From mice to men: insights into the insulin resistance syndromes

PHLPP phosphatases as a therapeutic target in insulin resistance-related diseases

INTRODUCTION

Pleckstrin homology domain leucine-rich repeat protein phosphatases (PHLPPs), originally identified as Akt kinase hydrophobic motif specific phosphatases, have subsequently been shown to regulate several molecules recurring within the insulin signaling pathway. This observation suggests that PHLPP phosphatases may have a clinically relevant role in the pathogenesis of insulin resistance-related diseases and may thus represent suitable targets for the treatment of these conditions.

AREAS COVERED

The literature pertaining to PHLPPs substrates is reviewed herein, along with information on the molecular players involved in regulating the activity and expression of PHLPP phosphatases. In the present review, knowledge of genetic variants in the genes that encode for PHLPP isozymes and the surrounding regulatory regions is also summarized. In addition, data from the studies addressing the role of PHLPPs in insulin resistance-related disorders and from those investigating the possibility to manipulate these phosphatases for therapeutic purposes are presented.

EXPERT OPINION

A number of issues should be resolved before PHLPPs are pursued as therapeutic targets including: the mechanisms regulating the specificity of PHLPP isozymes; the possibility of differentially regulating PHLPP family members and the possible impact of PHLPPs modulation on the risk of cancer.

Hypomagnesemia in Type 2 Diabetes: A Vicious Circle?

Over the past decades, hypomagnesemia (serum Mg(2+) <0.7 mmol/L) has been strongly associated with type 2 diabetes mellitus (T2DM). Patients with hypomagnesemia show a more rapid disease progression and have an increased risk for diabetes complications. Clinical studies demonstrate that T2DM patients with hypomagnesemia have reduced pancreatic β-cell activity and are more insulin resistant. Moreover, dietary Mg(2+) supplementation for patients with T2DM improves glucose metabolism and insulin sensitivity. Intracellular Mg(2+) regulates glucokinase, KATP channels, and L-type Ca(2+) channels in pancreatic β-cells, preceding insulin secretion. Moreover, insulin receptor autophosphorylation is dependent on intracellular Mg(2+) concentrations, making Mg(2+) a direct factor in the development of insulin resistance. Conversely, insulin is an important regulator of Mg(2+) homeostasis. In the kidney, insulin activates the renal Mg(2+) channel transient receptor potential melastatin type 6 that determines the final urinary Mg(2+) excretion. Consequently, patients with T2DM and hypomagnesemia enter a vicious circle in which hypomagnesemia causes insulin resistance and insulin resistance reduces serum Mg(2+) concentrations. This Perspective provides a systematic overview of the molecular mechanisms underlying the effects of Mg(2+) on insulin secretion and insulin signaling. In addition to providing a review of current knowledge, we provide novel directions for future research and identify previously neglected contributors to hypomagnesemia in T2DM.

The Role of HMGB1 in the Pathogenesis of Type 2 Diabetes

Significance. With an alarming increase in recent years, diabetes mellitus has become a global challenge. Despite advances in treatment of diabetes mellitus, currently, medications available are unable to control the progression of diabetes and its complications. Growing evidence suggests that inflammation is an important pathogenic mediator in the development of diabetes mellitus. The perspectives including suggestions for new therapies involving the shift from metabolic stress to inflammation should be taken into account. Critical Issues. High-mobility group box 1 (HMGB1), a nonhistone nuclear protein regulating gene expression, was rediscovered as an endogenous danger signal molecule to trigger inflammatory responses when released into extracellular milieu in the late 1990s. Given the similarities of inflammatory response in the development of T2D, we will discuss the potential implication of HMGB1 in the pathogenesis of T2D. Importantly, we will summarize and renovate the role of HMGB1 and HMGB1-mediated inflammatory pathways in adipose tissue inflammation, insulin resistance, and islet dysfunction. Future Directions. HMGB1 and its downstream receptors RAGE and TLRs may serve as potential antidiabetic targets. Current and forthcoming projects in this territory will pave the way for prospective approaches targeting the center of HMGB1-mediated inflammation to improve T2D and its complications.

Early inflammation-associated factors blunt sterol regulatory element-binding proteins-1-mediated lipogenesis in high-fat diet-fed APPSWE /PSEN1dE9 mouse model of Alzheimer's disease

Alzheimer's disease (AD) patients have an increased incidence of Type 2 diabetes (T2D); however, the underlying mechanisms are not well understood. Since AD is considered a multifactorial disease that affects both the central nerves system and periphery and the dysregulation of hepatic lipid and glucose metabolism play critical roles in T2D, we, therefore, aim to explore the influence of AD genotype on the liver during the progress of high-fat diet (HFD)-induced T2D. Fourteen-week-old female APP_SWE /PSEN1dE9 (AD) mice and age-, gender-matched wild-type controls C57BL/6J (WT) mice were fed a HFD (45% kcal fat content) or a standard chow diet (chow, 12% kcal fat content) for 22 weeks. The effects of diet and genotype were analyzed. Mouse primary hepatocytes were used to decipher the underlying mechanisms. HFD induced significantly higher body weight gain, more severe hyperglycemia, glucose intolerance and hepatic insulin resistance in AD mice than in WT mice. However, AD mice showed reduced HFD-induced hepatic steatosis, and SREBP-1-mediated lipogenic signaling was activated by HFD in WT mice but not in AD mice. In addition, 14-week-old AD mice exhibited higher expression of NF-κB p65, p-JNK and p-p38MAPK, as well as higher hepatic and serum contents of IL-6 and TNFα. In mouse primary hepatocyte cultures, IL-6 and TNFα inhibited high-glucose plus insulin-induced activation of SREBP-1-mediated lipogenic signaling and biosynthesis of non-esterified fatty acid and triglyceride. Early inflammation-associated factors most likely diminish HFD-induced hepatic lipid deposition by inhibiting SREBP-1-mediated de novo lipogenesis, thus driving substrate flux to glucose production for hyperglycemia and hepatic insulin resistance in T2D development. Alzheimer's disease (AD) is a multifactorial disease affecting both central nerves system and periphery organs. Therefore, we explored the hepatic susceptibility to high-fat diet (HFD) in AD mice. We found that AD mice were resistant to HFD-induced hepatic fat accumulation in spite of more severe obesity, hyperglycemia, glucose intolerance and hepatic insulin resistance. Mechanistically, AD mice exhibited hepatic inflammation at an early stage, which inhibited sterol regulatory element-binding proteins-1 (SREBP-1)-mediated de novo lipogenesis, and most likely drive substrate flux to glucose production for hyperglycemia and hepatic insulin resistance. Cover Image for this issue: doi: 10.1111/jnc.13306.

Unsuppressed lipolysis in adipocytes is linked with enhanced gluconeogenesis and altered bile acid physiology in Insr(P1195L/+) mice fed high-fat-diet

High-fat diet (HFD) triggers insulin resistance and diabetes mellitus, but their link remains unclear. Characterization of overt hyperglycemia in insulin receptor mutant (Insr(P1195L/+)) mice exposed to HFD (Insr(P1195L/+)/HFD mice) revealed increased glucose-6-phosphatase (G6pc) expression in liver and increased gluconeogenesis from glycerol. Lipolysis in white adipose tissues (WAT) and lipolysis-induced blood glucose rise were increased in Insr(P1195L/+)/HFD mice, while wild-type WAT transplantation ameliorated the hyperglycemia and the increased G6pc expression. We found that the expressions of genes involved in bile acid (BA) metabolism were altered in Insr(P1195L/+)/HFD liver. Among these, the expression of Cyp7a1, a BA synthesis enzyme, was insulin-dependent and was markedly decreased in Insr(P1195L/+)/HFD liver. Reduced Cyp7a1 expression in Insr(P1195L/+)/HFD liver was rescued by WAT transplantation, and the expression of Cyp7a1 was suppressed by glycerol administration in wild-type liver. These findings suggest that unsuppressed lipolysis in adipocytes elicited by HFD feeding is linked with enhanced gluconeogenesis from glycerol and with alterations in BA physiology in Insr(P1195L/+)/HFD liver.

