Nitric oxide directly promotes vascular endothelial insulin transport

Association between nitric oxide synthase (NOS3) gene polymorphisms and diabetic nephropathy: An updated meta-analysis

Polymorphisms located within NOS3 gene have been investigated as susceptibility variants for diabetic nephropathy (DN) in type 2 diabetes mellitus (T2DM) in a large number of studies. However, these previous articles yielded inconsistent results and we aimed at elucidating the impact of NOS3 variants on DN risk in T2DM by conducting an updated systematic data synthesis. A total of 36 studies (12,807 participants) were selected for qualitative data synthesis, while 33 records with 11,649 subjects were included in the meta-analysis. The pooled analysis demonstrated the association of minor alleles of rs2070744 and rs1799983 with an increased susceptibility to DN (P < 0.001 and P = 0.015 for allelic model, respectively). For both of these variants, a significant effect of subgrouping according to ethnicity was found. Rs869109213 displayed an association with DN susceptibility, with pooled effect measures indicating a predisposing effect of the minor allele a (Prec = 0.002, ORrec = 1.960, 95%CI 1.288-2.983; Paavs. bb = 0.001, ORaavs. bb = 2.014, 95%CI 1.316-3.083). These findings support the effects of NOS3 variants on the risk of developing DN in T2DM.

Endothelial-Specific Reduction in Arf6 Impairs Insulin-Stimulated Vasodilation and Skeletal Muscle Blood Flow Resulting in Systemic Insulin Resistance in Mice

BACKGROUND

Much of what we know about insulin resistance is based on studies from metabolically active tissues such as the liver, adipose tissue, and skeletal muscle. Emerging evidence suggests that the vascular endothelium plays a crucial role in systemic insulin resistance; however, the underlying mechanisms remain incompletely understood. Arf6 (ADP ribosylation factor 6) is a small GTPase that plays a critical role in endothelial cell function. Here, we tested the hypothesis that the deletion of endothelial Arf6 will result in systemic insulin resistance.

METHODS

We used mouse models of constitutive endothelial cell-specific Arf6 deletion (Arf6f/- Tie2Cre+) and tamoxifen-inducible Arf6 knockout (Arf6f/f Cdh5CreER+). Endothelium-dependent vasodilation was assessed using pressure myography. Metabolic function was assessed using a battery of metabolic assessments including glucose and insulin tolerance tests and hyperinsulinemic-euglycemic clamps. We used a fluorescence microsphere-based technique to measure tissue blood flow. Skeletal muscle capillary density was assessed using intravital microscopy.

RESULTS

Endothelial Arf6 deletion impaired insulin-stimulated vasodilation in white adipose tissue and skeletal muscle feed arteries. The impairment in vasodilation was primarily due to attenuated insulin-stimulated nitric oxide bioavailability but independent of altered acetylcholine-mediated or sodium nitroprusside-mediated vasodilation. Endothelial cell-specific deletion of Arf6 also resulted in systematic insulin resistance in normal chow-fed mice and glucose intolerance in high-fat diet-fed obese mice. The underlying mechanisms of glucose intolerance were reductions in insulin-stimulated blood flow and glucose uptake in the skeletal muscle and were independent of changes in capillary density or vascular permeability.

CONCLUSIONS

Results from this study support the conclusion that endothelial Arf6 signaling is essential for maintaining insulin sensitivity. Reduced expression of endothelial Arf6 impairs insulin-mediated vasodilation and results in systemic insulin resistance. These results have therapeutic implications for diseases that are associated with endothelial cell dysfunction and insulin resistance such as diabetes.

Roles of Protein S-Nitrosylation in Endothelial Homeostasis and Dysfunction

Significance: Nitric oxide (NO) plays several distinct roles in endothelial homeostasis. Except for activating the guanylyl cyclase enzyme-dependent cyclic guanosine monophosphate signaling pathway, NO can bind reactive cysteine residues in target proteins, a process known as S-nitrosylation (SNO). SNO is proposed to explain the multiple biological functions of NO in the endothelium. Investigating the targets and mechanism of protein SNO in endothelial cells (ECs) can provide new strategies for treating endothelial dysfunction-related diseases. Recent Advances: In response to different environments, proteomics has identified multiple SNO targets in ECs. Functional studies confirm that SNO regulates NO bioavailability, inflammation, permeability, oxidative stress, mitochondrial function, and insulin sensitivity in ECs. It also influences EC proliferation, migration, apoptosis, and transdifferentiation. Critical Issues: Single-cell transcriptomic analysis of ECs isolated from different mouse tissues showed heterogeneous gene signatures. However, litter research focuses on the heterogeneous properties of SNO proteins in ECs derived from different tissues. Although metabolism reprogramming plays a vital role in endothelial functions, little is known about how protein SNO regulates metabolism reprogramming in ECs. Future Directions: Precisely deciphering the effects of protein SNO in ECs isolated from different tissues under different conditions is necessary to further characterize the relationship between protein SNO and endothelial dysfunction-related diseases. In addition, identifying SNO targets that can influence endothelial metabolic reprogramming and the underlying mechanism can offer new views on the crosstalk between metabolism and post-translational protein modification. Antioxid. Redox Signal. 40, 186-205.

Roles of mitochondrial dynamics and mitophagy in diabetic myocardial microvascular injury

Myocardial microvessels are composed of a monolayer of endothelial cells, which play a crucial role in maintaining vascular barrier function, luminal latency, vascular tone, and myocardial perfusion. Endothelial dysfunction is a key factor in the development of cardiac microvascular injury and diabetic cardiomyopathy. In addition to their role in glucose oxidation and energy metabolism, mitochondria also participate in non-metabolic processes such as apoptosis, intracellular ion handling, and redox balancing. Mitochondrial dynamics and mitophagy are responsible for regulating the quality and quantity of mitochondria in response to hyperglycemia. However, these endogenous homeostatic mechanisms can both preserve and/or disrupt non-metabolic mitochondrial functions during diabetic endothelial damage and cardiac microvascular injury. This review provides an overview of the molecular features and regulatory mechanisms of mitochondrial dynamics and mitophagy. Furthermore, we summarize findings from various investigations that suggest abnormal mitochondrial dynamics and defective mitophagy contribute to the development of diabetic endothelial dysfunction and myocardial microvascular injury. Finally, we discuss different therapeutic strategies aimed at improving endothelial homeostasis and cardiac microvascular function through the enhancement of mitochondrial dynamics and mitophagy.

Circular RNA RSU1 promotes retinal vascular dysfunction by regulating miR-345-3p/TAZ

Diabetic mellitus-induced diabetic retinopathy is a significant cause of visual impairment and blindness in adults. Circular RNAs (circRNAs) have been shown to play initial roles in vascular progression. However, the mechanism underlying diabetes mellitus-induced vascular complications remains largely unknown. In circRNA chip experiments, circRSU1 was found to be generally overexpressed in diabetic retinopathy patients. Human retina endothelial cells were stably transfected with lentiviruses carrying a circRSU1 interference plasmid. CircRSU1 downregulation alleviated diabetes mellitus induced retina vascular dysfunction, resulting in decreased vascular endothelial growth factor levels, inflammatory responses and oxidative stress. Mechanistically, we showed that elevated circRSU1 expression upregulated the TAZ levels by sponging miR-345-3p. Downregulation of TAZ reversed the vascular dysfunction that was caused by increased circRSU1 expression under hyperglycaemic conditions. In conclusion, overexpression of circRSU1 promotes vascular dysfunction by sponging miR-345-3p to increase the TAZ levels under diabetic conditions. We provide evidence that circRSU1 is a potential therapeutic target for treating diabetes mellitus-induced vascular dysfunction.

