Regulation of interleukin-6-induced hepatic insulin resistance by mammalian target of rapamycin through the STAT3-SOCS3 pathway

PLD1 activation mediates Amb a 1-induced Th2-associated cytokine expression via the JNK/ATF-2 pathway in BEAS-2B cells

The purpose of this study was to identify the role of phospholipase D1 (PLD1) in Amb a 1-induced IL-5 and IL-13 expression. When BEAS-2B cells were stimulated with Amb a 1, PLD activity increased, and knockdown of PLD1 decreased Amb a 1-induced IL-5 and IL-13 expression. Amb a 1 also activated the PLCγ/p70S6K/JNK pathway. Furthermore, Amb a 1-induced PLD activation was also attenuated by PLCγ inhibition, and knockdown of PLD1 decreased Amb a 1-induced activation of P70S6K and JNK. When ATF-2 activity was blocked with ATF-2 siRNA, Amb a 1-induced IL-5 and IL-13 expression was completely abolished, indicating that ATF-2 is a transcriptional factor required for the expression of IL-5 and IL-13 in response to Amb a 1. Taken together, we suggest that PLD1 acts as an important regulator in Amb a 1-induced expression of IL-5 and IL-13 via a PLCγ/p70S6K/JNK/ATF-2 pathway in BEAS-2B cells.

Repression of Mammalian Target of Rapamycin Complex 1 Inhibits Intestinal Regeneration in Acute Inflammatory Bowel Disease Models

The mammalian target of rapamycin (mTOR) signaling pathway integrates environmental cues to regulate cell growth and survival through various mechanisms. However, how mTORC1 responds to acute inflammatory signals to regulate bowel regeneration is still obscure. In this study, we investigated the role of mTORC1 in acute inflammatory bowel disease. Inhibition of mTORC1 activity by rapamycin treatment or haploinsufficiency of Rheb through genetic modification in mice impaired intestinal cell proliferation and induced cell apoptosis, leading to high mortality in dextran sodium sulfate- and 2,4,6-trinitrobenzene sulfonic acid-induced colitis models. Through bone marrow transplantation, we found that mTORC1 in nonhematopoietic cells played a major role in protecting mice from colitis. Reactivation of mTORC1 activity by amino acids had a positive therapeutic effect in mTORC1-deficient Rheb(+/-) mice. Mechanistically, mTORC1 mediated IL-6-induced Stat3 activation in intestinal epithelial cells to stimulate the expression of downstream targets essential for cell proliferation and tissue regeneration. Therefore, mTORC1 signaling critically protects against inflammatory bowel disease through modulation of inflammation-induced Stat3 activity. As mTORC1 is an important therapeutic target for multiple diseases, our findings will have important implications for the clinical usage of mTORC1 inhibitors in patients with acute inflammatory bowel disease.

Adenosine 5'-monophosphate-activated protein kinase regulates IL-10-mediated anti-inflammatory signaling pathways in macrophages

AMP-activated protein kinase (AMPK) is a conserved serine/threonine kinase with a critical function in the regulation of metabolic pathways in eukaryotic cells. Recently, AMPK has been shown to play an additional role as a regulator of inflammatory activity in leukocytes. Treatment of macrophages with chemical AMPK activators, or forced expression of a constitutively active form of AMPK, results in polarization to an anti-inflammatory phenotype. In addition, we reported previously that stimulation of macrophages with anti-inflammatory cytokines such as IL-10, IL-4, and TGF-β results in rapid activation of AMPK, suggesting that AMPK contributes to the suppressive function of these cytokines. In this study, we investigated the role of AMPK in IL-10-induced gene expression and anti-inflammatory function. IL-10-stimulated wild-type macrophages displayed rapid activation of PI3K and its downstream targets Akt and mammalian target of rapamycin complex (mTORC1), an effect that was not seen in macrophages generated from AMPKα1-deficient mice. AMPK activation was not impacted by treatment with either the PI3K inhibitor LY294002 or the JAK inhibitor CP-690550, suggesting that IL-10-mediated activation of AMPK is independent of PI3K and JAK activity. IL-10 induced phosphorylation of both Tyr(705) and Ser(727) residues of STAT3 in an AMPKα1-dependent manner, and these phosphorylation events were blocked by inhibition of Ca(2+)/calmodulin-dependent protein kinase kinase β, an upstream activator of AMPK, and by the mTORC1 inhibitor rapamycin, respectively. The impaired STAT3 phosphorylation in response to IL-10 observed in AMPKα1-deficient macrophages was accompanied by reduced suppressor of cytokine signaling 3 expression and an inadequacy of IL-10 to suppress LPS-induced proinflammatory cytokine production. Overall, our data demonstrate that AMPKα1 is required for IL-10 activation of the PI3K/Akt/mTORC1 and STAT3-mediated anti-inflammatory pathways regulating macrophage functional polarization.

Mitochondrial dysfunction in some triple-negative breast cancer cell lines: role of mTOR pathway and therapeutic potential

INTRODUCTION

Triple-negative breast cancer (TNBC) is a subtype of highly malignant breast cancer with poor prognosis. TNBC is not amenable to endocrine therapy and often exhibit resistance to current chemotherapeutic agents, therefore, further understanding of the biological properties of these cancer cells and development of effective therapeutic approaches are urgently needed.

METHODS

We first investigated the metabolic alterations in TNBC cells in comparison with other subtypes of breast cancer cells using molecular and metabolic analyses. We further demonstrated that targeting these alterations using specific inhibitors and siRNA approach could render TNBC cells more sensitive to cell death compared to other breast cancer subtypes.

RESULTS

We found that TNBC cells compared to estrogen receptor (ER) positive cells possess special metabolic characteristics manifested by high glucose uptake, increased lactate production, and low mitochondrial respiration which is correlated with attenuation of mTOR pathway and decreased expression of p70S6K. Re-expression of p70S6K in TNBC cells reverses their glycolytic phenotype to an active oxidative phosphorylation (OXPHOS) state, while knockdown of p70S6K in ER positive cells leads to suppression of mitochondrial OXPHOS. Furthermore, lower OXPHOS activity in TNBC cells renders them highly dependent on glycolysis and the inhibition of glycolysis is highly effective in targeting TNBC cells despite their resistance to other anticancer agents.

CONCLUSIONS

Our study shows that TNBC cells have profound metabolic alterations characterized by decreased mitochondrial respiration and increased glycolysis. Due to their impaired mitochondrial function, TNBC cells are highly sensitive to glycolytic inhibition, suggesting that such metabolic intervention may be an effective therapeutic strategy for this subtype of breast cancer cells.

Nutrient Sensing via mTOR in T Cells Maintains a Tolerogenic Microenvironment

We have proposed that tolerance can be maintained through the induction, by Treg cells, of a tolerogenic microenvironment within tolerated tissues that inhibits effector cell activity but which supports the generation of further Treg cells by “infectious tolerance.” Two important components of this tolerogenic microenvironment depend on metabolism and nutrient sensing. The first is due to the up-regulation of multiple enzymes that consume essential amino acids, which are sensed in naïve T cells primarily via inhibition of the mechanistic target of rapamycin (mTOR) pathway, which in turn encourages their further differentiation into FOXP3⁺ Treg cells. The second mechanism is the metabolism of extracellular ATP to adenosine by the ectoenzymes CD39 and CD73. These two enzymes are constitutively co-expressed on Treg cells, but can also be induced on a wide variety of cell types by TGFβ and the adenosine generated can be shown to be a potent inhibitor of T cell proliferation. This review will focus on mechanisms of nutrient sensing in T cells, how these are integrated with TCR and cytokine signals via the mTOR pathway, and what impact this has on intracellular metabolism and subsequently the control of differentiation into different effector or regulatory T cell subsets.

Interleukin-6 upregulates Th17 response via mTOR/STAT3 pathway in acute-on-chronic hepatitis B liver failure

BACKGROUND

Interleukin (IL)-17-producing CD4(+) T cells (Th17) have been shown to play crucial roles in the pathogenesis of hepatitis B virus (HBV)-associated acute-on-chronic liver failure (ACLF). However, the mechanism underlying the enhanced Th17 responses in these patients remains elusive. In this study, the relevance of the IL-6/signal transducer and activator of transcription 3 (STAT3)/mammalian target of rapamycin (mTOR)/Th17 loop in HBV-associated ACLF was investigated.

