Association of interleukin-6, C-reactive protein, interleukin-10 and adiponectin plasma concentrations with measures of obesity, insulin sensitivity and glucose metabolism

Abnormal glucose tolerance is associated with subclinical chronic inflammation in patients with type 2 diabetes. The aim of this study was to investigate whether plasma concentrations of inflammatory markers are associated with measures of obesity, insulin sensitivity, and hyperglycemia. IL-6, adiponectin, CRP, and IL-10 plasma concentrations were evaluated in 142 patients with a wide range of obesity, insulin sensitivity and glucose tolerance. In parallel with the impairment of glucose tolerance, there was a significant increase in IL-6, and CRP, and a significant decrease in adiponectin and IL-10 plasma concentrations. There were significant correlations between the plasma concentrations of all inflammatory markers and % body fat, insulin sensitivity, and fasting plasma glucose. However, multivariate linear regression analysis identified insulin sensitivity as determined by glucose infusion rate during the steady state of an euglycemic-hyperinsulinemic clamp as the strongest predictor of adiponectin, CRP, IL-6, and IL-10 plasma concentrations. In addition, fasting plasma glucose was a significant determinant of adiponectin, CRP, and IL-6 plasma concentrations, whereas body fat content was only a significant predictor of CRP plasma concentration. In conclusion, our data suggest that abnormal inflammatory markers in patients with type 2 diabetes are primarily related to decreased insulin sensitivity.

Aldosterone inhibits uncoupling protein-1, induces insulin resistance, and stimulates proinflammatory adipokines in adipocytes

Aldosterone is a mineralocorticoid hormone that regulates blood pressure and salt/water balance. Increased aldosterone levels are found in states of disturbed energy balance such as the metabolic syndrome. Adipose tissue has been recognized to play a pivotal role in the regulation of energy homeostasis. We investigated direct aldosterone effects on brown adipocyte function. Aldosterone dose-dependently inhibited expression of uncoupling protein-1 (UCP-1) by 30% (p < 0.01). Furthermore, aldosterone dose-dependently impaired insulin-induced glucose uptake by about 25% (p < 0.01). On a transcriptional level, mRNA of the proinflammatory adipokines leptin and monocyte chemoattractant protein-1 (MCP-1) was increased by 5,000% and 40%, respectively, by aldosterone exposure (p < 0.05). This study demonstrates that aldosterone directly impacts on major adipose functions including stimulation of proinflammatory adipokines.

Meningitis Due to Haemophilus influenzae Type f in an 8-Year-Old Girl with Congenital Humoral Immunodeficiency

We report on an 8-year-old Haemophilus influenzae type b-vaccinated girl with meningitis due to H. influenzae type f. The girl had an oro-facio-digital syndrome and a hypogammaglobulinemia, obviously predisposing the patient to invasive infections. Treatment with cefotaxime was successful.Incidence, predisposing factors and the influence of H. influenzae type b immunization on invasive infections with non-type b H. influenzae strains in children are discussed.

Insulin resistance-inducing cytokines differentially regulate SOCS mRNA expression via growth factor- and Jak/Stat-signaling pathways in 3T3-L1 adipocytes

