Tissue-specificity of insulin action and resistance

Variations in adrenal gland medulla and dopamine effects induced by the lack of Irs2

The adrenomedullary chromaffin cells' hormonal pathway has been related to the pathophysiology of diabetes mellitus. In mice, the deletion of insulin receptor substrate type 2 (Irs2) causes peripheral insulin resistance and reduction in β-cell mass, leading to overt diabetes, with gender differences on adrenergic signaling. To further unravel the relevance of Irs2 on glycemic control, we analyzed in adult Irs2 deficient (Irs2^-/-) mice, of both sexes but still normoglycemic, dopamine effects on insulin secretion and glycerol release, as well as their adrenal medulla by an immunohistochemical and morphologic approach. In isolated islets, 10 μM dopamine significantly inhibited insulin release in wild-type (WT) and female Irs2^-/- mice; however, male Irs2^-/- islets were insensitive to that catecholamine. Similarly, on isolated adipocytes, gender differences were observed between WT and Irs2^-/- mice in basal and evoked glycerol release with crescent concentrations of dopamine. By immunohistochemistry, reactivity to tyrosine hydroxylase (TH) in female mice was significantly higher in the adrenal medulla of Irs2^-/- compared to WT; although no differences for TH-immunopositivity were observed between the male groups of mice. However, compared to their corresponding WT animals, adrenomedullary chromaffin cells of Irs2^-/- mice showed a significant decrease in the cellular and nuclear areas, and even in their percentage of apoptosis. Therefore, our observations suggest that, together with gender differences on dopamine responses in Irs2^-/- mice, disturbances in adrenomedullary chromaffin cells could be related to deficiency of Irs2. Accordingly, Irs2 could be necessary for adequate glucose homeostasis and maintenance of the population of the adrenomedullary chromaffin cells.

Endoplasmic reticulum stress activation in adipose tissue induces metabolic syndrome in individuals with familial partial lipodystrophy of the Dunnigan type

BACKGROUND

Familial partial lipodystrophy of the Dunnigan type is one of the most common inherited lipodystrophies variables. These individuals have important metabolic disorders that cause predisposition to various diseases. In this study we aimed to demonstrate the relation between the metabolic abnormalities, inflammatory profile and the expression of genes involved in the activation of the endoplasmic reticulum stress (ERS) in subjects with FPLD.

METHODS

We evaluated 14 female FPLD patients and compared with 13 female healthy individuals. The subjects were paired with their respective BMI and age and categorized into two groups: Familial partial lipodystrophy of the Dunnigan type (FPLD) and control. Patients were fasted for 12 h before blood collection for measurement of HbA1c, glucose, insulin, lipids and inflammatory markers. Subcutâneous adipose tissue was collected by puncture aspiration of submental region during ambulatorial surgical aesthetic procedure.

RESULTS

We demonstrate that patients with FPLD show increased HbA1c (p < 0.01), fasting glucose (p < 0.002) and triglycerides (p < 0.005) while HDL/cholesterol (p < 0.001) was lower when compared to healthy individuals. We found that 64.2% FPLD patients had metabolic syndrome according to International Diabetes Federation definition. We also observe increased AUC of glucose (p < 0.001) and insulin during oGTT, featuring a frame of hyperglycemia and hyperinsulinemia, suggesting insulin resistance. Also we found hyperactivation of several genes responsible for ERS such as ATF-4 (p < 0.01), ATF-6 (p < 0.01), EIF2α3K (p < 0.005), CCT4 (p < 0.001), CHOP (p < 0.01), CALR (p < 0.001) and CANX (p < 0.005), that corroborate the idea that diabetes mellitus and metabolic syndrome are associated with direct damage to the endoplasmic reticulum homeostasis. Ultimately, we note that individuals with lipodystrophy have an increase in serum interleukins, keys of the inflammatory process, as IL-1β, TNF-α and IL-6 (p < 0.05 all), compared with healthy individuals, which can be the trigger to insulin resistance in this population.

CONCLUSION

Individuals with FPLD besides having typical dysfunctions of metabolic syndrome, show a hyperactivation of ERS associated with increased systemic inflammatory profile, which together may explain the complex clinical aspect of this diseases.Trial registration HCRP no 6711/2012.

