Insulin regulates leptin secretion from 3T3-L1 adipocytes by a PI 3 kinase independent mechanism

A quantitative pipeline to assess secretion of human leptin coding variants reveals mechanisms underlying leptin deficiencies

The hormone leptin, primarily secreted by adipocytes, plays a crucial role in regulating whole-body energy homeostasis. Homozygous loss-of-function mutations in the leptin gene (LEP) cause hyperphagia and severe obesity, primarily through alterations in leptin's affinity for its receptor or changes in serum leptin concentrations. Although serum concentrations are influenced by various factors (e.g., gene expression, protein synthesis, stability in the serum), proper delivery of leptin from its site of synthesis in the endoplasmic reticulum via the secretory pathway to the extracellular serum is a critical step. However, the regulatory mechanisms and specific machinery involved in this trafficking route, particularly in the context of human LEP mutations, remain largely unexplored. We have employed the Retention Using Selective Hooks system to elucidate the secretory pathway of leptin. We have refined this system into a medium-throughput assay for examining the pathophysiology of a range of obesity-associated LEP variants. Our results reveal that leptin follows the default secretory pathway, with no additional regulatory steps identified prior to secretion. Through screening of leptin variants, we identified three mutations that lead to proteasomal degradation of leptin and one variant that significantly decreased leptin secretion, likely through aberrant disulfide bond formation. These observations have identified novel pathogenic effects of leptin variants, which can be informative for therapeutics and diagnostics. Finally, our novel quantitative screening platform can be adapted for other secreted proteins.

Analysis of gene expression changes during lipid droplet formation in HepG2 human liver cancer cells

Fatty liver is a condition of excessive triglyceride accumulation in hepatocytes. Additionally, hepatocytes exhibit a high degree of fat droplet accumulation during excessive alcohol consumption and metabolic syndrome. However, the molecular mechanisms involved in fat droplet formation remain unknown. The present study used an in vitro fatty liver formation model of the human liver cancer cell line, HepG2, to comprehensively search for fat droplet formation-related genes, and which exhibit changes in expression during fat droplet formation. Microarray analysis with extracted total RNA determined the genes that are involved in fat droplet formation and their expression was confirmed using quantitative polymerase chain reaction following the culture of the HepG2 cells in culture medium containing 0, 50, 200 and 500 µM of oleic acid for 24 h. The results revealed 142 genes demonstrating increased expression levels by >2.0-fold with oleic acid treatment and 426 genes demonstrating decreased expression levels. Perilipin 2 (PLIN2) was estimated as the gene most closely associated with fatty liver. Lipid droplet formation in the HepG2 cells induced by oleic acid led to the upregulation of PLIN2 in a concentration-dependent manner. On the whole, the findings of the present study indicate the involvement of genes in oleic acid-induced lipid droplet formation in HepG2 cells; PLIN2 in particular may play a crucial role in this process.

The Role of Cdc42 in the Insulin and Leptin Pathways Contributing to the Development of Age-Related Obesity

Age-related obesity significantly increases the risk of chronic diseases such as type 2 diabetes, cardiovascular diseases, hypertension, and certain cancers. The insulin-leptin axis is crucial in understanding metabolic disturbances associated with age-related obesity. Rho GTPase Cdc42 is a member of the Rho family of GTPases that participates in many cellular processes including, but not limited to, regulation of actin cytoskeleton, vesicle trafficking, cell polarity, morphology, proliferation, motility, and migration. Cdc42 functions as an integral part of regulating insulin secretion and aging. Some novel roles for Cdc42 have also been recently identified in maintaining glucose metabolism, where Cdc42 is involved in controlling blood glucose levels in metabolically active tissues, including skeletal muscle, adipose tissue, pancreas, etc., which puts this protein in line with other critical regulators of glucose metabolism. Importantly, Cdc42 plays a vital role in cellular processes associated with the insulin and leptin signaling pathways, which are integral elements involved in obesity development if misregulated. Additionally, a change in Cdc42 activity may affect senescence, thus contributing to disorders associated with aging. This review explores the complex relationships among age-associated obesity, the insulin-leptin axis, and the Cdc42 signaling pathway. This article sheds light on the vast molecular web that supports metabolic dysregulation in aging people. In addition, it also discusses the potential therapeutic implications of the Cdc42 pathway to mitigate obesity since some new data suggest that inhibition of Cdc42 using antidiabetic drugs or antioxidants may promote weight loss in overweight or obese patients.

SUCNR1 signaling in adipocytes controls energy metabolism by modulating circadian clock and leptin expression

Adipose tissue modulates energy homeostasis by secreting leptin, but little is known about the factors governing leptin production. We show that succinate, long perceived as a mediator of immune response and lipolysis, controls leptin expression via its receptor SUCNR1. Adipocyte-specific deletion of Sucnr1 influences metabolic health according to nutritional status. Adipocyte Sucnr1 deficiency impairs leptin response to feeding, whereas oral succinate mimics nutrient-related leptin dynamics via SUCNR1. SUCNR1 activation controls leptin expression via the circadian clock in an AMPK/JNK-C/EBPα-dependent manner. Although the anti-lipolytic role of SUCNR1 prevails in obesity, its function as a regulator of leptin signaling contributes to the metabolically favorable phenotype in adipocyte-specific Sucnr1 knockout mice under standard dietary conditions. Obesity-associated hyperleptinemia in humans is linked to SUCNR1 overexpression in adipocytes, which emerges as the major predictor of adipose tissue leptin expression. Our study establishes the succinate/SUCNR1 axis as a metabolite-sensing pathway mediating nutrient-related leptin dynamics to control whole-body homeostasis.

