Hepatic leptin signalling and subdiaphragmatic vagal efferents are not required for leptin-induced increases of plasma IGF binding protein-2 (IGFBP-2) in ob/ob mice

The gut microbiota modulate locomotion via vagus-dependent glucagon-like peptide-1 signaling

Locomotor activity is an innate behavior that can be triggered by gut-motivated conditions, such as appetite and metabolic condition. Various nutrient-sensing receptors distributed in the vagal terminal in the gut are crucial for signal transduction from the gut to the brain. The levels of gut hormones are closely associated with the colonization status of the gut microbiota, suggesting a complicated interaction among gut bacteria, gut hormones, and the brain. However, the detailed mechanism underlying gut microbiota-mediated endocrine signaling in the modulation of locomotion is still unclear. Herein, we show that broad-spectrum antibiotic cocktail (ABX)-treated mice displayed hypolocomotion and elevated levels of the gut hormone glucagon-like peptide-1 (GLP-1). Blockade of the GLP-1 receptor and subdiaphragmatic vagal transmission rescued the deficient locomotor phenotype in ABX-treated mice. Activation of the GLP-1 receptor and vagal projecting brain regions led to hypolocomotion. Finally, selective antibiotic treatment dramatically increased serum GLP-1 levels and decreased locomotion. Colonizing Lactobacillus reuteri and Bacteroides thetaiotaomicron in microbiota-deficient mice suppressed GLP-1 levels and restored the hypolocomotor phenotype. Our findings identify a mechanism by which specific gut microbes mediate host motor behavior via the enteroendocrine and vagal-dependent neural pathways.

Hepatic Leptin Signaling Improves Hyperglycemia by Stimulating MAPK Phosphatase-3 Protein Degradation via STAT3

BACKGROUND & AIMS

Obesity-related hyperglycemia, with hepatic insulin resistance, has become an epidemic disease. Central neural leptin signaling was reported to improve hyperglycemia. The aim of this study was to investigate the effect of hepatic leptin signaling on controlling hyperglycemia.

METHODS

First, the effect of leptin signaling on gluconeogenesis was investigated in primary mouse hepatocytes and hepatoma cells. Second, glucose tolerance, insulin tolerance, blood glucose levels, and hepatic gluconeogenic gene expression were analyzed in obese mice overexpressing hepatic OBRb. Third, expression of mitogen-activated protein kinase phosphatase (MKP)-3, phosphorylation level of signal transducer and activator of transcription (STAT) 3, and extracellular regulated protein kinase (ERK) were analyzed in hepatocytes and mouse liver. Fourth, the role of MKP-3 in hepatic leptin signaling regulating gluconeogenesis was analyzed. Lastly, the role of ERK and STAT3 in the regulation of MKP-3 protein by leptin signaling was analyzed.

RESULTS

Activation of hepatic leptin signaling suppressed gluconeogenesis in both hepatocytes and obese mouse liver, and improved hyperglycemia, insulin tolerance, and glucose tolerance in obese mice. The protein level of MKP-3, which can promote gluconeogenesis, was decreased by leptin signaling in both hepatocytes and mouse liver. Mkp-3 deficiency abolished the effect of hepatic leptin signaling on suppressing gluconeogenesis in hepatocytes. STAT3 decreased the MKP-3 protein level, while inactivation of STAT3 abolished the effect of leptin signaling on reducing the MKP-3 protein level in hepatocytes. Moreover, STAT3 could combine with MKP-3 and phospho-ERK1/2, which induced the degradation of MKP-3, and leptin signaling enhanced the combination.

CONCLUSIONS

Hepatic leptin signaling could suppress gluconeogenesis at least partially by decreasing the MKP-3 protein level via STAT3-enhanced MKP-3 and ERK1/2 combination.

Circulating IGFBP-2 levels reveal atherogenic metabolic risk in schizophrenic patients using atypical antipsychotics

OBJECTIVES

Second generation antipsychotics (SGAs) induce weight gain and dyslipidemia, albeit with important intervariability. Insulin-like growth factor binding protein (IGFBP)-2 is proposed as a circulating biomarker negatively associated with waist circumference and hypertriglyceridemia. Thus, we tested whether metabolic alterations developed upon the use of SGAs are associated with plasma IGFBP-2 levels.