METHODS

Eight patients with HBV-associated ACLF, eight asymptomatic chronic HBV carriers and eight healthy controls were enrolled in our study. The frequency of peripheral Th17 cells was determined by flow cytometry. IL-17 and IL-6 mRNA levels in peripheral blood mononuclear cells were quantified using quantitative real-time reverse polymerase chain reaction. The activation of STAT3 was seen upon stimulation with IL-6. Rapamycin, an mTOR inhibitor, was used for analysis of the suppressive effect on the Th17 response in vitro.

RESULTS

The percentage of peripheral Th17 cells significantly increased in ACLF patients. CD4(+) T cells from ACLF patients produced higher levels of IL-17 and IL-6 upon stimulation in vitro. Activation of STAT3 in response to IL-6 was elevated in ACLF patients. The IL-6-induced upregulation of IL-17 production by CD4(+) T cells could be reversed by an mTOR inhibitor through decreasing STAT3 activation.

CONCLUSIONS

STAT3 activation upon IL-6 stimulation contributed to the enhanced Th17 response in HBV-associated ACLF patients and mTOR regulated STAT3 phosphorylation. mTOR can be a novel target to suppress Th17-mediated liver injury in HBV-associated ACLF patients.

Glycerol-3-phosphate acyltransferase-4-deficient mice are protected from diet-induced insulin resistance by the enhanced association of mTOR and rictor

Glycerol-3-phosphate acyltransferase (GPAT) activity is highly induced in obese individuals with insulin resistance, suggesting a correlation between GPAT function, triacylglycerol accumulation, and insulin resistance. We asked whether microsomal GPAT4, an isoform regulated by insulin, might contribute to the development of hepatic insulin resistance. Compared with control mice fed a high fat diet, Gpat4(-/-) mice were more glucose tolerant and were protected from insulin resistance. Overexpression of GPAT4 in mouse hepatocytes impaired insulin-suppressed gluconeogenesis and insulin-stimulated glycogen synthesis. Impaired glucose homeostasis was coupled to inhibited insulin-stimulated phosphorylation of Akt(Ser⁴⁷³) and Akt(Thr³⁰⁸). GPAT4 overexpression inhibited rictor's association with the mammalian target of rapamycin (mTOR), and mTOR complex 2 (mTORC2) activity. Compared with overexpressed GPAT3 in mouse hepatocytes, GPAT4 overexpression increased phosphatidic acid (PA), especially di16:0-PA. Conversely, in Gpat4(-/-) hepatocytes, both mTOR/rictor association and mTORC2 activity increased, and the content of PA in Gpat4(-/-) hepatocytes was lower than in controls, with the greatest decrease in 16:0-PA species. Compared with controls, liver and skeletal muscle from Gpat4(-/-)-deficient mice fed a high-fat diet were more insulin sensitive and had a lower hepatic content of di16:0-PA. Taken together, these data demonstrate that a GPAT4-derived lipid signal, likely di16:0-PA, impairs insulin signaling in mouse liver and contributes to hepatic insulin resistance.

A model integration approach linking signalling and gene-regulatory logic with kinetic metabolic models

Systems biology has to increasingly cope with large- and multi-scale biological systems. Many successful in silico representations and simulations of various cellular modules proved mathematical modelling to be an important tool in gaining a solid understanding of biological phenomena. However, models spanning different functional layers (e.g. metabolism, signalling and gene regulation) are still scarce. Consequently, model integration methods capable of fusing different types of biological networks and various model formalisms become a key methodology to increase the scope of cellular processes covered by mathematical models. Here we propose a new integration approach to couple logical models of signalling or/and gene-regulatory networks with kinetic models of metabolic processes. The procedure ends up with an integrated dynamic model of both layers relying on differential equations. The feasibility of the approach is shown in an illustrative case study integrating a kinetic model of central metabolic pathways in hepatocytes with a Boolean logical network depicting the hormonally induced signal transduction and gene regulation events involved. In silico simulations demonstrate the integrated model to qualitatively describe the physiological switch-like behaviour of hepatocytes in response to nutritionally regulated changes in extracellular glucagon and insulin levels. A simulated failure mode scenario addressing insulin resistance furthermore illustrates the pharmacological potential of a model covering interactions between signalling, gene regulation and metabolism.

Leptin increases TNF-α expression and production through phospholipase D1 in Raw 264.7 cells

Epidemiological evidence suggests that obesity is associated with inflammation of the respiratory tract and the pathogenesis of asthma. The purpose of this study was to examine the role of phospholipase D1 (PLD1) in leptin-induced expression of the proinflammatory cytokine, tumor necrosis factor (TNF)-α, and to suggest a molecular link between obesity and respiratory tract inflammation. We investigated whether leptin, a typical adipocytokine, plays a role in the expression of TNF-α through increased PLD1 activity in Raw 264.7. Leptin enhanced the activity of PLD1 through activation of PLCγ and Src, while PLD1 siRNA decreased the leptin-induced expression and production of TNF-α. Leptin-induced PLD activation was also inhibited by a PLCγ inhibitor (PAO) and Src kinase inhibitor (PP2), indicating that PLCγ and Src kinase are upstream activators of PLD1. Down-regulation of PLD1 also completely blocked activation of p70S6K, an activator of JNK. Leptin-induced expression of TNF-α was also prevented by inhibition of p70S6K and JNK. Taken together, these results indicate that PLD1 acts as an important regulator of leptin-induced expression of TNF-α by participating in the PLCγ/Src/PLD1/PA/p70S6K/JNK pathway.

The mTOR pathway and integrating immune regulation

The mammalian target of rapamycin (mTOR) pathway is an important integrator of nutrient-sensing signals in all mammalian cells, and acts to coordinate the cell proliferation with the availability of nutrients such as glucose, amino acids and energy (oxygen and ATP). A large part of the immune response depends on the proliferation and clonal expansion of antigen-specific T cells, which depends on mTOR activation, and the pharmacological inhibition of this pathway by rapamycin is therefore potently immunosuppressive. It is only recently, however, that we have started to understand the more subtle details of how the mTOR pathway is involved in controlling the differentiation of effector versus memory CD8(+) T cells and the decision to generate different CD4(+) helper T-cell subsets. In particular, this review will focus on how nutrient sensing via mTOR controls the expression of the master transcription factor for regulatory T cells in order to maintain the balance between tolerance and inflammation.

Hypothalamic nesfatin-1/NUCB2 knockdown augments hepatic gluconeogenesis that is correlated with inhibition of mTOR-STAT3 signaling pathway in rats

Nesfatin-1, an 82-amino acid neuropeptide, has recently been characterized as a potent metabolic regulator. However, the metabolic mechanisms and signaling steps directly associated with the action of nesfatin-1 have not been well delineated. We established a loss-of-function model of hypothalamic nesfatin-1/NUCB2 signaling in rats through an adenoviral-mediated RNA interference. With this model, we found that inhibition of central nesfatin-1/NUCB2 activity markedly increased food intake and hepatic glucose flux and decreased glucose uptake in peripheral tissue in rats fed either a normal chow diet (NCD) or a high-fat diet (HFD). The change of hepatic glucose fluxes in the hypothalamic nesfatin-1/NUCB2 knockdown rats was accompanied by increased hepatic levels of glucose-6-phosphatase and PEPCK and decreased insulin receptor, insulin receptor substrate 1, and AKT kinase phosphorylation. Furthermore, knockdown of hypothalamic nesfatin-1 led to decreased phosphorylation of mammalian target of rapamycin (mTOR) and signal transducer and activator of transcription 3 (STAT3) and the subsequent suppressor of cytokine signaling 3 levels. These results demonstrate that hypothalamic nesfatin-1/NUCB2 plays an important role in glucose homeostasis and hepatic insulin sensitivity, which is, at least in part, associated with the activation of the mTOR-STAT3 signaling pathway.

