The role of endothelial insulin signaling in the regulation of glucose metabolism

Oxidative Stress, Endothelial Dysfunction, and N-Acetylcysteine in Type 2 Diabetes Mellitus

Significance: Cardiovascular diseases (CVDs) remain the leading cause of morbidity and mortality globally. Endothelial dysfunction is closely associated with the development and progression of CVDs. Patients with diabetes mellitus (DM) especially type 2 DM (T2DM) exhibit a significant endothelial cell (EC) dysfunction with substantially increased risk for CVDs. Recent Advances: Excessive reactive oxygen species (ROS) and oxidative stress are important contributing factors to EC dysfunction and subsequent CVDs. ROS production is significantly increased in DM and is critically involved in the development of endothelial dysfunction in diabetic patients. In this review, efforts are made to discuss the role of excessive ROS and oxidative stress in the pathogenesis of endothelial dysfunction and the mechanisms for excessive ROS production and oxidative stress in T2DM. Critical Issues: Although studies with diabetic animal models have shown that targeting ROS with traditional antioxidant vitamins C and E or other antioxidant supplements provides promising beneficial effects on endothelial function, the cardiovascular outcomes of clinical studies with these antioxidant supplements have been inconsistent in diabetic patients. Future Directions: Preclinical and limited clinical data suggest that N-acetylcysteine (NAC) treatment may improve endothelial function in diabetic patients. However, well-designed clinical studies are needed to determine if NAC supplementation would effectively preserve endothelial function and improve the clinical outcomes of diabetic patients with reduced cardiovascular morbidity and mortality. With better understanding on the mechanisms of ROS generation and ROS-mediated endothelial damages/dysfunction, it is anticipated that new selective ROS-modulating agents and effective personalized strategies will be developed for the management of endothelial dysfunction in DM.

Exercise mitigates flow recirculation and activates metabolic transducer SCD1 to catalyze vascular protective metabolites

Exercise promotes pulsatile shear stress in the arterial circulation and ameliorates cardiometabolic diseases. However, exercise-mediated metabolic transducers for vascular protection remain under-investigated. Untargeted metabolomic analysis demonstrated that wild-type mice undergoing voluntary wheel running exercise expressed increased endothelial stearoyl-CoA desaturase 1 (SCD1) that catalyzes anti-inflammatory lipid metabolites, namely, oleic (OA) and palmitoleic acids (PA), to mitigate NF-κB-mediated inflammatory responses. In silico analysis revealed that exercise augmented time-averaged wall shear stress but mitigated flow recirculation and oscillatory shear index in the lesser curvature of the mouse aortic arch. Following exercise, endothelial Scd1-deleted mice (Ldlr-/- Scd1EC-/-) on high-fat diet developed persistent VCAM1-positive endothelium in the lesser curvature and the descending aorta, whereas SCD1 overexpression via adenovirus transfection mitigated endoplasmic reticulum stress and inflammatory biomarkers. Single-cell transcriptomics of the aorta identified Scd1-positive and Vcam1-negative endothelial subclusters interacting with other candidate genes. Thus, exercise mitigates flow recirculation and activates endothelial SCD1 to catalyze OA and PA for vascular endothelial protection.

The Role of Perivascular Adipose Tissue in the Pathogenesis of Endothelial Dysfunction in Cardiovascular Diseases and Type 2 Diabetes Mellitus

Cardiovascular diseases (CVDs) and type 2 diabetes mellitus (T2DM) are two of the four major chronic non-communicable diseases (NCDs) representing the leading cause of death worldwide. Several studies demonstrate that endothelial dysfunction (ED) plays a central role in the pathogenesis of these chronic diseases. Although it is well known that systemic chronic inflammation and oxidative stress are primarily involved in the development of ED, recent studies have shown that perivascular adipose tissue (PVAT) is implicated in its pathogenesis, also contributing to the progression of atherosclerosis and to insulin resistance (IR). In this review, we describe the relationship between PVAT and ED, and we also analyse the role of PVAT in the pathogenesis of CVDs and T2DM, further assessing its potential therapeutic target with the aim of restoring normal ED and reducing global cardiovascular risk.

