Liver sinusoidal endothelial cells link hyperinsulinemia to hepatic insulin resistance

State of the Science on Brain Insulin Resistance and Cognitive Decline Due to Alzheimer's Disease

Type 2 diabetes mellitus (T2DM) is common and increasing in prevalence worldwide, with devastating public health consequences. While peripheral insulin resistance is a key feature of most forms of T2DM and has been investigated for over a century, research on brain insulin resistance (BIR) has more recently been developed, including in the context of T2DM and non-diabetes states. Recent data support the presence of BIR in the aging brain, even in non-diabetes states, and found that BIR may be a feature in Alzheimer's disease (AD) and contributes to cognitive impairment. Further, therapies used to treat T2DM are now being investigated in the context of AD treatment and prevention, including insulin. In this review, we offer a definition of BIR, and present evidence for BIR in AD; we discuss the expression, function, and activation of the insulin receptor (INSR) in the brain; how BIR could develop; tools to study BIR; how BIR correlates with current AD hallmarks; and regional/cellular involvement of BIR. We close with a discussion on resilience to both BIR and AD, how current tools can be improved to better understand BIR, and future avenues for research. Overall, this review and position paper highlights BIR as a plausible therapeutic target for the prevention of cognitive decline and dementia due to AD.

Deficiency of the lipid flippase ATP10A causes diet-induced dyslipidemia in female mice

Genetic association studies have linked ATP10A and closely related type IV P-type ATPases (P4-ATPases) to insulin resistance and vascular complications, such as atherosclerosis. ATP10A translocates phosphatidylcholine and glucosylceramide across cell membranes, and these lipids or their metabolites play important roles in signal transduction pathways regulating metabolism. However, the influence of ATP10A on lipid metabolism in mice has not been explored. Here, we generated gene-specific Atp10A knockout mice and show that Atp10A-/- mice fed a high-fat diet did not gain excess weight relative to wild-type littermates. However, Atp10A-/- mice displayed female-specific dyslipidemia characterized by elevated plasma triglycerides, free fatty acids and cholesterol, as well as altered VLDL and HDL properties. We also observed increased circulating levels of several sphingolipid species along with reduced levels of eicosanoids and bile acids. The Atp10A-/- mice also displayed hepatic insulin resistance without perturbations to whole-body glucose homeostasis. Thus, ATP10A has a sex-specific role in regulating plasma lipid composition and maintaining hepatic liver insulin sensitivity in mice.

Role of insulin action in the pathogenesis of diabetic complications

Among the various pathological conditions associated with type 2 diabetes, insulin resistance has long been reported to be a potent risk factor for diabetic complications. The liver, skeletal muscle, and adipose tissue are the major organs of action of insulin in systemic glucose metabolism, but insulin receptors and their downstream insulin signaling molecules are also constitutively expressed in vascular endothelial cells, vascular smooth muscle, and monocytes/macrophages. Forkhead box class O family member proteins (FoxOs) of transcription factors are essential regulators of cellular homeostasis, including glucose and lipid metabolism, oxidative stress response and redox signaling, cell cycle progression and apoptosis. In vascular endothelial cells, FoxOs strongly promote atherosclerosis via suppressing nitric oxide production and enhancing inflammatory responses. In liver sinusoidal endothelial cells, FoxOs induces hepatic insulin resistance by inducing nitration of insulin receptor in hepatocytes. Insulin resistance in adipose tissue limits capacity of lipid accumulation in adipose tissue, which promotes ectopic lipid accumulation and organ dysfunction in liver, vascular, and kidney. Modulation of insulin sensitivity in adipose tissue to induce healthy adipose expansion is expected to be a promising strategy for diabetic complications.

