Ulrike Lossner

Adiponectin gene expression and secretion is inhibited by interleukin-6 in 3T3-L1 adipocytes

Recently, it has been shown that adiponectin is an important insulin-sensitizing fat-derived protein which is downregulated in insulin resistance and obesity, and replenishment of which improves insulin sensitivity. In contrast, interleukin (IL)-6 appears as an adipocytokine serum concentrations of which are elevated in these states. However, it has not been determined whether IL-6 might impact on expression and secretion of adiponectin. To clarify this, 3T3-L1 adipocytes were treated with different concentrations of IL-6 for various periods of time. Adiponectin mRNA was measured by quantitative real-time reverse transcription-polymerase chain reaction and secretion was determined by radioimmunoassays. Interestingly, treatment of 3T3-L1 cells with 30 ng/ml IL-6 significantly decreased adiponectin secretion to 75% of control levels. Adiponectin secretion was also inhibited between 25% and 45% by chronic treatment with forskolin (50 microM), tumor necrosis factor alpha (100 ng/ml), and dexamethasone (100 nM). Furthermore, adiponectin mRNA expression was downregulated by up to 50% in a time- and dose-dependent manner, with significant inhibition detectable at concentrations as low as 3 ng/ml IL-6 and as early as 8h after effector addition. The inhibitory effect of IL-6 was partially reversed by pretreatment of 3T3-L1 cells with pharmacological inhibitors of a p44/42 mitogen-activated protein (MAP) kinase. Moreover, the negative effect of IL-6 on adiponectin mRNA expression could be reversed by withdrawal of the hormone for 24h. Taken together, our results suggest that adiponectin gene expression is reversibly downregulated by IL-6 and support the concept of adiponectin being an important selectively controlled modulator of insulin sensitivity.

Isoproterenol, TNFα, and insulin downregulate adipose triglyceride lipase in 3T3-L1 adipocytes

Recently, adipose triglyceride lipase (ATGL, also called desnutrin and calcium-independent phospholipase A2 [iPLA(2)] zeta) was isolated as a novel adipose-expressed triglyceride lipase which is downregulated in obesity and may contribute to obesity-associated metabolic disorders such as hyperlipidemia and insulin resistance. To clarify expression and regulation of this fat-derived lipase, ATGL mRNA was measured in 3T3-L1 adipocytes by quantitative real-time reverse transcription-polymerase chain reaction after treatment with isoproterenol, tumor necrosis factor (TNF) alpha, insulin, and growth hormone (GH) which have been shown to influence lipolysis and insulin sensitivity profoundly. Interestingly, treatment of adipocytes with 100 nM isoproterenol, 30 ng/ml TNF alpha, and 100 nM insulin for 16 h significantly decreased ATGL mRNA to 74%, 17%, and 49% of control levels, respectively. GH did not influence ATGL synthesis. The effect of isoproterenol, TNFalpha, and insulin on ATGL expression was time- and dose-dependent. Similarly, HSL mRNA was downregulated by the three hormones. Furthermore, signaling studies suggested that activation of Gs-protein-coupled pathways by forskolin and cholera toxin is sufficient to significantly downregulate ATGL mRNA. Moreover, p44/42 mitogen-activated protein kinase appears to partly mediate the negative effect of insulin but not TNFalpha on ATGL. Taken together, downregulation of ATGL by isoproterenol, TNFalpha, and insulin might contribute to dysregulated expression and function of this lipase in obesity, hyperlipidemia, and insulin resistance.

Interleukin-1ß induces the novel adipokine chemerin in adipocytes in vitro

Chemerin has recently been characterized as a novel adipokine playing a crucial role in adipocyte differentiation and insulin signalling. In the current study, the impact of insulin resistance-inducing and proinflammatory interleukin (IL)-1beta on chemerin protein secretion and mRNA expression was determined in 3T3-L1 adipocytes. Interestingly, IL-1beta significantly induced chemerin protein secretion almost 1.3-fold from 5.89 ng/ml (basal) to 7.52 ng/ml. Furthermore, chemerin mRNA synthesis was significantly stimulated by IL-1beta in a dose-dependent fashion with 1.5-fold induction seen at IL-1beta concentrations as low as 0.07 ng/ml and maximal 2.6-fold upregulation found at 2 ng/ml effector. Induction of chemerin mRNA by IL-1beta was time-dependent in both 3T3-L1 adipocytes and brown fat cells. Signalling studies suggested that Janus kinase 2, nuclear factor kappa B, p44/42 mitogen-activated protein kinase, and phosphatidylinositol 3-kinase are involved in IL-1beta-induced chemerin mRNA expression. Furthermore, recombinant chemerin downregulated insulin-stimulated glucose uptake. Taken together, we show that chemerin is upregulated in fat cells by IL-1beta and might modulate the effects of IL-1beta on adipocyte metabolism and insulin sensitivity.