Cysteine residues in signal transduction and its relevance in pancreatic beta cells

Cysteine is one of the least abundant but most conserved amino acid residues in proteins, playing a role in their structure, metal binding, catalysis, and redox chemistry. Thiols present in cysteines can be modified by post-translational modifications like sulfenylation, acylation, or glutathionylation, regulating protein activity and function and serving as signals. Their modification depends on their position in the structure, surrounding amino acids, solvent accessibility, pH, etc. The most studied modifications are the redox modifications by reactive oxygen, nitrogen, and sulfur species, leading to reversible changes that serve as cell signals or irreversible changes indicating oxidative stress and cell damage. Selected antioxidants undergoing reversible oxidative modifications like peroxiredoxin-thioredoxin system are involved in a redox-relay signaling that can propagate to target proteins. Cysteine thiols can also be modified by acyl moieties' addition (derived from lipid metabolism), resulting in protein functional modification or changes in protein anchoring in the membrane. In this review, we update the current knowledge on cysteine modifications and their consequences in pancreatic β-cells. Because β-cells exhibit well-balanced redox homeostasis, the redox modifications of cysteines here serve primarily for signaling purposes. Similarly, lipid metabolism provides regulatory intermediates that have been shown to be necessary in addition to redox modifications for proper β-cell function and, in particular, for efficient insulin secretion. On the contrary, the excess of reactive oxygen, nitrogen, and sulfur species and the imbalance of lipids under pathological conditions cause irreversible changes and contribute to oxidative stress leading to cell failure and the development of type 2 diabetes.

Endothelial specific reduction in Arf6 impairs insulin-stimulated vasodilation and skeletal muscle blood flow resulting in systemic insulin resistance

Background

Much of what we know about insulin resistance is based on studies from metabolically active tissues such as liver, adipose tissue, and skeletal muscle. Emerging evidence suggests that the vascular endothelium plays a crucial role in systemic insulin resistance, however, the underlying mechanisms remain incompletely understood. ADP ribosylation factor 6 (Arf6) is a small GTPase that plays a critical role in endothelial cell (EC) function. Here, we tested the hypothesis that the deletion of endothelial Arf6 will result in systemic insulin resistance.

Methods

We used mouse models of constitutive EC-specific Arf6 deletion (Arf6 f/- Tie2Cre) and tamoxifen inducible Arf6 knockout (Arf6 f/f Cdh5Cre). Endothelium-dependent vasodilation was assessed using pressure myography. Metabolic function was assessed using a battery of metabolic assessments including glucose- and insulin-tolerance tests and hyperinsulinemic-euglycemic clamps. A fluorescence microsphere-based technique was used to measure tissue blood flow. Intravital microscopy was used to assess skeletal muscle capillary density.

Results

Endothelial Arf6 deletion impaired insulin-stimulated vasodilation in white adipose tissue (WAT) and skeletal muscle feed arteries. The impairment in vasodilation was primarily due to attenuated insulin-stimulated nitric oxide (NO) bioavailability but independent of altered acetylcholine- or sodium nitroprusside-mediated vasodilation. In vitro Arf6 inhibition resulted in suppressed insulin stimulated phosphorylation of Akt and endothelial NO synthase. Endothelial cell-specific deletion of Arf6 also resulted in systematic insulin resistance in normal chow fed mice and glucose intolerance in high fat diet fed obese mice. The underlying mechanisms of glucose intolerance were reductions in insulin-stimulated blood flow and glucose uptake in the skeletal muscle and were independent of changes in capillary density or vascular permeability.

Conclusion

Results from this study support the conclusion that endothelial Arf6 signaling is essential for maintaining insulin sensitivity. Reduced expression of endothelial Arf6 impairs insulin-mediated vasodilation and results in systemic insulin resistance. These results have therapeutic implications for diseases that are associated with endothelial cell dysfunction and insulin resistance such as diabetes.

The level of nitric oxide and arginase activity in patients with arterial hypertension and diabetes mellitus during COVID-19

The aim of this study was to assess the level of nitric oxide production and arginase activity in patients with arterial hypertension and type II diabetes mellitus during infection with SARS-CoV-2. The study groups included patients with arterial hypertension, patients with arterial hypertension combined with a severe course of COVID-19 and patients who, in addition to arterial hypertension and COVID-19, were suffering from type II diabetes mellitus. The volunteers without any clinical signs of diseases and normal blood pressure formed the control group. It has been established that arterial hypertension, combined with COVID-19 occurs along with reduced L-arginine, nitric oxide, superoxide dismutase activity and increased arginase activity. At the same time, the presence of arterial hypertension in patients with diabetes and coronavirus disease is accompanied by a decline in the content of L-arginine and arginase activity. Our study’s results may help scientists find new pharmacological targets in the future treatment of coronavirus disease and comorbid disorders. Keywords: arginase, COVID-19, diabetes mellitus, hypertension, L-arginine, nitric oxide, superoxide dismutase

Potential health benefits of Nigella sativa on diabetes mellitus and its complications: A review from laboratory studies to clinical trials

This review aims to gather and summarize up-to-date information on the potential health benefits of Nigella sativa (NS) on diabetes mellitus (DM) and its complications from different animal models, clinical trials and in vitro studies. DM is one of the most prevalent metabolic disorders resulting from chronic hyperglycaemia due to problems in insulin secretion, insulin action or both. It affects people regardless of age, gender and race. The main consequence of DM development is the metabolic dysregulation of glucose homeostasis. Current treatments for DM include pharmacological therapy, insulin and diabetic therapy targeting β cells. Some of these therapeutic approaches are promising; however, their safety and effectiveness remain elusive. Since ancient times, medicinal plants have been used and proven effective against diseases. These plants are believed to be effective and benefit physiological and pathological processes, as they can be used to prevent, reduce or treat multiple diseases. Nigella sativa Linn. is an annual indigenous herbaceous plant belonging to Ranunculaceae, the buttercup family. NS exhibits multifactorial activities; it could ameliorate oxidative, inflammatory, apoptotic and insulinotropic effects and inhibit carbohydrate digestive enzymes. Thus, this review demonstrates the therapeutic potential of NS that could be used as a complement or adjuvant for the management of DM and its complications. However, future research should be able to replicate and fill in the gaps of the study conducted to introduce NS safely to patients with DM.

Metformin's Impact on the Microvascular Response to Insulin

Metformin improves insulin's action on whole-body glucose metabolism in various insulin-resistant populations. The detailed cellular mechanism(s) for its metabolic actions are multiple and still incompletely understood. Beyond metabolic actions, metformin also impacts microvascular function. However, the effects of metformin on microvascular function and microvascular insulin action specifically are poorly defined. In this mini-review, we summarize what is currently known about metformin's beneficial impact on both microvascular function and the microvascular response to insulin while highlighting methodologic issues in the literature that limit straightforward mechanistic understanding of these effects. We examine potential mechanisms for these effects based on pharmacologically dosed studies and propose that metformin may improve human microvascular insulin resistance by attenuating oxidative stress, inflammation, and endothelial dysfunction. Finally, we explore several important evidence gaps and discuss avenues for future investigation that may clarify whether metformin's ability to improve microvascular insulin sensitivity is linked to its positive impact on vascular outcomes.

Age-Related Hearing Loss: The Link between Inflammaging, Immunosenescence, and Gut Dysbiosis

This article provides a theoretical overview of the association between age-related hearing loss (ARHL), immune system ageing (immunosenescence), and chronic inflammation. ARHL, or presbyacusis, is the most common sensory disability that significantly reduces the quality of life and has a high economic impact. This disorder is linked to genetic risk factors but is also influenced by a lifelong cumulative effect of environmental stressors, such as noise, otological diseases, or ototoxic drugs. Age-related hearing loss and other age-related disorders share common mechanisms which often converge on low-grade chronic inflammation known as “inflammaging”. Various stimuli can sustain inflammaging, including pathogens, cell debris, nutrients, and gut microbiota. As a result of ageing, the immune system can become defective, leading to the accumulation of unresolved inflammatory processes in the body. Gut microbiota plays a central role in inflammaging because it can release inflammatory mediators and crosstalk with other organ systems. A proinflammatory gut environment associated with ageing could result in a leaky gut and the translocation of bacterial metabolites and inflammatory mediators to distant organs via the systemic circulation. Here, we postulate that inflammaging, as a result of immunosenescence and gut dysbiosis, accelerates age-related cochlear degeneration, contributing to the development of ARHL. Age-dependent gut dysbiosis was included as a hypothetical link that should receive more attention in future studies.

Perivascular brown adipocytes-derived kynurenic acid relaxes blood vessel via endothelium PI3K-Akt-eNOS pathway

OBJECTIVE

Several metabolites from the kynurenine pathway of tryptophan metabolism play a critical role in vascular function and vascular wall remodeling. This study aimed to test whether metabolite kynurenic acid (KYNA) from the kynurenine pathway relaxes blood vessels.