Hyperinsulinemia alters insulin receptor presentation and internalization in brain microvascular endothelial cells

Insulin receptors are internalized by endothelial cells to facilitate their physiological processes; however, the impact of hyperinsulinemia in brain endothelial cells is not known. Thus, the aim of this study was to elucidate the impact hyperinsulinemia plays on insulin receptor internalization through changes in phosphorylation, as well as the potential impact of protein tyrosine phosphatase 1B (PTP1B). Hippocampal microvessels were isolated from high-fat diet fed mice and assessed for insulin signaling activation, a process known to be involved with receptor internalization. Surface insulin receptors in brain microvascular endothelial cells were labelled to assess the role hyperinsulinemia plays on receptor internalization in response to stimulation, with and without the PTP1B antagonist, Claramine. Our results indicated that insulin receptor levels increased in tandem with decreased receptor signaling in the high-fat diet mouse microvessels. Insulin receptors of cells subjected to hyperinsulinemic treatment demonstrate splice variation towards decreased IR-A mRNA expression and demonstrate a higher membrane-localized proportion. This corresponded with decreased autophosphorylation at sites critical for receptor internalization and signaling. Claramine restored signaling and receptor internalization in cells treated with hyperinsulinemia. In conclusion, hyperinsulinemia impacts brain microvascular endothelial cell insulin receptor signaling and internalization, likely via alternative splicing and increased negative feedback from PTP1B.

Quantitative analysis methods for studying fenestrations in liver sinusoidal endothelial cells. A comparative study

Liver Sinusoidal Endothelial Cells (LSEC) line the hepatic vasculature providing blood filtration via transmembrane nanopores called fenestrations. These structures are 50-300 nm in diameter, which is below the resolution limit of a conventional light microscopy. To date, there is no standardized method of fenestration image analysis. With this study, we provide and compare three different approaches: manual measurements, a semi-automatic (threshold-based) method, and an automatic method based on user-friendly open source machine learning software. Images were obtained using three super resolution techniques - atomic force microscopy (AFM), scanning electron microscopy (SEM), and structured illumination microscopy (SIM). Parameters describing fenestrations such as diameter, area, roundness, frequency, and porosity were measured. Finally, we studied the user bias by comparison of the data obtained by five different users applying provided analysis methods.

New aspects of hepatic endothelial cells in physiology and nonalcoholic fatty liver disease

The liver is the central metabolic hub for carbohydrate, lipid, and protein metabolism. It is composed of four major types of cells, including hepatocytes, endothelial cells (ECs), Kupffer cells, and stellate cells. Hepatic ECs are highly heterogeneous in both mice and humans, representing the second largest population of cells in liver. The majority of them line hepatic sinusoids known as liver sinusoidal ECs (LSECs). The structure and biology of LSECs and their roles in physiology and liver disease were reviewed recently. Here, we do not give a comprehensive review of LSEC structure, function, or pathophysiology. Instead, we focus on the recent progress in LSEC research and other hepatic ECs in physiology and nonalcoholic fatty liver disease and other hepatic fibrosis-related conditions. We discuss several current areas of interest, including capillarization, scavenger function, autophagy, cellular senescence, paracrine effects, and mechanotransduction. In addition, we summarize the strengths and weaknesses of evidence for the potential role of endothelial-to-mesenchymal transition in liver fibrosis.

The vascular endothelium plays a role in insulin action

The endocrine system relies on the vasculature for delivery of hormones throughout the body, and the capillary microvasculature is the site where the hormones cross from the blood into the target tissue. Once considered an inert wall, various studies have now highlighted the functions of the capillary endothelium to regulate transport and therefore affect or maintain the interstitial environment. The role of the capillary may be clear in areas where there is a continuous endothelium, yet there also appears to be a role of endothelial cells in tissues with a sinusoidal structure. Here we focused on the most common endocrine disorder, diabetes, and several of the target organs associated with the disease, including skeletal muscle, liver and pancreas. However, it is important to note that the ability of hormones to cross the endothelium to reach their target tissue is a component of all endocrine functions. It is also a consideration in organs throughout the body and may have greater impact for larger hormones with target tissues containing a continuous endothelium. We noted that the blood levels do not always equal interstitial levels, which is what the cells are exposed to, and discussed how this may change in diseases such as obesity and insulin resistance. The capillary endothelium is, therefore, an essential and understudied aspect of endocrinology and metabolism that can be altered in disease, which may be an appropriate target for treatment.