Healthy Sleep Every Day Keeps the Doctor Away

When one considers the big picture of their health, sufficient sleep may often go overlooked as a keystone element in this picture. Insufficient sleep in either quality or duration is a growing problem for our modern society. It is essential to look at what this means for our health because insufficient sleep increases our risks of innumerable lifechanging diseases. Beyond increasing the risk of developing these diseases, it also makes the symptoms and pathogenesis of many diseases worse. Additionally, consistent quality sleep can not only improve our physical health but has also been shown to improve mental health and overall quality of life. Substandard sleep health could be a root cause for numerous issues individuals may be facing in their lives. It is essential that physicians take the time to learn about how to educate their patients on sleep health and try to work with them on an individual level to help motivate lifestyle changes. Facilitating access to sleep education for their patients is one way in which physicians can help provide patients with the tools to improve their sleep health. Throughout this paper, we will review the mechanisms behind the relationship between insufficient sleep health and chronic disease and what the science says about how inadequate sleep health negatively impacts the overall health and the quality of our lives. We will also explain the lifechanging effects of sufficient sleep and how we can help patients get there.

Matrigel® enhances 3T3-L1 cell differentiation

Culturing cells on bio-gels are believed to provide a more in vivo-like extracellular matrix. 3T3-L1 cells cultured on Matrigel® significantly alteregd their proliferation and differentiation as compared to growth on tissue culture-coated polystyrene surfaces. Growth on a 250-μm thick layer of Matrigel® facilitated the formation of cellular aggregates of 3T3-L1 cells. Differentiation of 3T3-L1 cells cultured on Matrigel® demonstrated increased levels of mRNA levels for key adipogenic transcription factors (PPARγ, C/EBPα, SREBP1), lipogenic markers (FAS, FABP4, LPL, PLIN1) and markers of adipocyte maturity (LEP), compared to cells cultured directly on a polystyrene tissue culture surface. The gene expression of extracellular matrix proteins (FN1, COL1A1, COL4A1, COL6, LAM) was decreased in 3T3-L1 cells cultured on Matrigel®. Furthermore, growth on Matrigel® increased lipid accumulation in 3T3-L1 cells in the presence and absence of rosiglitazone, a thiazolidinedione routinely used to optimize differentiation in these cells. These changes in adipocyte gene expression and lipid accumulation patterns may be a result of the increased cell-cell and cell-ECM interactions occurring on the Matrigel®, a scenario that is more reflective of an in vivo model. Taken together, our data advance the understanding of the value of culturing 3T3-L1 cells on Matrigel®.

Phenotypic characterization of Adig null mice suggests roles for adipogenin in the regulation of fat mass accrual and leptin secretion

Adipogenin (Adig) is an adipocyte-enriched transmembrane protein. Its expression is induced during adipogenesis in rodent cells, and a recent genome-wide association study associated body mass index (BMI)-adjusted leptin levels with the ADIG locus. In order to begin to understand the biological function of Adig, we studied adipogenesis in Adig-deficient cultured adipocytes and phenotyped Adig null (Adig^-/-) mice. Data from Adig-deficient cells suggest that Adig is required for adipogenesis. In vivo, Adig^-/- mice are leaner than wild-type mice when fed a high-fat diet and when crossed with Ob/Ob hyperphagic mice. In addition to the impact on fat mass accrual, Adig deficiency also reduces fat-mass-adjusted plasma leptin levels and impairs leptin secretion from adipose explants, suggesting an additional impact on the regulation of leptin secretion.

Reduced lipolysis in lipoma phenocopies lipid accumulation in obesity

BACKGROUND

Elucidation of lipid metabolism and accumulation mechanisms is of paramount importance to understanding obesity and unveiling therapeutic targets. In vitro cell models have been extensively used for these purposes, yet, they do not entirely reflect the in vivo setup. Conventional lipomas, characterized by the presence of mature adipocytes and increased adipogenesis, could overcome the drawbacks of cell cultures. Also, they have the unique advantage of easily accessible matched controls in the form of subcutaneous adipose tissue (SAT) from the same individual. We aimed to determine whether lipomas are a good model to understand lipid accumulation.

METHODS

We histologically compared lipomas and control SAT, followed by assessment of the lipidome using high-resolution ¹H NMR spectroscopy and ESI-IT mass spectrometry. RNA-sequencing was used to obtain the transcriptome of lipomas and the matched SAT.

RESULTS

We found a significant increase of small-size (maximal axis < 70 µm) and very big (maximal axis > 150 µm) adipocytes within lipomas. This suggests both enhanced adipocyte proliferation and increased lipid accumulation. We further show that there is no significant change in the lipid composition compared to matched SAT. To better delineate the pathophysiology of lipid accumulation, we considered two groups with different genetic backgrounds: (1) lipomas with HMGA2 fusions and (2) without gene fusions. To reduce the search space for genes that are relevant for lipid pathophysiology, we focused on the overlapping differentially expressed (DE) genes between the two groups. Gene Ontology analysis revealed that DE genes are enriched in pathways related to lipid accumulation.

CONCLUSIONS

We show that the common shared lipid accumulation mechanism in lipoma is a reduction in lipolysis, with most gene dysregulations leading to a reduced cAMP in the adipocyte. Superficial lipomas could thus be used as a model for lipid accumulation through altered lipolysis as found in obese patients.