Xanthine oxidase inhibition by febuxostat attenuates stress-induced hyperuricemia, glucose dysmetabolism, and prothrombotic state in mice

Chronic stress is closely linked to the metabolic syndrome, diabetes, hyperuricemia and thromboembolism, but the mechanisms remain elusive. We reported recently that stress targets visceral adipose tissue (VAT), inducing lipolysis, low-grade inflammation with production of inflammatory adipokines, metabolic derangements such as insulin resistance, and prothrombotic state. In the present study, we hypothesized the involvement of VAT xanthine oxidoreductase (XOR), a source of reactive oxygen species (ROS) and uric acid (UA) in the above processes. Restraint stress in mice resulted in upregulation of XOR and xanthine oxidase activity, accumulation of ROS in VAT as well as liver and intestine, increase in serum UA levels, upregulation of NADPH oxidase subunits and downregulation of antioxidant enzymes. Immunohistochemistry and RT-PCR analysis also showed that restraint stress induced VAT monocyte accumulation and proinflammatory adipokine production, resulting in reduced insulin sensitivity and induction of plasminogen activator inhibitor-1 and tissue factor in VAT. Treatment with febuxostat, a potent XO inhibitor, suppressed stress-induced ROS production and VAT inflammation, resulting in improvement of serum UA levels, insulin sensitivity, and prothrombotic tendency. Our results suggest that stress perturbs glucose and UA metabolism, and promotes prothrombotic status, and that XO inhibition by febuxostat might be a potential therapy for stress-related disorders.

Imbalanced Insulin Actions in Obesity and Type 2 Diabetes: Key Mouse Models of Insulin Signaling Pathway

Since the discovery of the tyrosine kinase activity of the insulin receptor (IR), researchers have been engaged in intensive efforts to resolve physiological functions of IR and its major downstream targets, insulin receptor substrate 1 (Irs1) and Irs2. Studies conducted using systemic and tissue-specific gene-knockout mice of IR, Irs1, and Irs2 have revealed the physiological roles of these molecules in each tissue and interactions among multiple tissues. In obesity and type 2 diabetes, selective downregulation of Irs2 and its downstream actions to cause reduced insulin actions was associated with increased insulin actions through Irs1 in variety tissues. Thus, we propose the novel concept of "organ- and pathway-specific imbalanced insulin action" in obesity and type 2 diabetes, which includes and extends "selective insulin resistance." This Review focuses on recent progress in understanding insulin signaling and insulin resistance using key mouse models for elucidating pathophysiology of human obesity and type 2 diabetes.

Dipeptidyl peptidase- IV inhibitor alogliptin improves stress-induced insulin resistance and prothrombotic state in a murine model

BACKGROUND

Stress evokes lipolytic release of free fatty acid (FFA) and low-grade inflammation in visceral adipose tissue, mediated by increased adipokine secretion, and contributes to glucose metabolism disorder and prothrombotic state. We tested the hypothesis that alogliptin, a dipeptidyl peptidase-4 inhibitor, can ameliorate the biological effects of chronic stress in mice.

METHOD AND RESULTS

C57BL/6J mice were subjected to 2-week intermittent restraint stress and orally treated with vehicle or alogliptin (dose: 15 or 45mg/kg/day). Plasma levels of lipids, proinflammatory cytokines (monocyte chemoattractant protein-1, tumor necrosis factor-α, and interleukin-6), and 8-hydroxydeoxyguanosine were measured with enzyme-linked immunosorbent assay. Monocyte/macrophage accumulation in inguinal white adipose tissue (WAT) was examined by CD11b-positive cell count and mRNA expression of CD68 and F4/80 was examined by immunohistochemistry and RT-PCR, respectively. The mRNA levels of the above-mentioned proinflammatory cytokines, NADPH oxidase 4, adiponectin, and coagulation factors (plasminogen activation inhibitor-1 and tissue factor) in WAT were also assessed with RT-PCR. Glucose metabolism was assessed by glucose and insulin tolerance tests, plasma levels of DPP-4 activity, glucagon-like peptide-1, expression of DPP-4, insulin receptor substrate-1 and glucose transporter 4 in WAT and skeletal muscle. Alogliptin administration suppressed stress-induced FFA release, oxidative stress, adipose tissue inflammation, DPP-4 activation, and prothrombotic state in a dose-dependent manner, and improved insulin sensitivity in stressed mice.