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.

Hallmarks of Aging in the Liver

While the liver demonstrates remarkable resilience during aging, there is growing evidence that it undergoes all the cellular hallmarks of aging, which increases the risk of liver and systemic disease. The aging process in the liver is driven by alterations of the genome and epigenome that contribute to dysregulation of mitochondrial function and nutrient sensing pathways, leading to cellular senescence and low-grade inflammation. These changes promote multiple phenotypic changes in all liver cells (hepatocytes, liver sinusoidal endothelial, hepatic stellate and Küpffer cells) and impairment of hepatic function. In particular, age-related changes in the liver sinusoidal endothelial cells are a significant but under-recognized risk factor for the development of age-related cardiometabolic disease.

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Liver aging is driven by transcription and metabolic epigenome alterations.

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This leads to cellular senescence and low-grade inflammation.

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Hepatocyte, sinusoidal endothelial, stellate and Küpffer cells undergoes the hallmarks of aging.

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Each cell type demonstrates phenotypical cellular changes with age.

A fluorescent reporter assay of differential gene expression response to insulin in hepatocytes

Insulin regulates multiple hepatic metabolic pathways in a seemingly heterogeneous manner. To understand this heterogeneity, we hypothesized that different subpopulations of hepatocytes have different sensitivity to insulin. To test this hypothesis, we developed a fluorescent reporter in which the insulin-responsive fatty acid synthase (FAS) promoter drove expression of a time-dependent fluorescent protein ("timer") and characterized timer expression in flow-sorted cell populations. In Hepa1c1c7 and AML12 hepatocytes, we found that different cell populations express distinct timer fluorescence following insulin treatment, consistent with cellular heterogeneity in the response to insulin. RNA measurements indicated an enrichment of forkhead box O transcription factors in cells with a greater response to insulin. Moreover, we found evidence of increased Akt activation. These data are consistent with a heterogeneous cellular response to insulin and raise the possibility that these different subpopulations underlie the peculiar pathophysiology of hepatic insulin resistance.

Endothelial Cells: New Players in Obesity and Related Metabolic Disorders

Metabolic disorders such as obesity are accompanied by endothelial cell (EC) dysfunction and decreased vascular density. The current paradigm posits that metabolic alterations associated with obesity secondarily lead to EC dysfunction. However, in view of recent evidence reporting that EC dysfunction per se is able to cause metabolic dysregulation, this paradigm should be revisited and further elaborated. In this article we summarize current views and discuss evidence in favor of a causal role for ECs in systemic metabolic dysregulation. We also integrate and contextualize current research in a pathophysiological framework and discuss potential therapeutic strategies targeting angiogenesis to help to counteract obesity.

Insulin Action Research and the Future of Diabetes Treatment: The 2017 Banting Medal for Scientific Achievement Lecture

Diabetes is caused by combined abnormalities in insulin production and action. The pathophysiology of these defects has been studied extensively and is reasonably well understood. Their causes are elusive and their manifestations pleiotropic, likely reflecting the triple threat of genes, environment, and lifestyle. Treatment, once restricted to monotherapy with secretagogues or insulin, now involves complex combinations of expensive regimens that stem the progression but do not fundamentally alter the underlying causes of the disease. As advances in our understanding of insulin action and β-cell failure reach a critical stage, here I draw on lessons learned from our research on insulin regulation of gene expression and pancreatic β-cell dedifferentiation to address the question of how we can translate this exciting biology into mechanism-based interventions to reverse the course of diabetes.

