Potentiation of growth hormone-induced liver suppressors of cytokine signaling messenger ribonucleic acid by cytokines

Effect of Galohgor Cookies Intake by Postpartum Mother on Newborn’s Growth

Background: Malnutrition is one of serious public health problems that still need treatment priority. Besides the problem of over-nutrition continues to increase, the problem of under-nutrition has not been resolved properly. Utilization of local food such as galohgor can be an alternative to meet the nutritional needs. Objectives: The present study was aimed to analyze the effect of galohgor cookies intake by postpartum mothers on the newborn’s growth. Methods: The design was quasi-experimental design. There were two groups including the control group (CG) (n = 9) and the intervention group (GG) (n = 9). Postpartum mothers aged 20-35 years (parity between 2 and 5) with normal delivery and had no medical indication were involved in this study. A total of four pieces of cookie (~ 4 g of galohgor powder) were consumed daily from the 1st day until the 40th day after delivery. Anthropometric data (height, body weight, and head circumference) were obtained on day 0, 14 and 40 after delivery. Meanwhile, human milk samples were taken on day 14 and 40 to analyze the levels of human milk insulin-like growth factor-I (IGF-I). Results: The newborns in the GG had significantly heavier weight and larger head circumference than CG on day 14 and 40 after delivery (p <0.05). IGF-I could be maintained by consuming galohgor during postpartum period. However, this study found no differences in body length of both control and intervention group (p> 0.05). Conclusions: Intake of galohgor cookies by postpartum mothers might support the newborn’s growth.

Signal Transduction for TNFα-Induced Type II SOCS Expression and Its Functional Implication in Growth Hormone Resistance in Carp Hepatocytes

In mammals, local production of tumor necrosis factor α (TNFα) inhibits growth hormone (GH)-induced IGF-I expression at tissue level and contributes to GH resistance caused by sepsis/endotoxemia and inflammation. Although the loss of GH responsiveness can be mediated by a parallel rise in SOCS expression, the signaling mechanisms for TNFα-induced SOCS expression at the hepatic level have not been characterized and the comparative aspects of the phenomenon, especially in lower vertebrates, are still unknown. Recently, type II SOCS, including SOCS1-3 and CISH, have been cloned in grass carp and shown to act as the feedback repressors for GH signaling via JAK₂/STAT₅ pathway. To shed light on the mechanisms for TNFα-induced GH resistance in fish model, grass carp TNFα was cloned and confirmed to be a single-copy gene expressed in various tissues including the liver. In carp hepatocytes, incubation with the endotoxin LPS induced TNFα expression with parallel rises in SOCS1-3 and CISH mRNA levels. Similar to LPS, TNFα treatment could block GH-induced IGF-I/-II mRNA expression and elevate SOCS1, SOCS3, and CISH transcript levels. However, TNFα was not effective in altering SOCS2 expression. In parallel experiment, LPS blockade of IGF-I/-II signals caused by GH could be partially reverted by TNFα receptor antagonism. At hepatocyte level, TNFα induction also triggered rapid phosphorylation of IκBα, MEK_1/2, ERK_1/2, MKK_3/6, P₃₈^MAPK, Akt, JAK₂, and STAT_1,3,5, and TNFα-induced SOCS1, SOCS3, and CISH mRNA expression could be negated by inhibiting the IKK/NFκB, MAPK, PI3K/Akt, and JAK/STAT cascades. Our findings, as a whole, suggest that local production of TNFα may interfere with IGF-I/-II induction by GH in the carp liver by up-regulation of SOCS1, SOCS3, and CISH via IKK/NFκB, MAPK, PI3K/Akt, and JAK/STAT-dependent mechanisms, which may contribute to GH resistance induced by endotoxin in carp species.

Body height in paediatric inflammatory bowel diseases: A structural equation model analysis

BACKGROUND

Linear growth restriction is a unique feature of paediatric inflammatory bowel diseases (IBD), and reduced insulin-like growth factor (IGF-1) is a major determinant of short stature. We aimed to assess factors influencing somatic height in children suffering from IBD.

MATERIALS AND METHODS

This was a retrospective, cross-sectional study conducted after approval by Institutional authorities. Anthropometric data, disease-related factors, biochemical and clinical indices of inflammation and endocrine parameters were recorded and considered as explanatory covariates. A structural equation model analysis was utilized. Somatic height was the outcome of interest, and possible associations of explanatory covariates directly or through the mediation effect of IGF-1 were assessed.

RESULTS

Systemic inflammation, as expressed by high-sensitivity intereukin-6 (IL-6), and nutritional status described by body mass index (BMI) were the pathways that significantly affected stature through the mediation effect of IGF-1. Cortisol showed a direct, positive and independent of IGF-1 association with height.

CONCLUSIONS

Insulin-like growth factor-1 is a key player in the process that results in impaired linear growth. Malnutrition and systemic inflammation have a restrictive action on growth by reducing circulating IGF-1. The positive relation of serum cortisol to height could correspond to suppressed pituitary-adrenal axis due to long-term use of glucocorticoids.