Exercise Mitigates Flow Recirculation and Activates Mechanosensitive Transcriptome to Uncover Endothelial SCD1-Catalyzed Anti-Inflammatory Metabolites

Exercise modulates vascular plasticity in multiple organ systems; however, the metabolomic transducers underlying exercise and vascular protection in the disturbed flow-prone vasculature remain under-investigated. We simulated exercise-augmented pulsatile shear stress (PSS) to mitigate flow recirculation in the lesser curvature of the aortic arch. When human aortic endothelial cells (HAECs) were subjected to PSS ( τ _ave = 50 dyne·cm ^-2 , ∂τ/∂t = 71 dyne·cm ^-2 ·s ^-1 , 1 Hz), untargeted metabolomic analysis revealed that Stearoyl-CoA Desaturase (SCD1) in the endoplasmic reticulum (ER) catalyzed the fatty acid metabolite, oleic acid (OA), to mitigate inflammatory mediators. Following 24 hours of exercise, wild-type C57BL/6J mice developed elevated SCD1-catalyzed lipid metabolites in the plasma, including OA and palmitoleic acid (PA). Exercise over a 2-week period increased endothelial SCD1 in the ER. Exercise further modulated the time-averaged wall shear stress (TAWSS or τ _ave) and oscillatory shear index (OSI _ave ), upregulated Scd1 and attenuated VCAM1 expression in the disturbed flow-prone aortic arch in Ldlr ^-/- mice on high-fat diet but not in Ldlr ^-/- Scd1 ^EC-/- mice. Scd1 overexpression via recombinant adenovirus also mitigated ER stress. Single cell transcriptomic analysis of the mouse aorta revealed interconnection of Scd1 with mechanosensitive genes, namely Irs2 , Acox1 and Adipor2 that modulate lipid metabolism pathways. Taken together, exercise modulates PSS ( τ _ave and OSI _ave ) to activate SCD1 as a metabolomic transducer to ameliorate inflammation in the disturbed flow-prone vasculature.

Novel pathophysiological roles of α-synuclein in age-related vascular endothelial dysfunction

Although α-synuclein (SNCA) is a well-known pathological molecule involved in synucleinopathy in neurons, its physiological roles remain largely unknown. We reported that serum SNCA levels have a close inverse correlation with blood pressure and age, which indicates the involvement of SNCA in age-related endothelial dysfunction. Therefore, this study aimed to elucidate the molecular functions of SNCA in the endothelium. We confirmed that SNCA was expressed in and secreted from endothelial cells (ECs). Exogenous treatment with recombinant SNCA (rSNCA) activated the Akt-eNOS axis and increased nitric oxide production in ECs. Treatment with rSNCA also suppressed TNF-α- and palmitic acid-induced NF-κB activation, leading to the suppression of VCAM-1 upregulation and restoration of eNOS downregulation in ECs. As for endogenous SNCA expression, replicative senescence resulted in the attenuation of SNCA expression in cultured ECs, similar to the effects of physiological aging on mice aortas. The siRNA-mediated silencing of SNCA consistently resulted in senescent phenotypes, such as eNOS downregulation, increased β-gal activity, decreased Sirt1 expression, and increased p53 expression, in ECs. Ex vivo assessment of endothelial functions using aortic rings revealed impaired endothelium-dependent acetylcholine-induced relaxation in SNCA knockout (KO) mice. Furthermore, SNCA KO mice, especially those on a high-fat diet, displayed elevated blood pressure compared with wild-type mice; this could be eNOS dysfunction-dependent because of the lower difference caused by L-NAME administration. These results indicate that exogenous and endogenous SNCA in ECs might physiologically maintain vascular integrity, and age-related endothelial dysfunction might be partially ascribed to loss-of-function of SNCA in ECs.

Therapeutic Applications of Nanomedicine in Metabolic Diseases by Targeting the Endothelium

The endothelial cells not only constitute the barrier between the blood and interstitial space, but also actively regulate vascular tone, blood flow, and the function of adjacent parenchymal cells. The close anatomical relationship between endothelial cells and highly vascularized metabolic organs suggests that the crosstalk between these units is vital for systemic metabolic homeostasis. Here, we review recent studies about the pivotal role of endothelial cells in metabolic diseases. Specifically, we discuss how the dysfunction of endothelial cells directly contributes to the development of insulin resistance (IR), type 2 diabetes mellitus (T2DM), atherosclerosis, and non-alcoholic fatty liver disease (NAFLD) via communication with parenchymal cells. Furthermore, although many biological macromolecules have been shown to ameliorate the progression of metabolic diseases by improving endothelial function, the low solubility, poor bioavailability, or lack of specificity of these molecules limit their clinical application. Given the advantages in drug delivery of nanomedicine, we focus on summarizing the reports that improving endothelial dysfunction through nanomedicine-based therapies provides an opportunity for preventing metabolic diseases.