Various cytokines, including tumor necrosis factor (TNF) alpha, growth hormone (GH) and interleukin (IL)-6, induce insulin resistance. Recently, it was demonstrated that induction of suppressor of cytokine signaling (SOCS)-3 by TNFalpha and GH is an important mechanism by which these cytokines impair insulin sensitivity. The current study investigated in 3T3-L1 adipocytes whether TNFalpha and GH also upregulate SOCS-1 and SOCS-6, which have both been shown to inhibit insulin signaling potently, and whether IL-6 might alter synthesis of SOCS-1, -3 and -6. Interestingly, 10 ng/ml TNFalpha, 500 ng/ml GH and 30 ng/ml IL-6 induced SOCS-1 mRNA time-dependently with maximal stimulation detectable after 8 h of TNFalpha and 1 h of GH and IL-6 addition respectively. Furthermore, TNFalpha and GH caused sustained upregulation of SOCS-1 for up to 24 h, whereas stimulation by IL-6 was only transient, with SOCS-1 mRNA returning to basal levels 2 h after effector addition. Induction of SOCS-1 was dose-dependent, and significant stimulation was detectable at concentrations as low as 3 ng/ml TNFalpha, 50 ng/ml GH and 10 ng/ml IL-6. Furthermore, stimulation experiments and studies using pharmacologic inhibitors suggested that the positive effect of TNFalpha, GH and IL-6 on SOCS-1 mRNA is, at least in part, mediated by Janus kinase (Jak) 2. Finally, SOCS-3 expression was dose- and time-dependently induced by IL-6, at least in part via Jak2, but none of the cytokines affected SOCS-6 expression. Taken together, our results show a differential regulation of SOCS mRNA by insulin resistance-inducing hormones, and suggest that SOCS-1, as well as SOCS-3, may be an important intracellular mediator of insulin resistance in fat cells and a potential pharmacologic target for the treatment of impaired insulin sensitivity.

The novel missense mutation methionine 442 threonine in the thyroid hormone receptor beta causes thyroid hormone resistance: a case report

We report a family with thyroid hormone resistance caused by a novel mutation M442T in the thyroid hormone receptor beta (TRbeta) gene. The 59-year-old propositus and one of his two daughters had typical clinical signs of reduced responsiveness of tissues to thyroid hormones. Thus, elevated free T (3) and T (4) plasma concentrations in coexistance with a diffuse nodular goiter, nonsuppressed TSH, and atrial fibrillation with tachycardia were present in the propositus. His affected daughter also had increased levels of free T (3) and T (4) with slightly elevated TSH concentrations. Both affected members harboured a heterozygous M442T TRbeta mutation. The unaffected child had no mutation in the TRbeta gene and no clinical manifestations.

Neurohumoral stimulation in type-2-diabetes as an emerging disease concept

Neurohumoral stimulation comprising both autonomic-nervous-system dysfunction and activation of hormonal systems including the renin-angiotensin-aldosterone system (RAAS) was found to be associated with Type-2-diabetes (T2D). Therapeutic strategies such as RAAS interference proved to be beneficial in both T2D treatment and prevention. In addition to an activated RAAS, hyperleptinemia in obesity, hyperinsulinemia in conditions of peripheral insulin resistance and overall oxidative stress in T2D represent known activators of the sympathetic component of the autonomic nervous system. Here, we hypothesize that sympathetic activation may cause peripheral insulin resistance defined as partial blocking of insulin effects on glucose uptake. Resulting hyperinsulinemia or hyperglycemia-related oxidative stress may further aggravate sympatho-excitation. This notion leads to a secondary hypothesis: sympathetic activation worsens from obesity towards insulin resistance, and further towards T2D. In this review, existing evidence relating to neurohumoral stimulation in T2D and consequences thereof, such as oxidative stress and inflammation, are discussed. The aim of this review is to provide a rationale for therapies, which are able to intercept neuroendocrine pathways in T2D and precursor states such as obesity.

Regulation of adipocytokines and insulin resistance

It has long been known that obesity and insulin resistance are linked. Recently, it has been shown that adipocytes secrete several proteins including tumour necrosis factor-alpha, interleukin-6, resistin, and adiponectin. Since several of these so-called adipocytokines influence insulin sensitivity and glucose metabolism profoundly, they might provide a molecular link between increased adiposity and impaired insulin sensitivity. Thiazolidinediones which decrease insulin resistance and are used in the treatment of Type 2 diabetes seem to mediate part of their insulin-sensitising effects via modulation of adipocytokine expression. Furthermore, hormones such as beta-adrenergic agonists, insulin, glucocorticoids, and growth hormone might impair insulin sensitivity at least in part via up-regulation or down-regulation of adipocytokine synthesis. We summarise the current knowledge on how major adipocyte-secreted proteins are regulated by hormones and drugs influencing insulin sensitivity and discuss its implications for insulin resistance and obesity.