Growth hormone is a positive regulator of adiponectin receptor 2 in 3T3-L1 adipocytes.

The fat‐derived protein adiponectin is an important insulin‐sensitizing adipocytokine which is downregulated in insulin resistance and obesity. Recently, two receptors of this adipose‐expressed protein called adiponectin receptor 1 (AdipoR1) and 2 (AdipoR2) have been cloned. To clarify expression and regulation of these receptors in fat cells, AdipoR1 and AdipoR2 mRNA was measured by quantitative real‐time reverse transcription‐polymerase chain reaction during differentiation of 3T3‐L1 adipocytes and after treatment with various hormones known to induce insulin resistance. Interestingly, AdipoR2 synthesis was significantly increased up to 4.8‐fold during differentiation of 3T3‐L1 preadipocytes, whereas AdipoR1 expression was only augmented up to 1.4‐fold. Furthermore, growth hormone (GH) induced AdipoR2, but not AdipoR1 mRNA by up to 2.4‐fold in a dose‐ and time‐dependent fashion with significant stimulation detectable at concentrations as low as 5 ng/ml GH and as early as 2 h after effector addition. The positive effect of GH on AdipoR2 expression could be reversed by withdrawal of the hormone for 24 h. In contrast, other key hormones involved in the regulation of insulin resistance and energy metabolism such as insulin, isoproterenol, dexamethasone, triiodothyronine, angiotensin 2, tumor necrosis factor α, and interleukin‐6 did not influence AdipoR1 and AdipoR2 synthesis in vitro. Taken together, our results suggest that AdipoR2 expression is differentiation‐dependent and selectively regulated by GH implying a potential role of this hormone in adiponectin‐associated alterations of insulin sensitivity and energy homeostasis.

Serum Levels of the Adipokine FGF21 Depend on Renal Function

OBJECTIVE—To investigate renal elimination of the adipokine fibroblast growth factor 21 (FGF21) by determining circulating FGF21 levels in patients on chronic hemodialysis (CD) as compared with control subjects with a glomerular filtration rate (GFR) >50 ml/min. RESEARCH DESIGN AND METHODS—FGF21 was determined by enzyme-linked immunosorbent assay in control (n = 60) and CD (n = 60) patients and correlated to clinical and biochemical measures of renal function, glucose and lipid metabolism, and inflammation in both groups. RESULTS—Median serum FGF21 levels were >15-fold higher in CD patients (3,710.6 ng/l) than in subjects with a GFR >50 ml/min (201.9 ng/l) (P < 0.001). Furthermore, serum creatinine positively and GFR negatively predicted FGF21 concentrations in multiple regression analyses in control subjects (P < 0.05). CONCLUSIONS—FGF21 serum levels increase in CD patients and are related to markers of renal function in control subjects.

Serum levels of the myokine irisin in relation to metabolic and renal function.

OBJECTIVE Irisin has recently been introduced as a novel myokine which reverses visceral obesity and improves glucose metabolism in mice. However, regulation of irisin in humans in relation to renal and metabolic disease has not been comprehensively studied. DESIGN AND METHODS Serum irisin levels were quantified by ELISA and correlated with anthropometric and biochemical parameters of renal function, glucose and lipid metabolism, as well as inflammation, in 532 patients with stages 1-5 of chronic kidney disease (CKD). RESULTS Median serum irisin levels adjusted for age, gender, and BMI significantly decreased with increasing CKD stage and lowest concentrations were seen in patients with CKD stage 5. Furthermore, irisin concentrations were associated with facets of the metabolic syndrome including diastolic blood pressure, markers of impaired glucose tolerance, and dyslipidemia in univariate analysis. Moreover, markers of renal function, e.g. glomerular filtration rate, and insulin resistance, e.g. homeostasis model assessment of insulin resistance, remained independently associated with circulating irisin levels in robust multivariate analysis. CONCLUSIONS We show that irisin serum concentrations decrease with increasing CKD stage and are independently and positively predicted by renal function and insulin resistance. The physiological relevance of our findings, as well as the factors contributing to irisin regulation in humans, needs to be further defined in future experiments.

