Vasoconstrictor effects of insulin in skeletal muscle arterioles are mediated by ERK1/2 activation in endothelium

New insights into mechanisms of endothelial insulin resistance in type 2 diabetes

Insulin resistance in the vasculature is a hallmark of type 2 diabetes (T2D), and blunting of insulin-induced vasodilation is its primary consequence. Individuals with T2D exhibit a marked impairment in insulin-induced dilation in resistance arteries across vascular beds. Importantly, reduced insulin-stimulated vasodilation and blood flow to skeletal muscle limits glucose uptake and contributes to impaired glucose control in T2D. The study of mechanisms responsible for the suppressed vasodilatory effects of insulin has been a growing topic of interest for not only its association with glucose control and extension to T2D but also its relationship with cardiovascular disease development and progression. In this mini-review, we integrate findings from recent studies by our group with the existing literature focused on the mechanisms underlying endothelial insulin resistance in T2D.

Investigation of proteins important for microcirculation using in vivo microdialysis after glucose provocation: a proteomic study

Insulin has metabolic and vascular effects in the human body. What mechanisms that orchestrate the effects in the microcirculation, and how the responds differ in different tissues, is however not fully understood. It is therefore of interest to search for markers in microdialysate that may be related to the microcirculation. This study aims to identify proteins related to microvascular changes in different tissue compartments after glucose provocation using in vivo microdialysis. Microdialysis was conducted in three different tissue compartments (intracutaneous, subcutaneous and intravenous) from healthy subjects. Microdialysate was collected during three time periods; recovery after catheter insertion, baseline and glucose provocation, and analyzed using proteomics. Altogether, 126 proteins were detected. Multivariate data analysis showed that the differences in protein expression levels during the three time periods, including comparison before and after glucose provocation, were most pronounced in the intracutaneous and subcutaneous compartments. Four proteins with vascular effects were identified (angiotensinogen, kininogen-1, alpha-2-HS-glycoprotein and hemoglobin subunit beta), all upregulated after glucose provocation compared to baseline in all three compartments. Glucose provocation is known to cause insulin-induced vasodilation through the nitric oxide pathway, and this study indicates that this is facilitated through the interactions of the RAS (angiotensinogen) and kallikrein-kinin (kininogen-1) systems.

How Perturbated Metabolites in Diabetes Mellitus Affect the Pathogenesis of Hypertension?

The presence of hypertension (HTN) in type 2 diabetes mellitus (DM) is a common phenomenon in more than half of the diabetic patients. Since HTN constitutes a predictor of vascular complications and cardiovascular disease in type 2 DM patients, it is of significance to understand the molecular and cellular mechanisms of type 2 DM binding to HTN. This review attempts to understand the mechanism via the perspective of the metabolites. It reviewed the metabolic perturbations, the biological function of perturbated metabolites in two diseases, and the mechanism underlying metabolic perturbation that contributed to the connection of type 2 DM and HTN. DM-associated metabolic perturbations may be involved in the pathogenesis of HTN potentially in insulin, angiotensin II, sympathetic nervous system, and the energy reprogramming to address how perturbated metabolites in type 2 DM affect the pathogenesis of HTN. The recent integration of the metabolism field with microbiology and immunology may provide a wider perspective. Metabolism affects immune function and supports immune cell differentiation by the switch of energy. The diverse metabolites produced by bacteria modified the biological process in the inflammatory response of chronic metabolic diseases either. The rapidly evolving metabolomics has enabled to have a better understanding of the process of diseases, which is an important tool for providing some insight into the investigation of diseases mechanism. Metabolites served as direct modulators of biological processes were believed to assess the pathological mechanisms involved in diseases.

GLP-1 modulates insulin-induced relaxation response through β-arrestin2 regulation in diabetic mice aortas

AIMS

Diabetes impairs insulin-induced endothelium-dependent relaxation by reducing nitric oxide (NO) production. GLP-1, an incretin hormone, has been shown to prevent the development of endothelial dysfunction. In this study, we hypothesized that GLP-1 would improve the impaired insulin-induced relaxation response in diabetic mice. We also examined the underlying mechanisms.

METHODS

Using aortic rings from ob/ob mice, an animal model of obesity and type 2 diabetes, and from lean mice, vascular relaxation responses and protein expressions were evaluated using insulin, GLP-1, and pathway-specific inhibitors to elucidate the mechanisms of response. In parallel experiments, β-arrestin2 siRNA-transfected aortas were treated with GLP-1 to evaluate its effects on aortic response pathways.

RESULTS

When compared to that of untreated ob/ob aortas, GLP-1 increased insulin-induced vasorelaxation and NO production. AMPK inhibition did not alter this vasorelaxation in both GLP-1-treated lean and ob/ob aortas, while Akt inhibition reduced vasorelaxation in both groups, and co-treatment with GLP-1 and insulin caused Akt/eNOS activation. Additionally, GLP-1 decreased GRK2 activity and enhanced β-arrestin2 translocation from the cytosol to membrane in ob/ob aortas. β-Arrestin2 siRNA decreased insulin-induced relaxation both in lean aortas and GLP-1-treated ob/ob aortas.

CONCLUSIONS

We demonstrated that insulin-induced relaxation is dependent on β-arrestin2 translocation and Akt activation via GLP-1-stimulated GRK2 inactivation in ob/ob aortas. We showed a novel cross-talk between GLP-1-responsive β-arrestin2 and insulin signalling in diabetic aortas.

β-blockade prevents coronary macro- and microvascular dysfunction induced by a high salt diet and insulin resistance in the Goto-Kakizaki rat

A high salt intake exacerbates insulin resistance, evoking hypertension due to systemic perivascular inflammation, oxidative-nitrosative stress and endothelial dysfunction. Angiotensin-converting enzyme inhibitor (ACEi) and angiotensin receptor blockers (ARBs) have been shown to abolish inflammation and redox stress but only partially restore endothelial function in mesenteric vessels. We investigated whether sympatho-adrenal overactivation evokes coronary vascular dysfunction when a high salt intake is combined with insulin resistance in male Goto-Kakizaki (GK) and Wistar rats treated with two different classes of β-blocker or vehicle, utilising synchrotron-based microangiography in vivo. Further, we examined if chronic carvedilol (CAR) treatment preserves nitric oxide (NO)-mediated coronary dilation more than metoprolol (MET). A high salt diet (6% NaCl w/w) exacerbated coronary microvessel endothelial dysfunction and NO-resistance in vehicle-treated GK rats while Wistar rats showed modest impairment. Microvascular dysfunction was associated with elevated expression of myocardial endothelin, inducible NO synthase (NOS) protein and 3-nitrotyrosine (3-NT). Both CAR and MET reduced basal coronary perfusion but restored microvessel endothelium-dependent and -independent dilation indicating a role for sympatho-adrenal overactivation in vehicle-treated rats. While MET treatment reduced myocardial nitrates, only MET treatment completely restored microvessel dilation to dobutamine (DOB) stimulation in the absence of NO and prostanoids (combined inhibition), indicating that MET restored the coronary flow reserve attributable to endothelium-derived hyperpolarisation (EDH). In conclusion, sympatho-adrenal overactivation caused by high salt intake and insulin resistance evoked coronary microvessel endothelial dysfunction and diminished NO sensitivity, which were restored by MET and CAR treatment in spite of ongoing inflammation and oxidative-nitrosative stress presumably caused by uninhibited renin-angiotensin-aldosterone system (RAAS) overactivation.

