Adrenalectomy improves diabetes in A-ZIP/F-1 lipoatrophic mice by increasing both liver and muscle insulin sensitivity

The interplay between leptin, glucocorticoids, and GLP1 regulates food intake and feeding behaviour

Nutritional, endocrine, and neurological signals converge in multiple brain centres to control feeding behaviour and food intake as part of the allostatic regulation of energy balance. Among the several neuroendocrine systems involved, the leptin, glucocorticoid, and glucagon-like peptide 1 (GLP1) systems have been extensively researched. Leptin is at the top hierarchical level since its complete absence is sufficient to trigger severe hyperphagia. Glucocorticoids are key regulators of the energy balance adaptation to stress and their sustained excess leads to excessive adiposity and metabolic perturbations. GLP1 participates in metabolic adaptation to food intake, regulating insulin secretion and satiety by parallel central and peripheral signalling systems. Herein, we review the brain and peripheral targets of these three hormone systems that integrate to regulate food intake, feeding behaviour, and metabolic homeostasis. We examine the functional relationships between leptin, glucocorticoids, and GLP1 at the central and peripheral levels, including the cross-regulation of their circulating levels and their cooperative or antagonistic actions at different brain centres. The pathophysiological roles of these neuroendocrine systems in dysregulated intake are explored in the two extremes of body adiposity - obesity and lipodystrophy - and eating behaviour disorders.

Across-species benefits of adrenalectomy on congenital generalized lipoatrophic diabetes: a review

Two adrenalectomies py -45erformed fourteen years apart notoriously alleviated insulin resistance in a female teenager with Congenital Generalized Lipoatrophy (CGL, 1988) and in a murine model of CGL (2002). Following a successful therapeutic trial with anti-glucocorticoids, we performed the first surgical procedure on an 18-year-old girl. Before surgery, the anti-glucocorticoid therapy produced a rapid and striking drop in fasting serum insulin levels (from over 400 to 7.0 mU/L) and a slower -but impressive- fall in fasting serum triglycerides from 7,400 to 220-230 mg/dL. In contrast, fasting serum glucose levels dropped more slowly, from 225-290 to 121-138 mg/dL. Two weeks following total adrenalectomy, the fasting serum glucose level was 98 mg/dL, with a corresponding serum insulin level of 10 mU/L. During an Oral Glucose Tolerance Test, the 2-hour serum glucose was 210 mg/dL, and serum insulin values during the test did not exceed 53 mU/L. In 2002, the A-ZIP/F1 hypoleptinemic mouse had its adrenal glands removed. Even though this CGL model does not respond well to leptin replacement, an infusion of recombinant leptin reduced the characteristic hypercorticosteronemia of this murine model of CGL. Adrenalectomy in this transgenic mouse improved insulin sensitivity in the liver and muscle. In summary, adrenalectomy -in both a human and a mouse case of CGL- limited adipose tissue exposure to corticosteroid action and led to a notorious metabolic improvement. On a broader scenario, given that leptin restrains the adrenal axis, the reduced leptin activity of the leptin resistance displayed by obese subjects should lead to adrenal axis overactivity. This overactivity should result in elevated serum levels of free cortisol, free fatty acids, and glycerol. In this manner, leptin resistance should lead to peripheral (adipose tissue, liver, and muscle) insulin resistance and islet beta-cell apoptosis, paving the way to Type 2 diabetes.

Mouse Embryonic Fibroblasts Protect ob/ob Mice From Obesity and Metabolic Complications

The global obesity epidemic is fueling alarming rates of diabetes, associated with increased risk of cardiovascular disease and cancer. Leptin is a hormone secreted by adipose tissue that is a key regulator of body weight (BW) and energy expenditure. Leptin-deficient humans and mice are obese, diabetic, and infertile and have hepatic steatosis. Although leptin replacement therapy can alleviate the pathologies seen in leptin-deficient patients and mouse models, treatment is costly and requires daily injections. Because adipocytes are the source of leptin secretion, we investigated whether mouse embryonic fibroblasts (MEFs), capable of forming adipocytes, could be injected into ob/ob mice and prevent the metabolic phenotype seen in these leptin-deficient mice. We performed a single subcutaneous injection of MEFs into leptin-deficient ob/ob mice. The MEF injection formed a single fat pad that is histologically similar to white adipose tissue. The ob/ob mice receiving MEFs (obRs) had significantly lower BW compared with nontreated ob/ob mice, primarily because of decreased adipose tissue mass. Additionally, obR mice had significantly less liver steatosis and greater glucose tolerance and insulin sensitivity. obR mice also manifested lower food intake and greater energy expenditure than ob/ob mice, providing a mechanism underlying their metabolic improvement. Furthermore, obRs have sustained metabolic protection and restoration of fertility. Collectively, our studies show the importance of functional adipocytes in preventing metabolic abnormalities seen in leptin deficiency and point to the possibility of cell-based therapies for the treatment of leptin-deficient states.

Regulation of Glucose Homeostasis by Glucocorticoids

Glucocorticoids are steroid hormones that regulate multiple aspects of glucose homeostasis. Glucocorticoids promote gluconeogenesis in liver, whereas in skeletal muscle and white adipose tissue they decrease glucose uptake and utilization by antagonizing insulin response. Therefore, excess glucocorticoid exposure causes hyperglycemia and insulin resistance. Glucocorticoids also regulate glycogen metabolism. In liver, glucocorticoids increase glycogen storage, whereas in skeletal muscle they play a permissive role for catecholamine-induced glycogenolysis and/or inhibit insulin-stimulated glycogen synthesis. Moreover, glucocorticoids modulate the function of pancreatic α and β cells to regulate the secretion of glucagon and insulin, two hormones that play a pivotal role in the regulation of blood glucose levels. Overall, the major glucocorticoid effect on glucose homeostasis is to preserve plasma glucose for brain during stress, as transiently raising blood glucose is important to promote maximal brain function. In this chapter we will discuss the current understanding of the mechanisms underlying different aspects of glucocorticoid-regulated mammalian glucose homeostasis.