Contraction stimulates muscle glucose uptake independent of atypical PKC

Exercise increases skeletal muscle glucose uptake, but the underlying mechanisms are only partially understood. The atypical protein kinase C (PKC) isoforms λ and ζ (PKC‐λ/ζ) have been shown to be necessary for insulin‐, AICAR‐, and metformin‐stimulated glucose uptake in skeletal muscle, but not for treadmill exercise‐stimulated muscle glucose uptake. To investigate if PKC‐λ/ζ activity is required for contraction‐stimulated muscle glucose uptake, we used mice with tibialis anterior muscle‐specific overexpression of an empty vector (WT), wild‐type PKC‐ζ (PKC‐ζ^WT), or an enzymatically inactive T410A‐PKC‐ζ mutant (PKC‐ζ^T410A). We also studied skeletal muscle‐specific PKC‐λ knockout (MλKO) mice. Basal glucose uptake was similar between WT, PKC‐ζ^WT, and PKC‐ζ^T410A tibialis anterior muscles. In contrast, in situ contraction‐stimulated glucose uptake was increased in PKC‐ζ^T410A tibialis anterior muscles compared to WT or PKC‐ζ^WT tibialis anterior muscles. Furthermore, in vitro contraction‐stimulated glucose uptake was greater in soleus muscles of MλKO mice than WT controls. Thus, loss of PKC‐λ/ζ activity increases contraction‐stimulated muscle glucose uptake. These data clearly demonstrate that PKC‐λ/ζ activity is not necessary for contraction‐stimulated glucose uptake.

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

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

Decreased adiposity and enhanced glucose tolerance in shikonin treated mice

OBJECTIVE

Obesity represents a major public health problem, and identifying natural compounds that modulate energy balance and glucose homeostasis is of interest for combating obesity and its associated disorders. The naphthoquinone shikonin has diverse beneficial properties including anti-inflammatory, anti-oxidant, and anti-microbial effects. The objective of this study is to investigate the effects of shikonin on adiposity and glucose homeostasis.

METHODS

The metabolic effects of shikonin treatment on mice fed regular chow or challenged with a high-fat diet (HFD) were determined.

RESULTS

Shikonin treated mice fed regular chow exhibited improved glucose tolerance compared with controls. In addition, shikonin treated mice fed HFD displayed decreased weight gain and resistance to HFD-induced glucose intolerance. Further, shikonin treatment decreased HFD-induced hepatic dyslipidemia. These findings correlated with enhanced hepatic insulin signaling in shikonin treated mice as evidenced by increased tyrosyl phosphorylation of the insulin receptor and enhanced downstream signaling.

CONCLUSIONS

These studies identify shikonin as a potential regulator of systemic glucose tolerance, energy balance, and adiposity in vivo.

Insulin Dissociates the Effects of Liver X Receptor on Lipogenesis, Endoplasmic Reticulum Stress, and Inflammation

Diabetes is characterized by increased lipogenesis as well as increased endoplasmic reticulum (ER) stress and inflammation. The nuclear hormone receptor liver X receptor (LXR) is induced by insulin and is a key regulator of lipid metabolism. It promotes lipogenesis and cholesterol efflux, but suppresses endoplasmic reticulum stress and inflammation. The goal of these studies was to dissect the effects of insulin on LXR action. We used antisense oligonucleotides to knock down Lxrα in mice with hepatocyte-specific deletion of the insulin receptor and their controls. We found, surprisingly, that knock-out of the insulin receptor and knockdown of Lxrα produced equivalent, non-additive effects on the lipogenic genes. Thus, insulin was unable to induce the lipogenic genes in the absence of Lxrα, and LXRα was unable to induce the lipogenic genes in the absence of insulin. However, insulin was not required for LXRα to modulate the phospholipid profile, or to suppress genes in the ER stress or inflammation pathways. These data show that insulin is required specifically for the lipogenic effects of LXRα and that manipulation of the insulin signaling pathway could dissociate the beneficial effects of LXR on cholesterol efflux, inflammation, and ER stress from the negative effects on lipogenesis.

Osthole improves glucose and lipid metabolism via modulation of PPARα/γ-mediated target gene expression in liver, adipose tissue, and skeletal muscle in fatty liver rats

CONTEXT

Osthole may be a dual agonist of peroxisome proliferator-activated receptors (PPAR) α/γ and ameliorate the insulin resistance (IR), but its mechanisms are not yet understood completely.

OBJECTIVE

We investigated the effects of osthole on PPARα/γ-mediated target genes involved in glucose and lipid metabolism in liver, adipose tissue, and skeletal muscle in fatty liver and IR rats.

MATERIALS AND METHODS

The rat model was established by orally feeding high-fat and high-sucrose emulsion for 9 weeks. The experimental rats were treated with osthole 5-10 mg/kg by gavage after feeding the emulsion for 6 weeks, and were sacrificed 4 weeks after administration.

RESULTS

After treatment with osthole 5-10 mg/kg for 4 weeks, the lipid levels in serum and liver were decreased by 37.9-67.2% and 31.4-38.5% for triglyceride, 33.1-47.5% and 28.5-31.2% for free fatty acid, respectively, the fasting blood glucose, fasting serum insulin, and homeostasis model assessment of IR were also decreased by 17.2-22.7%, 25.9-26.7%, and 37.5-42.8%, respectively. Osthole treatment might simultaneously decrease the sterol regulatory element binding protein-1c, diacylglycerol acyltransferase, and fatty acid synthase mRNA expressions in liver and adipose tissue, and increase the carnitine palmitoyltransferase-1A mRNA expression in liver and glucose transporter-4 mRNA expression in skeletal muscle, especially in the osthole 10 mg/kg group (p < 0.01).

DISCUSSION AND CONCLUSION

Osthole can improve glucose and lipid metabolism in fatty liver and IR rats, and its mechanisms may be associated with synergic modulation of PPARα/γ-mediated target genes involved in glucose and lipid metabolism in liver, adipose tissue, and skeletal muscle.

Conservation of gene and tissue networks regulating insulin signalling in flies and vertebrates

Fruit flies have emerged as a powerful tool to investigate metabolism. Not only are gene structures and gene networks that control metabolism conserved through evolution, but the interactions among organs to store and process metabolites have strong similarities between flies and humans. Accordingly, the Drosophila system has the potential to address human disorders associated with metabolic dysfunction including obesity, type 2 diabetes and lipotoxicity.

Paradoxical effect of rapamycin on inflammatory stress-induced insulin resistance in vitro and in vivo

Insulin resistance is closely related to inflammatory stress and the mammalian target of rapamycin/S6 kinase (mTOR/S6K) pathway. The present study investigated whether rapamycin, a specific inhibitor of mTOR, ameliorates inflammatory stress-induced insulin resistance in vitro and in vivo. We used tumor necrosis factor-alpha (TNF-α) and interleukin-6 (IL-6) stimulation in HepG2 hepatocytes, C2C12 myoblasts and 3T3-L1 adipocytes and casein injection in C57BL/6J mice to induce inflammatory stress. Our results showed that inflammatory stress impairs insulin signaling by reducing the expression of total IRS-1, p-IRS-1 (tyr632), and p-AKT (ser473); it also activates the mTOR/S6K signaling pathway both in vitro and in vivo. In vitro, rapamycin treatment reversed inflammatory cytokine-stimulated IRS-1 serine phosphorylation, increased insulin signaling to AKT and enhanced glucose utilization. In vivo, rapamycin treatment also ameliorated the impaired insulin signaling induced by inflammatory stress, but it induced pancreatic β-cell apoptosis, reduced pancreatic β-cell function and enhanced hepatic gluconeogenesis, thereby resulting in hyperglycemia and glucose intolerance in casein-injected mice. Our results indicate a paradoxical effect of rapamycin on insulin resistance between the in vitro and in vivo environments under inflammatory stress and provide additional insight into the clinical application of rapamycin.