Metabolic and Energy Imbalance in Dysglycemia-Based Chronic Disease

Metabolic flexibility is the ability to efficiently adapt metabolism based on nutrient availability and requirement that is essential to maintain homeostasis in times of either caloric excess or restriction and during the energy-demanding state. This regulation is orchestrated in multiple organ systems by the alliance of numerous metabolic pathways under the master control of the insulin-glucagon-sympathetic neuro-endocrine axis. This, in turn, regulates key metabolic enzymes and transcription factors, many of which interact closely with and culminate in the mitochondrial energy generation machinery. Metabolic flexibility is compromised due to the continuous mismatch between availability and intake of calorie-dense foods and reduced metabolic demand due to sedentary lifestyle and age-related metabolic slowdown. The resultant nutrient overload leads to mitochondrial trafficking of substrates manifesting as mitochondrial dysfunction characterized by ineffective substrate switching and incomplete substrate utilization. At the systemic level, the manifestation of metabolic inflexibility comprises reduced skeletal muscle glucose disposal rate, impaired suppression of hepatic gluconeogenesis and adipose tissue lipolysis manifesting as insulin resistance. This is compounded by impaired β-cell function and progressively reduced β-cell mass. A consequence of insulin resistance is the upregulation of the mitogen-activated protein kinase pathway leading to a pro-hypertensive, atherogenic, and thrombogenic environment. This is further aggravated by oxidative stress, advanced glycation end products, and inflammation, which potentiates the risk of micro- and macro-vascular complications. This review aims to elucidate underlying mechanisms mediating the onset of metabolic inflexibility operating at the main target organs and to understand the progression of metabolic diseases. This could potentially translate into a pharmacological tool that can manage multiple interlinked conditions of dysglycemia, hypertension, and dyslipidemia by restoring metabolic flexibility. We discuss the breadth and depth of metabolic flexibility and its impact on health and disease.

GLP-1 and insulin regulation of skeletal and cardiac muscle microvascular perfusion in type 2 diabetes

Muscle microvasculature critically regulates skeletal and cardiac muscle health and function. It provides endothelial surface area for substrate exchange between the plasma compartment and the muscle interstitium. Insulin fine-tunes muscle microvascular perfusion to regulate its own action in muscle and oxygen and nutrient supplies to muscle. Specifically, insulin increases muscle microvascular perfusion, which results in increased delivery of insulin to the capillaries that bathe the muscle cells and then facilitate its own transendothelial transport to reach the muscle interstitium. In type 2 diabetes, muscle microvascular responses to insulin are blunted and there is capillary rarefaction. Both loss of capillary density and decreased insulin-mediated capillary recruitment contribute to a decreased endothelial surface area available for substrate exchange. Vasculature expresses abundant glucagon-like peptide 1 (GLP-1) receptors. GLP-1, in addition to its well-characterized glycemic actions, improves endothelial function, increases muscle microvascular perfusion, and stimulates angiogenesis. Importantly, these actions are preserved in the insulin resistant states. Thus, treatment of insulin resistant patients with GLP-1 receptor agonists may improve skeletal and cardiac muscle microvascular perfusion and increase muscle capillarization, leading to improved delivery of oxygen, nutrients, and hormones such as insulin to the myocytes. These actions of GLP-1 impact skeletal and cardiac muscle function and systems biology such as functional exercise capacity. Preclinical studies and clinical trials involving the use of GLP-1 receptor agonists have shown salutary cardiovascular effects and improved cardiovascular outcomes in type 2 diabetes mellitus. Future studies should further examine the different roles of GLP-1 in cardiac as well as skeletal muscle function.

Mechanistic Links Between Obesity, Diabetes, and Blood Pressure: Role of Perivascular Adipose Tissue

Obesity is increasingly prevalent and is associated with substantial cardiovascular risk. Adipose tissue distribution and morphology play a key role in determining the degree of adverse effects, and a key factor in the disease process appears to be the inflammatory cell population in adipose tissue. Healthy adipose tissue secretes a number of vasoactive adipokines and anti-inflammatory cytokines, and changes to this secretory profile will contribute to pathogenesis in obesity. In this review, we discuss the links between adipokine dysregulation and the development of hypertension and diabetes and explore the potential for manipulating adipose tissue morphology and its immune cell population to improve cardiovascular health in obesity.

Relationship between Resistin, Endothelin-1, and Flow-Mediated Dilation in Patient with and without Metabolic Syndrome

Background:

Resistin is peptides that signal the functional status of adipose tissue to the brain and other target organs. It causes insulin resistance and affects the vascular endothelial dysfunction. However, the function and relation between resistin in endothelin-1 (ET-1), which leads to the endothelial dysfunction in humans are enigmatic.

Materials and Methods:

In a cross-sectional study of 76 participants (38 metabolic syndrome patients and 38 healthy participants), biochemical and clinical parameters, including lipid profile, fasting glucose, resistin, ET-1, C-reactive protein (CRP), flow-mediated dilation (FMD), and hypertension were determined and compared between the two groups.

Results:

Multiple linear regression analysis was performed with age- and sex-adjusted plasma resistin levels, FMD, and ET-1 as the dependent variables. Analysis showed that weight, body mass index, triglycerides (TGs), and ET-1 were statistically significant correlated with serum resistin. FMD has negative significantly correlated with weight (r = −0.491, P = 0.001), waist circumference (r = −0.491, P = 0.001), waist-to-hip ratio (r = −0.0444, P = 0.001), and ET-1 (r = −0.075, P = 0.050), but it has significantly correlated with systolic blood pressure (SBP) (r = 0.290, P = 0.016), diastolic blood pressure (DBP) (r = 0.275, P = 0.023), and high-density lipoprotein cholesterol (HDL-C) (r = −0.266, P = 0.050), and ET-1, but it has significantly correlated with SBP, DBP, and HDL-C. ET-1 is significantly correlated with TGs (r = −0.436, P = 0.006), total cholesterol (r = 0.452, P = 0.004), low-density lipoprotein cholesterol (r = 0.454, r = 0.004), and resistin (r = 0.282, P = 0.050), whereas it has negative significantly correlated with HDL-C (r = 0.346, P = 0.034), FMD (r = −0.075, P = 0.050.

Conclusion:

In this study, results shown plasma ET-1 and resistin are suggested as risk factors for the development of endothelial dysfunction and with further study, it is possible that can diagnose the risk of diabetes and cardiovascular disease in the early stages.

Muscle Insulin Resistance and the Inflamed Microvasculature: Fire from Within

Insulin is a vascular hormone and regulates vascular tone and reactivity. Muscle is a major insulin target that is responsible for the majority of insulin-stimulated glucose use. Evidence confirms that muscle microvasculature is an important insulin action site and critically regulates insulin delivery to muscle and action on myocytes, thereby affecting insulin-mediated glucose disposal. Insulin via activation of its signaling cascade in the endothelial cells increases muscle microvascular perfusion, which leads to an expansion of the endothelial exchange surface area. Insulin’s microvascular actions closely couple with its metabolic actions in muscle and blockade of insulin-mediated microvascular perfusion reduces insulin-stimulated muscle glucose disposal. Type 2 diabetes is associated with chronic low-grade inflammation, which engenders both metabolic and microvascular insulin resistance through endocrine, autocrine and paracrine actions of multiple pro-inflammatory factors. Here, we review the crucial role of muscle microvasculature in the regulation of insulin action in muscle and how inflammation in the muscle microvasculature affects insulin’s microvascular actions as well as metabolic actions. We propose that microvascular insulin resistance induced by inflammation is an early event in the development of metabolic insulin resistance and eventually type 2 diabetes and its related cardiovascular complications, and thus is a potential therapeutic target for the prevention or treatment of obesity and diabetes.

