-8)-encoded viral interleukin-6 (vIL-6) has been implicated as a key factor in virus-associated neoplasia because of its proproliferative and survival effects and also in view of its angiogenic properties. A major difference between vIL-6 and human IL-6 (hIL-6) is that vIL-6, uniquely, is largely retained and can signal intracellularly. While vIL-6 is generally considered to be a lytic gene, several reports have noted its low-level expression in latently infected primary effusion lymphoma (PEL) cultures, in the absence of other lytic gene expression. Thus, intracellular autocrine signal transduction by the viral cytokine may be of particular relevance to the growth and survival of latently infected cells and to pathogenesis. Here we report that most intracellular vIL-6 is located in the endoplasmic reticulum (ER), signals via the gp130 signal transducer in this compartment, and does so independently of the gp80 alpha-subunit of the IL-6 receptor, required for hIL-6 signal transduction. Signaling and biological assays incorporating ER-retained vIL-6 and hIL-6 confirmed vIL-6 activity, specifically, in this compartment. Knockdown of vIL-6 expression in PEL cells led to markedly reduced cell growth in normal culture, independently of extracellular cytokines. This could be reversed by reintroduction via virus vector of exclusively ER-retained vIL-6. These data indicate that in virus biology vIL-6 may act to support the growth and survival of cells latently infected with HHV-8 in an autocrine manner via intracrine signaling and that these activities may contribute to the maintenance of latently infected cells and to virus-induced neoplasia.

Analysis of extracellular signal-regulated kinase 2 function in neural stem/progenitor cells via nervous system-specific gene disruption

Extracellular signal-regulated kinase 2 (ERK2) is involved in a variety of cell fate decisions during development, but its exact role in this process remains to be determined. To specifically focus on the role of ERK2 in the brain, and to avoid early lethalities, we used a conditional gene-targeting approach to preferentially inactivate Erk2 in the embryonic mouse brain. The resulting mutant mice were viable and were relatively normal in overall appearance. However, the loss of Erk2 resulted in a diminished proliferation of neural stem cells in the embryonic ventricular zone (VZ), although the survival and differentiation of these cells was unaffected. The multipotent neural progenitor cells (NPCs) isolated from ERK2-deficient brains also showed impaired proliferation, reduced self-renewal ability, and increased apoptosis. By neurosphere differentiation analysis we further observed that lineage-restricted glial progenitors were increased in ERK2-deficient mice. The decline in the self-renewal ability and multipotency of NPCs resulting from the loss of ERK2 was found to be caused at least in part by upregulation of the JAK-STAT signaling pathway and reduced G1/S cell cycle progression. Furthermore, by global expression analysis we found that neural stem cell markers, including Tenascin C NR2E1 (Tlx), and Lgals1 (Galectin-1), were significantly downregulated, whereas several glial lineage markers were upregulated in neurospheres derived from ERK2-deficient mice. Our results thus suggest that ERK2 is required both for the proliferation of neural stem cells in the VZ during embryonic development and in the maintenance of NPC multipotency by suppressing the commitment of these cells to a glial lineage.

Estrogen and progesterone regulate the IL-6 signal transduction pathway in antibody secreting cells

Regulation of the immune response is necessary to allow successful pregnancy. Asymmetric IgG antibodies are involved in fetal maintenance. We have previously demonstrated that estrogen (E2) and progesterone (P4) modulate the synthesis of asymmetric antibodies but the underlying mechanisms remain unclear. Since IL-6 and a progesterone-induced blocking factor (PIBF) were shown to regulate asymmetric antibody synthesis, in this work we analyzed whether E2 and P4 were able to modulate IL-6 signal transduction pathways and the ability of P4 to induce PIBF synthesis, in hybridoma B cells was also evaluated. We found that the IL-6 treatment induced an increase in the expression of gp130 and JAK1 by the hybridoma. E2 and P4 diminished the IL-6-induced gp130 expression in a dose-dependent manner, whereas the expression of JAK1 was not significantly affected. At 10(-6)M concentration, the steroids inhibited the phosphorylation of gp130 and diminished the IL-6-induced STAT3 phosphorylation and translocation to the nucleus. Maximal PIBF expression was observed when the hybridoma was cultured with 10(-10)M P4, compared to the control (p<0.05). Results demonstrate two molecular mechanisms, the modulation of the IL-6R signal transduction pathway and PIBF induction, which could be involved in the immunoregulatory role of sexual steroids during pregnancy.

c-Cbl-dependent monoubiquitination and lysosomal degradation of gp130

Interleukin 6 (IL-6), a pleiotropic cytokine, functions in cells through its interaction with its receptor complex, which consists of two ligand-binding alpha subunits and two signal-transducing subunits known as gp130. There is a wealth of studies on signals mediated by gp130, but its downregulation is less well understood. Here we found that IL-6 stimulation induced lysosome-dependent degradation of gp130, which correlated with an increase in the K63-linked polyubiquitination of gp130. The stimulation-dependent ubiquitination of gp130 was mediated by c-Cbl, an E3 ligase, which was recruited to gp130 in a tyrosine-phosphorylated SHP2-dependent manner. We also found that IL-6 induced a rapid translocation of gp130 from the cell surface to endosomal compartments. Furthermore, the vesicular sorting molecule Hrs contributed to the lysosomal degradation of gp130 by directly recognizing its ubiquitinated form. Deficiency of either Hrs or c-Cbl suppressed gp130 degradation, which leads to a prolonged and amplified IL-6 signal. Thus, our present report provides the first evidence for involvement of a c-Cbl/SHP2 complex in ubiquitination and lysosomal degradation of gp130 upon IL-6 stimulation. The lysosomal degradation of gp130 is critical for cessation of IL-6-mediated signaling.

Insulin Antagonizes Interleukin-6 Signaling and Is Anti-inflammatory in 3T3-L1 Adipocytes

Adipose tissue secretes different adipokines, including interleukin-6 (IL-6), that have been implicated in the insulin resistance and inflammatory state characterizing obesity. We examined the putative cross-talk between insulin and IL-6 in adipose cells and found that insulin exerts an inhibitory effect on the IL-6 signaling pathway by altering the post-translational modifications of the signal transducer and activator of transcription 3 (STAT3). Insulin reduces the tyrosine phosphorylation and increases the serine phosphorylation of STAT3, thereby reducing its nuclear localization and transcriptional activity. Signaling through the MEK/MAPK pathway plays an important role as treatment with the MEK inhibitor PD98059 reduces the effects of insulin on IL-6 signaling. We also show that the protein tyrosine phosphatase SHP2 is activated upon insulin signaling and is required for the dephosphorylation of STAT3 and that insulin exerts a synergistic effect with IL-6 on suppressor of cytokine signaling 3 expression. As a consequence, the IL-6-induced expression of the inflammatory markers serum amyloid A 3 and haptoglobin are significantly decreased in cells incubated with both IL-6 and insulin. Thus, insulin exerts an important anti-inflammatory effect in adipose cells by impairing the IL-6 signal at several levels.

Signal transduction by human herpesvirus 8 viral interleukin-6 (vIL-6) is modulated by the nonsignaling gp80 subunit of the IL-6 receptor complex and is distinct from signaling induced by human IL-6

Human herpesvirus 8 (HHV-8) viral interleukin-6 (vIL-6) mediates signaling through the gp130 signal transducer but unlike human IL-6 (hIL-6) does not require the nonsignaling gp80 alpha subunit of the IL-6 receptor complex. By utilizing a gp80-refractory vIL-6 variant, vIL-6(R189L), we found that signal transduction, as measured by STAT1 and STAT3 activation and gp130 tyrosine phosphorylation in gp80+/gp130+ HEK293T cells, was modulated by gp80. Furthermore, the signaling and BAF-130 cell growth-promoting activities of vIL-6 and hIL-6 could be distinguished, and exogenous addition of soluble gp80 enhanced cell growth supported by vIL-6. Our findings demonstrate that gp80 can modulate vIL-6 activity and that vIL-6 and hIL-6 signaling are not directly equivalent.