Postprandial activation of metabolic and inflammatory signalling pathways in human peripheral mononuclear cells

High-fat, high-carbohydrate (HFHC) meals induce an inflammatory response in mononuclear cells (MNC). Here, we studied the interaction between metabolic and inflammatory signalling pathways by the measurement of postprandial effects of three different test meals on intracellular Akt, S6 kinase (S6K)/mammalian target of rapamycin and NF-κB signalling in human MNC. We recruited six healthy, lean individuals. Each individual ingested three different meals in the morning separated by at least 3 d: a HFHC meal; an oral lipid-tolerance test meal; a healthy breakfast. Blood samples were obtained before and 1, 2, 4, 6 and 8 h after ingestion. Plasma insulin and IL-6 levels were measured. Intracellular metabolic and inflammatory signalling pathways were assessed by measuring the phosphorylation of Akt kinase and S6K, the degradation of inhibitory κB-α (IκB-α) protein and the DNA binding activity of NF-κB in MNC. mRNA expression levels of the Akt and NF-κB target genes Mn superoxide dismutase (MnSOD), CC-chemokine-receptor 5 (CCR5), intercellular adhesion molecule 1 (ICAM-1) and plasminogen activator inhibitor-1 (PAI-1) were measured by quantitative RT-PCR. We found a positive correlation of Akt phosphorylation with NF-κB activation (NF-κB binding activity: r 0·4500, P= 0·0003; IκB-α protein levels: r − 0·5435, P< 0·0001), a negative correlation of plasma insulin levels with NF-κB binding activity (r − 0·3993, P= 0·0016) and a positive correlation of plasma insulin levels with S6K activation (r 0·4786, P< 0·0001). The activation of Akt and pro-inflammatory NF-κB signalling was supported by the up-regulation of the respective target genes MnSOD and CCR5. In conclusion, the present data suggest a postprandial interaction between the metabolic and inflammatory signalling pathways Akt and NF-κB in MNC.

Suppression of the mTORC1/STAT3/Notch1 pathway by activated AMPK prevents hepatic insulin resistance induced by excess amino acids

Nutrient overload is associated with the development of obesity, insulin resistance, and type 2 diabetes. However, the underlying mechanisms for developing insulin resistance in the presence of excess nutrients are incompletely understood. We investigated whether activation of AMP-activated protein kinase (AMPK) prevents the hepatic insulin resistance that is induced by the consumption of a high-protein diet (HPD) and the presence of excess amino acids. Exposure of HepG2 cells to excess amino acids reduced AMPK phosphorylation, upregulated Notch1 expression, and impaired the insulin-stimulated phosphorylation of Akt Ser(473) and insulin receptor substrate-1 (IRS-1) Tyr(612). Inhibition of Notch1 prevented amino acid-induced insulin resistance, which was accompanied by reduced expression of Rbp-Jk, hairy and enhancer of split-1, and forkhead box O1. Mechanistically, mTORC1 signaling was activated by excess amino acids, which then positively regulated Notch1 expression through the activation of the signal transducer and activator of transcription 3 (STAT3). Activation of AMPK by metformin inhibited mTORC1-STAT3 signaling, thereby preventing excess amino acid-impaired insulin signaling. Finally, HPD feeding suppressed AMPK activity, activated mTORC1/STAT3/Notch1 signaling, and induced insulin resistance. Chronic administration of either metformin or rapamycin inhibited the HPD-activated mTORC1/STAT3/Notch1 signaling pathway and prevented hepatic insulin resistance. We conclude that the upregulation of Notch1 expression by hyperactive mTORC1 signaling is an essential event in the development of hepatic insulin resistance in the presence of excess amino acids. Activation of AMPK prevents amino acid-induced insulin resistance through the suppression of the mTORC1/STAT3/Notch1 signaling pathway.

Neuroendocrine and metabolic components of dopamine agonist amelioration of metabolic syndrome in SHR rats

BACKGROUND

The hypertensive, pro-inflammatory, obese state is strongly coupled to peripheral and hepatic insulin resistance (in composite termed metabolic syndrome [MS]). Hepatic pro-inflammatory pathways have been demonstrated to initiate or exacerbate hepatic insulin resistance and contribute to fatty liver, a correlate of MS. Previous studies in seasonally obese animals have implicated an important role for circadian phase-dependent increases in hypothalamic dopaminergic tone in the maintenance of the lean, insulin sensitive condition. However, mechanisms driving this dopaminergic effect have not been fully delineated and the impact of such dopaminergic function upon the above mentioned parameters of MS, particularly upon key intra-hepatic regulators of liver inflammation and lipid and glucose metabolism have never been investigated.

OBJECTIVE

This study therefore investigated the effects of timed daily administration of bromocriptine, a potent dopamine D2 receptor agonist, on a) ventromedial hypothalamic catecholamine activity, b) MS and c) hepatic protein levels of key regulators of liver inflammation and glucose and lipid metabolism in a non-seasonal model of MS - the hypertensive, obese SHR rat.

METHODS

Sixteen week old SHR rats maintained on 14 hour daily photoperiods were treated daily for 16 days with bromocriptine (10 mg/kg, i.p.) or vehicle at 1 hour before light offset and, subsequent to blood pressure recordings on day 14, were then utilized for in vivo microdialysis of ventromedial hypothalamic catecholamine activity or sacrificed for the analyses of MS factors and regulators of hepatic metabolism. Normal Wistar rats served as wild-type controls for hypothalamic activity, body fat levels, and insulin sensitivity.

RESULTS

Bromocriptine treatment significantly reduced ventromedial hypothalamic norepinephrine and serotonin levels to the normal range and systolic and diastolic blood pressures, retroperitoneal body fat level, plasma insulin and glucose levels and HOMA-IR relative to vehicle treated SHR controls. Such treatment also reduced plasma levels of C-reactive protein, leptin, and norepinephrine and increased that of plasma adiponectin significantly relative to SHR controls. Finally, bromocriptine treatment significantly reduced hepatic levels of several pro-inflammatory pathway proteins and of the master transcriptional activators of lipogenesis, gluconeogenesis, and free fatty acid oxidation versus control SHR rats.

CONCLUSION

These findings indicate that in SHR rats, timed daily dopamine agonist treatment improves hypothalamic and neuroendocrine pathologies associated with MS and such neuroendocrine events are coupled to a transformation of liver metabolism potentiating a reduction of elevated lipogenic and gluconeogenic capacity. This liver effect may be driven in part by concurrent reductions in hyperinsulinemia and sympathetic tone as well as by reductions in intra-hepatic inflammation.

The use of adipose tissue-conditioned media to demonstrate the differential effects of fat depots on insulin-stimulated glucose uptake in a skeletal muscle cell line

SUMMARY

We aimed to study the depot-specific effect of adipose tissue on insulin sensitivity of skeletal muscle in vitro. Adipose tissue-conditioned medium (CM) was generated from visceral and subcutaneous fat from obese subjects. CM from visceral as compared to subcutaneous fat had higher concentrations of interleukin (IL)-6 (15-fold; P < 0.05) and IL-8 (8-fold; P < 0.05). CM from visceral fat (1:128 dilution) reduced insulin-stimulated glucose uptake in L6 myotubes by 19% (P < 0.05), an effect mediated by a nuclear factor kappa B (NFκB)/mammalian target of rapamycin complex 1 (mTORC1)-dependent pathway and partially reversed by neutralizing IL-6. IL-6 at a concentration comparable to that in CM from visceral fat reduced insulin-stimulated glucose uptake by 53% (P < 0.05), an effect abolished by inhibiting NFκB or mTORC1. We demonstrated the utility of the CM-myotube system and identified IL-6 as a major cytokine mediating visceral fat-induced muscle insulin resistance.:

Mechanisms of STAT3 activation in the liver of FXR knockout mice

Farnesoid X receptor (FXR, Nr1h4) is a ligand-activated transcription factor belonging to the nuclear receptor superfamily. FXR is essential in maintaining bile acid (BA) homeostasis, and FXR(-/-) mice develop cholestasis, inflammation, and spontaneous liver tumors. The signal transducer and activator of transcription 3 (STAT3) is well known to regulate liver growth, and STAT3 is feedback inhibited by its target gene, the suppressor of cytokine signaling 3 (SOCS3). Strong activation of STAT3 was detected in FXR(-/-) mouse livers. However, the mechanism of STAT3 activation with FXR deficiency remains elusive. Wild-type (WT) and FXR(-/-) mice were used to detect STAT3 pathway activation in the liver. In vivo BA feeding or deprivation was used to determine the role of BAs in STAT3 activation, and in vitro molecular approaches were used to determine the direct transcriptional regulation of SOCS3 by FXR. STAT3 was activated in FXR(-/-) but not WT mice. BA feeding increased, but deprivation by cholestyramine reduced, serum inflammatory markers and STAT3 activation. Furthermore, the Socs3 gene was determined as a direct FXR target gene. The elevated BAs and inflammation, along with reduced SOCS3, collectively contribute to the activation of the STAT3 signaling pathway in the liver of FXR(-/-) mice. This study suggests that the constitutive activation of STAT3 may be a mechanism of liver carcinogenesis in FXR(-/-) mice.