Leptin in Tumor Microenvironment

Leptin is a hormone that plays a major role as mediator of long-term regulation of energy balance, suppressing food intake, and stimulating weight loss. More recently, important physiological roles other than controlling appetite and energy expenditure have been suggested for leptin, including neuroendocrine, reparative, reproductive, and immune functions. These emerging peripheral roles let hypothesize that leptin can modulate also cancer progression. Indeed, many studies have demonstrated that elevated chronic serum concentrations of leptin, frequently seen in obese subjects, represent a stimulatory signal for tumor growth. Current knowledge indicates that also different non-tumoral cells resident in tumor microenvironment may respond to leptin creating a favorable soil for cancer cells. In addition, leptin is produced also within the tumor microenvironment creating the possibility for paracrine and autocrine action. In this review, we describe the main mechanisms that regulate peripheral leptin availability and how leptin can shape tumor microenvironment.

The development of a high throughput drug-responsive model of white adipose tissue comprising adipogenic 3T3-L1 cells in a 3D matrix

Adipose models have been applied to mechanistic studies of metabolic diseases (such as diabetes) and the subsequent discovery of new therapeutics. However, typical models are either insufficiently complex (2D cell cultures) or expensive and labor intensive (mice/in vivo). To bridge the gap between these models and in order to better inform pre-clinical studies we have developed a drug-responsive 3D model of white adipose tissue (WAT). Here, spheroids (680 ± 60 μm) comprising adipogenic 3T3-L1 cells encapsulated in 3D matrix were fabricated manually on a 96 well scale. Spheroids were highly characterised for lipid morphology, selected metabolite and adipokine secretion, and gene expression; displaying significant upregulation of certain adipogenic-specific genes compared with a 2D model. Furthermore, induction of lipolysis and promotion of lipogenesis in spheroids could be triggered by exposure to 8-br-cAMP and oleic-acid respectively. Metabolic and high content imaging data of spheroids exposed to an adipose-targeting drug, rosiglitazone, resulted in dose-responsive behavior. Thus, our 3D WAT model has potential as a powerful scalable tool for compound screening and for investigating adipose biology.

Orexin A-induced inhibition of leptin expression and secretion in adipocytes reducing plasma leptin levels and hypothalamic leptin resistance

Orexin A (OXA) is a neuropeptide associated with plasma insulin and leptin levels involved in body weight and appetite regulation. However, little is known about the effect of OXA on leptin secretion in adipocytes and its physiological roles. Leptin secretion and expression were analysed in 3T3-L1 adipocytes. Plasma leptin, adiponectin and insulin levels were measured by ELISA assay. Phosphorylated signal transducer and activator of transcription 3 (pSTAT3) levels in the hypothalamus were evaluated by western blotting. OXA dose-dependently suppressed leptin secretion from 3T3-L1 adipocytes by inhibiting its gene expression while facilitating adiponectin secretion. The leptin inhibition by OXA was mediated via orexin receptors (OXR1 and OXR2). In addition to the pathway via extracellular signal-regulated kinases, OXA triggered adenylyl cyclase-induced cAMP elevation, which results in protein kinase A-mediated activation of cAMP response element-binding proteins (CREB). Accordingly, CREB inhibition restored the OXA-induced downregulation of leptin gene expression and secretion. Exogenous OXA for 4 weeks decreased fasting plasma leptin levels and increased hypothalamic pSTAT3 levels in high-fat diet-fed mice, regardless of increase in body weight and food intake. These results suggest that high dose of OXA directly inhibits leptin mRNA expression and thus secretion in adipocytes, which may be a peripheral mechanism of OXA for its role in appetite drive during fasting. It may be also critical for lowering basal plasma leptin levels and thus maintaining postprandial hypothalamic leptin sensitivity.

Lactobacillus curvatus HY7601 and Lactobacillus plantarum KY1032 Cell Extracts Inhibit Adipogenesis in 3T3-L1 and HepG2 Cells

Some lactic acid bacteria (LAB) and their cellular components have antiobesity effects. In this study, we evaluated the antiadipogenic effects of a mixture of two LAB-Lactobacillus curvatus HY7601 and Lactobacillus plantarum KY1032-using 3T3-L1 preadipocytes and HepG2 hepatocarcinoma cells. 3T3-L1 cells treated with a 1:1 ratio of HY7601 and KY1032 during differentiation showed reduced lipid accumulation by Oil Red O staining, as well as decreased leptin secretion and mRNA expression of peroxisome proliferator-activated receptor-γ and CCAAT/enhancer binding protein-α. HY7601 and KY1032 treatment also suppressed mitochondrial biogenesis and inhibited the expression of genes encoding mitochondrial transcription factors, as well as those related to fatty acid synthesis in HepG2 cells. The antiadipogenic effects of LAB were associated with the cell membrane fraction. These results demonstrate that a mixture of two LAB (HY7601 and KY1032) inhibits adipogenesis in preadipocytes and liver cells and is a potential therapeutic strategy for the treatment of obesity.

Insulin as a hormone regulator of the synthesis and release of leptin by white adipose tissue

Leptin and its receptor are widely distributed in several tissues, mainly in white adipose tissue. The serum leptin is highly correlated with body mass index in rodents and humans, being documented that leptin levels reduces in the fasting state and increase during refeeding, similarly to insulin release by pancreatic islets. Insulin appears to increase leptin mRNA and protein expression and its release by adipocytes. Some studies have suggested that insulin acts through the activation of the transcription factors: sterol regulatory element binding protein 1 (SREBP1), CCAAT enhancer binding protein-α (C/EBP-α) and specificity protein 1 (Sp1). Insulin stimulates the release of preformed and newly synthesized leptin by adipocytes through its signaling cascade. Its effects are blocked by inhibitors of the insulin signaling pathway, as well as by inhibitors of protein synthesis and agents that increase the intracellular cAMP. The literature data suggest that chronic hyperinsulinemia increases serum leptin levels in humans and rodents. In this review, we summarized the most updated knowledge on the effects of insulin on serum leptin levels, presenting the cell mechanisms that control leptin synthesis and release by the white adipose tissue.