GH is a positive regulator of tumor necrosis factor alpha-induced adipose related protein in 3T3-L1 adipocytes

Tumor necrosis factor (TNF) alpha-induced adipose-related protein (TIARP) has recently been cloned as a TNFalpha-stimulated protein expressed in adipocytes. Its expression is differentiation-dependent and potentially involved in mediating TNFalpha-induced insulin resistance. To further characterize regulation of TIARP gene expression, 3T3-L1 adipocytes were treated with key hormones modulating insulin sensitivity and influencing adipocyte metabolism, and TIARP gene expression was determined by quantitative real-time RT-PCR. Interestingly, TIARP mRNA expression was stimulated almost 9-fold after 500 ng/ml GH were added for 16 h whereas addition of 10 microM isoproterenol, 100 nM insulin and 100 nM dexamethasone for 16 h significantly decreased TIARP gene expression to between 35 and 50% of control levels. In contrast, angiotensin 2 (10 microM) and triiodothyronine (1 microM) did not have any effect. The stimulatory effect of GH was time- and dose-dependent with stimulation occurring as early as 1 h after effector addition and at concentrations as low as 5 ng/ml GH. Moreover, pharmacological inhibition of Janus kinase 2 and p42/44 mitogen-activated protein kinase reversed the stimulatory effect of GH, suggesting that both signaling molecules are involved in activation of TIARP gene expression by GH. Furthermore, an increase of TIARP mRNA could be completely reversed to control levels by withdrawal of GH for 24 h. Taken together, these results show that TIARP is not only responsive to TNFalpha but also to important other hormones influencing glucose homeostasis and adipocyte metabolism. Thus, this factor may play an integrative role in the pathogenesis of insulin resistance and its link to obesity.

Suppression of aquaporin adipose gene expression by isoproterenol, TNFalpha, and dexamethasone

Aquaporin adipose (AQPap) is a putative glycerol channel in adipocytes. It has recently been shown to be upregulated in insulin resistance stimulated by thiazolidinediones and inhibited by insulin. To further clarify regulation of AQPap gene expression, 3T3-L1 adipocytes were chronically treated with various hormones known to influence insulin sensitivity and adipocyte metabolism, and AQPap mRNA was measured by quantitative real-time reverse transcription-polymerase chain reaction. Interestingly, treatment of 3T3-Ll adipocytes with 10 micro M isoproterenol, 10 ng/ml TNFalpha, and 100 nM dexamethasone for 16 h inhibited AQPap gene expression by 62 %, 60 %, and 39 %, respectively; angiotensin 2, growth hormone, and triiodothyronine did not have any effect. The inhibitory effects were dose-dependent with significant suppression detectable at concentrations as low as 1 nM isoproterenol, 1 ng/ml TNFalpha, and 10 nM dexamethasone. Furthermore, inhibition of AQPap gene expression could be almost completely reversed by pretreating 3T3-L1 adipocytes with the beta-adrenoceptor antagonist propranolol. Moreover, stimulation of Gs-proteins with cholera toxin and adenylyl cyclase with forskolin and dibutyryl-cAMP dramatically downregulated AQPap mRNA. Taken together, our results suggest that AQPap is an adipocyte-expressed glycerol channel selectively regulated and profoundly downregulated by hormones implicated in the pathogenesis of insulin resistance and dyslipidemia.

Interleukin (IL)-6 mRNA expression is stimulated by insulin, isoproterenol, tumour necrosis factor alpha, growth hormone, and IL-6 in 3T3-L1 adipocytes