Interleukin-6 inhibition of peroxisome proliferator-activated receptor alpha expression is mediated by JAK2- and PI3K-induced STAT1/3 in HepG2 hepatocyte cells

Interleukin-6 (IL-6) is the major activator of the acute phase response (APR). One important regulator of IL-6-activated APR is peroxisome proliferator-activated receptor alpha (PPARα). Currently, there is a growing interest in determining the role of PPARα in regulating APR; however, studies on the molecular mechanisms and signaling pathways implicated in mediating the effects of IL-6 on the expression of PPARα are limited. We previously revealed that IL-6 inhibits PPARα gene expression through CAAT/enhancer-binding protein transcription factors in hepatocytes. In this study, we determined that STAT1/3 was the direct downstream molecules that mediated the Janus kinase 2 (JAK2) and phosphatidylinositol-3 kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) signaling pathways in IL-6-induced repression of PPARα. Treatment of cells with pharmacological inhibitors of JAK2, PI3K, AKT, and mTOR attenuated the inhibitory effect of IL-6 on PPARα protein in a dose-dependent manner. These inhibitors also decreased the IL-6-induced repression of PPARα mRNA expression and promoter activity. Overexpression of STAT1 and STAT3 in HepG2 cells cotransfected with a reporter vector containing this PPARα promoter region revealed that both the expression plasmids inhibited the IL-6-induced repression of PPARα promoter activity. In the presence of inhibitors of JAK2 and mTOR (AG490 and rapamycin, respectively), IL-6-regulated protein expression and DNA binding of STAT1 and STAT3 were either completely or partially inhibited simultaneously, and the IL-6-induced repression of PPARα protein and mRNA was also inhibited. This study has unraveled novel pathways by which IL-6 inhibits PPARα gene transcription, involving the modulation of JAK2/STAT1-3 and PI3K/AKT/mTOR by inducing the binding of STAT1 and STAT3 to STAT-binding sites on the PPARα promoter. Together, these findings represent a new model of IL-6-induced suppression of PPARα expression by inducing STAT1 and STAT3 phosphorylation and subsequent down-regulation of PPARα mRNA expression.

Intranuclear crosstalk between extracellular regulated kinase1/2 and signal transducer and activator of transcription 3 regulates JEG-3 choriocarcinoma cell invasion and proliferation

Invasiveness of trophoblast and choriocarcinoma cells is in part mediated via leukemia inhibitory factor- (LIF-) induced activation of signal transducer and activator of transcription 3 (STAT3). The regulation of STAT3 phosphorylation at its ser727 binding site, possible crosstalk with intracellular MAPK signaling, and their functional implications are the object of the present investigation. JEG-3 choriocarcinoma cells were cultured in presence/absence of LIF and the specific ERK1/2 inhibitor (U0126). Phosphorylation of signaling molecules (p-STAT3 (ser727 and tyr705) and p-ERK1/2 (thr 202/tyr 204)) was assessed per Western blot. Immunocytochemistry confirmed results, but also pinpointed the location of phosphorylated signaling molecules. STAT3 DNA-binding capacity was studied with a colorimetric ELISA-based assay. Cell viability and invasion capability were assessed by MTS and Matrigel assays. Our results demonstrate that LIF-induced phosphorylation of STAT3 (tyr705 and ser727) is significantly increased after blocking ERK1/2. STAT3 DNA-binding capacity and cell invasiveness are enhanced after LIF stimulation and ERK1/2 blockage. In contrast, proliferation is enhanced by LIF but reduced after ERK1/2 inhibition. The findings herein show that blocking ERK1/2 increases LIF-induced STAT3 phosphorylation and STAT3 DNA-binding capacity by an intranuclear crosstalk, which leads to enhanced invasiveness and reduced proliferation.

HOXB13 mediates tamoxifen resistance and invasiveness in human breast cancer by suppressing ERα and inducing IL-6 expression

Most breast cancers expressing the estrogen receptor α (ERα) are treated successfully with the receptor antagonist tamoxifen (TAM), but many of these tumors recur. Elevated expression of the homeodomain transcription factor HOXB13 correlates with TAM-resistance in ERα-positive (ER+) breast cancer, but little is known regarding the underlying mechanism. Our comprehensive evaluation of HOX gene expression using tiling microarrays, with validation, showed that distant metastases from TAM-resistant patients also displayed high HOXB13 expression, suggesting a role for HOXB13 in tumor dissemination and survival. Here we show that HOXB13 confers TAM resistance by directly downregulating ERα transcription and protein expression. HOXB13 elevation promoted cell proliferation in vitro and growth of tumor xenografts in vivo. Mechanistic investigations showed that HOXB13 transcriptionally upregulated interleukin (IL)-6, activating the mTOR pathway via STAT3 phosphorylation to promote cell proliferation and fibroblast recruitment. Accordingly, mTOR inhibition suppressed fibroblast recruitment and proliferation of HOXB13-expressing ER+ breast cancer cells and tumor xenografts, alone or in combination with TAM. Taken together, our results establish a function for HOXB13 in TAM resistance through direct suppression of ERα and they identify the IL-6 pathways as mediator of disease progression and recurrence.

Mechanistic target of rapamycin controls homeostasis of adipogenesis

Signaling mediated by the mechanistic target of rapamycin (mTOR) is believed to play a critical and positive role in adipogenesis, based on pharmacological evidence and genetic manipulation of mTOR regulators and targets. However, there is no direct genetic evidence for an autonomous role of mTOR itself in preadipocyte differentiation. To seek such evidence, we employed a conditional knockdown approach to deplete mTOR in preadipocytes. Surprisingly, while knockdown of S6K1, a target of mTOR, impairs 3T3-L1 preadipocyte differentiation, reduction of mTOR levels leads to increased differentiation. This enhanced adipogenesis requires the remaining mTOR activity, as mTOR inhibitors abolish differentiation in the mTOR knockdown cells. We also found that mTOR knockdown elevates the levels of CCAAT/enhancer-binding protein α (C/EBPα) and peroxisome proliferator-activated receptor γ (PPARγ). Furthermore, partial reduction of mTOR levels alleviates inhibition of Akt by mTORC1 via IRS1, while at the same time maintaining its positive input through mTORC1 into the adipogenic program. The greater sensitivity of the IRS1-Akt pathway to mTOR levels provides a mechanism that explains the net outcome of enhanced adipogenesis through PPARγ upon mTOR knockdown. Our observations reveal an unexpected role of mTOR in suppressing adipogenesis and suggest that mTOR governs the homeostasis of the adipogenic process by modulating multiple signaling pathways.

Regulation of hepatic suppressor of cytokine signaling 3 by zinc

Promoter analysis of the family of suppressors of cytokine signaling (SOCS) revealed that the human SOCS3 gene contains four binding sites for the metal regulatory transcription factor 1 (MTF-1) located within 1600 bp relative to the transcription start site. A series of experiments were carried out with human hepatoma cells (HepG2) and C57BL/6 mice to examine the effect of zinc on the regulation of SOCS3. In addition, we tested the role of MTF-1 in the regulation of SOCS3 expression using EMSA, chromatin immunoprecipitation assay and siRNA. Lastly, the role of the zinc transporter SLC39A14 on the basal expression of SOCS3 was evaluated. Results indicate that SOCS3 expression is regulated by zinc through an MTF-1-dependent mechanism. In addition, results from siRNA experiments suggest that SLC39A14 is required for basal expression of SOCS3. Further studies are needed to determine whether zinc status affects SOCS3 function.

IL-6 induction of TLR-4 gene expression via STAT3 has an effect on insulin resistance in human skeletal muscle

We investigated the cytokines and mechanisms involved in the induction of insulin resistance in human skeletal muscle. Ten subjects with impaired glucose tolerance (IGT) and 10 control subjects were recruited. We performed biopsies on the vastus lateralis muscle and used immunoblotting to determine levels of inflammatory cytokines, Toll-like receptor (TLR) gene expression, and insulin signaling. We also used a human myotube culture system to examine the mechanisms underlying TLR-4 gene expression. To identify inflammatory cytokines associated with insulin resistance, we measured the levels of IL-6, TNF-α, TLR-2, and TLR-4 in skeletal muscle from non-obese patients with IGT and control subjects. Levels of IL-6, TNF-α, and TLR-4, but not TLR-2, were significantly increased in the IGT group. Insulin resistance decreased significantly in HSMMs following long-term IL-6 treatment. TLR-4 gene expression was significantly increased in human skeletal muscle myoblasts (HSMMs) treated with IL-6. To determine the main signaling pathway for IL-6-induced TLR-4 gene expression, we examined several signaling factors associated with IL-6 signaling pathways. We found that the active form of "signal transducer and activator of transcription 3" (STAT3) was increased. "Stattic" (a STAT3 inhibitor) markedly inhibited TLR-4 gene expression. IL-6 induction of TLR-4 gene expression via STAT3 is one of the main mechanisms underlying insulin resistance in human skeletal muscle.