Chronic hyper-leptinemia induces insulin signaling disruption in adipocytes: Implications of NOS2

Leptin, following its discovery, has developed a formidable interest in the scientific community to delineate its contribution towards overall metabolic homeostasis. Contradictory reports have been published on leptin administration effects on whole body insulin sensitivity. Following late reports, we surveyed human serum leptin levels along with other metabolic parameters including BMI and HOMA-IR. We found a positive correlation between leptin levels and insulin resistance parameters. Considering the presence of the long form of leptin receptor on adipocytes, we explored the effects of chronic physiological hyper-leptinemic exposure on adipocyte insulin sensitivity. Chronic leptin (50ng/ml) treatment in 3T3-L1 adipocytes decreased insulin-induced phosphorylation of nodal insulin signaling proteins along with reduced glucose uptake. Metabolic flux studies indicated mitochondrial dysfunction and reduced oxygen consumption rate. Leptin treatment also increased both cellular and mitochondrial superoxide levels concomitant to increased expression of nitric oxide synthase-2 (NOS2). Further, pharmacological depletion of NOS2 reversed leptin mediated effects on insulin signaling. In-vivo implantation of leptin osmotic pumps in C57BL/6 mice also decreased insulin responsiveness. Interestingly, these effects were lacking in NOS2 knockout strain. In conclusion, our studies put forward a potential link between leptin and adipocyte insulin responsiveness in an NOS2 dependent manner.

Insulin elevates leptin secretion and mRNA levels via cyclic AMP in 3T3-L1 adipocytes deprived of glucose

Aims

Leptin plays an important role in the pathogenesis of obesity and diabetes, yet the regulatory mechanisms of this hormone have not been fully elucidated. In this study, we aimed to clarify the roles of insulin and glucose in leptin secretion and mRNA production using inhibitors of insulin signal transduction in adipocytes cultured under glucose-free or normal conditions.

Methods

Differentiated 3T3-L1 adipocytes were stimulated with insulin in combination with inhibitors for phosphoinositide 3-kinase (PI3K), Akt, and phosphodiesterase 3B (PDE3B), as well as epinephrine and a cyclic AMP (cAMP) analog under glucose-free or normal conditions. After 8 h of stimulation, leptin protein levels in the media and leptin mRNA expression levels in the adipocytes were measured.

Results

Insulin significantly increased the secretion and mRNA levels of leptin under the depletion of glucose. Glucose augmented basal leptin secretion without insulin, while glucose nullified insulin-induced leptin mRNA upregulation. The PI3K inhibitor BEZ-235, the Akt inhibitor MK-2206, and the PDE3B inhibitor cilostazol attenuated the insulin stimulation of leptin secretion, but did not suppress the insulin-induced leptin mRNA upregulation with glucose depletion. In contrast to the glucose-free condition, insulin failed to upregulate leptin mRNA in the presence of glucose. The cAMP analog dibutyryl cAMP and epinephrine decreased both leptin secretion and mRNA regardless of glucose supplementation.

Conclusion

Insulin alone stimulates leptin secretion and elevates leptin mRNA levels via cAMP under the lack of glucose metabolism, while glucose is a significant and ambivalent effector on the insulin effects of leptin.

Role of Exchange Protein Directly Activated by Cyclic AMP Isoform 1 in Energy Homeostasis: Regulation of Leptin Expression and Secretion in White Adipose Tissue

Epacs (exchange proteins directly activated by cyclic AMP [cAMP]) act as downstream effectors of cAMP and play important roles in energy balance and glucose homeostasis. While global deletion of Epac1 in mice leads to heightened leptin sensitivity in the hypothalamus and partial protection against high-fat diet (HFD)-induced obesity, the physiological functions of Epac1 in white adipose tissue (WAT) has not been explored. Here, we report that adipose tissue-specific Epac1 knockout (AEKO) mice are more prone to HFD-induced obesity, with increased food intake, reduced energy expenditure, and impaired glucose tolerance. Despite the fact that AEKO mice on HFD display increased body weight, these mice have decreased circulating leptin levels compared to their wild-type littermates. In vivo and in vitro analyses further reveal that suppression of Epac1 in WAT decreases leptin mRNA expression and secretion by inhibiting cAMP response element binding (CREB) protein and AKT phosphorylation, respectively. Taken together, our results demonstrate that Epac1 plays an important role in regulating energy balance and glucose homeostasis by promoting leptin expression and secretion in WAT.

