Biomedical subjects

Mathias Fasshauer

Publications and source records attributed to Mathias Fasshauer.

24 records · Page 2Linked to original sources

PubMed2003

Positive and negative roles of p85 alpha and p85 beta regulatory subunits of phosphoinositide 3-kinase in insulin signaling.

Class IA phosphoinositide (PI) 3-kinase is composed of a p110 catalytic subunit and a p85 regulatory subunit and plays a pivotal role in insulin signaling. To explore the physiological roles of two major regulatory isoforms, p85 alpha and p85 beta, we have established brown adipose cell lines with disruption of the Pik3r1 or Pik3r2 gene. Pik3r1-/- (p85 alpha-/-) cells show a 70% reduction of p85 protein and a parallel reduction of p110. These cells have a 50% decrease in PI 3-kinase activity and a 30% decrease in Akt activity, leading to decreased insulin-induced glucose uptake and anti-apoptosis. Pik3r2-/- (p85 beta-/-) cells show a 25% reduction of p85 protein but normal levels of p85-p110 and PI 3-kinase activity, supporting the fact that p85 is more abundant than p110 in wild type. p85 beta-/- cells, however, exhibit significantly increased insulin-induced Akt activation, leading to increased anti-apoptosis. Reconstitution experiments suggest that the discrepancy between PI 3-kinase activity and Akt activity is at least in part due to the p85-dependent negative regulation of downstream signaling of PI 3-kinase. Indeed, both p85 alpha-/- cells and p85 beta-/- cells exhibit significantly increased insulin-induced glycogen synthase activation. p85 alpha-/- cells show decreased insulin-stimulated Jun N-terminal kinase activity, which is restored by expression of p85 alpha, p85 beta, or a p85 mutant that does not bind to p110, indicating the existence of p85-dependent, but PI 3-kinase-independent, signaling pathway. Furthermore, a reduction of p85 beta specifically increases insulin receptor substrate-2 phosphorylation. Thus, p85 alpha and p85 beta modulate PI 3-kinase-dependent signaling by multiple mechanisms and transmit signals independent of PI 3-kinase activation.

Adenoviridae↗

PubMed2003

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.

3T3 Cells↗

PubMed2002

Hormonal regulation of adiponectin gene expression in 3T3-L1 adipocytes.

Recently, it has been demonstrated that the fat-derived protein adiponectin is an important insulin-sensitizing adipocytokine which is downregulated in insulin resistance and obesity and replenishment of which in adiponectin-deficient states improves insulin sensitivity. To clarify the regulation of adiponectin gene expression, 3T3-L1 adipocytes were treated with various hormones known to induce insulin resistance in vivo and adiponectin mRNA was measured by quantitative real-time reverse transcription-polymerase chain reaction. Interestingly, treatment of 3T3-L1 cells with 100 nM insulin, 10 ng/ml tumor necrosis factor (TNF) alpha, or 100 nM dexamethasone for 16 h suppressed adiponectin gene expression by about 50 to 85% while angiotensin 2, growth hormone, and triiodothyronine did not have any effect. Furthermore, insulin reduced the level of adiponectin mRNA in a dose- and time-dependent fashion with inhibition detectable at concentrations as low as 10 nM insulin and as early as 4 h after effector addition. The inhibitory effect of insulin was partially reversed by pretreatment of 3T3-L1 cells with pharmacological inhibitors of p44/42 mitogen-activated protein (MAP) kinase, phosphatidylinositol (PI) 3-kinase, and p70S6 kinase. Moreover, the negative effects of insulin, TNFalpha, and dexamethasone on adiponectin gene expression could be completely reversed by withdrawal of the hormones for 24 h. Taken together, our results suggest that adiponectin gene expression is reversibly downregulated by insulin, TNFalpha, and dexamethasone. The data support the concept of adiponectin being an important selectively controlled modulator of insulin sensitivity.

3T3 Cells↗

PubMed2002

Novel adipocyte lines from brown fat: a model system for the study of differentiation, energy metabolism, and insulin action.