Gene methylation and silencing of SOCS3 in mantle cell lymphoma

The significance of loss of SOCS3, a negative regulator of signalling pathways including those of STAT3 and NF-κB, was examined in mantle cell lymphoma (MCL). The protein expression and gene methylation status of SOCS3 were detected using immunohistochemistry/Western blots and methylation-specific polymerase chain reaction, respectively. To evaluate its functional importance, SOCS3 was restored in two SOCS3-negative MCL cell lines using a lentiviral vector. Loss of SOCS3 protein expression was found in 3/4 MCL cell lines and 18/33 (54.5%) tumours. SOCS3 was found consistently methylated in cell lines (3/4) and tumours (7/7) negative for SOCS3, and was unmethylated in all SOCS3-positive cell line (1/1) and tumours (5/5) examined. Treatment of all three SOCS3-negative cell lines with 2'-deoxy-5-azacytidine restored SOCS3 expression. SOCS3 is biologically important in MCL, as lentiviral transfer of SOCS3 in SOCS3-negative cell lines increased their apoptotic activity, downregulated nuclear factor (NF)-κB-p65, cyclin D1 (CCND1), BCL2 and BCL-XL (BCL2L1), and substantially dampened interleukin 10-induced STAT3 activation. In 19 patients aged ≤ 69 years at time of diagnosis, we found that those that carried SOCS3-negative tumours showed a trend toward a worse outcome (P = 0.1, log-rank).

Constitutive STAT3 phosphorylation contributes to skeletal muscle insulin resistance in type 2 diabetes

Signal transducer and activator of transcription 3 (STAT3) is involved in cytokine- and nutrient-induced insulin resistance. The role of STAT3 in the development of skeletal muscle insulin resistance and type 2 diabetes (T2D) pathogenesis is incompletely defined. We tested the hypothesis that STAT3 signaling contributes to skeletal muscle insulin resistance in T2D. Protein abundance and phosphorylation of STAT3 signaling molecules were determined in skeletal muscle biopsy specimens from BMI- and age-matched overweight individuals with normal glucose tolerant (NGT) and T2D patients. The direct role of STAT3 in the development of lipid-induced skeletal muscle insulin resistance was determined using small interfering (si)RNA. Phosphorylated STAT3, phosphorylated Janus kinase 2 (JAK2), and suppressor of cytokine signaling 3 (SOCS3) protein abundance was increased in skeletal muscle from T2D patients. STAT3 phosphorylation positively correlated with free fatty acid level and measures of insulin sensitivity in NGT but not T2D patients. Palmitate exposure led to a constitutive phosphorylation of STAT3, increased protein abundance of SOCS3, and development of insulin resistance in L6 myotubes. These effects were prevented by siRNA-mediated STAT3 silencing. In summary, STAT3 is constitutively phosphorylated in skeletal muscle from T2D patients. STAT3 gene silencing prevents lipid-induced insulin resistance in cultured myotubes. Collectively, our results implicate excessive STAT3 signaling in the development of skeletal muscle insulin resistance in T2D.

Downregulating SOCS3 with siRNA ameliorates insulin signaling and glucose metabolism in hepatocytes of IUGR rats with catch-up growth

BACKGROUND

Individuals with intrauterine growth retardation (IUGR) who demonstrate a catch-up in body weight are prone to insulin resistance. High expressions of suppressor of cytokine signaling 3 (SOCS3) are thought to aggravate insulin resistance. We hypothesized that downregulating SOCS3 expression via small interfering RNA (siRNA) might have beneficial effects on insulin-resistant hepatocytes of catch-up growth IUGR rats (CG-IUGRs).

METHODS

An IUGR rat model was employed via maternal nutritional restriction. After evaluating metabolic states of CG-IUGR offspring, effective SOCS3-specific siRNA (siSOCS3) was transfected into cultured hepatocytes using liposomes. mRNA levels of SOCS3, insulin receptor substrates (IRSs), phosphatidylinositol 3-kinase (PI3K), and Akt2, key gluconeogenesis genes, were assessed via real-time PCR. Protein expression and phosphorylation changes were evaluated via western blot.

RESULTS

CG-IUGR hepatocytes showed increases in SOCS3 and gluconeogenic gene expressions, and decreases in IRS1 and PI3K expressions as compared with controls. After transfecting CG-IUGR hepatocytes with siSOCS3, protein levels of IRS1, PI3K, and phosphorylated Akt2 were higher as compared with those of untransfected CG-IUGR cells. Transcriptional suppression effects of gluconeogenesis genes were more obvious in siSOCS3-treated cells after insulin stimulation.

CONCLUSION

Additional insights were provided to understand mechanisms of insulin resistance in CG-IUGR rats. Downregulating SOCS3 might improve insulin signaling transduction and ameliorate hepatic glucose metabolism in insulin-resistant CG-IUGR rats in vitro.

An evaluation of sirolimus in renal transplantation

INTRODUCTION

Sirolimus is a powerful antiproliferative immunosuppressive drug approved for the prevention of kidney allograft rejection. By its unique mechanism of action, sirolimus provides a multitude of clinical potential and has been used effectively in different drug combinations. Extensive experience has been gained regarding the best timing of its application, side effect profile and potential benefits and limitations compared with other immunosuppressive drugs.

AREAS COVERED

The authors evaluate the recent experience with sirolimus in kidney transplantation. Pivotal randomized controlled trials were used to provide an overview with special attention to pharmacokinetic and dynamic aspects of sirolimus, its current clinical use as well as perspectives for its future role.

EXPERT OPINION

Sirolimus enriches the possibilities of immunosuppressive therapies after renal transplantation. Beneficial effects toward kidney function by allowing CNI sparing, lower incidence of malignancies and less viral infections have been suggested. Sirolimus should be used cautiously in de novo patients for reasons of wound healing. An early conversion to a sirolimus-based CNI-free regimen has shown promising results, whereas late conversion is more challenging. Finally, sirolimus-associated side effects are causing tolerability concerns and frequent discontinuations. Future research should aim to better define the therapeutic window and those patients most likely to benefit.

Rapamycin inhibits both motility through down-regulation of p-STAT3 (S727) by disrupting the mTORC2 assembly and peritoneal dissemination in sarcomatoid cholangiocarcinoma

Cholangiocarcinoma (CC) is a malignant epithelium neoplasm that originates from the bile epithelium and for which there are few therapeutic strategies. The mTOR pathway involved in many cellular processes was reported to be up-regulated in various cancers. We investigated the activation of the AKT/mTOR pathway in CC cell lines with different degrees of dedifferentiation and found that rapamycin could suppress the motility and the peritoneal dissemination of sarcomatoid SCK cells. Inhibition of the mTOR pathway with rapamycin decreased significantly the number of tumor nodules and prolonged the survival rates of nude mice inoculated with sarcomatoid CC cells. Prolonged treatments with rapamycin were found to disrupt the mTORC2 assembly and to reduce the phosphorylation of STAT3 at Ser 727. Rapamycin decreased both mRNA and protein levels of MMP2 and Twist1, which are regulated by STAT3 and associated with cancer metastasis. The overexpression of STAT3 S727A lacking the phosphorylation site resulted in significantly less sensitivity to rapamycin than the overexpression of STAT3 WT. Taken together, our results suggest that rapamycin could suppress the motility of sarcomatoid CC by down-regulating p-STAT3 (S727) through the impairment of mTORC2 assembly.

SOCS3 is an endogenous inhibitor of pathologic angiogenesis

Inflammatory cytokines and growth factors drive angiogenesis independently; however, their integrated role in pathologic and physiologic angiogenesis is not fully understood. Suppressor of cytokine signaling-3 (SOCS3) is an inducible negative feedback regulator of inflammation and growth factor signaling. In the present study, we show that SOCS3 curbs pathologic angiogenesis. Using a Cre/Lox system, we deleted SOCS3 in vessels and studied developmental and pathologic angiogenesis in murine models of oxygen-induced retinopathy and cancer. Conditional loss of SOCS3 leads to increased pathologic neovascularization, resulting in pronounced retinopathy and increased tumor size. In contrast, physiologic vascularization is not regulated by SOCS3. In vitro, SOCS3 knockdown increases proliferation and sprouting of endothelial cells costimulated with IGF-1 and TNFα via reduced feedback inhibition of the STAT3 and mTOR pathways. These results identify SOCS3 as a pivotal endogenous feedback inhibitor of pathologic angiogenesis and a potential therapeutic target acting at the converging crossroads of growth factor- and cytokine-induced vessel growth.

