Subcutaneous administration of leptin normalizes fasting plasma glucose in obese type 2 diabetic UCD-T2DM rats. Proc Natl Acad Sci U S A. 2011;108:14670-14675. [PMID: 21873226 DOI: 10.1073/pnas.1107163108]

Effects of Statin and Annatto-extracted Tocotrienol Supplementation on Glucose Homeostasis, Bone Microstructure, and Gut Microbiota Composition in Obese Mice

BACKGROUND/AIM

This study examined the effects of tocotrienols (TT) in conjunction with statin on glucose homeostasis, bone microstructure, gut microbiome, and systemic and liver inflammatory markers in obese C57BL/6J mice.

MATERIALS AND METHODS

Forty male C57BL/6J mice were fed a high-fat diet (HFD) and assigned into four groups in a 2 (no statin vs. 120 mg statin/kg diet)×2 (no TT vs. 400 mg TT/kg diet) factorial design for 14 weeks.

RESULTS

Statin and TT improved glucose tolerance only when each was given alone, and only statin supplementation decreased insulin resistance. Consistently, only statin supplementation decreased serum insulin levels and HOMA-IR. Pancreatic insulin was also increased with statin treatment. Statin and TT, alone or in combination, reduced the levels of serum IL-6, but only TT attenuated the increased serum leptin levels induced by a HFD. Statin supplementation increased bone area/total area and connectivity density at LV-4, while TT supplementation increased bone area/total area and trabecular number, but decreased trabecular separation at the distal femur. Statin supplementation, but not TT, reduced hepatic inflammatory cytokine gene expression. Neither TT supplementation nor statin supplementation statistically altered microbiome species evenness or richness. However, they altered the relative abundance of certain microbiome species. Most notably, both TT and statin supplementation increased the relative abundance of Lachnospiraceae UCG-006.

CONCLUSION

TT and statin collectively benefit bone microstructure, glucose homeostasis, and microbial ecology in obese mice. Such changes may be, in part, associated with suppression of inflammation in the host.

Contribution of animal models to diabetes research: Its history, significance, and translation to humans

Diabetes mellitus is still expanding globally and is epidemic in developing countries. The combat of this plague has caused enormous economic and social burdens related to a lowered quality of life in people with diabetes. Despite recent significant improvements of life expectancy in patients with diabetes, there is still a need for efforts to elucidate the complexities and mechanisms of the disease processes to overcome this difficult disorder. To this end, the use of appropriate animal models in diabetes studies is invaluable for translation to humans and for the development of effective treatment. In this review, a variety of animal models of diabetes with spontaneous onset in particular will be introduced and discussed for their implication in diabetes research.

Reactive Oxygen and Nitrogen Species (RONS) and Cytokines-Myokines Involved in Glucose Uptake and Insulin Resistance in Skeletal Muscle

Insulin resistance onset in skeletal muscle is characterized by the impairment of insulin signaling, which reduces the internalization of glucose, known as glucose uptake, into the cell. Therefore, there is a deficit of intracellular glucose, which is the main source for energy production in the cell. This may compromise cellular viability and functions, leading to pathological dysfunction. Skeletal muscle fibers continuously generate reactive oxygen and nitrogen species (RONS). An excess of RONS produces oxidative distress, which may evoke cellular damage and dysfunction. However, a moderate level of RONS, which is called oxidative eustress, is critical to maintain, modulate and regulate cellular functions through reversible interactions between RONS and the components of cellular signaling pathways that control those functions, such as the facilitation of glucose uptake. The skeletal muscle releases peptides called myokines that may have endocrine and paracrine effects. Some myokines bind to specific receptors in skeletal muscle fibers and might interact with cellular signaling pathways, such as PI3K/Akt and AMPK, and facilitate glucose uptake. In addition, there are cytokines, which are peptides produced by non-skeletal muscle cells, that bind to receptors at the plasma membrane of skeletal muscle cells and interact with the cellular signaling pathways, facilitating glucose uptake. RONS, myokines and cytokines might be acting on the same signaling pathways that facilitate glucose uptake in skeletal muscle. However, the experimental studies are limited and scarce. The aim of this review is to highlight the current knowledge regarding the role of RONS, myokines and cytokines as potential signals that facilitate glucose uptake in skeletal muscle. In addition, we encourage researchers in the field to lead and undertake investigations to uncover the fundamentals of glucose uptake evoked by RONS, myokines, and cytokines.

Cytokines and regulation of glucose and lipid metabolism in the obesity

The article presents data of the influence of cytokines of different directions of glucose and lipid metabolism in obesity. A change of the basic paradigm regarding adipose tissue has contributed to a number of recent discoveries. This concerns such basic concepts as healthy and diseased adipocytes, and, as a consequence, changes of their metabolism under the influence of cytokins. Distinguishing the concept of organokines demonstrates that despite the common features of cytokine regulation, each organ has its own specifics features of cytokine regulation, each organ has its own specific an important section of this concept is the idea of the heterogeneity of adipose tissue. Knowledge of the function of adipose tissue localized in different compartments of the body is expanding. There are date about the possibility of transition of one type of adipose tissue to another. A possible mechanism linking adipose tissue inflammation and the formation of insulin resistance (IR) is presented in this paper. The mechanism of IR development is closely connected with to proinflammatory cytokins disordering the insulin signal, accompanied by a decrease of the work of glucose transporters. A decrease of the income of glucose into cells leads to a change of glycolysis level to an increase of the fatty acids oxidation. Cytokins are able to participate in the process of the collaboration of some cells with others, that occurs both during physiological and pathological process.

Pancreatic Islets as a Target of Adipokines

Rising rates of obesity are intricately tied to the type 2 diabetes epidemic. The adipose tissues can play a central role in protection against or triggering metabolic diseases through the secretion of adipokines. Many adipokines may improve peripheral insulin sensitivity through a variety of mechanisms, thereby indirectly reducing the strain on beta cells and thus improving their viability and functionality. Such effects will not be the focus of this article. Rather, we will focus on adipocyte-secreted molecules that have a direct effect on pancreatic islets. By their nature, adipokines represent potential druggable targets that can reach the islets and improve beta-cell function or preserve beta cells in the face of metabolic stress. © 2022 American Physiological Society. Compr Physiol 12:1-27, 2022.

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.

Zucker Diabetic Sprague Dawley rat (ZDSD): type 2 diabetes translational research model

New Findings

What is the topic of this review?

The Zucker Diabetic‐Sprague Dawley (ZDSD) rat is in the early adoption phase of use by researchers in the fields of diabetes, including prediabetes, obesity and metabolic syndrome. It is essential that physiology researchers choose preclinical models that model human type 2 diabetes appropriately and are aware of the limitations on experimental design.

What advances does it highlight?

Our review of the scientific literature finds that although sex, age and diets contribute to variability, the ZDSD phenotype and disease progression model the characteristics of humans who have prediabetes and diabetes, including co‐morbidities.

Abstract

Type 2 diabetes (T2D) is a prevalent disease and a significant concern for global population health. For persons with T2D, clinical treatments target not only the characteristics of hyperglycaemia and insulin resistance, but also co‐morbidities, such as obesity, cardiovascular and renal disease, neuropathies and skeletal bone conditions. The Zucker Diabetic‐Sprague Dawley (ZDSD) rat is a rodent model developed for experimental studies of T2D. We reviewed the scientific literature to highlight the characteristics of T2D development and the associated phenotypes, such as metabolic syndrome, cardiovascular complications and bone and skeletal pathologies in ZDSD rats. We found that ZDSD phenotype characteristics are independent of leptin receptor signalling. The ZDSD rat develops prediabetes, then progresses to overt diabetes that is accelerated by introduction of a timed high‐fat diet. In male ZDSD rats, glycated haemoglobin (HbA1c) increases at a constant rate from 7 to >30 weeks of age. Diabetic ZDSD rats are moderately hypertensive compared with other rat strains. Diabetes in ZDSD rats leads to endothelial dysfunction in specific vasculatures, impaired wound healing, decreased systolic and diastolic cardiac function, neuropathy and nephropathy. Changes to bone composition and the skeleton increase the risk of bone fractures. Zucker Diabetic‐Sprague Dawley rats have not yet achieved widespread use by researchers. We highlight sex‐related differences in the ZDSD phenotype and gaps in knowledge for future studies. Overall, scientific data support the premise that the phenotype and disease progression in ZDSD rats models the characteristics in humans. We conclude that ZDSD rats are an advantageous model to advance understanding and discovery of treatments for T2D through preclinical research.