METHODS

A cross-sectional study was performed in 87 men newly diagnosed with schizophrenia and administered for approximately 20 months with olanzapine or risperidone as their first antipsychotic treatment. Plasma IGFBP-2 concentration, anthropometric data, as well as glucose and lipid profiles were determined at the end of the treatments.

RESULTS

IGFBP-2 levels were similar between patients using olanzapine or risperidone and were negatively correlated with waist circumference, insulin sensitivity, and plasma triglycerides (TG). A higher proportion of men with a hypertriglyceridemic (hyperTG) waist phenotype was found in patients with IGFBP-2 levels lower than 220 ng/mL (43% for olanzapine and 13% for risperidone) compared to those with IGFBP-2 above this threshold (10% and 0%, respectively).

CONCLUSIONS

IGFBP-2 may have a role in altering metabolic risk in schizophrenic patients using SGAs. Longitudinal studies are required to evaluate whether IGFBP-2 can predict the development of a hyperTG waist phenotype in this population.

Changes in IGFBP-2 levels following a one-year lifestyle modification program are independently related to improvements in plasma apo B and LDL apo B levels

BACKGROUND AND AIMS

Recent transversal studies have associated insulin-like growth factor binding protein (IGFBP)-2 levels with glucose tolerance and parameters of the lipoprotein-lipid profile. Here, we aimed at determining the longitudinal effects of a one-year lifestyle modification program on IGFBP-2 levels and to identify specific metabolic improvements impacted by the changes in IGFBP-2.

METHODS

99 middle-aged Caucasian men were involved in a lifestyle modification program consisting in personalized healthy eating and physical activity counseling, combined to elicit a daily 500 kcal deficit. Anthropometric and metabolic parameters as well as circulating IGFBP-2 levels were measured before and after one year of the lifestyle modification program.

RESULTS

The intervention triggered positive changes in many metabolic parameters and a 43% (p < 0.0001) increase of IGFBP-2 levels. Subjects with the most substantial increases in IGFBP-2 also experienced the most important metabolic improvements. Changes in IGFBP-2 levels (both absolute and relative) were correlated with markers of body fat distribution and lipoprotein-lipid profile, and independently associated with changes in LDL apolipoprotein (apo) B but not VLDL apo B concentrations. Further analyses showed that for similar changes in BMI, waist circumference and visceral adipose tissue volume, large changes in IGFBP-2 levels were required to observe improvements in LDL apo B levels.

CONCLUSIONS

The 1-year lifestyle modification program was associated with increased IGFBP-2 concentrations. Increases in IGFBP-2 levels were closely associated with reduced LDL apo B concentrations and independently of the modifications in fat mass and insulin sensitivity. Further mechanistic studies are required to assess the effects of IGFBP-2 levels on LDL metabolism.

Leptin contributes to the beneficial effects of insulin treatment in streptozotocin-diabetic male mice

It was long thought that the only hormone capable of reversing the catabolic consequences of diabetes was insulin. However, various studies have demonstrated that the adipocyte-derived hormone leptin can robustly lower blood glucose levels in rodent models of insulin-deficient diabetes. In addition, it has been suggested that some of the metabolic manifestations of insulin-deficient diabetes are due to hypoleptinemia as opposed to hypoinsulinemia. Because insulin therapy increases leptin levels, we sought to investigate the contribution of leptin to the beneficial effects of insulin therapy. To do this, we tested insulin therapy in streptozotocin (STZ) diabetic mice that were either on an ob/ ob background or that were given a leptin antagonist to determine if blocking leptin action would blunt the glucose-lowering effects of insulin therapy. We found that STZ diabetic ob/ ob mice have a diminished blood glucose-lowering effect in response to insulin therapy compared with STZ diabetic controls and exhibited more severe weight loss post-STZ injection. In addition, STZ diabetic mice administered a leptin antagonist through daily injection or plasmid expression respond less robustly to insulin therapy as assessed by both fasting blood glucose levels and blood glucose levels during an oral glucose tolerance test. However, leptin antagonism did not prevent the insulin-induced reduction in β-hydroxybutyrate and triglyceride levels. Therefore, we conclude that elevated leptin levels can contribute to the glucose-lowering effect of insulin therapy in insulin-deficient diabetes.