The effect of exercise on IL-6-induced cachexia in the Apc ( Min/+) mouse

BACKGROUND

Cachexia involves unintentional body weight loss including diminished muscle and adipose tissue mass and is associated with an underlying disease. Systemic overexpression of IL-6 accelerates cachexia in the Apc(Min/+) mouse, but does not induce wasting in control C57BL/6 mice. With many chronic diseases, chronic inflammation and metabolic dysfunction can be improved with moderate exercise. A direct effect of regular moderate exercise on the prevention of IL-6-induced cachexia in the Apc(Min/+) mouse has not been investigated. The purpose of this study was to assess the effects of exercise on the development of cachexia in the Apc(Min/+) mouse.

METHODS

Mice were randomly assigned to moderate treadmill exercise (18 m/min, 1 h, 6 days/week, 5% grade) or cage control (CC) groups from 6 to 14 weeks of age. At 12 weeks of age, mice were electroporated with either IL-6-containing or control plasmid into the quadriceps muscle. Mice were killed after 2 weeks of systemic IL-6 overexpression or control treatment.

RESULTS

IL-6 overexpression induced an 8% loss in body weight in CC mice, which was significantly attenuated by exercise. IL-6 overexpression in CC mice increased fasting insulin and triglyceride levels, which were normalized by exercise, and associated with increased oxidative capacity, an induction of AKT signaling, and a repression of AMPK signaling in muscle. These exercise-induced changes occurred despite elevated inflammatory signaling in skeletal muscle.

CONCLUSION

We conclude that moderate-intensity exercise can attenuate IL-6-dependent cachexia in Apc(Min/+) mice, independent of changes in IL-6 concentration and muscle inflammatory signaling. The exercise effect was associated with improved insulin sensitivity and improved energy status in the muscle.

Hormone replacement therapy and physical function in healthy older men. Time to talk hormones?

Improving physical function and mobility in a continuously expanding elderly population emerges as a high priority of medicine today. Muscle mass, strength/power, and maximal exercise capacity are major determinants of physical function, and all decline with aging. This contributes to the incidence of frailty and disability observed in older men. Furthermore, it facilitates the accumulation of body fat and development of insulin resistance. Muscle adaptation to exercise is strongly influenced by anabolic endocrine hormones and local load-sensitive autocrine/paracrine growth factors. GH, IGF-I, and testosterone (T) are directly involved in muscle adaptation to exercise because they promote muscle protein synthesis, whereas T and locally expressed IGF-I have been reported to activate muscle stem cells. Although exercise programs improve physical function, in the long-term most older men fail to comply. The GH/IGF-I axis and T levels decline markedly with aging, whereas accumulating evidence supports their indispensable role in maintaining physical function integrity. Several studies have reported that the administration of T improves lean body mass and maximal voluntary strength in healthy older men. On the other hand, most studies have shown that administration of GH alone failed to improve muscle strength despite amelioration of the detrimental somatic changes of aging. Both GH and T are anabolic agents that promote muscle protein synthesis and hypertrophy but work through separate mechanisms, and the combined administration of GH and T, albeit in only a few studies, has resulted in greater efficacy than either hormone alone. Although it is clear that this combined approach is effective, this review concludes that further studies are needed to assess the long-term efficacy and safety of combined hormone replacement therapy in older men before the medical rationale of prescribing hormone replacement therapy for combating the sarcopenia of aging can be established.

The peroxisome proliferator-activated receptor (PPAR) β/δ agonist GW501516 inhibits IL-6-induced signal transducer and activator of transcription 3 (STAT3) activation and insulin resistance in human liver cells

AIM/HYPOTHESIS

IL-6 induces insulin resistance by activating signal transducer and activator of transcription 3 (STAT3) and upregulating the transcription of its target gene SOCS3. Here we examined whether the peroxisome proliferator-activated receptor (PPAR)β/δ agonist GW501516 prevented activation of the IL-6-STAT3-suppressor of cytokine signalling 3 (SOCS3) pathway and insulin resistance in human hepatic HepG2 cells.

METHODS

Studies were conducted with human HepG2 cells and livers from mice null for Pparβ/δ (also known as Ppard) and wild-type mice.

RESULTS

GW501516 prevented IL-6-dependent reduction in insulin-stimulated v-akt murine thymoma viral oncogene homologue 1 (AKT) phosphorylation and in IRS-1 and IRS-2 protein levels. In addition, treatment with this drug abolished IL-6-induced STAT3 phosphorylation of Tyr⁷⁰⁵ and Ser⁷²⁷ and prevented the increase in SOCS3 caused by this cytokine. Moreover, GW501516 prevented IL-6-dependent induction of extracellular-related kinase 1/2 (ERK1/2), a serine-threonine protein kinase involved in serine STAT3 phosphorylation; the livers of Pparβ/δ-null mice showed increased Tyr⁷⁰⁵- and Ser⁷²⁷-STAT3 as well as phospho-ERK1/2 levels. Furthermore, drug treatment prevented the IL-6-dependent reduction in phosphorylated AMP-activated protein kinase (AMPK), a kinase reported to inhibit STAT3 phosphorylation on Tyr⁷⁰⁵. In agreement with the recovery in phospho-AMPK levels observed following GW501516 treatment, this drug increased the AMP/ATP ratio and decreased the ATP/ADP ratio.

CONCLUSIONS/INTERPRETATION

Overall, our findings show that the PPARβ/δ activator GW501516 prevents IL-6-induced STAT3 activation by inhibiting ERK1/2 phosphorylation and preventing the reduction in phospho-AMPK levels. These effects of GW501516 may contribute to the prevention of cytokine-induced insulin resistance in hepatic cells.

Cytoplasmic polyadenylation element binding protein deficiency stimulates PTEN and Stat3 mRNA translation and induces hepatic insulin resistance

The cytoplasmic polyadenylation element binding protein CPEB1 (CPEB) regulates germ cell development, synaptic plasticity, and cellular senescence. A microarray analysis of mRNAs regulated by CPEB unexpectedly showed that several encoded proteins are involved in insulin signaling. An investigation of Cpeb1 knockout mice revealed that the expression of two particular negative regulators of insulin action, PTEN and Stat3, were aberrantly increased. Insulin signaling to Akt was attenuated in livers of CPEB-deficient mice, suggesting that they might be defective in regulating glucose homeostasis. Indeed, when the Cpeb1 knockout mice were fed a high-fat diet, their livers became insulin-resistant. Analysis of HepG2 cells, a human liver cell line, depleted of CPEB demonstrated that this protein directly regulates the translation of PTEN and Stat3 mRNAs. Our results show that CPEB regulated translation is a key process involved in insulin signaling.

NOD1 activation induces proinflammatory gene expression and insulin resistance in 3T3-L1 adipocytes

Chronic inflammation is associated with obesity and insulin resistance; however, the underlying mechanisms are not fully understood. Pattern recognition receptors Toll-like receptors and nucleotide-oligomerization domain-containing proteins play critical roles in innate immune response. Here, we report that activation of nucleotide binding oligomerization domain-containing protein-1 (NOD1) in adipocytes induces proinflammatory response and impairs insulin signaling and insulin-induced glucose uptake. NOD1 and NOD2 mRNA are markedly increased in differentiated murine 3T3-L1 adipocytes and human primary adipocyte culture upon adipocyte conversion. Moreover, NOD1 mRNA is markedly increased only in the fat tissues in diet-induced obese mice, but not in genetically obese ob/ob mice. Stimulation of NOD1 with a synthetic ligand Tri-DAP induces proinflammatory chemokine MCP-1, RANTES, and cytokine TNF-α and MIP-2 (human IL-8 homolog) and IL-6 mRNA expression in 3T3-L1 adipocytes in a time- and dose-dependent manner. Similar proinflammatory profiles are observed in human primary adipocyte culture stimulated with Tri-DAP. Furthermore, NOD1 activation suppresses insulin signaling, as revealed by attenuated tyrosine phosphorylation and increased inhibitory serine phosphorylation, of IRS-1 and attenuated phosphorylation of Akt and downstream target GSK3α/3β, resulting in decreased insulin-induced glucose uptake in 3T3-L1 adipocytes. Together, our results suggest that NOD1 may play an important role in adipose inflammation and insulin resistance in diet-induced obesity.