Adipokine Levels in Men with Coronary Atherosclerosis on the Background of Abdominal Obesity

Background. Obesity is associated with dyslipidemia, and excess body fat is associated with unfavorable levels of adipokines and markers of inflammation. The goal of research. To study the level of adipokines and markers of inflammation, their associations with unstable atherosclerotic plaques in men with coronary atherosclerosis on the background of abdominal obesity. Materials and methods. The study involved 82 men aged 40–77 years with coronary atherosclerosis after endarterectomy from the coronary arteries. We divided all men into two groups: 37 men (45.1%) with unstable atherosclerotic plaques, and 45 men (54.9%) who had stable plaques. Obesity was established at a BMI of ≥30 kg/m2. The levels of adipokines and markers of inflammation in the blood were determined by multiplex analysis. Results. In patients with obesity and unstable plaques, the levels of C-peptide, TNFa and IL-6 were 1.8, 1.6, and 2.8 times higher, respectively, than in patients with obesity and stable plaques. The chance of having an unstable plaque increases with an increase in TNFa by 49% in obese patients and decreases with an increase in insulin by 3% in non-obese patients. Conclusions. In men with coronary atherosclerosis and obesity, unstable atherosclerotic plaques in the coronary arteries are directly associated with the level of TNF-α.

Pathophysiological Association between Diabetes Mellitus and Endothelial Dysfunction

Endothelial dysfunction plays a critical role in atherosclerosis progression, leading to cardiovascular complications. There are significant associations between diabetes mellitus, oxidative stress, and endothelial dysfunction. Oxidative stress is increased by chronic hyperglycemia and acute glucose fluctuations induced by postprandial hyperglycemia in patients with diabetes mellitus. In addition, selective insulin resistance in the phosphoinositide 3-kinase/Akt/endothelial nitric oxide (NO) synthase pathway in endothelial cells is involved in decreased NO production and increased endothelin-1 production from the endothelium, resulting in endothelial dysfunction. In a clinical setting, selecting an appropriate therapeutic intervention that improves or augments endothelial function is important for preventing diabetic vascular complications. Hypoglycemic drugs that reduce glucose fluctuations by decreasing the postprandial rise in blood glucose levels, such as glinides, α-glucosidase inhibitors and dipeptidyl peptidase 4 inhibitors, and hypoglycemic drugs that ameliorate insulin sensitivity, such as thiazolidinediones and metformin, are expected to improve or augment endothelial function in patients with diabetes. Glucagon-like peptide 1 receptor agonists, metformin, and sodium-glucose cotransporter 2 inhibitors may improve endothelial function through multiple mechanisms, some of which are independent of glucose control or insulin signaling. Oral administration of antioxidants is not recommended in patients with diabetes due to the lack of evidence for the efficacy against diabetic complications.

Endothelial Cell Glucose Metabolism and Angiogenesis

Angiogenesis, a process of new blood vessel formation from the pre-existing vascular bed, is a critical event in various physiological and pathological settings. Over the last few years, the role of endothelial cell (EC) metabolism in angiogenesis has received considerable attention. Accumulating studies suggest that ECs rely on aerobic glycolysis, rather than the oxidative phosphorylation pathway, to produce ATP during angiogenesis. To date, numerous critical regulators of glucose metabolism, fatty acid oxidation, and glutamine metabolism have been identified to modulate the EC angiogenic switch and pathological angiogenesis. The unique glycolytic feature of ECs is critical for cell proliferation, migration, and responses to environmental changes. In this review, we provide an overview of recent EC glucose metabolism studies, particularly glycolysis, in quiescent and angiogenic ECs. We also summarize and discuss potential therapeutic strategies that take advantage of EC metabolism. The elucidation of metabolic regulation and the precise underlying mechanisms could facilitate drug development targeting EC metabolism to treat angiogenesis-related diseases.