Changes in FGF21 serum concentrations and liver mRNA expression in an experimental model of complete lipodystrophy and insulin-resistant diabetes

Patients with obesity and type 2 diabetes often display high levels of the anti-diabetic factor fibroblast growth factor-21 (FGF21), suggesting that the overproduction of FGF21 may result from increased adiposity in an attempt by white adipose tissue (WAT) to counteract insulin resistance. However, the production of FGF21 diabetes in the absence of WAT has not been examined. In this study, we investigated the effects of lipodystrophy in A-ZIP F-1 mice on FGF21 production in relation to diabetes. A-ZIP F-1 mice displayed high FGF21 plasma levels resulting from enhanced FGF21 mRNA expression in the liver. Concomitant enhancement of FGF21 receptor (FGFR1) and glucose transporter 1 (GLUT-1) mRNA expression was observed in the muscles of A-ZIP F-1 mice. Furthermore, the activation of hypothalamic NPY and AgRP mRNA expression positively correlated with plasma levels of FGF21 but not active ghrelin. Our study demonstrates that an increased FGF21 plasma level in lipodystrophic A-ZIP F-1 mice results mainly from up-regulated liver production but does not suffice to overcome the lipodystrophy-induced severe type 2-diabetes and insulin resistance in the liver linked to the augmented liver fat deposition.

Leptin ameliorates insulin resistance and hepatic steatosis in Agpat2-/- lipodystrophic mice independent of hepatocyte leptin receptors

Leptin is essential for energy homeostasis and regulation of food intake. Patients with congenital generalized lipodystrophy (CGL) due to mutations in 1-acylglycerol-3-phosphate-O-acyltransferase 2 (AGPAT2) and the CGL murine model (Agpat2(-/-) mice) both have severe insulin resistance, diabetes mellitus, hepatic steatosis, and low plasma leptin levels. In this study, we show that continuous leptin treatment of Agpat2(-/-) mice for 28 days reduced plasma insulin and glucose levels and normalized hepatic steatosis and hypertriglyceridemia. Leptin also partially, but significantly, reversed the low plasma thyroxine and high corticosterone levels found in Agpat2(-/-) mice. Levels of carbohydrate response element binding protein (ChREBP) were reduced, whereas lipogenic gene expression were increased in the livers of Agpat2(-/-) mice, suggesting that deregulated ChREBP contributed to the development of fatty livers in these mice and that this transcription factor is a target of leptin's beneficial metabolic action. Leptin administration did not change hepatic fatty acid oxidation enzymes mRNA levels in Agpat2(-/-) mice. The selective deletion of leptin receptors only in hepatocytes did not prevent the positive metabolic actions of leptin in Agpat2(-/-) mice, supporting the notion that the majority of metabolic actions of leptin are dependent on its action in nonhepatocyte cells and/or the central nervous system.

Metabolic functions of glucocorticoid receptor in skeletal muscle

Glucocorticoids (GCs) exert key metabolic influences on skeletal muscle. GCs increase protein degradation and decrease protein synthesis. The released amino acids are mobilized from skeletal muscle to liver, where they serve as substrates for hepatic gluconeogenesis. This metabolic response is critical for mammals' survival under stressful conditions, such as fasting and starvation. GCs suppress insulin-stimulated glucose uptake and utilization and glycogen synthesis, and play a permissive role for catecholamine-induced glycogenolysis, thus preserving the level of circulating glucose, the major energy source for the brain. However, chronic or excess exposure of GCs can induce muscle atrophy and insulin resistance. GCs convey their signal mainly through the intracellular glucocorticoid receptor (GR). While GR can act through different mechanisms, one of its major actions is to regulate the transcription of its primary target genes through genomic glucocorticoid response elements (GREs) by directly binding to DNA or tethering onto other DNA-binding transcription factors. These GR primary targets trigger physiological and pathological responses of GCs. Much progress has been made to understand how GCs regulate protein and glucose metabolism. In this review, we will discuss how GR primary target genes confer metabolic functions of GCs, and the mechanisms governing the transcriptional regulation of these targets. Comprehending these processes not only contributes to the fundamental understanding of mammalian physiology, but also will provide invaluable insight for improved GC therapeutics.

Effects of adrenal hormones on the expression of adiponectin and adiponectin receptors in adipose tissue, muscle and liver

BACKGROUND

Adiponectin, an insulin-sensitive hormone that is primarily synthesized in adipose tissue, exerts its effects by binding to two receptors, adipoR1 and adipoR2. Little is known regarding the effects of glucocorticoids on the expression of adiponectin receptors.

METHODS

Male Wistar rats were bilaterally adrenalectomized and treated with dexamethasone (0.2 mg/100 g) twice daily for 3 days. To analyze the potential effects of glucocorticoids, rats received two daily injections of the glucocorticoid receptor antagonist (RU-486, 5.0 mg) over the course of 3 days. Additionally, 3T3-L1 adipocytes and C2C12 myotubes were treated with dexamethasone, adrenaline or RU-486. The gene expression of adiponectin, adipoR1 and adipoR2 was determined by real-time PCR, and protein secretion was examined by Western blotting using lysates from retroperitoneal, epididymal and subcutaneous adipose tissue depots, liver and muscle.

RESULTS

In rats, excess glucocorticoids increased the levels of insulin in serum and decreased serum adiponectin concentrations, whereas adrenalectomy decreased the mRNA expression of adiponectin (3-fold) and adipoR2 (7-fold) in epididymal adipose tissue and increased adipoR2 gene expression in muscle (3-fold) compared to control group sham-operated. Dexamethasone treatment did not reverse the effects of adrenalectomy, and glucocorticoid receptor blockade did not reproduce the effects of adrenalectomy. In 3T3-L1 adipocytes, dexamethasone and adrenaline both increased adipoR2 mRNA levels, but RU-486 reduced adipoR2 gene expression in vitro.

CONCLUSION

Dexamethasone treatment induces a state of insulin resistance but does not affect adiponectin receptor expression in adipose tissue. However, the effects of catecholamines on insulin resistance may be due to their effects on adipoR2.