Recent Insights into Insulin-Like Growth Factor Binding Protein 2 Transcriptional Regulation

Insulin-like growth factor binding proteins (IGFBPs) are major regulators of insulin-like growth factor bioavailability and activity in metabolic signaling. Seven IGFBP family isoforms have been identified. Recent studies have shown that IGFBPs play a pivotal role in metabolic signaling and disease, including the pathogenesis of obesity, diabetes, and cancer. Although many studies have documented the various roles played by IGFBPs, transcriptional regulation of IGFBPs is not well understood. In this review, we focus on the regulatory mechanisms of IGFBP gene expression, and we summarize the findings of transcription factor activity in the IGFBP promoter region.

The role of autonomic efferents and uncoupling protein 1 in the glucose-lowering effect of leptin therapy

Objective

Leptin reverses hyperglycemia in rodent models of type 1 diabetes (T1D). Direct application of leptin to the brain can lower blood glucose in diabetic rodents, and can activate autonomic efferents and non-shivering thermogenesis in brown adipose tissue (BAT). We investigated whether leptin reverses hyperglycemia through a mechanism that requires autonomic innervation, or uncoupling protein 1 (UCP1)-mediated thermogenesis.

Methods

To examine the role of parasympathetic and sympathetic efferents in the glucose-lowering action of leptin, mice with a subdiaphragmatic vagotomy or 6-hydroxydopamine induced chemical sympathectomy were injected with streptozotocin (STZ) to induce hyperglycemia, and subsequently leptin treated. To test whether the glucose-lowering action of leptin requires activation of UCP1-mediated thermogenesis in BAT, we administered leptin in STZ-diabetic Ucp1 knockout (Ucp1^−/−) mice and wildtype controls.

Results

Leptin ameliorated STZ-induced hyperglycemia in both intact and vagotomised mice. Similarly, mice with a partial chemical sympathectomy did not have an attenuated response to leptin-mediated glucose lowering relative to sham controls, and showed intact leptin-induced Ucp1 expression in BAT. Although leptin activated BAT thermogenesis in STZ-diabetic mice, the anti-diabetic effect of leptin was not blunted in Ucp1^−/− mice.

Conclusions

These results suggest that leptin lowers blood glucose in insulin-deficient diabetes through a manner that does not require parasympathetic or sympathetic innervation, and thus imply that leptin lowers blood glucose through an alternative CNS-mediated mechanism or redundant target tissues. Furthermore, we conclude that the glucose lowering action of leptin is independent of UCP1-dependent thermogenesis.

•

Leptin does not require vagal innervation to reverse hyperglycemia.

•

Leptin therapy reverses hyperglycemia in mice with a partial chemical sympathectomy.

•

Leptin reverses hyperglycemia independent of uncoupling protein 1.

Circulating IGF-axis protein levels and their relation with levels of plasma adipocytokines and macronutrient consumption in women

OBJECTIVE

Circulating free insulin-like growth factor (IGF)-I and its binding proteins, most notably, IGFBP-1 and IGFBP-2, have been prospectively associated with incident type 2 diabetes in women. However, little is known regarding the factors that may influence these IGF-axis protein levels. The aim is to study the relation of IGF-axis protein levels with adipcytokines, macronutrient consumption, and other factors related to diabetes.

DESIGN

Fasting plasma from 558 controls enrolled in a nested case-control study within the Nurses' Health Study of incident type 2 diabetes in women was tested for: IGF-axis proteins (free and total IGF-I, IGFBP-1, IGFBP-2, IGFBP-3), adipocytokines (leptin, adiponectin, resistin), soluble leptin receptor (sOB-R), inflammatory factors (IL-18 and C-reactive protein (CRP)), insulin, and glycated hemoglobin (HbA1C).