Prenatal exposure to persistent organochlorine pollutants is associated with high insulin levels in 5-year-old girls

BACKGROUND

Several persistent organochlorine pollutants (POPs) possess endocrine disrupting abilities, thereby potentially leading to an increased risk of obesity and metabolic diseases, especially if the exposure occurs during prenatal life. We have previously found associations between prenatal POP exposures and increased BMI, waist circumference and change in BMI from 5 to 7 years of age, though only among girls with overweight mothers.

OBJECTIVES

In the same birth cohort, we investigated whether prenatal POP exposure was associated with serum concentrations of insulin and leptin among 5-year-old children, thus possibly mediating the association with overweight and obesity at 7 years of age.

METHODS

The analyses were based on a prospective Faroese Birth Cohort (n=656), recruited between 1997 and 2000. Major POPs, polychlorinated biphenyls (PCBs), p,p'-dichlorodiphenyldichloroethylene (DDE) and hexachlorobenzene (HCB), were measured in maternal pregnancy serum and breast milk. Children were followed-up at the age of 5 years where a non-fasting blood sample was drawn; 520 children (273 boys and 247 girls) had adequate serum amounts available for biomarker analyses by Luminex® technology. Insulin and leptin concentrations were transformed from continuous to binary variables, using the 75th percentile as a cut-off point. Multiple logistic regression was used to investigate associations between prenatal POP exposures and non-fasting serum concentrations of insulin and leptin at age 5 while taking into account confounders.

RESULTS

Girls with highest prenatal POP exposure were more likely to have high non-fasting insulin levels (PCBs 4th quartile: OR=3.71; 95% CI: 1.36, 10.01. DDE 4th quartile: OR=2.75; 95% CI: 1.09, 6.90. HCB 4th quartile: OR=1.98; 95% CI: 1.06, 3.69) compared to girls in the lowest quartile. No significant associations were observed with leptin, or among boys. A mediating effect of insulin or leptin on later obesity was not observed.

CONCLUSION

These findings suggest, that for girls, prenatal exposure to POPs may play a role for later development of metabolic diseases by affecting the level of insulin.

Hepatic Insulin Resistance Following Chronic Activation of the CREB Coactivator CRTC2

Under fasting conditions, increases in circulating concentrations of glucagon maintain glucose homeostasis via the induction of hepatic gluconeogenesis. Triggering of the cAMP pathway in hepatocytes stimulates the gluconeogenic program via the PKA-mediated phosphorylation of CREB and dephosphorylation of the cAMP-regulated CREB coactivators CRTC2 and CRTC3. In parallel, decreases in circulating insulin also increase gluconeogenic gene expression via the de-phosphorylation and activation of the forkhead transcription factor FOXO1. Hepatic gluconeogenesis is increased in insulin resistance where it contributes to the attendant hyperglycemia. Whether selective activation of the hepatic CREB/CRTC pathway is sufficient to trigger metabolic changes in other tissues is unclear, however. Modest hepatic expression of a phosphorylation-defective and therefore constitutively active CRTC2S171,275A protein increased gluconeogenic gene expression under fasting as well as feeding conditions. Circulating glucose concentrations were constitutively elevated in CRTC2S171,275A-expressing mice, leading to compensatory increases in circulating insulin concentrations that enhance FOXO1 phosphorylation. Despite accompanying decreases in FOXO1 activity, hepatic gluconeogenic gene expression remained elevated in CRTC2S171,275A mice, demonstrating that chronic increases in CRTC2 activity in the liver are indeed sufficient to promote hepatic insulin resistance and to disrupt glucose homeostasis.

Syntaxin 4 Overexpression Ameliorates Effects of Aging and High-Fat Diet on Glucose Control and Extends Lifespan

Indirect evidence suggests that improved insulin sensitivity may contribute to improved lifespan of mice in which aging has been slowed by mutations, drugs, or dietary means, even in stocks of mice that do not show signs of late-life diabetes. Peripheral responses to insulin can be augmented by overexpression of Syntaxin 4 (Syn4), a plasma-membrane-localized SNARE protein. We show here that Syn4 transgenic (Tg) mice with high level expression of Syn4 had a significant extension of lifespan (33% increase in median) and showed increased peripheral insulin sensitivity, even at ages where controls exhibited age-related insulin resistance. Moreover, skeletal muscle GLUT4 and islet insulin granule exocytosis processes were fully protected in Syn4 Tg mice challenged with a high-fat diet. Hence, high-level expressing Syn4 Tg mice may exert better glycemic control, which slows multiple aspects of aging and extends lifespan, even in non-diabetic mice.

Control of hepatic gluconeogenesis by the promyelocytic leukemia zinc finger protein

The promyelocytic leukemia zinc finger (PLZF) protein is involved in major biological processes including energy metabolism, although its role remains unknown. In this study, we demonstrated that hepatic PLZF expression was induced in fasted or diabetic mice. PLZF promoted gluconeogenic gene expression and hepatic glucose output, leading to hyperglycemia. In contrast, hepatic PLZF knockdown improved glucose homeostasis in db/db mice. Mechanistically, peroxisome proliferator-activated receptor γ coactivator 1α and the glucocorticoid receptor synergistically activated PLZF expression. We conclude that PLZF is a critical regulator of hepatic gluconeogenesis. PLZF manipulation may benefit the treatment of metabolic diseases associated with gluconeogenesis.

Pancreatic PYY Is Critical in the Control of Insulin Secretion and Glucose Homeostasis in Female Mice

Insulin secretion is tightly controlled through coordinated actions of a number of systemic and local factors. Peptide YY (PYY) is expressed in α-cells of the islet, but its role in control of islet function such as insulin release is not clear. In this study, we generated a transgenic mouse model (Pyy(tg/+)/Rip-Cre) overexpressing the Pyy gene under the control of the rat insulin 2 gene promoter and assessed the impact of islet-released PYY on β-cell function, insulin release, and glucose homeostasis in mice. Our results show that up-regulation of PYY in islet β-cells leads to an increase in serum insulin levels as well as improved glucose tolerance. Interestingly, PYY-overproducing mice show increased lean mass and reduced fat mass with no significant changes in food intake or body weight. Energy expenditure is also increased accompanied by increased respiratory exchange ratio. Mechanistically, the enhanced insulin levels and improved glucose tolerance are primarily due to increased β-cell mass and secretion. This is associated with alterations in the expression of genes important for β-cell proliferation and function as well as the maintenance of the β-cell phenotype. Taken together, these data demonstrate that pancreatic islet-derived PYY plays an important role in controlling glucose homeostasis through the modulation of β-cell mass and function.

Docosahexaenoic acid inhibits proteolytic processing of sterol regulatory element-binding protein-1c (SREBP-1c) via activation of AMP-activated kinase

In hyperinsulinemic states including obesity and T2DM, overproduction of fatty acid and triglyceride contributes to steatosis of the liver, hyperlipidemia and hepatic insulin resistance. This effect is mediated in part by the transcriptional regulator sterol responsive element binding protein-1c (SREBP-1c), which stimulates the expression of genes involved in hepatic fatty acid and triglyceride synthesis. SREBP-1c is up regulated by insulin both via increased transcription of nascent full-length SREBP-1c and by enhanced proteolytic processing of the endoplasmic reticulum (ER)-bound precursor to yield the transcriptionally active n-terminal form, nSREBP-1c. Polyunsaturated fatty acids of marine origin (n-3 PUFA) prevent induction of SREBP-1c by insulin thereby reducing plasma and hepatic triglycerides. Despite widespread use of n-3 PUFA supplements to reduce triglycerides in clinical practice, the exact mechanisms underlying their hypotriglyceridemic effect remain elusive. Here we demonstrate that the n-3 PUFA docosahexaenoic acid (DHA; 22:5 n-3) reduces nSREBP-1c by inhibiting regulated intramembrane proteolysis (RIP) of the nascent SREBP-1c. We further show that this effect of DHA is mediated both via activation of AMP-activated protein kinase (AMPK) and by inhibition of mechanistic target of rapamycin complex 1 (mTORC1). The inhibitory effect of AMPK on SREBP-1c processing is linked to phosphorylation of serine 365 of SREBP-1c in the rat. We have defined a novel regulatory mechanism by which n-3 PUFA inhibit induction of SREBP-1c by insulin. These findings identify AMPK as an important negative regulator of hepatic lipid synthesis and as a potential therapeutic target for hyperlipidemia in obesity and T2DM.