High-fat diet and glucocorticoid treatment cause hyperglycemia associated with adiponectin receptor alterations

BACKGROUND

Adiponectin is the most abundant plasma protein synthesized for the most part in adipose tissue, and it is an insulin-sensitive hormone, playing a central role in glucose and lipid metabolism. In addition, it increases fatty acid oxidation in the muscle and potentiates insulin inhibition of hepatic gluconeogenesis. Two adiponectin receptors have been identified: AdipoR1 is the major receptor expressed in skeletal muscle, whereas AdipoR2 is mainly expressed in liver. Consumption of high levels of dietary fat is thought to be a major factor in the promotion of obesity and insulin resistance. Excessive levels of cortisol are characterized by the symptoms of abdominal obesity, hypertension, glucose intolerance or diabetes and dyslipidemia; of note, all of these features are shared by the condition of insulin resistance. Although it has been shown that glucocorticoids inhibit adiponectin expression in vitro and in vivo, little is known about the regulation of adiponectin receptors. The link between glucocorticoids and insulin resistance may involve the adiponectin receptors and adrenalectomy might play a role not only in regulate expression and secretion of adiponectin, as well regulate the respective receptors in several tissues.

RESULTS

Feeding of a high-fat diet increased serum glucose levels and decreased adiponectin and adipoR2 mRNA expression in subcutaneous and retroperitoneal adipose tissues, respectively. Moreover, it increased both adipoR1 and adipoR2 mRNA levels in muscle and adipoR2 protein levels in liver. Adrenalectomy combined with the synthetic glucocorticoid dexamethasone treatment resulted in increased glucose and insulin levels, decreased serum adiponectin levels, reduced adiponectin mRNA in epididymal adipose tissue, reduction of adipoR2 mRNA by 7-fold in muscle and reduced adipoR1 and adipoR2 protein levels in muscle. Adrenalectomy alone increased adiponectin mRNA expression 3-fold in subcutaneous adipose tissue and reduced adipoR2 mRNA expression 2-fold in liver.

CONCLUSION

Hyperglycemia as a result of a high-fat diet is associated with an increase in the expression of the adiponectin receptors in muscle. An excess of glucocorticoids, rather than their absence, increase glucose and insulin and decrease adiponectin levels.

Standard operating procedures for describing and performing metabolic tests of glucose homeostasis in mice

The Mouse Metabolic Phenotyping Center (MMPC) Consortium was established to address the need to characterize the growing number of mouse models of metabolic diseases, particularly diabetes and obesity. A goal of the MMPC Consortium is to propose standard methods for assessing metabolic phenotypes in mice. In this article, we discuss issues pertaining to the design and performance of various tests of glucose metabolism. We also propose guidelines for the description of methods, presentation of data and interpretation of results. The recommendations presented in this article are based on the experience of the MMPC Consortium and other investigators.

An adipose tissue-independent insulin-sensitizing action of telmisartan: a study in lipodystrophic mice

Adipose tissue plays an important role in energy balance and metabolism and is the major target for insulin-sensitizing peroxisome proliferator-activated receptor (PPAR) gamma agonists. The angiotensin II type 1 receptor blocker telmisartan, a partial agonist of PPAR-gamma, has been demonstrated to improve insulin sensitivity. However, there is uncertainty about the sites of its action. Here, we demonstrate that treatment with telmisartan (3 mg/kg p.o.) for 7 weeks decreased plasma glucose levels in oral glucose and insulin tolerance tests and the index of the homeostasis model assessment of insulin resistance in A-ZIP/F-1 transgenic mice, an animal model of lipodystrophy. These effects were accompanied by decreases in circulating triglyceride and fatty acid levels. However, this treatment did not affect body weight and plasma adiponectin, leptin, and corticosterone levels. In A-ZIP/F-1 mouse liver the transcripts encoding PPAR-gamma and its downstream lipogenic genes were highly up-regulated, consistent with increased hepatic triglyceride content and lipid droplet accumulation. Telmisartan reversed these effects and also down-regulated mRNAs encoding gluconeogenic genes. Thus, the present findings are consistent with a novel mode of insulin-sensitizing action of telmisartan, involving an adipose tissue-independent pathway. Telmisartan-elicited down-regulation of hepatic expression of PPAR-gamma-regulated lipogenic genes is associated with amelioration of fatty liver.

A cAMP-specific phosphodiesterase (PDE8B) that is mutated in adrenal hyperplasia is expressed widely in human and mouse tissues: a novel PDE8B isoform in human adrenal cortex

Bilateral adrenocortical hyperplasia (BAH) is the second most common cause of corticotropin-independent Cushing syndrome (CS). Genetic forms of BAH have been associated with complex syndromes such as Carney Complex and McCune-Albright syndrome or may present as isolated micronodular adrenocortical disease (iMAD) usually in children and young adults with CS. A genome-wide association study identified inactivating phosphodiesterase (PDE) 11A (PDE11A)-sequencing defects as low-penetrance predisposing factors for iMAD and related abnormalities; we also described a mutation (c.914A > C/H305P) in cyclic AMP (cAMP)-specific PDE8B, in a patient with iMAD. In this study we further characterize this mutation; we also found a novel PDE8B isoform that is highly expressed in the adrenal gland. This mutation is shown to significantly affect the ability of the protein to degrade cAMP in vitro. Tumor tissues from patients with iMAD and no mutations in the coding PDE8B sequence or any other related genes (PRKAR1A, PDE11A) showed downregulated PDE8B expression (compared to normal adrenal cortex). Pde8b is detectable in the adrenal gland of newborn mice and is widely expressed in other mouse tissues. We conclude that PDE8B is another PDE gene linked to iMAD; it is a candidate causative gene for other adrenocortical lesions linked to the cAMP signaling pathway and possibly for tumors in other tissues.