Free fatty acid receptors in the endocrine regulation of glucose metabolism: Insight from gastrointestinal-pancreatic-adipose interactions

Glucose metabolism is primarily controlled by pancreatic hormones, with the coordinated assistance of the hormones from gastrointestine and adipose tissue. Studies have unfolded a sophisticated hormonal gastrointestinal-pancreatic-adipose interaction network, which essentially maintains glucose homeostasis in response to the changes in substrates and nutrients. Free fatty acids (FFAs) are the important substrates that are involved in glucose metabolism. FFAs are able to activate the G-protein coupled membrane receptors including GPR40, GPR120, GPR41 and GPR43, which are specifically expressed in pancreatic islet cells, enteroendocrine cells as well as adipocytes. The activation of FFA receptors regulates the secretion of hormones from pancreas, gastrointestine and adipose tissue to influence glucose metabolism. This review presents the effects of the FFA receptors on glucose metabolism via the hormonal gastrointestinal-pancreatic-adipose interactions and the underlying intracellular mechanisms. Furthermore, the development of therapeutic drugs targeting FFA receptors for the treatment of abnormal glucose metabolism such as type 2 diabetes mellitus is summarized.

Insulin Increases Adipose Adiponectin in Pregnancy by Inhibiting Ubiquitination and Degradation: Impact of Obesity

CONTEXT

Circulating adiponectin levels are decreased in pregnant women with obesity or gestational diabetes, and this is believed to contribute to the insulin resistance and increased risk of fetal overgrowth associated with these conditions. However, the molecular mechanisms regulating adiponectin secretion from maternal adipose tissues in pregnancy are poorly understood.

OBJECTIVE

We tested the hypothesis that obesity in pregnancy is associated with adipose tissue insulin resistance and increased adiponectin ubiquitination and degradation, caused by inflammation and endoplasmic reticulum (ER) stress.

METHODS

Visceral adipose tissues were collected from lean and obese pregnant humans and mice. Total and ubiquitinated adiponectin, and markers of inflammation, ER stress, and insulin resistance were examined in adipose tissues. The role of insulin, inflammation, and ER stress in mediating adiponectin ubiquitination and degradation was examined using 3T3L-1 adipocytes.

RESULTS

Obesity in pregnancy is associated with adipose tissue inflammation, ER stress, insulin resistance, increased adiponectin ubiquitination, and decreased total abundance of adiponectin. Adiponectin ubiquitination was increased in visceral fat of obese pregnant women as compared to lean pregnant women. We further observed that insulin prevents, whereas ER stress and inflammation promote, adiponectin ubiquitination and degradation in differentiated 3T3-L1 adipocytes.

CONCLUSION

We have identified adiponectin ubiquitination as a key mechanism by which obesity diminishes adiponectin secretion in pregnancy. This information will help us better understand the mechanisms controlling maternal insulin resistance and fetal growth in pregnancy and may provide a foundation for the development of strategies aimed at improving adiponectin production in pregnant women with obesity or gestational diabetes.

Hyperpolarized NMR study of the impact of pyruvate dehydrogenase kinase inhibition on the pyruvate dehydrogenase and TCA flux in type 2 diabetic rat muscle

The role of pyruvate dehydrogenase in mediating lipid-induced insulin resistance stands as a central question in the pathogenesis of type 2 diabetes mellitus. Many researchers have invoked the Randle hypothesis to explain the reduced glucose disposal in skeletal muscle by envisioning an elevated acetyl CoA pool arising from increased oxidation of fatty acids. Over the years, in vivo NMR studies have challenged that monolithic view. The advent of the dissolution dynamic nuclear polarization NMR technique and a unique type 2 diabetic rat model provides an opportunity to clarify. Dynamic nuclear polarization enhances dramatically the NMR signal sensitivity and allows the measurement of metabolic kinetics in vivo. Diabetic muscle has much lower pyruvate dehydrogenase activity than control muscle, as evidenced in the conversion of [1-¹³C]lactate and [2-¹³C]pyruvate to HCO₃^- and acetyl carnitine. The pyruvate dehydrogenase kinase inhibitor, dichloroacetate, restores rapidly the diabetic pyruvate dehydrogenase activity to control level. However, diabetic muscle has a much larger dynamic change in pyruvate dehydrogenase flux than control. The dichloroacetate-induced surge in pyruvate dehydrogenase activity produces a differential amount of acetyl carnitine but does not affect the tricarboxylic acid flux. Further studies can now proceed with the dynamic nuclear polarization approach and a unique rat model to interrogate closely the biochemical mechanism interfacing oxidative metabolism with insulin resistance and metabolic inflexibility.

Specific Binding of Leptin to Red Blood Cells Delivers a Pancreatic Hormone and Stimulates ATP Release

Since its discovery in 1994, leptin continues to have new potential physiological roles uncovered, including a role in the regulation of blood flow. Leptin's role in regulating blood flow is not completely understood. Red blood cell (RBC)-derived ATP is a recognized stimulus of blood flow, and multiple studies suggest that C-peptide, a hormone secreted in equimolar amounts with insulin from the pancreatic β-cells, can stimulate that release when delivered by albumin and in combination with Zn²⁺. Here, we report leptin delivers C-peptide and Zn²⁺ to RBCs in a saturable and specific manner. We labeled leptin with technetium-99 m (^99mTc) to perform binding studies while using albumin to block the specific binding of ^99mTc-leptin in the presence or absence of C-peptide. Our results suggest that leptin has a saturable and specific binding site on the RBC ((K_d = 1.79 ± 0.46) × 10^-7 M) that is statistically equal to the binding affinity in the presence of 20 nM C-peptide ((K_d = 2.05 ± 0.20) × 10^-7 M). While the binding affinity between leptin and the RBC did not change with C-peptide, the moles of bound leptin did increase with C-peptide, suggesting a separate binding site on the cell for a leptin/C-peptide complex. The RBC-derived ATP increased in the presence of a leptin/C-peptide/Zn²⁺ addition, in a concentration-dependent manner. Control RBCs ATP release increased (71 ± 5.6%) in the presence of C-peptide and Zn²⁺, which increased further to (94 ± 5.6%) in the presence of Zn²⁺, C-peptide, and leptin.

Da-Chai-Hu Decoction Ameliorates High Fat Diet-Induced Nonalcoholic Fatty Liver Disease Through Remodeling the Gut Microbiota and Modulating the Serum Metabolism

The dysbiosis in gut microbiota could affect host metabolism and contribute to the development of nonalcoholic fatty liver disease (NAFLD). Da-Chai-Hu decoction (DCH) has demonstrated protective effects on NAFLD, however, the exact mechanisms remain unclear. In this study, we established a NAFLD rat model using a high fat diet (HFD) and provided treatment with DCH. The changes in gut microbiota post DCH treatment were then investigated using 16S rRNA sequencing. Additionally, serum untargeted metabolomics were performed to examine the metabolic regulations of DCH on NAFLD. Our results showed that DCH treatment improved the dyslipidemia, insulin resistance (IR) and ameliorated pathological changes in NAFLD model rats. 16S rRNA sequencing and untargeted metabolomics showed significant dysfunction in gut microbiota community and serum metabolites in NAFLD model rats. DCH treatment restored the dysbiosis of gut microbiota and improved the dysfunction in serum metabolism. Correlation analysis indicated that the modulatory effects of DCH on the arachidonic acid (AA), glycine/serine/threonine, and glycerophospholipid metabolic pathways were related to alterations in the abundance of Romboutsia, Bacteroides, Lactobacillus, Akkermansia, Lachnoclostridium and Enterobacteriaceae in the gut microflora. In conclusion, our study revealed the ameliorative effects of DCH on NAFLD and indicated that DCH's function on NAFLD may link to the improvement of the dysbiosis of gut microbiota and the modulation of the AA, glycerophospholipid, and glycine/serine/threonine metabolic pathways.