Endothelial APLNR regulates tissue fatty acid uptake and is essential for apelin's glucose-lowering effects

Treatment of type 2 diabetes mellitus continues to pose an important clinical challenge, with most existing therapies lacking demonstrable ability to improve cardiovascular outcomes. The atheroprotective peptide apelin (APLN) enhances glucose utilization and improves insulin sensitivity. However, the mechanism of these effects remains poorly defined. We demonstrate that the expression of APLNR (APJ/AGTRL1), the only known receptor for apelin, is predominantly restricted to the endothelial cells (ECs) of multiple adult metabolic organs, including skeletal muscle and adipose tissue. Conditional endothelial-specific deletion of Aplnr (Aplnr^ECKO ) resulted in markedly impaired glucose utilization and abrogation of apelin-induced glucose lowering. Furthermore, we identified inactivation of Forkhead box protein O1 (FOXO1) and inhibition of endothelial expression of fatty acid (FA) binding protein 4 (FABP4) as key downstream signaling targets of apelin/APLNR signaling. Both the Apln^-/- and Aplnr^ECKO mice demonstrated increased endothelial FABP4 expression and excess tissue FA accumulation, whereas concurrent endothelial Foxo1 deletion or pharmacologic FABP4 inhibition rescued the excess FA accumulation phenotype of the Apln^-/- mice. The impaired glucose utilization in the Aplnr^ECKO mice was associated with excess FA accumulation in the skeletal muscle. Treatment of these mice with an FABP4 inhibitor abrogated these metabolic phenotypes. These findings provide mechanistic insights that could greatly expand the therapeutic repertoire for type 2 diabetes and related metabolic disorders.

Biochemical and cellular properties of insulin receptor signalling

The mechanism of insulin action is a central theme in biology and medicine. In addition to the rather rare condition of insulin deficiency caused by autoimmune destruction of pancreatic β-cells, genetic and acquired abnormalities of insulin action underlie the far more common conditions of type 2 diabetes, obesity and insulin resistance. The latter predisposes to diseases ranging from hypertension to Alzheimer disease and cancer. Hence, understanding the biochemical and cellular properties of insulin receptor signalling is arguably a priority in biomedical research. In the past decade, major progress has led to the delineation of mechanisms of glucose transport, lipid synthesis, storage and mobilization. In addition to direct effects of insulin on signalling kinases and metabolic enzymes, the discovery of mechanisms of insulin-regulated gene transcription has led to a reassessment of the general principles of insulin action. These advances will accelerate the discovery of new treatment modalities for diabetes.

Endothelial Cell Metabolism

Endothelial cells (ECs) are more than inert blood vessel lining material. Instead, they are active players in the formation of new blood vessels (angiogenesis) both in health and (life-threatening) diseases. Recently, a new concept arose by which EC metabolism drives angiogenesis in parallel to well-established angiogenic growth factors (e.g., vascular endothelial growth factor). 6-Phosphofructo-2-kinase/fructose-2,6-bisphosphatase-3-driven glycolysis generates energy to sustain competitive behavior of the ECs at the tip of a growing vessel sprout, whereas carnitine palmitoyltransferase 1a-controlled fatty acid oxidation regulates nucleotide synthesis and proliferation of ECs in the stalk of the sprout. To maintain vascular homeostasis, ECs rely on an intricate metabolic wiring characterized by intracellular compartmentalization, use metabolites for epigenetic regulation of EC subtype differentiation, crosstalk through metabolite release with other cell types, and exhibit EC subtype-specific metabolic traits. Importantly, maladaptation of EC metabolism contributes to vascular disorders, through EC dysfunction or excess angiogenesis, and presents new opportunities for anti-angiogenic strategies. Here we provide a comprehensive overview of established as well as newly uncovered aspects of EC metabolism.