APPROACH AND RESULTS

We employed histological staining, in vitro cell culture, Western blotting, real-time PCR, and nitric oxide detection to validate kynurenine aminotransferase (KAT) localization in the vasculature as well as KYNA action on endothelial cells. We also detected vascular reactivity by organ chamber and monitored blood pressure by telemetry to investigate the regulation effect of KYNA on vascular tone. The results presented that perivascular adipose tissue (PVAT) from mice thoracic aorta had robust staining of anti-KAT1 and KYNA than PVAT from the abdominal aorta and mesenteric artery, which is consistent with the expression profile of brown adipocyte marker uncoupling protein 1. KYNA, metabolized from kynurenine by KAT, relaxed pre-contracted both aortic ring and mesenteric artery. In addition, KYNA derived from KAT in PVAT participates in the cross-talk between PVAT and vessel by mediating PVAT inhibition on agonist-induced thoracic aorta contraction. Furthermore, intraperitoneal injection of KYNA in mice reduced blood pressure. The vessel relaxation effect of KYNA was through the endothelium-dependent PI3K-Akt-eNOS pathway. Finally, the high-fat diet decreased KAT1 expression in perithoracic aortic fat and led to KYNA reduction in blood.

CONCLUSIONS

Our research identified KYNA generated by KAT as a novel perivascular brown adipocyte-derived vascular relaxation factor and suggests that KYNA reduction is a critical event in vascular dysfunction under obese condition.

The ginger extract could improve diabetic retinopathy by inhibiting the expression of e/iNOS and G6PDH, apoptosis, inflammation, and angiogenesis

Diabetic retinopathy is a complication of diabetes, caused by high blood sugar levels damaging the retina. It is the result of damage to the small blood vessels and neurons of the retina. Ginger and its phytochemical compounds can improve oxidative damage and inflammation. However, the effects of this plant on ocular expression G6PDH and e/iNOS, eye cell apoptosis, and angiogenesis are not well known in this tissue. Therefore, the aim of this study was to evaluate the therapeutic potential of ginger extract on rats with type 2 diabetic retinopathy. Thirty-two Wistar rats were randomly divided into four controlled and treated groups. The serum level of metabolic factors such as lipid profiles, insulin and glucose, and the level of oxidative biomarkers along with the TNF-α level in eye tissue were measured. The expression of NF-κB, VEGF, BAX, Bcl-2, caspase-3, e/iNOS, and G6PDH in eye tissue was measured. Serum levels of lipid profiles, glucose, and insulin, oxidative and inflammatory markers were significantly increased in the diabetic group compared to control. While, treatment with ginger extract could significantly improve these factors in diabetic rats. Moreover, the ocular expression of e/iNOS, G6PDH, VEGF, NF-κB, and genes involved in apoptosis was changed in diabetic rats. However, treatment with ginger extract could ameliorate these changes in the diabetic-treated group. It can be concluded that ginger extract could improve diabetic retinopathy by inhibiting oxidative damage, inflammation, iNOS, VEGF, apoptosis, and improving eNOS and G6PDH. PRACTICAL APPLICATIONS: Microvascular complications of diabetes such as retinopathy can be one of the main causes of disability in people with diabetes. Chronic hyperglycemia, oxidative stress, inflammation, and apoptosis cause diabetic retinopathy through retinal damage. Ginger, on the other hand, is an available, inexpensive, and uncomplicated medicinal plant that contains more than 20 different phytochemicals, such as gingerol and shogaol, which have anti-inflammatory, antioxidant, antihypertensive, hypoglycemic, and hypolipidemic properties. The results of our study showed well that the ginger extract could improve diabetic retinopathy by inhibiting the expression of e/iNOS and G6PDH and oxidative damage, apoptosis, inflammation, and angiogenesis. Therefore, ginger and its compounds can be a good option to improve the complications of diabetes.

Effect of low-dose tadalafil once daily on glycemic control in patients with type 2 diabetes and erectile dysfunction: a randomized, double-blind, placebo-controlled pilot study

BACKGROUND

Phosphodiesterase type 5 inhibitors restore nitric oxide signaling, that plays a significant role in erectile function, and appears to counteract insulin resistance in animal and human models. This study was aimed to evaluate the glycemic and metabolic effects of low-dose tadalafil once daily in patients with type 2 diabetes and erectile dysfunction.

METHODS

A 6-month, randomized, double-blind, placebo-controlled pilot trial was conducted. Eligible patients were randomly assigned in a ratio of 2:1 to the tadalafil 5 mg and placebo groups; all patients received either tadalafil or placebo once a day. The primary efficacy endpoint was the absolute change in glycated hemoglobin (HbA1c) levels during the 6-month study period. The secondary efficacy endpoints included metabolic parameters and erectile function.

RESULTS

Of the 68 patients who completed this study, 45 and 23 patients were allocated to the tadalafil and placebo groups, respectively. The mean HbA1c level was significantly different between the groups over the 6-month study period (P = 0.021). After 6 months of treatment, the HbA1c decrement in the tadalafil group was greater than that in the placebo group (- 0.14 ± 0.53% vs. 0.20 ± 0.69%, P = 0.030). The International Index of Erectile Function-5 scores improvement was significantly greater in the tadalafil group than in the placebo group at 6 months (P = 0.003).

CONCLUSION

This prospective pilot study showed that low-dose tadalafil administered once a day was effective in improving glycemic control and erectile function in patients with type 2 diabetes and erectile dysfunction. Trial registration KCT0005666.

A New Approach to Overcome Insulin Resistance in Patients with Impaired Glucose Tolerance: The Results of a Multicenter, Double-Blind, Placebo-Controlled, Randomized Clinical Trial of Efficacy and Safety of Subetta

Impaired glucose tolerance (IGT) is a common carbohydrate metabolism disorder world-wide. To evaluate the efficacy and safety of 12-week Subetta therapy in correcting 2-h plasma glucose in patients with IGT, a multicenter, double-blind, placebo-controlled, randomized clinical trial was performed. Derived by technological treatment of antibodies to insulin receptor β-subunit and endothelial NO synthase, Subetta increases the sensitivity of insulin receptors by activating the insulin signaling pathway. Oral glucose tolerance test (OGTT), fasting plasma glucose (FPG), and glycated hemoglobin (HbA1c) were examined at screening, after 4 and 12 weeks. In Per Protocol population, 2-h plasma glucose in the Subetta group decreased by 2.05 ± 2.11 mmol/L (versus 0.56 ± 2.55 mmol/L in the Placebo group) after 12 weeks. The difference between the two groups was 1.49 ± 2.33 mmol/L (p < 0.0001). After 12 weeks, 65.2% of patients had 2-h plasma glucose <7.8 mmol/L. FPG remained almost unchanged. HbA1c tended to decrease. The number of adverse events did not differ in both groups. Subetta treatment is beneficial for patients with IGT; it also prevents progression of carbohydrate metabolism disorders.

Capillary Endothelial Insulin Transport: The Rate-limiting Step for Insulin-stimulated Glucose Uptake

The rate-limiting step for skeletal muscle glucose uptake is transport from microcirculation to muscle interstitium. Capillary endothelium poses a barrier that delays the onset of muscle insulin action. Defining physiological barriers that control insulin access to interstitial space is difficult because of technical challenges that confront study of microscopic events in an integrated physiological system. Two physiological variables determine muscle insulin access. These are the number of perfused capillaries and the permeability of capillary walls to insulin. Disease states associated with capillary rarefaction are closely linked to insulin resistance. Insulin permeability through highly resistant capillary walls of muscle poses a significant barrier to insulin access. Insulin may traverse the endothelium through narrow intercellular junctions or vesicular trafficking across the endothelial cell. Insulin is large compared with intercellular junctions, making this an unlikely route. Transport by endothelial vesicular trafficking is likely the primary route of transit. Studies in vivo show movement of insulin is not insulin receptor dependent. This aligns with single-cell transcriptomics that show the insulin receptor is not expressed in muscle capillaries. Work in cultured endothelial cell lines suggest that insulin receptor activation is necessary for endothelial insulin transit. Controversies remain in the understanding of transendothelial insulin transit to muscle. These controversies closely align with experimental approaches. Control of circulating insulin accessibility to skeletal muscle is an area that remains ripe for discovery. Factors that impede insulin access to muscle may contribute to disease and factors that accelerate access may be of therapeutic value for insulin resistance.