CONCLUSIONS

The results indicate that alogliptin improves stress-induced prothrombotic state and insulin resistance; suggesting that alogliptin could have beneficial therapeutic effects against cardiovascular complications in diabetic patients under stress.

Angiotensin II receptor blocker ameliorates stress-induced adipose tissue inflammation and insulin resistance

A strong causal link exists between psychological stress and insulin resistance as well with hypertension. Meanwhile, stress-related responses play critical roles in glucose metabolism in hypertensive patients. As clinical trials suggest that angiotensin-receptor blocker delays the onset of diabetes in hypertensive patients, we investigated the effects of irbesartan on stress-induced adipose tissue inflammation and insulin resistance. C57BL/6J mice were subjected to 2-week intermittent restraint stress and orally treated with vehicle, 3 and 10 mg/kg/day irbesartan. The plasma concentrations of lipid and proinflammatory cytokines [Monocyte Chemoattractant Protein-1 (MCP-1), tumor necrosis factor-α, and interleukin-6] were assessed with enzyme-linked immunosorbent assay. Monocyte/macrophage accumulation in inguinal white adipose tissue (WAT) was observed with CD11b-positive cell counts and mRNA expressions of CD68 and F4/80 using immunohistochemistry and RT-PCR methods respectively. The mRNA levels of angiotensinogen, proinflammatory cytokines shown above, and adiponectin in WAT were also assessed with RT-PCR method. Glucose metabolism was assessed by glucose tolerance tests (GTTs) and insulin tolerance tests, and mRNA expression of insulin receptor substrate-1 (IRS-1) and glucose transporter 4 (GLUT4) in WAT. Restraint stress increased monocyte accumulation, plasma free fatty acids, expression of angiotensinogen and proinflammatory cytokines including MCP-1, and reduced adiponectin. Irbesartan reduced stress-induced monocyte accumulation in WAT in a dose dependent manner. Irbesartan treatment also suppressed induction of adipose angiotensinogen and proinflammatory cytokines in WAT and blood, and reversed changes in adiponectin expression. Notably, irbesartan suppressed stress-induced reduction in adipose tissue weight and free fatty acid release, and improved insulin tolerance with restoration of IRS-1 and GLUT4 mRNA expressions in WAT. The results indicate that irbesartan improves stress-induced adipose tissue inflammation and insulin resistance. Our results suggests that irbesartan treatment exerts additive benefits for glucose metabolism in hypertensive patients with mental stress.

Preserved Na/HCO3 cotransporter sensitivity to insulin may promote hypertension in metabolic syndrome

Hyperinsulinemia can contribute to hypertension through effects on sodium transport. To test whether the stimulatory effect of insulin on renal proximal tubule sodium transport is preserved in insulin resistance, we compared the effects of insulin on abdominal adipocytes and proximal tubules in rats and humans. Insulin markedly stimulated the sodium-bicarbonate cotransporter (NBCe1) activity in isolated proximal tubules through the phosphoinositide 3-kinase (PI3-K) pathway. Gene silencing in rats showed that while insulin receptor substrate (IRS)1 mediates the insulin effect on glucose uptake into adipocytes, IRS2 mediates the insulin effect on proximal tubule transport. The stimulatory effect of insulin on glucose uptake into adipocytes was severely reduced, but its stimulatory effect on NBCe1 activity was completely preserved in insulin-resistant Otsuka Long-Evans Tokushima Fatty (OLETF) rats and patients with insulin resistance. Despite widespread reduction of IRS1 and IRS2 expression in insulin-sensitive tissues, IRS2 expression in the kidney cortex was exceptionally preserved in both OLETF rats and patients with insulin resistance. Unlike liver, acute insulin injection failed to change the expression levels of IRS2 and sterol regulatory element-binding protein 1 in rat kidney cortex, indicating that regulatory mechanisms of IRS2 expression are distinct in liver and kidney. Thus, preserved stimulation of proximal tubule transport through the insulin/IRS2/PI3-K pathway may play an important role in the pathogenesis of hypertension associated with metabolic syndrome.