Leptin concentrations in type 2 diabetes and non-diabetes Nigerian-Africans

Most studies on leptin in diabetes mellitus (DM) compared to healthy controls were done in Caucasians, with conflicting findings. Paucity of data on this exists in Nigerian-Africans. Therefore, the study determined plasma leptin concentrations in newly diagnosed type-2 diabetes versus controls and its relation to obesity/demographic-metabolic indices. A cross-sectional comparative study on 154 subjects: 67 diabetes and 87 healthy controls at the Ahmadu Bello University Teaching Hospital, Nigeria. Leptin was determined by the sandwich enzyme-linked immunosorbent assay. Mann-Whitney U test, Spearman's Correlation and Step-wise Multiple Logistic Regression analysis of Log-transformed variables determined outcomes. Leptin trended towards lower levels in DM subjects than controls when both sexes were combined, though insignificant (P=0.12). Leptin was significantly (P<0.001) positively correlated with waist circumference (WC) and body mass index (BMI) in DM (WC, r=0.71; BMI, r=0.84) as well as controls (WC, r=0.46, BMI, r=0.51), respectively. Leptin was significantly (P<0.001) higher in females than males, with approximately 2 times Odds of female sex association with log-transformed (Ln₁₀) hyperleptinaemia {Odds Ratio (OR): 1.9, 95% CI, 0.97-3.92, P<0.001}. Age was positively (r=0.21, P=0.05) correlated to leptin in controls, while fasting blood glucose (FBG) negatively correlated to uncontrolled DM (r=-0.26). Leptin showed no significant (P>0.05) correlation to fasting insulin (FI) and HOMA-IR. WC was an independent predictor of Ln₁₀hyperleptinaemia in DM subjects (OR: 1.12, 95% CI, 1.03-1.23, P=0.01). BMI showed significant (P<0.001) association with Ln₁₀hyperleptinaemia in both subjects. Conclusively, leptin trends towards lower levels but are not different in newly diagnosed DM than controls. The association of leptin with obesity is similar but stronger in diabetes than controls, with no relations to FI and HOMA-IR. WC and BMI are independent predictors of hyperleptinaemia.

Novel idebenone analogs block Shc's access to insulin receptor to improve insulin sensitivity

There has been little innovation in identifying novel insulin sensitizers. Metformin, developed in the 1920s, is still used first for most Type 2 diabetes patients. Mice with genetic reduction of p52Shc protein have improved insulin sensitivity and glucose tolerance. By high-throughput screening, idebenone was isolated as the first small molecule 'Shc Blocker'. Idebenone blocks p52Shc's access to Insulin Receptor to increase insulin sensitivity. In this work the avidity of 34 novel idebenone analogs and 3 metabolites to bind p52Shc, and to block the interaction of p52Shc with the Insulin receptor was tested. Our hypothesis was that if an idebenone analog bound and blocked p52Shc's access to insulin receptor better than idebenone, it should be a more effective insulin sensitizing agent than idebenone itself. Of 34 analogs tested, only 2 both bound p52Shc more tightly and/or blocked the p52Shc-Insulin Receptor interaction more effectively than idebenone. Of those 2 only idebenone analog #11 was a superior insulin sensitizer to idebenone. Also, the long-lasting insulin-sensitizing potency of idebenone in rodents over many hours had been puzzling, as the parent molecule degrades to metabolites within 1 h. We observed that two of the idebenone's three metabolites are insulin sensitizing almost as potently as idebenone itself, explaining the persistent insulin sensitization of this rapidly metabolized molecule. These results help to identify key SAR = structure-activity relationship requirements for more potent small molecule Shc inhibitors as Shc-targeted insulin sensitizers for type 2 diabetes.

CNS control of the endocrine pancreas

Increasing evidence suggests that, although pancreatic islets can function autonomously to detect and respond to changes in the circulating glucose level, the brain cooperates with the islet to maintain glycaemic control. Here, we review the role of the central and autonomic nervous systems in the control of the endocrine pancreas, including mechanisms whereby the brain senses circulating blood glucose levels. We also examine whether dysfunction in these systems might contribute to complications of type 1 diabetes and the pathogenesis of type 2 diabetes. Graphical abstract.

Moringa Isothiocyanate-rich Seed Extract Delays the Onset of Diabetes in UC Davis Type-2 Diabetes Mellitus Rats

Moringa seeds have been used traditionally in the management of type 2 diabetes mellitus (T2DM) and contain potent bioactive isothiocyanates. This study evaluated the efficacy of an isothiocyanate-rich moringa seed extract in delaying the onset of T2DM in UC Davis T2DM rats, a well validated model which closely mimics T2DM in humans. Rats were separated into three groups; control, moringa seed extract at 0.4%, and a weight matched group. Rats were fed respective diets for 8 months, during which energy intake, body weight, the onset of diabetes circulating hormones, metabolites and markers of inflammation and liver function, and were monitored. The MS group had a significantly slower rate of diabetes onset p = 0.027), lower plasma glucose (p = 0.043), and lower HbA1c (p = 0.008) compared with CON animals. There were no significant differences in food intake and body weight between all groups. This study demonstrated MS can delay the onset of diabetes in the UC Davis T2DM rat model to a greater extent than moderate caloric restriction (by comparison to the WM group). The results support its documented traditional uses and a bioactive role of moringa isothiocyanates and suggest the potential efficacy for moringa supplementation for diabetes management in populations at risk for T2DM.

Adipokines as key players in β cell function and failure

The growing prevalence of obesity and its related metabolic diseases, mainly Type 2 diabetes (T2D), has increased the interest in adipose tissue (AT) and its role as a principal metabolic orchestrator. Two decades of research have now shown that ATs act as an endocrine organ, secreting soluble factors termed adipocytokines or adipokines. These adipokines play crucial roles in whole-body metabolism with different mechanisms of action largely dependent on the tissue or cell type they are acting on. The pancreatic β cell, a key regulator of glucose metabolism due to its ability to produce and secrete insulin, has been identified as a target for several adipokines. This review will focus on how adipokines affect pancreatic β cell function and their impact on pancreatic β cell survival in disease contexts such as diabetes. Initially, the "classic" adipokines will be discussed, followed by novel secreted adipocyte-specific factors that show therapeutic promise in regulating the adipose-pancreatic β cell axis.

Mesenteric arterial dysfunction in the UC Davis Type 2 Diabetes Mellitus rat model is dependent on pre-diabetic versus diabetic status and is sexually dimorphic

Previous reports suggest that diabetes may differentially affect the vascular beds of females and males. However, there is insufficient evidence to establish the timeline of the vascular dysfunction in diabetes, specifically in relation to sex. Here, we determined whether mesenteric arterial function is altered in UC Davis Type-2 Diabetes Mellitus (UCD-T2DM) rats and if this occurs as early as the pre-diabetic stage of the disease. Specifically, we investigated whether vascular dysfunction differs between pre-diabetic or diabetic status and if this varies by sex. We measured the responses to endothelium-dependent and -independent vasorelaxant as well as vasoconstrictor agents and explored the potential mechanisms involved in sex-specific development of arterial dysfunction in UCD-T2DM rats. In addition, indices of insulin sensitivity were assessed. We report the reduced insulin sensitivity in pre-diabetic males and diabetic females. Vascular relaxation to acetylcholine was impaired to a greater extent in mesenteric artery from males in the pre-diabetic stage than in their female counterparts. In contrast, the arteries from females with diabetes exhibited a greater impairment to acetylcholine compared with diabetic males. Additionally, the sensitivity of mesenteric artery to contractile agents in females, but not in males, after the onset of diabetes was increased. Our data suggest that the reduced insulin sensitivity through AKT may predispose vessels to injury in the pre-diabetic stage in males. On the other hand, reduced insulin sensitivity as well as enhanced responsiveness to contractile agents may predispose arteries to injury in the diabetic stage in females.

Associations of leptin and adiponectin with incident type 2 diabetes and interactions among African Americans: the Jackson heart study

BACKGROUND

Growing evidence suggests that leptin is critical for glycemic control. Impaired leptin signaling may also contribute to low adiponectin expression in obese individuals. We assessed the association of leptin and adiponectin with incident type 2 diabetes (T2D), their interactions with sex and obesity status, and mediation by insulin resistance.

METHODS

We included study participants from the Jackson Heart Study, a prospective cohort of adult African Americans in Jackson, Mississippi, that were free of T2D at the baseline Exam 1. Incident T2D was defined as new cases at Exam 2 or Exam 3. We created separate Cox regression models (hazard ratios per log-transformed ng/mL of leptin and adiponectin) with and without insulin resistance, HOMA-IR. Mediation by insulin resistance was analyzed. Several interactions were assessed, including by sex, HbA1c, and obesity.