Endothelial TFEB (Transcription Factor EB) Improves Glucose Tolerance via Upregulation of IRS (Insulin Receptor Substrate) 1 and IRS2

OBJECTIVE

Vascular endothelial cells (ECs) play a critical role in maintaining vascular homeostasis. Aberrant EC metabolism leads to vascular dysfunction and metabolic diseases. TFEB (transcription factor EB), a master regulator of lysosome biogenesis and autophagy, has protective effects on vascular inflammation and atherosclerosis. However, the role of endothelial TFEB in metabolism remains to be explored. In this study, we sought to investigate the role of endothelial TFEB in glucose metabolism and underlying molecular mechanisms. Approach and Results: To determine whether endothelial TFEB is critical for glucose metabolism in vivo, we utilized EC-selective TFEB knockout and EC-selective TFEB transgenic mice fed a high-fat diet. EC-selective TFEB knockout mice exhibited significantly impaired glucose tolerance compared with control mice. Consistently, EC-selective TFEB transgenic mice showed improved glucose tolerance. In primary human ECs, small interfering RNA-mediated TFEB knockdown blunts Akt (AKT serine/threonine kinase) signaling. Adenovirus-mediated overexpression of TFEB consistently activates Akt and significantly increases glucose uptake in ECs. Mechanistically, TFEB upregulates IRS1 and IRS2 (insulin receptor substrate 1 and 2). TFEB increases IRS2 transcription measured by reporter gene and chromatin immunoprecipitation assays. Furthermore, we found that TFEB increases IRS1 protein via downregulation of microRNAs (miR-335, miR-495, and miR-548o). In vivo, Akt signaling in the skeletal muscle and adipose tissue was significantly impaired in EC-selective TFEB knockout mice and consistently improved in EC-selective TFEB transgenic mice on high-fat diet.

CONCLUSIONS

Our data revealed a critical role of TFEB in endothelial metabolism and suggest that TFEB constitutes a potential molecular target for the treatment of vascular and metabolic diseases.

Aging differentially impacts vasodilation and angiogenesis in arteries from the white and brown adipose tissues

Aging adipose tissues (ATs) manifest reduced vascularity and increased hypoxia and inflammation that contribute to local and systemic metabolic dysfunction. However, the mechanisms that underlie these age-related changes are incompletely understood. In this study, we sought to examine insulin-stimulated vasodilation and angiogenesis in the arterial vasculature from three major AT depots, perigonadal white (pgWAT), subcutaneous white (scWAT) and brown (BAT) from young and old mice. Here, we demonstrate that in young mice, insulin-stimulated vasodilation is lower in feed arteries from pgWAT compared to scWAT (p < 0.05), but no differences were found between feed arteries in other AT depots (p > 0.05). Insulin-stimulated vasodilation was lower in old compared to young feed arteries from all three AT depots (p < 0.05 for all). In the presence of endothelial nitric oxide synthase inhibitor, L-NAME, insulin-stimulated vasodilation was decreased in young (p < 0.05), but was unaffected in old (p > 0.05) from all AT depots. We also observed no age-related differences in endothelium-independent dilation, as assessed by sodium nitroprusside (p > 0.05). We next investigated angiogenic capacity of the vasculature in these AT depots. In young mice, BAT vasculature demonstrated the highest angiogenic potential, followed by pgWAT and scWAT. We found that aging decreased angiogenic sprout formation in pgWAT and BAT (both p < 0.05), but increased angiogenic potential in scWAT (p < 0.05), indicating dissimilar impact of aging on angiogenesis in different AT depots. Collectively, these data suggest that aging leads to a consistent impairment in insulin-stimulated vasodilation and reduction in NO bioavailability in all three AT, although aging differentially impacts angiogenic capacity across different AT depots.

Emerging Roles of Vascular Endothelium in Metabolic Homeostasis

Our understanding of the role of the vascular endothelium has evolved over the past 2 decades, with the recognition that it is a dynamically regulated organ and that it plays a nodal role in a variety of physiological and pathological processes. Endothelial cells (ECs) are not only a barrier between the circulation and peripheral tissues, but also actively regulate vascular tone, blood flow, and platelet function. Dysregulation of ECs contributes to pathological conditions such as vascular inflammation, atherosclerosis, hypertension, cardiomyopathy, retinopathy, neuropathy, and cancer. The close anatomic relationship between vascular endothelium and highly vascularized metabolic organs/tissues suggests that the crosstalk between ECs and these organs is vital for both vascular and metabolic homeostasis. Numerous reports support that hyperlipidemia, hyperglycemia, and other metabolic stresses result in endothelial dysfunction and vascular complications. However, how ECs may regulate metabolic homeostasis remains poorly understood. Emerging data suggest that the vascular endothelium plays an unexpected role in the regulation of metabolic homeostasis and that endothelial dysregulation directly contributes to the development of metabolic disorders. Here, we review recent studies about the pivotal role of ECs in glucose and lipid homeostasis. In particular, we introduce the concept that the endothelium adjusts its barrier function to control the transendothelial transport of fatty acids, lipoproteins, LPLs (lipoprotein lipases), glucose, and insulin. In addition, we summarize reports that ECs communicate with metabolic cells through EC-secreted factors and we discuss how endothelial dysregulation contributes directly to the development of obesity, insulin resistance, dyslipidemia, diabetes mellitus, cognitive defects, and fatty liver disease.