Interleukin (IL)-6 has recently been shown to be an adipocyte-expressed cytokine. Its serum concentrations are elevated in insulin resistance and obesity. For further evaluation of IL-6 gene expression regulation, fully differentiated 3T3-L1 adipocytes were treated with various hormones known to induce insulin resistance. IL-6 mRNA content was measured by quantitative real-time reverse transcription-polymerase chain reaction. Interestingly, treatment of adipocytes with 100 nM insulin, 10 micro M isoproterenol, 10 ng/ml tumour necrosis factor alpha (TNFalpha), and 500 ng/ml growth hormone (GH) for 16 h stimulated IL-6 mRNA expression 2.3-fold, 47-fold, 74-fold, and 1.4-fold, respectively (p < 0.01). In contrast, treatment with 100 nM dexamethasone significantly decreased IL-6 expression to 32 % of control levels (p < 0.01), whereas triiodothyronine and angiotensin 2 did not have any effect. Furthermore, stimulation of IL-6 expression was time-dependent with maximal stimulatory effects detectable after 1 h of insulin, isoproterenol, and GH addition and 12 h of TNFalpha, respectively. Moreover, isoproterenol's effect could be almost completely reversed by pretreatment of 3T3-L1 cells with the beta-adrenergic antagonist propranolol and mimicked by stimulation of G S -proteins with cholera toxin and adenylyl cyclase with forskolin and dibutyryl cAMP, respectively. Finally, IL-6 strongly induced its own expression in a time-dependent fashion. Taken together, our results demonstrate that IL-6 expression in adipocytes is governed by an autocrine positive feedback loop and upregulated by insulin, isoproterenol, TNFalpha, and GH. In concert with this adipocytokine's upregulation in states of decreased insulin sensitivity such as obesity and diabetes, the data support a possible role of IL-6 as a selectively regulated mediator of insulin resistance.

Isoproterenol is a positive regulator of the suppressor of cytokine signaling-3 gene expression in 3T3-L1 adipocytes

SOCS (suppressor of cytokine signaling)-3 has recently been shown to be an insulin- and tumor necrosis factor (TNF)-alpha-induced negative regulator of insulin signaling. To further clarify a potential involvement of SOCS-3 in the development of insulin resistance, we measured differentiation-dependent SOCS-3 mRNA expression in 3T3-L1 adipocytes and studied its regulation by various hormones known to impair insulin signaling using quantitative real-time RT-PCR. There was a differentiation-dependent downregulation of SOCS-3 mRNA by 50% over the 9 day adipocyte differentiation course. Interestingly, besides insulin and TNF-alpha, chronic treatment of differentiated 3T3-L1 cells with 10 microM isoproterenol for 16 h stimulated SOCS-3 gene expression by about 3.5-fold. Furthermore, isoproterenol stimulated SOCS-3 mRNA expression in a dose-dependent manner with significant activation detectable at concentrations as low as 10 nM isoproterenol. Moreover, a strong 27- and 47-fold activation of SOCS-3 mRNA expression could be seen after 1 h of isoproterenol and GH treatment respectively. The stimulatory effect of isoproterenol could be almost completely reversed by pretreatment of 3T3-L1 cells with the beta-adrenergic antagonist propranolol. Finally, isoproterenol's action could be mimicked by stimulation of G(S)-proteins with cholera toxin and of adenylyl cyclase with forskolin and dibutyryl cAMP. Taken together, our results demonstrate a differentiation-dependent downregulation of SOCS-3 in adipocytes and suggest that SOCS-3 gene expression is stimulated by beta-adrenergic agents via activation of a G(S)-protein-adenylyl cyclase-dependent pathway. As SOCS-3 is a novel inhibitor of insulin signaling, the data support a possible role of this protein as a selectively regulated mediator of catecholamine-induced insulin resistance.

Direct peripheral effects of ghrelin include suppression of adiponectin expression

The stomach-derived peptide, ghrelin, has recently been discovered as an important regulator of energy homeostasis. Central nervous system pathways involving stimulation of hypothalamic neuropeptides play a prominent role in mediating ghrelin's orexigenic effects. However, potential direct peripheral effects remain poorly understood. Using a brown adipocyte model, we tested ghrelin-mediated influences on adipose tissue. Chronic ghrelin stimulation of differentiating adipocytes did not affect the pattern or extent of fat accumulation. Furthermore, insulin-induced glucose uptake as a hallmark of adipocyte function was not altered by ghrelin pre-treatment. However, acute ghrelin treatment resulted in a significant time-dependent increase in p44/42 mitogen-activated protein kinase phosphorylation. There was no stimulation of phosphatidylinositol 3-kinase, JAK/STAT, or stress kinase signaling pathways. Furthermore, ghrelin did not significantly alter gene expression of the thermogenic uncoupling protein-1. By contrast, expression of the novel adipokine adiponectin, which has been implicated in the pathogenesis of insulin resistance and obesity, was strongly impaired. This inhibition occurred acutely, and was sustained for several hours. In summary, our data provide evidence for selective effects of ghrelin on adipocyte signaling and function and thus propose a role for adipose tissue as a novel mediator of ghrelin's effects on energy balance and glucose homeostasis.