Endothelin-1 induces hypertrophy and inhibits apoptosis in human airway smooth muscle cells

Endothelin-1 (ET-1), a G protein-coupled receptor-activating peptide, is increased in airway epithelium, plasma, and bronchoalveolar lavage fluid of asthmatic patients. We hypothesized that ET-1 may contribute to the increased airway smooth muscle mass found in severe asthma by inducing hypertrophy and inhibiting apoptosis of smooth muscle cells. To investigate this hypothesis, we determined that treatment of primary human bronchial smooth muscle cells with ET-1 dose dependently [10(-11)-10(-7) M] inhibited the apoptosis induced by serum withdrawal. ET-1 treatment also resulted in a significant increase in total protein synthesis, mediated through both ET(A) and ET(B) receptors, cell size, as well as increased expression of myosin heavy chain, alpha-smooth muscle actin, and calponin. ET-1-induced hypertrophy was accompanied by activation of JAK1/STAT-3 and MAPK1/2 (ERK1/2) cell signaling pathways. Inhibition of JAK1/STAT-3 pathways by piceatannol or ERK1/2 by the MAPK/ERK kinase 1/2 inhibitor U0126 blunted the increase in total protein synthesis. The hypertrophic effect of ET-1 was equivalent to that of the gp130 cytokine oncostatin M and greater than that induced by cardiotrophin-1. ET-1 induced release of IL-6 but not IL-11, leukemia inhibitory factor, oncostatin M, or cardiotrophin-1, although treatment of cells with IL-6 alone did not induce hypertrophy. These results suggest that ET-1 is a candidate mediator for the induction of increased smooth muscle mass in asthma and identify signaling pathways activated by this mediator.

Rho family GTPase inhibition reveals opposing effects of mitogen-activated protein kinase kinase/extracellular signal-regulated kinase and Janus kinase/signal transducer and activator of transcription signaling cascades on neuronal survival

Rho family GTPases promote the survival of certain neuronal populations. However, pro-survival and pro-death signaling pathways regulated downstream of Rho GTPases are largely unknown. Cerebellar granule neurons (CGNs) exposed to Clostridium difficile toxin B (ToxB), a monoglucosyltransferase that specifically inhibits Rho GTPases, die by a mitochondrial apoptotic cascade. Using a high-throughput immunoblotting screen (BD Powerblot), we found that ToxB markedly reduced the expression of Rac1 and c-Raf, upstream components of a Rac-dependent mitogen-activated protein (MAP) kinase pathway. Moreover, ToxB rapidly suppressed a p21-activated kinase/MAP kinase kinase (MEK)/extracellular signal-regulated kinase (ERK)1/2 signaling cascade that normally promotes degradation of the Bcl-2 homology-3 (BH3)-only protein Bim, a key initiator of mitochondrial apoptosis. In contrast to c-Raf down-regulation, ToxB enhanced expression of the transcription factor, signal transducer and activator of transcription-1 (STAT1). Both STAT1 up-regulation and apoptosis induced by ToxB were prevented by a pan-inhibitor of Janus kinases (JAKs), indicating that JAK/STAT signaling was pro-apoptotic in CGNs. Most significantly, direct inhibition of MEK was sufficient to trigger JAK-dependent STAT1 expression, suggesting that cross-talk between MEK/ERK and JAK/STAT pathways plays a key role in regulating neuronal survival. Finally, ERK dephosphorylation and STAT1 up-regulation induced by ToxB were mimicked by a dominant-negative (N17) mutant of Rac1. These data suggest that the MEK/ERK cascade functions downstream of Rac GTPase to actively repress pro-apoptotic JAK/STAT signaling in healthy CGNs.

Cerebral preconditioning using cortical application of hypertonic salt solutions: upregulation of mRNAs encoding inhibitors of inflammation

Previous studies have demonstrated that local application of hypertonic KCl or NaCl to the cerebral cortex induces tolerance to a subsequent episode of ischemia. The objective of the present study was to determine whether application of these salts increases the levels of mRNAs encoding inhibitors of inflammation. Hypertonic KCl or NaCl was applied for 2 h to the frontal cortex of Sprague-Dawley rats. After recovery periods up to 24 h, levels of selected mRNAs were measured in samples from frontal and parietal cortex using Northern blots. Application of hypertonic KCl caused a rapid and widespread increase in the levels of mRNA coding for tumor necrosis factor (TNF), tristetraprolin (TTP), suppressor of cytokine signaling-3 (SOCS3), and brain-derived neurotrophic factor (BDNF), and a 24-h delayed induction of ciliary neurotrophic factor (CNTF) mRNA. Application of hypertonic NaCl caused alterations in mRNA levels that were restricted to the frontal cortex. In this region, application of NaCl rapidly increased levels of mRNA encoding TNF, TTP, and SOCS3, but not BDNF, and caused a delayed induction of CNTF mRNA. These results raise the possibility that upregulation of inhibitors of inflammation after preconditioning may contribute to the induction of tolerance to ischemia.

Sp3 is involved in the regulation of SOCS3 gene expression

Cytokine-induced expression of SOCS (suppressor of cytokine signalling) molecules is important for the negative regulatory control of STAT (signal transduction and activators of transcription)-dependent cytokine signalling, e.g. for the signal transduction of IL-6 (interleukin-6)-type cytokines through the JAK (Janus kinase)/STAT cascade. STAT activation itself represents an important step in the transcriptional activation of SOCS3 gene expression. However, downstream of the STAT-responsive element, the SOCS3 gene contains a GC-rich element in its 5'-upstream region. The aim of the present study was to investigate the implications of this GC-rich element in the transcriptional control of SOCS3 gene expression. In the present study, we show that mutation of this GC-rich element abolishes IL-6-dependent transcriptional activation of the SOCS3 promoter and that Sp3 (specificity protein 3), a ubiquitously expressed transcription factor, but not Sp1 binds to this GC-rich motif, suggesting that Sp3 is involved in the regulation of SOCS3 expression. The results suggest that Sp3 is important for IL-6-induced transcriptional activation of the SOCS3 (gene) promoter and acts as an enhancer of basal as well as induced transcriptional activity, resulting in enhanced SOCS3 mRNA and protein expression. Mutation of Lys-483, a potential target for Sp3 acetylation, inhibited Sp3-mediated enhancement of SOCS3 mRNA expression and SOCS3 promoter activation, indicating that the acetylation of this lysine residue of Sp3 is important for the enhancing effect of Sp3 on SOCS3 expression.

Oncostatin M receptor-mediated signal transduction is negatively regulated by SOCS3 through a receptor tyrosine-independent mechanism

Down-regulation of interleukin (IL)-6-type cytokine signaling has been shown to occur, among other mechanisms, via induction of the feedback inhibitor SOCS3 (suppressor of cytokine signaling 3). Binding of SOCS3 to the phosphorylated Tyr(759) in the cytoplasmic region of gp130, the common signal transducing receptor chain of all IL-6-type cytokines, is necessary for inhibition of Janus kinase-mediated signaling. In the present study, we analyzed the effect of SOCS3 on signal transduction by the proinflammatory cytokine oncostatin M (OSM), which signals through a receptor complex of gp130 and the OSM receptor (OSMR). OSM leads to a much stronger and prolonged induction of SOCS3 in HepG2 hepatoma cells and murine embryonal fibroblasts (MEF) compared with IL-6. A negative effect of SOCS3 on OSM signaling was confirmed using MEF cells lacking SOCS3. We can show that the OSMR-mediated signaling is inhibited by SOCS3 to a similar extent as previously described for gp130. However, the inhibition occurs independent of tyrosine motifs within the OSMR. Instead, SOCS3 interacts directly with JAK1 in a stimulation-dependent manner, a mechanism so far only known for SOCS1.