RESULTS

Among our 3363 participants (mean age 53 years, 63% women), 584 developed incident T2D. Leptin was directly associated with incident T2D when modeled without HOMA-IR (HR = 1.29, 95% CI = 1.05-1.58). This direct association between leptin and T2D was significant among men (HR = 1.33, 95% CI = 1.05-1.69), but nonsignificant among women (HR = 1.24, 95% CI = 0.94-1.64); statistical interaction with sex was nonsignificant (p = 0.65). The associations in all participants and in men were nullified by HOMA-IR (HR = 0.99, 95% CI = 0.80-1.22; HR = 1.00, 95% CI = 0.78-1.28, respectively), indicating mediation through insulin resistance (proportion mediated: 1.04), and were not observed in abdominally obese participants. Adiponectin was inversely associated with T2D even after adjustment for HOMA-IR in women (HR = 0.68, 95% CI = 0.55-0.84), but not in men (HR = 0.80, 95% CI = 0.62-1.04). The inverse association was present only among abdominally obese participants, and persisted after adjustment for HOMA-IR.

CONCLUSIONS

Among African Americans in the Jackson Heart Study the association of leptin with incident type 2 diabetes was mediated by insulin resistance. This association was present only among abdominally non-obese participants. Differences by sex appeared: men showed a significant association mediated by insulin resistance. Among abdominally obese participants, adiponectin was inversely associated with incident T2D even after adjustment for HOMA-IR. Our results should inform future clinical trials that aim to reduce the burden of type 2 diabetes through the modification of serum levels of leptin and adiponectin.

The Novel Perspectives of Adipokines on Brain Health

First seen as a fat-storage tissue, the adipose tissue is considered as a critical player in the endocrine system. Precisely, adipose tissue can produce an array of bioactive factors, including cytokines, lipids, and extracellular vesicles, which target various systemic organ systems to regulate metabolism, homeostasis, and immune response. The global effects of adipokines on metabolic events are well defined, but their impacts on brain function and pathology remain poorly defined. Receptors of adipokines are widely expressed in the brain. Mounting evidence has shown that leptin and adiponectin can cross the blood–brain barrier, while evidence for newly identified adipokines is limited. Significantly, adipocyte secretion is liable to nutritional and metabolic states, where defective circuitry, impaired neuroplasticity, and elevated neuroinflammation are symptomatic. Essentially, neurotrophic and anti-inflammatory properties of adipokines underlie their neuroprotective roles in neurodegenerative diseases. Besides, adipocyte-secreted lipids in the bloodstream can act endocrine on the distant organs. In this article, we have reviewed five adipokines (leptin, adiponectin, chemerin, apelin, visfatin) and two lipokines (palmitoleic acid and lysophosphatidic acid) on their roles involving in eating behavior, neurotrophic and neuroprotective factors in the brain. Understanding and regulating these adipokines can lead to novel therapeutic strategies to counteract metabolic associated eating disorders and neurodegenerative diseases, thus promote brain health.

Cytokines and Abnormal Glucose and Lipid Metabolism

Clear evidence indicates that cytokines, for instance, adipokines, hepatokines, inflammatory cytokines, myokines, and osteokines, contribute substantially to the development of abnormal glucose and lipid metabolism. Some cytokines play a positive role in metabolism action, while others have a negative metabolic role linking to the induction of metabolic dysfunction. The mechanisms involved are not fully understood, but are associated with lipid accumulation in organs and tissues, especially in the adipose and liver tissue, changes in energy metabolism, and inflammatory signals derived from various cell types, including immune cells. In this review, we describe the roles of certain cytokines in the regulation of metabolism and inter-organ signaling in regard to the pathophysiological aspects. Given the disease-related changes in circulating levels of relevant cytokines, these factors may serve as biomarkers for the early detection of metabolic disorders. Moreover, based on preclinical studies, certain cytokines that can induce improvements in glucose and lipid metabolism and immune response may emerge as novel targets of broader and more efficacious treatments and prevention of metabolic disease.

Emerging hormonal-based combination pharmacotherapies for the treatment of metabolic diseases

Obesity and its comorbidities, such as type 2 diabetes mellitus and cardiovascular disease, constitute growing challenges for public health and economies globally. The available treatment options for these metabolic disorders cannot reverse the disease in most individuals and have not substantially reduced disease prevalence, which underscores the unmet need for more efficacious interventions. Neurobiological resilience to energy homeostatic perturbations, combined with the heterogeneous pathophysiology of human metabolic disorders, has limited the sustainability and efficacy of current pharmacological options. Emerging insights into the molecular origins of eating behaviour, energy expenditure, dyslipidaemia and insulin resistance suggest that coordinated targeting of multiple signalling pathways is probably necessary for sizeable improvements to reverse the progression of these diseases. Accordingly, a broad set of combinatorial approaches targeting feeding circuits, energy expenditure and glucose metabolism in concert are currently being explored and developed. Notably, several classes of peptide-based multi-agonists and peptide-small molecule conjugates with superior preclinical efficacy have emerged and are currently undergoing clinical evaluation. Here, we summarize advances over the past decade in combination pharmacotherapy for the management of obesity and type 2 diabetes mellitus, exclusively focusing on large-molecule formats (notably enteroendocrine peptides and proteins) and discuss the associated therapeutic opportunities and challenges.

Adipose tissue transplantation ameliorates lipodystrophy-associated metabolic disorders in seipin-deficient mice

Seipin deficiency is responsible for type 2 congenital generalized lipodystrophy with severe loss of adipose tissue and can lead to hepatic steatosis, insulin resistance (IR), and dyslipidemia in humans. Adipose tissue secretes many adipokines that are central to the regulation of metabolism. In this study, we investigated whether transplantation of normal adipose tissue could ameliorate severe hepatic steatosis, IR, and dyslipidemia in lipoatrophic seipin knockout (SKO) mice. Normal adipose tissue from wild-type mice was transplanted into 6-wk-old SKO mice. At 4 mo after adipose tissue transplantation (AT), the transplanted fat survived with detectable blood vessels, and the reduced levels of plasma leptin, a major adipokine, were dramatically increased. Severe hepatic steatosis, IR, and dyslipidemia in SKO mice were ameliorated after AT. In addition, abnormal hepatic lipogenesis and β-oxidation gene expression in SKO mice were improved after AT. Our results suggest that AT may be an effective treatment to improve lipodystrophy-associated metabolic disorders.

BTBR ob/ob mouse model of type 2 diabetes exhibits early loss of retinal function and retinal inflammation followed by late vascular changes

AIMS/HYPOTHESIS

Diabetic retinopathy is increasing in prevalence worldwide and is fast becoming a global epidemic and a leading cause of visual loss. Current therapies are limited, and the development of effective treatments for diabetic retinopathy requires a greater in-depth knowledge of disease progression and suitable modelling of diabetic retinopathy in animals. The aim of this study was to assess the early pathological changes in retinal morphology and neuronal, inflammatory and vascular features consistent with diabetic retinopathy in the ob/ob mouse model of type 2 diabetes, to investigate whether features similar to those in human diabetic retinopathy were present.

METHODS

Male and female wild-type (+/+), heterozygous (+/-) and homozygous (-/-) BTBR ob/ob mice were examined at 6, 10, 15 and 20 weeks of age. Animals were weighed and blood glucose was measured. TUNEL and brain-specific homeobox/POU domain protein 3A (BRN3A) markers were used to examine retinal ganglion cells. We used immunostaining (collagen IV and platelet endothelial cell adhesion molecule [PECAM]/CD31) to reveal retinal vessel degeneration. Spectral domain optical coherence tomography was used to reveal changes in the thickness and structure of the retinal layer. Vitreous fluorophotometry was used to investigate vascular permeability. A-waves, b-waves and oscillatory potentials were measured under photopic and scotopic conditions. Concanavalin A leucostasis and immunostaining with glial fibrillary acidic protein (GFAP) and ionised calcium-binding adapter molecule 1 (IBA-1) identified differences in inflammatory status. Paraffin sections and transmission electron microscopy were used to reveal changes in the thickness and structure of the retinal layer.

