The role of interleukin-6 in insulin resistance, body fat distribution and energy balance

Interleukin-6 (IL-6) is a central player in the regulation of inflammation, haematopoiesis, immune response and host defense mechanisms. During the last decade, an accumulating amount of data suggested a pivotal role for IL-6 in metabolic processes, thus fortifying the picture of IL-6 as a multifaceted, pleiotropic cytokine. Because of its secretion by adipose tissue and contracting skeletal muscle and its broad action on central and peripheral organs, IL-6 has been termed an adipokine and a myokine. Its quantitative release from adipose tissue results in a subclinical, systemic elevation of IL-6 plasma levels with increasing body fat content, which may be implicated in the proinflammatory state leading to insulin resistance. On the other hand, IL-6 produced in the working muscle during physical activity could act as an energy sensor by activating AMP-activated kinase and enhancing glucose disposal, lipolysis and fat oxidation. In addition, both impaired IL-6 secretion and action are risk factors for weight gain. Thus, IL-6 clearly has lipolytic effects and anti-obesity potential. However, the application of IL-6 itself is at least limited by a narrow therapeutic range and its important function for a balanced inflammatory response. Further studies on the molecular basis of the metabolic effects of IL-6 could elucidate novel therapeutic strategies for custom-designed, IL-6-related substances.

Polymorphisms in the gene encoding adiponectin receptor 1 are associated with insulin resistance and high liver fat

AIMS/HYPOTHESIS

The adipokine adiponectin has insulin-sensitising, anti-atherogenic and anti-inflammatory properties. Recently, the genes for mouse and human adiponectin receptor-1 (ADIPOR1) and -2 (ADIPOR2) have been cloned. The aim of this study was to investigate whether genetic variants of the genes encoding ADIPOR1 and ADIPOR2 play a role in human metabolism.

MATERIALS AND METHODS

We screened ADIPOR1 and ADIPOR2 for polymorphisms and determined their association with glucose metabolism, lipid metabolism, an atherogenic lipid profile and inflammatory markers in 502 non-diabetic subjects. A subgroup participated in a longitudinal study; these subjects received diet counselling and increased their physical activity.

RESULTS

We identified six variants of ADIPOR1 and seven variants of ADIPOR2. A single-nucleotide polymorphism (SNP) in the putative promoter region 8503 bp upstream of the translational start codon (-8503 G/A) of ADIPOR1 (frequency of allele A=0.31) was in almost complete linkage disequilibrium with another SNP (-1927 T/C) in intron 1. Subjects carrying the -8503 A and -1927 C alleles had lower insulin sensitivity, as estimated from a 75 g OGTT (p=0.04) and determined during a euglycaemic clamp (n=295, p=0.04); they also had higher HbA(1)c levels (p=0.02) and, although the difference was not statistically significant, higher liver fat (n=85, determined by proton magnetic resonance spectroscopy, p=0.056) (all p values are adjusted for age, sex and percentage of body fat). In the longitudinal study (n=45), the -8503 A and -1927 C alleles were associated with lower insulin sensitivity (p=0.03) and higher liver fat (p=0.02) at follow-up compared with the -8503 G and -1927 T alleles, independently of basal measurements, sex and baseline and follow-up percentage of body fat.

CONCLUSIONS/INTERPRETATION

The present findings suggest that the -8503 G/A SNP in the promoter or the -1927 T/C SNP in intron 1 of ADIPOR1 may affect insulin sensitivity and liver fat in humans.

Glutamine:fructose-6-phosphate aminotransferase enzyme activity is necessary for the induction of TGF-beta1 and fibronectin expression in mesangial cells

AIMS/HYPOTHESIS

Increased flux through the hexosamine biosynthetic pathway with glutamine:fructose-6-phosphate aminotransferase (GFAT) as a rate-limiting enzyme has been linked to the enhanced bioactivity of the prosclerotic cytokine TGF-beta1, a key mediator in the development of diabetic nephropathy and possibly other diabetic angiopathies. In this study we investigated the effect of enhanced expression of wild-type GFAT and two enzymatically inactive GFAT mutants on TGF-beta1 synthesis in mesangial cells.

METHODS

Mutated human GFAT expression vectors were prepared by PCR-site directed mutagenesis. Wild-type and mutated vectors were transfected into human embryonic kidney 293 cells and mesangial cells and GFAT enzyme activity was assessed by formation of glucosamine-6-phosphate. Production of TGF-beta1 and fibronectin protein was examined by ELISA.