A search for genes modulated by interleukin-6 alone or with interleukin-1beta in HepG2 cells using differential display analysis

Interleukin-1 and interleukin-6 are principal cytokines involved in regulation of expression of acute-phase proteins. In the joint action of both cytokines IL-1 can suppress or enhance the IL-6-dependent induction of gene expression. Here, we report changes in the transcriptome profile of HepG2 cells exposed to IL-6 alone, or IL-1 and IL-6. Cytokine-responsive genes were identified by differential display analysis. Validation of observed changes in the transcript level was carried out using the slot blot method. Out of 88 cDNA species modulated by IL-6, only 38 represent different known genes whereas 18 clones match genomic clones in NCBI data with hypothetical cDNA sequences (the remaining 32 clones showed no homology with the database or represented several clones of the same gene). In the experiments with HepG2 cells prestimulated for 3 h with IL-1 and then stimulated with IL-6, 43 cDNA fragments were amplified. Twenty-three of them represent known genes while 10 clones have inserts matching hypothetical cDNA sequences in NCBI data. The identified transcripts modulated by IL-6 or both cytokines in HepG2 cells code for intracellular proteins of various function. The largest groups represent genes engaged in metabolism, protein synthesis and signaling pathways. Among all genes identified as differentially regulated under stimulation by IL-6, or IL-1/IL-6, six were detected in both types of stimulation. None of the typical genes coding for plasma acute phase proteins was identified in our experiments. This indicates that differential display cannot be used to characterize the profile of a given transcriptome. On the other hand, it is a useful technique for detection of new genes responding to IL-6 alone or IL-6 in combination with IL-1.

Cellular mechanisms associated with spontaneous and ciliary neurotrophic factor-cAMP-induced survival and axonal regeneration of adult retinal ganglion cells

We have shown previously that intraocular elevation of cAMP using the cAMP analog 8-(4-chlorophenylthio)-cAMP (CPT-cAMP) failed to promote axonal regeneration of axotomized adult retinal ganglion cells (RGCs) into peripheral nerve (PN) grafts but significantly potentiated ciliary neurotrophic factor (CNTF)-induced axonal regeneration. Using the PN graft model, we now examine the mechanisms underlying spontaneous and CNTF/CPT-cAMP-induced neuronal survival and axonal regrowth. We found that blockade of the cAMP pathway executor protein kinase A (PKA) using the cell-permeable inhibitor KT5720 did not affect spontaneous survival and axonal regeneration but essentially abolished the CNTF/CPT-cAMP-induced RGC survival and axonal regeneration. Blockade of CNTF signaling pathways such as phosphotidylinositol 3-kinase (PI3K)/akt by 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY294002), mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) by 2-(2-diamino-3-methoxyphenyl-4H-1-benzopyran-4-one (PD98059), or Janus kinase (JAK)/signal transducer and activators of transcription (STAT3) by tyrphostin AG490 also blocked the CNTF/CPT-cAMP-dependent survival and regeneration effects. PKA activity assay and Western blots showed that KT5720, LY294002, and PD98059 almost completely inhibited PKA, PI3K/akt, and MAPK/ERK signal transduction, respectively, whereas AG490 substantially decreased JAK/STAT3 signal transduction. Intraocular injection of CPT-cAMP resulted in a small PKA-dependent increase in CNTF receptor alpha mRNA expression in the retinas, an effect that may facilitate CNTF action on survival and axonal regeneration. Surprisingly, in the absence of CNTF/CPT-cAMP, LY294002, PD98059, and AG490, but not KT5720, significantly enhanced spontaneous RGC survival, suggesting differential roles of these pathways in RGC survival under different conditions. Our data suggest that CNTF/CPT-cAMP-induced RGC survival and axonal regeneration are a result of multiple pathway actions, with PKA as an essential component, but that these pathways can function in an antagonistic manner under different conditions.

Suppressors of cytokine signaling regulate Fc receptor signaling and cell activation during immune renal injury

Suppressors of cytokine signaling (SOCS) are cytokine-inducible proteins that modulate receptor signaling via tyrosine kinase pathways. We investigate the role of SOCS in renal disease, analyzing whether SOCS regulate IgG receptor (FcgammaR) signal pathways. In experimental models of immune complex (IC) glomerulonephritis, the renal expression of SOCS family genes, mainly SOCS-3, significantly increased, in parallel with proteinuria and renal lesions, and the proteins were localized in glomeruli and tubulointerstitium. Induction of nephritis in mice with a deficiency in the FcgammaR gamma-chain (gamma(-/-) mice) resulted in a decrease in the renal expression of SOCS-3 and SOCS-1. Moreover, blockade of FcgammaR by Fc fragment administration in rats with ongoing nephritis selectively inhibited SOCS-3 and SOCS-1, without affecting cytokine-inducible Src homology 2-containing protein and SOCS-2. In cultured human mesangial cells (MC) and monocytes, IC caused a rapid and transient induction of SOCS-3 expression. Similar kinetics was observed for SOCS-1, whereas SOCS-2 expression was very low. MC from gamma(-/-) mice failed to respond to IC activation, confirming the participation of FcgammaR. Interestingly, IC induced tyrosine phosphorylation of SOCS-3 and Tec tyrosine kinase, and both proteins coprecipitated in lysates from IC-stimulated MC, suggesting intracellular association. IC also activated STAT pathway in MC, which was suppressed by SOCS overexpression, mainly SOCS-3. In SOCS-3 knockdown studies, specific antisense oligonucleotides inhibited mesangial SOCS-3 expression, leading to an increase in the IC-induced STAT activation. Our results indicate that SOCS may play a regulatory role in FcgammaR signaling, and implicate SOCS as important modulators of cell activation during renal inflammation.

Cytoplasmic c-Fos induced by the YXXQ-derived STAT3 signal requires the co-operative MEK/ERK signal for its nuclear translocation

A STAT3 (signal transducer and activator of transcription 3)- and a MEK/Erk-mediated signal can be activated by cytokines, including IL-6 (interleukin-6), PDGF, and EGF. Recently, STAT3 and an ERK-signal were shown to co-operatively activate the c-fos gene. Activation of a truncated form of the IL-6 receptor subunit, gp130, that had only one YXXQ motif, induced both c-Fos and JunB in NIH3T3 cells through STAT3 without an apparent increase in the AP-1 (activator protein-1) activity. In contrast, concomitant stimulation of the STAT3 signal and a MEK/Erk-signal markedly increased AP-1 activity with enhanced c-Fos expression. Surprisingly, the c-Fos induced by the YXXQ-signal alone was localized to the cytoplasm, from which it translocated into the nucleus following TPA (12-O-tetradecanoyl-phorbol 13-acetate) treatment in a MEK/Erk-dependent manner. c-Fos that was expressed from a constitutive promoter localized to the nucleus and did not move into the cytoplasm in response to the YXXQ-signal. Rather, the YXXQ-signal was required during c-Fos production for it to be retained in the cytoplasm. Thus, the YXXQ-signal induces c-Fos expression through STAT3 and anchors the new c-Fos in the cytoplasm. In addition, the YXXQ-signal and an Erk signal co-operatively cause c-Fos activation in the nucleus.

Statins induce suppressor of cytokine signaling-3 in macrophages

Our previous study has shown that lipophilic 3-hydroxy-3-methyl-glutaryl coenzyme A reductase inhibitors of statins can inhibit interferon-gamma-induced inducible nitric oxide synthase gene expression in RAW264.7 macrophages. In this study, we showed that lovastatin and fluvastatin are able to upregulate the mRNA expression of the suppressor of cytokine signaling-3 (SOCS-3) gene. This effect is specific for SOCS-3 and could be blocked by mevalonate, farnesyl pyrophosphate and geranylgeranyl pyrophosphate, while it was not affected by inhibitors of protein kinase C and A, mitogen-activated protein/extracellular signal-regulated kinase kinase, p38 mitogen-activated protein kinase, c-Jun N-terminal kinase, Src, Raf and Rho kinase. SOCS-3 expression results in the inhibition of interferon-gamma-, interleukin-6- and macrophage colony-stimulating factor-elicited signal transducer and activator of transcription phosphorylation, suggesting a novel anti-inflammatory mechanism of statins to down-modulate the functions of interferon-gamma-activated macrophages.