Serum levels of the adipokine visfatin are increased in pre‐eclampsia

Objective  Pre‐eclampsia (PE) is a serious cardiovascular complication in pregnancy which shares risk factors with the metabolic syndrome including insulin resistance and obesity. Recently, visfatin was introduced as a novel insulin‐mimetic adipokine which is up‐regulated when weight is gained. In the current study, we investigated visfatin serum levels in pre‐eclamptic patients as compared to healthy gestational age‐matched controls.

Serum Levels of the Adipokine Chemerin in Relation to Renal Function

OBJECTIVE To investigate serum levels of the adipokine chemerin in patients on chronic hemodialysis (CD) as compared with control patients with a glomerular filtration rate (GFR) >50 ml/min. RESEARCH DESIGN AND METHODS Chemerin was quantified by ELISA in control patients (n = 60) and CD patients (n = 60) and correlated with clinical and biochemical measures of renal function, glucose, and lipid metabolism, as well as inflammation, in both groups. RESULTS Median serum chemerin levels were more than twofold higher in CD patients (542.2 μg/l) compared with subjects with a GFR >50 ml/min (254.3 μg/l) (P < 0.001). Furthermore, GFR, as assessed by the original Modification of Diet in Renal Disease formula, independently predicted circulating chemerin concentrations in multiple regression analyses in both control patients (P < 0.05) and CD patients (P < 0.01). CONCLUSIONS We demonstrate that markers of renal function are independently related to circulating chemerin levels.

Serum levels of the adipokine chemerin are increased in preeclampsia during and 6 months after pregnancy.

UNLABELLED Preeclampsia is a serious cardiovascular complication in pregnancy which is associated with an increased future metabolic and cardiovascular risk for mother and newborn. Recently, chemerin was introduced as a novel adipokine inducing insulin resistance in vitro and in vivo. In the current study, we investigated serum concentrations of chemerin by ELISA in control and preeclampsia patients during pregnancy ( CONTROL n=37, preeclampsia: n=37) and 6 months after delivery ( CONTROL n=35, preeclampsia: n=36). Furthermore, the association between chemerin and markers of renal function, glucose and lipid metabolism, as well as inflammation was studied in pregnant patients. Median maternal chemerin concentrations were significantly elevated in preeclampsia patients (249.5 [range: 123.1-366.9] μg/l) as compared to controls (204.8 [138.5-280.8] μg/l) (p<0.001). Furthermore, chemerin serum levels positively correlated with blood pressure, creatinine, free fatty acids, cholesterol, triglycerides (TG), leptin, adiponectin, and C-reactive protein in univariate analyses. In multivariate analyses, TG and leptin remained independently associated with circulating chemerin. Interestingly, median chemerin concentrations 6 months after delivery remained significantly higher in former preeclampsia patients (196.0 [119.8-368.7] μg/l) as compared to controls (152.2 [102.8-216.4] μg/l). Taken together, maternal chemerin serum concentrations are significantly increased in preeclampsia during and after pregnancy. Furthermore, TG and leptin are independent predictors of circulating chemerin during pregnancy.

Circulating high-molecular-weight adiponectin is upregulated in preeclampsia and is related to insulin sensitivity and renal function

OBJECTIVE Preeclampsia (PE) is a serious cardiovascular complication in pregnancy which is associated with an increased future metabolic and cardiovascular risk for mother and newborn. Recently, a paradoxical upregulation of the insulin-sensitizing and anti-atherogenic adipokine adiponectin has been shown in PE. Furthermore, high-molecular-weight (HMW) adiponectin has been suggested as the biologically active form of this adipokine. DESIGN AND METHODS HMW adiponectin and total adiponectin serum concentrations were quantified by ELISA in PE (n=16) patients and pregnant control women without PE (n=20). Furthermore, HMW adiponectin and total adiponectin were correlated to clinical and biochemical measures of renal function, glucose, and lipid metabolism, as well as inflammation. RESULTS Median maternal HMW adiponectin and total adiponectin levels were significantly and independently upregulated almost twofold in PE when compared with controls. HMW adiponectin and total adiponectin correlated positively with creatinine and negatively with fasting insulin in univariate and multivariate analyses. CONCLUSIONS We show that maternal HMW adiponectin and total adiponectin serum concentrations are significantly increased in PE and are positively associated with markers of insulin sensitivity and renal dysfunction. Adiponectin might be part of a physiological feedback mechanism improving insulin sensitivity and cardiovascular health in PE.