Forkhead box class O family member proteins: The biology and pathophysiological roles in diabetes

Forkhead box class O family member proteins (FoxOs) of transcription factors are essential regulators of cellular homeostasis, including glucose and lipid metabolism, oxidative stress response and redox signaling, cell cycle progression, and apoptosis. Altered FoxO1 expression and activity have been associated with glucose intolerance, dyslipidemia and complications of diabetes. In the liver, they direct carbons to glucose or lipid utilization, thus providing a unifying mechanism for the two abnormalities of the diabetic liver: excessive glucose production, and increased lipid synthesis and secretion. In the pancreas, FoxO1 is necessary to maintain β-cell differentiation, and could be promising targets for β-cell regeneration. In endothelial cells, FoxOs strongly promote atherosclerosis through suppressing nitric oxide production and enhancing inflammatory responses. In the present review, we summarize the basic biology and pathophysiological significance of FoxOs in diabetes.

Antifibrotic effect of pirfenidone in a mouse model of human nonalcoholic steatohepatitis

Non-alcoholic steatohepatitis (NASH) is characterized by steatosis with lobular inflammation and hepatocyte injury. Pirfenidone (PFD) is an orally bioavailable pyridone derivative that has been clinically used for the treatment of idiopathic pulmonary fibrosis. However, it remains unknown whether PFD improves liver fibrosis in a mouse model with human NASH-like phenotypes. In this study, we employed melanocortin 4 receptor-deficient (MC4R-KO) mice as a mouse model with human NASH-like phenotypes to elucidate the effect and action mechanisms of PFD on the development of NASH. PFD markedly attenuated liver fibrosis in western diet (WD)-fed MC4R-KO mice without affecting metabolic profiles or steatosis. PFD prevented liver injury and fibrosis associated with decreased apoptosis of liver cells in WD-fed MC4R-KO mice. Pretreatment of PFD inhibited the tumor necrosis factor-α (TNF-α)-induced liver injury and fibrogenic responses associated with decreased apoptosis of liver cells in wild-type mice. PFD also prevented TNF-α-induced hepatocyte apoptosis in vitro with reduced activation of caspase-8 and -3. This study provides evidence for the antifibrotic effect of PFD in a mouse model of human NASH. The data of this study highlight hepatocyte apoptosis as a potential therapeutic target, and suggest that PFD can be repositioned as an antifibrotic drug for human NASH.

Synthesis of substituted 2H-benzo[e]indazole-9-carboxylate as a potent antihyperglycemic agent that may act through IRS-1, Akt and GSK-3β pathways

Based on high throughput screening of our chemical library, we identified two 4,5-dihydro-2H-benzo[e]indazole derivatives (5d and 5g), which displayed a significant effect on glucose uptake in L6 skeletal muscle cells. Based on these lead molecules, a series of benzo[e]indazole derivatives were prepared. Among all the synthesized dihydro-2H-benzo[e]indazoles, 8-(methylthio)-2-phenyl-6-p-tolyl-4,5-dihydro-2H-benzo[e]indazole-9-carboxylate (5e) showed significant glucose uptake stimulation in L6 skeletal muscle cells, even better than lead compounds. Additionally, 5e decreased glucagon-induced glucose release in HepG2 hepatoma cells. The 2H-benzo[e]indazole 5e exerted an antihyperglycemic effect in normal, sucrose challenged streptozotocin-induced diabetic rats and type 2 diabetic db/db mice. Treatment with 5e at a dose of 30 mg kg-1 in db/db mice caused a significant decrease in triglyceride and total cholesterol levels and increased the HDL-C level in a significant manner. The mechanistic studies revealed that the 2H-benzo[e]indazole 5e significantly stimulated insulin-induced signaling at the level of IRS-1, Akt and GSK-3β in L6 skeletal muscle cells, possibly by inhibiting protein tyrosine phosphatase-1B. This new 2H-benzo[e]indazole derivative has potential for the treatment of diabetes with improved lipid profile.