Inhibition of interleukin-4-induced class switch recombination by a human immunoglobulin Fc gamma-Fc epsilon chimeric protein

Immunoglobulin E (IgE) is important in mediating human allergic diseases. We tested the hypothesis that a human Ig Fc gamma-Fc epsilon bifunctional chimeric protein, GE2, would inhibit IgE class switch recombination (CSR) by co-aggregating B-cell CD32 and CD23. Indeed, GE2 directly inhibited epsilon germ-line transcription, subsequent CSR to epsilon and IgE protein production. This CSR inhibition was dependent on CD23 binding and the phosphorylation of extracellular signal-related kinase (ERK), and it was mediated via suppression of interleukin-4-induced STAT6 phosphorylation. Treatment with PD98059, a specific inhibitor of mitogen-activated protein kinase kinase 1 (MAPKK1 (MEK1)) and MEK2 reversed the ability of GE2 to decrease CSR and STAT6 phosphorylation. GE2 stimulation induced ERK phosphorylation, whereas it did not alter the phosphorylation of c-Jun N-terminal kinase or p38 MAPK. The ability of GE2 to block human isotype switching to epsilon, in addition to its already demonstrated ability to inhibit mast cell and basophil function, suggests that it will provide an important novel benefit in the treatment of IgE-mediated diseases.

Helicobacter pylori infection interferes with epithelial Stat6-mediated interleukin-4 signal transduction independent of cagA, cagE, or VacA

Helicobacter pylori is a bacterial pathogen evolved to chronically colonize the gastric epithelium, evade immune clearance by the host, and cause gastritis, peptic ulcers, and even gastric malignancies in some infected humans. In view of the known ability of this bacterium to manipulate gastric epithelial cell signal transduction cascades, we determined the effects of H. pylori infection on epithelial IL-4-Stat6 signal transduction. HEp-2 and MKN45 epithelial cells were infected with H. pylori strains LC11 or 8823 (type 1; cagA(+)/cagE(+)/VacA(+)), LC20 (type 2; cagA(-), cagE(-), VacA(-)), and cagA, cagE, and vacA isogenic mutants of strain 8823, with some cells receiving subsequent treatment with the Th2 cytokine IL-4, a known Stat6 activator. Immunofluorescence showed a disruption of Stat6-induced nuclear translocation by IL-4 in LC11-infected HEp-2 cells. IL-4-inducible Stat6 DNA binding in HEp-2 and MKN45 cells was abrogated by infection, but MKN45 cell viability was unaffected. A decrease in IL-4-mediated Stat6 tyrosine phosphorylation in nuclear and whole cell lysates was also observed following infection with strains LC11 and LC20, while neither strain altered IL-4 receptor chain alpha or Janus kinase 1 protein expression. Furthermore, parental strain 8823 and its isogenic cagA, cagE, and vacA mutants also suppressed IL-4-induced Stat6 tyrosine phosphorylation to comparable degrees. Thus, H. pylori did not directly activate Stat6, but blocked the IL-4-induced activation of epithelial Stat6. This may represent an evolutionarily conserved strategy to disrupt a Th2 response and evade the host immune system, allowing for successful chronic infection.

Desensitization of signaling by oncostatin M in human vascular cells involves cytoplasmic Tyr residue 759 in gp130 but is not mediated by either Src homology 2 domain-containing tyrosine phosphatase 2 or suppressor of cytokine signaling 3

Oncostatin M (OnM) signals through cell surface receptors, which utilize the gp130 subunit. In cultured human umbilical vein endothelial cells (HUVEC), OnM transiently elevates mRNA encoding for suppressor of cytokine signaling-3 (SOCS-3). By 1 h of OnM treatment, HUVEC become refractory to the restimulation by OnM, measured as failure to reinduce SOCS-3 mRNA. OnM-induced desensitization also prevents responses to other gp130-signaling cytokines (e.g. leukemia inhibitory factor and interleukin 11). OnM treatment does not affect gp130 expression levels and desensitizes signaling mediated by a transduced chimeric receptor containing extracellular domains of platelet-derived growth factor receptor-beta (PDGFRbeta) and the cytoplasmic region of gp130. Interestingly, a chimeric PDGFRbeta-gp130 mutant receptor, in which intracellular Tyr residue 759 of gp130 is replaced by a Phe residue, mediates prolonged signaling and is not cross-desensitized by OnM. Phospho-Tyr759 is the binding site for both SOCS-3 and for Src homology domain 2-containing tyrosine phosphatase 2 (SHP-2). In human aortic smooth muscle cells, neither prevention of SOCS-3 protein induction, using STAT3 or SOCS-3 antisense, nor prevention of SHP-2 expression, also with antisense, ablates desensitization. These data suggest that desensitization of vascular cells to OnM is mediated in trans and involves Tyr residue 759 in gp130 but is not mediated by either SOCS-3 or SHP-2, the only two proteins currently known to bind to gp130 at this site.

15d-PGJ2 and rosiglitazone suppress Janus kinase-STAT inflammatory signaling through induction of suppressor of cytokine signaling 1 (SOCS1) and SOCS3 in glia

Peroxisome proliferator-activated receptor (PPAR)-gamma agonists are now emerging as therapeutic drugs for various inflammatory diseases. However, their molecular mechanism of action remains to be elucidated. Here we report a novel mechanism that underlies the PPAR-gamma agonist-mediated suppression of brain inflammation. We show that 15-deoxy-Delta12,14-prostaglandin J(2) (15d-PGJ(2)) and rosiglitazone reduce the phosphorylation of STAT1 and STAT3 as well as Janus kinase 1 (JAK1) and JAK2 in activated astrocytes and microglia. The PPAR-gamma agonist-mediated reduction in phosphorylation leads to the suppression of JAK-STAT-dependent inflammatory responses. The effects of 15d-PGJ(2) and rosiglitazone are not mediated by activation of PPAR-gamma. 15d-PGJ(2) and rosiglitazone rapidly induce the transcription of suppressor of cytokine signaling (SOCS) 1 and 3, which in turn inhibit JAK activity in activated glial cells. In addition, Src homology 2 domain-containing protein phosphatase 2 (SHP2), another negative regulator of JAK activity, is also involved in their anti-inflammatory action. Our data suggest that 15d-PGJ(2) and rosiglitazone suppress the initiation of JAK-STAT inflammatory signaling independently of PPAR-gamma, thus attenuating brain inflammation.

Suppressor of cytokine signaling-3 (SOCS-3), a potential mediator of interleukin-6-dependent insulin resistance in hepatocytes

Interleukin-6 (IL-6) is one of several pro-inflammatory cytokines implicated in insulin resistance during infection, cachexia, and obesity. We recently demonstrated that IL-6 inhibits insulin signaling in hepatocytes (Senn, J. J., Klover, P. J., Nowak, I. A., and Mooney, R. A. (2002) Diabetes 51, 3391-3399). Members of the suppressors of cytokine signaling (SOCS) family associate with the insulin receptor (IR), and their ectopic expression inhibits IR signaling. Since several SOCS proteins are induced by IL-6, a working hypothesis is that IL-6-dependent insulin resistance is mediated, at least in part, by induction of SOCS protein(s) in insulin target cells. To examine the involvement of SOCS protein(s) in IL-6-dependent inhibition of insulin receptor signaling, HepG2 cells were treated with IL-6 (20 ng/ml) for periods from 1 min to 8 h. IL-6 induced SOCS-3 transcript at 30 min with a maximum effect at 1 h. SOCS-3 protein levels were also markedly elevated at 1 h. Transcript and protein levels returned to near basal levels by 2 h. SOCS-3 induction by IL-6 paralleled IL-6-dependent inhibition of IR signal transduction. Ectopically expressed SOCS-3 associated with the IR and suppressed insulin-dependent receptor autophosphorylation, insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation, association of IRS-1 with the p85 subunit of phosphatidylinositol 3-kinase, and activation of Akt. SOCS-3 was also a direct inhibitor of insulin receptor autophosphorylation in vitro. In mice exposed to IL-6 for 60-90 min, hepatic SOCS-3 expression was increased. This was associated with inhibition of hepatic insulin-dependent receptor autophosphorylation and IRS-1 tyrosine phosphorylation. These data suggest that induction of SOCS-3 in liver may be an important mechanism of IL-6-mediated insulin resistance.