CD226 reduces endothelial cell glucose uptake under hyperglycemic conditions with inflammation in type 2 diabetes mellitus

CD226 is a co-stimulatory adhesion molecule found on immune and endothelial cells. Here, we evaluated a possible role for CD226 in inhibiting glucose uptake in isolated human umbilical vein endothelial cells (HUVECs) and in wild-type (WT) and CD226 knockout (KO) mice with high-fat diet (HFD)-induced type 2 diabetes (T2DM). CD226 expression increased under hyperglycemic conditions in the presence of TNF-α. Furthermore, CD226 knockdown improved glucose uptake in endothelial cells, and CD226 KO mice exhibited increased glucose tolerance. Levels of soluble CD226 in plasma were higher in T2DM patients following an oral glucose tolerance test (OGTT) than under fasting conditions. Our results indicate that low-grade inflammation coupled with elevated blood glucose increases CD226 expression, resulting in decreased endothelial cell glucose uptake in T2DM.

Map4k4 impairs energy metabolism in endothelial cells and promotes insulin resistance in obesity

The blood vasculature responds to insulin, influencing hemodynamic changes in the periphery, which promotes tissue nutrient and oxygen delivery and thus metabolic function. The lymphatic vasculature regulates fluid and lipid homeostasis, and impaired lymphatic function can contribute to atherosclerosis and obesity. Recent studies have suggested a role for endothelial cell (EC) mitogen-activated protein kinase kinase kinase kinase 4 (Map4k4) in developmental angiogenesis and lymphangiogenesis as well as atherosclerosis. Here, we show that inducible EC Map4k4 deletion in adult mice ameliorates metabolic dysfunction in obesity despite the development of chylous ascites and a concomitant striking increase in adipose tissue lymphocyte content. Despite these defects, animals lacking endothelial Map4k4 were protected from skeletal muscle microvascular rarefaction in obesity, and primary ECs lacking Map4k4 displayed reduced senescence and increased metabolic capacity. Thus endothelial Map4k4 has complex and opposing functions in the blood and lymphatic endothelium postdevelopment. Whereas blood endothelial Map4k4 promotes vascular dysfunction and impairs glucose homeostasis in adult animals, lymphatic endothelial Map4k4 is required to maintain lymphatic vascular integrity and regulate immune cell trafficking in obesity.

Vascular Endothelial Regulation of Obesity-Associated Insulin Resistance

Obesity is a worldwide epidemic that predisposes individuals to metabolic complications, such as type 2 diabetes mellitus and non-alcoholic fatty liver disease, all of which are related to an imbalance between food intake and energy expenditure. Identification of the pathogenic molecular mechanisms and effective therapeutic approaches are urgently needed. A well-accepted paradigm is that crosstalk between organs/tissues contributes to diseases. Endothelial dysfunction characterizes metabolic disorders and the related vascular complications. Over the past two decades, overwhelming studies have focused on mechanisms that lead to endothelial dysfunction. New investigations, however, have begun to appreciate the opposite direction of the crosstalk: endothelial regulation of metabolism, although the underlying mechanisms remain to be elucidated. This review summarizes the evidence that supports the concept of endothelial regulation of obesity and the associated insulin resistance in fat, liver, and skeletal muscles, the classic targets of insulin. Outstanding questions and future research directions are highlighted. Identification of the mechanisms of vascular endothelial regulation of metabolism may offer strategies for prevention and treatment of obesity and the related metabolic complications.