Irisin inhibits hepatic gluconeogenesis and increases glycogen synthesis via the PI3K/Akt pathway in type 2 diabetic mice and hepatocytes

Increased glucose production and reduced hepatic glycogen storage contribute to metabolic abnormalities in diabetes. Irisin, a newly identified myokine, induces the browning of white adipose tissue, but its effects on gluconeogenesis and glycogenesis are unknown. In the present study, we investigated the effects and underlying mechanisms of irisin on gluconeogenesis and glycogenesis in hepatocytes with insulin resistance, and its therapeutic role in type 2 diabetic mice. Insulin resistance was induced by glucosamine (GlcN) or palmitate in human hepatocellular carcinoma (HepG2) cells and mouse primary hepatocytes. Type 2 diabetes was induced by streptozotocin/high-fat diet (STZ/HFD) in mice. In HepG2 cells, irisin ameliorated the GlcN-induced increases in glucose production, phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) expression, and glycogen synthase (GS) phosphorylation; it prevented GlcN-induced decreases in glycogen content and the phosphoinositide 3-kinase (PI3K) p110α subunit level, and the phosphorylation of Akt/protein kinase B, forkhead box transcription factor O1 (FOXO1) and glycogen synthase kinase-3 (GSK3). These effects of irisin were abolished by the inhibition of PI3K or Akt. The effects of irisin were confirmed in mouse primary hepatocytes with GlcN-induced insulin resistance and in human HepG2 cells with palmitate-induced insulin resistance. In diabetic mice, persistent subcutaneous perfusion of irisin improved the insulin sensitivity, reduced fasting blood glucose, increased GSK3 and Akt phosphorylation, glycogen content and irisin level, and suppressed GS phosphorylation and PEPCK and G6Pase expression in the liver. Irisin improves glucose homoeostasis by reducing gluconeogenesis via PI3K/Akt/FOXO1-mediated PEPCK and G6Pase down-regulation and increasing glycogenesis via PI3K/Akt/GSK3-mediated GS activation. Irisin may be regarded as a novel therapeutic strategy for insulin resistance and type 2 diabetes.

Effects of intranasal insulin on hepatic fat accumulation and energy metabolism in humans

Studies in rodents suggest that insulin controls hepatic glucose metabolism through brain-liver crosstalk, but human studies using intranasal insulin to mimic central insulin delivery have provided conflicting results. In this randomized controlled crossover trial, we investigated the effects of intranasal insulin on hepatic insulin sensitivity (HIS) and energy metabolism in 10 patients with type 2 diabetes and 10 lean healthy participants (CON). Endogenous glucose production was monitored with [6,6-(2)H2]glucose, hepatocellular lipids (HCLs), ATP, and inorganic phosphate concentrations with (1)H/(31)P magnetic resonance spectroscopy. Intranasal insulin transiently increased serum insulin levels followed by a gradual lowering of blood glucose in CON only. Fasting HIS index was not affected by intranasal insulin in CON and patients. HCLs decreased by 35% in CON only, whereas absolute hepatic ATP concentration increased by 18% after 3 h. A subgroup of CON received intravenous insulin to mimic the changes in serum insulin and blood glucose levels observed after intranasal insulin. This resulted in a 34% increase in HCLs without altering hepatic ATP concentrations. In conclusion, intranasal insulin does not affect HIS but rapidly improves hepatic energy metabolism in healthy humans, which is independent of peripheral insulinemia. These effects are blunted in patients with type 2 diabetes.

Insulin resistance and its consequences in chronic hepatitis C

Chronic hepatitis C (CHC) is generally a slowly progressive disease, but some factors associated with rapid progression have been identified. Hepatitis C virus (HCV) may contribute to a broad spectrum of metabolic disturbances - namely, steatosis, insulin resistance (IR), increased prevalence of impaired glucose tolerance, type 2 diabetes mellitus (T2DM), lipid metabolism abnormalities and atherosclerosis. HCV can directly or indirectly cause both IR and steatosis, but it is still not resolved whether this viral impact bears the same prognostic value as the metabolic counterparts. As the population exposed to HCV ages, the morbidity due to this disease is increasing. The rising epidemic of obesity contributes to higher prevalence of IR and T2DM. Our understanding of the mutual association between both disease states continues to grow, but is still far from complete. This review briefly discusses the most probable mechanisms involved in IR development in the course of CHC. Molecular mechanisms for the direct and indirect influence of HCV on intracellular insulin signaling are described. Subsequently, the consequences of IR/T2DM for disease progression and management are summarized.

The role of mouse Akt2 in insulin-dependent suppression of adipocyte lipolysis in vivo

AIM/HYPOTHESIS

The release of fatty acids from adipocytes, i.e. lipolysis, is maintained under tight control, primarily by the opposing actions of catecholamines and insulin. A widely accepted model is that insulin antagonises catecholamine-dependent lipolysis through phosphorylation and activation of cAMP phosphodiesterase 3B (PDE3B) by the serine-threonine protein kinase Akt (protein kinase B). Recently, this hypothesis has been challenged, as in cultured adipocytes insulin appears, under some conditions, to suppress lipolysis independently of Akt.

METHODS

To address the requirement for Akt2, the predominant isoform expressed in classic insulin target tissues, in the suppression of fatty acid release in vivo, we assessed lipolysis in mice lacking Akt2.

RESULTS

In the fed state and following an oral glucose challenge, Akt2 null mice were glucose intolerant and hyperinsulinaemic, but nonetheless exhibited normal serum NEFA and glycerol levels, suggestive of normal suppression of lipolysis. Furthermore, insulin partially inhibited lipolysis in Akt2 null mice during an insulin tolerance test (ITT) and hyperinsulinaemic-euglycaemic clamp, respectively. In support of these in vivo observations, insulin antagonised catecholamine-induced lipolysis in primary brown fat adipocytes from Akt2-deficient mice.

CONCLUSIONS/INTERPRETATION

These data suggest that suppression of lipolysis by insulin in hyperinsulinaemic states can take place in the absence of Akt2.

Inhibition of the glucose transporter SGLT2 with dapagliflozin in pancreatic alpha cells triggers glucagon secretion

Type 2 diabetes (T2D) is characterized by chronic hyperglycemia resulting from a deficiency in insulin signaling, because of insulin resistance and/or defects in insulin secretion; it is also associated with increases in glucagon and endogenous glucose production (EGP). Gliflozins, including dapagliflozin, are a new class of approved oral antidiabetic agents that specifically inhibit sodium-glucose co-transporter 2 (SGLT2) function in the kidney, thus preventing renal glucose reabsorption and increasing glycosuria in diabetic individuals while reducing hyperglycemia. However, gliflozin treatment in subjects with T2D increases both plasma glucagon and EGP by unknown mechanisms. In spite of the rise in EGP, T2D patients treated with gliflozin have lower blood glucose levels than those receiving placebo, possibly because of increased glycosuria; however, the resulting increase in plasma glucagon levels represents a possible concerning side effect, especially in a patient population already affected by hyperglucagonemia. Here we demonstrate that SGLT2 is expressed in glucagon-secreting alpha cells of the pancreatic islets. We further found that expression of SLC5A2 (which encodes SGLT2) was lower and glucagon (GCG) gene expression was higher in islets from T2D individuals and in normal islets exposed to chronic hyperglycemia than in islets from non-diabetics. Moreover, hepatocyte nuclear factor 4-α (HNF4A) is specifically expressed in human alpha cells, in which it controls SLC5A2 expression, and its expression is downregulated by hyperglycemia. In addition, inhibition of either SLC5A2 via siRNA-induced gene silencing or SGLT2 via dapagliflozin treatment in human islets triggered glucagon secretion through KATP channel activation. Finally, we found that dapagliflozin treatment further promotes glucagon secretion and hepatic gluconeogenesis in healthy mice, thereby limiting the decrease of plasma glucose induced by fasting. Collectively, these results identify a heretofore unknown role of SGLT2 and designate dapagliflozin an alpha cell secretagogue.