RESULTS

In multivariate models, each 1% increase in sOB-R (mean 34.9ng/mL, standard deviation (SD) ±11.3) was associated with -0.20% total IGF-I (P=0.0003) and -0.42% free IGF-I (P=0.002), as well as 0.73% higher IGFBP-1 (P<0.0001) and 0.27% IGFBP-2 (P=0.003). For example, a one SD change from the mean sOB-R level was associated with 11% lower free IGF-I. Insulin levels (mean 6.8μU/mL±5.3) were inversely and adiponectin (mean 18.3μg/mL±7.4) positively associated with IGFBP-1 and IGFBP-2 (all P<0.01). Consumption of dairy protein, monounsaturated fats, and saturated fats, was also correlated with IGF-axis protein levels (all P<0.05).

CONCLUSIONS

Several molecular factors and macronutrients were independently associated with plasma IGF-axis protein levels. Which of these, if any, reflect biologic relationships that can be intervened upon to influence IGF-axis protein concentrations warrants further investigation.

The role of leptin in glucose homeostasis

The fat‐derived hormone, leptin, is well known to regulate body weight. However, there is now substantial evidence that leptin also plays a primary role in the regulation of glucose homeostasis, independent of actions on food intake, energy expenditure or body weight. As such, leptin might have clinical utility in treating hyperglycemia, particularly in conditions of leptin deficiency, such as lipodystrophy and diabetes mellitus. The mechanisms through which leptin modulates glucose metabolism have not been fully elucidated. Leptin receptors are widely expressed in peripheral tissues, including the endocrine pancreas, liver, skeletal muscle and adipose, and both direct and indirect leptin action on these tissues contributes to the control of glucose homeostasis. Here we review the role of leptin in glucose homeostasis, along with our present understanding of the mechanisms involved. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2012.00203.x, 2012)

Leptin enhances insulin sensitivity by direct and sympathetic nervous system regulation of muscle IGFBP-2 expression: evidence from nonrodent models

Leptin is produced from white adipose tissue and acts primarily to regulate energy balance. Obesity is associated with leptin resistance and increased circulating levels of leptin. Leptin has recently been shown to influence levels of IGF binding protein-2 (IGFBP-2), a protein that is reduced in obesity and type 2 diabetes. Overexpression of IGFBP-2 protects against obesity and type 2 diabetes. As such, IGFBP-2 signaling may represent a novel pathway by which leptin regulates insulin sensitivity. We sought to investigate how leptin regulates skeletal muscle IGFBP-2 levels and to assess the impact of this on insulin signaling and glucose uptake. In vitro experiments were undertaken in cultured human skeletal myotubes, whereas in vivo experiments assessed the effect of intracerebroventricular leptin on peripheral skeletal muscle IGFBP-2 expression and insulin sensitivity in sheep. Leptin directly increased IGFBP-2 mRNA and protein in human skeletal muscle through both signal transducer and activator of transcription-3 and phosphatidylinositol 3-kinase signaling, in parallel with enhanced insulin signaling. Silencing IGFBP-2 lowered leptin- and insulin-stimulated protein kinase B phosphorylation and glucose uptake. In in vivo experiments, intracerebroventricular leptin significantly increased hind-limb skeletal muscle IGFBP-2, an effect completely blocked by concurrent peripheral infusion of a β-adrenergic blocking agent. Sheep receiving central leptin showed improvements in glucose tolerance and circulating insulin levels after an iv glucose load. In summary, leptin regulates skeletal muscle IGFBP-2 by both direct peripheral and central (via the sympathetic nervous system) mechanisms, and these likely impact on peripheral insulin sensitivity and glucose metabolism.

Maternal nutritional history modulates the hepatic IGF-IGFBP axis in adult male rat offspring