Caloric restriction attenuates aging-induced cardiac insulin resistance in male Wistar rats through activation of PI3K/Akt pathway

BACKGROUND AND AIM

Caloric restriction (CR) improves insulin sensitivity and is one of the dietetic strategies most commonly used to enlarge life and to prevent aging-induced cardiovascular alterations. The aim of this study was to analyze the possible beneficial effects of caloric restriction (CR) preventing the aging-induced insulin resistance in the heart of male Wistar rats.

METHODS AND RESULTS

Three experimental groups were used: 3 months old rats (3m), 24 months old rats (24m) and 24 months old rats subjected to 20% CR during their three last months of life (24m-CR). After sacrifice hearts were mounted in a perfusion system (Langendorff) and heart function in basal conditions and in response to accumulative doses of insulin (10^-9-10^-7 M), in the presence or absence of Wortmannin (10^-6 M), was recorded. CR did not attenuate the aging-induced decrease in coronary artery vasodilation in response to insulin administration, but it prevented the aging-induced downregulation of cardiac contractility (dp/dt) through activation of the PI3K/Akt intracellular pathway. Insulin stimulated in a greater extent the PI3K/Akt pathway vs the activation of the MAPK pathway and increased the protein expression of IR, GLUT-4 and eNOS in the hearts of 3m and 24m-CR rats, but not in the hearts of 24m rats. Furthermore, CR prevented the aging induced increase in endothelin-1 protein expression in myocardial tissue.

CONCLUSION

In conclusion CR partially improves cardiac insulin sensitivity and prevents the aging induced decrease in myocardial contractility in response to insulin administration through activation of PI3K/Akt pathway.

Microvascular Dysfunction and Hyperglycemia: A Vicious Cycle With Widespread Consequences

Microvascular and metabolic physiology are tightly linked. This Perspective reviews evidence that 1) the relationship between hyperglycemia and microvascular dysfunction (MVD) is bidirectional and constitutes a vicious cycle; 2) MVD in diabetes affects many, if not all, organs, which may play a role in diabetes-associated comorbidities such as depression and cognitive impairment; and 3) MVD precedes, and contributes to, hyperglycemia in type 2 diabetes (T2D) through impairment of insulin-mediated glucose disposal and, possibly, insulin secretion. Obesity and adverse early-life exposures are important drivers of MVD. MVD can be improved through weight loss (in obesity) and through exercise. Pharmacological interventions to improve MVD are an active area of investigation.

The Vasculature in Prediabetes

The frequency of prediabetes is increasing as the prevalence of obesity rises worldwide. In prediabetes, hyperglycemia, insulin resistance, and inflammation and metabolic derangements associated with concomitant obesity cause endothelial vasodilator and fibrinolytic dysfunction, leading to increased risk of cardiovascular and renal disease. Importantly, the microvasculature affects insulin sensitivity by affecting the delivery of insulin and glucose to skeletal muscle; thus, endothelial dysfunction and extracellular matrix remodeling promote the progression from prediabetes to diabetes mellitus. Weight loss is the mainstay of treatment in prediabetes, but therapies that improved endothelial function and vasodilation may not only prevent cardiovascular disease but also slow progression to diabetes mellitus.

An Overview of Novel Dietary Supplements and Food Ingredients in Patients with Metabolic Syndrome and Non-Alcoholic Fatty Liver Disease

Metabolic syndrome (MetS) is characterized by interconnected factors related to metabolic disturbances, and is directly related to the occurrence of some diseases such as cardiovascular diseases and type 2 diabetes. MetS is described as one or both of insulin resistance and visceral adiposity, considered the initial causes of abnormalities that include hyperglycemia, elevated blood pressure, dyslipidemia, elevated inflammatory markers, and prothrombotic state, as well as polycystic ovarian syndrome in women. Other than in MetS, visceral adiposity and the pro-inflammatory state are also key in the development of non-alcoholic fatty liver disease (NAFLD), which is the most prevalent chronic liver disease in modern society. Both MetS and NAFLD are related to diet and lifestyle, and their treatment may be influenced by dietary pattern changes and the use of certain dietary supplements. This study aimed to review the role of food ingredients and supplements in the management of MetS and NAFLD specifically in human clinical trials. Moreover, bioactive compounds and polyunsaturated fatty acids (PUFAs) may be used as strategies for preventing the onset of and treatment of metabolic disorders, such as MetS and NAFLD, improving the inflammatory state and other comorbidities, such as obesity, dyslipidemias, and cardiovascular diseases (CVD).

Insulin Receptor Substrate 2 Controls Insulin-Mediated Vasoreactivity and Perivascular Adipose Tissue Function in Muscle

Introduction: Insulin signaling in adipose tissue has been shown to regulate insulin's effects in muscle. In muscle, perivascular adipose tissue (PVAT) and vascular insulin signaling regulate muscle perfusion. Insulin receptor substrate (IRS) 2 has been shown to control adipose tissue function and glucose metabolism, and here we tested the hypothesis that IRS2 mediates insulin's actions on the vessel wall as well as the vasoactive properties of PVAT.

Methods: We studied PVAT and muscle resistance arteries (RA) from littermate IRS2^+/+ and IRS2^−/− mice and vasoreactivity by pressure myography, vascular insulin signaling, adipokine expression, and release and PVAT morphology. As insulin induced constriction of IRS2^+/+ RA in our mouse model, we also exposed RA's of C57/Bl6 mice to PVAT from IRS2^+/+ and IRS2^−/− littermates to evaluate vasodilator properties of PVAT.

Results: IRS2^−/− RA exhibited normal vasomotor function, yet a decreased maximal diameter compared to IRS2^+/+ RA. IRS2^+/+ vessels unexpectedly constricted endothelin-dependently in response to insulin, and this effect was absent in IRS2^−/− RA due to reduced ERK1/2activation. For evaluation of PVAT function, we also used C57/Bl6 vessels with a neutral basal effect of insulin. In these experiments insulin (10.0 nM) increased diameter in the presence of IRS2^+/+ PVAT (17 ± 4.8, p = 0.014), yet induced a 10 ± 7.6% decrease in diameter in the presence of IRS2^−/− PVAT. Adipocytes in IRS2^−/− PVAT (1314 ± 161 μm²) were larger (p = 0.0013) than of IRS2^+/+ PVAT (915 ± 63 μm²). Adiponectin, IL-6, PAI-1 secretion were similar between IRS2^+/+ and IRS2^−/− PVAT, as were expression of pro-inflammatory genes (TNF-α, CCL2) and adipokines (adiponectin, leptin, endothelin-1). Insulin-induced AKT phosphorylation in RA was similar in the presence of IRS2^−/− and IRS2^+/+ PVAT.

Conclusion: In muscle, IRS2 regulates both insulin's vasoconstrictor effects, mediating ERK1/2-ET-1 activation, and its vasodilator effects, by mediating the vasodilator effect of PVAT. The regulatory role of IRS2 in PVAT is independent from adiponectin secretion.

Protective Role of Perivascular Adipose Tissue in Endothelial Dysfunction and Insulin-Induced Vasodilatation of Hypercholesterolemic LDL Receptor-Deficient Mice

Background: Endothelial dysfunction plays a pivotal role in the initiation of atherosclerosis. Vascular insulin resistance might contribute to a reduction in endothelial nitric oxide (NO) production, leading to impaired endothelium-dependent relaxation in cardiometabolic diseases. Because perivascular adipose tissue (PVAT) controls endothelial function and NO bioavailability, we hypothesized a role for this fat deposit in the vascular complications associated with the initial stages of atherosclerosis. Therefore, we investigated the potential involvement of PVAT in the early endothelial dysfunction in hypercholesterolemic LDL receptor knockout mice (LDLr-KO).