The manifold roles of protein S-nitrosylation in the life of insulin

Insulin, which is released by pancreatic islet β-cells in response to elevated levels of glucose in the blood, is a critical regulator of metabolism. Insulin triggers the uptake of glucose and fatty acids into the liver, adipose tissue and muscle, and promotes the storage of these nutrients in the form of glycogen and lipids. Dysregulation of insulin synthesis, secretion, transport, degradation or signal transduction all cause failure to take up and store nutrients, resulting in type 1 diabetes mellitus, type 2 diabetes mellitus and metabolic dysfunction. In this Review, we make the case that insulin signalling is intimately coupled to protein S-nitrosylation, in which nitric oxide groups are conjugated to cysteine thiols to form S-nitrosothiols, within effectors of insulin action. We discuss the role of S-nitrosylation in the life cycle of insulin, from its synthesis and secretion in pancreatic β-cells, to its signalling and degradation in target tissues. Finally, we consider how aberrant S-nitrosylation contributes to metabolic diseases, including the roles of human genetic mutations and cellular events that alter S-nitrosylation of insulin-regulating proteins. Given the growing influence of S-nitrosylation in cellular metabolism, the field of metabolic signalling could benefit from renewed focus on S-nitrosylation in type 2 diabetes mellitus and insulin-related disorders.

The Link between Gut Dysbiosis Caused by a High-Fat Diet and Hearing Loss

This review aims to provide a conceptual and theoretical overview of the association between gut dysbiosis and hearing loss. Hearing loss is a global health issue; the World Health Organisation (WHO) estimates that 2.5 billion people will be living with some degree of hearing loss by 2050. The aetiology of sensorineural hearing loss (SNHL) is complex and multifactorial, arising from congenital and acquired causes. Recent evidence suggests that impaired gut health may also be a risk factor for SNHL. Inflammatory bowel disease (IBD), type 2 diabetes, diet-induced obesity (DIO), and high-fat diet (HFD) all show links to hearing loss. Previous studies have shown that a HFD can result in microangiopathy, impaired insulin signalling, and oxidative stress in the inner ear. A HFD can also induce pathological shifts in gut microbiota and affect intestinal barrier (IB) integrity, leading to a leaky gut. A leaky gut can result in chronic systemic inflammation, which may affect extraintestinal organs. Here, we postulate that changes in gut microbiota resulting from a chronic HFD and DIO may cause a systemic inflammatory response that can compromise the permeability of the blood-labyrinth barrier (BLB) in the inner ear, thus inducing cochlear inflammation and hearing deficits.

Serum uric acid and its change with the risk of type 2 diabetes: A prospective study in China

OBJECTIVE

To assess the association of baseline uric acid levels and their changes from baseline to Year 1 with the risk of type 2 diabetes.

RESEARCH DESIGN AND METHODS

This study cohort included 9471 subjects without a history of diabetes at baseline. The incident diabetes was diagnosed according to the American Diabetes Association standard.

RESULTS

During a mean follow-up of 2.9 years, we identified 762 type 2 diabetes cases. Multivariate-adjusted hazard ratios (HRs) of diabetes across baseline tertiles of serum uric acid were 1.00, 1.15, and 1.32 (P for trend = 0.018), respectively. Participants with hyperuricemia compared with those without had a 1.20-fold (95% confidence interval [CI] 1.01-1.44) risk of diabetes. When uric acid was examined as a continuous variable, multivariable-adjusted HR of diabetes for each 1 mg/dL (60 μmol/L) increase in serum uric acid was 1.09 (95% CI 1.03-1.15). Compared with subjects with stable serum uric acid from baseline to Year 1 (±10%), those with uric acid gain ≥30% had a 30% (95% CI 1.01-1.79) increased risk of diabetes and those with uric acid loss ≥10% had a 21% (95% 0.62-0.99) decreased risk of diabetes. This positive association between baseline serum uric acid and diabetes risk was consistent among subjects younger and older than 45 years, non-obese and obese participants, and men.

CONCLUSIONS

High level of baseline serum uric acid and serum uric acid gain from baseline to Year 1 are associated with an increased risk of type 2 diabetes among Chinese adults.

Insulin Sensitivity Is Retained in Mice with Endothelial Loss of Carcinoembryonic Antigen Cell Adhesion Molecule 1

CEACAM1 regulates endothelial barrier integrity. Because insulin signaling in extrahepatic target tissues is regulated by insulin transport through the endothelium, we aimed at investigating the metabolic role of endothelial CEACAM1. To this end, we generated endothelial cell-specific Ceacam1 null mice (VECadCre+Cc1^fl/fl) and carried out their metabolic phenotyping and mechanistic analysis by comparison to littermate controls. Hyperinsulinemic-euglycemic clamp analysis showed intact insulin sensitivity in VECadCre+Cc1^fl/fl mice. This was associated with the absence of visceral obesity and lipolysis and normal levels of circulating non-esterified fatty acids, leptin, and adiponectin. Whereas the loss of endothelial Ceacam1 did not affect insulin-stimulated receptor phosphorylation, it reduced IRS-1/Akt/eNOS activation to lower nitric oxide production resulting from limited SHP2 sequestration. It also reduced Shc sequestration to activate NF-κB and increase the transcription of matrix metalloproteases, ultimately inducing plasma IL-6 and TNFα levels. Loss of endothelial Ceacam1 also induced the expression of the anti-inflammatory CEACAM1-4L variant in M2 macrophages in white adipose tissue. Together, this could cause endothelial barrier dysfunction and facilitate insulin transport, sustaining normal glucose homeostasis and retaining fat accumulation in adipocytes. The data assign a significant role for endothelial cell CEACAM1 in maintaining insulin sensitivity in peripheral extrahepatic target tissues.

Effects of imatinib on vascular insulin sensitivity and free fatty acid transport in early weight gain

BACKGROUND

Vascular endothelial dysfunction is an essential part of the pathophysiology of type 2 diabetes and its complications. In type 2 diabetes, endothelial dysfunction is characterized by reduced insulin signaling and increased transendothelial transport of fatty acids (FA). As the Abl kinase inhibitor imatinib was previously shown to reverse type 2 diabetes and to inhibit VEGF signaling via Abl kinases, we studied the effect of imatinib on vascular insulin sensitivity and fatty acid transport in vivo and in vitro.

METHODS

C57/BL6J mice were fed a chow diet or Western diet (WD), and received daily imatinib injections for two weeks. Insulin-mediated vasoreactivity of resistance arteries was studied using intravital microscopy, and metabolic insulin sensitivity using the hyperinsulinemic-euglycemic clamp. The effect of imatinib on triglyceride content in skeletal muscle and heart in vivo was also determined. In vitro, the effect of imatinib on fatty acid transport was studied in human umbilical vein endothelial cells (HUVECs) by evaluating the effect of imatinib on fluorescently labeled FA uptake both under basal and VEGF-B-stimulated conditions.

RESULTS

Imatinib prevented the WD-induced weight gain in mice, independently from food intake. In line with this, imatinib enhanced insulin-mediated vasoreactivity of resistance arteries in the WD-fed mice. However, imatinib did not affect triglyceride content in muscle. In cultured endothelial cells, VEGF-B stimulation resulted in a time-dependent uptake of fatty acids in parallel with increased phosphorylation of the Abl kinase substrate Crk-like protein (CrkL) at Tyr207. Although imatinib effectively prevented VEGF-B-mediated Abl kinase activation, it had no effect on VEGF-B mediated endothelial FA uptake.

CONCLUSION

Imatinib prevents weight gain and preserves insulin-mediated vasodilation in WD-fed mice, but does not affect endothelial FA transport despite inhibiting VEGF-B signaling. The beneficial effect of imatinib on insulin-mediated vasodilation may contribute to the anti-diabetic effects of imatinib.

Considerations for Maximizing the Exercise "Drug" to Combat Insulin Resistance: Role of Nutrition, Sleep, and Alcohol

Insulin resistance is a key etiological factor in promoting not only type 2 diabetes mellitus but also cardiovascular disease (CVD). Exercise is a first-line therapy for combating chronic disease by improving insulin action through, in part, reducing hepatic glucose production and lipolysis as well as increasing skeletal muscle glucose uptake and vasodilation. Just like a pharmaceutical agent, exercise can be viewed as a "drug" such that identifying an optimal prescription requires a determination of mode, intensity, and timing as well as consideration of how much exercise is done relative to sitting for prolonged periods (e.g., desk job at work). Furthermore, proximal nutrition (nutrient timing, carbohydrate intake, etc.), sleep (or lack thereof), as well as alcohol consumption are likely important considerations for enhancing adaptations to exercise. Thus, identifying the maximal exercise "drug" for reducing insulin resistance will require a multi-health behavior approach to optimize type 2 diabetes and CVD care.