Human Cytomegalovirus Immediate-Early 1 Protein Rewires Upstream STAT3 to Downstream STAT1 Signaling Switching an IL6-Type to an IFNγ-Like Response

The human cytomegalovirus (hCMV) major immediate-early 1 protein (IE1) is best known for activating transcription to facilitate viral replication. Here we present transcriptome data indicating that IE1 is as significant a repressor as it is an activator of host gene expression. Human cells induced to express IE1 exhibit global repression of IL6- and oncostatin M-responsive STAT3 target genes. This repression is followed by STAT1 phosphorylation and activation of STAT1 target genes normally induced by IFNγ. The observed repression and subsequent activation are both mediated through the same region (amino acids 410 to 445) in the C-terminal domain of IE1, and this region serves as a binding site for STAT3. Depletion of STAT3 phenocopies the STAT1-dependent IFNγ-like response to IE1. In contrast, depletion of the IL6 receptor (IL6ST) or the STAT kinase JAK1 prevents this response. Accordingly, treatment with IL6 leads to prolonged STAT1 instead of STAT3 activation in wild-type IE1 expressing cells, but not in cells expressing a mutant protein (IE1dl410-420) deficient for STAT3 binding. A very similar STAT1-directed response to IL6 is also present in cells infected with a wild-type or revertant hCMV, but not an IE1dl410-420 mutant virus, and this response results in restricted viral replication. We conclude that IE1 is sufficient and necessary to rewire upstream IL6-type to downstream IFNγ-like signaling, two pathways linked to opposing actions, resulting in repressed STAT3- and activated STAT1-responsive genes. These findings relate transcriptional repressor and activator functions of IE1 and suggest unexpected outcomes relevant to viral pathogenesis in response to cytokines or growth factors that signal through the IL6ST-JAK1-STAT3 axis in hCMV-infected cells. Our results also reveal that IE1, a protein considered to be a key activator of the hCMV productive cycle, has an unanticipated role in tempering viral replication.

Our previous work has shown that the human cytomegalovirus (hCMV) major immediate-early 1 protein (IE1) modulates host cell signaling pathways involving proteins of the signal transducer and activator of transcription (STAT) family. IE1 has also long been known to facilitate viral replication by activating transcription. In this report we demonstrate that IE1 is as significant a repressor as it is an activator of host gene expression. Many genes repressed by IE1 are normally induced via STAT3 signaling triggered by interleukin 6 (IL6) or related cytokines, whereas many genes activated by IE1 are normally induced via STAT1 signaling triggered by interferon gamma (IFNγ). Our results suggest that the repression of STAT3- and the activation of STAT1-responsive genes by IE1 are coupled. By targeting STAT3, IE1 rewires upstream STAT3 to downstream STAT1 signaling. Consequently, genes normally induced by IL6 are repressed while genes normally induced by IFNγ become responsive to IL6 in the presence of IE1. We also demonstrate that, by switching an IL6 to an IFNγ-like response, IE1 tempers viral replication. These results suggest an unanticipated dual role for IE1 in either promoting or limiting hCMV propagation and demonstrate how a key viral regulatory protein merges two central cellular signaling pathways to divert cytokine responses relevant to hCMV pathogenesis.

Distinct mechanisms of induction of hepatic growth hormone resistance by endogenous IL-6, TNF-α, and IL-1β

During inflammation, the liver becomes resistant to growth hormone (GH) actions, leading to downregulation of the GH target gene IGF-I and activation of catabolism. Proinflammatory cytokines IL-6, TNF-α, and IL-1β are critically involved in the pathogenesis of hepatic GH resistance. However, the mechanisms used by endogenous IL-6, TNF-α, and IL-1β to inhibit the hepatic GH-IGF-I pathway during inflammation are not fully understood. Here, we show that TNF-α and IL-1β inhibited GH receptor (GHR) expression but had minor effects on the downstream suppressor of cytokine signaling (SOCS)3, while IL-6 induced SOCS3 expression but had no effect on GHR expression in Huh-7 cells. Consistent with the in vitro observations, neutralization of TNF-α and IL-1β in mouse models of inflammation did not significantly alter SOCS3 expression stimulated by inflammation but restored GHR and IGF-I expression suppressed by inflammation. Neutralization of IL-6 did not alter inflammation-suppressed GHR expression but drastically reduced the inflammation-stimulated SOCS3 expression and restored IGF-I expression. Interestingly, when the GH-IGF-I pathway was turned off by maximal inhibition of GHR expression, IL-6 and SOCS3 were no longer able to regulate IGF-I expression. Taken together, our results suggest that TNF-α/IL-1β and IL-6 use distinct mechanisms to induce hepatic GH resistance, with TNF-α and IL-1β acting primarily on GHR and IL-6 acting primarily on SOCS3. IL-6 action may be superseded by factors such as TNF-α and IL-1β that inhibit GHR expression.

Increased circulating IL-8 is associated with reduced IGF-1 and related to poor metabolic control in adolescents with type 1 diabetes mellitus

BACKGROUND

A dysregulated growth hormone (GH)/insulin-like growth factor 1 (IGF-1) axis is well-recognized in children and adolescents with type 1 diabetes mellitus (T1DM). Decreased IGF-1 levels can also be found in chronic inflammatory diseases, while hyperglycemia promotes inflammatory cytokine production. Therefore, inflammatory cytokines may link poor metabolic control with GH/IGF-1 axis changes. This study examined the relationship between serum inflammatory cytokines and IGF-1 in adolescents (age 13-18) with TIDM in chronic poor (n=17) or favorable (n=19) glucose control. Poor control (PC) was defined as >or=3, consistent HbA1C>9% during the previous 2 years, while favorable control (FC) was consistent levels of HbA1C<9%.

RESULTS

HbA1C (FC: 7.5+/-0.6%; PC: 10.5+/-0.9%, p<0.001) and interleukin (IL)-8 (FC: 3.7+/-4.0 pg/ml; PC: 7.4+/-4.3 pg/ml, p=0.01) were increased and IGF-1 (FC: 536.5+/-164.3 ng/ml; PC: 408.9+/-157.1 ng/ml, p=0.03) was decreased in patients with poor control compared to patients with favorable control. Moreover, IL-8 was inversely correlated with IGF-1 (r=-0.40, p=0.03) and positively correlated with HbA1C (r=0.36, p=0.03).