Salidroside ameliorates insulin resistance through activation of a mitochondria-associated AMPK/PI3K/Akt/GSK3β pathway

BACKGROUND AND PURPOSE

Recent reports have suggested that salidroside could protect cardiomyocytes from oxidative injury and stimulate glucose uptake in skeletal muscle cells by activating AMP-activated protein kinase (AMPK). The aim of this study was to evaluate the therapeutic effects of salidroside on diabetic mice and to explore the underlying mechanisms.

EXPERIMENTAL APPROACH

The therapeutic effects of salidroside on type 2 diabetes were investigated. Increasing doses of salidroside (25, 50 and 100 mg·kg(-1) ·day(-1)) were administered p.o. to db/db mice for 8 weeks. Biochemical analysis and histopathological examinations were conducted to evaluate the therapeutic effects of salidroside. Primary cultured mouse hepatocytes were used to further explore the underlying mechanisms in vitro.

KEY RESULTS

Salidroside dramatically reduced blood glucose and serum insulin levels and alleviated insulin resistance. Hypolipidaemic effects and amelioration of liver steatosis were observed after salidroside administration. In vitro, salidroside dose-dependently induced an increase in the phosphorylations of AMPK and PI3K/Akt, as well as glycogen synthase kinase 3β (GSK3β) in hepatocytes. Furthermore, salidroside-stimulated AMPK activation was found to suppress the expression of PEPCK and glucose-6-phosphatase. Salidroside-induced AMPK activation also resulted in phosphorylation of acetyl CoA carboxylase, which can reduce lipid accumulation in peripheral tissues. In isolated mitochondria, salidroside inhibited respiratory chain complex I and disturbed oxidation/phosphorylation coupling and moderately depolarized the mitochondrial membrane potential, resulting in a transient increase in the AMP/ATP ratio.

CONCLUSIONS AND IMPLICATIONS

Salidroside exerts an antidiabetic effect by improving the cellular metabolic flux through the activation of a mitochondria-related AMPK/PI3K/Akt/GSK3β pathway.

Clinical pathological characteristics of breast cancer patients with secondary diabetes after systemic therapy: a retrospective multicenter study

The objective of this study was to investigate the clinical pathological characteristics of breast cancer (BC) patients with secondary diabetes after systemic therapy without preexisting diabetes. A total of 1434 BC patients received systemic therapy and were analyzed retrospectively. Fasting plasma glucose (FPG) levels were monitored prior to the treatments, during the course of systemic therapy, and at the follow-up visits. Cox regression models were used to estimate the associations between the clinical pathological characteristics of BC and the cause-specific hazard of developing secondary diabetes. Among the 1434 BC patients, 151 had preexisting type 2 diabetes. Of the remaining 1283 patients with normal FPG levels prior to the systemic therapy, 59 developed secondary diabetes and 72 displayed secondary impaired fasting glucose (IFG) over a mean follow-up of 41 months. The prevalence of secondary type 2 diabetes in BC patients was 4.6 % (59/1283), which was obviously higher than that of the normal control group (1.4 %, P < 0.001). The percentage of older patients (P < 0.05), menopausal patients (P < 0.001), and obese patients (P < 0.01) tended to be lower in the secondary diabetic group. In addition, these patients with secondary diabetes had later pathological stages (P < 0.01), more lymph node metastasis (P < 0.05), negative estrogen receptor (ER) expression (P < 0.05), and smaller size of tumors (P < 0.05). After adjusting for age and BMI, the risk of developing secondary diabetes and IFG in subjects with later pathological stage BC (hazard ratio (HR) = 1.623; 95 % confidence interval (CI) 1.128–2.335 (P < 0.01)), negative progesterone receptor (PR) expression (HR = 0.530; 95 % CI 0.372–0.755 (P < 0.001)), positive human epidermal growth factor receptor 2 (HER2) expression (HR = 1.822; 95 % CI 1.230–2.700 (P < 0.01)), and more lymph node metastasis (HR = 1.595; 95 % CI 1.128–2.258 (P < 0.01)) was significantly higher. In conclusion, this study shows that an increase in the incidence of diabetes among breast cancer survivors after systemic therapy, especially the patients with later pathological stages, more lymph node metastasis, negative hormone receptor expression, and positive HER2 expression. Our study suggests that greater diabetes screening and prevention strategies among breast cancer patients after systemic treatment are needed in China.

Flavin-containing monooxygenase 3 as a potential player in diabetes-associated atherosclerosis

Despite the well-documented association between insulin resistance and cardiovascular disease, the key targets of insulin relevant to the development of cardiovascular disease are not known. Here, using non-biased profiling methods, we identify the enzyme flavin-containing monooxygenase 3 (Fmo3) to be a target of insulin. FMO3 produces trimethylamine N-oxide (TMAO), which has recently been suggested to promote atherosclerosis in mice and humans. We show that FMO3 is suppressed by insulin in vitro, increased in obese/insulin resistant male mice and increased in obese/insulin-resistant humans. Knockdown of FMO3 in insulin-resistant mice suppresses FoxO1, a central node for metabolic control, and entirely prevents the development of hyperglycaemia, hyperlipidemia and atherosclerosis. Taken together, these data indicate that FMO3 is required for FoxO1 expression and the development of metabolic dysfunction.

18-carbon polyunsaturated fatty acids ameliorate palmitate-induced inflammation and insulin resistance in mouse C2C12 myotubes

Skeletal muscle is a major site of insulin action. Intramuscular lipid accumulation results in inflammation, which has a strong correlation with skeletal muscle insulin resistance (IR). The aim of this study was to explore the effects of linoleic acid, alpha-linolenic acid, and gamma-linolenic acid (GLA), 18-carbon polyunsaturated fatty acids (PUFAs), on palmitic acid (PA)-induced inflammatory responses and IR in C2C12 myotubes. Our data demonstrated that these three test 18-carbon PUFAs can inhibit PA-induced interleukin-6 and tumor necrosis factor-α messenger RNA (mRNA) expression and IR as evidenced by increases in phosphorylated AKT and the 160-kD AKT substrate, mRNA and plasma membrane protein expression of glucose transporter 4, and glucose uptake. Moreover, the 18-carbon PUFAs blocked the effects of PA on activation of mitogen-activated protein kinases (MAPKs), protein kinase C-θ (PKC-θ), AMP-activated protein kinase (AMPK) and nuclear factor-κB (NF-κB). Of note, supplementation with GLA-rich borage oil decreased proinflammatory cytokine production and hindered the activation of MAPKs, PKC-θ and NF-κB in the skeletal muscles of diabetic mice. The 18-carbon PUFAs did not reverse PA-induced inflammation or IR in C2C12 myotubes transfected with a constitutively active mutant IκB kinase-β plasmid, which suggests the importance of the inhibition of NF-κB activation by the 18-carbon PUFAs. Moreover, blockade of AMPK activation by short hairpin RNA annulled the inhibitory effects of the 18-carbon PUFAs on PA-induced IR but not inflammation. Our findings suggest that the 18-carbon PUFAs may be useful in the management of PA-induced inflammation and IR in myotubes.

Pancreatic T cell protein-tyrosine phosphatase deficiency affects beta cell function in mice

AIMS/HYPOTHESIS

T cell protein tyrosine phosphatase (TCPTP, encoded by PTPN2) regulates cytokine-induced pancreatic beta cell apoptosis and may contribute to the pathogenesis of type 1 diabetes. However, the role of TCPTP in pancreatic endocrine function and insulin secretion remains largely unknown.

