Leptin administration enhances islet transplant performance in diabetic mice

Astragalus mongholicus powder, a traditional Chinese medicine formula ameliorate type 2 diabetes by regulating adipoinsular axis in diabetic mice

The global morbidity of obesity and type 2 diabetes mellitus (T2DM) has dramatically increased. Insulin resistance is the most important pathogenesis and therapeutic target of T2DM. The traditional Chinese medicine formula Astragalus mongholicus powder (APF), consists of Astragalus mongholicus Bunge [Fabaceae], Pueraria montana (Lour.) Merr. [Fabaceae], and Morus alba L. [Moraceae] has a long history to be used to treat diabetes in ancient China. This work aims to investigate the effects of APF on diabetic mice and its underlying mechanism. Diabetic mice were induced by High-fat-diet (HFD) and streptozotocin (STZ). The body weight of mice and their plasma levels of glucose, insulin, leptin and lipids were examined. Reverse transcription-polymerase chain reaction, histology, and Western blot analysis were performed to validate the effects of APF on diabetic mice and investigate the underlying mechanism. APF reduced hyperglycemia, hyperinsulinemia, and hyerleptinemia and attenuate the progression of obesity and non-alcoholic fatty liver disease (NAFLD). However, these effects disappeared in leptin deficient ob/ob diabetic mice and STZ-induced insulin deficient type 1 diabetic mice. Destruction of either these hormones would abolish the therapeutic effects of APF. In addition, APF inhibited the protein expression of PTP1B suppressing insulin–leptin sensitivity, the gluconeogenic gene PEPCK, and the adipogenic gene FAS. Therefore, insulin–leptin sensitivity was normalized, and the gluconeogenic and adipogenic genes were suppressed. In conclusion, APF attenuated obesity, NAFLD, and T2DM by regulating the balance of adipoinsular axis in STZ + HFD induced T2DM mice.

White Adipose Tissue as a Site for Islet Transplantation

Although islet transplantation is recognized as a useful cellular replacement therapy for severe diabetes, surgeons face difficulties in islet engraftment. The transplant site is a pivotal factor that influences the engraftment. Although the liver is the current representative site for clinical islet transplantation, it is not the best site because of limitations in immunity, inflammation, and hypoxia. White adipose tissue, including omentum, is recognized as a useful candidate site for islet transplantation. Its effectiveness has been evaluated in not only various basic and translational studies using small and large animals but also in some recent clinical trials. In this review, we attempt to shed light on the characteristics and usefulness of white adipose tissue as a transplant site for islets.

White Adipose Tissue as a Site for Islet Transplantation

Although islet transplantation is recognized as a useful cellular replacement therapy for severe diabetes, surgeons face difficulties in islet engraftment. The transplant site is a pivotal factor that influences the engraftment. Although the liver is the current representative site for clinical islet transplantation, it is not the best site because of limitations in immunity, inflammation, and hypoxia. White adipose tissue, including omentum, is recognized as a useful candidate site for islet transplantation. Its effectiveness has been evaluated in not only various basic and translational studies using small and large animals but also in some recent clinical trials. In this review, we attempt to shed light on the characteristics and usefulness of white adipose tissue as a transplant site for islets.

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

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

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.

Disrupted Leptin Signaling in the Lateral Hypothalamus and Ventral Premammillary Nucleus Alters Insulin and Glucagon Secretion and Protects Against Diet-Induced Obesity

Leptin signaling in the central nervous system, and particularly the arcuate hypothalamic nucleus, is important for regulating energy and glucose homeostasis. However, the roles of extra-arcuate leptin responsive neurons are less defined. In the current study, we generated mice with widespread inactivation of the long leptin receptor isoform in the central nervous system via Synapsin promoter-driven Cre (Lepr(flox/flox) Syn-cre mice). Within the hypothalamus, leptin signaling was disrupted in the lateral hypothalamic area (LHA) and ventral premammillary nucleus (PMV) but remained intact in the arcuate hypothalamic nucleus and ventromedial hypothalamic nucleus, dorsomedial hypothalamic nucleus, and nucleus of the tractus solitarius. To investigate the role of LHA/PMV neuronal leptin signaling, we examined glucose and energy homeostasis in Lepr(flox/flox) Syn-cre mice and Lepr(flox/flox) littermates under basal and diet-induced obese conditions and tested the role of LHA/PMV neurons in leptin-mediated glucose lowering in streptozotocin-induced diabetes. Lepr(flox/flox) Syn-cre mice did not have altered body weight or blood glucose levels but were hyperinsulinemic and had enhanced glucagon secretion in response to experimental hypoglycemia. Surprisingly, when placed on a high-fat diet, Lepr(flox/flox) Syn-cre mice were protected from weight gain, glucose intolerance, and diet-induced hyperinsulinemia. Peripheral leptin administration lowered blood glucose in streptozotocin-induced diabetic Lepr(flox/flox) Syn-cre mice as effectively as in Lepr(flox/flox) littermate controls. Collectively these findings suggest that leptin signaling in LHA/PMV neurons is not critical for regulating glucose levels but has an indispensable role in the regulation of insulin and glucagon levels and, may promote the development of diet-induced hyperinsulinemia and weight gain.