CONCLUSIONS

In adolescents with T1DM and chronic, poor glucose control, increased serum IL-8 is associated with reduced IGF-1 suggesting a pro-inflammatory milieu that may contribute to alterations in the GH/IGF-1 axis.

Mechanisms of growth impairment in pediatric Crohn's disease

Crohn's disease manifests during childhood or adolescence in up to 25% of patients. The potential for linear growth impairment as a complication of chronic intestinal inflammation is unique to pediatric patient populations. Insulin-like growth factor I (IGF-I), produced by the liver in response to growth hormone (GH) stimulation, is the key mediator of GH effects at the growth plate of bones. An association between impaired growth in children with Crohn's disease and low IGF-I levels is well recognized. Early studies emphasized the role of malnutrition in suppression of IGF-I production. However, a simple nutritional hypothesis fails to explain all the observations related to growth in children with Crohn's disease. The direct, growth-inhibitory effects of proinflammatory cytokines are increasingly recognized and explored. The potential role of noncytokine factors, such as lipopolysaccharides, and their potential to negatively influence the growth axis have recently been investigated with intriguing results. There is now reason for optimism that the modern anticytokine therapeutic agents available for treating children and adolescents with Crohn's disease will reduce the prevalence of this otherwise common complication. As our understanding of the mechanisms that underlie growth impairment advance, so too should the opportunity for developing further novel and targeted therapies.

Endotoxin-induced growth hormone resistance in skeletal muscle

Inflammation-induced skeletal muscle wasting is a serious clinical problem and arises in part because of resistance to GH-stimulated IGF-I expression. Although it is established that in the liver, resistance develops because of impaired signaling through the Janus kinase 2 (JAK2)/signal transducer and activator of transcription 5 (STAT5) transduction pathway, together with a more distal defect in STAT5 DNA-binding activity, the situation in skeletal muscle is unclear. Accordingly, we set out to characterize the mechanisms behind the skeletal muscle resistance to GH in rats with acute inflammation induced by endotoxin. Endotoxin caused significant declines in GH-stimulated STAT5a/b phosphorylation and IGF-I gene expression, and this occurred despite a lack of change in signaling protein levels or phosphorylation of JAK2. In whole muscle, GH-stimulated phospho-STAT5a/b levels were reduced by half, and in the nucleus, phospho-STAT5b levels were similarly reduced. Furthermore, the binding of phosphorylated STAT5b to DNA was reduced and to a similar extent to the reduction in nuclear phosphorylated STAT5b. Interestingly, GH-induced androgen receptor gene expression was also suppressed. Thus, it appears that skeletal muscle resistance to GH-stimulated IGF-I expression in acute endotoxemia arises from a defect in STAT5b signaling, with a proportionate reduction in STAT5b DNA binding. Finally, it appears that resistance to GH-induced androgen receptor expression also develops and, together with the attenuated GH-induced IGF-I expression, likely plays an important role in the muscle wasting that arises in endotoxin-induced inflammation.

Endotoxin-induced proteolytic reduction in hepatic growth hormone (GH) receptor: a novel mechanism for GH insensitivity

GH is an important anabolic hormone. We previously demonstrated in cell culture that the cell surface GH receptor (GHR) is susceptible to inducible metalloproteolytic cleavage that yields the shed receptor extracellular domain (called GH binding protein) and renders the cells desensitized to subsequent GH stimulation. Sepsis and inflammatory states are associated with hepatic desensitization to GH, although disparate mechanisms have been postulated in various animal models. Using C3H/HeJ mice, we now demonstrate that administration of lipopolysaccharide (LPS) causes marked hepatic desensitization to GH, assessed by monitoring signal transducer and activator of transcription 5 tyrosine phosphorylation and nuclear accumulation and with a novel noninvasive bioluminescence imaging system to track in vivo hepatic GH signaling serially in individual mice. This endotoxin-induced desensitization was accompanied by marked loss of hepatic GHR, which was not explained by changes in GHR mRNA abundance. Furthermore, we observe that LPS causes GH-binding protein shedding of a hepatically expressed wild-type GHR but not a GHR with a mutation in the metalloprotease cleavage site. These data suggest that in this model system, LPS-induced desensitization to GH is associated with proteolytic GHR cleavage. These data are the first to demonstrate inducible in vivo GHR proteolysis and suggest this is a mechanism to regulate GH sensitivity and its anabolic effects during sepsis or inflammation.

Caveolae nitration of Janus kinase-2 at the 1007Y-1008Y site: coordinating inflammatory response and metabolic hormone readjustment within the somatotropic axis

Life-threatening proinflammatory response (PR) induces severe GH resistance. Although low-level PR is much more commonly encountered clinically, relatively few studies have investigated the accompanying change in GH signal transduction progression and, in particular, the impact of low-level PR on Janus kinase (JAK)-2. Using a low-level, in vivo endotoxin [lipopolysaccharide (LPS)] challenge protocol, we demonstrated that the liver tissue content of JAK2 declined 24 h (62%, P < 0.02) after LPS and that tyrosine-nitrated JAK2 could be immunoprecipitated from post-LPS liver biopsy homogenates. With antibodies developed to probe specifically for nitration at the (1007)Y-(1008)Y phosphorylation epitope of JAK2, we demonstrated that the nitrated (1007)Y-(1008)Y-JAK-2 (nitro-JAK2) coimmunoprecipitated with caveolin-1 and (1177)phospho-SER-endothelial nitric oxide synthase when post-LPS liver homogenates were treated with anticaveolin-1 and protein A/G. The magnitude of increase in nitro-JAK2 was attenuated in animals treated with vitamin E prior to LPS. The increase in nitro-JAK2 after LPS was greater in a line of experimental animals with a genetic propensity for higher PR at the given LPS dose than responses measured in their normal counterparts. The development and remission of nitro-JAK2 was temporally concordant with changes in plasma concentrations of IGF-I; hepatocellular IGF-I mRNA content was inversely proportional to nitro-JAK2 content. Localized changes in the state of nitration of regulatory phosphorylation domains of JAK2 in caveolar microenvironments and tissue content of JAK2 during PR suggest a unique mechanism through which discrete signal transduction switching might occur in the liver to fine tune cellular responses to the endocrine-immune signals that develop during low-level, transient proinflammatory stress.

