The insulin receptor--a critical link in glucose homeostasis and insulin action

Physiological and pathophysiological actions of insulin in the liver

The liver plays an important role in the control of glucose homeostasis. When insulin levels are low, such as in the fasting state, gluconeogenesis and glycogenolysis are stimulated to maintain the blood glucose levels. Conversely, in the presence of increased insulin levels, such as after a meal, synthesis of glycogen and lipid occurs to maintain the blood glucose levels within normal range. Insulin receptor signaling regulates glycogenesis, gluconeogenesis and lipogenesis through downstream pathways such as the insulin receptor substrate (IRS)-phosphoinositide 3 (PI3) kinase-Akt pathway. IRS-1 and IRS-2 are abundantly expressed in the liver and are thought to be responsible for transmitting the insulin signal from the insulin receptor to the intracellular effectors involved in the regulation of glucose and lipid homeostasis. Impaired insulin receptor signaling can cause hepatic insulin resistance and lead to type 2 diabetes. In the present study, we focus on a concept called "selective insulin resistance," which has received increasing attention recently: the frequent coexistence of hyperglycemia and hepatic steatosis in people with type 2 diabetes and obesity suggests that it is possible for the insulin signaling regulating gluconeogenesis to be impaired even while that regulating lipogenesis is preserved, suggestive of selective insulin resistance. In this review, we review the progress in research on the insulin actions and insulin signaling in the liver.

Special features on insulin resistance, metabolic syndrome and vascular complications in hypopituitary patients

Pituitary hormone deficiency, hypopituitarism, is a dysfunction resulting from numerous etiologies, which can be complete or partial, and is therefore heterogeneous. This heterogeneity makes it difficult to interpret the results of scientific studies with these patients.Adequate treatment of etiologies and up-to-date hormone replacement have improved morbidity and mortality rates in patients with hypopituitarism. As GH replacement is not performed in a reasonable proportion of patients, especially in some countries, it is essential to understand the known consequences of GH replacement in each subgroup of patients with this heterogeneous dysfunction.In this review on hypopituitarism, we will address some particularities regarding insulin resistance, which is no longer common in these patients with hormone replacement therapy based on current guidelines, metabolic syndrome and its relationship with changes in BMI and body composition, and to vascular complications that need to be prevented taking into account the individual characteristics of each case to reduce mortality rates in these patients.

Glycomacropeptide in PKU-Does It Live Up to Its Potential?

The use of casein glycomacropeptide (CGMP) as a protein substitute in phenylketonuria (PKU) has grown in popularity. CGMP is derived from κ casein and is a sialic-rich glycophosphopeptide, formed by the action of chymosin during the production of cheese. It comprises 20–25% of total protein in whey products and has key biomodulatory properties. In PKU, the amino acid sequence of CGMP has been adapted by adding the amino acids histidine, leucine, methionine, tyrosine and tryptophan naturally low in CGMP. The use of CGMP compared to mono amino acids (L-AAs) as a protein substitute in the treatment of PKU promises several potential clinical benefits, although any advantage is supported only by evidence from non-PKU conditions or PKU animal models. This review examines if there is sufficient evidence to support the bioactive properties of CGMP leading to physiological benefits when compared to L-AAs in PKU, with a focus on blood phenylalanine control and stability, body composition, growth, bone density, breath odour and palatability.

INitial Steps of Insulin Action in Parotid Glands of Male Wistar Rats

The parotid gland is the largest salivary gland. It produces watery saliva, rich in proteins (amylase, lysozymes, and antibodies). Due to the gland's morphological cytoarchitecture composed of only serous acini, it contributes almost 50% of total salivary volume upon stimulation. It has been reported that the prevalence of saliva secretion impairments, periodontitis, delayed wound healing, and xerostomia increase in diabetic patients. Herein we evaluated the acute effects of insulin on insulin receptor phosphorylation status and its substrates IRS-1 and IRS-2 in the parotid glands of adult male Wistar rats, using Western blot analyses. We confirmed an acute effect of insulin on IR/IRS/PI3K/Akt and MAPK intracellular pathway activation in the parotid glands of male Wistar rats similar to the classical metabolic targets of the hormone, like the liver.