RESULTS

Following the development of obesity and hyperglycaemia in 2-week-old and 3-week-old ob^-/ob^- mice, respectively (p < 0.001), early functional deficits (p < 0.001) and thinning of the inner retina (p < 0.001) were identified. Glial activation, leucostasis (p < 0.05) and a shift in microglia/macrophage phenotype were observed before microvascular degeneration (p < 0.05) and elevated vascular permeability occurred (p < 0.05).

CONCLUSIONS/INTERPRETATION

The present characterisation of the development of diabetic retinopathy in the ob/ob mouse represents a platform that will enable the development of new therapies, particularly for the early stages of disease.

Obesity in Type 1 Diabetes: Pathophysiology, Clinical Impact, and Mechanisms

There has been an alarming increase in the prevalence of obesity in people with type 1 diabetes in recent years. Although obesity has long been recognized as a major risk factor for the development of type 2 diabetes and a catalyst for complications, much less is known about the role of obesity in the initiation and pathogenesis of type 1 diabetes. Emerging evidence suggests that obesity contributes to insulin resistance, dyslipidemia, and cardiometabolic complications in type 1 diabetes. Unique therapeutic strategies may be required to address these comorbidities within the context of intensive insulin therapy, which promotes weight gain. There is an urgent need for clinical guidelines for the prevention and management of obesity in type 1 diabetes. The development of these recommendations will require a transdisciplinary research strategy addressing metabolism, molecular mechanisms, lifestyle, neuropsychology, and novel therapeutics. In this review, the prevalence, clinical impact, energy balance physiology, and potential mechanisms of obesity in type 1 diabetes are described, with a special focus on the substantial gaps in knowledge in this field. Our goal is to provide a framework for the evidence base needed to develop type 1 diabetes-specific weight management recommendations that account for the competing outcomes of glycemic control and weight management.

Animal models of obesity and diabetes mellitus

More than one-third of the worldwide population is overweight or obese and therefore at risk of developing type 2 diabetes mellitus. In order to mitigate this pandemic, safer and more potent therapeutics are urgently required. This necessitates the continued use of animal models to discover, validate and optimize novel therapeutics for their safe use in humans. In order to improve the transition from bench to bedside, researchers must not only carefully select the appropriate model but also draw the right conclusions. In this Review, we consolidate the key information on the currently available animal models of obesity and diabetes and highlight the advantages, limitations and important caveats of each of these models.

Adipose depot-specific effects of ileal interposition surgery in UCD-T2D rats: unexpected implications for obesity and diabetes

Ileal interposition (IT) surgery delays the onset of diabetes in a rat model of type-2 diabetes (UCD-T2DM). Here, to gain a deeper understanding of the molecular events underlying the effects of IT surgery, we examined the changes in the proteome of four white adipose depots (retroperitoneal, mesenteric, inguinal, and epididymal) and plasma-free fatty acid profile in pre-diabetic rats 1.5 months following IT or sham surgery. The IT-mediated changes were exerted mainly in mesenteric fat and spanned from delayed adipocyte maturation to a neuroendocrine remodeling. Conversely, inguinal, retroperitoneal, and epididymal depots showed opposite trends consistent with increased adipocyte maturation and adipogenesis development prior to overt signs of diabetes, probably orchestrated by peroxisome proliferator-activated receptor gamma signaling and higher plasma n-6/n-3 free fatty acid ratios. The resulting scenario suggests a targeted use of surgical strategies that seek to delay or improve diabetes in order to manipulate adipose depot-specific responses to maximize the duration and beneficial effects of the surgery.

Estrogen receptor signaling mediates leptin-induced growth of breast cancer cells via autophagy induction

Leptin, a hormone derived from adipose tissue, promotes growth of cancer cells via multiple mechanisms. Estrogen receptor signaling is also known to stimulate the growth of breast cancer cells. However, the involvement of estrogen receptor signaling in the oncogenic actions of leptin and its underlying mechanisms are not clearly understood. Herein, we investigated mechanisms for estrogen receptor signaling-mediated growth of breast cancer cells, particularly focusing on autophagy, which plays a crucial role in leptin-induced tumor growth. Inhibition of estrogen receptor signaling via gene silencing or treatment with a pharmacological inhibitor (tamoxifen) abolished leptin-induced growth of MCF-7 human breast cancer cells. Interestingly, leptin-induced autophagy activation, determined by up-regulation of autophagy-related genes and autophagosome formation, was also significantly suppressed by inhibiting estrogen receptor signaling. Moreover, inhibition of estrogen receptor markedly prevented leptin-induced activation of AMPK/FoxO3A axis, which plays a crucial role in autophagy induction. Leptin-induced cell cycle progression and Bax down-regulation were also prevented by treatment with tamoxifen. The pivotal roles of estrogen receptor signaling in leptin-induced cell cycle progression, apoptosis suppression, and autophagy induction were further confirmed in MCF-7 tumor xenograft model. Taken together, these results demonstrate that estrogen receptor signaling plays a key role in leptin-induced growth of breast cancer cells via autophagy activation.

Lipid nanoparticle delivery of glucagon receptor siRNA improves glucose homeostasis in mouse models of diabetes

Objective

Hyperglucagonemia is present in many forms of diabetes and contributes to hyperglycemia, and glucagon suppression can ameliorate diabetes in mice. Leptin, a glucagon suppressor, can also reverse diabetes in rodents. Lipid nanoparticle (LNP) delivery of small interfering RNA (siRNA) effectively targets the liver and is in clinical trials for the treatment of various diseases. We compared the effectiveness of glucagon receptor (Gcgr)-siRNA delivered via LNPs to leptin in two mouse models of diabetes.

Methods

Gcgr siRNA encapsulated into LNPs or leptin was administered to mice with diabetes due to injection of the β-cell toxin streptozotocin (STZ) alone or combined with high fat diet (HFD/STZ).

Results

In STZ-diabetic mice, a single injection of Gcgr siRNA lowered blood glucose levels for 3 weeks, improved glucose tolerance, and normalized plasma ketones levels, while leptin therapy normalized blood glucose levels, oral glucose tolerance, and plasma ketones, and suppressed lipid metabolism. In contrast, in HFD/STZ-diabetic mice, Gcgr siRNA lowered blood glucose levels for 2 months, improved oral glucose tolerance, and reduced HbA1c, while leptin had no beneficial effects.

Conclusions

While leptin may be more effective than Gcgr siRNA at normalizing both glucose and lipid metabolism in STZ diabetes, Gcgr siRNA is more effective at reducing blood glucose levels in HFD/STZ diabetes.

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Gcgr siRNA improves glucose metabolism but not lipid metabolism in STZ diabetic mice.

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Leptin improves both glucose and lipid metabolism in STZ diabetic mice.

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Gcgr siRNA improves glucose metabolism in HFD/STZ diabetic mice.

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Leptin does not improve glucose metabolism in HFD/STZ diabetic mice.

Role of Hormone-sensitive Lipase in Leptin-Promoted Fat Loss and Glucose Lowering

Aim: Myriad biological effects of leptin may lead to broad therapeutic applications for various metabolic diseases, including diabetes and its complications; however, in contrast to its anorexic effect, the molecular mechanisms underlying adipopenic and glucose-lowering effects of leptin have not been fully understood. Here we aim to clarify the role of hormone-sensitive lipase (HSL) in leptin's action.

Methods: Wild-type (WT) and HSL-deficient (HSLKO) mice were made hyperleptinemic by two commonly-used methods: adenovirus-mediated overexpression of leptin and continuous subcutaneous infusion of leptin by osmotic pumps. The amount of food intake, body weights, organ weights, and parameters of glucose and lipid metabolism were measured.

Results: Hyperleptinemia equally suppressed the food intake in WT and HSLKO mice. On the other hand, leptin-mediated fat loss and glucose-lowering were significantly blunted in the absence of HSL when leptin was overexpressed by recombinant adenovirus carrying leptin. By osmotic pumps, the fat-losing and glucose-lowering effects of leptin were milder due to lower levels of hyperleptinemia; although the difference between WT and HSLKO mice did not reach statistical significance, HSLKO mice had a tendency to retain more fat than WT mice in the face of hyperleptinemia.

Conclusions: We clarify for the first time the role of HSL in leptin's effect using a genetic model: leptin-promoted fat loss and glucose-lowering are at least in part mediated via HSL-mediated lipolysis. Further studies to define the pathophysiological role of adipocyte lipases in leptin action may lead to a new therapeutic approach to circumvent leptin resistance.