Methods: Thoracic aortas with and without PVAT were isolated from 4-month-old C57BL/6J (WT) and LDLr-KO mice. The contribution of PVAT to relaxation responses to acetylcholine, insulin, and sodium nitroprusside was investigated. Western blotting was used to examine endothelial NO synthase (eNOS) and adiponectin expression, as well the insulin signaling pathway in aortic PVAT.

Results: PVAT-free aortas of LDLr-KO mice exhibited impaired acetylcholine- and insulin-induced relaxation compared with those of WT mice. Both vasodilatory responses were restored by the presence of PVAT in LDLr-KO mice, associated with enhanced acetylcholine-induced NO levels. PVAT did not change vasodilatory responses to acetylcholine and insulin in WT mice, while vascular relaxation evoked by the NO donor sodium nitroprusside was not modified by either genotype or PVAT. The expression of insulin receptor substrate-1 (IRS-1), phosphatidylinositol 3-kinase (PI3K), AKT, ERK1/2, phosphorylation of AKT (Ser473) and ERK1/2 (Thr202/Tyr204), and adiponectin was similar in the PVAT of WT and LDLr-KO mice, suggesting no changes in PVAT insulin signaling. However, eNOS expression was enhanced in the PVAT of LDLr-KO mice, while eNOS expression was less abundant in PVAT-free aortas.

Conclusion: These results suggest that elevated eNOS-derived NO production in aortic PVAT might be a compensatory mechanism for the endothelial dysfunction and impaired vasodilator action of insulin in hypercholesterolemic LDLr-deficient mice. This protective effect may limit the progression of atherosclerosis in genetic hypercholesterolemia in the absence of an atherogenic diet.

Benefits of Nut Consumption on Insulin Resistance and Cardiovascular Risk Factors: Multiple Potential Mechanisms of Actions

Epidemiological and clinical studies have indicated that nut consumption could be a healthy dietary strategy to prevent and treat type 2 diabetes (T2DM) and related cardiovascular disease (CVD). The objective of this review is to examine the potential mechanisms of action of nuts addressing effects on glycemic control, weight management, energy balance, appetite, gut microbiota modification, lipid metabolism, oxidative stress, inflammation, endothelial function and blood pressure with a focus on data from both animal and human studies. The favourable effects of nuts could be explained by the unique nutrient composition and bioactive compounds in nuts. Unsaturated fatty acids (monounsaturated fatty acids and polyunsaturated fatty acids) present in nuts may play a role in glucose control and appetite suppression. Fiber and polyphenols in nuts may also have an anti-diabetic effect by altering gut microbiota. Nuts lower serum cholesterol by reduced cholesterol absorption, inhibition of HMG-CoA reductase and increased bile acid production by stimulation of 7-α hydroxylase. Arginine and magnesium improve inflammation, oxidative stress, endothelial function and blood pressure. In conclusion, nuts contain compounds that favourably influence glucose homeostasis, weight control and vascular health. Further investigations are required to identify the most important mechanisms by which nuts decrease the risk of T2DM and CVD.

Targeting endothelial metaflammation to counteract diabesity cardiovascular risk: Current and perspective therapeutic options

The association of obesity and diabetes, termed "diabesity", defines a combination of primarily metabolic disorders with insulin resistance as the underlying common pathophysiology. Cardiovascular disorders associated with diabesity represent the leading cause of morbidity and mortality in the Western world. This makes diabesity, with its rising impacts on both health and economics, one of the most challenging biomedical and social threats of present century. The emerging comprehension of the genes whose alteration confers inter-individual differences on risk factors for diabetes or obesity, together with the potential role of genetically determined variants on mechanisms controlling responsiveness, effectiveness and safety of anti-diabetic therapy underlines the need of additional knowledge on molecular mechanisms involved in the pathophysiology of diabesity. Endothelial cell dysfunction, resulting from the unbalanced production of endothelial-derived vascular mediators, is known to be present at the earliest stages of insulin resistance and obesity, and may precede the clinical diagnosis of diabetes by several years. Once considered as a mere consequence of metabolic abnormalities, it is now clear that endothelial dysfunctional activity may play a pivotal role in the progression of diabesity. In the vicious circle where vascular defects and metabolic disturbances worsen and reinforce each other, a low-grade, chronic, and 'cold' inflammation (metaflammation) has been suggested to serve as the pathophysiological link that binds endothelial and metabolic dysfunctions. In this paradigm, it is important to consider how traditional antidiabetic treatments (specifically addressing metabolic dysregulation) may directly impact on inflammatory processes or cardiovascular function. Indeed, not all drugs currently available to treat diabetes possess the same anti-inflammatory potential, or target endothelial cell function equally. Perspective strategies pointing at reducing metaflammation or directly addressing endothelial dysfunction may disclose beneficial consequences on metabolic regulation. This review focuses on existing and potential new approaches ameliorating endothelial dysfunction and vascular inflammation in the context of diabesity.

Methylglyoxal-Glyoxalase 1 Balance: The Root of Vascular Damage

The highly reactive dicarbonyl methylglyoxal (MGO) is mainly formed as byproduct of glycolysis. Therefore, high blood glucose levels determine increased MGO accumulation. Nonetheless, MGO levels are also increased as consequence of the ineffective action of its main detoxification pathway, the glyoxalase system, of which glyoxalase 1 (Glo1) is the rate-limiting enzyme. Indeed, a physiological decrease of Glo1 transcription and activity occurs not only in chronic hyperglycaemia but also with ageing, during which MGO accumulation occurs. MGO and its advanced glycated end products (AGEs) are associated with age-related diseases including diabetes, vascular dysfunction and neurodegeneration. Endothelial dysfunction is the first step in the initiation, progression and clinical outcome of vascular complications, such as retinopathy, nephropathy, impaired wound healing and macroangiopathy. Because of these considerations, studies have been centered on understanding the molecular basis of endothelial dysfunction in diabetes, unveiling a central role of MGO-Glo1 imbalance in the onset of vascular complications. This review focuses on the current understanding of MGO accumulation and Glo1 activity in diabetes, and their contribution on the impairment of endothelial function leading to diabetes-associated vascular damage.

Role of habitual physical activity in modulating vascular actions of insulin

NEW FINDINGS

What is the topic of this review? This review highlights the importance of increased vascular insulin sensitivity for maintaining glycaemic control and cardiovascular health. What advances does it highlight? We discuss the role of habitual physical activity in modulating vascular actions of insulin. Type 2 diabetes and cardiovascular disease commonly coexist. Current evidence suggests that impaired insulin signalling in the vasculature may be a common link between metabolic and cardiovascular diseases, including glycaemic dysregulation and atherosclerosis. Herein, we highlight the importance of the actions of insulin on the vasculature for glycaemic control and arterial health. In addition, we summarize and discuss findings from our group and others demonstrating that increased physical activity may be an effective approach to enhancing vascular insulin sensitivity. Furthermore, in light of the existing literature, we formulate the hypothesis that increased shear stress may be a prime mechanism through which habitual physical activity improves insulin signalling in the vasculature. Ultimately, we propose that targeting vascular insulin resistance may represent a viable strategy for improving glycaemic control and reducing cardiovascular risk in patients with type 2 diabetes.