Leptin secretion and negative autocrine crosstalk with insulin in brown adipocytes

Leptin is an important adipocytokine whose main regulative effects on energy metabolism are exerted via activation of signalling pathways in the central nervous system. Another important regulator of energy homeostasis is insulin. The role of direct autocrine leptin effects on adipose tissue and crosstalk with insulin, in particular in the thermogenically active brown adipose tissue, remains unclear. In the present study, we have investigated leptin secretion and interaction with insulin in highly insulin-responsive immortalised mouse brown adipocytes. Leptin was secreted in a differentiation-dependent manner, and acute leptin treatment of mature adipocytes dose- and time-dependently stimulated phosphorylation of STAT3 and MAP kinase. Interestingly, acute pretreatment of fully differentiated brown adipocytes with leptin (100 nM) significantly diminished insulin-induced glucose uptake by approximately 25%. This inhibitory effect was time-dependent and maximal after 60 min of leptin prestimulation. Furthermore, it correlated with a 35% reduction in insulin-stimulated insulin receptor kinase activity after acute leptin pretreatment. Insulin-induced insulin receptor substrate-1 tyrosine phosphorylation and binding to the regulatory subunit p85 of phosphatidylinositol 3-kinase (PI 3-kinase) were diminished by approximately 60% and 40%, respectively. Taken together, this study has demonstrated strong differentiation-dependent leptin secretion in brown adipocytes and PI 3-kinase-mediated negative autocrine effects of this hormone on insulin action. Direct peripheral leptin-insulin crosstalk may play an important role in the regulation of energy homeostasis.

Direct effects of ciliary neurotrophic factor on brown adipocytes: evidence for a role in peripheral regulation of energy homeostasis

Ciliary neurotrophic factor (CNTF) plays an important role in regulating neuronal growth. Recently, central anorexigenic effects of this cytokine have been characterized. However, peripheral effects on tissues that actively contribute to the regulation of energy homeostasis have not been described. Here, we report direct potent and selective effects of CNTF on growth factor and metabolic signalling intermediates in mouse brown adipocytes. CNTF stimulates STAT3, MAP kinase, Akt, and p70 S6 kinase. We find that, next to mediating Akt and p70 S6 kinase activation, both phosphatidylinositol 3-kinase and protein kinase C are separately acting, main intermediates for inducing mitogen-activated protein (MAP) kinase activation. On a functional level, CNTF enhances beta3-adrenergic induction of uncoupling protein-1. Thus, these results demonstrate direct effects of CNTF on adipose tissue signalling and metabolism and suggest a novel role for this cytokine in the peripheral regulation of energy homeostasis.

Tumor necrosis factor alpha is a negative regulator of resistin gene expression and secretion in 3T3-L1 adipocytes