Endoplasmic reticulum stress and oxidative stress drive endothelial dysfunction induced by high selenium

Selenium is an essential trace element important for human health. A balanced intake is, however, crucial to maximize the health benefits of selenium. At physiological concentrations, selenium mediates antioxidant, anti-inflammatory, and pro-survival actions. However, supra-nutritional selenium intake was associated with increased diabetes risk leading potentially to endothelial dysfunction, the initiating step in atherosclerosis. High selenium causes apoptosis in cancer cells via endoplasmic reticulum (ER) stress, a mechanism also implicated in endothelial dysfunction. Nonetheless, whether ER stress drives selenium-induced endothelial dysfunction, remains unknown. Here, we investigated the effects of increasing concentrations of selenium on endothelial cells. High selenite reduced nitric oxide bioavailability and impaired angiogenesis. High selenite also induced ER stress, increased reactive oxygen species (ROS) production, and apoptosis. Pretreatment with the chemical chaperone, 4-phenylbutyrate, prevented the toxic effects of selenium. Our findings support a model where high selenite leads to endothelial dysfunction through activation of ER stress and increased ROS production. These results highlight the importance of tailoring selenium supplementation to achieve maximal health benefits and suggest that prophylactic use of selenium supplements as antioxidants may entail risk.

Synergistic association of high-sensitivity C-reactive protein and body mass index with insulin resistance in non-diabetic adults

Epidemiological evidence has indicated that inflammatory markers and obesity are strongly correlated with insulin resistance (IR). However, there is a paucity of studies assessing the complex interaction between elevated hs-CRP and body mass index (BMI), particularly among Asians. This study investigated the additive interaction between hs-CRP and BMI on IR, using cross-sectional data from the 7th Korea National Health and Nutrition Examination Survey (2016–2018). A total of 5706 men and 6707 women aged 20 years or older were evaluated, and a multiple logistic regression analysis was used to assess the association of serum hs-CRP and BMI with IR, as measured by the triglyceride-glucose index (TyG index). Sex-specific median values were used to dichotomise the continuous TyG index variable into insulin-sensitive and IR categories. Biological interaction was evaluated using the Relative excess risk due to interaction (RERI), attributable proportion due to interaction (AP), and synergy index (SI). The joint effects of high hs-CRP and overweight/obesity on IR were greater than would be expected from the effects of the individual exposures alone. Relative to those with low hs-CRP and BMI < 23, having both exposures was related to increased IR with an adjusted OR of 2.97 (95% CI 2.50–3.52) in men and 3.08 (95% CI 2.67–3.56) in women with significant additive interactions. These findings demonstrate that IR prevention strategies that reduce both systematic inflammation and BMI may exceed the expected benefits based on targeting these risk factors separately.

Role of Nitric Oxide in Insulin Secretion and Glucose Metabolism

Nitric oxide (NO) contributes to carbohydrate metabolism and decreased NO bioavailability is involved in the development of type 2 diabetes mellitus (T2DM). NO donors may improve insulin signaling and glucose homeostasis in T2DM and insulin resistance (IR), suggesting the potential clinical importance of NO-based interventions. In this review, site-specific roles of the NO synthase (NOS)-NO pathway in carbohydrate metabolism are discussed. In addition, the metabolic effects of physiological low levels of NO produced by constitutive NOS (cNOS) versus pathological high levels of NO produced by inducible NOS (iNOS) in pancreatic β-cells, adipocytes, hepatocytes, and skeletal muscle cells are summarized. A better understanding of the NOS-NO system in the regulation of glucose homeostasis can hopefully facilitate the development of new treatments for T2DM.

Obesity-related cognitive impairment: The role of endothelial dysfunction

Obesity is a global pandemic associated with macro- and microvascular endothelial dysfunction. Microvascular endothelial dysfunction has recently emerged as a significant risk factor for the development of cognitive impairment. In this review, we present evidence from clinical and preclinical studies supporting a role for obesity in cognitive impairment. Next, we discuss how obesity-related hyperinsulinemia/insulin resistance, systemic inflammation, and gut dysbiosis lead to cognitive impairment through induction of endothelial dysfunction and disruption of the blood brain barrier. Finally, we outline the potential clinical utility of dietary interventions, exercise, and bariatric surgery in circumventing the impacts of obesity on cognitive function.

Metabolic Syndrome, Brain Insulin Resistance, and Alzheimer's Disease: Thioredoxin Interacting Protein (TXNIP) and Inflammasome as Core Amplifiers

Empirical evidence indicates a strong association between insulin resistance and pathological alterations related to Alzheimer's disease (AD) in different cerebral regions. While cerebral insulin resistance is not essentially parallel with systemic metabolic derangements, type 2 diabetes mellitus (T2DM) has been established as a risk factor for AD. The circulating "toxic metabolites" emerging in metabolic syndrome may engage several biochemical pathways to promote oxidative stress and neuroinflammation leading to impair insulin function in the brain or "type 3 diabetes". Thioredoxin-interacting protein (TXNIP) as an intracellular amplifier of oxidative stress and inflammasome activation may presumably mediate central insulin resistance. Emerging data including those from our recent studies has demonstrated a sharp TXNIP upregulation in stroke, aging and AD and well underlining the significance of this hypothesis. With the main interest to illustrate TXNIP place in type 3 diabetes, the present review primarily briefs the potential mechanisms contributing to cerebral insulin resistance in a metabolically deranged environment. Then with a particular focus on plausible TXNIP functions to drive and associate with AD pathology, we present the most recent evidence supporting TXNIP as a promising therapeutic target in AD as an age-associated dementia.

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.

Exercise and Vascular Insulin Sensitivity in the Skeletal Muscle and Brain

We present the hypothesis that exercise-induced hyperemia, perhaps through vascular shear stress, represents an important factor responsible for the effects of physical activity (PA) on vascular insulin sensitivity. Specifically, we postulate PA involving the greatest amount of skeletal muscle mass and the greatest central neural recruitment maximizes perfusion and consequently enhances vascular insulin sensitivity in the skeletal muscle and brain.

Inhibiting Protein Tyrosine Phosphatase 1B to Improve Regenerative Functions of Endothelial Cells

Protein tyrosine phosphatase-1B (PTP1B) is an important negative regulator of insulin receptor- and vascular endothelial growth factor receptor-dependent signalings in endothelial cells. Genetic or pharmacological inhibition of PTP1B has been shown to enhance endothelial cell proliferation and migration and increase nitric oxide production. In vivo, inhibiting PTP1B can reverse endothelial dysfunction, promote angiogenesis, and accelerate wound healing. Intense research is currently continuing in an effort to discover novel selective PTP1B inhibitors, primarily for treating insulin resistance. We propose that these drugs may also represent a new horizon for boosting the regenerative capacities of endothelial cells.

Acute Nitric Oxide Synthase Inhibition Accelerates Transendothelial Insulin Efflux In Vivo

Before insulin can stimulate glucose uptake in muscle, it must be delivered to skeletal muscle (SkM) through the microvasculature. Insulin delivery is determined by SkM perfusion and the rate of movement of insulin across the capillary endothelium. The endothelium therefore plays a central role in regulating insulin access to SkM. Nitric oxide (NO) is a key regulator of endothelial function and stimulates arterial vasodilation, which increases SkM perfusion and the capillary surface area available for insulin exchange. The effects of NO on transendothelial insulin efflux (TIE), however, are unknown. We hypothesized that acute reduction of endothelial NO would reduce TIE. However, intravital imaging of TIE in mice revealed that reduction of NO by l-N^G-nitro-l-arginine methyl ester (l-NAME) enhanced the rate of TIE by ∼30% and increased total extravascular insulin delivery. This accelerated TIE was associated with more rapid insulin-stimulated glucose lowering. Sodium nitroprusside, an NO donor, had no effect on TIE in mice. The effects of l-NAME on TIE were not due to changes in blood pressure alone, as a direct-acting vasoconstrictor (phenylephrine) did not affect TIE. These results demonstrate that acute NO synthase inhibition increases the permeability of capillaries to insulin, leading to an increase in delivery of insulin to SkM.