Skeletal Muscle Vascular Function: A Counterbalance of Insulin Action

Insulin is a vasoactive hormone that regulates vascular homeostasis by maintaining balance of endothelial-derived NO and ET-1. Although there is general agreement that insulin resistance and the associated hyperinsulinemia disturb this balance, the vascular consequences for hyperinsulinemia in isolation from insulin resistance are still unclear. Presently, there is no simple answer for this question, especially in a background of mixed reports examining the effects of experimental hyperinsulinemia on endothelial-mediated vasodilation. Understanding the mechanisms by which hyperinsulinemia induces vascular dysfunction is essential in advancing treatment and prevention of insulin resistance-related vascular complications. Thus, we review literature addressing the effects of hyperinsulinemia on vascular function. Furthermore, we give special attention to the vasoregulatory effects of hyperinsulinemia on skeletal muscle, the largest insulin-dependent organ in the body. This review also characterizes the differential vascular effects of hyperinsulinemia on large conduit vessels versus small resistance microvessels and the effects of metabolic variables in an effort to unravel potential sources of discrepancies in the literature. At the cellular level, we provide an overview of insulin signaling events governing vascular tone. Finally, we hypothesize a role for hyperinsulinemia and insulin resistance in the development of CVD.

Activation of extracellular signal-related kinase in abdominal aortic aneurysm

BACKGROUND

Extracellular matrix degeneration, caused by matrix metalloproteinase-2, facilitates smooth muscle cell migration leading to medial layer decline and, ultimately, abdominal aortic aneurysm. It remains unclear what exactly causes aneurysms to rupture, which leads to death in most patients. The extracellular signal-related kinase may be linked to the latter process. We aimed to clarify the role of extracellular signal-related kinase in aortic aneurysm development and rupture in patients.

DESIGN

Aortic fragments were harvested during open repair of nonruptured (n = 20) and ruptured (n = 8) aneurysms. As control, nondilated aortas (n = 6) were obtained during autopsy. We determined levels of phosphorylated and total extracellular signal-related kinase by Western blot, matrix metalloproteinase-2 by immunohistochemistry and medial layer thickness by conventional microscopy.

RESULTS

Nonruptured aneurysms had 1·8 times higher activation of extracellular signal-related kinase (ratio: phosphorylated/total) than controls (P = 0·011). However, the ruptured aneurysms had only 0·9 times the activation of controls (ns). Both nonruptured and ruptured aneurysms showed significantly higher matrix metalloproteinase-2 than controls (3·8 and 4·0-times, respectively; P < 0·005). Of the medial layer thickness in controls, the median was 1·5 mm, in nonruptured 1·0 mm and in ruptured aneurysms 0·7 mm. Activation of extracellular signal-related kinase correlated positively to medial layer thickness (Rs  = 0·48; P = 0·014), but not to matrix metalloproteinase-2 (Rs  = -0·36; P = 0·10).

CONCLUSIONS

In this study, nonruptured aneurysms are associated with increased extracellular signal-related kinase activation while ruptured aneurysms are not. Extracellular signal-related kinase was not related to total matrix metalloproteinase-2 expression. We therefore speculate that increased extracellular signal-related kinase, in response to medial layer decline, could be protective against aneurysm rupture.

Insights into the molecular mechanisms of diabetes-induced endothelial dysfunction: focus on oxidative stress and endothelial progenitor cells

Diabetes mellitus is a heterogeneous, multifactorial, chronic disease characterized by hyperglycemia owing to insulin insufficiency and insulin resistance (IR). Recent epidemiological studies showed that the diabetes epidemic affects 382 million people worldwide in 2013, and this figure is expected to be 600 million people by 2035. Diabetes is associated with microvascular and macrovascular complications resulting in accelerated endothelial dysfunction (ED), atherosclerosis, and cardiovascular disease (CVD). Unfortunately, the complex pathophysiology of diabetic cardiovascular damage is not fully understood. Therefore, there is a clear need to better understand the molecular pathophysiology of ED in diabetes, and consequently, better treatment options and novel efficacious therapies could be identified. In the light of recent extensive research, we re-investigate the association between diabetes-associated metabolic disturbances (IR, subclinical inflammation, dyslipidemia, hyperglycemia, dysregulated production of adipokines, defective incretin and gut hormones production/action, and oxidative stress) and ED, focusing on oxidative stress and endothelial progenitor cells (EPCs). In addition, we re-emphasize that oxidative stress is the final common pathway that transduces signals from other conditions-either directly or indirectly-leading to ED and CVD.

Metabolic, anabolic, and mitogenic insulin responses: A tissue-specific perspective for insulin receptor activators

Insulin acts as the major regulator of the fasting-to-fed metabolic transition by altering substrate metabolism, promoting energy storage, and helping activate protein synthesis. In addition to its glucoregulatory and other metabolic properties, insulin can also act as a growth factor. The metabolic and mitogenic responses to insulin are regulated by divergent post-receptor signaling mechanisms downstream from the activated insulin receptor (IR). However, the anabolic and growth-promoting properties of insulin require tissue-specific inter-relationships between the two pathways, and the nature and scope of insulin-regulated processes vary greatly across tissues. Understanding the nuances of this interplay between metabolic and growth-regulating properties of insulin would have important implications for development of novel insulin and IR modulator therapies that stimulate insulin receptor activation in both pathway- and tissue-specific manners. This review will provide a unique perspective focusing on the roles of "metabolic" and "mitogenic" actions of insulin signaling in various tissues, and how these networks should be considered when evaluating selective pharmacologic approaches to prevent or treat metabolic disease.

Globular adiponectin controls insulin-mediated vasoreactivity in muscle through AMPKα2

Decreased tissue perfusion increases the risk of developing insulin resistance and cardiovascular disease in obesity, and decreased levels of globular adiponectin (gAdn) have been proposed to contribute to this risk. We hypothesized that gAdn controls insulin's vasoactive effects through AMP-activated protein kinase (AMPK), specifically its α2 subunit, and studied the mechanisms involved. In healthy volunteers, we found that decreased plasma gAdn levels in obese subjects associate with insulin resistance and reduced capillary perfusion during hyperinsulinemia. In cultured human microvascular endothelial cells (HMEC), gAdn increased AMPK activity. In isolated muscle resistance arteries gAdn uncovered insulin-induced vasodilation by selectively inhibiting insulin-induced activation of ERK1/2, and the AMPK inhibitor compound C as well as genetic deletion of AMPKα2 blunted insulin-induced vasodilation. In HMEC deletion of AMPKα2 abolished insulin-induced Ser(1177) phosphorylation of eNOS. In mice we confirmed that AMPKα2 deficiency decreases insulin sensitivity, and this was accompanied by decreased muscle microvascular blood volume during hyperinsulinemia in vivo. This impairment was accompanied by a decrease in arterial Ser(1177) phosphorylation of eNOS, which closely related to AMPK activity. In conclusion, globular adiponectin controls muscle perfusion during hyperinsulinemia through AMPKα2, which determines the balance between NO and ET-1 activity in muscle resistance arteries. Our findings provide a novel mechanism linking reduced gAdn-AMPK signaling to insulin resistance and impaired organ perfusion.

Insulin-induced changes in skeletal muscle microvascular perfusion are dependent upon perivascular adipose tissue in women

AIMS/HYPOTHESIS

Obesity increases the risk of cardiovascular disease and type 2 diabetes, partly through reduced insulin-induced microvascular vasodilation, which causes impairment of glucose delivery and uptake. We studied whether perivascular adipose tissue (PVAT) controls insulin-induced vasodilation in human muscle, and whether altered properties of PVAT relate to reduced insulin-induced vasodilation in obesity.

METHODS

Insulin-induced microvascular recruitment was measured using contrast enhanced ultrasound (CEU), before and during a hyperinsulinaemic-euglycaemic clamp in 15 lean and 18 obese healthy women (18-55 years). Surgical skeletal muscle biopsies were taken on a separate day to study perivascular adipocyte size in histological slices, as well as to study ex vivo insulin-induced vasoreactivity in microvessels in the absence and presence of PVAT in the pressure myograph. Statistical mediation of the relation between BMI and microvascular recruitment by PVAT was studied in a mediation model.