Association of Increased Serum Leptin with Ameliorated Anemia and Malnutrition in Stage 5 Chronic Kidney Disease Patients after Parathyroidectomy

Leptin is an adipokine that regulates various metabolism, but its association with secondary hyperparathyroidism (SHPT), a clinical manifestation of chronic kidney disease-mineral and bone disorder (CKD-MBD), remains obscure. Parathyroidectomy (PTX) is recommended for severe SHPT patients. Here, the associations between circulating leptin and clinical characteristics in CKD patients were investigated. Effects of PTX on leptin production were analyzed in vivo and in vitro. Controls and CKD patients had approximate serum leptin levels in that a larger proportion of CKD patients with body mass index (BMI) <23 kg/m². Serum leptin was related to anemia, albumin, and bone metabolism disorders in CKD patients. Lower intact parathyroid hormone (PTH) was related with higher leptin in PTX patients group. Severe SHPT inhibited uremia-enhanced leptin production in 3T3-L1 adipocytes, which was attenuated after PTX. High levels of PTH were found to reduce Akt phosphorylation and leptin production in vitro but high levels of calcium and phosphorus were not. Successful PTX was found to improve anemia and malnutrition in severe SHPT patients, and this was correlated with increased circulating leptin levels via up-regulated Akt signaling in adipocytes. These findings indicated the therapeutic potential of leptin and related target pathway for improving survival and quality of life in CKD.

Leptin Production by Encapsulated Adipocytes Increases Brown Fat, Decreases Resistin, and Improves Glucose Intolerance in Obese Mice

The neuroendocrine effects of leptin on metabolism hold promise to be translated into a complementary therapy to traditional insulin therapy for diabetes and obesity. However, injections of leptin can provoke inflammation. We tested the effects of leptin, produced in the physiological adipocyte location, on metabolism in mouse models of genetic and dietary obesity. We generated 3T3-L1 adipocytes constitutively secreting leptin and encapsulated them in a poly-L-lysine membrane, which protects the cells from immune rejection. Ob/ob mice (OB) were injected with capsules containing no cells (empty, OB^[Emp]), adipocytes (OB^[3T3]), or adipocytes overexpressing leptin (OB^[Lep]) into both visceral fat depots. Leptin was found in the plasma of OB^[Lep], but not OB^[Emp] and OB^[3T3] mice at the end of treatment (72 days). The OB^[Lep] and OB^[3T3] mice have transiently suppressed appetite and weight loss compared to OB^[Emp]. Only OB^[Lep] mice have greater brown fat mass, metabolic rate, and reduced resistin plasma levels compared to OB^[Emp]. Glucose tolerance was markedly better in OB^[Lep]vs. OB^[Emp] and OB^[3T3] mice as well as in wild type mice with high-fat diet-induced obesity and insulin resistance treated with encapsulated leptin-producing adipocytes. Our proof-of-principle study provides evidence of long-term improvement of glucose tolerance with encapsulated adipocytes producing leptin.

Pancreatic regulation of glucose homeostasis

In order to ensure normal body function, the human body is dependent on a tight control of its blood glucose levels. This is accomplished by a highly sophisticated network of various hormones and neuropeptides released mainly from the brain, pancreas, liver, intestine as well as adipose and muscle tissue. Within this network, the pancreas represents a key player by secreting the blood sugar-lowering hormone insulin and its opponent glucagon. However, disturbances in the interplay of the hormones and peptides involved may lead to metabolic disorders such as type 2 diabetes mellitus (T2DM) whose prevalence, comorbidities and medical costs take on a dramatic scale. Therefore, it is of utmost importance to uncover and understand the mechanisms underlying the various interactions to improve existing anti-diabetic therapies and drugs on the one hand and to develop new therapeutic approaches on the other. This review summarizes the interplay of the pancreas with various other organs and tissues that maintain glucose homeostasis. Furthermore, anti-diabetic drugs and their impact on signaling pathways underlying the network will be discussed.

Weight regaining: From statistics and behaviors to physiology and metabolism

Achieving maintenance of weight loss is crucial to combat obesity. However, most individuals tend to regain weight. Data from successful maintainers show that they remain vigilant and constantly apply techniques to oppose the course of regaining. On the other hand, current advances in obesity research show that the reduced obese state is a state of altered physiology in terms of energy balance. This review describes the physiological adaptations occurring after weight loss that predispose to regaining. Specifically, changes regarding body composition, hormonal background, energy expenditure and control of food intake are discussed. Moreover, metabolites that can act as regain predictors and dietary techniques to oppose regaining are presented.

Adipocyte iron regulates leptin and food intake

Dietary iron supplementation is associated with increased appetite. Here, we investigated the effect of iron on the hormone leptin, which regulates food intake and energy homeostasis. Serum ferritin was negatively associated with serum leptin in a cohort of patients with metabolic syndrome. Moreover, the same inverse correlation was observed in mice fed a high-iron diet. Adipocyte-specific loss of the iron exporter ferroportin resulted in iron loading and decreased leptin, while decreased levels of hepcidin in a murine hereditary hemochromatosis (HH) model increased adipocyte ferroportin expression, decreased adipocyte iron, and increased leptin. Treatment of 3T3-L1 adipocytes with iron decreased leptin mRNA in a dose-dependent manner. We found that iron negatively regulates leptin transcription via cAMP-responsive element binding protein activation (CREB activation) and identified 2 potential CREB-binding sites in the mouse leptin promoter region. Mutation of both sites completely blocked the effect of iron on promoter activity. ChIP analysis revealed that binding of phosphorylated CREB is enriched at these two sites in iron-treated 3T3-L1 adipocytes compared with untreated cells. Consistent with the changes in leptin, dietary iron content was also directly related to food intake, independently of weight. These findings indicate that levels of dietary iron play an important role in regulation of appetite and metabolism through CREB-dependent modulation of leptin expression.

Osteocalcin, under-carboxylated osteocalcin and osteopontin are not associated with gestational diabetes mellitus but are inversely associated with leptin in non-diabetic women

OBJECTIVE

To measure serum osteocalcin (OC), under-carboxylated osteocalcin (ucOC), osteopontin (OPN), and leptin in pregnant women with gestational diabetes mellitus (GDM) and in healthy pregnant women during pregnancy and after birth and relate these markers to glucose metabolism.