Proline-rich Akt substrate of 40kDa (PRAS40): a novel downstream target of PI3k/Akt signaling pathway

Modifications in signaling of the proline-rich Akt substrate of 40-kDa (PRAS40) pathway is implicated in type 2 diabetes and melanoma. PRAS40 is known for its ability to regulate the mammalian target of rapamycin complex 1 (mTORC1) kinase activity, possessing a key regulatory role at the cross point of signal transduction pathways activated by growth factor receptors. Recently it has been found that PRAS40 is regulated by its upstream phosphatidylinositol 3-kinase/Akt (PI3K/Akt) which is activated by many tyrosine kinase receptors growth factors including insulin-like growth factor 1. Also, PRAS40 functions downstream of mTORC1 and upstream from its effectors ribosomal protein S6 kinase 1 (S6K1) and eukaryotic initiation factor 4E binding protein 1 (4E-BP1). Phosphorylation of PRAS40 by Akt and mTORC1 disrupts the binding between mTORC1 and PRAS40, and relieves the inhibitory constraint of PRAS40 on mTORC1 activity. This review summarizes the signaling regulating PRAS40 phosphorylation, as well as the dual function of PRAS40 as substrate and inhibitor of mTORC1 upon growth factor stimulation and under pathophysiological conditions.

Interpreting mixed signals: the cell's cytokine conundrum

Cytokines are essential for the activation, differentiation and control of the immune system. Many cytokines, alone or in combination with other cytokines, have multiple functions and can often act on many different cell types with distinct developmental or functional consequences. Despite the myriad of cytokines and cytokine receptors, there are relatively few signaling molecules that transduce these diverse cytokine signals. In this review, we will discuss the potential mechanisms used by cytokines to mediate distinct cellular outcomes from a small number of signaling molecules. Understanding this paradigm in cytokine signaling can aid in the development of potential therapeutic approaches involving cytokine targeting or use.

Activation of peroxisome proliferator-activated receptor-β/-δ (PPAR-β/-δ) ameliorates insulin signaling and reduces SOCS3 levels by inhibiting STAT3 in interleukin-6-stimulated adipocytes

OBJECTIVE

It has been suggested that interleukin (IL)-6 is one of the mediators linking obesity-derived chronic inflammation with insulin resistance through activation of STAT3, with subsequent upregulation of suppressor of cytokine signaling 3 (SOCS3). We evaluated whether peroxisome proliferator-activated receptor (PPAR)-β/-δ prevented activation of the IL-6-STAT3-SOCS3 pathway and insulin resistance in adipocytes.

RESEARCH DESIGN AND METHODS

Adipocytes and white adipose tissue from wild-type and PPAR-β/-δ-null mice were used to evaluate the effect of PPAR-β/-δ on the IL-6-STAT3-SOCS3 pathway.

RESULTS

First, we observed that the PPAR-β/-δ agonist GW501516 prevented both IL-6-dependent reduction in insulin-stimulated Akt phosphorylation and glucose uptake in adipocytes. In addition, this drug treatment abolished IL-6-induced SOCS3 expression in differentiated 3T3-L1 adipocytes. This effect was associated with the capacity of the drug to prevent IL-6-induced STAT3 phosphorylation on Tyr(705) and Ser(727) residues in vitro and in vivo. Moreover, GW501516 prevented IL-6-dependent induction of extracellular signal-related kinase (ERK)1/2, a serine-threonine-protein kinase involved in serine STAT3 phosphorylation. Furthermore, in white adipose tissue from PPAR-β/-δ-null mice, STAT3 phosphorylation (Tyr(705) and Ser(727)), STAT3 DNA-binding activity, and SOCS3 protein levels were higher than in wild-type mice. Several steps in STAT3 activation require its association with heat shock protein 90 (Hsp90), which was prevented by GW501516 as revealed in immunoprecipitation studies. Consistent with this finding, the STAT3-Hsp90 association was enhanced in white adipose tissue from PPAR-β/-δ-null mice compared with wild-type mice.

CONCLUSIONS

Collectively, our findings indicate that PPAR-β/-δ activation prevents IL-6-induced STAT3 activation by inhibiting ERK1/2 and preventing the STAT3-Hsp90 association, an effect that may contribute to the prevention of cytokine-induced insulin resistance in adipocytes.

Anti-inflammatory effect of insulin in the human hepatoma cell line HepG2 involves decreased transcription of IL-6 target genes and nuclear exclusion of FOXO1

The liver is an important target for interleukin-6 (IL-6) action leading to an increased inflammatory response with impaired insulin signaling and action. The aims of this study are to address if insulin is anti-inflammatory and attenuates IL-6-induced inflammation in the human hepatoma cell line HepG2 and if this involves signal transducer and activator of transcription 3 (STAT3) signal transduction. It was found that insulin significantly reduced IL-6-induced gene transcription of serum amyloid 1 (SAA1), serum amyloid 2 (SAA2), haptoglobin, orosomucoid, and plasmin activator inhibitor-1 (PAI-1). However, the authors did not find any evidence that insulin inhibited IL-6 signal transduction, i.e., no effect of insulin was detected on STAT3 phosphorylation or its translocation to cell nucleus. The potential role of PKCδ was also analyzed but no evidence of its involvement was found. Taken together, these results suggest that the anti-inflammatory effect of insulin on IL-6 action is exerted at the level of the transcriptional activation of the genes. Further analysis revealed that insulin regulates nuclear localization of FOXO1, which is an important co-activator for STAT3 mediated transcription. Insulin induced nuclear exit and Thr24 phosphorylation of FOXO1, thus, inhibiting STAT3-mediated transcription.

Probiotics as an emerging therapeutic strategy to treat NAFLD: focus on molecular and biochemical mechanisms

Nonalcoholic fatty liver disease (NAFLD) is currently the most common liver disease worldwide, both in adults and in children. NAFLD is characterized by aberrant lipid storage in hepatocytes (hepatic steatosis) and inflammatory progression to nonalcoholic steatohepatitis. Evidences so far suggest that intrahepatic lipid accumulation does not always derive from obesity. Gut microbiota has been considered as a regulator of energy homeostasis and ectopic fat deposition, suggesting its implications in metabolic diseases. Probiotics are live microbial that alter the enteric microflora and have beneficial effects on human health. Although the molecular mechanisms of probiotics have not been completely elucidated yet, many of their effects have proved to be beneficial in NAFLD, including the modulation of the intestinal microbiota, an antibacterial substance production, an improved epithelial barrier function and a reduced intestinal inflammation. Given the close anatomical and functional correlation between the bowel and the liver, and the immunoregulatory effects elicited by probiotics, the aim of this review is to summarize today's knowledge about probiotics in NAFLD, focusing in particular on their molecular and biochemical mechanisms, as well as highlighting their efficacy as an emerging therapeutic strategy to treat this condition.

NOD2 activation induces muscle cell-autonomous innate immune responses and insulin resistance

Insulin resistance is associated with chronic low-grade inflammation in vivo, largely mediated by activated innate immune cells. Cytokines and pathogen-derived ligands of surface toll-like receptors can directly cause insulin resistance in muscle cells. However, it is not known if intracellular pathogen sensors can, on their own, provoke insulin resistance. Here, we show that the cytosolic pattern recognition receptors nucleotide-binding oligomerization domain-containing protein (NOD)1 and NOD2 are expressed in immune and metabolic tissues and hypothesize that their activation in muscle cells would result in cell-autonomous responses leading to insulin resistance. Bacterial peptidoglycan motifs that selectively activate NOD2 were directly administered to L6- GLUT4myc myotubes in culture. Within 3 h, insulin resistance arose, characterized by reductions in each insulin-stimulated glucose uptake, GLUT4 translocation, Akt Ser(473) phosphorylation, and insulin receptor substrate 1 tyrosine phosphorylation. Muscle cell-autonomous responses to NOD2 ligand included activation of the stress/inflammation markers c-Jun N-terminal kinase, ERK1/2, p38 MAPK, degradation of inhibitor of κBα, and production of proinflammatory cytokines. These results show that NOD2 alone is capable of acutely inducing insulin resistance within muscle cells, possibly by activating endogenous inflammatory signals and/or through cytokine production, curbing upstream insulin signals. NOD2 is hence a new inflammation target connected to insulin resistance, and this link occurs without the need of additional contributing cell types. This study provides supporting evidence for the integration of innate immune and metabolic responses through the involvement of NOD proteins and suggests the possible participation of cell autonomous immune responses in the development of insulin resistance in skeletal muscle, the major depot for postprandial glucose utilization.