Adrenocortical changes and arterial hypertension in lipoatrophic A-ZIP/F-1 mice

The A-ZIP/F-1 transgenic mouse is a model of lipoatrophic diabetes with severe insulin resistance, hyperglycemia and hyperlipidemia. Recently, a regulatory role of adipose tissue on adrenal gland function and blood pressure has been suggested. To further explore the importance of adipose tissue in the regulation of adrenal function and blood pressure, we studied this mouse model of lipodystrophy. A-ZIP/F-1 mice exhibit significantly elevated systolic and diastolic blood pressure values despite lack of white adipose tissue and its hormones. Furthermore, A-ZIP/F-1 lipoatrophic mice have a significant reduction of adrenal zona glomerulosa, while plasma aldosterone levels and aldosterone synthase mRNA expression remain unchanged. On the other hand, lipoatrophic mice present elevated corticosterone levels but no adrenocortical hyperplasia. Ultrastructural analysis of adrenal gland show significant alterations in adrenocortical cells, with conformational changes of mitochondrial internal membranes and high amounts of liposomes. In conclusion, lipodystrophy in A-ZIP/F-1 mice is associated with hypertension, possibly due to hypercorticosteronemia and/or others metabolic-vascular changes.

Insulin hypersensitivity in mice lacking the V1b vasopressin receptor

We have reported that [Arg(8)]-vasopressin-stimulated insulin release is blunted in islet cells isolated from V1b receptor-deficient (V1bR(-/-)) mice. In this study, we used V1bR(-/-) mice to examine the physiological role of the V1b receptor in regulating blood glucose levels in vivo, and we found that the fasting plasma glucose, insulin and glucagon levels were lower in V1bR(-/-) mice than in wild-type (V1bR(+/+)) mice. Next, we evaluated glucose tolerance by performing an intraperitoneal glucose tolerance test (GTT). The plasma glucose and insulin levels during the GTT were lower in V1bR(-/-) mice than in V1bR(+/+) mice. An insulin tolerance test (ITT) revealed that, after insulin administration, plasma glucose levels were lower in V1bR(-/-) mice than in V1bR(+/+) mice. In addition, a hyperinsulinaemic-euglycaemic clamp study showed that the glucose infusion rate was increased in V1bR(-/-) mice, indicating that insulin sensitivity was enhanced at the in vivo level in V1bR(-/-) mice. Furthermore, we found that the V1b receptor was expressed in white adipose tissue and that insulin-stimulated phosphorylation of Akt as an important signaling molecule was increased in adipocytes isolated from V1bR(-/-) mice. Thus, the blockade of the V1b receptor could result, at least in part, in enhanced insulin sensitivity by altering insulin signalling in adipocytes.

Transplantation of wild-type white adipose tissue normalizes metabolic, immune and inflammatory alterations in leptin-deficient ob/ob mice

Leptin-deficient ob/ob mice exhibit several metabolic and immune abnormalities, including thymus atrophy and markedly reduced inflammatory responses. We evaluated whether transplantation of wild-type (WT) white adipose tissue (WAT) into ob/ob mice could mimic the effect of recombinant leptin administration in normalizing metabolic, immune and inflammatory abnormalities. Female ob/ob mice received a subcutaneous transplantation of WAT obtained from WT littermates. A separate group of ob/ob mice was sham-operated. Despite raising leptin levels to only 15% of those observed in WT mice, WAT transplantation normalized metabolic abnormalities (glycemia, ALT, liver weight) in ob/ob mice and prevented further body weight gain. The transplanted group demonstrated normalization of thymus and spleen cellularity, thymocyte subpopulations and rates of thymocyte apoptosis. In the model of dextran sulfate sodium-induced colitis, WAT transplantation restored inflammation to levels equivalent to those of WT mice. Colonic production of IL-6 and MIP-2 was markedly reduced in the non-transplanted ob/ob group compared to transplanted ob/ob and WT mice. Our data indicate that WAT transplantation is an effective way to normalize metabolic as well as immune and inflammatory parameters in ob/ob mice. The threshold of leptin sufficient to normalize metabolic, immune and inflammatory function is significantly lower than levels present in lean WT mice. Finally, leptin derived exclusively from WAT is sufficient to normalize metabolic, immune and inflammatory parameters in ob/ob mice.

Alterations in regulation of energy homeostasis in cyclic nucleotide phosphodiesterase 3B-null mice

Cyclic nucleotide phosphodiesterase 3B (PDE3B) has been suggested to be critical for mediating insulin/IGF-1 inhibition of cAMP signaling in adipocytes, liver, and pancreatic beta cells. In Pde3b-KO adipocytes we found decreased adipocyte size, unchanged insulin-stimulated phosphorylation of protein kinase B and activation of glucose uptake, enhanced catecholamine-stimulated lipolysis and insulin-stimulated lipogenesis, and blocked insulin inhibition of catecholamine-stimulated lipolysis. Glucose, alone or in combination with glucagon-like peptide-1, increased insulin secretion more in isolated pancreatic KO islets, although islet size and morphology and immunoreactive insulin and glucagon levels were unchanged. The beta(3)-adrenergic agonist CL 316,243 (CL) increased lipolysis and serum insulin more in KO mice, but blood glucose reduction was less in CL-treated KO mice. Insulin resistance was observed in KO mice, with liver an important site of alterations in insulin-sensitive glucose production. In KO mice, liver triglyceride and cAMP contents were increased, and the liver content and phosphorylation states of several insulin signaling, gluconeogenic, and inflammation- and stress-related components were altered. Thus, PDE3B may be important in regulating certain cAMP signaling pathways, including lipolysis, insulin-induced antilipolysis, and cAMP-mediated insulin secretion. Altered expression and/or regulation of PDE3B may contribute to metabolic dysregulation, including systemic insulin resistance.

Increased subcutaneous and epicardial adipose tissue production of proinflammatory cytokines in cardiac surgery patients: possible role in postoperative insulin resistance

CONTEXT

Hyperglycemia and insulin resistance frequently occur in critically ill patients even without a history of diabetes.