Alterations in early life nutrition lead to an increased risk of obesity and metabolic syndrome in offspring. We have shown that both relative maternal undernutrition (UN) and maternal obesity result in metabolic derangements in offspring, independent of the postnatal dietary environment. Since insulin-like growth factor binding protein 2 (IGFBP2) has been shown to be independently associated with obesity and diabetes risk, we examined the IGF-IGFBP axis in male rat offspring following either maternal UN or maternal obesity to explain possible common pathways in the development of metabolic disorders. Wistar rats were time-mated and fed either a control diet (CONT), 50 % of CONT (UN) or a high-fat (HF) diet throughout pregnancy. Male offspring were weaned onto a standard chow diet and blood and tissues were collected at postnatal day 160. Plasma and hepatic tissue samples were analysed for key players in the IGF-IGFBP system. Both maternal UN and HF resulted in increased fat mass, hyperinsulinemia, hyperleptinemia and altered blood lipid profiles in offspring compared to CONT. Circulating IGF-1 and IGFBP3 levels and hepatic mRNA expression of IGFBP1 and IGFBP2 were significantly decreased in UN and HF offspring compared to CONT. DNA methylation of the IGFBP2 promotor region was similar between maternal dietary groups. Although chaperone gene heat-shock protein 90 and hepatic IGFBP1 were significantly correlated in CONT offspring this effect was absent in both UN and HF offspring. In conclusion, this study is one of the first to directly compare two experimental models of developmental programming representing both ends of the maternal dietary spectrum. Our data suggest that two disparate nutritional models that elicit similar adverse metabolic phenotypes in offspring are characterised by common alterations in the IGF-IGFBP pathway.

IGFBP2 is neither sufficient nor necessary for the physiological actions of leptin on glucose homeostasis in male ob/ob mice

The ability of leptin to improve metabolic abnormalities in models of leptin deficiency, lipodystrophy, and even type 1 diabetes is of significant interest. However, the mechanism by which leptin mediates these effects remains ill-defined. Leptin was recently reported to regulate insulin-like growth factor-binding protein-2 (IGFBP2), and adenoviral overexpression of pharmacological levels of IGFBP2 ameliorates diabetic symptoms in many models of diabetes. We sought to determine the role of physiological levels of IGFBP2 in the glucoregulatory action of leptin. To investigate whether physiological levels of IGFBP2 are sufficient to mimic the action of leptin, we treated male ob/ob mice with low-dose IGFBP2 adenovirus (Ad-IGFBP2) or low-dose leptin. Despite similar levels of circulating IGFBP2, leptin but not Ad-IGFBP2 lowered body weight and plasma insulin and improved glucose and insulin tolerance. To elucidate the role of IGFBP2 in normal glucose homeostasis, we knocked down IGFBP2 in male C57BL/6 mice using small interfering RNA to determine whether this would recapitulate any aspect of the ob/ob phenotype. Despite successful IGFBP2 knockdown, body weight, blood glucose, and plasma insulin were unchanged. Finally, to determine whether IGFBP2 is required for the glucoregulatory actions of leptin, we prevented leptin-mediated increases in IGFBP2 in male ob/ob mice using RNA interference. Even though increases in IGFBP2 were blocked, the ability of leptin to decrease body weight, blood glucose, and plasma insulin levels were unaltered. In conclusion, physiological levels of IGFBP2 are neither sufficient to mimic nor required for the physiological action of leptin.

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.

The heparin-binding domains of IGFBP-2 mediate its inhibitory effect on preadipocyte differentiation and fat development in male mice

IGF-binding protein (IGFBP)-2 overexpression confers resistance to high-fat feeding and inhibits the differentiation of preadipocytes in vitro. However, whether administration of IGFBP-2 can regulate adipogenesis in vivo and the domains that mediate this response have not been defined. IGFBP-2 contains 2 heparin-binding domains (HBD), which are localized in the linker region (HBD1) and C-terminal region (HBD2) of IGFBP-2. To determine the relative importance of these domains, we used synthetic peptides as well as mutagenesis. Both HBD1 and HBD2 peptides inhibited preadipocyte differentiation, but the HBD2 peptide was more effective. Selective substitution of charged residues in the HBD1 or HBD2 regions attenuated the ability of the full-length protein to inhibit cell differentiation, but the HBD2 mutant had the greatest reduction. To determine their activities in vivo, pegylated forms of each peptide were administered to IGFBP-2(-/-) mice for 12 weeks. Magnetic resonance imaging scanning showed that only the HBD2 peptide significantly reduced (48 ± 9%, P < .05) gain in total fat mass. Both inguinal (32 ± 7%, P < .01) and visceral fat (44 ± 7%, P < .01) were significantly decreased by HBD2 whereas HBD1 reduced only visceral fat accumulation (24 ± 5%, P < .05). The HBD2 peptide was more effective peptide in reducing triglyceride content and serum adiponectin, but only the HBD2 peptide increased serum leptin. These findings demonstrate that the HBD2 domain of IGFBP-2 is the primary region that accounts for its ability to inhibit adipogenesis and that a peptide encompassing this region has activity that is comparable with native IGFBP-2.