Resistin has recently been implicated as an adipocytokine leading to insulin resistance and, therefore, potentially linking obesity and diabetes. To further characterize the regulation of this fat-secreted protein by insulin sensitivity-modulating hormones, 3T3-L1 adipocytes were treated with tumor necrosis factor (TNF) alpha, angiotensin (AT) 2, as well as growth hormone (GH), and resistin gene expression and protein secretion were determined by quantitative real-time reverse transcription-polymerase chain reaction and Western blotting. Interestingly, both, resistin mRNA expression and protein secretion, were inhibited by 70-90% after TNFalpha-treatment whereas AT2 and GH did not have any effect. The inhibitory effect of TNFalpha was time- and dose-dependent with significant inhibition occurring as early as 4 h after effector addition and at concentrations as low as 1 ng/ml TNFalpha. Pharmacological inhibition of protein kinase A (PKA), p44/42, and p38 mitogen-activated protein (MAP) kinase did not reverse the inhibitory effect of TNFalpha suggesting that neither of these signaling molecules is involved in suppression of resistin gene expression by TNFalpha. Furthermore, suppression of resistin mRNA levels could be completely reversed to control levels by withdrawal of TNFalpha for 24 h. Taken together, these results suggest that TNFalpha is a pivotal negative regulator of resistin gene expression. This may have important implications for the pathogenesis of insulin resistance and its link to obesity.

Adiponectin gene expression is inhibited by beta-adrenergic stimulation via protein kinase A in 3T3-L1 adipocytes

Recently, it has been shown that the fat-derived factor adiponectin is downregulated in insulin resistance and obesity and that replenishment of this adipocytokine reverses insulin resistance in mice. Growing evidence, on the other hand, suggests that raised levels of catecholamines due to increased activity of the sympathetic nervous system are an integral part in the development of insulin resistance. To clarify whether catecholamines might exert their insulin resistance-inducing effects at least partly via downregulation of adiponectin gene expression, 3T3-L1 adipocytes were treated with isoproterenol, and adiponectin mRNA was measured by quantitative real-time reverse transcription-polymerase chain reaction. In fact, isoproterenol treatment reduced the level of adiponectin mRNA by about 75% in a dose-dependent fashion with significant inhibition detectable at concentrations as low as 10 nM isoproterenol. Furthermore, the inhibitory effect of isoproterenol was almost completely reversed by pretreatment of 3T3-L1 cells with the beta-adrenergic antagonist propranolol and the protein kinase A (PKA) inhibitor H-89. Moreover, the effects of isoproterenol could be mimicked by stimulation of stimulatory guanine nucleotide-binding (G(S))-proteins with cholera toxin and adenylyl cyclase with forskolin. Thus, our results suggest that adiponectin gene expression is severely suppressed by beta-adrenergic agents via activation of a G(S)-protein-PKA-dependent pathway. The data support a possible role of adiponectin in catecholamine-induced insulin resistance.

Isoproterenol inhibits resistin gene expression through a G(S)-protein-coupled pathway in 3T3-L1 adipocytes

Resistin was recently identified as a hormone secreted by adipocytes which leads to insulin resistance in vivo and in vitro and might therefore be an important link between obesity and diabetes. To clarify the regulation of resistin gene expression, 3T3-L1 adipocytes were treated with various agents known to modulate insulin sensitivity, and resistin mRNA was measured by quantitative real-time reverse transcription-polymerase chain reaction. Interestingly, isoproterenol treatment reduced the level of resistin mRNA to 20% of non-treated control cells. This effect was dose-dependent with significant inhibition occurring at concentrations as low as 10 nM isoproterenol. Moreover, pretreatment of adipocytes with the beta-adrenergic antagonist propranolol almost completely reversed the inhibitory effect of isoproterenol, whereas addition of the alpha-adrenergic antagonist phentolamine did not have any effect. Furthermore, the effect of isoproterenol could be mimicked by activation of G(S)-proteins and adenylyl cyclase. Thus, both cholera toxin and forskolin decreased resistin mRNA expression in a dose-dependent fashion by up to 90% of control levels. Taken together, these results suggest that resistin gene expression is regulated by a protein kinase A-dependent pathway in 3T3-L1 adipocytes.