Glucose, insulin, insulin receptor subunits α and β in normal and spontaneously diabetic and obese ob/ob and db/db infertile mouse testis and hypophysis

BACKGROUND

Type 2 diabetes touches young subjects of reproductive age in epidemic proportion. This study assesses glucose, total InsulinT, Insulin2 and insulin receptor subunits α and β in testis during mouse development then, in the spontaneously type 2 diabetes models associated with infertility db/db and ob/ob mice. IR-β and α were also assessed in spermatozoa (SPZ), anterior pituitary (AP) and serum.

METHODS

Serum and tissue glucose were measured with enzymatic colorimetric assays and InsulinT and Insulin2 by ELISAs in serum, interstitial tissue- (ITf) and seminiferous tubule (STf) fractions in14- > 60-day-old normal and db/db, ob/ob and wild type (WT) mice. IR subunits were assessed by immunoblotting in tissues and by immunoprecipitation followed by immunoblotting in serum.

RESULTS

Development: Glucose increased in serum, ITf and STf. InsulinT and Insulin2 dropped in serum; both were higher in STf than in ITf. In > 60-day-old mouse ITf, insulinT rose whereas Insulin2 decreased; InsulinT and Insulin2 rose concurrently in STf. Glucose and insulin were high in > 60-day-old ITf; in STf high insulin2 accompanied low glucose. One hundred ten kDa IR-β peaked in 28-day-old ITf and 14-day-old STf. One hundred thirty five kDa IR-α was high in ITf but decreased in STf. Glucose escalated in db/db and ob/ob sera. Glucose doubled in ITf while being halved in STf in db/db mice. Glucose significantly dropped in db/db and ob/ob mice spermatozoa. InsulinT and Insulin2 rose significantly in the serum, ITf and STf in db/db and ob/ob mice. One hundred ten kDa IR-β and 135 kDa IR-α decreased in db/db and ob/ob ITf. Only 110 kDa IR-β dropped in db/db and ob/ob STf and AP. One hundred ten kDa IR-β fell in db/db and ob/ob SPZ. One hundred ten kDa sIR-α rose in the db/db and ob/ob mouse sera.

CONCLUSION

Insulin regulates glucose in tubules not in the interstitium. The mouse interstitium contains InsulinT and Insulin2 whereas tubules contain Insulin2. Decreased 110 kDa IR-β and 135 kDa IR-α in the db/db and ob/ob interstitial tissue suggest a loss of active receptor sites that could alter the testicular cell insulin binding and response to the hormone. Decreased IR-β levels were insufficient to stimulate downstream effectors in AP and tubules. IR-α shedding increased in db/db and ob/ob mice.

Perinatal programming of metabolic diseases: The role of glucocorticoids

The worldwide increase in metabolic diseases has urged the scientific community to improve our understanding about the mechanisms underlying its cause and effects. A well supported area of studies had related maternal stress with early programming to the later metabolic diseases. Mechanisms upon origins of metabolic disturbances are not yet fully understood, even though stressful factors rising glucocorticoids have been put out as pivotal trigger by programming metabolic diseases as long-term consequence. Considering energy balance and glucose homeostasis, by producing and/or sensing regulator signals, hypothalamus-pituitary-adrenal axis and endocrine pancreas are directly affected by glucocorticoids excess. We focus on the evidences reporting the role of increased glucocorticoids due to perinatal insults on the physiological systems involved in the metabolic homeostasis and in the target organs such as endocrine pancreas, white adipose tissue and blood vessels. Besides, we review some mechanisms underlining the malprogramming of type 2 diabetes, obesity and hypertension. Studies on this field are currently ongoing and even there is a good understanding regarding the effects of glucocorticoids addressing metabolic diseases, few is known about the relationship between maternal insults rising glucocorticoids to pups' metabolic disturbances, a thorough understanding about that may provide pivotal clinical clues regarding those disorders.

