Deletion of interleukin-6 improves pyruvate tolerance without altering hepatic insulin signaling in the leptin receptor-deficient mouse

Functional role of skeletal muscle-derived interleukin-6 and its effects on lipid metabolism

The detrimental impact of obesity on human health is increasingly evident with the rise in obesity-related diseases. Skeletal muscle, the crucial organ responsible for energy balance metabolism, plays a significant role as a secretory organ by releasing various myokines. Among these myokines, interleukin 6 (IL-6) is closely associated with skeletal muscle contraction. IL-6 triggers the process of lipolysis by mobilizing energy-storing adipose tissue, thereby providing energy for physical exercise. This phenomenon also elucidates the health benefits of regular exercise. However, skeletal muscle and adipose tissue maintain a constant interaction, both directly and indirectly. Direct interaction occurs through the accumulation of excess fat within skeletal muscle, known as ectopic fat deposition. Indirect interaction takes place when adipose tissue is mobilized to supply the energy for skeletal muscle during exercise. Consequently, maintaining a functional balance between skeletal muscle and adipose tissue becomes paramount in regulating energy metabolism and promoting overall health. IL-6, as a representative cytokine, participates in various inflammatory responses, including non-classical inflammatory responses such as adipogenesis. Skeletal muscle influences adipogenesis through paracrine mechanisms, primarily by secreting IL-6. In this research paper, we aim to review the role of skeletal muscle-derived IL-6 in lipid metabolism and other physiological activities, such as insulin resistance and glucose tolerance. By doing so, we provide valuable insights into the regulatory function of skeletal muscle-derived myokines in lipid metabolism.

Impaired Glucose Homeostasis in a Tau Knock-In Mouse Model

Alzheimer’s disease (AD) is the leading cause of dementia. While impaired glucose homeostasis has been shown to increase AD risk and pathological loss of tau function, the latter has been suggested to contribute to the emergence of the glucose homeostasis alterations observed in AD patients. However, the links between tau impairments and glucose homeostasis, remain unclear. In this context, the present study aimed at investigating the metabolic phenotype of a new tau knock-in (KI) mouse model, expressing, at a physiological level, a human tau protein bearing the P301L mutation under the control of the endogenous mouse Mapt promoter. Metabolic investigations revealed that, while under chow diet tau KI mice do not exhibit significant metabolic impairments, male but not female tau KI animals under High-Fat Diet (HFD) exhibited higher insulinemia as well as glucose intolerance as compared to control littermates. Using immunofluorescence, tau protein was found colocalized with insulin in the β cells of pancreatic islets in both mouse (WT, KI) and human pancreas. Isolated islets from tau KI and tau knock-out mice exhibited impaired glucose-stimulated insulin secretion (GSIS), an effect recapitulated in the mouse pancreatic β-cell line (MIN6) following tau knock-down. Altogether, our data indicate that loss of tau function in tau KI mice and, particularly, dysfunction of pancreatic β cells might promote glucose homeostasis impairments and contribute to metabolic changes observed in AD.

Role of myeloid cell leptin signaling in the regulation of glucose metabolism

Although innate immunity is linked to metabolic health, the effect of leptin signaling in cells from the innate immune system on glucose homeostasis has not been thoroughly investigated. We generated two mouse models using Cre-lox methodology to determine the effect of myeloid cell-specific leptin receptor (Lepr) reconstitution and Lepr knockdown on in vivo glucose metabolism. Male mice with myeloid cell-specific Lepr reconstitution (Lyz2Cre⁺Lepr^loxTB/loxTB) had better glycemic control as they aged compared to male mice with whole-body transcriptional blockade of Lepr (Lyz2Cre⁻Lepr^loxTB/loxTB). In contrast, Lyz2Cre⁺Lepr^loxTB/loxTB females only had a trend for diminished hyperglycemia after a prolonged fast. During glucose tolerance tests, Lyz2Cre⁺Lepr^loxTB/loxTB males had a mildly improved plasma glucose profile compared to Cre⁻ controls while Lyz2Cre⁺Lepr^loxTB/loxTB females had a similar glucose excursion to their Cre⁻ controls. Myeloid cell-specific Lepr knockdown (Lyz2Cre⁺Lepr^flox/flox) did not significantly alter body weight, blood glucose, insulin sensitivity, or glucose tolerance in males or females. Expression of the cytokine interleukin 10 (anti-inflammatory) tended to be higher in adipose tissue of male Lyz2Cre⁺Lepr^loxTB/loxTB mice (p = 0.0774) while interleukin 6 (pro-inflammatory) was lower in male Lyz2Cre⁺Lepr^flox/flox mice (p < 0.05) vs. their respective controls. In conclusion, reconstitution of Lepr in cells of myeloid lineage has beneficial effects on glucose metabolism in male mice.