Essential role of insulin receptor substrate 1 in differentiation of brown adipocytes

The most widely distributed members of the family of insulin receptor substrate (IRS) proteins are IRS-1 and IRS-2. These proteins participate in insulin and insulin-like growth factor 1 signaling, as well as the actions of some cytokines, growth hormone, and prolactin. To more precisely define the specific role of IRS-1 in adipocyte biology, we established brown adipocyte cell lines from wild-type and IRS-1 knockout (KO) animals. Using differentiation protocols, both with and without insulin, preadipocyte cell lines derived from IRS-1 KO mice exhibited a marked decrease in differentiation and lipid accumulation (10 to 40%) compared to wild-type cells (90 to 100%). Furthermore, IRS-1 KO cells showed decreased expression of adipogenic marker proteins, such as peroxisome proliferator-activated receptor gamma (PPARgamma), CCAAT/enhancer-binding protein alpha (C/EBPalpha), fatty acid synthase, uncoupling protein-1, and glucose transporter 4. The differentiation deficit in the KO cells could be reversed almost completely by retrovirus-mediated reexpression of IRS-1, PPARgamma, or C/EBPalpha but not the thiazolidinedione troglitazone. Phosphatidylinositol 3-kinase (PI 3-kinase) assays performed at various stages of the differentiation process revealed a strong and transient activation in IRS-1, IRS-2, and phosphotyrosine-associated PI 3-kinase in the wild-type cells, whereas the IRS-1 KO cells showed impaired phosphotyrosine-associated PI 3-kinase activation, all of which was associated with IRS-2. Akt phosphorylation was reduced in parallel with the total PI 3-kinase activity. Inhibition of PI 3-kinase with LY294002 blocked differentiation of wild-type cells. Thus, IRS-1 appears to be an important mediator of brown adipocyte maturation. Furthermore, this signaling molecule appears to exert its unique role in the differentiation process via activation of PI 3-kinase and its downstream target, Akt, and is upstream of the effects of PPARgamma and C/EBPalpha.

Essential role of insulin receptor substrate-2 in insulin stimulation of Glut4 translocation and glucose uptake in brown adipocytes

Insulin and insulin-like growth factor I signals are mediated via phosphorylation of a family of insulin receptor substrate (IRS) proteins, which may serve both complementary and overlapping functions in the cell. To study the metabolic effects of these proteins in more detail, we established brown adipocyte cell lines from wild type and various IRS knockout (KO) animals and characterized insulin action in these cells in vitro. Preadipocytes derived from both wild type and IRS-2 KO mice could be fully differentiated into mature brown adipocytes. In differentiated IRS-2 KO adipocytes, insulin-induced glucose uptake was decreased by 50% compared with their wild type counterparts. This was the result of a decrease in insulin-stimulated Glut4 translocation to the plasma membrane. This decrease in insulin-induced glucose uptake could be partially reconstituted in these cells by retrovirus-mediated re-expression of IRS-2, but not overexpression of IRS-1. Insulin signaling studies revealed a total loss of IRS-2-associated phosphatidylinositol (PI) 3-kinase activity and a reduction in phosphotyrosine-associated PI 3-kinase by 30% (p < 0.05) in the KO cells. The phosphorylation and activity of Akt, a major downstream effector of PI 3-kinase, as well as Akt-dependent phosphorylation of glycogen synthase kinase-3 and p70S6 kinase were not affected by the lack of IRS-2; however, there was a decrease in insulin stimulation of Akt associated with the plasma membrane. These results provide evidence for a critical role of IRS-2 as a mediator of insulin-stimulated Glut4 translocation and glucose uptake in adipocytes. This occurs without effects in differentiation, total activation of Akt and its downstream effectors, but may be caused by alterations in compartmentalization of these downstream signals.

Insulin and the beta3-adrenoceptor differentially regulate uncoupling protein-1 expression

Cross-talk between insulin and the adrenergic system is important in the regulation of energy homeostasis. In cultured, differentiated mouse brown adipocytes, beta3-adrenergic stimulation induced a 4.5-fold increase in uncoupling protein-1 (UCP-1) expression, which was diminished by 25% in the presence of insulin. Beta3-adrenergic stimulation also activated mitogen-activated protein (MAP) kinase by 3.5-fold and caused a decrease in basal phosphoinositide (PI) 3-kinase activity detected in p110gamma- and Gbeta-subunit-immunoprecipitates in a time-dependent manner, whereas insulin stimulated p110alpha- and phosphotyrosine-associated PI 3-kinase activity. Inhibition of MAP kinase or PI 3-kinase potentiated the beta3-adrenergic effect on UCP-1 expression, both alone and in the presence of insulin. Thus, insulin inhibits beta3-adrenergic stimulation of UCP-1, and both MAP kinase and PI 3-kinase are negative regulatory elements in the beta3-adrenergic control of UCP-1 expression. Cross-talk between the adrenergic and insulin signaling systems and impaired regulation of UCP-1 might contribute to the development of a reduced energy balance, resulting in obesity and insulin resistance.