Endotoxin attenuates growth hormone-induced hepatic insulin-like growth factor I expression by inhibiting JAK2/STAT5 signal transduction and STAT5b DNA binding

Gram-negative sepsis with release of endotoxin is a frequent cause of cachexia that develops partly because of resistance to growth hormone (GH) with reduced insulin-like growth factor-I (IGF-I) expression. We set out to more fully characterize the mechanisms for the resistance and to determine whether in addition to a defect in the janus kinase 2 (JAK2)-signal transducer and activator of transcription (STAT) 5b pathway, required for GH-induced IGF-I expression, there might also be a more distal defect. Conscious rats were given endotoxin and studied 4 h later. In liver of these animals, GH-induced JAK2 and STAT5 phosphorylation was impaired and appeared to be caused, at least in part, by a marked increase in hepatic tumor necrosis factor-alpha and interleukin-6 mRNA expression accompanied by elevated levels of inhibitors of GH signaling, namely cytokine-inducible suppressors of cytokine signaling-1 and -3 and cytokine-inducible SH2 protein (CIS). Nuclear phosphorylated STAT5b levels were significantly depressed to 61% of the control values and represent a potential cause of the reduced GH-induced IGF-I expression. In addition, binding of phosphorylated STAT5b to DNA was reduced to an even greater extent and averaged 17% of the normal control value. This provides a further explanation for the impaired IGF-I gene transcription. Interestingly, when endotoxin-treated rats were treated with GH, there was a marked increase in proinflammatory cytokine gene expression in the liver. If such a response were to occur in humans, this might provide a partial explanation for the adverse effect of GH treatment reported in critically ill patients.

Interleukin-1 inhibits the induction of insulin-like growth factor-I by growth hormone in CWSV-1 hepatocytes

Sepsis results in hepatic "growth hormone (GH) resistance" with reductions in plasma IGF-I despite a two- to fourfold increase in circulating GH. In this study, we examine the effects of IL-1 on GH receptor (GHR) expression, GH signaling (via the JAK/STAT and MAPK pathways), and the induction of gene expression [IGF-I mRNA and serine protease inhibitor (Spi) 2.1] by GH in CWSV-1 hepatocytes. Incubation of cells with IL-1beta (10 ng/ml, 24 h) had no effect on the relative abundance of GHR or signaling proteins JAK2, STAT5b, and ERK1/2 in cell lysates. Baseline phosphorylation of GHR, JAK2, STAT5b, and ERK1/2 was minimal. After GH stimulation, tyrosine phosphorylation of GHR, JAK2, STAT5b, and ERK1/2 increased 2- to 10-fold. However, neither the time course nor the magnitude of GHR, JAK2, and ERK1/2 phosphorylation by GH were significantly altered by IL-1. The GH-induced translocation of STAT5b to the nucleus was not prevented by IL-1. Although phosphorylated STAT5 in nuclear extracts from GH + IL-1 cells was decreased by 24% (vs. controls) 15 min after GH stimulation, this did not result in reduced STAT5-DNA binding activity. Pretreatment with IL-1 did not significantly decrease IGF-I mRNA stability. We conclude that IL-1 only minimally affects the time course of JAK2/STAT5 and MAPK signaling by GH. Therefore, an inhibitory effect of IL-1 on IGF-I and Spi 2.1 mRNA synthesis by GH represents the most likely mechanism for IL-1-mediated GH resistance.

Estrogen up-regulates hepatic expression of suppressors of cytokine signaling-2 and -3 in vivo and in vitro

Suppressors of cytokine signaling (SOCS) are important negative regulators of cytokine action. We recently reported that estrogen stimulates SOCS-2 expression and inhibits GH signaling in kidney cells. The effects of estrogen on SOCS expression in other tissues are unclear. The aim of this study was to investigate in vivo and in vitro whether estrogen affected SOCS expression in the liver, a major target organ of GH. The in vivo hepatic effects of estrogen on ovariectomized mice lacking estrogen receptor (ER)-alpha, ERbeta, or both and their wild-type littermates were examined by DNA microarray analysis. In vitro, the effects of estrogen on SOCS expression in human hepatoma cells were examined by reverse transcription quantitative PCR. Long-term (3 wk) estrogen treatment induced a 2- to 3-fold increase in hepatic expression of SOCS-2 and -3 in wild-type and ERbeta knockout mice but not in those lacking ERalpha or both ER subtypes. Short-term treatment (at 24 h) increased the mRNA level of SOCS-3 but not SOCS-2. In cultured hepatoma cells, estrogen increased SOCS-2 and -3 mRNA levels by 2-fold in a time- and dose-dependent manner (P < 0.05). Estrogen induced murine SOCS-3 promoter activity by 2-fold (P < 0.05) in constructs containing a region between nucleotides -1862 and -855. Moreover, estrogen and GH had additive effects on the SOCS-3 promoter activity. In summary, estrogen, via ERalpha, up-regulated hepatic expression of SOCS-2 and -3, probably through transcriptional activation. This indicates a novel mechanism of estrogen regulation of cytokine action.