Roles of renal proximal tubule transport in acid/base balance and blood pressure regulation

Sodium-coupled bicarbonate absorption from renal proximal tubules (PTs) plays a pivotal role in the maintenance of systemic acid/base balance. Indeed, mutations in the Na⁺-HCO₃⁻ cotransporter NBCe1, which mediates a majority of bicarbonate exit from PTs, cause severe proximal renal tubular acidosis associated with ocular and other extrarenal abnormalities. Sodium transport in PTs also plays an important role in the regulation of blood pressure. For example, PT transport stimulation by insulin may be involved in the pathogenesis of hypertension associated with insulin resistance. Type 1 angiotensin (Ang) II receptors in PT are critical for blood pressure homeostasis. Paradoxically, the effects of Ang II on PT transport are known to be biphasic. Unlike in other species, however, Ang II is recently shown to dose-dependently stimulate human PT transport via nitric oxide/cGMP/ERK pathway, which may represent a novel therapeutic target in human hypertension. In this paper, we will review the physiological and pathophysiological roles of PT transport.

Protein tyrosine phosphatase-1B modulates pancreatic β-cell mass

Protein tyrosine phosphatase 1B (PTP1B) is a negative regulator of the insulin signalling pathway. It has been demonstrated that PTP1B deletion protects against the development of obesity and Type 2 Diabetes, mainly through its action on peripheral tissues. However, little attention has been paid to the role of PTP1B in β-cells. Therefore, our aim was to study the role of PTP1B in pancreatic β-cells. Silencing of PTP1B expression in a pancreatic β-cell line (MIN6 cells) reveals the significance of this endoplasmic reticulum bound phosphatase in the regulation of cell proliferation and apoptosis. Furthermore, the ablation of PTP1B is able to regulate key proteins involved in the proliferation and/or apoptosis pathways, such as STAT3, AKT, ERK1/2 and p53 in isolated islets from PTP1B knockout (PTP1B ⁻/⁻) mice. Morphometric analysis of pancreatic islets from PTP1B ⁻/⁻ mice showed a higher β-cell area, concomitantly with higher β-cell proliferation and a lower β-cell apoptosis when compared to islets from their respective wild type (WT) littermates. At a functional level, isolated islets from 8 weeks old PTP1B ⁻/⁻ mice exhibit enhanced glucose-stimulated insulin secretion. Moreover, PTP1B ⁻/⁻ mice were able to partially reverse streptozotocin-induced β-cell loss. Together, our data highlight for the first time the involvement of PTP1B in β-cell physiology, reinforcing the potential of this phosphatase as a therapeutical target for the treatment of β-cell failure, a central aspect in the pathogenesis of Type 2 Diabetes.

Tissue-specific effects of protein malnutrition on insulin signaling pathway and lipid accumulation in growing rats

Our previous studies have revealed that protein malnutrition enhances insulin signaling in rat liver and muscle in response to a bolus insulin injection. However, it has not been established whether protein malnutrition up-regulates insulin signaling under physiological conditions, such as feeding. Here, we studied the effects of protein malnutrition on insulin signaling after feeding in rat liver, muscle and white adipose tissue (WAT). Six-week-old rats were fed a 15% casein diet (15C) or a calorie-matched 5% casein diet (5C) for 8 h/day during 14 days. On the 15th day, blood and tissues were collected at various time points after feeding. Feeding-induced insulin secretion was reduced in 5C-fed rats compared to 15C-fed rats. The 5C-feeding suppressed immediate activation of insulin receptor after feeding in the liver, muscle, and WAT. However, 5C-feeding constantly increased tyrosine phosphorylation of insulin receptor substrate (IRS)-2 and threonine phosphorylation of eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) in the liver during the examined periods, corresponding to the changes of their amounts. In skeletal muscle, 5C-feeding did not appreciably alter insulin signaling. In WAT, 5C-feeding decreased tyrosine phosphorylation of IRS-1 compared to 15C-feeding. Furthermore, hepatic triglyceride content was increased and feeding-induced acetyl-CoA carboxylase 1 gene expression was enhanced in 5C-fed rats. The 5C-feeding decreased insulin-dependent glucose uptake in adipocytes. These results suggest that enhanced insulin signaling through increased IRS-2 and 4E-BP1 levels in the liver and repressed insulin signaling through decreased IRS-1 levels in WAT contribute to the preferential hepatic lipid accumulation under protein malnutrition.