METHODS

This was a prospective study including 60 women with GDM and 60 subjects with normal gestation who were evaluated at gestational week 30 and 6 weeks postpartum. Serum OC, ucOC, OPN, leptin, insulin and insulin resistance were evaluated during the study.

RESULTS

Bone biomarkers and leptin were similar between GDM and normal pregnancy. After delivery, OC, ucOC and OPN increased in both groups, while leptin decreased only in healthy controls. Bone markers did not correlate with insulin and insulin resistance in the two groups, but leptin was positively correlated with insulin and insulin resistance and negatively correlated with bone biomarkers only in healthy women. Furthermore, the women who developed diabetes postpartum had lower levels of OC than women with normal glucose tolerance.

CONCLUSION

GDM is not associated with OC, ucOC, OPN, and leptin and does not correlate with insulin resistance. At postpartum, women who develop diabetes have lower osteocalcin concentrations. Leptin correlates with insulin resistance and bone biomarkers in non-diabetic women.

Insulin-stimulated leptin secretion requires calcium and PI3K/Akt activation

Insulin stimulates leptin secretion through the PI3K/Akt, but not the MAPK, pathway. Although Ca2+ alone does not trigger leptin secretion, it is required for robust Akt phosphorylation and leptin secretion.

Long-term artificial sweetener acesulfame potassium treatment alters neurometabolic functions in C57BL/6J mice

With the prevalence of obesity, artificial, non-nutritive sweeteners have been widely used as dietary supplements that provide sweet taste without excessive caloric load. In order to better understand the overall actions of artificial sweeteners, especially when they are chronically used, we investigated the peripheral and central nervous system effects of protracted exposure to a widely used artificial sweetener, acesulfame K (ACK). We found that extended ACK exposure (40 weeks) in normal C57BL/6J mice demonstrated a moderate and limited influence on metabolic homeostasis, including altering fasting insulin and leptin levels, pancreatic islet size and lipid levels, without affecting insulin sensitivity and bodyweight. Interestingly, impaired cognitive memory functions (evaluated by Morris Water Maze and Novel Objective Preference tests) were found in ACK-treated C57BL/6J mice, while no differences in motor function and anxiety levels were detected. The generation of an ACK-induced neurological phenotype was associated with metabolic dysregulation (glycolysis inhibition and functional ATP depletion) and neurosynaptic abnormalities (dysregulation of TrkB-mediated BDNF and Akt/Erk-mediated cell growth/survival pathway) in hippocampal neurons. Our data suggest that chronic use of ACK could affect cognitive functions, potentially via altering neuro-metabolic functions in male C57BL/6J mice.

A case report of recovery of menstrual function following a nutritional intervention in two exercising women with amenorrhea of varying duration

Increasing caloric intake is a promising treatment for exercise-associated amenorrhea, but strategies have not been fully explored. The purpose of this case report was to compare and contrast the responses of two exercising women with amenorrhea of varying duration to an intervention of increased energy intake. Two exercising women with amenorrhea of short (3 months) and long (11 months) duration were chosen to demonstrate the impact of increased caloric intake on recovery of menstrual function and bone health. Repeated measures of dietary intake, eating behavior, body weight, body composition, bone mineral density, resting energy expenditure, exercise volume, serum metabolic hormones and markers of bone turnover, and daily urinary metabolites were obtained. Participant 1 was 19 years old and had a body mass index (BMI) of 20.4 kg/m² at baseline. She increased caloric intake by 276 kcal/day (1,155 kJ/day, 13%), on average, during the intervention, and her body mass increased by 4.2 kg (8%). Participant 2 was 24 years old and had a BMI of 19.7 kg/m². She increased caloric intake by 1,881 kcal/day (7,870 kJ/day, 27%) and increased body mass by 2.8 kg (5%). Resting energy expenditure, triiodothyronine, and leptin increased; whereas, ghrelin decreased in both women. Resumption of menses occurred 23 and 74 days into the intervention for the women with short-term and long-term amenorrhea, respectively. The onset of ovulation and regular cycles corresponded with changes in body weight. Recovery of menses coincided closely with increases in caloric intake, weight gain, and improvements in the metabolic environment; however, the nature of restoration of menstrual function differed between the women with short-term versus long-term amenorrhea.

Insulin resistance is associated with elevated serum pigment epithelium-derived factor (PEDF) levels in morbidly obese patients

Obesity is a significant risk factor for developing diabetes. Pigment epithelium-derived factor (PEDF) has been identified by experimental and clinical studies as both a causative and counter-regulatory factor in the metabolic syndrome. We set out to determine whether serum PEDF levels correlated with the degree of insulin resistance in morbidly obese patients. Sera from 53 patients who were evaluated prior to gastric bypass surgery were analyzed for PEDF levels using a commercial ELISA. None of the patients were on diabetes medications prior to enrollment. Baseline data included BMI, serum glucose and insulin, and homeostasis model assessment (HOMA) scores. Patients were stratified based on HOMA score and glucose levels into three groups: insulin sensitive (IS): HOMA <2 and glucose <126; insulin resistant (IR): HOMA >2 and glucose ≤126; and diabetes mellitus (DM): HOMA >2 and glucose >126. Pre- and post-gastric bypass sera from 12 patients were obtained for serial assessment of metabolic parameters and PEDF levels. PEDF secretion was assessed in primary human hepatocytes, HCC cells, and cultured adipocytes in the absence and presence of high glucose media. No significant differences in age, gender, and BMI were found among the three groups. PEDF levels were similar between IR patients and the other groups, but were significantly higher in DM compared to IS patients (p = 0.01). Serum PEDF in individual patients declined significantly after gastric bypass (p = 0.006). High glucose media led to significantly higher PEDF release by human hepatocytes in vitro (p = 0.016). These data demonstrate that serum PEDF concentrations better relate to insulin resistance than to adiposity and suggest that PEDF expression is closely linked to the development of insulin resistance.