Estrogen regulation of growth hormone action

GH plays a pivotal role in regulating body growth and development, which is modulated by sex steroids. A close interplay between estrogen and GH leads to attainment of gender-specific body composition during puberty. The physiological basis of the interaction is not well understood. Most previous studies have focused on the effects of estrogen on GH secretion. There is also strong evidence that estrogen modulates GH action independent of secretion. Oral but not transdermal administration of estrogen impairs the metabolic action of GH in the liver, causing a fall in IGF-I production and fat oxidation. This results in a loss of lean tissue and a gain of body fat in postmenopausal women and an impairment of GH effect in hypopituitary women on GH replacement. The negative metabolic sequelae are potentially important because of the widespread use of oral estrogen and estrogen-related compounds. Estrogen affects GH action at the level of receptor expression and signaling. More recently, estrogen has been shown to inhibit Janus kinase/signal transducer and activator of transcription signaling by GH via the induction of suppressor of cytokine signaling-2, a protein inhibitor for cytokine signaling. This represents a novel paradigm of steroid regulation of cytokine receptors and is likely to have significance for a diverse range of cytokine function.

Sepsis-induced muscle growth hormone resistance occurs independently of STAT5 phosphorylation

Growth hormone (GH) stimulates insulin-like growth factor I (IGF-I) synthesis in both liver and muscle. During sepsis, proinflammatory cytokines inhibit GH action in liver, but it is unknown whether sepsis also produces GH resistance in muscle. Sepsis was induced by cecal ligation and puncture, and 18 h later the effect of GH on signal transducer and activator of transcription (STAT) phosphorylation and IGF-I mRNA content was assessed in rat gastrocnemius and liver. The relative abundance of phosphorylated (p)STAT5a, pSTAT5b, pSTAT3, and pSTAT1 was increased in liver from control rats after GH. Sepsis alone also increased hepatic pSTAT5a, pSTAT3, and pSTAT1. Sepsis dramatically impaired the ability of GH to stimulate the phosphorylation of STAT5a and -5b, as well as to increase IGF-I mRNA in liver. In muscle from control rats, GH increased pSTAT5a and -5b, whereas content of pSTAT3 and pSTAT1 was not affected. Sepsis increased basal content of pSTAT3 but not pSTAT5a, pSTAT5b, or pSTAT1 in muscle. The GH-induced increase of pSTAT5a and -5b in muscle from septic rats was not inhibited, suggesting that muscle was not GH resistant. In contrast to these changes in pSTAT5, the ability of GH to increase IGF-I mRNA was completely absent in muscle from septic rats. Because the suppressor of cytokine signaling (SOCS) proteins may function as negative regulators of GH signaling, we examined the content of these proteins. Sepsis produced small (30-50%), albeit statistically significant, increases in SOCS-1, -2, and -3 protein in muscle. In contrast to muscle, the SOCS proteins in the liver did not change under the various experimental conditions, suggesting that these proteins are not responsible for the impaired phosphorylation of STAT5 by GH. In conclusion, sepsis produces GH resistance in both muscle and liver, with the locus of this impairment in muscle differing from that in liver and being independent of a defect in STAT5 phosphorylation.

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.

Interleukin-6 inhibits hepatic growth hormone signaling via upregulation of Cis and Socs-3

Cytokines may cause an acquired growth hormone (GH) resistance in patients with inflammatory diseases. Anabolic effects of GH are mediated through activation of STAT5 transcription factors. We have reported that TNF-alpha suppresses hepatic GH receptor (GHR) gene expression, whereas the cytokine-inducible SH2-containing protein 1 (Cis)/suppressors of cytokine signaling (Socs) genes are upregulated by TNF-alpha and IL-6 and inhibit GH activation of STAT5. However, the relative importance of these mechanisms in inflammatory GH resistance was not known. We hypothesized that IL-6 would prevent GH activation of STAT5 and that this would involve Cis/Socs protein upregulation. GH +/- LPS was administered to TNF receptor 1 (TNFR1) or IL-6 null mice and wild-type (WT) controls. STAT5, STAT3, GHR, Socs 1-3, and Cis phosphorylation and abundance were assessed by using immunoblots, EMSA, and/or real time RT-PCR. TNF-alpha and IL-6 abundance were assessed by using ELISA. GH activated STAT5 in WT and TNFR1 or IL-6 null mice. LPS pretreatment prevented STAT5 activation in WT and TNFR1 null mice; however, STAT5 activation was preserved in IL-6 null mice. GHR abundance did not change with LPS administration. Inhibition of STAT5 activation by LPS was temporally associated with phosphorylation of STAT3 and upregulation of Cis and Socs-3 protein in WT and TNFR1 null mice; STAT3, Cis, and Socs-3 were not induced in IL-6 null mice. IL-6 inhibits hepatic GH signaling by upregulating Cis and Socs-3, which may involve activation of STAT3. Therapies that block IL-6 may enhance GH signaling in inflammatory diseases.

Suppressors of cytokine signaling: Relevance to gastrointestinal function and disease

BACKGROUND & AIMS

The suppressor of cytokine signaling (SOCS) proteins are a family of Src homology 2 domain-containing proteins. Currently, there are 8 members of the SOCS family, of which a number have been implicated strongly in the negative regulation of cytokine signal transduction pathways.

METHODS

This review focuses on recent discoveries about 4 SOCS family members, SOCS-1, -2, and -3, and cytokine-inducible SH2-domain containing (CIS), and provides more limited information about other SOCS family members.