RESULTS

Mutation of histidine 577 or lysine 676 to alanine led to a complete loss of GFAT enzyme activity. An increased concentration of wild-type GFAT in mesangial cells enhanced both TGF-beta1 and fibronectin production 1.5-fold, while mesangial cells transfected with the mutated GFAT constructs showed no effect.

CONCLUSION/INTERPRETATION

The data indicate that the hexosamine pathway-mediated induction of TGF-beta1 synthesis in mesangial cells is dependent on GFAT enzyme activity. Our results add to previous observations showing that the hexosamine pathway could increase the transcriptional activity of nuclear proteins leading to enhanced cytokine synthesis.

Inhibition of interferon-gamma expression by osmotic shrinkage of peripheral blood lymphocytes

A hypertonic environment, as it prevails in renal medulla or in hyperosmolar states such as hyperglycemia of diabetes mellitus, has been shown to impair the immune response, thus facilitating the development of infection. The present experiments were performed to test whether hypertonicity influences activation of T lymphocytes. To this end, peripheral blood lymphocytes (PBL) of cytomegalovirus (CMV)-positive donors were stimulated by human leukocyte antigen (HLA)-A2-restricted CMV epitope NLVPMVATV to produce interferon (IFN)-gamma at varying extracellular osmolarity. As a result, increasing extracellular osmolarity during exposure to the CMV antigen indeed decreased IFN-gamma formation. Addition of NaCl was more effective than urea. A 50% inhibition was observed at 350 mosM by addition of NaCl. The combined application of the Ca(2+) ionophore ionomycin (1 microg/ml) and the phorbol ester phorbol 12-myristate 13-acetate (PMA; 5 microg/ml) stimulated IFN-gamma production, an effect again reversed by hyperosmolarity. Moreover, hyperosmolarity abrogated the stimulating effect of ionomycin (1 microg/ml) and PMA (5 microg/ml) on the transcription factors activator protein (AP)-1, nuclear factor of activated T cells (NFAT), and NF-kappaB but not Sp1. In conclusion, osmotic cell shrinkage blunts the stimulatory action of antigen exposure on IFN-gamma production, an effect explained at least partially by suppression of transcription factor activation.

Angiotensin II induces human TGF-beta 1 promoter activation: similarity to hyperglycaemia

AIMS/HYPOTHESIS

Activation of the renal renin-angiotensin system has been implicated in the pathogenesis of diabetic nephropathy. Because previous in vitro studies demonstrated the angiotensin II (ang II)-mediated up-regulation of the prosclerotic transforming growth factor beta 1 (TGF) we studied the molecular mechanism of ang II-induced TGF-beta 1 gene activation.

METHODS

Mesangial cells were stimulated with 100 nmol/l ang II with or without inhibitors of protein kinase C (PKC) and p38 MAPK and the TGF-beta 1 promoter activity was determined by promoter-reporter assays. The effect of ang II on the binding of nuclear proteins to the regulatory AP-1 site B, previously shown to mediate the high glucose-response of the TGF-beta 1 promoter, was studied by electrophoretic mobility shift assays.

RESULTS

Ang II enhanced the activity of the TGF-beta1 promoter fragment -453/+11 approximately 1.6-fold. Mutation of each of two AP-1 binding sites or inhibition of the PKC- and p38 MAPK-dependent pathways blocked the ang II-stimulated activity completely. Furthermore, ang II activated the binding of nuclear proteins to the AP-1 box B of the TGF-beta 1 promoter. These effects were similar to those previously observed with high glucose. Co-incubation with ang II and high glucose had no additive effect on TGF-beta 1 promoter activity, protein binding to the AP-1 box B or activation of p38 MAPK.

CONCLUSION/INTERPRETATION

The findings indicate that ang II and hyperglycaemia stimulate the TGF-beta 1 gene activation through the same PKC- and p38 MAPK-dependent pathways by the same regulatory elements of the TGF-beta 1 promoter. Our data could also be relevant for e.g. hypertension-induced glomerulosclerosis.

Low-molecular-weight heparin prevents high glucose- and phorbol ester-induced TGF-beta 1 gene activation

BACKGROUND

Hyperglycemia-induced overexpression of prosclerotic transforming growth factor-beta 1 (TGF-beta 1) has been implicated in the pathogenesis of diabetic nephropathy. Since previous in vivo studies demonstrated a renoprotective effect of low-molecular-weight (LMW) heparin in experimental animals, and recent in vitro data showed an interaction of this drug with the overactivated TGF-beta 1 cascade in high glucose- and phorbol ester-stimulated mesangial cells, we studied the molecular mechanism of these effects on TGF-beta 1 gene expression.