Leptin Levels in Patients with Type 1 Diabetes Mellitus After Fetal Pancreatic Stem Cell Transplant

OBJECTIVES

Our objective was to determine leptin levels in patients with type 1 diabetes mellitus after fetal pancreatic stem cell transplant.

MATERIALS AND METHODS

Seven patients, aged 20 to 42 years, with type 1 diabetes mellitus received a fetal pancreatic stem cell transplant by intravenous infusion. The quantity of fetal stem cells infused was ≥ 5 × 10⁶, and the cells were of 12 to 14 weeks of gestation. We analyzed the levels of leptin, C-peptide, and antibodies to the islets of Langerhans before and 3 months after the transplant procedure.

RESULTS

Fetal pancreatic stem cell transplant led to significant increases in leptin and C-peptide levels, from 4.63 ± 1.17 ng/mL and 0.09 ± 0.02 ng/mL to 7.71 ± 1.45 ng/mL (P < .05) and 0.22 ± 0.05 ng/mL (P < .005), respectively, without an increase in antibodies to the islets of Langerhans, which measured 0.64 ± 0.13 U/mL before transplant and 0.57 ± 0.18 U/mL 3 months later (P > .05).

CONCLUSIONS

Leptin levels increase significantly within 3 months of fetal pancreatic stem cell transplant in patients with type 1 diabetes mellitus.

Mechanism for leptin's acute insulin-independent effect to reverse diabetic ketoacidosis

The mechanism by which leptin reverses diabetic ketoacidosis (DKA) is unknown. We examined the acute insulin-independent effects of leptin replacement therapy in a streptozotocin-induced rat model of DKA. Leptin infusion reduced rates of lipolysis, hepatic glucose production (HGP), and hepatic ketogenesis by 50% within 6 hours and were independent of any changes in plasma glucagon concentrations; these effects were abrogated by coinfusion of corticosterone. Treating leptin- and corticosterone-infused rats with an adipose triglyceride lipase inhibitor blocked corticosterone-induced increases in plasma glucose concentrations and rates of HGP and ketogenesis. Similarly, adrenalectomized type 1 diabetic (T1D) rats exhibited decreased rates of lipolysis, HGP, and ketogenesis; these effects were reversed by corticosterone infusion. Leptin-induced decreases in lipolysis, HGP, and ketogenesis in DKA were also nullified by relatively small increases (15 to 70 pM) in plasma insulin concentrations. In contrast, the chronic glucose-lowering effect of leptin in a STZ-induced mouse model of poorly controlled T1D was associated with decreased food intake, reduced plasma glucagon and corticosterone concentrations, and decreased ectopic lipid (triacylglycerol/diacylglycerol) content in liver and muscle. Collectively, these studies demonstrate marked differences in the acute insulin-independent effects by which leptin reverses fasting hyperglycemia and ketoacidosis in a rodent model of DKA versus the chronic pleotropic effects by which leptin reverses hyperglycemia in a non-DKA rodent model of T1D.

Suppressing hyperinsulinemia prevents obesity but causes rapid onset of diabetes in leptin-deficient Lepob/ob mice

Objective

Hyperinsulinemia is commonly associated with obesity. Mice deficient in the adipose-derived hormone leptin (Lep^ob/ob) develop hyperinsulinemia prior to onset of obesity and glucose intolerance. Whether the excess of circulating insulin is a major contributor to obesity and impaired glucose homeostasis in Lep^ob/ob mice is unclear. It has been reported previously that diet-induced obesity in mice can be prevented by reducing insulin gene dosage. In the present study, we examined the effects of genetic insulin reduction in Lep^ob/ob mice on circulating insulin, body composition, and glucose homeostasis.

Methods

Leptin expressing (Lep^wt/wt) mice lacking 3 insulin alleles were crossed with Lep^ob/ob mice to generate Lep^ob/ob and Lep^wt/wt littermates lacking 1 (Ins1^+/+;Ins2^+/−), 2 (Ins1^+/+;Ins2^−/−) or 3 (Ins1^+/−;Ins2^−/−) insulin alleles. Animals were assessed for body weight gain, body composition, glucose homeostasis, and islet morphology.

Results

We found that in young Lep^ob/ob mice, loss of 2 or 3 insulin alleles reduced plasma insulin levels by 75–95% and attenuated body weight gain by 50–90% compared to Ins1^+/+;Ins2^+/−;Lep^ob/ob mice. This corresponded with ∼30% and ∼50% reduced total body fat in Ins1^+/+;Ins2^−/−;Lep^ob/ob and Ins1^+/−;Ins2^−/−;Lep^ob/ob mice, respectively. Loss of 2 or 3 insulin alleles in young Lep^ob/ob mice resulted in onset of fasting hyperglycemia by 4 weeks of age, exacerbated glucose intolerance, and abnormal islet morphology. In contrast, loss of 1,2 or 3 insulin alleles in Lep^wt/wt mice did not significantly alter plasma insulin levels, body weight, fat mass, fasting glycemia, or glucose tolerance.

Conclusion

Taken together, our findings indicate that hyperinsulinemia is required for excess adiposity in Lep^ob/ob mice and sufficient insulin production is necessary to maintain euglycemia in the absence of leptin.

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Loss of 2 or 3 insulin alleles results in a dose dependent decrease of circulating insulin levels and body fat in Lep^ob/ob mice.

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Loss of 2 or 3 insulin alleles produced a greater reduction in body weight in male as compared to female Lep^ob/ob mice.

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Attenuation of hyperinsulinemia accelerates onset of hyperglycemia in Lep^ob/ob mice.

Efficient Targeting of Adipose Tissue Macrophages in Obesity with Polysaccharide Nanocarriers

Obesity leads to an increased risk for type 2 diabetes, heart disease, stroke, and cancer. The causal link between obesity and these pathologies has recently been identified as chronic low-grade systemic inflammation initiated by pro-inflammatory macrophages in visceral adipose tissue. Current medications based on small-molecule drugs yield significant off-target side effects with long-term use, and therefore there is a major need for targeted therapies. Here we report that nanoscale polysaccharides based on biocompatible glucose polymers can efficiently target adipose macrophages in obese mice. We synthesized a series of dextran conjugates with tunable size linked to contrast agents for positron emission tomography, fluorophores for optical microscopy, and anti-inflammatory drugs for therapeutic modulation of macrophage phenotype. We observed that larger conjugates efficiently distribute to visceral adipose tissue and selectively associate with macrophages after regional peritoneal administration. Up to 63% of the injected dose remained in visceral adipose tissue 24 h after administration, resulting in >2-fold higher local concentration compared to liver, the dominant site of uptake for most nanomedicines. Furthermore, a single-dose treatment of anti-inflammatory conjugates significantly reduced pro-inflammatory markers in adipose tissue of obese mice. Importantly, all components of these therapeutic agents are approved for clinical use. This work provides a promising nanomaterials-based delivery strategy to inhibit critical factors leading to obesity comorbidities and demonstrates a unique transport mechanism for drug delivery to visceral tissues. This approach may be further applied for high-efficiency targeting of other inflammatory diseases of visceral organs.

Perinatal triphenyl phosphate exposure accelerates type 2 diabetes onset and increases adipose accumulation in UCD-type 2 diabetes mellitus rats

Triphenyl phosphate (TPhP) is a flame retardant additive frequently found in consumer products and household dust. We administered 170μg of TPhP in maternal food from gestational day 8.5 to weaning and evaluated metabolic phenotypes of 3.5 month old male and female rats, and weight-matched males up to 6 months, to assess the development of obesity and type 2 diabetes mellitus (T2DM), respectively. Perinatal TPhP exposure increased body and fat mass in 3.5 month old male and female rats, while leptin and cumulative energy intake were elevated in males and females, respectively. Independent of body mass, perinatal TPhP exposure accelerated T2DM onset in males and increased plasma non-esterified- fasting fatty acids. These observations suggest that perinatal exposure to TPhP exacerbates the development of obesity in male and female UCDavis-T2DM rats and accelerates T2DM onset in male UCD-T2DM rats.

Leptin but not adiponectin is related to type 2 diabetes mellitus in obese adolescents

BACKGROUND

Adipokines have been suggested to be involved in the development of type 2 diabetes mellitus (T2DM). However, studies in humans are controversial and analyzes at the onset of disease are scarce.