Activation of STAT3 by IL-6 and IL-10 in primary human macrophages is differentially modulated by suppressor of cytokine signaling 3

On human macrophages IL-10 acts as a more potent anti-inflammatory cytokine than IL-6, although both cytokines signal mainly via activation of the transcription factor STAT3. In this study we compare IL-10 and IL-6 signaling in primary human macrophages derived from blood monocytes. Pretreatment of macrophages with PMA or the proinflammatory mediators LPS and TNF-alpha blocks IL-6-induced STAT3 activation, whereas IL-10-induced activation of STAT3 remains largely unaffected. Although LPS induces the feedback inhibitor suppressor of cytokine signaling 3 (SOCS3) in macrophages, inhibition of IL-6 signal transduction by LPS occurs rapidly and does not depend on gene transcription. We also found that pretreatment of macrophages with IL-10 inhibits subsequent STAT3 activation by IL-6, whereas IL-10-induced STAT3 activation is not affected by preincubation with IL-6. This cross-inhibition is dependent on active transcription and might therefore be explained by different sensitivities of IL-10 and IL-6 signaling toward the feedback inhibitor SOCS3, which is induced by both cytokines. In contrast to the IL-6 signal transducer gp130, which has been previously shown to recruit SOCS3 to one of its phosphotyrosine residues (Y759), peptide precipitation experiments suggest that SOCS3 does not interact with phosphorylated tyrosine motifs of the IL-10R. Taken together, different sensitivities of IL-10 and IL-6 signaling toward mechanisms that inhibit the Janus kinase/STAT pathway define an important mechanism that contributes to the different anti-inflammatory potencies of these two cytokines.

Accelerated in vivo growth of prostate tumors that up-regulate interleukin-6 is associated with reduced retinoblastoma protein expression and activation of the mitogen-activated protein kinase pathway

Interleukin-6 (IL-6) is a multifunctional cytokine that activates the signaling pathways of Janus kinases-signal transducers and activators of transcription (STAT) and/or mitogen-activated protein kinases (MAPK) in various tumors. Thus, it modulates cell growth and apoptosis. IL-6 levels are elevated in tissues and sera from prostate cancer patients and IL-6 receptor expression has been detected in prostate cancer cell lines and clinical specimens. Continuous exposure of prostate cancer cells to IL-6 might alter their responsiveness to this cytokine. To gain more insight into the function of IL-6 in prostate carcinoma, we have inoculated LNCaP-IL-6+ cells, generated after prolonged treatment with IL-6, into nude mice (total n = 16, two independent experiments). Controls included animals bearing LNCaP-IL-6- cells, passaged at the same time as LNCaP-IL-6+ cells without supplementation of IL-6. LNCaP-IL-6+ tumor volumes were larger than those of their counterparts at all time points. There were no signs of cachexia in any of the experimental animals and all mice were free of metastases. To better understand the mechanisms responsible for accelerated growth of LNCaP-IL-6+ tumors, we have investigated the expression of cell-cycle regulatory molecules by Western blot analysis. The levels of cyclin-dependent kinase 2 were elevated in LNCaP-IL-6+ cells. There was a strong down-regulation of cyclins D1 and E in the LNCaP-IL-6+ subline. The cell-cycle inhibitor p27 was expressed at a low level in LNCaP-IL-6+ cells and could not be up-regulated by addition of IL-6. Most notably, LNCaP-IL-6+ cells exhibited a reduced expression of the hypophosphorylated form of the retinoblastoma protein (pRb). Accelerated tumor growth in our model system was also associated with alterations in IL-6-signaling pathways. The ability of IL-6 to induce tyrosine phosphorylation of STAT3 was abolished in the LNCaP-IL-6+ subline. In contrast, the levels of the MAPK extracellular signal-regulated kinases 1/2 increased in cells generated after long-term IL-6 treatment. The inhibitor of MAPK kinase PD 98059 retarded the proliferation of LNCaP-IL-6+ but not that of control cells. In summary, we show in the present study that chronic exposure of prostate cancer cells to IL-6 facilitates tumor growth in vivo by abolishment of the growth control by pRb and activation of the MAPK signaling pathway. These findings could be relevant to understand the role of IL-6 in prostate cancer progression.

Independent roles of SOCS-3 and SHP-2 in the regulation of neuronal gene expression by leukemia inhibitory factor

The neurokine leukemia inhibitory factor (LIF) initiates signaling through heterodimerization of the low affinity LIF receptor (LIFR) and gp130. Tyrosine 759 of gp130 is required for the negative regulation of LIF-mediated signaling by both the protein tyrosine phosphatase SHP-2 and the suppressor of cytokine signaling-3 (SOCS-3). We find that SOCS-3 is expressed in the neuronal cell lines SN56 and IMR32 and negatively regulates LIF-stimulated neuronal gene expression. Studies using antisense oligonucleotides targeted to SHP-2 or SOCS-3 indicate that either protein can negatively regulate LIF-stimulated neuronal gene expression independently of the other. Mutagenesis of the cytoplasmic domain of gp130 demonstrates that the four signal transducer and activators of transcription (STAT) binding sites within gp130 are necessary for the induction of vasoactive intestinal peptide (VIP) and choline acetyltransferase (ChAT) reporter genes, with the sites surrounding tyrosines 905 and 915 (Y905 and Y915) being most important in gp130-mediated reporter gene expression. While there are four STAT binding sites within gp130, only those surrounding Y905 and Y915 can mediate STAT1 activation; these results indicate that STAT1 may be essential for normal gp130-stimulated VIP and ChAT expression. Additionally, the negative regulation of signaling mediated by Y759 of gp130 is dependent upon intact STAT sites within the receptor. This indicates that STAT signaling is necessary for LIF- and CNTF-stimulated VIP and ChAT expression and Y759 of gp130 mediates the activities of SHP-2 and SOCS-3, which act to negatively regulate STAT activity.

Molecular basis of the cell specificity of cytokine action

The molecular cloning and biological analyses of cytokines have led us to a general understanding of their pleiotropism and redundancy. These features have been ascribed to the composition of cytokine receptor complexes, which include a signal-transducing receptor subunit that is used by all members of a cytokine family and a binding subunit that is specific for each cytokine. Even though a given cytokine uses the same receptor complex when binding to various cell types, the cytokine elicits quite specific and distinct biological responses in different types of cells. Even in the same type of cell, the responses to a given cytokine could vary depending on the location of the cell and the condition of its microenvironment. Important mediators for the main cytokine signal-transduction pathway are the Janus kinases (Jaks) and signal transducer and activator of transcription (STATs). Selective usage of members of the Jak and STAT families by a given cytokine receptor is partly responsible for the specificity of cytokine action. In addition to the Jak-STAT pathway, a cytokine receptor complex can simultaneously operate multiple signal-transduction pathways, which usually express contradictory properties. These contradictory signals from a single cytokine are orchestrated to evoke a unified biological response in the cell. Here we discuss the molecular mechanisms that regulate how the cell specificity of cytokine signals is regulated, especially focusing on the IL-6/gp130 system.