RESULTS

Obese women showed impaired insulin-induced microvascular recruitment and lower metabolic insulin sensitivity compared with lean women. Microvascular recruitment was a mediator in the association between obesity and insulin sensitivity. Perivascular adipocyte size, determined in skeletal muscle biopsies, was larger in obese than in lean women, and statistically explained the difference in microvascular recruitment between obese and lean women. PVAT from lean women enhanced insulin-induced vasodilation in isolated skeletal muscle resistance arteries, while PVAT from obese women revealed insulin-induced vasoconstriction.

CONCLUSIONS/INTERPRETATION

PVAT from lean women enhances insulin-induced vasodilation and microvascular recruitment whereas PVAT from obese women does not. PVAT adipocyte size partly explains the difference in insulin-induced microvascular recruitment between lean and obese women.

Vascular and metabolic actions of the green tea polyphenol epigallocatechin gallate

Epidemiological studies demonstrate robust correlations between green tea consumption and reduced risk of type 2 diabetes and its cardiovascular complications. However, underlying molecular, cellular, and physiological mechanisms remain incompletely understood. Health promoting actions of green tea are often attributed to epigallocatechin gallate (EGCG), the most abundant polyphenol in green tea. Insulin resistance and endothelial dysfunction play key roles in the pathogenesis of type 2 diabetes and its cardiovascular complications. Metabolic insulin resistance results from impaired insulin-mediated glucose disposal in skeletal muscle and adipose tissue, and blunted insulin-mediated suppression of hepatic glucose output that is often associated with endothelial/vascular dysfunction. This endothelial dysfunction is itself caused, in part, by impaired insulin signaling in vascular endothelium resulting in reduced insulin-stimulated production of NO in arteries, and arterioles that regulate nutritive capillaries. In this review, we discuss the considerable body of literature supporting insulin-mimetic actions of EGCG that oppose endothelial dysfunction and ameliorate metabolic insulin resistance in skeletal muscle and liver. We conclude that EGCG is a promising therapeutic to combat cardiovascular complications associated with the metabolic diseases characterized by reciprocal relationships between insulin resistance and endothelial dysfunction that include obesity, metabolic syndrome and type 2 diabetes. There is a strong rationale for well-powered randomized placebo controlled intervention trials to be carried out in insulin resistant and diabetic populations.

Plants with potential use on obesity and its complications

Obesity is the most prevalent nutritional disease and a growing public health problem worldwide. This disease is a causal component of the metabolic syndrome related with abnormalities, including hyperglycemia, dyslipidemia, hypertension, inflammation, among others. There are anti-obesity drugs, affecting the fundamental processes of the weight regulation; however they have shown serious side effects, which outweigh their beneficial effects. Most recent studies on the treatment of obesity and its complications have focused on the potential role of different plants preparation that can exert a positive effect on the mechanisms involved in this pathology. For instance, anti-obesity effects of green tea and its isolated active principles have been reported in both in vitro (cell cultures) and in vivo (animal models) that possess healthy effects, decreasing adipose tissue through reduction of adipocytes differentiation and proliferation. A positive effect in lipid profile, and lipid and carbohydrates metabolisms were demonstrated as well. In addition, anti-inflammatory and antioxidant activities were studied. However, the consumption of green tea and its products is not that common in Western countries, where other plants with similar bioactivity predominate; nevertheless, the effect extension has not been analyzed in depth, despite of their potential as alternative treatment for obesity. In this review the anti-obesity potential and reported mechanisms of action of diverse plants such as: Camellia sinensis, Hibiscus sabdariffa, Hypericum perforatum, Persea americana, Phaseolus vulgaris, Capsicum annuum, Rosmarinus officinalis, Ilex paraguariensis, Citrus paradisi, Citrus limon, Punica granatum, Aloe vera, Taraxacum officinale and Arachis hypogaea is summarized. We consider the potential of these plants as natural alternative treatments of some metabolic alterations associated with obesity.

Glucagon-like peptide 1 recruits muscle microvasculature and improves insulin's metabolic action in the presence of insulin resistance

Glucagon-like peptide 1 (GLP-1) acutely recruits muscle microvasculature, increases muscle delivery of insulin, and enhances muscle use of glucose, independent of its effect on insulin secretion. To examine whether GLP-1 modulates muscle microvascular and metabolic insulin responses in the setting of insulin resistance, we assessed muscle microvascular blood volume (MBV), flow velocity, and blood flow in control insulin-sensitive rats and rats made insulin-resistant acutely (systemic lipid infusion) or chronically (high-fat diet [HFD]) before and after a euglycemic-hyperinsulinemic clamp (3 mU/kg/min) with or without superimposed systemic GLP-1 infusion. Insulin significantly recruited muscle microvasculature and addition of GLP-1 further expanded muscle MBV and increased insulin-mediated glucose disposal. GLP-1 infusion potently recruited muscle microvasculature in the presence of either acute or chronic insulin resistance by increasing muscle MBV. This was associated with an increased muscle delivery of insulin and muscle interstitial oxygen saturation. Muscle insulin sensitivity was completely restored in the presence of systemic lipid infusion and significantly improved in rats fed an HFD. We conclude that GLP-1 infusion potently expands muscle microvascular surface area and improves insulin's metabolic action in the insulin-resistant states. This may contribute to improved glycemic control seen in diabetic patients receiving incretin-based therapy.

Methylglyoxal impairs endothelial insulin sensitivity both in vitro and in vivo

AIMS/HYPOTHESIS

Insulin exerts a direct action on vascular cells, thereby affecting the outcome and progression of diabetic vascular complications. However, the mechanism through which insulin signalling is impaired in the endothelium of diabetic individuals remains unclear. In this work, we have evaluated the role of the AGE precursor methylglyoxal (MGO) in generating endothelial insulin resistance both in cells and in animal models.

METHODS

Time course experiments were performed on mouse aortic endothelial cells (MAECs) incubated with 500 μmol/l MGO. The glyoxalase-1 inhibitor S-p-bromobenzylglutathione-cyclopentyl-diester (SpBrBzGSHCp2) was used to increase the endogenous levels of MGO. For the in vivo study, an MGO solution was administrated i.p. to C57BL/6 mice for 7 weeks.

RESULTS

MGO prevented the insulin-dependent activation of the IRS1/protein kinase Akt/endothelial nitric oxide synthase (eNOS) pathway, thereby blunting nitric oxide (NO) production, while extracellular signal-regulated kinase (ERK1/2) activation and endothelin-1 (ET-1) release were increased by MGO in MAECs. Similar results were obtained in MAECs treated with SpBrBzGSHCp2. In MGO- and SpBrBzGSHCp2-exposed cells, inhibition of ERK1/2 decreased IRS1 phosphorylation on S616 and rescued insulin-dependent Akt activation and NO generation, indicating that MGO inhibition of the IRS1/Akt/eNOS pathway is mediated, at least in part, by ERK1/2. Chronic administration of MGO to C57BL/6 mice impaired whole-body insulin sensitivity and induced endothelial insulin resistance.

CONCLUSIONS/INTERPRETATION

MGO impairs the action of insulin on the endothelium both in vitro and in vivo, at least in part through an ERK1/2-mediated mechanism. These findings may be instrumental in developing novel strategies for preserving endothelial function in diabetes.