OBJECTIVE

Our objective was to study the role of adipose tissue hormonal production in the development of insulin resistance in cardiac surgery patients. PARTICIPANTS, INTERVENTIONS, AND SETTINGS: Fifteen patients with elective cardiac surgery underwent blood sampling before, at the end, and 6, 12, 24, 48, and 120 h after the end of their operation. Epicardial and sc adipose tissue sampling was done at the beginning and at the end of surgery in the Department of Cardiac Surgery.

MAIN OUTCOME MEASURES

We measured serum concentrations and sc and epicardial adipose tissue mRNA expression of IL-6, monocyte chemoattractant protein-1 (MCP-1), TNF-alpha, leptin, resistin, and adiponectin and sc and epicardial adipose tissue mRNA expression of CD14, CD45, and CD68.

RESULTS

The rate of insulin infusion required to maintain euglycemia increased up to 7-fold 12 h after the operation, suggesting the development of insulin resistance. Serum IL-6 levels increased 43-fold 12 h after surgery. MCP-1 peaked 6-fold at the end of surgery. Smaller peaks of TNF-alpha and leptin appeared 6 and 12 h after surgery, respectively. Resistin levels peaked 4-fold 24 h after surgery, but adiponectin levels were not significantly affected. TNF-alpha and CD45 mRNA expression increased markedly during the operation in sc adipose tissue. IL-6, resistin, and MCP-1 mRNA expression increased in both sc and epicardial adipose tissue. Leptin, adiponectin, CD14, and CD68 mRNA expression did not change significantly.

CONCLUSIONS

Both sc and epicardial adipose tissue is a source of proinflammatory cytokines in cardiac surgery patients and may contribute to the development of postoperative insulin resistance.

Improvement of insulin sensitivity after peroxisome proliferator-activated receptor-alpha agonist treatment is accompanied by paradoxical increase of circulating resistin levels

We studied the effect of peroxisome proliferator-activated receptor-alpha (PPAR-alpha) activation on serum concentrations and tissue expression of resistin, adiponectin, and adiponectin receptor-1 and -2 (AdipoR1 and AdipoR2) mRNA in normal mice and mice with insulin resistance induced by lipogenic, simple-carbohydrate diet (LD). Sixteen weeks of LD feeding induced obesity with liver steatosis and increased insulin levels but did not significantly affect circulating adiponectin or resistin. Treatment with PPAR-alpha agonist fenofibrate decreased body weight and fat pad weight and ameliorated liver steatosis in LD-fed mice with concomitant reduction in blood glucose, free fatty acid, triglyceride, serum insulin levels, and homeostasis model assessment index values. Euglycemic-hyperinsulinemic clamp demonstrated the development of whole-body and liver insulin resistance in LD-fed mice, which were both normalized by fenofibrate. Fenofibrate treatment markedly increased circulating resistin levels on both diets and adiponectin levels in chow-fed mice only. Fat adiponectin mRNA expression was not affected by fenofibrate treatment. Resistin mRNA expression increased in subcutaneous but not gonadal fat after fenofibrate treatment. In addition to fat, a significant amount of adiponectin mRNA was also expressed in the muscle. This expression markedly increased after fenofibrate treatment in chow- but not in LD-fed mice. Adipose tissue expression of AdipoR1 mRNA was significantly reduced in LD-fed mice and increased after fenofibrate treatment. In conclusion, PPAR-alpha activation ameliorated the development of insulin resistance in LD-fed mice despite a major increase in serum resistin levels. This effect could be partially explained by increased AdipoR1 expression in adipose tissue after fenofibrate treatment.

Increased subcutaneous and epicardial adipose tissue production of proinflammatory cytokines in cardiac surgery patients: possible role in postoperative insulin resistance

CONTEXT

Hyperglycemia and insulin resistance frequently occur in critically ill patients even without a history of diabetes.

OBJECTIVE

Our objective was to study the role of adipose tissue hormonal production in the development of insulin resistance in cardiac surgery patients. PARTICIPANTS, INTERVENTIONS, AND SETTINGS: Fifteen patients with elective cardiac surgery underwent blood sampling before, at the end, and 6, 12, 24, 48, and 120 h after the end of their operation. Epicardial and sc adipose tissue sampling was done at the beginning and at the end of surgery in the Department of Cardiac Surgery.

MAIN OUTCOME MEASURES

We measured serum concentrations and sc and epicardial adipose tissue mRNA expression of IL-6, monocyte chemoattractant protein-1 (MCP-1), TNF-alpha, leptin, resistin, and adiponectin and sc and epicardial adipose tissue mRNA expression of CD14, CD45, and CD68.

RESULTS

The rate of insulin infusion required to maintain euglycemia increased up to 7-fold 12 h after the operation, suggesting the development of insulin resistance. Serum IL-6 levels increased 43-fold 12 h after surgery. MCP-1 peaked 6-fold at the end of surgery. Smaller peaks of TNF-alpha and leptin appeared 6 and 12 h after surgery, respectively. Resistin levels peaked 4-fold 24 h after surgery, but adiponectin levels were not significantly affected. TNF-alpha and CD45 mRNA expression increased markedly during the operation in sc adipose tissue. IL-6, resistin, and MCP-1 mRNA expression increased in both sc and epicardial adipose tissue. Leptin, adiponectin, CD14, and CD68 mRNA expression did not change significantly.

CONCLUSIONS

Both sc and epicardial adipose tissue is a source of proinflammatory cytokines in cardiac surgery patients and may contribute to the development of postoperative insulin resistance.

Hemodynamic effects of methylprednisolone acetate administration in cats

OBJECTIVE

To investigate the mechanisms by which corticosteroid administration may predispose cats to congestive heart failure (CHF).

ANIMALS

12 cats receiving methylprednisolone acetate (MPA) for the treatment of dermatologic disorders.

PROCEDURE

The study was conducted as a repeated-measures design. Various baseline variables were measured, after which MPA (5 mg/kg, IM) was administered. The same variables were then measured at 3 to 6 days and at 16 to 24 days after MPA administration. Evaluations included physical examination, systolic blood pressure measurement, hematologic analysis, serum biochemical analysis, thoracic radiography, echocardiography, and total body water and plasma volume determination.