Mildly compromised tetrahydrobiopterin cofactor biosynthesis due to Pts variants leads to unusual body fat distribution and abdominal obesity in mice

Tetrahydrobiopterin (BH4) is an essential cofactor for the aromatic amino acid hydroxylases, alkylglycerol monooxygenase, and nitric oxide synthases (NOS). Inborn errors of BH4 metabolism lead to severe insufficiency of brain monoamine neurotransmitters while augmentation of BH4 by supplementation or stimulation of its biosynthesis is thought to ameliorate endothelial NOS (eNOS) dysfunction, to protect from (cardio-) vascular disease and/or prevent obesity and development of the metabolic syndrome. We have previously reported that homozygous knock-out mice for the 6-pyruvolytetrahydropterin synthase (PTPS; Pts-ko/ko) mice with no BH4 biosynthesis die after birth. Here we generated a Pts-knock-in (Pts-ki) allele expressing the murine PTPS-p.Arg15Cys with low residual activity (15% of wild-type in vitro) and investigated homozygous (Pts-ki/ki) and compound heterozygous (Pts-ki/ko) mutants. All mice showed normal viability and depending on the severity of the Pts alleles exhibited up to 90% reduction of PTPS activity concomitant with neopterin elevation and mild reduction of total biopterin while blood L-phenylalanine and brain monoamine neurotransmitters were unaffected. Yet, adult mutant mice with compromised PTPS activity (i.e., Pts-ki/ko, Pts-ki/ki or Pts-ko/wt) had increased body weight and elevated intra-abdominal fat. Comprehensive phenotyping of Pts-ki/ki mice revealed alterations in energy metabolism with proportionally higher fat content but lower lean mass, and increased blood glucose and cholesterol. Transcriptome analysis indicated changes in glucose and lipid metabolism. Furthermore, differentially expressed genes associated with obesity, weight loss, hepatic steatosis, and insulin sensitivity were consistent with the observed phenotypic alterations. We conclude that reduced PTPS activity concomitant with mildly compromised BH4-biosynthesis leads to abnormal body fat distribution and abdominal obesity at least in mice. This study associates a novel single gene mutation with monogenic forms of obesity.

MicroRNA-181b Improves Glucose Homeostasis and Insulin Sensitivity by Regulating Endothelial Function in White Adipose Tissue

RATIONALE

The pathogenesis of insulin resistance involves dysregulated gene expression and function in multiple cell types, including endothelial cells (ECs). Post-transcriptional mechanisms such as microRNA-mediated regulation of gene expression could affect insulin action by modulating EC function.

OBJECTIVE

To determine whether microRNA-181b (miR-181b) affects the pathogenesis of insulin resistance by regulating EC function in white adipose tissue during obesity.

METHODS AND RESULTS

MiR-181b expression was reduced in adipose tissue ECs of obese mice, and rescue of miR-181b expression improved glucose homeostasis and insulin sensitivity. Systemic intravenous delivery of miR-181b robustly accumulated in adipose tissue ECs, enhanced insulin-mediated Akt phosphorylation at Ser473, and reduced endothelial dysfunction, an effect that shifted macrophage polarization toward an M2 anti-inflammatory phenotype in epididymal white adipose tissue. These effects were associated with increased endothelial nitric oxide synthase and FoxO1 phosphorylation as well as nitric oxide activity in epididymal white adipose tissue. In contrast, miR-181b did not affect insulin-stimulated Akt phosphorylation in liver and skeletal muscle. Bioinformatics and gene profiling approaches revealed that Pleckstrin homology domain leucine-rich repeat protein phosphatase, a phosphatase that dephosphorylates Akt at Ser473, is a novel target of miR-181b. Knockdown of Pleckstrin homology domain leucine-rich repeat protein phosphatase increased Akt phosphorylation at Ser473 in ECs, and phenocopied miR-181b's effects on glucose homeostasis, insulin sensitivity, and inflammation of epididymal white adipose tissue in vivo. Finally, ECs from diabetic subjects exhibited increased Pleckstrin homology domain leucine-rich repeat protein phosphatase expression.