METHODS

We compared adiponectin and leptin levels between 74 predominately Caucasian adolescents with T2DM and 74 body mass index (BMI)-, age-, and gender-matched controls without T2DM. Adiponectin and leptin were correlated to age, BMI, hemoglobin A1c (HbA1c), blood pressure, and lipids.

RESULTS

Adolescents with T2DM showed significant lower leptin levels as compared with controls (18 ± 12 vs. 37 ± 23 ng/mL, p < 0.001), whereas the adiponectin levels did not differ between the adolescents with and without T2DM (5.0 ± 2.5 vs. 4.9 ± 2.5 µg/mL, p = 0.833). The associations between adiponectin and high-density lipoprotein (HDL) cholesterol (r = 0.42), systolic (r = -0.15), and diastolic blood pressure (r = -0.20) were stronger as the associations of leptin to these parameters (all r < 0.07). In multiple linear regression analysis, leptin was significantly and positively associated with BMI [β-coefficient: 1.3 (95% confidence interval (95% CI): ±0.5), p < 0.001] and female sex [β-coefficient: 9.7 (95% CI: ±6.7), p = 0.005], and negatively with age [β-coefficient: -2.3 (95% CI: ±2.1), p < 0.001] and HbA1c [β-coefficient -3.1 (95% CI: ±2.1), p = 0.011]. Adiponectin was not significantly associated with BMI, HbA1c, age, or gender in multiple linear regression analysis.

CONCLUSIONS

Because adiponectin levels did not differ between obese adolescents with and without T2DM, hypoadiponectinemia as observed in obesity seems not to be involved in the genesis of T2DM. The relative hypoleptinemia in obese adolescents with T2DM as compared with obese adolescents without T2DM may contribute to the development of T2DM. Future longitudinal studies in humans are necessary to prove this hypothesis.

The role of leptin in diabetes: metabolic effects

While it is well established that the adiposity hormone leptin plays a key role in the regulation of energy homeostasis, growing evidence suggests that leptin is also critical for glycaemic control. In this review we examine the role of the brain in the glucose-lowering actions of leptin and the potential mediators responsible for driving hyperglycaemia in states of uncontrolled insulin-deficient diabetes (uDM). These considerations highlight the possibility of targeting leptin-sensitive pathways as a therapeutic option for the treatment of diabetes. This review summarises a presentation given at the 'Is leptin coming back?' symposium at the 2015 annual meeting of the EASD. It is accompanied by two other reviews on topics from this symposium (by Christoffer Clemmensen and colleagues, DOI: 10.1007/s00125-016-3906-7 , and by Gerald Shulman and colleagues, DOI: 10.1007/s00125-016-3909-4 ) and an overview by the Session Chair, Ulf Smith (DOI: 10.1007/s00125-016-3894-7 ).

Metabolic interplay between white, beige, brown adipocytes and the liver

In mammalian evolution, three types of adipocytes have developed, white, brown and beige adipocytes. White adipocytes are the major constituents of white adipose tissue (WAT), the predominant store for energy-dense triglycerides in the body that are released as fatty acids during catabolic conditions. The less abundant brown adipocytes, the defining parenchymal cells of brown adipose tissue (BAT), internalize triglycerides that are stored intracellularly in multilocular lipid droplets. Beige adipocytes (also known as brite or inducible brown adipocytes) are functionally very similar to brown adipocytes and emerge in specific WAT depots in response to various stimuli including sustained cold exposure. The activation of brown and beige adipocytes (together referred to as thermogenic adipocytes) causes both the hydrolysis of stored triglycerides as well as the uptake of lipids and glucose from the circulation. Together, these fuels are combusted for heat production to maintain body temperature in mammals including adult humans. Given that heating by brown and beige adipocytes is a very-well controlled and energy-demanding process which entails pronounced shifts in energy fluxes, it is not surprising that an intensive interplay exists between the various adipocyte types and parenchymal liver cells, and that this influences systemic metabolic fluxes and endocrine networks. In this review we will emphasize the role of hepatic factors that regulate the metabolic activity of white and thermogenic adipocytes. In addition, we will discuss the relevance of lipids and hormones that are secreted by white, brown and beige adipocytes regulating liver metabolism in order to maintain systemic energy metabolism in health and disease.

Chronic central leptin infusion modulates the glycemia response to insulin administration in male rats through regulation of hepatic glucose metabolism

Leptin and insulin use overlapping signaling mechanisms to modify hepatic glucose metabolism, which is critical in maintaining normal glycemia. We examined the effect of an increase in central leptin and insulin on hepatic glucose metabolism and its influence on serum glucose levels. Chronic leptin infusion increased serum leptin and reduced hepatic SH-phosphotyrosine phosphatase 1, the association of suppressor of cytokine signaling 3 to the insulin receptor in liver and the rise in glycemia induced by central insulin. Leptin also decreased hepatic phosphoenolpyruvate carboxykinase levels and increased insulin's ability to phosphorylate insulin receptor substrate-1, Akt and glycogen synthase kinase on Ser9 and to stimulate glucose transporter 2 and glycogen levels. Peripheral leptin treatment reproduced some of these changes, but to a lesser extent. Our data indicate that leptin increases the hepatic response to a rise in insulin, suggesting that pharmacological manipulation of leptin targets may be of interest for controlling glycemia.

Transient receptor potential vanilloid type-1 channel regulates diet-induced obesity, insulin resistance, and leptin resistance

Insulin resistance is a major characteristic of obesity and type 2 diabetes, but the underlying mechanism is unclear. Recent studies have shown a metabolic role of capsaicin that may be mediated via the transient receptor potential vanilloid type-1 (TRPV1) channel. In this study, TRPV1 knockout (KO) and wild-type (WT) mice (as controls) were fed a high-fat diet (HFD), and metabolic studies were performed to measure insulin and leptin action. The TRPV1 KO mice became more obese than the WT mice after HFD, partly attributed to altered energy balance and leptin resistance in the KO mice. The hyperinsulinemic-euglycemic clamp experiment showed that the TRPV1 KO mice were more insulin resistant after HFD because of the ∼40% reduction in glucose metabolism in the white and brown adipose tissue, compared with that in the WT mice. Leptin treatment failed to suppress food intake, and leptin-mediated hypothalamic signal transducer and activator of transcription (STAT)-3 activity was blunted in the TRPV1 KO mice. We also found that the TRPV1 KO mice were more obese and insulin resistant than the WT mice at 9 mo of age. Taken together, these results indicate that lacking TRPV1 exacerbates the obesity and insulin resistance associated with an HFD and aging, and our findings further suggest that TRPV1 has a major role in regulating glucose metabolism and hypothalamic leptin's effects in obesity.

A critical role for PKR complexes with TRBP in Immunometabolic regulation and eIF2α phosphorylation in obesity

Aberrant stress and inflammatory responses are key factors in the pathogenesis of obesity and metabolic dysfunction, and the double-stranded RNA-dependent kinase (PKR) has been proposed to play an important role in integrating these pathways. Here, we report the formation of a complex between PKR and TAR RNA-binding protein (TRBP) during metabolic and obesity-induced stress, which is critical for the regulation of eukaryotic translation initiation factor 2 alpha (eIF2α) phosphorylation and c-Jun N-terminal kinase (JNK) activation. We show that TRBP phosphorylation is induced in the setting of metabolic stress, leading to PKR activation. Suppression of hepatic TRBP reduced inflammation, JNK activity, and eIF2α phosphorylation and improved systemic insulin resistance and glucose metabolism, while TRBP overexpression exacerbated the impairment in glucose homeostasis in obese mice. These data indicate that the association between PKR and TRBP integrates metabolism with translational control and inflammatory signaling and plays important roles in metabolic homeostasis and disease.

Physiology of leptin: energy homeostasis, neuroendocrine function and metabolism

Leptin is secreted by adipose tissue and regulates energy homeostasis, neuroendocrine function, metabolism, immune function and other systems through its effects on the central nervous system and peripheral tissues. Leptin administration has been shown to restore metabolic and neuroendocrine abnormalities in individuals with leptin-deficient states, including hypothalamic amenorrhea and lipoatrophy. In contrast, obese individuals are resistant to leptin. Recombinant leptin is beneficial in patients with congenital leptin deficiency or generalized lipodystrophy. However, further research on molecular mediators of leptin resistance is needed for the development of targeted leptin sensitizing therapies for obesity and related metabolic diseases.