RESULTS

A large number of cytokines and growth factors are now known to induce SOCS proteins. In turn, SOCS inhibit the actions of a growing number of cytokines and growth factors in vitro or in vivo. SOCS proteins exert their inhibitory effects at the level of activation of janus kinases (JAKs) or by competing with transcription factors for binding sites on activated cytokine receptors. SOCS proteins also may mediate the ubiquitination and subsequent degradation of the SOCS protein and its bound signaling complex. Genetic modification of SOCS genes in mice has revealed crucial roles in the negative regulation of a number of important physiologic parameters including interferon gamma activity, growth, blood cell production, and placental development.

CONCLUSIONS

Information about SOCS action in gastrointestinal function and disease is only just emerging, but available data indicate a role in growth of gastrointestinal tissues, inflammatory bowel disease, and cancer.

SOCS-1 and SOCS-3 block insulin signaling by ubiquitin-mediated degradation of IRS1 and IRS2

Inflammation associates with peripheral insulin resistance, which dysregulates nutrient homeostasis and leads to diabetes. Inflammation induces the expression of SOCS proteins. We show that SOCS1 or SOCS3 targeted IRS1 and IRS2, two critical signaling molecules for insulin action, for ubiquitin-mediated degradation. SOCS1 or SOCS3 bound both recombinant and endogenous IRS1 and IRS2 and promoted their ubiquitination and subsequent degradation in multiple cell types. Mutations in the conserved SOCS box of SOCS1 abrogated its interaction with the elongin BC ubiquitin-ligase complex without affecting its binding to IRS1 or IRS2. The SOCS1 mutants also failed to promote the ubiquitination and degradation of either IRS1 or IRS2. Adenoviral-mediated expression of SOCS1 in mouse liver dramatically reduced hepatic IRS1 and IRS2 protein levels and caused glucose intolerance; by contrast, expression of the SOCS1 mutants had no effect. Thus, SOCS-mediated degradation of IRS proteins, presumably via the elongin BC ubiquitin-ligase, might be a general mechanism of inflammation-induced insulin resistance, providing a target for therapy.

Regulation of the acid-labile subunit in sustained endotoxemia

The effect of sustained endotoxemia on expression of the acid-labile subunit (ALS) in relation to hepatic markers of altered GH and insulin sensitivity was examined. Juvenile rats were injected with endotoxin twice daily for 48 h, causing reduced food intake and attenuated growth. In pair-fed controls, food restriction caused marked suppression of ALS gene expression and circulating levels within 12 h, and endotoxemia augmented this effect. This acute effect of endotoxin corresponded temporally with transient induction of suppressor of cytokine signaling (SOCS)-3, cytokine-inducible SH2-containing protein (CIS), phosphoenolpyruvate carboxykinase (PEPCK), and insulin-like growth factor-binding protein (IGFBP)-1 and suppression of GH receptor (GHR). During the subsequent 36 h of sustained endotoxin treatment, expression of ALS recovered to, and then rose above, that of their pair-fed controls. This effect was paralleled by other ternary complex components. The inductive effect of sustained endotoxemia relative to pair-fed controls could not be explained by differences in expression of GHR, SOCS-3, or CIS but coincided with normalized PEPCK and IGFBP-1 levels, suggesting better hepatic insulin sensitivity in these animals. These data may indicate that, in sustained endotoxemia, ALS levels are regulated through modulation of hepatic insulin sensitivity.

Thermal injury-induced increases of hepatocyte SOCS3 lead to decreases in STAT3

Previous work in this laboratory has shown an increase of both mRNA and protein for suppressor of cytokine signaling 3 (SOCS3) in rat liver after thermal injury. This study identifies which liver cell type (parenchymal or non-parenchymal) is responsible for the postburn increase in SOCS3 and how this increase is connected to the signal transducer and activator of transcription (STAT) pathway. Parenchymal (hepatocytes) and non-parenchymal cells were isolated by Liberase digestion from postburn day 1 (PBD1) rats (including sham controls) and were analyzed for the expression of SOCS3 mRNA and protein and STAT3 and p-STAT3 protein. Reverse transcriptase (RT)-PCR performed on the isolated cells showed a significant increase of SOCS3 in the hepatocytes, but not in the non-parenchymal cells. When isolated hepatocytes from rats and the human hepatocyte cell line, HepG2, were cultured in the presence of IL-6, both showed an increase in SOCS3 mRNA expression. Anti-SOCS3, anti-STAT3, and anti-phosphorylated STAT3 labeling in both postburn rat liver and isolated hepatocyte cells that were cultured in the presence of IL-6 revealed that an increase in SOCS3 protein was accompanied by decrease in STAT3 protein. We propose that thermal injury stimulates non-parenchymal cells to produce cytokines, including IL-6, which in tum stimulate the Jak/STAT pathway in hepatocytes. The signal transduction pathway triggered by non-parenchymal cells causes an increase in SOCS3 production, which in turn induces the reduction of STAT3 protein in the hepatocytes.