CONCLUSIONS

Our data underscore the importance of adipose tissue EC function in controlling the development of insulin resistance. Delivery of miR-181b or Pleckstrin homology domain leucine-rich repeat protein phosphatase inhibitors may represent a new therapeutic approach to ameliorate insulin resistance by improving adipose tissue endothelial Akt-endothelial nitric oxide synthase-nitric oxide signaling.

The new biology of diabetes

Until recently, type 2 diabetes was seen as a disease caused by an impaired ability of insulin to promote the uptake and utilisation of glucose. Work on forkhead box protein O (FOXO) transcription factors revealed new aspects of insulin action that have led us to articulate a liver- and beta cell-centric narrative of diabetes pathophysiology and treatment. FOXO integrate a surprisingly diverse subset of biological functions to promote metabolic flexibility. In the liver, they controls the glucokinase/glucose-6-phosphatase switch and bile acid pool composition, directing carbons to glucose or lipid utilisation, thus providing a unifying mechanism for the two abnormalities of the diabetic liver: excessive glucose production and increased lipid synthesis and secretion. Moreover, FOXO are necessary to maintain beta cell differentiation, and diabetes development is associated with a gradual loss of FOXO function that brings about beta cell dedifferentiation. We proposed that dedifferentiation is the main cause of beta cell failure and conversion into non-beta endocrine cells, and that treatment should restore beta cell differentiation. Our studies investigating these proposals have revealed new dimensions to the pathophysiology of diabetes that can be leveraged to design new therapies.

Discovery of biaryl-4-carbonitriles as antihyperglycemic agents that may act through AMPK-p38 MAPK pathway

A series of functionalized biaryl-4-carbonitriles was synthesized in three steps and evaluated for PTP-1B inhibitory activity. Among the synthesized compounds, four biaryls 6a-d showed inhibition (IC50 58-75 μM) against in vitro PTP-1B assay possibly due to interaction with amino acid residues Lys120, Tyr46 through hydrogen bonding and aromatic-aromatic interactions, respectively. Two biaryl-4-carbonitriles 6b and 6c showed improved glucose tolerance, fasting as well as postprandial blood glucose, serum total triglycerides, and increased high-density lipoprotein-cholesterol in SLM, STZ, STZ-S and C57BL/KsJ-db/db animal models. The bioanalysis of 4'-bromo-2,3-dimethyl-5-(piperidin-1-yl)biphenyl-4-carbonitrile (6b) revealed that like insulin, it increased 2-deoxyglucose uptake in skeletal muscle cells (L6 and C2C12 myotubes). The compound 6b significantly up-regulated the genes related to the insulin signaling pathways like AMPK, MAPK including glucose transporter-4 (GLUT-4) gene in muscle tissue of C57BL/KsJ-db/db mice. Furthermore, it was observed that the compound 6b up-regulated PPARα, UCP2 and HNF4α, which are key regulator of glucose, lipid, and fatty acid metabolism. Western blot analysis of the compound 6b showed that it significantly increased the phosphorylation of AMPK and p38 MAPK and ameliorated glucose uptake in C57BL/KsJ-db/db mice through the AMPK-p38 MAPK pathway.

Activation of hepatic inflammatory pathways by catecholamines is associated with hepatic insulin resistance in male ischemic stroke rats

Patients who experience acute ischemic stroke may develop hyperglycemia, even in the absence of diabetes. In the current study we determined the effects of acute stroke on hepatic insulin signaling, TNF-α expression, endoplasmic reticulum (ER) stress, the activities of c-Jun N-terminal kinase (JNK), inhibitor κB kinase β (IKK-β), and nuclear factor-κB (NF-κB) pathways. Rats with cerebral ischemia developed higher blood glucose, and insulin levels, and insulin resistance index, as well as hepatic gluconeogenic enzyme expression compared with the sham-treated group. The hepatic TNF-α mRNA and protein levels were elevated in stroke rats in association with increased ER stress, phosphorylation of JNK1/2 and IKK-β proteins, IκB/NF-κB signaling, and phosphorylation of insulin receptor-1 (IRS-1) at serine residue. The basal and insulin-stimulated tyrosine phosphorylation of IRS-1 and AKT proteins was reduced. In addition, acute stroke increased circulating catecholamines in association with hepatic adrenergic signaling activation. After administration of a nonselective β-adrenergic receptor blocker (propranolol) before induction of cerebral ischemic injury, hepatic adrenergic transduction, TNF-α expression, ER stress, and the activation of the JNK1/2, IKK-β, and NF-κB pathways, and serine phosphorylation of IRS-1 were all attenuated. In contrast, the phosphorylated IRS-1 at tyrosine site and AKT levels were partially restored with improved poststroke hyperglycemia and insulin resistance index. These results suggest that acute ischemic stroke can activate proinflammatory pathways in the liver by the catecholamines and is associated with the development of hepatic insulin resistance.