Pulmonary vascular effect of insulin in a rodent model of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is associated with metabolic derangements including insulin resistance, although their effects on the cardiopulmonary disease are unclear. We hypothesized that insulin resistance promotes pulmonary hypertension (PH) development and mutations in type 2 bone morphogenetic protein receptor (BMPR2) cause cellular insulin resistance. Using a BMPR2 transgenic murine model of PAH and two models of inducible diabetes mellitus, we explored the impact of hyperglycemia and/or hyperinsulinemia on development and severity of PH. We assessed insulin signaling and insulin-mediated glucose uptake in human endothelial cells with and without mutations in BMPR2. PH developed in control mice fed a Western diet and PH in BMPR2 mutant mice was increased by Western diet. Pulmonary artery pressure correlated strongly with fasting plasma insulin but not glucose. Reactive oxygen species were increased in lungs of insulin-resistant animals. BMPR2 mutation impaired insulin-mediated endothelial glucose uptake via reduced glucose transporter translocation despite intact insulin signaling. Experimental hyperinsulinemia is strongly associated with PH in both control and BMPR2-mutant mice, though to a greater degree in those with BMPR2 mutation. Human pulmonary endothelial cells with BMPR2 mutation have evidence of reduced glucose uptake due to impaired glucose transporter translocation. These experiments support a role for hyperinsulinemia in pulmonary vascular disease.

Deletion of TXNIP Mitigates High-Fat Diet-Impaired Angiogenesis and Prevents Inflammation in a Mouse Model of Critical Limb Ischemia

Background: Previous work demonstrated that high-fat diet (HFD) triggered thioredoxin-interacting protein (TXNIP) and that silencing TXNIP prevents diabetes-impaired vascular recovery. Here, we examine the impact of genetic deletion of TXNIP on HFD-impaired vascular recovery using hind limb ischemia model. Methods: Wild type mice (WT, C57Bl/6) and TXNIP knockout mice (TKO) were fed either normal chow diet (WT-ND and TKO-ND) or 60% high-fat diet (WT-HFD and TKO-HFD). After four weeks of HFD, unilateral hind limb ischemia was performed and blood flow was measured using Laser doppler scanner at baseline and then weekly for an additional three weeks. Vascular density, nitrative stress, infiltration of CD68+ macrophages, and expression of inflammasome, vascular endothelial growth factor (VEGF), VEGF receptor-2 were examined by slot blot, Western blot and immunohistochemistry. Results: By week 8, HFD caused similar increases in weight, cholesterol and triglycerides in both WT and TKO. At week 4 and week 8, HFD significantly impaired glucose tolerance in WT and to a lesser extent in TKO. HFD significantly impaired blood flow and vascular density (CD31 labeled) in skeletal muscle of WT mice compared to ND but not in TKO. HFD and ischemia significantly induced tyrosine nitration, and systemic IL-1β and infiltration of CD68+ cells in skeletal muscle from WT but not from TKO. HFD significantly increased cleaved-caspase-1 and IL-1 β compared to ND. Under both ND, ischemia tended to increase VEGF expression and increased VEGFR2 activation in WT only but not TKO. Conclusion: Similar to prior observation in diabetes, HFD-induced obesity can compromise vascular recovery in response to ischemic insult. The mechanism involves increased TXNIP-NLRP3 (nucleotide-binding oligomerization domain-like receptor protein 3) inflammasome activation, nitrative stress and impaired VEGFR2 activation. Deletion of TXNIP restored blood flow, reduced nitrative stress and blunted inflammasome-mediated inflammation; however, it did not impact VEGF/VEGFR2 in HFD. Targeting TXNIP-NLRP3 inflammasome can provide potential therapeutic target in obesity-induced vascular complication.

Microfluidic on-chip biomimicry for 3D cell culture: a fit-for-purpose investigation from the end user standpoint

A plethora of 3D and microfluidics-based culture models have been demonstrated in the recent years with the ultimate aim to facilitate predictive in vitro models for pharmaceutical development. This article summarizes to date the progress in the microfluidics-based tissue culture models, including organ-on-a-chip and vasculature-on-a-chip. Specific focus is placed on addressing the question of what kinds of 3D culture and system complexities are deemed desirable by the biological and biomedical community. This question is addressed through analysis of a research survey to evaluate the potential use of microfluidic cell culture models among the end users. Our results showed a willingness to adopt 3D culture technology among biomedical researchers, although a significant gap still exists between the desired systems and existing 3D culture options. With these results, key challenges and future directions are highlighted.

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.