METHODS

To investigate the endocrine role of pancreatic TCPTP we generated mice with pancreas Ptpn2/TCPTP deletion (panc-TCPTP KO).

RESULTS

When fed regular chow, panc-TCPTP KO and control mice exhibited comparable glucose tolerance. However, when challenged with prolonged high fat feeding panc-TCPTP KO mice exhibited impaired glucose tolerance and attenuated glucose-stimulated insulin secretion (GSIS). The defect in GSIS was recapitulated in primary islets ex vivo and after TCPTP pharmacological inhibition or lentiviral-mediated TCPTP knockdown in the glucose-responsive MIN6 beta cells, consistent with this being cell autonomous. Reconstitution of TCPTP in knockdown cells reversed the defect in GSIS demonstrating that the defect was a direct consequence of TCPTP deficiency. The reduced insulin secretion in TCPTP knockdown MIN6 beta cells was associated with decreased insulin content and glucose sensing. Furthermore, TCPTP deficiency led to enhanced tyrosyl phosphorylation of signal transducer and activator of transcription 1 and 3 (STAT 1/3), and substrate trapping studies in MIN6 beta cells identified STAT 1/3 as TCPTP substrates. STAT3 pharmacological inhibition and small interfering RNA-mediated STAT3 knockdown in TCPTP deficient cells restored GSIS to control levels, indicating that the effects of TCPTP deficiency were mediated, at least in part, through enhanced STAT3 phosphorylation and signalling.

CONCLUSIONS/INTERPRETATION

These studies identify a novel role for TCPTP in insulin secretion and uncover STAT3 as a physiologically relevant target for TCPTP in the endocrine pancreas.

Molecular physiopathology of obesity-related diseases: multi-organ integration by GRK2

Obesity is a worldwide problem that has reached epidemic proportions both in developed and developing countries. The excessive accumulation of fat poses a risk to health since it favours the development of metabolic alterations including insulin resistance and tissue inflammation, which further contribute to the progress of the complex pathological scenario observed in the obese. In this review we put together the different outcomes of fat accumulation and insulin resistance in the main insulin-responsive tissues, and discuss the role of some of the key molecular routes that control disease progression both in an organ-specific and also in a more systemic manner. In particular, we focus on the importance of studying the integrated regulation of different organs and pathways that contribute to the global pathophysiology of this condition with a specific emphasis on the role of emerging key molecular nodes such as the G protein-coupled receptor kinase 2 (GRK2) signalling hub.

In vivo and in vitro attenuation of naloxone-precipitated experimental opioid withdrawal syndrome by insulin and selective KATP channel modulator

RATIONALE

Opiate exposure for longer duration develops state of dependence in humans and animals, which is revealed by signs and symptoms of withdrawal precipitated by opioid receptor antagonists. The sudden withdrawal of opioids produces a withdrawal syndrome in opioid-dependent subjects. Insulin and ATP-sensitive potassium (KATP) channel-mediated glucose homeostasis have been shown to modulate morphine withdrawal.

OBJECTIVE

Present study has been structured to investigate the role of insulin and pharmacological modulator of KATP channel (gliclazide) in experimental morphine withdrawal syndrome, both invivo and invitro.

METHODS

In this study, naloxone-precipitated morphine withdrawal syndrome in mice (invivo) as well as in rat ileum (invitro) were utilized to assess opioid withdrawal phenomenon. Morphine withdrawal syndromes like jumping and rearing frequency, forepaw licking, circling, fore paw tremor, wet dog shake, sneezing, overall morphine withdrawal severity (OMWS), serum glucose, brain malondialdehyde (MDA), glutathione (GSH), nitrite/nitrate, and calcium (Ca(+2)) were assessed.

RESULTS

Naloxone has significantly increased morphine withdrawal syndrome, both invivo and invitro. Insulin and gliclazide have significantly attenuated, naloxone induced behavioral changes like jumping and rearing frequency, forepaw licking, wet dog shake, sneezing, straightening, circling, OMWS, and various biochemical impairments such as serum glucose, brain MDA, GSH, nitrite/nitrate, and Ca(+2) in morphine-dependent animals (invivo). In vitro, insulin and gliclazide have significantly reduced naloxone-induced contraction in morphine-withdrawn rat ileum preparation.

CONCLUSIONS

Insulin and gliclazide (KATP channel blocker) have attenuated naloxone-precipitated morphine withdrawal syndrome, both invivo and invitro. Thus, insulin and KATP channel modulation may provide new avenues for research in morphine withdrawal.

Hypoglycemic effects of Grifola frondosa (Maitake) polysaccharides F2 and F3 through improvement of insulin resistance in diabetic rats

This study demonstrated the hypoglycemic effects of Grifola frondosa polysaccharides F2 and F3 through insulin sensitivity improvement in diabetic rats.

Is there an association between variants in candidate insulin pathway genes IGF-I, IGFBP-3, INSR, and IRS2 and risk of colorectal cancer in the Iranian population?

BACKGROUND

Several epidemiological studies have shown associations between colorectal cancer (CRC) risk and type 2 diabetes and obesity. Any effects would be expected to be mediated through the insulin pathway. Therefore it is possible that variants of genes encoding components of the insulin pathway play roles in CRC susceptibility. In this study, we hypothesized that polymorphisms in the genes involving the insulin pathway are associated with risk of CRC.

MATERIALS AND METHODS

The associations of four single nucleotide polymorphisms (SNPs) in IGF-I (rs6214), IGFBP-3 (rs3110697), INSR (rs1052371), and IRS2 (rs2289046) genes with the risk of CRC were evaluated using a case-control design with 167 CRC cases and 277 controls by the PCR-RFLP method.

RESULTS

Overall, we observed no significant difference in genotype and allele frequencies between the cases and controls for the IGF-I, IGFBP-3, INSR, IRS2 gene variants and CRC before or after adjusting for confounders (age, BMI, sex, and smoking status). However, we observed that the IRS2 (rs2289046) GG genotype compared with AA+AG genotypes has a protective effect for CRC in normal weight subjects (p=0.035, OR=0.259, 95%CI= 0.074-0.907).

CONCLUSIONS

These findings do not support plausible associations between polymorphic variations in IGF-I, IGFBP-3, INSR, IRS2 genes and risk of CRC. However, the evidence for a link between the IRS2 (rs2289046) variant and risk of CRC dependent on the BMI of the subjects, requires confirmation in subsequent studies with greater sample size.

RETRACTED: Combination of carvacrol and rosiglitazone ameliorates high fat diet induced changes in lipids and inflammatory markers in C57BL/6J mice

This article has been retracted: please see Elsevier Policy on Article Withdrawal (https://www.elsevier.com/about/our-business/policies/article-withdrawal). This article has been retracted at the request of the Editor-in-Chief. Following concerns raised by Dr. E. Bik, the journal conducted an investigation and found evidence that there had been improper manipulation and duplication of images in Figure 4. The editors would like to thank Dr. Bik for her valuable insight in this matter. The authors have not responded to requests for an explanation of these irregularities so this article is retracted without their approval.