Cytokine G-protein signaling crosstalk in cardiomyocytes: attenuation of Jak-STAT activation by endothelin-1

The IL-6-related cytokines, LIF and cardiotrophin-1, are important growth promoting and cardioprotective agents for cardiomyocytes. However, factors that regulate their actions in the heart are poorly understood. In this study, we tested the hypothesis that endothelin-1, a peptide hormone that produces a pattern of cardiac hypertrophy distinct from LIF and cardiotrophin-1, modulates LIF-induced signaling in cardiomyocytes. Upon binding LIF or cardiotrophin-1, the LIF receptor alpha subunit (LIFRalpha) dimerizes with gp130, leading to activation of constitutively associated Jak1 proteins and LIFRalpha-gp130 tyrosine phosphorylation. We found that pretreatment of neonatal rat ventricular myocytes with endothelin-1 rapidly inhibited LIF-induced LIFRalpha tyrosine phosphorylation and Jak1 activation. This effect of endothelin-1 on LIFalpha and Jak1 was attenuated by the MEK1 inhibitor, PD98059, implicating involvement of the ERK kinases. Radioligand binding studies showed that inhibition of LIF signaling resulted from a reduction in cell surface LlFRalpha levels. Additionally, endothelin-1 was found to reduce LIF-induced STAT3 activation, as indexed by STAT3 Y705 phosphorylation. Finally, endothelin-1 and LIF were shown to induce opposite patterns of STAT3 activation in cardiomyocytes. LIF induced rapid, robust STAT3 Y705 phosphorylation; endothelin-1 produced a delayed, modest increase, and initially decreased STAT3 Y705 phosphorylation. Overall our findings indicate that endothelin-1 acts to temper IL-6-related cytokine signaling in cardiomyocytes, in particular STAT3 activation.

The role of endotoxin, TNF-α, and IL-6 in inducing the state of growth hormone insensitivity

AIM: Critical illnesses such as sepsis, trauma, and burns cause a growth hormone insensitivity, which leads to an increased negative nitrogen balance. Endotoxin is generously released into blood under these conditions and stimulates the production of proinflammatory cytokines such as TNF-α, IL-6, and IL-1, which may play a very important role in inducing the growth hormone insensitivity. The objective of this current study was to investigate the role of endotoxin, TNF-α and IL-6 in inducing the growth hormone insensitivity at the receptor and post-receptor levels.

METHODS: Spague-Dawley rats were injected with endotoxin, TNF-α, and IL-6, respectively and part of rats injected with endotoxin was treated with exogenous somatotropin simultaneously. All rats were killed at different time points. The expression of IGF-I, GHR, SOCS-3 and β-actin mRNA in the liver was detected by RT-PCR and the GH levels were measured by radioimmunoassay, the levels of TNF-α and IL-6 were detected by ELISA.

RESULTS: There was no significant difference in serous GH levels between experimental group and control rats after endotoxin injection, however, liver IGF-I mRNA expression had been obviously down-regulated in endotoxemic rats. Liver GHR mRNA expression also had a predominant down-regulation after endotoxin injection. The lowest regulation of liver IGF-I mRNA expression occurred at 12 h after LPS injection, being decreased by 53% compared with control rats. For GHR mRNA expression, the lowest expression occurred at 8 h and had a 81% decrease. Although SOCS-3 mRNA was weakly expressed in control rats, it was strongly up-regulated after LPS injection and had a 7.84 times increase compared with control rats. Exogenous GH could enhance IGF-I mRNA expression in control rats, but it did fail to prevent the decline in IGF-I mRNA expression in endotoxemic rats. Endotoxin stimulated the production of TNF-α and IL-6, and the elevated IL-6 levels was shown a positive correlation with increased SOCS-3 mRNA expression. The liver GHR mRNA expression was obviously down-regulated after TNF-α iv injection and had a 40% decrease at 8 h, but the liver SOCS-3 mRNA expression was the 4.94 times up-regulation occurred at 40 min after IL-6 injection.

CONCLUSION: The growth hormone insensitivity could be induced by LPS injection, which was associated with down-regulated GHR mRNA expression at receptor level and with up-regulated SOCS-3 mRNA expression at post-receptor level. The in vivo biological activities of LPS were mediated by TNF-α and IL-6 indirectly, and TNF-α and IL-6 may exert their effects on the receptor and post-receptor levels respectively.

Involvement of suppressor of cytokine signaling-3 as a mediator of the inhibitory effects of IL-10 on lipopolysaccharide-induced macrophage activation

Previous studies have shown that IL-10 can induce the expression of the suppressor of cytokine signaling 3 (SOCS-3) mRNA in human monocytes and neutrophils, suggesting that the capacity of IL-10 to inhibit the expression of LPS-inducible proinflammatory genes may depend on SOCS-3 induction. However, no direct experimental evidence has been provided to support such hypothesis. Herein, we show that stable transfection of SOCS-3 into the mouse macrophage cell line J774 resulted in an inhibition of NO, TNF-alpha, IL-6, and GM-CSF secretion in response to LPS at levels similar to those exerted by IL-10 in LPS-stimulated wild-type J774. Constitutive SOCS-3 expression also down-regulated the mRNA expression of inducible NO synthase and IL-6 and impaired the production of TNF-alpha, mainly at a post-transcriptional level. In addition, SOCS-3-transfected cells displayed a constitutive expression of the IL-1R antagonist gene, consistent with the observation that IL-10 enhances IL-1R antagonist mRNA in LPS-stimulated wild-type cells. Furthermore, in peritoneal macrophages harvested from mice carrying heterozygous disruption of the SOCS-3 gene, IL-10 was less effective in repressing LPS-stimulated TNF-alpha and NO production. Taken together, our data show that SOCS-3 inhibits LPS-induced macrophage activation, strongly supporting the idea that it plays a role in the molecular mechanism by which IL-10 down-modulates the effector functions of LPS-activated macrophages. Finally, we show that forced expression of SOCS-3 significantly suppresses the ability of IL-10 to trigger tyrosine phosphorylation of STAT3. Therefore, SOCS-3 functions both as an LPS signal inhibitor and as a negative feedback regulator of IL-10/STAT3 signaling.

The role of endotoxin, TNF-alpha, and IL-6 in inducing the state of growth hormone insensitivity

AIM

Critical illnesses such as sepsis, trauma, and burns cause a growth hormone insensitivity, which leads to an increased negative nitrogen balance. Endotoxin is generously released into blood under these conditions and stimulates the production of proinflammatory cytokines such as TNF-alpha, IL-6, and IL-1, which may play a very important role in inducing the growth hormone insensitivity. The objective of this current study was to investigate the role of endotoxin, TNF-alpha and IL-6 in inducing the growth hormone insensitivity at the receptor and post-receptor levels.

METHODS

Spague-Dawley rats were injected with endotoxin, TNF-alpha, and IL-6, respectively and part of rats injected with endotoxin was treated with exogenous somatotropin simultaneously. All rats were killed at different time points. The expression of IGF-I, GHR, SOCS-3 and beta-actin mRNA in the liver was detected by RT-PCR and the GH levels were measured by radioimmunoassay, the levels of TNF-alpha and IL-6 were detected by ELISA.

RESULTS

There was no significant difference in serous GH levels between experimental group and control rats after endotoxin injection, however, liver IGF-I mRNA expression had been obviously down-regulated in endotoxemic rats. Liver GHR mRNA expression also had a predominant down-regulation after endotoxin injection. The lowest regulation of liver IGF-I mRNA expression occurred at 12h after LPS injection, being decreased by 53% compared with control rats. For GHR mRNA expression, the lowest expression occurred at 8h and had a 81% decrease. Although SOCS-3 mRNA was weakly expressed in control rats, it was strongly up-regulated after LPS injection and had a 7.84 times increase compared with control rats. Exogenous GH could enhance IGF-I mRNA expression in control rats, but it did fail to prevent the decline in IGF-I mRNA expression in endotoxemic rats. Endotoxin stimulated the production of TNF-alpha and IL-6, and the elevated IL-6 levels was shown a positive correlation with increased SOCS-3 mRNA expression. The liver GHR mRNA expression was obviously down-regulated after TNF-alpha iv injection and had a 40% decrease at 8h, but the liver SOCS-3 mRNA expression was the 4.94 times up-regulation occurred at 40 min after IL-6 injection.