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

Objective

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

Methods

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

Results

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

Conclusions

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

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Leptin does not require vagal innervation to reverse hyperglycemia.

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

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Leptin reverses hyperglycemia independent of uncoupling protein 1.

Leptin induces fasting hypoglycaemia in a mouse model of diabetes through the depletion of glycerol

AIMS/HYPOTHESIS

Leptin has profound glucose-lowering effects in rodent models of type 1 diabetes, and is currently being tested clinically to treat this disease. In addition to reversing hyperglycaemia, leptin therapy corrects multiple lipid, energy and neuroendocrine imbalances in rodent models of type 1 diabetes, yet the precise mechanism has not been fully defined. Thus, we performed metabolic analyses to delineate the downstream metabolic pathway mediating leptin-induced glucose lowering in diabetic mice.

METHODS

Mice were injected with streptozotocin (STZ) to induce insulin-deficient diabetes, and were subsequently treated with 20 μg/day recombinant murine leptin or vehicle for 5 to 14 days. Energy-yielding substrates were measured in the liver and plasma, and endogenous glucose production was assessed by tolerance to extended fasting.

RESULTS

STZ-leptin-treated mice developed severe hypoketotic hypoglycaemia during prolonged fasting, indicative of suppressed endogenous ketone and glucose production. STZ-leptin mice displayed normal gluconeogenic and glycogenolytic capacity, but had depleted circulating glycerol and NEFA. The depletion of glycerol and NEFA correlated tightly with the kinetics of glucose lowering in response to chronic leptin administration, and was not mimicked by single leptin injection. Administration of glycerol acutely reversed fasting-induced hypoglycaemia in leptin-treated mice.

CONCLUSIONS/INTERPRETATION

The findings of this study suggest that the diminution of circulating glycerol reduces endogenous glucose production, contributing to severe fasting-induced hypoglycaemia in leptin-treated rodent models of type 1 diabetes, and support that depletion of glycerol contributes to the glucose-lowering action of leptin.

Treating diet-induced diabetes and obesity with human embryonic stem cell-derived pancreatic progenitor cells and antidiabetic drugs

Human embryonic stem cell (hESC)-derived pancreatic progenitor cells effectively reverse hyperglycemia in rodent models of type 1 diabetes, but their capacity to treat type 2 diabetes has not been reported. An immunodeficient model of type 2 diabetes was generated by high-fat diet (HFD) feeding in SCID-beige mice. Exposure to HFDs did not impact the maturation of macroencapsulated pancreatic progenitor cells into glucose-responsive insulin-secreting cells following transplantation, and the cell therapy improved glucose tolerance in HFD-fed transplant recipients after 24 weeks. However, since diet-induced hyperglycemia and obesity were not fully ameliorated by transplantation alone, a second cohort of HFD-fed mice was treated with pancreatic progenitor cells combined with one of three antidiabetic drugs. All combination therapies rapidly improved body weight and co-treatment with either sitagliptin or metformin improved hyperglycemia after only 12 weeks. Therefore, a stem cell-based therapy may be effective for treating type 2 diabetes, particularly in combination with antidiabetic drugs.

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High-fat diets (HFDs) caused rapid metabolic dysfunction in immunodeficient mice

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HFDs and/or antidiabetic drugs did not affect function of hESC-derived beta cells

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hESC-derived beta cells improved glucose tolerance in HFD-fed mice

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Cell therapy combined with an antidiabetic drug was the most effective treatment

Partial ablation of leptin signaling in mouse pancreatic α-cells does not alter either glucose or lipid homeostasis

The role of glucagon in the pathological condition of diabetes is gaining interest, and it has been recently reported that its action is essential for hyperglycemia to occur. Glucagon levels, which are elevated in some diabetic models, are reduced following leptin therapy. Likewise, hyperglycemia is corrected in type 1 diabetic mice treated with leptin, although the mechanisms have not been fully determined. A direct inhibitory effect of leptin on mouse and human α-cells has been demonstrated at the levels of electrical activity, calcium signaling, and glucagon secretion. In the present study we employed the Cre-loxP strategy to generate Lepr(flox/flox) Gcg-cre mice, which specifically lack leptin receptors in glucagon-secreting α-cells, to determine whether leptin resistance in α-cells contributes to hyperglucagonemia, and also whether leptin action in α-cells is required to improve glycemia in type 1 diabetes with leptin therapy. Immunohistochemical analysis of pancreas sections revealed Cre-mediated recombination in ∼ 43% of the α-cells. We observed that in vivo Lepr(flox/flox) Gcg-cre mice display normal glucose and lipid homeostasis. In addition, leptin administration in streptozotocin-induced diabetic Lepr(flox/flox) Gcg-cre mice restored euglycemia similarly to control mice. These findings suggest that loss of leptin receptor signaling in close to one-half of α-cells does not alter glucose metabolism in vivo, nor is it sufficient to prevent the therapeutic action of leptin in type 1 diabetes.