Insulin-resistance HCV infection-related affects vascular stiffness in normotensives

BACKGROUND AND AIMS. Arterial stiffness evaluated as pulse wave velocity, is an early marker of vascular damage and an independent predictor for cardiovascular events. We investigated if the insulin resistance/hyperinsulinemia chronic hepatitis C virus infection-related could influence arterial stiffness. METHODS. We enrolled 260 outpatients matched for age, body mass index, gender, ethnicity: 52 with never-treated uncomplicated chronic hepatitis C virus infection (HCV(+)), 104 never-treated hypertensives (HT) and 104 healthy subjects (NT). Pulse wave velocity was evaluated by a validated system employing high-fidelity applanation tonometry. We also measured: fasting plasma glucose and insulin, total, LDL- and HDL-cholesterol, triglyceride, creatinine, e-GFR-EPI, HOMA, quantitative HCV-RNA. RESULTS. HCV(+) patients with respect to NT had an increased pulse wave velocity (7.9 ± 2.1 vs 6.4 ± 2.1 m/s; P < 0.0001), similar to that observed in HT group (8.8 ± 3.2 m/s). HCV(+) patients, in comparison with NT, had higher triglyceride, creatinine, fasting insulin and HOMA (3.2 ± 1.3 vs 2.5 ± 1.0; P < 0.0001). At linear regression analysis, the correlation between pulse wave velocity and HOMA was similar in HT (r = 0.380, P < 0.0001) and HCV(+) (r = 0.369, P = 0.004) groups. At multiple regression analysis, HOMA resulted the major determinant of pulse wave velocity in all groups, explaining respectively 11.8%, 14.4% and 13.6% of its variation in NT, HT and HCV(+). At correlational analysis hepatitis C virus-RNA and HOMA demonstrated a strong and linear relationship between them, explaining the 72.4% of their variation (P = 0.022). CONCLUSIONS. We demonstrated a significant and direct correlation between HOMA and pulse wave velocity in HCV(+) patients, similar to that observed in hypertensives.

Maternal insulin resistance and transient hyperglycemia impact the metabolic and endocrine phenotypes of offspring

Studies in both humans and rodents suggest that maternal diabetes leads to a higher risk of the fetus developing impaired glucose tolerance and obesity during adulthood. However, the impact of hyperinsulinemia in the mother on glucose homeostasis in the offspring has not been fully explored. We aimed to determine the consequences of maternal insulin resistance on offspring metabolism and endocrine pancreas development using the LIRKO mouse model, which exhibits sustained hyperinsulinemia and transient increase in blood glucose concentrations during pregnancy. We examined control offspring born to either LIRKO or control mothers on embryonic days 13.5, 15.5, and 17.5 and postpartum days 0, 4, and 10. Control offspring born to LIRKO mothers displayed low birth weights and subsequently rapidly gained weight, and their blood glucose and plasma insulin concentrations were higher than offspring born to control mothers in early postnatal life. In addition, concentrations of plasma leptin, glucagon, and active GLP-1 were higher in control pups from LIRKO mothers. Analyses of the endocrine pancreas revealed significantly reduced β-cell area in control offspring of LIRKO mothers shortly after birth. β-Cell proliferation and total islet number were also lower in control offspring of LIRKO mothers during early postnatal days. Together, these data indicate that maternal hyperinsulinemia and the transient hyperglycemia impair endocrine pancreas development in the control offspring and induce multiple metabolic alterations in early postnatal life. The relatively smaller β-cell mass/area and β-cell proliferation in these control offspring suggest cell-autonomous epigenetic mechanisms in the regulation of islet growth and development.

Ubiquitination in disease pathogenesis and treatment

Ubiquitination is crucial for a plethora of physiological processes, including cell survival and differentiation and innate and adaptive immunity. In recent years, considerable progress has been made in the understanding of the molecular action of ubiquitin in signaling pathways and how alterations in the ubiquitin system lead to the development of distinct human diseases. Here we describe the role of ubiquitination in the onset and progression of cancer, metabolic syndromes, neurodegenerative diseases, autoimmunity, inflammatory disorders, infection and muscle dystrophies. Moreover, we indicate how current knowledge could be exploited for the development of new clinical therapies.

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

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

Novel strategies for managing pancreatic cancer

With the incidence reports of pancreatic cancer increasing every year, research over the last several decades has been focused on the means to achieve early diagnosis in patients that are at a high risk of developing the malignancy. This review covers current strategies for managing pancreatic cancer and further discusses efforts in understanding the role of early onset symptoms leading to tumor progression. Recent investigations in this discussion include type 3c diabetes, selected biomarkers and pathways related to pancreatic intraepithelial neoplasia lesions, drug resistance, and advances in nanomedicine which may provide significant solutions for improving early detection and treatments in future medicine.

Chronic HCV infection increases cardiac left ventricular mass index in normotensive patients

BACKGROUND & AIMS

Left ventricular hypertrophy (LVH), is an independent predictor for cardiovascular events. We investigated if chronic hepatitis C virus (HCV) infection and the related insulin resistance (IR)/hyperinsulinemia could influence the increase of left ventricular mass (LVM).

METHODS

We enrolled 260 outpatients matched for age, body mass index, gender, ethnicity: 52 with never-treated uncomplicated chronic HCV infection (HCV(+)), 104 never-treated hypertensives (HT) and 104 healthy subjects (NT). LVM was calculated according to the Devereux formula and indexed for body surface area. The following laboratory parameters were measured: fasting plasma glucose and insulin, total, LDL- and HDL-cholesterol, triglyceride, creatinine, e-GFR-EPI, HOMA. Quantitative HCV-RNA was assessed by PCR.

RESULTS

HCV(+) patients with respect to healthy normotensive subjects had an increased LVMI (100 ± 23 vs. 83 ± 15 g/m(2); p < 0.0001), similar to that observed in HT group (103 ± 25 g/m(2)). Regarding biochemical variables, HCV(+) patients, in comparison with normotensive healthy subjects, had higher triglyceride, creatinine, fasting insulin and HOMA (3.2 ± 1.3 vs. 2.5 ± 1.0; p < 0.0001). At linear regression analysis, the correlation between LVMI and HOMA was similar in HT (r = 0.528, p < 0.0001) and HCV(+) (r = 0.489, p < 0.0001) groups. At multiple regression analysis, HOMA resulted the major determinant of LMVI in all groups, explaining respectively 21.8%, 27.8%, and 23.9% of its variation in NT, HT and HCV(+). At correlational analysis HCV-RNA and HOMA demonstrated a strong and linear relationship between them, explaining the 72.4% of their variation (p = 0.022).

CONCLUSIONS

We demonstrated a significant and direct correlation between HOMA and LVMI in patients with chronic HCV infection, similar to that observed in hypertensives.

Effects of a community-based weight loss intervention on adipose tissue circulating factors

AIMS

Obesity is associated with metabolic dysfunctions, which may be mediated by changes in adipose tissue signaling factors. These molecules are denoted as Adipose Tissue Generated Mediators of CardioVascular Risk (ATGMCVR) here, and include leptin, adiponectin, C-reactive protein (CRP), interleukin 6 (IL-6), tumor necrosis factor alpha (TNFα), and plasminogen activator inhibitor 1 (PAI-1). This study examined the effect of a weight loss program on ATGMCVR in obese adults with prediabetes.

MATERIALS AND METHODS

Subjects were randomized to usual care (UC; n=15) or lifestyle weight loss groups (LWL; n=15). LWL was a community-based weight loss intervention to promote physical activity and healthy eating. ATGMCVR at 1-year were compared between groups by analysis of covariance; baseline value of the mediator was the covariate. Baseline means for ATGMCVR were compared between those with (n=21) and without (n=9) metabolic syndrome (MetS).

RESULTS

At baseline, subjects were 58±9 (SD) years, 70% female, with a BMI of 34±4kg/m(2). One-year weight loss (%) was 7.8±6.0% for LWL and 1.7±4.5% for UC. Group differences at 1-year were noted (adjusted means [95%CI] for UC and LWL, respectively) for adiponectin (8526.3 [7397.7, 9827]; 10,870.9 [9432.0, 12,529.3]ng/ml; p=0.02), leptin (30.4 [26.1, 35.4]; 23.7 [20.3, 27.5]ng/ml; p=0.02), IL-6 (0.4 [0.3, 0.5]; 0.2 [0.1, 0.2] pg/ml; p=0.001), and PAI-1 (50 [42.7, 58.7]; 36.2 [30.8, 42.4]pg/ml; p=0.01). No differences in baseline ATGMCVR were seen between subjects with and without MetS.

CONCLUSION

These findings suggest ATGMCVR can be improved with weight loss; larger studies are needed to determine if improvements in metabolic dysfunction are related to changes in ATGMCVR.