CONCLUSION

The growth hormone insensitivity could be induced by LPS injection, which was associated with down-regulated GHR mRNA expression at receptor level and with up-regulated SOCS-3 mRNA expression at post-receptor level. The in vivo biological activities of LPS were mediated by TNF-alpha and IL-6 indirectly, and TNF-alpha and IL-6 may exert their effects on the receptor and post-receptor levels respectively.

The role of endotoxin, TNF-alpha, and IL-6 in inducing the state of growth hormone insensitivity

AIM

Critical illnesses such as sepsis, trauma, and burns cause a growth hormone insensitivity, which leads to an increased negative nitrogen balance. Endotoxin is generously released into blood under these conditions and stimulates the production of proinflammatory cytokines such as TNF-alpha, IL-6, and IL-1, which may play a very important role in inducing the growth hormone insensitivity. The objective of this current study was to investigate the role of endotoxin, TNF-alpha and IL-6 in inducing the growth hormone insensitivity at the receptor and post-receptor levels.

METHODS

Spague-Dawley rats were injected with endotoxin, TNF-alpha, and IL-6, respectively and part of rats injected with endotoxin was treated with exogenous somatotropin simultaneously. All rats were killed at different time points. The expression of IGF-I, GHR, SOCS-3 and beta-actin mRNA in the liver was detected by RT-PCR and the GH levels were measured by radioimmunoassay, the levels of TNF-alpha and IL-6 were detected by ELISA.

RESULTS

There was no significant difference in serous GH levels between experimental group and control rats after endotoxin injection, however, liver IGF-I mRNA expression had been obviously down-regulated in endotoxemic rats. Liver GHR mRNA expression also had a predominant down-regulation after endotoxin injection. The lowest regulation of liver IGF-I mRNA expression occurred at 12h after LPS injection, being decreased by 53% compared with control rats. For GHR mRNA expression, the lowest expression occurred at 8h and had a 81% decrease. Although SOCS-3 mRNA was weakly expressed in control rats, it was strongly up-regulated after LPS injection and had a 7.84 times increase compared with control rats. Exogenous GH could enhance IGF-I mRNA expression in control rats, but it did fail to prevent the decline in IGF-I mRNA expression in endotoxemic rats. Endotoxin stimulated the production of TNF-alpha and IL-6, and the elevated IL-6 levels was shown a positive correlation with increased SOCS-3 mRNA expression. The liver GHR mRNA expression was obviously down-regulated after TNF-alpha iv injection and had a 40% decrease at 8h, but the liver SOCS-3 mRNA expression was the 4.94 times up-regulation occurred at 40 min after IL-6 injection.

CONCLUSION

The growth hormone insensitivity could be induced by LPS injection, which was associated with down-regulated GHR mRNA expression at receptor level and with up-regulated SOCS-3 mRNA expression at post-receptor level. The in vivo biological activities of LPS were mediated by TNF-alpha and IL-6 indirectly, and TNF-alpha and IL-6 may exert their effects on the receptor and post-receptor levels respectively.

The role of endotoxin, TNF-alpha, and IL-6 in inducing the state of growth hormone insensitivity

AIM

Critical illnesses such as sepsis, trauma, and burns cause a growth hormone insensitivity, which leads to an increased negative nitrogen balance. Endotoxin is generously released into blood under these conditions and stimulates the production of proinflammatory cytokines such as TNF-alpha, IL-6, and IL-1, which may play a very important role in inducing the growth hormone insensitivity. The objective of this current study was to investigate the role of endotoxin, TNF-alpha and IL-6 in inducing the growth hormone insensitivity at the receptor and post-receptor levels.

METHODS

Spague-Dawley rats were injected with endotoxin, TNF-alpha, and IL-6, respectively and part of rats injected with endotoxin was treated with exogenous somatotropin simultaneously. All rats were killed at different time points. The expression of IGF-I, GHR, SOCS-3 and beta-actin mRNA in the liver was detected by RT-PCR and the GH levels were measured by radioimmunoassay, the levels of TNF-alpha and IL-6 were detected by ELISA.

RESULTS

There was no significant difference in serous GH levels between experimental group and control rats after endotoxin injection, however, liver IGF-I mRNA expression had been obviously down-regulated in endotoxemic rats. Liver GHR mRNA expression also had a predominant down-regulation after endotoxin injection. The lowest regulation of liver IGF-I mRNA expression occurred at 12h after LPS injection, being decreased by 53% compared with control rats. For GHR mRNA expression, the lowest expression occurred at 8h and had a 81% decrease. Although SOCS-3 mRNA was weakly expressed in control rats, it was strongly up-regulated after LPS injection and had a 7.84 times increase compared with control rats. Exogenous GH could enhance IGF-I mRNA expression in control rats, but it did fail to prevent the decline in IGF-I mRNA expression in endotoxemic rats. Endotoxin stimulated the production of TNF-alpha and IL-6, and the elevated IL-6 levels was shown a positive correlation with increased SOCS-3 mRNA expression. The liver GHR mRNA expression was obviously down-regulated after TNF-alpha iv injection and had a 40% decrease at 8h, but the liver SOCS-3 mRNA expression was the 4.94 times up-regulation occurred at 40 min after IL-6 injection.

CONCLUSION

The growth hormone insensitivity could be induced by LPS injection, which was associated with down-regulated GHR mRNA expression at receptor level and with up-regulated SOCS-3 mRNA expression at post-receptor level. The in vivo biological activities of LPS were mediated by TNF-alpha and IL-6 indirectly, and TNF-alpha and IL-6 may exert their effects on the receptor and post-receptor levels respectively.

The role of endotoxin, TNF-alpha, and IL-6 in inducing the state of growth hormone insensitivity

AIM

Critical illnesses such as sepsis, trauma, and burns cause a growth hormone insensitivity, which leads to an increased negative nitrogen balance. Endotoxin is generously released into blood under these conditions and stimulates the production of proinflammatory cytokines such as TNF-alpha, IL-6, and IL-1, which may play a very important role in inducing the growth hormone insensitivity. The objective of this current study was to investigate the role of endotoxin, TNF-alpha and IL-6 in inducing the growth hormone insensitivity at the receptor and post-receptor levels.

METHODS

Spague-Dawley rats were injected with endotoxin, TNF-alpha, and IL-6, respectively and part of rats injected with endotoxin was treated with exogenous somatotropin simultaneously. All rats were killed at different time points. The expression of IGF-I, GHR, SOCS-3 and beta-actin mRNA in the liver was detected by RT-PCR and the GH levels were measured by radioimmunoassay, the levels of TNF-alpha and IL-6 were detected by ELISA.

RESULTS

There was no significant difference in serous GH levels between experimental group and control rats after endotoxin injection, however, liver IGF-I mRNA expression had been obviously down-regulated in endotoxemic rats. Liver GHR mRNA expression also had a predominant down-regulation after endotoxin injection. The lowest regulation of liver IGF-I mRNA expression occurred at 12h after LPS injection, being decreased by 53% compared with control rats. For GHR mRNA expression, the lowest expression occurred at 8h and had a 81% decrease. Although SOCS-3 mRNA was weakly expressed in control rats, it was strongly up-regulated after LPS injection and had a 7.84 times increase compared with control rats. Exogenous GH could enhance IGF-I mRNA expression in control rats, but it did fail to prevent the decline in IGF-I mRNA expression in endotoxemic rats. Endotoxin stimulated the production of TNF-alpha and IL-6, and the elevated IL-6 levels was shown a positive correlation with increased SOCS-3 mRNA expression. The liver GHR mRNA expression was obviously down-regulated after TNF-alpha iv injection and had a 40% decrease at 8h, but the liver SOCS-3 mRNA expression was the 4.94 times up-regulation occurred at 40 min after IL-6 injection.