RESULTS

MPA resulted in a substantial increase in serum glucose concentration at 3 to 6 days after administration. Concurrently, RBC count, Hct, and hemoglobin concentration as well as serum concentrations of the major extracellular electrolytes, sodium and chloride, decreased. Plasma volume increased by 13.4% (> 40% in 3 cats), whereas total body water and body weight slightly decreased. All variables returned to baseline by 16 to 24 days after MPA administration.

CONCLUSIONS AND CLINICAL RELEVANCE

These data suggest that MPA administration in cats causes plasma volume expansion as a result of an intra to extracellular fluid shift secondary to glucocorticoid-mediated extracellular hyperglycemia. This mechanism is analogous to the plasma volume expansion that accompanies uncontrolled diabetes mellitus in humans. Any cardiovascular disorders that impair the normal compensatory mechanisms for increased plasma volume may predispose cats to CHF following MPA administration.

Considerations in the design of hyperinsulinemic-euglycemic clamps in the conscious mouse

Despite increased use of the hyperinsulinemic-euglycemic clamp to study insulin action in mice, the effects of experimental parameters on the results obtained have not been addressed. In our studies, we determined the influences of sampling sites, fasting duration, and insulin delivery on results obtained from clamps in conscious mice. Carotid artery and jugular vein catheters were implanted in C57BL/6J mice (n = 6-10/group) fed a normal diet for sampling and infusions. After a 5-day recovery period, mice underwent a 120-min clamp (2.5-mU . kg(-1) . min(-1) insulin infusion; approximately 120-130 mg/dl glucose) while receiving [3-(3)H]glucose to determine glucose appearance (endoR(a)) and disappearance (R(d)). Sampling large volumes (approximately 100 mul) from the cut tail resulted in elevated catecholamines and basal glucose compared with artery sampling. Catecholamines were not elevated when taking small samples ( approximately 5 mul) from the cut tail. Overnight (18-h) fasting resulted in greater loss of total body, lean, and fat masses and hepatic glycogen but resulted in enhanced insulin sensitivity compared with 5-h fasting. Compared with a 16-mU/kg insulin prime, a 300-mU/kg prime resulted in hepatic insulin resistance and slower acquisition of steady-state glucose infusion rates (GIR) after a 5-h fast. The steady-state GIR was expedited after the 300-mU/kg prime in 18-h-fasted mice. The GIR and R(d) rose with increasing insulin infusions (0.8, 2.5, 4, and 20 mU . kg(-1) . min(-1)), but endoR(a) was fully suppressed with doses higher than 0.8 mU . kg(-1) . min(-1). Thus, common variations in experimental factors yield different results and should be considered in designing and interpreting clamps.

A polygenic model of the metabolic syndrome with reduced circulating and intra-adipose glucocorticoid action

Despite major advances in understanding monogenic causes of morbid obesity, the complex genetic and environmental etiology of idiopathic metabolic syndrome remains poorly understood. One hypothesis suggests that similarities between the metabolic disease of plasma glucocorticoid excess (Cushing's syndrome) and idiopathic metabolic syndrome results from increased glucocorticoid reamplification within adipose tissue by 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD-1). Indeed, 11beta-HSD-1 is now a major therapeutic target. Because much supporting evidence for a role of adipose 11beta-HSD-1 comes from transgenic or obese rodents with single-gene mutations, we investigated whether the predicted traits of metabolic syndrome and glucocorticoid metabolism were coassociated in a unique polygenic model of obesity developed by long-term selection for divergent fat mass (Fat and Lean mice with 23 vs. 4% fat as body weight, respectively). Fat mice exhibited an insulin-resistant metabolic syndrome including fatty liver and hypertension. Unexpectedly, Fat mice had a marked intra-adipose (11beta-HSD-1) and plasma glucocorticoid deficiency but higher liver glucocorticoid action. Furthermore, metabolic disease was exacerbated only in Fat mice when challenged with exogenous glucocorticoids or a high-fat diet. Our data suggest that idiopathic metabolic syndrome might associate with such a novel pattern of glucocorticoid action and sensitivity in humans, with implications for tissue-specific therapeutic targeting of 11beta-HSD-1.

Muscle-specific overexpression of CD36 reverses the insulin resistance and diabetes of MKR mice

Insulin resistance is one of the primary characteristics of type 2 diabetes. Mice overexpressing a dominant-negative IGF-I receptor specifically in muscle (MKR mice) demonstrate severe insulin resistance with high levels of serum and tissue lipids and eventually develop type 2 diabetes at 5-6 wk of age. To determine whether lipotoxicity plays a role in the progression of the disease, we crossed MKR mice with mice overexpressing a fatty acid translocase, CD36, in skeletal muscle. The double-transgenic MKR/CD36 mice showed normalization of the hyperglycemia and the hyperinsulinemia as well as a marked improvement in liver insulin sensitivity. The MKR/CD36 mice also exhibited normal rates of fatty acid oxidation in skeletal muscle when compared with the decreased rate of fatty acid oxidation in MKR. With the reduction in insulin resistance, beta-cell function returned to normal. These and other results suggest that the insulin resistance in the MKR mice is associated with increased muscle triglycerides levels and that whole-body insulin resistance can be, at least partially, reversed in association with a reduction in muscle triglycerides levels, although the mechanisms are yet to be determined.

Genetic background (C57BL/6J versus FVB/N) strongly influences the severity of diabetes and insulin resistance in ob/ob mice

We studied the effects of genetic background on the phenotype of ob/ob mice, a model of severe obesity, insulin resistance, and diabetes caused by leptin deficiency. Despite a comparable degree of obesity and hyperinsulinemia, C57BL/6J ob/ob mice had much milder hyperglycemia and, surprisingly, normal circulating adiponectin levels despite still-prominent signs of insulin resistance. Hyperinsulinemic-euglycemic clamp revealed relatively less whole-body and muscle insulin resistance in C57BL/6J ob/ob mice, whereas liver insulin resistance tended to be more severe than in FVB/N ob/ob mice. C57BL/6J ob/ob mice had also more rapid clearance of circulating triglycerides and more severe hepatic steatosis. We suggest that strain-related distinction in lipid handling is the most important player in the differences in diabetic phenotype and insulin sensitivity, whereas the impact of circulating adiponectin levels on the overall phenotype of ob/ob mice is less important.