Leptin administration enhances islet transplant performance in diabetic mice

Islet transplantation is an effective method to obtain long-term glycemic control for patients with type 1 diabetes, yet its widespread use is limited by an inadequate supply of donor islets. The hormone leptin has profound glucose-lowering and insulin-sensitizing action in type 1 diabetic rodent models. We hypothesized that leptin administration could reduce the dose of transplanted islets required to achieve metabolic control in a mouse model of type 1 diabetes. We first performed a leptin dose-response study in C57Bl/6 mice with streptozotocin (STZ)-induced diabetes to determine a leptin dose insufficient to reverse hyperglycemia. Subsequently, we compared the ability of suboptimal islet transplants of 50 or 125 syngeneic islets to achieve glycemic control in STZ-induced diabetic C57Bl/6 mice treated with or without this dose of leptin. The dose-response study revealed that leptin reverses STZ-induced diabetes in a dose-dependent manner. Supraphysiological leptin levels were necessary to restore euglycemia but simultaneously increased risk of hypoglycemia, and also lost efficacy after 12 days of administration. In contrast, 1 µg/day leptin only modestly reduced blood glucose but maintained efficacy throughout the study duration. We then administered 1 µg/day leptin to diabetic mice that underwent transplantation of 50 or 125 islets. Although these islet doses were insufficient to ameliorate hyperglycemia alone, coadministration of leptin with islet transplantation robustly improved control of glucose and lipid metabolism, without increasing circulating insulin levels. This study reveals that low-dose leptin administration can reduce the number of transplanted islets required to achieve metabolic control in STZ-induced diabetic mice.

Leptin promotes K(ATP) channel trafficking by AMPK signaling in pancreatic β-cells

Leptin is a pivotal regulator of energy and glucose homeostasis, and defects in leptin signaling result in obesity and diabetes. The ATP-sensitive potassium (K(ATP)) channels couple glucose metabolism to insulin secretion in pancreatic β-cells. In this study, we provide evidence that leptin modulates pancreatic β-cell functions by promoting K(ATP) channel translocation to the plasma membrane via AMP-activated protein kinase (AMPK) signaling. K(ATP) channels were localized mostly to intracellular compartments of pancreatic β-cells in the fed state and translocated to the plasma membrane in the fasted state. This process was defective in leptin-deficient ob/ob mice, but restored by leptin treatment. We discovered that the molecular mechanism of leptin-induced AMPK activation involves canonical transient receptor potential 4 and calcium/calmodulin-dependent protein kinase kinase β. AMPK activation was dependent on both leptin and glucose concentrations, so at optimal concentrations of leptin, AMPK was activated sufficiently to induce K(ATP) channel trafficking and hyperpolarization of pancreatic β-cells in a physiological range of fasting glucose levels. There was a close correlation between phospho-AMPK levels and β-cell membrane potentials, suggesting that AMPK-dependent K(ATP) channel trafficking is a key mechanism for regulating β-cell membrane potentials. Our results present a signaling pathway whereby leptin regulates glucose homeostasis by modulating β-cell excitability.

Pregnancy-associated plasma protein-A is related to gender and to adipocytokine levels: results of the Health Survey of Catalonia

OBJECTIVE

Pregnancy-associated plasma protein-A (PAPP-A) is a protease promoting IGF1 tissue availability and considered as a new biomarker of cardiovascular disease.

AIM

To evaluate the relationship between PAPP-A concentrations and anthropometric variables, physical activity, smoking status, glucose homoeostasis and adipocytokines in healthy adults.

DESIGN AND METHODS

One hundred and forty-nine subjects (77 women; mean age 39·7 ± 14 years; mean BMI 23·7 ± 1·9 kg/m(2) ) were randomly selected from 8000 adults of The Health Survey of Catalonia. Possible effects of gender, age, body composition, smoking status, physical activity, glucose homoeostasis and adipocytokines on PAPP-A concentrations were assessed.