Expression of Dual-Specificity Phosphatase 9 in Placenta and Its Relationship with Gestational Diabetes Mellitus

INTRODUCTION

The aim of the present study was to examine placental levels of DUSP9 mRNA and protein and to investigate the potential role of DUSP9 in the development of gestational diabetes mellitus (GDM).

METHODS

Placental tissues from pregnant women with GDM (n = 17) and normal healthy pregnant women (n = 16) were collected at delivery. The expression of DUSP9 mRNA in placental tissue was analyzed by real-time PCR, while the expression of DUPS9 protein was evaluated by immunohistochemistry and western blot. Differences in the expression levels of DUSP9 mRNA and protein between the two groups were assessed, as well as potential correlations between DUSP9 mRNA expression levels and relevant clinical indicators.

RESULTS

Blood glucose levels were significantly higher in the GDM group than in the control group, based on an oral glucose tolerance test. DUSP9 protein was expressed in the placental cytotrophoblasts in both groups, and placental levels of DUSP9 protein and mRNA were significantly higher in women with GDM. Placental DUSP9 mRNA levels in all 33 women correlated moderately with delivery gestational week (R = 0.465, P = 0.006), fasting plasma glucose (R = 0.350, P = 0.046), 1-hour postload plasma glucose (R = 0.363, P = 0.038), and 2-hour postload plasma glucose (R = 0.366, P = 0.036), but not with maternal age, preconception body mass index, prenatal body mass index, or neonatal birth weight. Multiple linear regression analysis indicated that delivery gestational week was an influence factor of DUSP9 mRNA levels (β ₁ = 0.026, P < 0.05).

CONCLUSIONS

DUSP9 upregulation in the placenta of GDM pregnant women may promote insulin resistance, which may correlate with the occurrence of GDM. But there is still possibility that DUSP9 upregulation was the results of insulin resistance and/or hyperglycemia. Further research is needed to explore the role of DUSP9 in GDM.

Lactational programming of glucose homeostasis: a window of opportunity

The window of lactation is a critical period during which nutritional and environmental exposures impact lifelong metabolic disease risk. Significant organ and tissue development, organ expansion and maturation of cellular functions occur during the lactation period, making this a vulnerable time during which transient insults can have lasting effects. This review will cover current literature on factors influencing lactational programming such as milk composition, maternal health status and environmental endocrine disruptors. The underlying mechanisms that have the potential to contribute to lactational programming of glucose homeostasis will also be addressed, as well as potential interventions to reduce offspring metabolic disease risk.

Cardiovascular Action of Insulin in Health and Disease: Endothelial L-Arginine Transport and Cardiac Voltage-Dependent Potassium Channels

Impairment of insulin signaling on diabetes mellitus has been related to cardiovascular dysfunction, heart failure, and sudden death. In human endothelium, cationic amino acid transporter 1 (hCAT-1) is related to the synthesis of nitric oxide (NO) and insulin has a vascular effect in endothelial cells through a signaling pathway that involves increases in hCAT-1 expression and L-arginine transport. This mechanism is disrupted in diabetes, a phenomenon potentiated by excessive accumulation of reactive oxygen species (ROS), which contribute to lower availability of NO and endothelial dysfunction. On the other hand, electrical remodeling in cardiomyocytes is considered a key factor in heart failure progression associated to diabetes mellitus. This generates a challenge to understand the specific role of insulin and the pathways involved in cardiac function. Studies on isolated mammalian cardiomyocytes have shown prolongated action potential in ventricular repolarization phase that produces a long QT interval, which is well explained by attenuation in the repolarizing potassium currents in cardiac ventricles. Impaired insulin signaling causes specific changes in these currents, such a decrease amplitude of the transient outward K(+) (Ito) and the ultra-rapid delayed rectifier (IKur) currents where, together, a reduction of mRNA and protein expression levels of α-subunits (Ito, fast; Kv 4.2 and IKs; Kv 1.5) or β-subunits (KChIP2 and MiRP) of K(+) channels involved in these currents in a MAPK mediated pathway process have been described. These results support the hypothesis that lack of insulin signaling can produce an abnormal repolarization in cardiomyocytes. Furthermore, the arrhythmogenic potential due to reduced Ito current can contribute to an increase in the incidence of sudden death in heart failure. This review aims to show, based on pathophysiological models, the regulatory function that would have insulin in vascular system and in cardiac electrophysiology.