Adipose tissue has emerged as an important endocrine regulator of glucose metabolism and energy homeostasis. By virtue of the mitochondrial protein uncoupling protein-1 (UCP-1), brown fat additionally plays a unique role in thermoregulation. Interest has focused on this tissue not only as a target for pharmacotherapy of obesity and insulin resistance but also as an endocrine tissue with leptin secretion and high insulin sensitivity. Most studies of adipocytes have been limited either to primary cell culture or to a small number of established cell lines. Recently, we have generated immortalized brown adipocyte cell lines from single newborn mice of different knockout mouse models. These cell lines retain the main characteristics of primary cells including UCP-1 expression. They display sensitive and diverse metabolic responses to insulin and adrenergic stimulation and have proven to be useful in the characterization of UCP regulation and the role of key insulin signaling elements for insulin action. Here, we outline common approaches to the generation of adipose tissue cell lines. Furthermore, we propose that the novel technique of generating brown adipocyte lines from a single newborn mouse will be instrumental in gaining further insight into the role of a broad range of signaling molecules in adipose tissue biology and in the pathogenesis of insulin resistance.

Adipocytes↗

PubMed2002

Atypical beta-adrenergic effects on insulin signaling and action in beta(3)-adrenoceptor-deficient brown adipocytes.

Cross talk between adrenergic and insulin signaling systems may represent a fundamental molecular basis of insulin resistance. We have characterized a newly established beta(3)-adrenoceptor-deficient (beta(3)-KO) brown adipocyte cell line and have used it to selectively investigate the potential role of novel-state and typical beta-adrenoceptors (beta-AR) on insulin signaling and action. The novel-state beta(1)-AR agonist CGP-12177 strongly induced uncoupling protein-1 in beta(3)-KO brown adipocytes as opposed to the beta(3)-selective agonist CL-316,243. Furthermore, CGP-12177 potently reduced insulin-induced glucose uptake and glycogen synthesis. Neither the selective beta(1)- and beta(2)-antagonists metoprolol and ICI-118,551 nor the nonselective antagonist propranolol blocked these effects. The classical beta(1)-AR agonist dobutamine and the beta(2)-AR agonist clenbuterol also considerably diminished insulin-induced glucose uptake. In contrast to CGP-12177 treatment, these negative effects were completely abrogated by metoprolol and ICI-118,551. Stimulation with CGP-12177 did not impair insulin receptor kinase activity but decreased insulin receptor substrate-1 binding to phosphatidylinositol (PI) 3-kinase and activation of protein kinase B. Thus the present study characterizes a novel cell system to selectively analyze molecular and functional interactions between novel and classical beta-adrenoceptor types with insulin action. Furthermore, it indicates insulin receptor-independent, but PI 3-kinase-dependent, potent negative effects of the novel beta(1)-adrenoceptor state on diverse biological end points of insulin action.

Adipocytes↗

PubMed2002

Negative regulation of adipose-expressed galectin-12 by isoproterenol, tumor necrosis factor alpha, insulin and dexamethasone.

OBJECTIVE: Galectin-12 has recently been shown to be a predominantly adipocyte-expressed protein which is stimulated by insulin-sensitizing thiazolidinediones and possesses apoptosis-inducing activity. METHODS: To further clarify galectin-12 regulation and its potential involvement in the development of insulin resistance, 3T3-L1 adipocytes were chronically treated with various hormones known to impair insulin sensitivity, and galectin-12 mRNA was measured by quantitative real-time reverse transcription-polymerase chain reaction. RESULTS: Treatment of 3T3-L1 cells for 16 h with 10 micromol/l isoproterenol, 100 nmol/l insulin, 0.6 nmol/l tumor necrosis factor alpha (TNFalpha), and 100 nmol/l dexamethasone reduced galectin-12 gene expression between 47% and 85%. These negative effects were dose-dependent with significant inhibition detectable at concentrations as low as 10 nmol/l isoproterenol, 0.06 nmol/l TNFalpha, and 1 nmol/l dexamethasone. Furthermore, the inhibitory effect of isoproterenol could be almost completely reversed by pretreatment with the beta-adrenergic antagonist propranolol and mimicked by stimulation of G(S)-proteins with cholera toxin or by activation of adenylyl cyclase with forskolin and dibutyryl-cAMP. CONCLUSIONS: Our results suggest that galectin-12 is an adipocyte-expressed protein which is downregulated by various insulin resistance-inducing hormones. These findings imply a role for galectin-12 in the pathogenesis of insulin resistance.

Adenylyl Cyclases↗