Stimulation of leptin secretion by insulin

Leptin has a crucial role in regulating food intake and maintaining metabolic homeostasis. Although little is known about the process of leptin secretion, insulin, which has an important role in the metabolism of glucose and lipids, is believed to regulate leptin secretion through a posttranscriptional mechanism in the short term, and via glucose metabolism in the long term. The gastric mucosa secretes leptin, but this mechanism has not been completely elucidated. Understanding the mechanism of insulin-regulated leptin secretion could lead to the development of new treatment methods for obesity and its comorbidities, which are serious public health concerns.

Differential effects of environmental chemicals and food contaminants on adipogenesis, biomarker release and PPARγ activation

Eleven environmental relevant chemicals were investigated for their ability to affect adipogenesis in vitro, biomarker release from adipocytes and PPARα and γ activation. We found that butylparaben stimulated adipogenesis in 3T3-L1 adipocytes and increased release of leptin, adiponectin and resistin from the cells. Butylparaben activated PPARγ as well, which may be a mediator of the adipogenic effect. Polychlorinated biphenyl (PCB)153 also stimulate adipogenesis and biomarker release, but did not affect PPARs. The data indicates that PPARγ activating chemicals often stimulate adipocyte differentiation although PPARγ activation is neither a requirement nor a guarantee for stimulation. Four out of the eleven chemicals (bisphenol A, mono-ethylhexyl phthalate, butylparaben, PCB 153) caused increased adipogenesis. The release of adipocyte-secreted hormones was sometimes but not always correlated with the effect on adipocyte differentiation. Eight chemicals were able to cause increased leptin release. These findings strengthen the hypothesis that chemicals can interfere with pathways related to obesity development.

Relationship between circulating adipokines and insulin resistance during pregnancy and postpartum in women with gestational diabetes

BACKGROUND AND AIMS

We undertook this study to assess the relationship between circulating adipokines and insulin resistance during pregnancy and postpartum in women with gestational diabetes mellitus (GDM).

METHODS

This was a prospective study including 60 women with GDM and 60 subjects with normal gestation who were evaluated at gestational week 30, 6 weeks and 6 months postpartum. Circulating adipokines that were evaluated during the study were leptin, adiponectin, retinol-binding protein-4 (RBP4), and tumor necrosis factor-alpha (TNF-α).

RESULTS

Women with GDM showed higher insulin resistance measured by HOMA-IR than subjects with normal gestation (2.3 ± 2.3 vs. 1.3 ± 0.95). There was no difference between groups in adipokines; however, in women with a healthy pregnancy, RBP4 was associated with insulin resistance (r = 0.47, p <0.05). At 6 weeks and 6 months postpartum, women with previous GDM exhibited persistent elevated leptin and insulin resistance. RBP4 was associated with insulin resistance only in women with a previous healthy pregnancy (r = 0.51, p <0.05). In addition, progressively impaired glucose tolerance was observed after delivery in women with previous GDM.

CONCLUSIONS

It was demonstrated that GDM is associated with greater insulin resistance than observed in normal pregnancy; however, adipokines are similar in both groups. RBP4 levels are significantly associated with insulin resistance in healthy women during pregnancy and postpartum. After a pregnancy complicated by GDM, leptin and insulin resistance remain elevated and glucose tolerance worsens.

Dopamine receptors in human adipocytes: expression and functions

Introduction

Dopamine (DA) binds to five receptors (DAR), classified by their ability to increase (D1R-like) or decrease (D2R-like) cAMP. In humans, most DA circulates as dopamine sulfate (DA-S), which can be de-conjugated to bioactive DA by arylsulfatase A (ARSA). The objective was to examine expression of DAR and ARSA in human adipose tissue and determine whether DA regulates prolactin (PRL) and adipokine expression and release.

Methods

DAR were analyzed by RT-PCR and Western blotting in explants, primary adipocytes and two human adipocyte cell lines, LS14 and SW872. ARSA expression and activity were determined by qPCR and enzymatic assay. PRL expression and release were determined by luciferase reporter and Nb2 bioassay. Analysis of cAMP, cGMP, leptin, adiponectin and interleukin 6 (IL-6) was done by ELISA. Activation of MAPK and PI3 kinase/Akt was determined by Western blotting.

Results

DAR are variably expressed at the mRNA and protein levels in adipose tissue and adipocytes during adipogenesis. ARSA activity in adipocyte increases after differentiation. DA at nM concentrations suppresses cAMP, stimulates cGMP, and activates MAPK in adipocytes. Acting via D2R-like receptors, DA and DA-S inhibit PRL gene expression and release. Acting via D1R/D5R receptors, DA suppresses leptin and stimulates adiponectin and IL-6 release.

Conclusions

This is the first report that human adipocytes express functional DAR and ARSA, suggesting a regulatory role for peripheral DA in adipose functions. We speculate that the propensity of some DAR-activating antipsychotics to increase weight and alter metabolic homeostasis is due, in part, to their direct action on adipose tissue.