Tumor necrosis factor mediates hepatic growth hormone resistance during sepsis

During sepsis, growth hormone (GH) resistance contributes to the catabolism of muscle protein. To determine the role of tumor necrosis factor (TNF) as a mediator of GH resistance, we examined the effects of a TNF antagonist [TNF-binding protein (TNFbp)] on the GH/insulin-like growth factor (IGF) I system during abdominal sepsis. To investigate potential mechanisms, the effects of TNF on the IGF-I response to GH and GH signaling were examined in cultured rat hepatocytes (CWSV-1). Three groups of rats were studied: Control, Sepsis, and Sepsis + TNFbp. Liver, gastrocnemius, and plasma were collected on day 5. In gastrocnemius, neither sepsis nor TNFbp altered the abundance of IGF-I mRNA. However, septic rats demonstrated an increase in circulating GH and a reduction in plasma IGF-I concentrations that was ameliorated by pretreatment with TNFbp. Liver from septic rats demonstrated a 50% reduction in GH receptor (GHR) and IGF-I mRNA on day 5 that was attenuated by TNFbp. However, the abundance of GHR protein was not different in liver from Control, Sepsis, or Sepsis + TNFbp rats. Consequently, a decreased amount of hepatic GHR does not explain the GH-resistant septic state. In CWSV-1 hepatocytes, TNF-alpha had no effect on GHR protein level but inhibited the induction of IGF-I mRNA by GH. Nuclear protein from TNF-treated hepatocytes demonstrated similar levels of phosphorylated signal transducer and activator of transcription-5 (STAT5) and DNA binding relative to controls 5 min after GH treatment. However, both of these parameters were decreased (vs. control) in TNF-treated cells 60 min after GH treatment. Collectively, these results suggest that TNF mediates hepatic GH resistance during sepsis by inhibiting the duration of signaling via the janus kinase-2/STAT5 pathway.

The effect of suppressor of cytokine signaling 3 on GH signaling in beta-cells

GH is an important regulator of cell growth and metabolism. In the pancreas, GH stimulates mitogenesis as well as insulin production in beta-cells. The cellular effects of GH are exerted mainly through activation of the Janus kinase-signal transducer and activator of transcription (STAT) pathway. Recently it has been found that suppressors of cytokine signaling (SOCS) proteins are able to inhibit GH-induced signal transduction. In the present study, the role of SOCS-3 in GH signaling was investigated in the pancreatic beta-cell lines RIN-5AH and INS-1 by means of inducible expression systems. Via stable transfection of the beta-cell lines with plasmids expressing SOCS-3 under the control of an inducible promoter, a time- and dose-dependent expression of SOCS-3 in the cells was obtained. EMSA showed that SOCS-3 is able to inhibit GH-induced DNA binding of both STAT3 and STAT5 in RIN-5AH cells. Furthermore, using Northern blot analysis it was shown that SOCS-3 can completely inhibit GH-induced insulin production in these cells. Finally, 5-bromodeoxyuridine incorporation followed by fluorescence-activated cell sorting analysis showed that SOCS-3 inhibits GH-induced proliferation of INS-1 cells. These findings support the hypothesis that SOCS-3 is a major regulator of GH signaling in insulin-producing cells.

Impairment of liver GH receptor signaling by fasting

Fasting causes a state of GH resistance responsible for low circulating IGF-I levels. To investigate whether this resistance may result from alterations in the GH signaling pathway, we determined the effects of fasting on the GH transduction pathway in rat liver. Forty-eight-hour fasted or fed male rats were injected with recombinant rat GH via the portal vein. Liver was removed 0 and 15 min after injection. Although GH stimulated Janus kinase 2 (JAK2) phosphorylation in all animals, this was severely blunted in fasted animals. Similarly, the phosphorylation of the GH receptor, although observed in both fasted and fed rats after GH injection, was markedly reduced in fasted rats. A rapid signal transducer and activator of transcription 5 (STAT5) tyrosine phosphorylation was also induced in the liver of fed animals in response to GH. In contrast, in fasted rats only a slight phosphorylated STAT5 signal was observed. The inhibitory effect of fasting on these GH signaling molecules occurred without changes in their protein content. Furthermore, the impairment of the JAK-STAT pathway in fasted animals was associated with increased liver suppressor of cytokine signaling 3 mRNA levels. Although glucocorticoids, which are increased by fasting, may cause GH resistance, adrenalectomy failed to prevent alterations in the JAK-STAT pathway caused by fasting. In conclusion, the GH resistance induced by fasting is associated with impairment of the JAK-STAT signaling pathway. This might contribute to the decrease in liver IGF-I production observed in fasting.

PRL receptor-mediated effects in female mouse adipocytes: PRL induces suppressors of cytokine signaling expression and suppresses insulin-induced leptin production in adipocytes in vitro

PRL has been reported to regulate fat metabolism in several species. We recently reported PRL receptor (PRLR) expression in mouse adipocytes and increased levels of PRLR expression in the adipose tissue of lactating and PRL-transgenic mice compared with controls. These results suggest PRLR-mediated effects in adipose tissue. However, to date most studies have been performed in vivo, and it is unclear whether PRL has direct effects on adipocytes. The PRLR belongs to the cytokine receptor family, and a family of suppressors of cytokine signaling (SOCS) was recently identified. The present study was performed to investigate whether PRL has direct effects on adipocytes. The expression of cytokine-inducible SH2-domain-containing protein (CIS), SOCS-3, and SOCS-2 mRNA and protein was analyzed using ribonuclease protection assay and immunoblotting, respectively. Ovine PRL induced CIS mRNA expression and a combination of oPRL and insulin induced SOCS-3 mRNA expression in adipocytes cultured in vitro for 0-240 min, demonstrating PRLR-mediated direct effects in these cells. Furthermore, CIS, SOCS-3, and SOCS-2 mRNA and protein were all transiently expressed in adipose tissue obtained from female mice stimulated with oPRL (1 microg/g BW) for 0-24 h. In adipose tissue of female mice with endogenously high PRL levels, PRL-transgenic mice, only SOCS-2 expression was increased. The level of SOCS-2 mRNA was also increased in adipose tissue during pregnancy and lactation compared with that in wild-type virgin female mice. A possible reason for increased SOCS-2 expression after prolonged PRL exposure during lactation and in the PRL transgenes could be to restore the sensitivity of adipose tissue to PRL. In addition, the direct effect of PRL on leptin production was investigated in adipocytes cultured in vitro for 6 h. PRL inhibited insulin-induced leptin production in vitro. However, PRL had no effect on leptin production in the absence of insulin. In contrast, serum leptin concentrations were increased in PRL-transgenic females compared with control mice. In conclusion, our results demonstrate functional PRLRs in mouse adipocytes and suggest a role for CIS, SOCS-3, and SOCS-2 in regulating PRL signal transduction in adipose tissue.