Trans-omic analysis reveals obesity-associated dysregulation of inter-organ metabolic cycles between the liver and skeletal muscle

Systemic metabolic homeostasis is regulated by inter-organ metabolic cycles involving multiple organs. Obesity impairs inter-organ metabolic cycles, resulting in metabolic diseases. The systemic landscape of dysregulated inter-organ metabolic cycles in obesity has yet to be explored. Here, we measured the transcriptome, proteome, and metabolome in the liver and skeletal muscle and the metabolome in blood of fasted wild-type and leptin-deficient obese (ob/ob) mice, identifying components with differential abundance and differential regulation in ob/ob mice. By constructing and evaluating the trans-omic network controlling the differences in metabolic reactions between fasted wild-type and ob/ob mice, we provided potential mechanisms of the obesity-associated dysfunctions of metabolic cycles between liver and skeletal muscle involving glucose-alanine, glucose-lactate, and ketone bodies. Our study revealed obesity-associated systemic pathological mechanisms of dysfunction of inter-organ metabolic cycles.

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Multi-omic data in liver and skeletal muscle of WT and ob/ob mice were measured

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We developed the trans-omic network of differentially regulated metabolic reactions

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Dysregulation of inter-organ metabolic cycles associated with obesity was revealed

Glimepiride Administered in Chow Reversibly Impairs Glucose Tolerance in Mice

Sulfonylureas are a class of antidiabetes medications prescribed to millions of individuals worldwide. Rodents have been used extensively to study sulfonylureas in the laboratory. Here, we report the results of studies treating mice with a sulfonylurea (glimepiride) in order to understand how the drug affects glucose homeostasis and tolerance. We tested the effect of glimepiride on fasting blood glucose, glucose tolerance, and insulin secretion, using glimepiride sourced from a local pharmacy. We also examined the effect on glucagon, gluconeogenesis, and insulin sensitivity. Unexpectedly, glimepiride exposure in mice was associated with fasting hyperglycemia, glucose intolerance, and decreased insulin. There was no change in circulating glucagon levels or gluconeogenesis. The effect was dose-dependent, took effect by two weeks, and was reversed within three weeks after removal. Glimepiride elicited the same effects in all strains evaluated: four wild-type strains, as well as the transgenic Grn-/- and diabetic db/db mice. Our findings suggest that the use of glimepiride as a hypoglycemic agent in mice should proceed with caution and may have broader implications about mouse models as a proxy to study the human pharmacopeia.

Delayed fracture healing and increased callus adiposity in a C57BL/6J murine model of obesity-associated type 2 diabetes mellitus

Introduction

Impaired healing and non-union of skeletal fractures is a major public health problem, with morbidity exacerbated in patients with diabetes mellitus (DM). DM is prevalent worldwide and affects approximately 25.8 million US adults, with >90% having obesity-related type 2 DM (T2DM). While fracture healing in type 1 DM (T1DM) has been studied using animal models, an investigation into delayed healing in an animal model of T2DM has not yet been performed.

Methods

Male C57BL/6J mice at 5 weeks of age were placed on either a control lean diet or an experimental high-fat diet (HFD) for 12 weeks. A mid-diaphyseal open tibia fracture was induced at 17 weeks of age and a spinal needle was used for intra-medullary fixation. Mice were sacrificed at days 7, 10, 14, 21, 28, and 35 for micro-computed tomography (μCT), histology-based histomorphometry and molecular analyses, and biomechanical testing.