PI3K Phosphorylation Is Linked to Improved Electrical Excitability in an In Vitro Engineered Heart Tissue Disease Model System

Myocardial infarction, a prevalent cardiovascular disease, is associated with cardiomyocyte cell death, and eventually heart failure. Cardiac tissue engineering has provided hopes for alternative treatment options, and high-fidelity tissue models for drug discovery. The signal transduction mechanisms relayed in response to mechanoelectrical (physical) stimulation or biochemical stimulation (hormones, cytokines, or drugs) in engineered heart tissues (EHTs) are poorly understood. In this study, an EHT model was used to elucidate the signaling mechanisms involved when insulin was applied in the presence of electrical stimulation, a stimulus that mimics functional heart tissue environment in vitro. EHTs were insulin treated, electrically stimulated, or applied in combination (insulin and electrical stimulation). Electrical excitability parameters (excitation threshold and maximum capture rate) were measured. Protein kinase B (AKT) and phosphatidylinositol-3-kinase (PI3K) phosphorylation revealed that insulin and electrical stimulation relayed electrical excitability through two separate signaling cascades, while there was a negative crosstalk between sustained activation of AKT and PI3K.

Effects of different intensities of physical exercise on insulin sensitivity and protein kinase B/Akt activity in skeletal muscle of obese mice

Objective:

To investigate the effects of different intensities of acute exercise on insulin sensitivity and protein kinase B/Akt activity in skeletal muscle of obese mice.

Methods:

Swiss mice were randomly divided into four groups, and fed either a standard diet (control group) or high fat diet (obese sedentary group and obese exercise group 1 and 2) for 12 weeks. Two different exercise protocols were used: swimming for 1 hour with or without an overload of 5% body weight. The insulin tolerance test was performed to estimate whole-body sensitivity. Western blot technique was used to determine protein levels of protein kinase B/Akt and phosphorylation by protein Kinase B/Akt in mice skeletal muscle.

Results:

A single bout of exercise inhibited the high fat diet-induced insulin resistance. There was increase in phosphorylation by protein kinase B/Akt serine, improve in insulin signaling and reduce of fasting glucose in mice that swam for 1 hour without overload and mice that swan for 1 hour with overload of 5%. However, no significant differences were seen between exercised groups.

Conclusion:

Regardless of intensity, aerobic exercise was able to improve insulin sensitivity and phosphorylation by protein kinase B/Ak, and proved to be a good form of treatment and prevention of type 2 diabetes.

Insulin and wound healing

Skin is a dynamic and complex organ that relies on the interaction of different cell types, biomacromolecules and signaling molecules. Injury triggers a cascade of events designed to quickly restore skin integrity. Depending on the size and severity of the wound, extensive physiological and metabolic changes can occur, resulting in impaired wound healing and increased morbidity resulting in higher rates of death. While wound dressings provide a temporary barrier, they are inherently incapable of significantly restoring metabolic upsets, post-burn insulin resistance, and impaired wound healing in patients with extensive burns. Exogenous insulin application has therefore been investigated as a potential therapeutic intervention for nearly a century to improve wound recovery. This review will highlight the important achievements that demonstrate insulin's ability to stimulate cellular migration and burn wound recovery, as well as providing a perspective on future therapeutic applications and research directions.