METHODS

Mesangial cells were stimulated with 30 mmol/L glucose or with 0.5 micromol/L phorbol ester [phorbol myristate acetate (PMA)] in the absence or presence of LMW heparin. TGF-beta 1 promoter activity was determined in promoter-reporter luciferase assays. The effect of LMW heparin on the binding of nuclear proteins to a regulatory activator protein-1 (AP-1) site, which mediates the high glucose and PMA responsiveness of the TGF-beta 1 promoter, was studied by electrophoretic mobility shift assays.

RESULTS

The presence of LMW heparin completely prevented TGF-beta 1 gene activation in both high glucose- and PMA-stimulated cells. Preincubation of the cells with LMW heparin and subsequent stimulation of the cells with both stimuli yielded the same result. Furthermore, treatment with LMW heparin prevented the enhanced binding of nuclear proteins to the regulatory AP-1 site, while binding to a Sp1 site was unaffected. Basal promoter activity and basal AP-1 binding also was reduced by LMW heparin. The LMW heparin effect on basal promoter activity was abolished by mutation of the regulatory AP-1 box B and by deletion of this AP-1 binding site.

CONCLUSIONS

LMW heparin prevents high glucose- and PMA-mediated TGF-beta 1 expression by inhibiting the activation of the TGF-beta 1 promoter and by preventing the enhanced binding of nuclear proteins to the regulatory AP-1 site.

Overexpression of glutamine:fructose-6-phosphate-amidotransferase induces transforming growth factor-beta1 synthesis in NIH-3T3 fibroblasts

Increased flux through the hexosamine biosynthetic pathway with glutamine:fructose-6-phosphate-amidotransferase (GFAT) as rate-limiting enzyme has been linked to the enhanced bioactivity of the prosclerotic cytokine transforming growth factor beta1 (TGF-beta1) in fibrotic complications, particularly in diabetic kidney disease. Here, we investigate in a stable transfection system the effect of overexpression of GFAT on TGF-beta1 synthesis in NIH-3T3 fibroblasts. We demonstrate a 1.8-fold stimulation of TGF-beta1 mRNA and a 1.9-fold increased protein expression, whereas TGF-beta2 production was not upregulated. The 1.5-fold enhanced TGF-beta1 promoter activity suggests a transcriptional regulation. The elevated TGF-beta1 protein is biologically active since GFAT-overexpressing cells exhibit a 2-fold fibronectin production. The results indicate a GFAT-dependent induction of TGF-beta1 synthesis.

Induction of apoptosis by glyoxal in human embryonic lung epithelial cell line L132

Oxidative stress has been suggested to play a central role in the pathogenesis of lung fibrosis and lung epithelial cell apoptosis is considered to be a key event during fibrogenesis. Studies from various laboratories have indicated that metabolic conditions may initiate oxidative stress, thereby contributing to epithelial cell death. This study was designed to test the hypothesis that glyoxal, an intermediate product in the glycation reaction leading to advanced glycation end products (AGEs), may induce lung epithelial cell apoptosis. We investigated the in vitro effects of glyoxal on fetal human lung epithelial L132 cells. Immunocytochemical analysis of paraffin-embedded cells and fluorescence-activated cell sorter analysis revealed a dose-dependent accumulation of the glycoxidation product (epsilon)N-carboxymethyllysine (CML) in all compartments of the cell. It has been shown that CML modification of proteins may serve as an indicator for oxidative stress. To examine the role of apoptosis in epithelial lung cells we investigated glyoxal-dependent changes in pro- and antiapoptotic mediators bax and activated caspase-3, and galectin-3 and bcl-2, respectively. Increasing concentrations of glyoxal (50 to 400 microM) induced an increase in the number of apoptotic cells. The apoptotic changes were confirmed by transmission electron microscopy. Immunocytochemical analysis of treated cells revealed the presence of other AGEs such as pentosidine as well as products of lipid peroxidation.