beta(3)-adrenergic stimulation differentially inhibits insulin signaling and decreases insulin-induced glucose uptake in brown adipocytes

Activity of the sympathetic nervous system is an important factor involved in the pathogenesis of insulin resistance and associated metabolic and vascular abnormalities. In this study, we investigate the molecular basis of cross-talk between beta(3)-adrenergic and insulin signaling systems in mouse brown adipocytes immortalized by SV40 T infection. Insulin-induced tyrosine phosphorylation of the insulin receptor, insulin receptor substrate 1 (IRS-1), and IRS-2 was reduced by prestimulation of beta(3)-adrenergic receptors (CL316243). Similarly, insulin-induced IRS-1-associated and phosphotyrosine-associated phosphatidylinositol 3-kinase (PI 3-kinase) activity, but not IRS-2-associated PI 3-kinase activity, was reduced by beta(3)-adrenergic prestimulation. Furthermore, insulin-stimulated activation of Akt, but not mitogen-activated protein kinase, was diminished. Insulin-induced glucose uptake was completely inhibited by beta(3)-adrenergic prestimulation. These effects appear to be protein kinase A-dependent. Furthermore inhibition of protein kinase C restored the beta(3)-receptor-mediated reductions in insulin-induced IRS-1 tyrosine phosphorylation and IRS-1-associated PI 3-kinase activity. Together, these findings indicate cross-talk between adrenergic and insulin signaling pathways. This interaction is protein kinase A-dependent and, at least in part, protein kinase C-dependent, and could play an important role in the pathogenesis of insulin resistance associated with sympathetic overactivity and regulation of brown fat metabolism.

Synthesis of proto-oncogene proteins and cyclins depends on intact microfilaments

It is well established that microfilament disintegration by cytochalasin D (CD) as well as latrunculin (LAT)-A and LAT-B causes an inhibition of S phase entry of various nontransformed cell lines. Our experiments extended these observations to human embryonal diploid fibroblasts (Wi-38). To investigate the question whether this stop of DNA synthesis is due to a decline of the synthesis of proteins that are necessary for G1 progression and S phase entry, we examined the expression of two proto-oncogenes (c-fos, c-jun) and three cyclins (D1, E, A) after altering the microfilament system. Disintegration of microfilaments by CD, LAT-A, or LAT-B of asynchronously growing fibroblasts caused a strong dose-dependent and time-dependent inhibition of total protein synthesis. Expression of c-jun, cyclins D1, E, and A decreased by about the same percentage as total protein synthesis. The strong induction of total protein synthesis after reactivating serum-starved fibroblasts by adding fetal calf serum was suppressed, when CD or LAT-A were added to the culture medium during this reactivation process. While expression of cyclin E as well as cyclin A decreased by about the same percentage as total protein synthesis, cyclin D1 was more suppressed after microfilament disintegration. After reactivating growth-arrested Wi-38 fibroblasts, cultured in suspension for 12 h, by transferring them to a rigid substratum they could adhere to, total protein synthesis was strongly induced. Again alteration of microfilaments by CD suppressed that increase. The expression of cyclin D1 was slightly more suppressed than total protein synthesis after addition of CD during that reactivation process. Our results suggest that alteration of microfilaments causes a strong decline of total protein synthesis accompanied by a decrease of the expression of proteins that are required for G1 progression and S phase entry. The diminished presence of proteins that are important for cell cycle progression could explain the inhibition of DNA synthesis after microfilament disintegration by various drugs.