The physiological and pathophysiological roles of adipocyte miRNAs

MicroRNAs (miRNAs) are highly conserved, small, noncoding RNAs that regulate gene expression at the posttranscriptional level. Their actions affect numerous important biological processes, including adipocyte differentiation and function, sugar and lipid metabolism, and insulin production and secretion. Recent reports suggest miRNAs may also be involved in the pathogenic processes of obesity, diabetes, and insulin resistance. In this review, we summarize research progresses on adipocyte miRNAs and their physiological and pathological implications.

Insulin resistance and the polycystic ovary syndrome revisited: an update on mechanisms and implications

Polycystic ovary syndrome (PCOS) is now recognized as an important metabolic as well as reproductive disorder conferring substantially increased risk for type 2 diabetes. Affected women have marked insulin resistance, independent of obesity. This article summarizes the state of the science since we last reviewed the field in the Endocrine Reviews in 1997. There is general agreement that obese women with PCOS are insulin resistant, but some groups of lean affected women may have normal insulin sensitivity. There is a post-binding defect in receptor signaling likely due to increased receptor and insulin receptor substrate-1 serine phosphorylation that selectively affects metabolic but not mitogenic pathways in classic insulin target tissues and in the ovary. Constitutive activation of serine kinases in the MAPK-ERK pathway may contribute to resistance to insulin's metabolic actions in skeletal muscle. Insulin functions as a co-gonadotropin through its cognate receptor to modulate ovarian steroidogenesis. Genetic disruption of insulin signaling in the brain has indicated that this pathway is important for ovulation and body weight regulation. These insights have been directly translated into a novel therapy for PCOS with insulin-sensitizing drugs. Furthermore, androgens contribute to insulin resistance in PCOS. PCOS may also have developmental origins due to androgen exposure at critical periods or to intrauterine growth restriction. PCOS is a complex genetic disease, and first-degree relatives have reproductive and metabolic phenotypes. Several PCOS genetic susceptibility loci have been mapped and replicated. Some of the same susceptibility genes contribute to disease risk in Chinese and European PCOS populations, suggesting that PCOS is an ancient trait.

Stress augments insulin resistance and prothrombotic state: role of visceral adipose-derived monocyte chemoattractant protein-1

Stressors contribute to thrombosis and insulin resistance. Since obesity-related adipose inflammation is also involved in these pathological states, we assumed that stress correlates with adipose inflammation. Male mice were subjected to 2-week intermittent restraint stress. Expression of plasma lipids, monocyte/macrophage markers (CD11b, CD68, and F4/80), proinflammatory cytokines (monocyte chemoattractant protein-1 [MCP-1], tumor necrosis factor-α, and interleukin-6), adiponectin, heat shock protein 70.1 (HSP70.1), and coagulation factors (plasminogen activation inhibitor-1 [PAI-1] and tissue factor [TF]) in blood and inguinal white adipose tissue (WAT) was determined using immunohistochemistry, enzyme-linked immunosorbent assay, and RT-PCR, respectively. Glucose metabolism was assessed by glucose tolerance tests (GTTs) and insulin tolerance tests, and expression of insulin receptor substrate-1 (IRS-1) and glucose transporter 4 (GLUT4) in WAT. To examine effects of MCP-1 blockade, animals were treated with control or neutralizing antibody, or transplanted with control or 7ND (dominant-negative form of MCP-1)-overexpressing adipose-derived stromal cells (ADSCs). Stress increased monocyte accumulation, free fatty acids, proinflammatory cytokine, and HSP70.1 and reduced adiponectin. Adipose stromal cells highly expressed MCP-1. The stress-induced adipose inflammation increased PAI-1 and TF but did not give rise to thrombus formation. Without any changes in GTT, stress worsened insulin sensitivity and decreased IRS-1 and GLUT4 in WAT. Neutralizing antibody and 7ND-ADSCs reversed stress-induced adipose inflammation, procoagulant state, and insulin resistance. Stress evoked adipose inflammation to increase coagulation factors and impair insulin sensitivity through adipose-derived MCP-1.