Efficacy and safety of Subetta add-on therapy in type 1 diabetes mellitus: The results of a multicenter, double-blind, placebo-controlled, randomized clinical trial

BACKGROUND

To examine efficacy of Subetta as an add-on to insulin therapy in patients with type 1 diabetes mellitus (T1DM) a multicenter, double-blind, placebo-controlled, randomized clinical trial was performed. Derived by technological treatment of antibodies to insulin receptor β-subunit and endothelial NO synthase Subetta was previously proved to activate insulin signaling pathway.

METHODS

A total of 144 randomized patients with poor glycemic control in basal-bolus insulin regime were included in intention-to-treat analysis in Subetta add-on therapy or placebo (n = 72 in both groups). Hemoglobin A1c (HbA1c), fasting plasma glucose (FPG), basal and prandial insulin doses, number of hypoglycemia episodes confirmed by self-monitoring of blood glucose were recorded for 36 weeks.

RESULTS

The baseline characteristics of subjects did not differ between the two groups. HbA1c mean (±standard deviation) change was -0.59 ± 0.99% (95% CI -0.84 to -0.37) after 36 weeks in Subetta (vs. -0.20 ± 1.14%; 95% CI -0.44 to 0.11 in placebo; p = 0.028). The rate of overall hypoglycemia events was 7.9 per patient year (95% CI 7.1-8.6) in Subetta group and 7.6 (95% CI 6.9-8.4) in Placebo group (p = 0.63). The basal and total insulin doses did not change at the end of 36 weeks in both groups.

CONCLUSIONS

Subetta add-on therapy boosting insulin activity and improving glycemic control in patients with T1DM is proved to be beneficial.

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov identifier: NCT01868594.

Mechanisms of Aerobic Exercise Impairment in Diabetes: A Narrative Review

The prevalence of diabetes in the United States and globally has been rapidly increasing over the last several decades. There are now estimated to be 30.3 million people in the United States and 422 million people worldwide with diabetes. Diabetes is associated with a greatly increased risk of cardiovascular mortality, which is the leading cause of death in adults with diabetes. While exercise training is a cornerstone of diabetes treatment, people with diabetes have well-described aerobic exercise impairments that may create an additional diabetes-specific barrier to adding regular exercise to their lifestyle. Physiologic mechanisms linked to exercise impairment in diabetes include insulin resistance, cardiac abnormalities, mitochondrial function, and the ability of the body to supply oxygen. In this paper, we highlight the abnormalities of exercise in type 2 diabetes as well as potential therapeutic approaches.

Hyposialylated IgG activates endothelial IgG receptor FcγRIIB to promote obesity-induced insulin resistance

Type 2 diabetes mellitus (T2DM) is a common complication of obesity. Here, we have shown that activation of the IgG receptor FcγRIIB in endothelium by hyposialylated IgG plays an important role in obesity-induced insulin resistance. Despite becoming obese on a high-fat diet (HFD), mice lacking FcγRIIB globally or selectively in endothelium were protected from insulin resistance as a result of the preservation of insulin delivery to skeletal muscle and resulting maintenance of muscle glucose disposal. IgG transfer in IgG-deficient mice implicated IgG as the pathogenetic ligand for endothelial FcγRIIB in obesity-induced insulin resistance. Moreover, IgG transferred from patients with T2DM but not from metabolically healthy subjects caused insulin resistance in IgG-deficient mice via FcγRIIB, indicating that similar processes may be operative in T2DM in humans. Mechanistically, the activation of FcγRIIB by IgG from obese mice impaired endothelial cell insulin transcytosis in culture and in vivo. These effects were attributed to hyposialylation of the Fc glycan, and IgG from T2DM patients was also hyposialylated. In HFD-fed mice, supplementation with the sialic acid precursor N-acetyl-D-mannosamine restored IgG sialylation and preserved insulin sensitivity without affecting weight gain. Thus, IgG sialylation and endothelial FcγRIIB may represent promising therapeutic targets to sever the link between obesity and T2DM.

Nitric oxide is required for the insulin sensitizing effects of contraction in mouse skeletal muscle

KEY POINTS

People with insulin resistance or type 2 diabetes can substantially increase their skeletal muscle glucose uptake during exercise and insulin sensitivity after exercise. Skeletal muscle nitric oxide (NO) is important for glucose uptake during exercise, although how prior exercise increases insulin sensitivity is unclear. In the present study, we examined whether NO is necessary for normal increases in skeletal muscle insulin sensitivity after contraction ex vivo in mouse muscle. The present study uncovers, for the first time, a novel role for NO in the insulin sensitizing effects of ex vivo contraction, which is independent of blood flow.

ABSTRACT

The factors regulating the increase in skeletal muscle insulin sensitivity after exercise are unclear. We examined whether nitric oxide (NO) is required for the increase in insulin sensitivity after ex vivo contractions. Isolated C57BL/6J mouse EDL muscles were contracted for 10 min or remained at rest (basal) with or without the NO synthase (NOS) inhibition (N^G -monomethyl-l-arginine; l-NMMA; 100 μm). Then, 3.5 h post contraction/basal, muscles were exposed to saline or insulin (120 μU ml^-1 ) with or without l-NMMA during the last 30 min. l-NMMA had no effect on basal skeletal muscle glucose uptake. The increase in muscle glucose uptake with insulin (57%) was significantly (P < 0.05) greater after prior contraction (140% increase). NOS inhibition during the contractions had no effect on this insulin-sensitizing effect of contraction, whereas NOS inhibition during insulin prevented the increase in skeletal muscle insulin sensitivity post-contraction. Soluble guanylate cyclase inhibition, protein kinase G (PKG) inhibition or cyclic nucleotide phosphodiesterase inhibition each had no effect on the insulin-sensitizing effect of prior contraction. In conclusion, NO is required for increases in insulin sensitivity several hours after contraction of mouse skeletal muscle via a cGMP/PKG independent pathway.

SR-BI Mediated Transcytosis of HDL in Brain Microvascular Endothelial Cells Is Independent of Caveolin, Clathrin, and PDZK1

The vascular endothelium supplying the brain exhibits very low paracellular and transcellular permeability and is a major constituent of the blood-brain barrier. High-density lipoprotein (HDL) crosses the blood-brain barrier by transcytosis, but technical limitations have made it difficult to elucidate its regulation. Using a combination of spinning-disc confocal and total internal reflection fluorescence microscopy, we examined the uptake and transcytosis of HDL by human primary brain microvascular endothelial cell monolayers. Using these approaches, we report that HDL internalization requires dynamin but not clathrin heavy chain and that its internalization and transcytosis are saturable. Internalized HDL partially co-localized with the scavenger receptor BI (SR-BI) and knockdown of SR-BI significantly attenuated HDL internalization. However, we observed that the adaptor protein PDZK1—which is critical to HDL-SR-BI signaling in other tissues—is not required for HDL uptake in these cells. Additionally, while these cells express caveolin, the abundance of caveolae in this tissue is negligible and we find that SR-BI and caveolin do not co-fractionate. Furthermore, direct silencing of caveolin-1 had no impact on the uptake of HDL. Finally, inhibition of endothelial nitric oxide synthase increased HDL internalization while increasing nitric oxide levels had no impact. Together, these data indicate that SR-BI-mediated transcytosis in brain microvascular endothelial cells is distinct from uptake and signaling pathways described for this receptor in other cell types.

The endothelial border to health: Mechanistic evidence of the hyperglycemic culprit of inflammatory disease acceleration

The endothelial cell (EC) layer constitutes a barrier that controls movements of fluid, solutes and cells between blood and tissue. Further, the endothelial layer regulates vascular tone and directs local humoral and cellular inflammatory processes. The strategic position makes it an important player for maintenance of health and for development of a number of diseases. Endothelial dysfunction is known to be an important component of type 2 diabetes, but is also assumed to be involved in many other diseases, for example, rheumatoid arthritis, inflammatory bowel disease, asthma, and cardiovascular diseases. We here suggest that the EC plays a pivotal role in disease pathophysiology through initiation, potentiation, and maintenance of several inflammatory mechanisms. Our contention is based on the observation that hyperglycemia-intermittent or sustained, local or systemic-is a major culprit for several endothelial dysfunctions. There is also mounting epidemiological evidence that dietary intake of refined sugars is important for the development of a number of diseases beyond obesity and type 2 diabetes. Various diseases involving inflammatory and immunological components are accelerated by hyperglycemic events because the endothelium transduces "high glucose" signaling into significant pathophysiological phenomena leading to reduced endothelial barrier function, compromised vascular tone regulation and inflammation (e.g., cytokine secretion and RAGE activation). In addition, endothelial extracellular proteins form epitopes for potential specific antibody formation upon interactions with reducing sugars. This paper reviews the endothelial metabolism, biology, inflammatory processes, physical barrier functions, and summarizes evidence that although stochastic in nature, endothelial responses to hyperglycemia are major contributors to disease pathophysiology. We present molecular and mechanistic evidence that both biological and physical barriers, protein function, specific immunity, and inflammatory processes are compromised by hyperglycemic events and thus, hyperglycemic events alone should be considered risk factors for numerous human diseases. © 2017 IUBMB Life, 69(3):148-161, 2017.