Results

HFD-fed mice displayed increased body weight and impaired glucose tolerance, both characteristic of T2DM. Compared to control mice, HFD-fed mice with tibia fractures showed significantly (p<0.001) decreased woven bone at day 28 by histomorphometry and significantly (p<0.01) decreased callus bone volume at day 21 by μCT. Interestingly, fracture calluses contained markedly increased adiposity in HFD-fed mice at days 21, 28, and 35. HFD-fed mice also showed increased PPARγ immunohistochemical staining at day 14. Finally, calluses from HFD-fed mice at day 35 showed significantly (p<0.01) reduced torsional rigidity compared to controls.

Discussion

Our murine model of T2DM demonstrated delayed fracture healing and weakened biomechanical properties, and was distinctly characterized by increased callus adiposity. This suggests altered mesenchymal stem cell fate determination with a shift to the adipocyte lineage at the expense of the osteoblast lineage. The up-regulation of PPARγ in fracture calluses of HFD-fed mice is likely involved in the proposed fate switching.

Blockade of B-cell-activating factor signaling enhances hepatic steatosis induced by a high-fat diet and improves insulin sensitivity

Chronic inflammation is an important contributor to the development and progression of metabolic syndrome. Recent evidence indicates that, in addition to innate immune cells, adaptive immune cells have an important role in this process. We previously showed that the serum level of B-cell-activating factor (BAFF) was increased in patients with nonalcoholic steatohepatitis. However, it is currently unknown whether BAFF and BAFF-R (BAFF-R) have a role in lipid metabolism in the liver. To address this issue, the role played by BAFF and BAFF-R signaling in the development of insulin resistance and hepatic steatosis was examined in BAFF-R(-/-) mice fed a high-fat diet (HFD). Furthermore, the effect of BAFF on lipid metabolism in hepatocytes was analyzed in vitro. BAFF-R(-/-) mice showed improvements in HFD-induced obesity and insulin resistance. In addition, the number of B cells, levels of serum IgG, and inflammation of visceral fat were reduced in these mice. However, the expression of steatogenic genes and fatty acid deposition in the liver was higher in these mice than in control mice. BAFF was also found to downregulate the expression of steatogenesis genes and enhance steatosis in hepatocytes through BAFF-R. Collectively, these data indicated that, in addition to its known functions in inflammation and glucose metabolism, BAFF has a protective role in hepatic steatosis by regulating lipid metabolism in the liver.

Direct control of hepatic glucose production by interleukin-13 in mice

Hyperglycemia is a result of impaired insulin action on glucose production and disposal, and a major target of antidiabetic therapies. The study of insulin-independent regulatory mechanisms of glucose metabolism may identify new strategies to lower blood sugar levels. Here we demonstrate an unexpected metabolic function for IL-13 in the control of hepatic glucose production. IL-13 is a Th2 cytokine known to mediate macrophage alternative activation. Genetic ablation of Il-13 in mice (Il-13-/-) resulted in hyperglycemia, which progressed to hepatic insulin resistance and systemic metabolic dysfunction. In Il-13-/- mice, upregulation of enzymes involved in hepatic gluconeogenesis was a primary event leading to dysregulated glucose metabolism. IL-13 inhibited transcription of gluconeogenic genes by acting directly on hepatocytes through Stat3, a noncanonical downstream effector. Consequently, the ability of IL-13 to suppress glucose production was abolished in liver cells lacking Stat3 or IL-13 receptor α1 (Il-13rα1), which suggests that the IL-13Rα1/Stat3 axis directs IL-13 signaling toward metabolic responses. These findings extend the implication of a Th1/Th2 paradigm in metabolic homeostasis beyond inflammation to direct control of glucose metabolism and suggest that the IL-13/Stat3 pathway may serve as a therapeutic target for glycemic control in insulin resistance and type 2 diabetes.