Suppressor of cytokine signaling 3 suppresses hepatitis C virus replication in an mTOR-dependent manner

We and others have observed that hepatic levels of suppressor of cytokine signaling 3 (SOCS3) are significantly higher in persons with chronic hepatitis C, particularly those who are nonresponders to interferon (IFN) treatment, than in healthy individuals. However, the relationship between SOCS3 and hepatitis C virus (HCV) replication remains unclear. Given its putative role, we hypothesized that SOCS3 is permissive for viral replication. We therefore used the OR6 cell line, which harbors a genotype 1b full-length HCV replicon, and the genotype 2a full-length HCV strain JFH1 infection system to analyze the effects of SOCS3 overexpression and short hairpin RNA (shRNA)-mediated knockdown on HCV replication. We further analyzed the role of mTOR in the effects of SOCS3 by treating selected cells with rapamycin. OR6 cells and JFH1-infected Huh7.5.1 cells expressed significantly less SOCS3 than control cells. Furthermore, inhibition of HCV replication with the HCV protease inhibitor BILN 2061 restored SOCS3 protein levels. SOCS3 overexpression in OR6 cells and JFH1-infected Huh7.5.1 cells resulted in significantly lower HCV replication than that in the control cells, despite SOCS3-related inhibition of STAT1 phosphorylation and type I IFN signaling. In contrast, JFH1-infected cells with stable SOCS3 knockdown expressed higher levels of HCV proteins and RNA than did control cells. SOCS3-targeting shRNA also knocked down mTOR and phospho-mTOR. The mTOR inhibitor rapamycin reversed the inhibitory effects of SOCS3. In independent investigations, SOCS3 unexpectedly suppressed HCV replication in an mTOR-dependent manner. These findings suggest that increased SOCS3 levels consistently observed in chronic IFN nonresponders may reflect a compensatory host antiviral response to persistent infection and that manipulation of SOCS3/mTOR may offer benefit against HCV infection.

Loss of SOCS3 expression is associated with an increased risk of recurrent disease in breast carcinoma

PURPOSE

Constitutive activation of JAK/STAT pathway is observed in various solid tumors and hematological malignancies. SOCS3 acts as a key negative regulator of JAK/STAT pathway and represents one of the candidate tumor suppressor genes. In the current study, we aimed to evaluate SOCS3 expression in breast carcinoma and to explore the prognostic significance of SOCS3.

METHODS

The expression of SOCS3 was measured by Western blot and immunohistochemistry in breast carcinoma cells and a large cohort of tissue microarray, respectively.

RESULTS

Among 367 human primary breast tumors, SOCS3 protein was detected in 103 patients. Deficient SOCS3 expression correlated significantly with lymph node metastasis (P = 0.003), blood vessel invasion (P = 0.029), VEGF (P = 0.001) and Ki-67 (P = 0.027). Univariate and multivariate analyses revealed that SOCS3 expression was an independent prognostic factor for disease-free survival (P < 0.0001). A positive SOCS3 protein expression correlated significantly with a low pSTAT3 protein expression in breast carcinoma (P = 0.015). The patients with a SOCS3 (+)/pSTAT3 (-) phenotype had a better prognosis than any other combination (DFI: P < 0.0001, BCSS: P = 0.013).

CONCLUSIONS

Deficient expression of SOCS3 is associated with an aggressive phenotype and portends a poor clinical outcome in breast carcinoma.

Signal transducer and activator of transcription 3 (STAT3) mediates amino acid inhibition of insulin signaling through serine 727 phosphorylation

Nutrient overload is associated with the development of obesity, insulin resistance, and type II diabetes. High plasma concentrations of amino acids have been found to correlate with insulin resistance. At the cellular level, excess amino acids impair insulin signaling, the mechanisms of which are not fully understood. Here, we report that STAT3 plays a key role in amino acid dampening of insulin signaling in hepatic cells. Excess amino acids inhibited insulin-stimulated Akt phosphorylation and glycogen synthesis in mouse primary hepatocytes as well as in human hepatocarcinoma HepG2 cells. STAT3 knockdown protected insulin sensitivity from inhibition by amino acids. Amino acids stimulated the phosphorylation of STAT3 at Ser(727), but not Tyr(705). Replacement of the endogenous STAT3 with wild-type, but not S727A, recombinant STAT3 restored the ability of amino acids to inhibit insulin signaling, suggesting that Ser(727) phosphorylation was critical for STAT3-mediated amino acid effect. Furthermore, overexpression of STAT3-S727D was sufficient to inhibit insulin signaling in the absence of excess amino acids. Our results also indicated that mammalian target of rapamycin was likely responsible for the phosphorylation of STAT3 at Ser(727) in response to excess amino acids. Finally, we found that STAT3 activity and the expression of its target gene socs3, known to be involved in insulin resistance, were both stimulated by excess amino acids and inhibited by rapamycin. In conclusion, our study reveals STAT3 as a novel mediator of nutrient signals and identifies a Ser(727) phosphorylation-dependent and Tyr(705) phosphorylation-independent STAT3 activation mechanism in the modulation of insulin signaling.

A novel mouse model of inflammatory bowel disease links mammalian target of rapamycin-dependent hyperproliferation of colonic epithelium to inflammation-associated tumorigenesis

Inflammatory bowel disease (IBD) is a high-risk condition for human colorectal cancer. However, our mechanistic understanding of the link between inflammation and tumorigenesis in the colon is limited. Here we established a novel mouse model of colitis-associated cancer by genetically inactivating signal transducer and activator of transcription 3 (Stat3) in macrophages, with partial deletion in other myeloid and lymphoid cells. Inflammation developed in the colon of mutant mice spontaneously, and tumor lesions, including invasive carcinoma, arose in the inflamed region of the intestine with a frequency similar to that observed in human IBD patients. The development of both inflammation and tumors in the mutant mice required the presence of microflora. Indeed, inflammation was associated with disruption of colonic homeostasis, fulminant epithelial/tumor cell proliferation, and activation of the mammalian target of rapamycin (mTOR)-Stat3 pathway in epithelial and tumor cells. The activation of this pathway was essential for both the excess proliferation of epithelial/tumor cells and the disruption of colonic homeostasis in the mutant mice. Notably, a similar abnormal up-regulation of mTOR-Stat3 signaling was consistently observed in the colonic epithelial cells of human IBD patients with active disease. These studies demonstrate a novel mouse model of IBD-colorectal cancer progression in which disrupted immune regulation, mTOR-Stat3 signaling, and epithelial hyperproliferation are integrated and simultaneously linked to the development of malignancy.

A novel mouse model of inflammatory bowel disease links mammalian target of rapamycin-dependent hyperproliferation of colonic epithelium to inflammation-associated tumorigenesis

Inflammatory bowel disease (IBD) is a high-risk condition for human colorectal cancer. However, our mechanistic understanding of the link between inflammation and tumorigenesis in the colon is limited. Here we established a novel mouse model of colitis-associated cancer by genetically inactivating signal transducer and activator of transcription 3 (Stat3) in macrophages, with partial deletion in other myeloid and lymphoid cells. Inflammation developed in the colon of mutant mice spontaneously, and tumor lesions, including invasive carcinoma, arose in the inflamed region of the intestine with a frequency similar to that observed in human IBD patients. The development of both inflammation and tumors in the mutant mice required the presence of microflora. Indeed, inflammation was associated with disruption of colonic homeostasis, fulminant epithelial/tumor cell proliferation, and activation of the mammalian target of rapamycin (mTOR)-Stat3 pathway in epithelial and tumor cells. The activation of this pathway was essential for both the excess proliferation of epithelial/tumor cells and the disruption of colonic homeostasis in the mutant mice. Notably, a similar abnormal up-regulation of mTOR-Stat3 signaling was consistently observed in the colonic epithelial cells of human IBD patients with active disease. These studies demonstrate a novel mouse model of IBD-colorectal cancer progression in which disrupted immune regulation, mTOR-Stat3 signaling, and epithelial hyperproliferation are integrated and simultaneously linked to the development of malignancy.