Programming of fetal insulin resistance in pregnancies with maternal obesity by ER stress and inflammation

The global epidemics of obesity during pregnancy and excessive gestational weight gain (GWG) are major public health problems worldwide. Obesity and excessive GWG are related to several maternal and fetal complications, including diabetes (pregestational and gestational diabetes) and intrauterine programming of insulin resistance (IR). Maternal obesity (MO) and neonatal IR are associated with long-term development of obesity, diabetes mellitus, and increased global cardiovascular risk in the offspring. Multiple mechanisms of insulin signaling pathway impairment have been described in obese individuals, involving complex interactions of chronically elevated inflammatory mediators, adipokines, and the critical role of the endoplasmic reticulum (ER) stress-dependent unfolded protein response (UPR). However, the underlying cellular processes linking MO and IR in the offspring have not been fully elucidated. Here, we summarize the state-of-the-art evidence supporting the possibility that adverse metabolic postnatal outcomes such as IR in the offspring of pregnancies with MO and/or excessive GWG may be related to intrauterine activation of ER stress response.

Increase in insulin-induced relaxation of consecutive arterial segments toward the periphery: Role of vascular oxidative state

RATIONALE

The oxidative state has been implicated in the signaling of various vasomotor functions, yet its role regarding the vasomotor action of insulin is less known.

OBJECTIVE

To investigate the insulin-evoked relaxations of consecutive arterial segments of different oxidative state and the role of extracellular signal-regulated kinase (ERK) pathway.

METHODS AND RESULTS

The oxidative state, as assessed by the level of ortho-tyrosine, was higher in the thoracic aorta of rats than in the abdominal aorta, and was the lowest in the femoral artery. The vasomotor function of vessels of same origin was studied using a small-vessel myograph. Insulin-induced relaxations increased toward the periphery (i.e., thoracic < abdominal < femoral). Aortic banding and hydrogen peroxide/aminotriazole increased the oxidative state of the thoracic aorta that was accompanied by ERK activation and decreased relaxation to insulin, and vice versa, acutely lowered oxidative state by superoxide dismutase/catalase improved relaxation. In contrast, insulin-induced relaxation of the femoral artery could be enhanced with a higher oxidative state, and reduced with a lower state.

CONCLUSIONS

Oxidative state of vessels modulates the magnitude of vasomotor responses to insulin, which appears to be mediated via the ERK signaling pathway.

Endothelial dysfunction in (pre)diabetes: characteristics, causative mechanisms and pathogenic role in type 2 diabetes

Endothelial dysfunction associated with diabetes and cardiovascular disease is characterized by changes in vasoregulation, enhanced generation of reactive oxygen intermediates, inflammatory activation, and altered barrier function. These endothelial alterations contribute to excess cardiovascular disease in diabetes, but may also play a role in the pathogenesis of diabetes, especially type 2. The mechanisms underlying endothelial dysfunction in diabetes differ between type 1 (T1D) and type 2 diabetes (T2D): hyperglycemia contributes to endothelial dysfunction in all individuals with diabetes, whereas the causative mechanisms in T2D also include impaired insulin signaling in endothelial cells, dyslipidemia and altered secretion of bioactive substances (adipokines) by adipose tissue. The close association of so-called perivascular adipose tissue with arteries and arterioles facilitates the exposure of vascular endothelium to adipokines, particularly if inflammation activates the adipose tissue. Glucose and adipokines activate specific intracellular signaling pathways in endothelium, which in concert result in endothelial dysfunction in diabetes. Here, we review the characteristics of endothelial dysfunction in diabetes, the causative mechanisms involved and the role of endothelial dysfunction(s) in the pathogenesis of T2D. Finally, we will discuss the therapeutic potential of endothelial dysfunction in T2D.

Perivascular adipose tissue from human systemic and coronary vessels: the emergence of a new pharmacotherapeutic target

Fat cells or adipocytes are distributed ubiquitously throughout the body and are often regarded purely as energy stores. However, recently it has become clear that these adipocytes are engine rooms producing large numbers of metabolically active substances with both endocrine and paracrine actions. White adipocytes surround almost every blood vessel in the human body and are collectively termed perivascular adipose tissue (PVAT). It is now well recognized that PVAT not only provides mechanical support for any blood vessels it invests, but also secretes vasoactive and metabolically essential cytokines known as adipokines, which regulate vascular function. The emergence of obesity as a major challenge to our healthcare systems has contributed to the growing interest in adipocyte dysfunction with a view to discovering new pharmacotherapeutic agents to help rescue compromised PVAT function. Very few PVAT studies have been carried out on human tissue. This review will discuss these and the hypotheses generated from such research, as well as highlight the most significant and clinically relevant animal studies showing the most pharmacological promise.

LINKED ARTICLES

This article is part of a themed section on Fat and Vascular Responsiveness. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-3.

Insulin-induced generation of reactive oxygen species and uncoupling of nitric oxide synthase underlie the cerebrovascular insulin resistance in obese rats

Hyperinsulinemia accompanying insulin resistance (IR) is an independent risk factor for stroke. The objective is to examine the cerebrovascular actions of insulin in Zucker obese (ZO) rats with IR and Zucker lean (ZL) control rats. Diameter measurements of cerebral arteries showed diminished insulin-induced vasodilation in ZO compared with ZL. Endothelial denudation revealed vasoconstriction to insulin that was greater in ZO compared with ZL. Nonspecific inhibition of nitric oxide synthase (NOS) paradoxically improved vasodilation in ZO. Scavenging of reactive oxygen species (ROS), supplementation of tetrahydrobiopterin (BH(4)) precursor, and inhibition of neuronal NOS or NADPH oxidase or cyclooxygenase (COX) improved insulin-induced vasodilation in ZO. Immunoblot experiments revealed that insulin-induced phosphorylation of Akt, endothelial NOS, and expression of GTP cyclohydrolase-I (GTP-CH) were diminished, but phosphorylation of PKC and ERK was enhanced in ZO arteries. Fluorescence studies showed increased ROS in ZO arteries in response to insulin that was sensitive to NOS inhibition and BH(4) supplementation. Thus, a vicious cycle of abnormal insulin-induced ROS generation instigating NOS uncoupling leading to further ROS production underlies the cerebrovascular IR in ZO rats. In addition, decreased bioavailability and impaired synthesis of BH(4) by GTP-CH induced by insulin promoted NOS uncoupling.

Regulation of muscle microcirculation in health and diabetes

Insulin increases microvascular perfusion and substrate exchange surface area in muscle, which is pivotal for hormone action and substrate exchange, by activating insulin signaling cascade in the endothelial cells to produce nitric oxide. This action of insulin is closely coupled with its metabolic action and type 2 diabetes is associated with both metabolic and microvascular insulin resistance. Muscle microvascular perfusion/volume can be assessed by 1-methylxanthine metabolism, contrast-enhanced ultrasound and positron emission tomography. In addition to insulin, several factors have been shown to recruit muscle microvasculature, including exercise or muscle contraction, mixed meals, glucagon-like peptide 1 and angiotensin II type 1 receptor (AT(1)R) blocker. On the other hand, factors that cause metabolic insulin resistance, such as inflammatory cytokines, free fatty acids, and selective activation of the AT(1)R, are capable of causing microvascular insulin resistance. Therapies targeting microvascular insulin resistance may help prevent or control diabetes and decrease the associated cardiovascular morbidity and mortality.