Adipokine ganglioside GM2 activator protein stimulates insulin secretion

Recently, we identified ganglioside GM2 activator protein (GM2AP) as a novel adipokine, and revealed that treatment of cultured cells with GM2AP impairs insulin signal transduction. The aim of this study was to examine the impact of GM2AP on glucose metabolism in vivo. Injection of recombinant GM2AP in mice significantly lowered blood glucose levels in glucose tolerance tests. Administration of GM2AP to mice for 10 days increased serum insulin levels, whereas the contents of glucose, leptin and FFA were significantly decreased. Stimulation of calcium influx and insulin secretion by GM2AP was observed in hamster insulinoma HIT-T15 cells. Blockage of GM2AP function by specific antibodies inhibited GM2AP-induced insulin secretion. These results provide novel insights into the physiological functions of GM2AP in obesity.

Impact of clock gene Bmal1 deficiency on nutritionally induced obesity in mice

To evaluate the hypothesis that the clock gene Bmal1 (brain and muscle arnt like protein-1) plays a role in the development of obesity, 5-week-old male Bmal1-deficient (Bmal1(-/-)) mice and wild-type littermates (Bmal1(+/+)) were kept on a high-fat diet (HFD) for 15 weeks. Despite an initial accelerated weight gain of Bmal1(-/-) mice, body weight and subcutaneous (SC) and gonadal (GON) adipose tissue mass were comparable to Bmal1(+/+) mice at the end of the diet period. Noninvasive magnetic resonance imaging scanning revealed a modest increase in fat content in Bmal1(-/-) mice after 10 weeks of HFD, whereas at the start and the end of the HFD feeding no differences were observed between both genotypes. After 15 weeks of HFD, adipocyte and blood vessel size and density were similar for Bmal1(+/+) and Bmal1(-/-) mice. However, the weight of major organs was significantly reduced in Bmal1(-/-) mice, confirming the premature ageing phenotype. Thus, we hypothesize that an initial accelerated increase in body weight and fat mass of Bmal1(-/-) mice on HFD may have been offset by the effect of premature ageing on organ weight, resulting in comparable weights after 15 weeks of HFD.

Low-abundant adiponectin receptors in visceral adipose tissue of humans and rats are further reduced in diabetic animals

BACKGROUND AND AIMS

Adipose tissue is an endocrine organ that releases various proteins that may also exert autocrine/paracrine effects. The antidiabetic adipokine adiponectin acts through two receptors, AdipoR1 and AdipoR2, but so far mainly mRNA expression has been measured in adipocytes and adipose tissues. Therefore, we aimed to analyze AdipoR1 and AdipoR2 proteins in adipocytes and paired samples of subcutaneous and visceral adipocytes/adipose tissue.

METHODS

AdipoR1 and AdipoR2 mRNA and protein expression were determined in adipocytes and paired samples of subcutaneous and visceral adipose tissue of humans and rats.

RESULTS

AdipoR1 and AdipoR2 proteins were similarly abundant in preadipocytes and mature adipocytes despite an induction of mRNA expression during differentiation. Differentiation of 3T3-L1 cells in the presence of palmitic acid did not alter adiponectin receptor proteins but metformin and fenofibrate upregulated AdipoR2 within 24 h of incubation. AdipoR2 protein was significantly lower in human visceral compared to subcutaneous fat, and both receptors were reduced in visceral adipocytes. In rat tissues both receptors were reduced in visceral fat. In diabetic animals AdipoR2 protein, but not mRNA, was lower in both fat depots compared to similarly obese rats with normal glucose disposal. AdipoR1 was only reduced in subcutaneous adipose tissue of diabetic animals where mRNA expression was induced.

CONCLUSIONS

These data indicate that mRNA expression is not suitable to predict adiponectin receptor protein. Low adiponectin receptors in visceral adipocytes and adipose tissue and further suppression in adipose tissue of insulin-resistant animals indicate disturbed adiponectin bioactivity.

Integration of hormonal and nutrient signals that regulate leptin synthesis and secretion

This review summarizes recent advances in our understanding of the pre- and posttranscriptional mechanisms that regulate leptin production and secretion in adipocytes. Basal leptin production is proportional to the status of energy stores, i.e., fat cell size, and this is mainly regulated by alterations in leptin mRNA levels. Leptin mRNA levels are regulated by hormones, including glucocorticoids and catecholamines, but little is known about the transcriptional mechanisms involved. Leptin synthesis and secretion is also acutely modulated in response to hormones such as insulin and the availability of metabolic fuels. Acute variations in leptin production over a time course of minutes to hours are mediated at the levels of both translation and secretion. Increases in amino acids and insulin after a meal activate the mammalian target of rapamycin (mTOR) pathway, leading to an increase in specific rates of leptin biosynthesis. Cross-talk among mTOR, PKA, and AMP-activated protein kinase pathways appears to integrate hormonal and nutrient signals that regulate leptin mRNA translation, at least in part through mechanisms involving its 5'- and 3'-untranslated regions. In addition, the rate of leptin secretion from preformed stores in response to hormonal cues is also regulated. Insulin stimulates, and adrenergic agonists inhibit, leptin secretion, and this likely contributes to variations in the magnitude of nutrition-related leptin excursions and oscillations. Overall, the study of leptin production has contributed to a deepening understanding of leptin biology and, more broadly, to our understanding of the cellular and molecular mechanisms by which the adipocyte integrates hormonal and nutrient signals to regulate adipokine production.