Unravelling the regulation of insulin transport across the brain endothelial cell

AIMS/HYPOTHESIS

For circulating insulin to act on the brain it must cross the blood-brain barrier (BBB). Remarkably little is known about how circulating insulin crosses the BBB's highly restrictive brain endothelial cells (BECs). Therefore, we examined potential mechanisms regulating BEC insulin uptake, signalling and degradation during BEC transcytosis, and how transport is affected by a high-fat diet (HFD) and by astrocyte activity.

METHODS

125I-TyrA14-insulin uptake and transcytosis, and the effects of insulin receptor (IR) blockade, inhibition of insulin signalling, astrocyte stimulation and an HFD were tested using purified isolated BECs (iBECs) in monoculture and co-cultured with astrocytes.

RESULTS

At physiological insulin concentrations, the IR, not the IGF-1 receptor, facilitated BEC insulin uptake, which required lipid raft-mediated endocytosis, but did not require insulin action on phosphoinositide-3-kinase (PI3K) or mitogen-activated protein kinase kinase (MEK). Feeding rats an HFD for 4 weeks decreased iBEC insulin uptake and increased NF-κB binding activity without affecting insulin PI3K signalling, IR expression or content, or insulin degrading enzyme expression. Using an in vitro BBB (co-culture of iBECs and astrocytes), we found insulin was not degraded during transcytosis, and that stimulating astrocytes with L-glutamate increased transcytosis, while inhibiting nitric oxide synthase decreased insulin transcytosis.

CONCLUSIONS/INTERPRETATION

Insulin crosses the BBB intact via an IR-specific, vesicle-mediated transport process in the BECs. HFD feeding, nitric oxide inhibition and astrocyte stimulation can regulate BEC insulin uptake and transcytosis.

Hypoxia in Obesity and Diabetes: Potential Therapeutic Effects of Hyperoxia and Nitrate

The prevalence of obesity and diabetes is increasing worldwide. Obesity and diabetes are associated with oxidative stress, inflammation, endothelial dysfunction, insulin resistance, and glucose intolerance. Obesity, a chronic hypoxic state that is associated with decreased nitric oxide (NO) bioavailability, is one of the main causes of type 2 diabetes. The hypoxia-inducible factor-1α (HIF-1α) is involved in the regulation of several genes of the metabolic pathways including proinflammatory adipokines, endothelial NO synthase (eNOS), and insulin signaling components. It seems that adipose tissue hypoxia and NO-dependent vascular and cellular dysfunctions are responsible for other consequences linked to obesity-related disorders. Although hyperoxia could reverse hypoxic-related disorders, it increases the production of reactive oxygen species (ROS) and decreases the production of NO. Nitrate can restore NO depletion and has antioxidant properties, and recent data support the beneficial effects of nitrate therapy in obesity and diabetes. Although it seems reasonable to combine hyperoxia and nitrate treatments for managing obesity/diabetes, the combined effects have not been investigated yet. This review discusses some aspects of tissue oxygenation and the potential effects of hyperoxia and nitrate interventions on obesity/diabetes management. It can be proposed that concomitant use of hyperoxia and nitrate is justified for managing obesity and diabetes.

Development of microdialysis methodology for interstitial insulin measurement in rodents

INTRODUCTION

Accurate assessment of muscle insulin sensitivity requires measurement of insulin concentration in interstitial fluid (ISF), but has proved difficult. We aimed to optimise measurement of ISF insulin concentrations in rat muscles in vivo using microdialysis.

METHODS

Factorial experimental design experiments were performed in vitro to determine optimal conditions for insulin recovery with microdialysis probes. These conditions were tested in vivo, adjusted appropriately and used in lean and obese Zucker rats to compare ISF insulin concentrations basally and during hyperinsulinaemic-euglycaemic (HE) clamp.

RESULTS

Optimal conditions in vivo were: a 100kDa microdialysis probe inserted in muscle, perfused with 1% BSA, 1.5mM glucose in 0.9% sodium chloride at 1μl/min. Samples were collected into siliconised glass microvials. As a reference for insulin, we established a protocol of inulin infusion, beginning at -80min and reaching equilibrium within 60min. HE clamp, beginning at 0min, increased ISF insulin concentration from 122±56 basally to 429±180pmol/l (P<0.05) in lean rats and from 643±165 to 1087±243pmol/l (P=0.07) in obese rats; ISF insulin concentrations were significantly higher throughout in obese rats. The difference between ISF and plasma insulin concentration (ISF:plasma ratio) was substantially higher in obese rats, but fell to similar values in obese and lean rats during HE clamp.

DISCUSSION

Optimising insulin recovery with microdialysis allowed accurate measurement of basal ISF insulin in muscle of lean and obese Zucker rats and indicates insulin transport across capillaries is impaired in obese rats, basally and during hyperinsulinaemia.

Dietary nitrite reverses features of postmenopausal metabolic syndrome induced by high-fat diet and ovariectomy in mice

Menopausal women are at greater risk of developing metabolic syndrome with reduced endothelial nitric oxide synthase (eNOS) activity. Hormone replacement therapy increases eNOS activity and normalizes some characteristics of metabolic syndrome. We hypothesized that nitric oxide (NO) supplementation should have a therapeutic effect on this syndrome. We examined the effect of dietary nitrite in a mouse model with postmenopausal metabolic syndrome induced by ovariectomy (OVX) and a high fat diet (HF). C57BL/6 female mice were divided into five groups, sham+normal fat diet (NF), sham+ HF, OVX+HF with or without sodium nitrite (50 mg and 150 mg/l) in the drinking water. Daily food intake and weekly body weight were monitored for 18 wk. OVX and HF significantly reduced plasma levels of nitrate/nitrite (NOx), and mice developed obesity with visceral hypertrophic adipocytes and increased transcriptional levels of monocyte chemoattractant protein-1, TNF-α, and IL-6 in visceral fat tissues. The proinflammatory state in the adipocytes provoked severe hepatosteatosis and insulin resistance in OVX+HF group compared with sham+NF group. However, dietary nitrite significantly suppressed adipocyte hypertrophy and transcriptions of proinflammatory cytokines in visceral fat in a dose-dependent manner. The improvement of visceral inflammatory state consequently reversed the hepatosteatosis and insulin resistance observed in OVX+HF mice. These results suggest that an endogenous NO defect might underlie postmenopausal metabolic syndrome and that dietary nitrite provides an alternative source of NO, subsequently compensating for metabolic impairments of this syndrome.

The impact of insulin resistance on the kidney and vasculature

Insulin resistance is a systemic disorder that affects many organs and insulin-regulated pathways. The disorder is characterized by a reduced action of insulin despite increased insulin concentrations (hyperinsulinaemia). The effects of insulin on the kidney and vasculature differ in part from the effects on classical insulin target organs. Insulin causes vasodilation by enhancing endothelial nitric oxide production through activation of the phosphatidylinositol 3-kinase pathway. In insulin-resistant states, this pathway is impaired and the mitogen-activated protein kinase pathway stimulates vasoconstriction. The action of insulin on perivascular fat tissue and the subsequent effects on the vascular wall are not fully understood, but the hepatokine fetuin-A, which is released by fatty liver, might promote the proinflammatory effects of perivascular fat. The strong association of salt-sensitive arterial hypertension with insulin resistance indicates an involvement of the kidney in the insulin resistance syndrome. The insulin receptor is expressed on renal tubular cells and podocytes and insulin signalling has important roles in podocyte viability and tubular function. Renal sodium transport is preserved in insulin resistance and contributes to the salt-sensitivity of blood pressure in hyperinsulinaemia. Therapeutically, renal and vascular insulin resistance can be improved by an integrated holistic approach aimed at restoring overall insulin sensitivity and improving insulin signalling.