Role of the hexosamine biosynthetic pathway in diabetic nephropathy

The hexosamine biosynthetic pathway has been hypothesized to be involved in the development of insulin resistance and diabetic vascular complications. In particular, it was demonstrated that hyperglycemia-induced production of transforming growth factor-beta (TGF-beta1), a prosclerotic cytokine causally involved in the development of diabetic nephropathy. Several lines of evidence indicate that TGF-beta1 induction is mediated by the hexosamine pathway. In cultured mesangial cells, high glucose levels induce TGF-beta1 production. This effect is eliminated by inhibition of glutamine: fructose-6-phosphate-amidotransferase (GFAT), the rate-limiting enzyme of this pathway. Furthermore, stable overexpression of GFAT increased levels of TGF-beta1 protein, mRNA, and promoter activity. Inasmuch as stimulation or inhibition of GFAT increased or decreased high glucose-stimulated activity of protein kinase C (PKC), respectively, the observed effects appear to be transduced by PKC. In similar experiments, involvement of the hexosamine pathway in hyperglycemia-induced production of cytokines (TGF-alpha and basic fibroblast growth factor [bFGF]) was demonstrated in vascular smooth muscle cells. These studies also revealed a rapid increase in GFAT activity by treatment with agents that elevated levels of cyclic adenosine 3',5' monophosphate (cAMP), thus indicating that GFAT activity is tightly regulated by cAMP-dependent phosphorylation. Using immunohistochemistry and in situ hybridization, high expression of GFAT was found in human adipocytes, skeletal muscle, vascular smooth muscle cells, and renal tubular epithelial cells. whereas glomerular cells remained essentially unstained. However, significant staining occurred in glomerular cells of patients with diabetic nephropathy. Current data indicate that the flux through the hexosamine pathway, regulated by GFAT, may be causally involved in the development of diabetic vascular disease, particularly diabetic nephropathy.

Simultaneous, quantitative analysis of UDP-N-acetylglucosamine, UDP-N-acetylgalactosamine, UDP-glucose and UDP-galactose in human peripheral blood cells, muscle biopsies and cultured mesangial cells by capillary zone electrophoresis

The aim of this study was to develop and evaluate a capillary zone electrophoretic (CE) procedure for the accurate quantification of the UDP-hexosamines as well as for the corresponding UDP-hexoses in samples from various biological origins. Testing different buffer conditions, voltages, capillary dimensions and temperatures, optimal results were achieved with a 90 mM borate buffer, pH 9.0, at 18 degrees C and 15.5 kV in an uncoated fused-silica capillary of 50 cm x 50 microm and a detection wavelength of lambda = 262 nm. The total procedure, i.e., including variations of the sample preparation, showed coefficients of variation for the peak areas between 4. 1% and 10.4% in mesangial cells (n = 7) and between 7.8 and 10.3% (n = 6) in leukocytes for the components of interest. To improve precision, an internal standard was used for calibration. The limit of detection for all compounds is an absolute amount of 180 fmol, sufficient for the precise analysis of UDP-sugars in a limited amount of biological samples, such as human leukocytes (obtained from a 10 mL blood sample), muscle biopsies (< or = 100 mg), and mesangial kidney cells (ca. 2.5 x 10(5) cells). This reproducible, quantitative analysis of all four UDP-sugars from various biomedically relevant origins by CZE is a definite improvement over the generally used high performance liquid chromatography (HPLC) procedures. The CZE method allows the study of the flux through the hexosamine pathway in diabetes mellitus and other diseases in a simple, quantitative and accurate way.

Determination of inorganic anions in environmental waters with a hydroxide-selective column

US Environmental Protection Agency Method 300.0 specifies the use of an IonPac AS4A anion-exchange column with a carbonate-hydrogencarbonate eluent and suppressed conductivity detection for the determination of inorganic anions in environmental waters by ion chromatography. Hydroxide eluents have not typically been used for the routine analysis of common inorganic anions due to the lack of an appropriate hydroxide selective column and the difficulty in preparing contaminant free hydroxide eluents. The use of ion chromatography with a hydroxide-selective IonPac AS17 column, automated eluent generation and potassium hydroxide gradient represents a new approach to the routine determination of inorganic anions in environmental waters. This new approach, which is a modification of Method 300.0, allows equivalent method performance with improved linearity, precision, and method detection limits. The AS17 column provides superior retention of fluoride from the column void volume and improved resolution from small organic acids, such as formate and acetate, compared to the AS4A column. Quantitative recoveries were obtained for all the common inorganic anions spiked into typical environmental waters using this new approach, and the Performance Based Measurement System Tier 1 method validation quality control acceptance criteria are well within the acceptable ranges defined by Method 300.0. In addition, the EG40 eluent generator eliminates the need to manually prepare eluents, increasing the level of automation and ease-of-use of the ion chromatography system.