Impaired coordination of nutrient intake and substrate oxidation in melanocortin-4 receptor knockout mice

Mutations in the melanocortin-4 receptor (MC4R) are associated with obesity. The obesity syndrome observed in humans with MC4R haploinsufficiency is similar to that observed in MC4R knockout mice, including increased longitudinal growth, hyperphagia, and fasting hyperinsulinemia. For comparison with other commonly investigated models of obesity and insulin resistance, we have backcrossed Mc4r-/- mice into the C57BL/6J (B6) background. Female obese Mc4r-/- mice exhibit reduced energy expenditure and an attenuated increase in fatty acid (FA) oxidation after exposure to high-fat diets compared with obese Lepob/Lepob mice. The reduced energy expenditure and FA oxidation correlates with changes in hepatic gene expression. The expression of genes involved in FA oxidation increased in obese Lepob/Lepob mice compared with wild-type and obese Mc4r-/- mice. In contrast, a key lipogenic enzyme, FA synthase (FAS), is increased in obese Mc4r-/- mice compared with obese Lepob/Lepob mice. Hyperinsulinemia, increased FAS mRNA expression and hepatic steatosis appear to be secondary to obesity in B6 Mc4r-/- mice. However, Mc4r-/- mice in a mixed genetic background develop severe hepatic steatosis at an early age. This might suggest an important role of the MC4R in regulating liver FA metabolism that is masked on the B6 background. Interestingly, the 10- to 20-fold increase in liver triglyceride in the outbred strain of Mc4r-/- mice is not always associated with fasting hyperinsulinemia or increased FAS mRNA expression. This observation suggests that changes in liver secondary to triglyceride accumulation lead to hyperinsulinemia and increased hepatic FAS expression in Mc4r-/- mice.

Insulin resistance in the liver-specific IGF-1 gene-deleted mouse is abrogated by deletion of the acid-labile subunit of the IGF-binding protein-3 complex: relative roles of growth hormone and IGF-1 in insulin resistance

Liver IGF-1 deficient (LID) mice demonstrate a 75% reduction in circulating IGF-1 levels and a corresponding fourfold increase in growth hormone (GH) levels. At 16 weeks of age, LID mice demonstrate, using the hyperinsulinemic-euglycemic clamp, insulin insensitivity in muscle, liver, and fat tissues. In contrast, mice with a gene deletion of the acid-labile subunit (ALSKO) demonstrate a 65% reduction in circulating IGF-1 levels, with normal GH levels and no signs of insulin resistance. To further clarify the relative roles of increased GH and decreased IGF-1 levels in the development of insulin resistance, we crossed the two mouse lines and created a double knockout mouse (LID+ALSKO). LID+ALSKO mice demonstrate a further reduction in circulating IGF-1 levels (85%) and a concomitant 10-fold increase in GH levels. Insulin tolerance tests showed an improvement in insulin responsiveness in the LID+ALSKO mice compared with controls; LID mice were very insulin insensitive. Surprisingly, insulin sensitivity, while improved in white adipose tissue and in muscle, was unchanged in the liver. The lack of improvement in liver insulin sensitivity may reflect the absence of IGF-1 receptors or increased triglyceride levels in the liver. The present study suggests that whereas GH plays a major role in inducing insulin resistance, IGF-1 may have a direct modulatory role.

Liver peroxisome proliferator-activated receptor gamma contributes to hepatic steatosis, triglyceride clearance, and regulation of body fat mass

Peroxisome proliferator-activated receptor gamma (PPAR gamma) is a nuclear receptor that mediates the antidiabetic effects of thiazolidinediones. PPAR gamma is present in adipose tissue and becomes elevated in fatty livers, but the roles of specific tissues in thiazolidinedione actions are unclear. We studied the function of liver PPAR gamma in both lipoatrophic A-ZIP/F-1 (AZIP) and wild type mice. In AZIP mice, ablation of liver PPAR gamma reduced the hepatic steatosis but worsened the hyperlipidemia, triglyceride clearance, and muscle insulin resistance. Inactivation of AZIP liver PPAR gamma also abolished the hypoglycemic and hypolipidemic effects of rosiglitazone, demonstrating that, in the absence of adipose tissue, the liver is a primary and major site of thiazolidinedione action. In contrast, rosiglitazone remained effective in non-lipoatrophic mice lacking liver PPAR gamma, suggesting that adipose tissue is the major site of thiazolidinedione action in typical mice with adipose tissue. Interestingly, mice without liver PPAR gamma, but with adipose tissue, developed relative fat intolerance, increased adiposity, hyperlipidemia, and insulin resistance. Thus, liver PPAR gamma regulates triglyceride homeostasis, contributing to hepatic steatosis, but protecting other tissues from triglyceride accumulation and insulin resistance.

Liver-specific disruption of PPARgamma in leptin-deficient mice improves fatty liver but aggravates diabetic phenotypes