Perivascular Fat and the Microcirculation: Relevance to Insulin Resistance, Diabetes, and Cardiovascular Disease

Type 2 diabetes and its major risk factor, obesity, are a growing burden for public health. The mechanisms that connect obesity and its related disorders, such as insulin resistance, type 2 diabetes, and hypertension, are still undefined. Microvascular dysfunction may be a pathophysiologic link between insulin resistance and hypertension in obesity. Many studies have shown that adipose tissue-derived substances (adipokines) interact with (micro)vascular function and influence insulin sensitivity. In the past, research focused on adipokines from perivascular adipose tissue (PVAT). In this review, we focus on the interactions between adipokines, predominantly from PVAT, and microvascular function in relation to the development of insulin resistance, diabetes, and cardiovascular disease.

APPL1 counteracts obesity-induced vascular insulin resistance and endothelial dysfunction by modulating the endothelial production of nitric oxide and endothelin-1 in mice

OBJECTIVE

Insulin stimulates both nitric oxide (NO)-dependent vasodilation and endothelin-1 (ET-1)-dependent vasoconstriction. However, the cellular mechanisms that control the dual vascular effects of insulin remain unclear. This study aimed to investigate the roles of the multidomain adaptor protein APPL1 in modulating vascular actions of insulin in mice and in endothelial cells.

RESEARCH DESIGN AND METHODS

Both APPL1 knockout mice and APPL1 transgenic mice were generated to evaluate APPL1's physiological roles in regulating vascular reactivity and insulin signaling in endothelial cells.

RESULTS

Insulin potently induced NO-dependent relaxations in mesenteric arteries of 8-week-old mice, whereas this effect of insulin was progressively impaired with ageing or upon development of obesity induced by high-fat diet. Transgenic expression of APPL1 prevented age- and obesity-induced impairment in insulin-induced vasodilation and reversed obesity-induced augmentation in insulin-evoked ET-1-dependent vasoconstriction. By contrast, genetic disruption of APPL1 shifted the effects of insulin from vasodilation to vasoconstriction. At the molecular level, insulin-elicited activation of protein kinase B (Akt) and endothelial NO synthase and production of NO were enhanced in APPL1 transgenic mice but were abrogated in APPL1 knockout mice. Conversely, insulin-induced extracellular signal-related kinase (ERK)1/2 phosphorylation and ET-1 expression was augmented in APPL1 knockout mice but was diminished in APPL1 transgenic mice. In endothelial cells, APPL1 potentiated insulin-stimulated Akt activation by competing with the Akt inhibitor Tribbles 3 (TRB3) and suppressed ERK1/2 signaling by altering the phosphorylation status of its upstream kinase Raf-1.

CONCLUSIONS

APPL1 plays a key role in coordinating the vasodilator and vasoconstrictor effects of insulin by modulating Akt-dependent NO production and ERK1/2-mediated ET-1 secretion in the endothelium.

Obesity, blood vessels and metabolic syndrome

Obesity is rising worldwide at an alarming rate and so is the incidence of obesity-related disorders, such as the metabolic syndrome, type 2 diabetes and cardiovascular diseases. The obesity-dependent vascular damage appears to be derived from a variety of changes in the adipose tissue, leading to a chronic inflammatory state and dysregulation of adipocyte-derived factors. This, in turn, impairs vascular homeostasis by determining an unbalance between the protective effect of the nitric oxide pathway and the unfavourable action of the endothelin-1 system. In addition, hyperinsulinemia and insulin resistance contribute to vascular dysfunction because the opposing endothelium-dependent vasodilating and vasoconstrictor effects of insulin are shifted towards a predominant vasoconstriction in patients with obesity. Importantly, emerging evidence suggests that the vascular dysfunction of obesity is not only limited to the endothelium but also involves the other layers of the vessel wall. In particular, obesity-related changes in vascular smooth muscle seem to disrupt the physiological facilitatory action of insulin on the responsiveness to vasodilator stimuli, whereas the adventitia and the perivascular fat appear to be a source of proinflammatory and vasoactive factors that may contribute to endothelial and smooth muscle cell dysfunction and to the pathogenesis of vascular disease.

Dysfunction of human subcutaneous fat arterioles in obesity alone or obesity associated with Type 2 diabetes

The aim of the present study was to examine the effects of obesity alone and obesity associated with Type 2 diabetes on the structure, vascular reactivity and response to insulin of isolated human subcutaneous fat arterioles; these effects were correlated with the expression of insulin signalling proteins. Periumbilical subcutaneous adipose tissue was explanted during surgery, small arterioles (internal diameter 220 ± 40 μm) were dissected out and investigated by electron microscopy, myography and immunoblotting. Compared with the subcutaneous arterioles of lean subjects, obesity activated the endothelium, enhanced the accumulation of collagen within vascular wall and increased the sensitivity of adrenergic response; obesity also diminished eNOS (endothelial NO synthase) protein expression, NO production, and endothelium-dependent and insulin-induced vasodilatation, as well as the protein expression of both IRS (insulin receptor substrates)-1 and IRS-2 and of the downstream molecules in the insulin signalling pathway, such as PI3K (phosphoinositide 3-kinase), phospho-Akt and Akt. When obesity was associated with Type 2 diabetes, these changes were significantly augmented. In conclusion, obesity alone or obesity associated with Type 2 diabetes alters human periumbilical adipose tissue arterioles in terms of structure, function and biochemsitry, including diminished eNOS expression and reduced levels of IRS-1, IRS-2, PI3K and Akt in the insulin signalling pathway.

An early response transcription factor, Egr-1, enhances insulin resistance in type 2 diabetes with chronic hyperinsulinism

One of the most important characteristics of type 2 diabetes is insulin resistance, during which the patients normally experienced hyperinsulinism stress that would alter insulin signal transduction in insulin target tissues. We have previously found that early growth responsive gene-1 (Egr-1), a zinc finger transcription factor, is highly expressed in db/db mice and in the fat tissue of individuals with type 2 diabetes. In this report, we found that chronic exposure to hyperinsulinism caused persistent Erk/MAPK activity in adipocytes and enhanced insulin resistance in an Egr-1-dependent manner. An elevation in Egr-1 augmented Erk1/2 activation via geranylgeranyl diphosphate synthase (GGPPS). Egr-1-promoted GGPPS transcription increased Ras prenylation and caused Erk1/2 activation. The sustained activation of Erk1/2 resulted in the phosphorylation of insulin receptor substrate-1 at Serine 612. Phosphorylation at this site impaired insulin signaling in adipocytes and reduced glucose uptake. The loss of Egr-1 function, knockdown of GGPPS, or inhibition of Erk1/2 activity in insulin-resistant adipocytes restored insulin receptor substrate-1 tyrosine phosphorylation and increased insulin sensitivity. Our results suggest a new mechanism by which the Egr-1/GGPPS/Erk1/2 pathway is responsible for insulin resistance during hyperinsulinism. This pathway provides a new therapeutic target for increasing insulin sensitivity: inhibiting the function of Egr-1.

The role of blood vessels, endothelial cells, and vascular pericytes in insulin secretion and peripheral insulin action

The pathogenesis of type 2 diabetes is intimately intertwined with the vasculature. Insulin must efficiently enter the bloodstream from pancreatic beta-cells, circulate throughout the body, and efficiently exit the bloodstream to reach target tissues and mediate its effects. Defects in the vasculature of pancreatic islets can lead to diabetic phenotypes. Similarly, insulin resistance is accompanied by defects in the vasculature of skeletal muscle, which ultimately reduce the ability of insulin and nutrients to reach myocytes. An underappreciated participant in these processes is the vascular pericyte. Pericytes, the smooth muscle-like cells lining the outsides of blood vessels throughout the body, have not been directly implicated in insulin secretion or peripheral insulin delivery. Here, we review the role of the vasculature in insulin secretion, islet function, and peripheral insulin delivery, and highlight a potential role for the vascular pericyte in these processes.