Role of Insulin-Stimulated Adipose Tissue Perfusion in the Development of Whole-Body Insulin Resistance

After food ingestion, macronutrients are transported to and stored in the skeletal muscle and adipose tissue. They can be subsequently used as an energy source in times of energy deprivation. Uptake of these nutrients in myocytes and adipocytes depends largely on adequate tissue perfusion. Interestingly, insulin is able to dilate skeletal muscle arterioles, which facilitates the delivery of macronutrients and insulin itself to muscle tissue. Insulin-stimulated skeletal muscle perfusion is impaired in several insulin-resistant states and is believed to contribute to impaired skeletal muscle glucose uptake and consequently impaired whole-body glucose disposal. Insulin-resistant individuals also exhibit blunted postprandial adipose tissue perfusion. However, the relevance of this impairment to metabolic dysregulation is less clear. In this review, we provide an overview of adipose tissue perfusion in healthy and insulin-resistant individuals, its regulation among others by insulin, and the possible influences of impaired adipose tissue perfusion on whole-body insulin sensitivity. Finally, we propose a novel hypothesis that acute overfeeding impacts distribution of macronutrients by reducing skeletal muscle perfusion, while adipose tissue perfusion remains intact.

VISUAL OVERVIEW

An online visual overview is available for this article.

Insulin access to skeletal muscle is impaired during the early stages of diet-induced obesity

OBJECTIVE

Insulin must move from the blood to the interstitium to initiate signaling, yet access to the interstitium may be impaired in cases of insulin resistance, such as obesity. This study investigated whether consuming a short- and long-term high-fat diet (HFD) impairs insulin access to skeletal muscle, the major site of insulin-mediated glucose uptake.

METHODS

Male mongrel dogs were divided into three groups consisting of control diet (n = 16), short-term (n = 8), and long-term HFD (n = 8). Insulin sensitivity was measured with intravenous glucose tolerance tests. A hyperinsulinemic euglycemic clamp was performed in each animal at the conclusion of the study. During the clamp, lymph fluid was measured as a representation of the interstitial space to assess insulin access to muscle.

RESULTS

Short- and long-term HFD induced obesity and reduced insulin sensitivity. Lymph insulin concentrations were approximately 50% of plasma insulin concentrations under control conditions. Long-term HFD caused fasting plasma hyperinsulinemia; however, interstitial insulin concentrations were not increased, suggesting impaired insulin access to muscle.

CONCLUSIONS

A HFD rapidly induces insulin resistance at the muscle and impairs insulin access under basal insulin concentrations. Hyperinsulinemia induced by a long-term HFD may be a compensatory mechanism necessary to maintain healthy insulin levels in muscle interstitium.

Map4k4 Signaling Nodes in Metabolic and Cardiovascular Diseases

Mitogen-activated kinase kinase kinase kinase 4 (Map4k4), originally identified in small interfering (si)RNA screens and characterized by tissue-specific gene deletions, is emerging as a regulator of glucose homeostasis and cardiovascular health. Recent studies have shown that Map4k4 gene ablation or inhibition of its kinase activity attenuates hyperglycemia and plaque formation in mouse models of insulin resistance and atherosclerosis, and suggest roles for Map4k4 in multiple signaling systems, including NFκB activation, small GTPase regulation, the Hippo cascade, and regulation of cell dynamics by FERM domain proteins. This new and promising area of inquiry raises key questions that need to be addressed, such as defining which of the above or other effectors mediate Map4k4 control of metabolic and vascular functions, and identifying upstream activators of Map4k4.

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.

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.

An integrated genomic and proteomic approach to identify signatures of endosulfan exposure in hepatocellular carcinoma cells

Present study reports the identification of genomic and proteomic signatures of endosulfan exposure in hepatocellular carcinoma cells (HepG2). HepG2 cells were exposed to sublethal concentration (15μM) of endosulfan for 24h. DNA microarray and MALDI-TOF-MS analyses revealed that endosulfan induced significant alterations in the expression level of genes and proteins involved in multiple cellular pathways (apoptosis, transcription, immune/inflammatory response, carbohydrate metabolism, etc.). Furthermore, downregulation of PHLDA gene, upregulation of ACIN1 protein and caspase-3 activation in exposed cells indicated that endosulfan can trigger apoptotic cascade in hepatocellular carcinoma cells. In total 135 transcripts and 19 proteins were differentially expressed. This study presents an integrated approach to identify the alteration of biological/cellular pathways in HepG2 cells upon endosulfan exposure.