To elucidate the function of PPARgamma in leptin-deficient mouse (ob/ob) liver, a PPARgamma liver-null mouse on an ob/ob background, ob/ob-PPARgamma(fl/fl)AlbCre(+), was produced using a floxed PPARgamma allele, PPARgamma(fl/fl), and Cre recombinase under control of the albumin promoter (AlbCre). The liver of ob/ob-PPARgamma(fl/fl)AlbCre(+) mice had a deletion of exon 2 and a corresponding loss of full-length PPARgamma mRNA and protein. The PPARgamma-deficient liver in ob/ob mice was smaller and had a dramatically decreased triglyceride (TG) content compared with equivalent mice lacking the AlbCre transgene (ob/ob-PPARgamma(fl/fl)AlbCre(-)). Messenger RNA levels of the hepatic lipogenic genes, fatty acid synthase, acetyl-CoA carboxylase, and stearoyl-CoA desaturase-1, were reduced in ob/ob-PPARgamma(fl/fl)AlbCre(+) mice, and the levels of serum TG and FFA in ob/ob-PPARgamma(fl/fl)AlbCre(+) mice were significantly higher than in the control ob/ob-PPARgamma(fl/fl)AlbCre(-) mice. Rosiglitazone treatment exacerbated the fatty liver in ob/ob-PPARgamma(fl/fl)AlbCre(-) mice compared with livers from nonobese Cre(-) mice; there was no effect of rosiglitazone in ob/ob-PPARgamma(fl/fl)AlbCre(+) mice. The deficiency of hepatic PPARgamma further aggravated the severity of diabetes in ob/ob mice due to decreased insulin sensitivity in muscle and fat. These data indicate that hepatic PPARgamma plays a critical role in the regulation of TG content and in the homeostasis of blood glucose and insulin resistance in steatotic diabetic mice.

Enhanced insulin sensitivity in mice lacking ganglioside GM3

Gangliosides are sialic acid-containing glycosphingolipids that are present on all mammalian plasma membranes where they participate in recognition and signaling activities. We have established mutant mice that lack GM3 synthase (CMP-NeuAc:lactosylceramide alpha2,3-sialyltransferase; EC 2.4.99.-). These mutant mice were unable to synthesize GM3 ganglioside, a simple and widely distributed glycosphingolipid. The mutant mice were viable and appeared without major abnormalities but showed a heightened sensitivity to insulin. A basis for the increased insulin sensitivity in the mutant mice was found to be enhanced insulin receptor phosphorylation in skeletal muscle. Importantly, the mutant mice were protected from high-fat diet-induced insulin resistance. Our results show that GM3 ganglioside is a negative regulator of insulin signaling, making it a potential therapeutic target in type 2 diabetes.

Liver-specific disruption of PPARgamma in leptin-deficient mice improves fatty liver but aggravates diabetic phenotypes

To elucidate the function of PPARgamma in leptin-deficient mouse (ob/ob) liver, a PPARgamma liver-null mouse on an ob/ob background, ob/ob-PPARgamma(fl/fl)AlbCre(+), was produced using a floxed PPARgamma allele, PPARgamma(fl/fl), and Cre recombinase under control of the albumin promoter (AlbCre). The liver of ob/ob-PPARgamma(fl/fl)AlbCre(+) mice had a deletion of exon 2 and a corresponding loss of full-length PPARgamma mRNA and protein. The PPARgamma-deficient liver in ob/ob mice was smaller and had a dramatically decreased triglyceride (TG) content compared with equivalent mice lacking the AlbCre transgene (ob/ob-PPARgamma(fl/fl)AlbCre(-)). Messenger RNA levels of the hepatic lipogenic genes, fatty acid synthase, acetyl-CoA carboxylase, and stearoyl-CoA desaturase-1, were reduced in ob/ob-PPARgamma(fl/fl)AlbCre(+) mice, and the levels of serum TG and FFA in ob/ob-PPARgamma(fl/fl)AlbCre(+) mice were significantly higher than in the control ob/ob-PPARgamma(fl/fl)AlbCre(-) mice. Rosiglitazone treatment exacerbated the fatty liver in ob/ob-PPARgamma(fl/fl)AlbCre(-) mice compared with livers from nonobese Cre(-) mice; there was no effect of rosiglitazone in ob/ob-PPARgamma(fl/fl)AlbCre(+) mice. The deficiency of hepatic PPARgamma further aggravated the severity of diabetes in ob/ob mice due to decreased insulin sensitivity in muscle and fat. These data indicate that hepatic PPARgamma plays a critical role in the regulation of TG content and in the homeostasis of blood glucose and insulin resistance in steatotic diabetic mice.

Opposite effects of background genotype on muscle and liver insulin sensitivity of lipoatrophic mice. Role of triglyceride clearance

The metabolic phenotype of the A-ZIP/F-1 (AZIP) lipoatrophic mouse is different depending on its genetic background. On both the FVB/N (FVB) and C57BL/6J (B6) backgrounds, AZIP mice have a similarly severe lack of white adipose tissue and comparably increased insulin levels and triglyceride secretion rates. However, on the B6 background, the AZIP mice have less hyperglycemia, lower circulating triglyceride and fatty acid levels, and lower mortality. AZIP characteristics that are more severe on the B6 background include increased liver size and liver triglyceride content. A unifying hypothesis is that the B6 strain has higher triglyceride clearance into the liver, with lower triglyceride levels elsewhere. This may account for the observation that the B6 AZIP mice have less insulin-resistant muscles and more insulin-resistant livers, than do the FVB AZIP mice. B6 wild type, as well as B6 AZIP, mice have increased triglyceride clearance relative to FVB, which may be explained in part by higher serum lipase levels and liver CD36/fatty acid translocase mRNA levels. Thus, it is likely that increased triglyceride clearance in B6, as compared with FVB, mice contributes to the strain differences in insulin resistance and lipid metabolism.

Adiponectin is stimulated by adrenalectomy in ob/ob mice and is highly correlated with resistin mRNA

Plasma levels of the adipocyte product adiponectin, a putative insulin-sensitizing agent, are reduced in obesity, whereas plasma levels of resistin, an agent that some believe to confer insulin resistance, are thought to increase with obesity. Because adrenalectomy can increase insulin sensitivity, we hypothesized that adrenalectomy would increase expression of adiponectin and decrease expression of resistin. Therefore, we measured adiponectin mRNA, adiponectin peptide, and resistin mRNA in adrenalectomized ob/ob mice. Adrenalectomy restored adiponectin expression in ob/ob mice to wild-type levels and stimulated adiponectin peptide to above wild-type levels. Surprisingly, expression of adiponectin and resistin was highly positively correlated even after statistical removal of effects of insulin, glucose, and adiposity. In addition, adiponectin and resistin expression were also highly correlated in diet-induced obese mice. The data support a role for adiponectin in mediating some effects of adrenalectomy on insulin sensitivity.