RESULTS

Pregnancy-associated plasma protein-A was significantly higher in men than in women [1·04 (0·61-0·44) vs 0·61 (0·41-0·90) μIU/ml; P < 0·0001]; there were no differences in relation to physical activity or smoking status. PAPP-A showed a negative correlation with leptin in men (P = 0·01) and women (P = 0·05), and a positive correlation with adiponectin (P = 0·006) in women and a trend (P = 0·073) in men. Homoeostasis model assessment of insulin resistance (HOMA-IR) showed a negative correlation with PAPP-A only in women (P = 0·019). No association was found with blood pressure, IGF1, lipids or glucose in either gender. When a multiple regression analysis was performed including gender, age, BMI, waist-hip ratio, HOMA-IR, adiponectin and leptin as confounders, PAPP-A was independently correlated with adiponectin (β = 0·23; P = 0·02) and leptin (β = -0·33; P = 0·04).

CONCLUSIONS

Our study shows a sexual dimorphism of PAPP-A, and a possible influence of leptin and adiponectin on its concentrations in healthy subjects. The mechanisms responsible for this relationship remain to be determined.

Leptin, resistin and visfatin: the missing link between endocrine metabolic disorders and immunity

Adipose tissue is still regarded as a principle site for lipid storage and mobilizing tissue with an important role in the control of energy homeostasis. Additionally, adipose tissue-secreted hormones such as leptin, visfatin, resistin, apelin, omentin, sex steroids, and various growth factors are now regarded as a functional part of the endocrine system. These hormones also play an important role in the immune system. Several in vitro and in vivo studies have suggested the complex role of adipocyte-derived hormones in immune system and inflammation. Adipokines mediate beneficial and detrimental effects in immunity and inflammation. Many of these adipocytokines have a physiological role in metabolism. The uncontrolled secretions of several adipocytokines were associated with the stimulation of inflammatory processes leading to metabolic disorders including obesity, atherosclerosis, insulin resistance and type 2 diabetes. Obesity leads to the dysfunction of adipocytes andcorrelated with the imbalance of adipokines levels. In obese and diabetic conditions, leptin deficiency inhibited the Jak/Stat3/PI3K and insulin pathways. In this review, ample evidence exists to support the recognition of the adipocyte’s role in various tissues and pathologies. New integral insights may add dimensions to translate any potential agents into the future clinical armamentarium of chronic endocrine metabolic and inflammatory diseases. Functional balance of both adipocytes and immune cells is important to exert their effects on endocrine metabolic disorders; furthermore, adipose tissue should be renamed not only as a functional part of the endocrine system but also as a new part of the immune system.

A role for hepatic leptin signaling in lipid metabolism via altered very low density lipoprotein composition and liver lipase activity in mice

Obesity is highly associated with dyslipidemia and cardiovascular disease. However, the mechanism behind this association is not completely understood. The hormone leptin may be a molecular link between obesity and dysregulation of lipid metabolism. Leptin can affect lipid metabolism independent of its well-known effects on food intake and energy expenditure, but exactly how this occurs is ill-defined. We hypothesized that since leptin receptors are found on the liver and the liver plays an integral role in regulating lipid metabolism, leptin may affect lipid metabolism by acting directly on the liver. To test this hypothesis, we generated mice with a hepatocyte-specific loss of leptin signaling. We previously showed that these mice have increased insulin sensitivity and elevated levels of liver triglycerides compared with controls. Here, we show that mice lacking hepatic leptin signaling have decreased levels of plasma apolipoprotein B yet increased levels of very low density lipoprotein (VLDL) triglycerides, suggesting alterations in triglyceride incorporation into VLDL or abnormal lipoprotein remodeling in the plasma. Indeed, lipoprotein profiles revealed larger apolipoprotein B-containing lipoprotein particles in mice with ablated liver leptin signaling. Loss of leptin signaling in the liver was also associated with a substantial increase in lipoprotein lipase activity in the liver, which may have contributed to increased lipid droplets in the liver.

CONCLUSION

Lack of hepatic leptin signaling results in increased lipid accumulation in the liver and larger, more triglyceride-rich VLDL particles. Collectively, these data reveal an interesting role for hepatic leptin signaling in modulating triglyceride metabolism.