CONCLUSION

The growth hormone insensitivity could be induced by LPS injection, which was associated with down-regulated GHR mRNA expression at receptor level and with up-regulated SOCS-3 mRNA expression at post-receptor level. The in vivo biological activities of LPS were mediated by TNF-alpha and IL-6 indirectly, and TNF-alpha and IL-6 may exert their effects on the receptor and post-receptor levels respectively.

The role of endotoxin, TNF-alpha, and IL-6 in inducing the state of growth hormone insensitivity

AIM

Critical illnesses such as sepsis, trauma, and burns cause a growth hormone insensitivity, which leads to an increased negative nitrogen balance. Endotoxin is generously released into blood under these conditions and stimulates the production of proinflammatory cytokines such as TNF-alpha, IL-6, and IL-1, which may play a very important role in inducing the growth hormone insensitivity. The objective of this current study was to investigate the role of endotoxin, TNF-alpha and IL-6 in inducing the growth hormone insensitivity at the receptor and post-receptor levels.

METHODS

Spague-Dawley rats were injected with endotoxin, TNF-alpha, and IL-6, respectively and part of rats injected with endotoxin was treated with exogenous somatotropin simultaneously. All rats were killed at different time points. The expression of IGF-I, GHR, SOCS-3 and beta-actin mRNA in the liver was detected by RT-PCR and the GH levels were measured by radioimmunoassay, the levels of TNF-alpha and IL-6 were detected by ELISA.

RESULTS

There was no significant difference in serous GH levels between experimental group and control rats after endotoxin injection, however, liver IGF-I mRNA expression had been obviously down-regulated in endotoxemic rats. Liver GHR mRNA expression also had a predominant down-regulation after endotoxin injection. The lowest regulation of liver IGF-I mRNA expression occurred at 12h after LPS injection, being decreased by 53% compared with control rats. For GHR mRNA expression, the lowest expression occurred at 8h and had a 81% decrease. Although SOCS-3 mRNA was weakly expressed in control rats, it was strongly up-regulated after LPS injection and had a 7.84 times increase compared with control rats. Exogenous GH could enhance IGF-I mRNA expression in control rats, but it did fail to prevent the decline in IGF-I mRNA expression in endotoxemic rats. Endotoxin stimulated the production of TNF-alpha and IL-6, and the elevated IL-6 levels was shown a positive correlation with increased SOCS-3 mRNA expression. The liver GHR mRNA expression was obviously down-regulated after TNF-alpha iv injection and had a 40% decrease at 8h, but the liver SOCS-3 mRNA expression was the 4.94 times up-regulation occurred at 40 min after IL-6 injection.

CONCLUSION

The growth hormone insensitivity could be induced by LPS injection, which was associated with down-regulated GHR mRNA expression at receptor level and with up-regulated SOCS-3 mRNA expression at post-receptor level. The in vivo biological activities of LPS were mediated by TNF-alpha and IL-6 indirectly, and TNF-alpha and IL-6 may exert their effects on the receptor and post-receptor levels respectively.

The role of endotoxin, TNF-alpha, and IL-6 in inducing the state of growth hormone insensitivity

AIM

Critical illnesses such as sepsis, trauma, and burns cause a growth hormone insensitivity, which leads to an increased negative nitrogen balance. Endotoxin is generously released into blood under these conditions and stimulates the production of proinflammatory cytokines such as TNF-alpha, IL-6, and IL-1, which may play a very important role in inducing the growth hormone insensitivity. The objective of this current study was to investigate the role of endotoxin, TNF-alpha and IL-6 in inducing the growth hormone insensitivity at the receptor and post-receptor levels.

METHODS

Spague-Dawley rats were injected with endotoxin, TNF-alpha, and IL-6, respectively and part of rats injected with endotoxin was treated with exogenous somatotropin simultaneously. All rats were killed at different time points. The expression of IGF-I, GHR, SOCS-3 and beta-actin mRNA in the liver was detected by RT-PCR and the GH levels were measured by radioimmunoassay, the levels of TNF-alpha and IL-6 were detected by ELISA.

RESULTS

There was no significant difference in serous GH levels between experimental group and control rats after endotoxin injection, however, liver IGF-I mRNA expression had been obviously down-regulated in endotoxemic rats. Liver GHR mRNA expression also had a predominant down-regulation after endotoxin injection. The lowest regulation of liver IGF-I mRNA expression occurred at 12h after LPS injection, being decreased by 53% compared with control rats. For GHR mRNA expression, the lowest expression occurred at 8h and had a 81% decrease. Although SOCS-3 mRNA was weakly expressed in control rats, it was strongly up-regulated after LPS injection and had a 7.84 times increase compared with control rats. Exogenous GH could enhance IGF-I mRNA expression in control rats, but it did fail to prevent the decline in IGF-I mRNA expression in endotoxemic rats. Endotoxin stimulated the production of TNF-alpha and IL-6, and the elevated IL-6 levels was shown a positive correlation with increased SOCS-3 mRNA expression. The liver GHR mRNA expression was obviously down-regulated after TNF-alpha iv injection and had a 40% decrease at 8h, but the liver SOCS-3 mRNA expression was the 4.94 times up-regulation occurred at 40 min after IL-6 injection.

CONCLUSION

The growth hormone insensitivity could be induced by LPS injection, which was associated with down-regulated GHR mRNA expression at receptor level and with up-regulated SOCS-3 mRNA expression at post-receptor level. The in vivo biological activities of LPS were mediated by TNF-alpha and IL-6 indirectly, and TNF-alpha and IL-6 may exert their effects on the receptor and post-receptor levels respectively.

The role of endotoxin, TNF-alpha, and IL-6 in inducing the state of growth hormone insensitivity

AIM

Critical illnesses such as sepsis, trauma, and burns cause a growth hormone insensitivity, which leads to an increased negative nitrogen balance. Endotoxin is generously released into blood under these conditions and stimulates the production of proinflammatory cytokines such as TNF-alpha, IL-6, and IL-1, which may play a very important role in inducing the growth hormone insensitivity. The objective of this current study was to investigate the role of endotoxin, TNF-alpha and IL-6 in inducing the growth hormone insensitivity at the receptor and post-receptor levels.

METHODS

Spague-Dawley rats were injected with endotoxin, TNF-alpha, and IL-6, respectively and part of rats injected with endotoxin was treated with exogenous somatotropin simultaneously. All rats were killed at different time points. The expression of IGF-I, GHR, SOCS-3 and beta-actin mRNA in the liver was detected by RT-PCR and the GH levels were measured by radioimmunoassay, the levels of TNF-alpha and IL-6 were detected by ELISA.

RESULTS

There was no significant difference in serous GH levels between experimental group and control rats after endotoxin injection, however, liver IGF-I mRNA expression had been obviously down-regulated in endotoxemic rats. Liver GHR mRNA expression also had a predominant down-regulation after endotoxin injection. The lowest regulation of liver IGF-I mRNA expression occurred at 12h after LPS injection, being decreased by 53% compared with control rats. For GHR mRNA expression, the lowest expression occurred at 8h and had a 81% decrease. Although SOCS-3 mRNA was weakly expressed in control rats, it was strongly up-regulated after LPS injection and had a 7.84 times increase compared with control rats. Exogenous GH could enhance IGF-I mRNA expression in control rats, but it did fail to prevent the decline in IGF-I mRNA expression in endotoxemic rats. Endotoxin stimulated the production of TNF-alpha and IL-6, and the elevated IL-6 levels was shown a positive correlation with increased SOCS-3 mRNA expression. The liver GHR mRNA expression was obviously down-regulated after TNF-alpha iv injection and had a 40% decrease at 8h, but the liver SOCS-3 mRNA expression was the 4.94 times up-regulation occurred at 40 min after IL-6 injection.

CONCLUSION

The growth hormone insensitivity could be induced by LPS injection, which was associated with down-regulated GHR mRNA expression at receptor level and with up-regulated SOCS-3 mRNA expression at post-receptor level. The in vivo biological activities of LPS were mediated by TNF-alpha and IL-6 indirectly, and TNF-alpha and IL-6 may exert their effects on the receptor and post-receptor levels respectively.