Cyclin D1 represses gluconeogenesis via inhibition of the transcriptional coactivator PGC1α

Hepatic gluconeogenesis is crucial to maintain normal blood glucose during periods of nutrient deprivation. Gluconeogenesis is controlled at multiple levels by a variety of signal transduction and transcriptional pathways. However, dysregulation of these pathways leads to hyperglycemia and type 2 diabetes. While the effects of various signaling pathways on gluconeogenesis are well established, the downstream signaling events repressing gluconeogenic gene expression are not as well understood. The cell-cycle regulator cyclin D1 is expressed in the liver, despite the liver being a quiescent tissue. The most well-studied function of cyclin D1 is activation of cyclin-dependent kinase 4 (CDK4), promoting progression of the cell cycle. We show here a novel role for cyclin D1 as a regulator of gluconeogenic and oxidative phosphorylation (OxPhos) gene expression. In mice, fasting decreases liver cyclin D1 expression, while refeeding induces cyclin D1 expression. Inhibition of CDK4 enhances the gluconeogenic gene expression, whereas cyclin D1-mediated activation of CDK4 represses the gluconeogenic gene-expression program in vitro and in vivo. Importantly, we show that cyclin D1 represses gluconeogenesis and OxPhos in part via inhibition of peroxisome proliferator-activated receptor γ coactivator-1α (PGC1α) activity in a CDK4-dependent manner. Indeed, we demonstrate that PGC1α is novel cyclin D1/CDK4 substrate. These studies reveal a novel role for cyclin D1 on metabolism via PGC1α and reveal a potential link between cell-cycle regulation and metabolic control of glucose homeostasis.

Methionine restriction restores a younger metabolic phenotype in adult mice with alterations in fibroblast growth factor 21

Methionine restriction (MR) decreases body weight and adiposity and improves glucose homeostasis in rodents. Similar to caloric restriction, MR extends lifespan, but is accompanied by increased food intake and energy expenditure. Most studies have examined MR in young animals; therefore, the aim of this study was to investigate the ability of MR to reverse age-induced obesity and insulin resistance in adult animals. Male C57BL/6J mice aged 2 and 12 months old were fed MR (0.172% methionine) or control diet (0.86% methionine) for 8 weeks or 48 h. Food intake and whole-body physiology were assessed and serum/tissues analyzed biochemically. Methionine restriction in 12-month-old mice completely reversed age-induced alterations in body weight, adiposity, physical activity, and glucose tolerance to the levels measured in healthy 2-month-old control-fed mice. This was despite a significant increase in food intake in 12-month-old MR-fed mice. Methionine restriction decreased hepatic lipogenic gene expression and caused a remodeling of lipid metabolism in white adipose tissue, alongside increased insulin-induced phosphorylation of the insulin receptor (IR) and Akt in peripheral tissues. Mice restricted of methionine exhibited increased circulating and hepatic gene expression levels of FGF21, phosphorylation of eIF2a, and expression of ATF4, with a concomitant decrease in IRE1α phosphorylation. Short-term 48-h MR treatment increased hepatic FGF21 expression/secretion and insulin signaling and improved whole-body glucose homeostasis without affecting body weight. Our findings suggest that MR feeding can reverse the negative effects of aging on body mass, adiposity, and insulin resistance through an FGF21 mechanism. These findings implicate MR dietary intervention as a viable therapy for age-induced metabolic syndrome in adult humans.

Protein-tyrosine phosphatase 1B substrates and metabolic regulation

Metabolic homeostasis requires integration of complex signaling networks which, when deregulated, contribute to metabolic syndrome and related disorders. Protein-tyrosine phosphatase 1B (PTP1B) has emerged as a key regulator of signaling networks that are implicated in metabolic diseases such as obesity and type 2 diabetes. In this review, we examine mechanisms that regulate PTP1B-substrate interaction, enzymatic activity and experimental approaches to identify PTP1B substrates. We then highlight findings that implicate PTP1B in metabolic regulation. In particular, insulin and leptin signaling are discussed as well as recently identified PTP1B substrates that are involved in endoplasmic reticulum stress response, cell-cell communication, energy balance and vesicle trafficking. In summary, PTP1B exhibits exquisite substrate specificity and is an outstanding pharmaceutical target for obesity and type 2 diabetes.

Insulin resistance induced by hyperinsulinemia coincides with a persistent alteration at the insulin receptor tyrosine kinase domain

Insulin resistance, the diminished response of target tissues to insulin, is associated with the metabolic syndrome and a predisposition towards diabetes in a growing proportion of the worldwide population. Under insulin resistant states, the cellular response of the insulin signaling pathway is diminished and the body typically responds by increasing serum insulin concentrations to maintain insulin signaling. Some evidence indicates that the increased insulin concentration may itself further dampen insulin response. If so, insulin resistance would worsen as the level of circulating insulin increases during compensation, which could contribute to the transition of insulin resistance to more severe disease. Here, we investigated the consequences of excess insulin exposure to insulin receptor (IR) activity. Cells chronically exposed to insulin show a diminished the level of IR tyrosine and serine autophosphorylation below that observed after short-term insulin exposure. The diminished IR response did not originate with IR internalization since IR amounts at the cell membrane were similar after short- and long-term insulin incubation. Förster resonance energy transfer between fluorophores attached to the IR tyrosine kinase (TK) domain showed that a change in the TK domain occurred upon prolonged, but not short-term, insulin exposure. Even though the altered ‘insulin refractory’ IR TK FRET and IR autophosphorylation levels returned to baseline (non-stimulated) levels after wash-out of the original insulin stimulus, subsequent short-term exposure to insulin caused immediate re-establishment of the insulin-refractory levels. This suggests that some cell-based ‘memory’ of chronic hyperinsulinemic exposure acts directly at the IR. An improved understanding of that memory may help define interventions to reset the IR to full insulin responsiveness and impede the progression of insulin resistance to more severe disease states.

Insulin resistance in Alzheimer's disease

Insulin is a key hormone regulating metabolism. Insulin binding to cell surface insulin receptors engages many signaling intermediates operating in parallel and in series to control glucose, energy, and lipids while also regulating mitogenesis and development. Perturbations in the function of any of these intermediates, which occur in a variety of diseases, cause reduced sensitivity to insulin and insulin resistance with consequent metabolic dysfunction. Chronic inflammation ensues which exacerbates compromised metabolic homeostasis. Since insulin has a key role in learning and memory as well as directly regulating ERK, a kinase required for the type of learning and memory compromised in early Alzheimer's disease (AD), insulin resistance has been identified as a major risk factor for the onset of AD. Animal models of AD or insulin resistance or both demonstrate that AD pathology and impaired insulin signaling form a reciprocal relationship. Of note are human and animal model studies geared toward improving insulin resistance that have led to the identification of the nuclear receptor and transcription factor, peroxisome proliferator-activated receptor gamma (PPARγ) as an intervention tool for early AD. Strategic targeting of alternate nodes within the insulin signaling network has revealed disease-stage therapeutic windows in animal models that coalesce with previous and ongoing clinical trial approaches. Thus, exploiting the connection between insulin resistance and AD provides powerful opportunities to delineate therapeutic interventions that slow or block the pathogenesis of AD.

[Implication of MAP kinases in obesity-induced inflammation and insulin resistance]

Insulin resistance is often associated with obesity and is a major risk factor for development of type 2 diabetes as well as cardiovascular and hepatic diseases. Insulin resistance may also increase the incidence or the aggressiveness of some cancers. Insulin resistance occurs owing to defects in insulin signaling in target tissues of this hormone. During the last ten years, it became evident that the chronic low-grade inflammatory state that develops during obesity plays an important role in insulin resistance development. Indeed, inflammatory cytokines activate several signaling pathways that impinge on the insulin signaling pathway. Among them, this review will focus on the implication of the MAP kinases JNK and ERK1/2 signaling in the development of insulin signaling alterations and will discuss the possibility to target these pathways in order to fight insulin resistance.