Leptin in nonalcoholic fatty liver disease: a narrative review

Leptin, the first described adipokine, interplays with hepatic metabolism. The aim of this review was to summarize available data on the association between leptin and nonalcoholic fatty liver disease (NAFLD). Leptin has a potential dual action on NAFLD experimental models, exerting a possible anti-steatotic, but also a proinflammatory and profibrogenic action. Observational clinical studies have shown higher or similar leptin levels between simple steatosis and nonalcoholic steatohepatitis (NASH) compared with controls. Interventional studies showed that circulating leptin diminishes together with body mass index after successful weight loss following lifestyle modifications or bariatric surgery. Studies providing evidence for the effect of other medications on leptin levels in NAFLD populations are limited and of low power. Data from small studies claim that recombinant leptin administration had a possibly beneficial effect on steatosis, but not fibrosis, in NAFLD patients with hypoleptinemia. Although the aforementioned dual leptin action has not yet been validated in humans, leptin administration in NAFLD patients with normoleptinemia or hyperleptinemia is discouraged. Further well-controlled studies in cautiously selected populations are needed to elucidate whether leptin has any prognostic and therapeutic role in NAFLD patients.

Butyrate increases intracellular calcium levels and enhances growth hormone release from rat anterior pituitary cells via the G-protein-coupled receptors GPR41 and 43

Butyrate is a short-chain fatty acid (SCFA) closely related to the ketone body ß-hydroxybutyrate (BHB), which is considered to be the major energy substrate during prolonged exercise or starvation. During fasting, serum growth hormone (GH) rises concomitantly with the accumulation of BHB and butyrate. Interactions between GH, ketone bodies and SCFA during the metabolic adaptation to fasting have been poorly investigated to date. In this study, we examined the effect of butyrate, an endogenous agonist for the two G-protein-coupled receptors (GPCR), GPR41 and 43, on non-stimulated and GH-releasing hormone (GHRH)-stimulated hGH secretion. Furthermore, we investigated the potential role of GPR41 and 43 on the generation of butyrate-induced intracellular Ca²⁺ signal and its ultimate impact on hGH secretion. To study this, wt-hGH was transfected into a rat pituitary tumour cell line stably expressing the human GHRH receptor. Treatment with butyrate promoted hGH synthesis and improved basal and GHRH-induced hGH-secretion. By acting through GPR41 and 43, butyrate enhanced intracellular free cytosolic Ca²⁺. Gene-specific silencing of these receptors led to a partial inhibition of the butyrate-induced intracellular Ca²⁺ rise resulting in a decrease of hGH secretion. This study suggests that butyrate is a metabolic intermediary, which contributes to the secretion and, therefore, to the metabolic actions of GH during fasting.

Oral administration of soybean peptide Vglycin normalizes fasting glucose and restores impaired pancreatic function in Type 2 diabetic Wistar rats

Vglycin, a natural 37-residue polypeptide isolated from pea seeds in which six half-cysteine residues are embedded in three pairs of disulfide bonds, is resistant to digestive enzymes and has antidiabetic potential. To investigate the pharmacological activity of Vglycin in vivo and to examine the mechanisms involved, the therapeutic effect of Vglycin in diabetic rats was examined. Diabetes was induced in Wistar rats by high-fat diet and multiple streptozotocin intraperitoneal injections. Diabetic rats were treated daily with Vglycin for 4 weeks. Body weight, food intake, fasting plasma glucose and insulin levels were assayed weekly. Glucose and insulin tolerance tests were conducted on Day 29. Subsequently, levels of p-Akt in the liver and pancreas and cleaved PARP, Pdx-1 and insulin in the pancreas were detected by immunoblotting. The morphology of the pancreas and the insulin expression in the pancreas were analyzed by hematoxylin-eosin staining and immunohistochemistry, respectively. Furthermore, human liver-derived cell lines were used to explore the in vitro effects of Vglycin on insulin sensitivity and glucose uptake. Chronic treatment with Vglycin normalized fasting glucose levels in diabetic rats. The improvement in glucose homeostasis and the increased insulin sensitivity mediated by restored insulin signaling likely contributed to decreased food intake and reduced body weight. Vglycin protected pancreatic cells from damage by streptozotocin. Although insulin synthesis and secretion in impaired β-cell were not significantly elevated, islets morphology was improved in the Vglycin-treated groups. These results suggest that Vglycin could be useful in Type 2 diabetes for restoring impaired insulin signaling, glucose tolerance and pancreatic function.

Benefits of healthy adipose tissue in the treatment of diabetes

The major malfunction in diabetes mellitus is severe perturbation of glucose homeostasis caused by deficiency of insulin. Insulin deficiency is either absolute due to destruction or failure of pancreatic β cells, or relative due to decreased sensitivity of peripheral tissues to insulin. The primary lesion being related to insulin, treatments for diabetes focus on insulin replacement and/or increasing sensitivity to insulin. These therapies have their own limitations and complications, some of which can be life-threatening. For example, exogenous insulin administration can lead to fatal hypoglycemic episodes; islet/pancreas transplantation requires life-long immunosuppressive therapy; and anti-diabetic drugs have dangerous side effects including edema, heart failure and lactic acidosis. Thus the need remains for better safer long term treatments for diabetes. The ultimate goal in treating diabetes is to re-establish glucose homeostasis, preferably through endogenously generated hormones. Recent studies increasingly show that extra-pancreatic hormones, particularly those arising from adipose tissue, can compensate for insulin, or entirely replace the function of insulin under appropriate circumstances. Adipose tissue is a versatile endocrine organ that secretes a variety of hormones with far-reaching effects on overall metabolism. While unhealthy adipose tissue can exacerbate diabetes through limiting circulation and secreting of pro-inflammatory cytokines, healthy uninflamed adipose tissue secretes beneficial adipokines with hypoglycemic and anti-inflammatory properties, which can complement and/or compensate for the function of insulin. Administration of specific adipokines is known to alleviate both type 1 and 2 diabetes, and leptin mono-therapy is reported to reverse type 1 diabetes independent of insulin. Although specific adipokines may correct diabetes, administration of individual adipokines still carries risks similar to those of insulin monotherapy. Thus a better approach is to achieve glucose homeostasis with endogenously-generated adipokines through transplantation or regeneration of healthy adipose tissue. Our recent studies on mouse models show that type 1 diabetes can be reversed without insulin through subcutaneous transplantation of embryonic brown adipose tissue, which leads to replenishment of recipients’ white adipose tissue; increase of a number of beneficial adipokines; and fast and long-lasting euglycemia. Insulin-independent glucose homeostasis is established through a combination of endogenously generated hormones arising from the transplant and/or newly-replenished white adipose tissue. Transplantation of healthy white adipose tissue is reported to alleviate type 2 diabetes in rodent models on several occasions, and increasing the content of endogenous brown adipose tissue is known to combat obesity and type 2 diabetes in both humans and animal models. While the underlying mechanisms are not fully documented, the beneficial effects of healthy adipose tissue in improving metabolism are increasingly reported, and are worthy of attention as a powerful tool in combating metabolic disease.

Hypothalamic agouti-related peptide neurons and the central melanocortin system are crucial mediators of leptin's antidiabetic actions

Leptin has beneficial effects on glucose metabolism via actions in the hypothalamus, but the roles of specific subgroups of neurons responsible for these antidiabetic effects remain unresolved. We generated diabetic Lep(ob/ob) or Lepr(db/db) mice lacking or re-expressing leptin receptors (LepRb) in subgroups of neurons to explore their contributions to leptin's glucose-lowering actions. We show that agouti-related peptide (AgRP)-expressing neurons are both required and sufficient to correct hyperglycemia by leptin. LepRb in pro-opiomelanocortin (POMC) neurons or steroidogenic factor-1 (SF1) neurons are not required. Furthermore, normalization of blood glucose by leptin is blunted in Lep(ob/ob)/MC4R-null mice, but not in Lep(ob/ob) mice lacking neuropeptide Y (NPY) or gamma-aminobutyric acid (GABA) in AgRP neurons. Leptin's ability to improve glucose balance is accompanied by a reduction in circulating glucagon. We conclude that AgRP neurons play a crucial role in glucose-lowering actions by leptin and that this requires the melanocortin system, but not NPY and GABA.