Impaired JAK-STAT signal transduction contributes to growth hormone resistance in chronic uremia

Chronic renal failure (CRF) is associated with resistance to the growth-promoting and anabolic actions of growth hormone (GH). In rats with CRF induced by partial renal ablation, 7 days of GH treatment had a diminished effect on weight gain and hepatic IGF-1 and IGFBP-1 mRNA levels, compared with sham-operated pair-fed controls. To assess whether GH resistance might be due to altered signal transduction, activation of the JAK-STAT pathway was studied 10 or 15 minutes after intravenous injection of 5 mg/kg GH or vehicle. Hepatic GH receptor (GHR) mRNA levels were significantly decreased in CRF, but GHR protein abundance and GH binding to microsomal and plasma membranes was unaltered. JAK2, STAT1, STAT3, and STAT5 protein abundance was also unchanged. However, GH-induced tyrosine phosphorylation of JAK2, STAT5, and STAT3 was 75% lower in the CRF animals. Phosphorylated STAT5 and STAT3 were also diminished in nuclear extracts. The expression of the suppressor of cytokine signaling-2 (SOCS-2) was increased twofold in GH-treated CRF animals, and SOCS-3 mRNA levels were elevated by 60% in CRF, independent of GH treatment. In conclusion, CRF causes a postreceptor defect in GH signal transduction characterized by impaired phosphorylation and nuclear translocation of GH-activated STAT proteins, which is possibly mediated, at least in part, by overexpression of SOCS proteins.

Tissue-specific regulation of IGF-I and IGF-binding proteins in response to TNFalpha

The circulating concentration of insulin-like growth factor-I (IGF-I) is regulated by both its rate of synthesis and its ability to form stable complexes with IGF-binding proteins (IGFBPs). An equilibrium between IGF-I and IGFBPs is thought to help maintain muscle protein balance. In contrast, catabolic conditions disrupt the IGF system and result in the loss of skeletal muscle protein. We have examined the mechanisms by which tumour necrosis factor alpha (TNFalpha), a catabolic cytokine, alters the IGF system. Conscious rats were infused intravenously with recombinant human TNFalpha or vehicle for 24 h. TNFalpha decreased the concentration of both total and free IGF-I in the plasma (30-40%). This change was associated with a reduction in IGF-I mRNA expression in liver (39%), gastrocnemius (73%), soleus (46%) and heart (63%), but a 2.5-fold increase in the whole kidney. In contrast, TNFalpha did not alter IGF-II mRNA expression in skeletal muscle. TNFalpha also increased IGFBP-1 in the blood (4-fold) and this response was associated with an increase in IGFBP-1 mRNA expression in both liver (3-fold) and kidney (9-fold). In contrast, IGFBP-3 levels in the blood were reduced 38% in response to the infusion of TNFalpha. This change was accompanied by a 60-80% reduction of IGFBP-3 mRNA in liver and kidney but no significant change in muscle. Hepatic mRNA levels of the acid-labile subunit were also reduced by TNFalpha (46%). Finally, tissue expression of mac25 (also referred to IGFBP-related protein-1) mRNA was increased in gastrocnemius (50%) but remained unchanged in liver and kidney. These results more fully characterize the changes in various elements of the IGF system and, thereby, provide potential mechanisms for the alterations in the circulating IGF system as well as for changes in tissue metabolism observed during catabolic insults associated with increased TNFalpha expression.

Suppressors of cytokine signaling-1 and -6 associate with and inhibit the insulin receptor. A potential mechanism for cytokine-mediated insulin resistance

Insulin resistance contributes to a number of metabolic disorders, including type II diabetes, hypertension, and atherosclerosis. Cytokines, such as tumor necrosis factor-alpha, interleukin-1 beta, and interleukin-6, and hormones, such as growth hormone, are known to cause insulin resistance, but the mechanisms by which they inhibit the cellular response to insulin have not been elucidated. One mechanism by which these agents could cause insulin resistance is by inducing the expression of cellular proteins that inhibit insulin receptor (IR) signaling. Suppressors of cytokine signaling (SOCS) proteins are negative regulators of cytokine signaling pathways, the expression of which is regulated by certain cytokines. SOCS proteins are therefore attractive candidates as mediators of cytokine-induced insulin resistance. We have found that SOCS-1 and SOCS-6 interact with the IR when expressed in human hepatoma cells (HepG2) or in rat hepatoma cells overexpressing the human IR. In SOCS-1-expressing cells, insulin treatment increases the extent of interaction with the IR, whereas in SOCS-6-expressing cells the association with the IR appears to require insulin treatment. SOCS-1 and SOCS-6 do not inhibit insulin-dependent IR autophosphorylation, but both proteins inhibit insulin-dependent activation of ERK1/2 and protein kinase B in vivo and IR-directed phosphorylation of IRS-1 in vitro. These results suggest that SOCS proteins may be inhibitors of IR signaling and could mediate cytokine-induced insulin resistance and contribute to the pathogenesis of type II diabetes.