New insights into insulin action and resistance in the vasculature

Two-thirds of adults in the United States are overweight or obese, and another 26 million have type 2 diabetes. Decreased insulin sensitivity in cardiovascular tissue is an underlying abnormality in these individuals. Insulin metabolic signaling increases endothelial cell nitric oxide (NO) production. Impaired vascular insulin sensitivity is an early defect leading to impaired vascular relaxation. In overweight and obese persons, as well as in those with hypertension, systemic and vascular insulin resistance often occur in conjunction with activation of the cardiovascular tissue renin-angiotensin-aldosterone system (RAAS). Activated angiotensin II type 1 receptor and mineralocorticoid receptor signaling promote the development of vascular insulin resistance and impaired endothelial NO-mediated relaxation. Research in this area has implicated excessive serine phosphorylation and proteasomal degradation of the docking protein insulin receptor substrate and enhanced signaling through hybrid insulin/insulin-like growth factor receptor as important mechanisms underlying RAAS impediment of downstream vascular insulin metabolic signaling. This review will present recent evidence supporting the notion that RAAS signaling represents a potential pathway for the development of vascular insulin resistance and impaired endothelial-mediated vasodilation.

Reduced hepatic eNOS phosphorylation is associated with NAFLD and type 2 diabetes progression and is prevented by daily exercise in hyperphagic OLETF rats

We tested the hypothesis that nonalcoholic fatty liver disease (NAFLD) is associated with reduced hepatic endothelial nitric oxide synthase (eNOS) activation status via S1177 phosphorylation (p-eNOS) and is prevented by daily voluntary wheel running (VWR). Hyperphagic Otsuka Long-Evans Tokushima Fatty (OLETF) rats, an established model of obesity, type 2 diabetes (T2D) and NAFLD, and normophagic controls [Long-Evans Tokushima Otsuka (LETO)] were studied at 8, 20, and 40 wk of age. Basal hepatic eNOS phosphorylation (p-eNOS/eNOS) was similar between LETO and OLETFs with early hepatic steatosis (8 wk of age) and advanced steatosis, hyperinsulinemia, and hyperglycemia (20 wk of age). In contrast, hepatic p-eNOS/eNOS was significantly lower (P < 0.05) in OLETF rats with T2D advancement and the transition to more advanced NAFLD with inflammation and fibrosis [increased tumor necrosis factor-α (TNF-α), CD68, and CD163 mRNA expression; 40 wk of age]. Reduced hepatic eNOS activation status in 40-wk OLETF rats was significantly correlated with reduced p-Akt/Akt (r = 0.73, P < 0.05), reduced serum insulin (r = 0.59, P < 0.05), and elevated serum glucose (r = -0.78, P < 0.05), suggesting a link between impaired glycemic control and altered hepatic nitric oxide metabolism. VWR by OLETF rats, in conjunction with NAFLD and T2D prevention, normalized p-eNOS/eNOS and p-Akt/Akt to LETO levels. Basal activation of hepatic eNOS and Akt are maintained until advanced NAFLD and T2D development in obese OLETF rats. The prevention of this reduction by VWR may result from maintained insulin sensitivity and glycemic control.