The R3 receptor-like protein tyrosine phosphatase subfamily inhibits insulin signalling by dephosphorylating the insulin receptor at specific sites

The autophosphorylation of specific tyrosine residues occurs in the cytoplasmic region of the insulin receptor (IR) upon insulin binding, and this in turn initiates signal transduction. The R3 subfamily (Ptprb, Ptprh, Ptprj and Ptpro) of receptor-like protein tyrosine phosphatases (RPTPs) is characterized by an extracellular region with 6-17 fibronectin type III-like repeats and a cytoplasmic region with a single phosphatase domain. We herein identified the IR as a substrate for R3 RPTPs by using the substrate-trapping mutants of R3 RPTPs. The co-expression of R3 RPTPs with the IR in HEK293T cells suppressed insulin-induced tyrosine phosphorylation of the IR. In vitro assays using synthetic phosphopeptides revealed that R3 RPTPs preferentially dephosphorylated a particular phosphorylation site of the IR: Y960 in the juxtamembrane region and Y1146 in the activation loop. Among four R3 members, only Ptprj was co-expressed with the IR in major insulin target tissues, such as the skeletal muscle, liver and adipose tissue. Importantly, the activation of IR and Akt by insulin was enhanced, and glucose and insulin tolerance was improved in Ptprj-deficient mice. These results demonstrated Ptprj as a physiological enzyme that attenuates insulin signalling in vivo, and indicate that an inhibitor of Ptprj may be an insulin-sensitizing agent.

Arp2/3 complex regulates adipogenesis by controlling cortical actin remodelling

Extensive actin cytoskeleton remodelling occurs during adipocyte development. We have previously shown that disruption of stress fibres by the actin-severing protein cofilin is a requisite step in adipogenesis. However, it remains unclear whether actin nucleation and assembly into the cortical structure are essential for adipocyte development. In the present study we investigated the role of cortical actin assembly and of actin nucleation by the actin-related protein 2/3 (Arp2/3) complex in adipogenesis. Cortical actin structure formation started with accumulation of filamentous actin (F-actin) patches near the plasma membrane during adipogenesis. Depletion of Arp2/3 by knockdown of its subunits Arp3 or ARPC3 strongly impaired adipocyte differentiation, although adipogenesis-initiating factors were unaffected. Moreover, the assembly of F-actin-rich structures at the plasma membrane was suppressed and the cortical actin structure poorly developed after adipogenic induction in Arp2/3-deficient cells. Finally, we provide evidence that the cortical actin cytoskeleton is essential for efficient glucose transporter 4 (GLUT4) vesicle exocytosis and insulin signal transduction. These results show that the Arp2/3 complex is an essential regulator of adipocyte development through control of the formation of cortical actin structures, which may facilitate nutrient uptake and signalling events.

Differences in muscle transcriptome among pigs phenotypically extreme for fatty acid composition

BACKGROUND

Besides having an impact on human health, the porcine muscle fatty acid profile determines meat quality and taste. The RNA-Seq technologies allowed us to explore the pig muscle transcriptome with an unprecedented detail. The aim of this study was to identify differentially-expressed genes between two groups of 6 sows belonging to an Iberian × Landrace backcross with extreme phenotypes according to FA profile.

RESULTS

We sequenced the muscle transcriptome acquiring 787.5 M of 75 bp paired-end reads. About 85.1% of reads were mapped to the reference genome. Of the total reads, 79.1% were located in exons, 6.0% in introns and 14.9% in intergenic regions, indicating expressed regions not annotated in the reference genome. We identified a 34.5% of the intergenic regions as interspersed repetitive regions. We predicted a total of 2,372 putative proteins. Pathway analysis with 131 differentially-expressed genes revealed that the most statistically-significant metabolic pathways were related with lipid metabolism. Moreover, 18 of the differentially-expressed genes were located in genomic regions associated with IMF composition in an independent GWAS study in the same genetic background. Thus, our results indicate that the lipid metabolism of FAs is differently modulated when the FA composition in muscle differs. For instance, a high content of PUFA may reduce FA and glucose uptake resulting in an inhibition of the lipogenesis. These results are consistent with previous studies of our group analysing the liver and the adipose tissue transcriptomes providing a view of each of the main organs involved in lipid metabolism.

CONCLUSIONS

The results obtained in the muscle transcriptome analysis increase the knowledge of the gene regulation of IMF deposition, FA profile and meat quality, in terms of taste and nutritional value. Besides, our results may be important in terms of human health.