S-nitrosylation of endogenous protein tyrosine phosphatases in endothelial insulin signaling

Nitric oxide (NO) exerts its biological function through S-nitrosylation of cellular proteins. Due to the labile nature of this modification under physiological condition, identification of S-nitrosylated residue in enzymes involved in signaling regulation remains technically challenging. The present study investigated whether intrinsic NO produced in endothelium-derived MS-1 cells response to insulin stimulation might target endogenous protein tyrosine phosphatases (PTPs). For this, we have developed an approach using a synthetic reagent that introduces a phenylacetamidyl moiety on S-nitrosylated Cys, followed by detection with anti-phenylacetamidyl Cys (PAC) antibody. Coupling with sequential blocking of free thiols with multiple iodoacetyl-based Cys-reactive chemicals, we employed this PAC-switch method to show that endogenous SHP-2 and PTP1B were S-nitrosylated in MS-1 cells exposed to insulin. The mass spectrometry detected a phenylacetamidyl moiety specifically present on the active-site Cys463 of SHP-2. Focusing on the regulatory role of PTP1B, we showed S-nitrosylation to be the principal Cys reversible redox modification in endothelial insulin signaling. The PAC-switch method in an imaging format illustrated that a pool of S-nitrosylated PTP1B was colocalized with activated insulin receptor to the cell periphery, and that such event was endothelial NO synthase (eNOS)-dependent. Moreover, ectopic expression of the C215S mutant of PTP1B that mimics the active-site Cys215 S-nitrosylated form restored insulin responsiveness in eNOS-ablated cells, which was otherwise insensitive to insulin stimulation. This work not only introduces a new method that explores the role of physiological NO in regulating signal transduction, but also highlights a positive NO effect on promoting insulin responsiveness through S-nitrosylation of PTP1B's active-site Cys215.

Cerebral cortex: a target and source of insulin?

Recent results suggest that insulin is synthesised by a subpopulation of neurons in the cerebral cortex and neural progenitor cells of the hippocampus. Supplementing the slow supply of insulin to the brain by pancreatic beta cells, the insulin locally released by neurons provides a rapid means of regulating local microcircuits, effectively modulating synaptic transmission and on-demand energy homeostasis of neural networks. Modulation of insulin production by brain neurons via glucagon-like peptide 1 (GLP-1) agonists might be useful in counteracting diabetes, obesity and neurodegenerative diseases. Replacement of lost pancreatic beta cells by autologous transplantation of insulin-producing neural progenitor cells could be a viable therapy for diabetes.

Sodium nitrate co-ingestion with protein does not augment postprandial muscle protein synthesis rates in older, type 2 diabetes patients

The age-related anabolic resistance to protein ingestion is suggested to be associated with impairments in insulin-mediated capillary recruitment and postprandial muscle tissue perfusion. The present study investigated whether dietary nitrate co-ingestion with protein improves muscle protein synthesis in older, type 2 diabetes patients. Twenty-four men with type 2 diabetes (72 ± 1 yr, 26.7 ± 1.4 m/kg(2) body mass index, 7.3 ± 0.4% HbA1C) received a primed continuous infusion of l-[ring-(2)H5]phenylalanine and l-[1-(13)C]leucine and ingested 20 g of intrinsically l-[1-(13)C]phenylalanine- and l-[1-(13)C]leucine-labeled protein with (PRONO3) or without (PRO) sodium nitrate (0.15 mmol/kg). Blood and muscle samples were collected to assess protein digestion and absorption kinetics and postprandial muscle protein synthesis rates. Upon protein ingestion, exogenous phenylalanine appearance rates increased in both groups (P < 0.001), resulting in 55 ± 2% and 53 ± 2% of dietary protein-derived amino acids becoming available in the circulation over the 5h postprandial period in the PRO and PRONO3 groups, respectively. Postprandial myofibrillar protein synthesis rates based on l-[ring-(2)H5]phenylalanine did not differ between groups (0.025 ± 0.004 and 0.021 ± 0.007%/h over 0-2 h and 0.032 ± 0.004 and 0.030 ± 0.003%/h over 2-5 h in PRO and PRONO3, respectively, P = 0.7). No differences in incorporation of dietary protein-derived l-[1-(13)C]phenylalanine into de novo myofibrillar protein were observed at 5 h (0.016 ± 0.002 and 0.014 ± 0.002 mole percent excess in PRO and PRONO3, respectively, P = 0.8). Dietary nitrate co-ingestion with protein does not modulate protein digestion and absorption kinetics, nor does it further increase postprandial muscle protein synthesis rates or the incorporation of dietary protein-derived amino acids into de novo myofibrillar protein in older, type 2 diabetes patients.

Endothelial Fcγ Receptor IIB Activation Blunts Insulin Delivery to Skeletal Muscle to Cause Insulin Resistance in Mice

Modest elevations in C-reactive protein (CRP) are associated with type 2 diabetes. We previously revealed in mice that increased CRP causes insulin resistance and mice globally deficient in the CRP receptor Fcγ receptor IIB (FcγRIIB) were protected from the disorder. FcγRIIB is expressed in numerous cell types including endothelium and B lymphocytes. Here we investigated how endothelial FcγRIIB influences glucose homeostasis, using mice with elevated CRP expressing or lacking endothelial FcγRIIB. Whereas increased CRP caused insulin resistance in mice expressing endothelial FcγRIIB, mice deficient in the endothelial receptor were protected. The insulin resistance with endothelial FcγRIIB activation was due to impaired skeletal muscle glucose uptake caused by attenuated insulin delivery, and it was associated with blunted endothelial nitric oxide synthase (eNOS) activation in skeletal muscle. In culture, CRP suppressed endothelial cell insulin transcytosis via FcγRIIB activation and eNOS antagonism. Furthermore, in knock-in mice harboring constitutively active eNOS, elevated CRP did not invoke insulin resistance. Collectively these findings reveal that by inhibiting eNOS, endothelial FcγRIIB activation by CRP blunts insulin delivery to skeletal muscle to cause insulin resistance. Thus, a series of mechanisms in endothelium that impairs insulin movement has been identified that may contribute to type 2 diabetes pathogenesis.

Reversibility of endothelial dysfunction in diabetes: role of polyphenols

The endothelium, a thin single sheet of endothelial cells, is a metabolically active layer that coats the inner surface of blood vessels and acts as an interface between the circulating blood and the vessel wall. The endothelium through the secretion of vasodilators and vasoconstrictors serves as a critical mediator of vascular homeostasis. During the development of the vascular system, it regulates cellular adhesion and vessel wall inflammation in addition to maintaining vasculogenesis and angiogenesis. A shift in the functions of the endothelium towards vasoconstriction, proinflammatory and prothrombic states characterise improper functioning of these cells, leading to endothelial dysfunction (ED), implicated in the pathogenesis of many diseases including diabetes. Major mechanisms of ED include the down-regulation of endothelial nitric oxide synthase levels, differential expression of vascular endothelial growth factor, endoplasmic reticulum stress, inflammatory pathways and oxidative stress. ED tends to be the initial event in macrovascular complications such as coronary artery disease, peripheral arterial disease, stroke and microvascular complications such as nephropathy, neuropathy and retinopathy. Numerous strategies have been developed to protect endothelial cells against various stimuli, of which the role of polyphenolic compounds in modulating the differentially regulated pathways and thus maintaining vascular homeostasis has been proven to be beneficial. This review addresses the factors stimulating ED in diabetes and the molecular mechanisms of natural polyphenol antioxidants in maintaining vascular homeostasis.

Muscle microvasculature's structural and functional specializations facilitate muscle metabolism

We review the evolving findings from studies that examine the relationship between the structural and functional properties of skeletal muscle's vasculature and muscle metabolism. Unique aspects of the organization of the muscle microvasculature are highlighted. We discuss the role of vasomotion at the microscopic level and of flowmotion at the tissue level as modulators of perfusion distribution in muscle. We then consider in some detail how insulin and exercise each modulate muscle perfusion at both the microvascular and whole tissue level. The central role of the vascular endothelial cell in modulating both perfusion and transendothelial insulin and nutrient transport is also reviewed. The relationship between muscle metabolic insulin resistance and the vascular action of insulin in muscle continues to indicate an important role for the microvasculature as a target for insulin action and that impairing insulin's microvascular action significantly affects body glucose metabolism.