Role of protein tyrosine phosphatases in the modulation of insulin signaling and their implication in the pathogenesis of obesity-linked insulin resistance

Insulin resistance is a major disorder that links obesity to type 2 diabetes mellitus (T2D). It involves defects in the insulin actions owing to a reduced ability of insulin to trigger key signaling pathways in major metabolic tissues. The pathogenesis of insulin resistance involves several inhibitory molecules that interfere with the tyrosine phosphorylation of the insulin receptor and its downstream effectors. Among those, growing interest has been developed toward the protein tyrosine phosphatases (PTPs), a large family of enzymes that can inactivate crucial signaling effectors in the insulin signaling cascade by dephosphorylating their tyrosine residues. Herein we briefly review the role of several PTPs that have been shown to be implicated in the regulation of insulin action, and then focus on the Src homology 2 (SH2) domain-containing SHP1 and SHP2 enzymes, since recent reports have indicated major roles for these PTPs in the control of insulin action and glucose metabolism. Finally, the therapeutic potential of targeting PTPs for combating insulin resistance and alleviating T2D will be discussed.

Something old, something new and something borrowed: emerging paradigm of insulin-like growth factor type 1 receptor (IGF-1R) signaling regulation

The insulin-like growth factor type 1 receptor (IGF-1R) plays a key role in the development and progression of cancer; however, therapeutics targeting it have had disappointing results in the clinic. As a receptor tyrosine kinase (RTK), IGF-1R is traditionally described as an ON/OFF system, with ligand stabilizing the ON state and exclusive kinase-dependent signaling activation. Newly added to the traditional model, ubiquitin-mediated receptor downregulation and degradation was originally described as a response to ligand/receptor interaction and thus inseparable from kinase signaling activation. Yet, the classical model has proven over-simplified and insufficient to explain experimental evidence accumulated over the last decade, including kinase-independent signaling, unbalanced signaling, or dissociation between signaling and receptor downregulation. Based on the recent findings that IGF-1R “borrows” components of G-protein coupled receptor (GPCR) signaling, including β-arrestins and G-protein-related kinases, we discuss the emerging paradigm for the IGF-1R as a functional RTK/GPCR hybrid, which integrates the kinase signaling with the IGF-1R canonical GPCR characteristics. The contradictions to the classical IGF-1R signaling concept as well as the design of anti-IGF-1R therapeutics treatment are considered in the light of this paradigm shift and we advocate recognition of IGF-1R as a valid target for cancer treatment.

Inference of gene-phenotype associations via protein-protein interaction and orthology

One of the fundamental goals of genetics is to understand gene functions and their associated phenotypes. To achieve this goal, in this study we developed a computational algorithm that uses orthology and protein-protein interaction information to infer gene-phenotype associations for multiple species. Furthermore, we developed a web server that provides genome-wide phenotype inference for six species: fly, human, mouse, worm, yeast, and zebrafish. We evaluated our inference method by comparing the inferred results with known gene-phenotype associations. The high Area Under the Curve values suggest a significant performance of our method. By applying our method to two human representative diseases, Type 2 Diabetes and Breast Cancer, we demonstrated that our method is able to identify related Gene Ontology terms and Kyoto Encyclopedia of Genes and Genomes pathways. The web server can be used to infer functions and putative phenotypes of a gene along with the candidate genes of a phenotype, and thus aids in disease candidate gene discovery. Our web server is available at http://jjwanglab.org/PhenoPPIOrth.

Staying alive: metabolic adaptations to quiescence

Quiescence is a state of reversible cell cycle arrest that can grant protection against many environmental insults. In some systems, cellular quiescence is associated with a low metabolic state characterized by a decrease in glucose uptake and glycolysis, reduced translation rates and activation of autophagy as a means to provide nutrients for survival. For cells in multiple different quiescence model systems, including Saccharomyces cerevisiae, mammalian lymphocytes and hematopoietic stem cells, the PI3Kinase/TOR signaling pathway helps to integrate information about nutrient availability with cell growth rates. Quiescence signals often inactivate the TOR kinase, resulting in reduced cell growth and biosynthesis. However, quiescence is not always associated with reduced metabolism; it is also possible to achieve a state of cellular quiescence in which glucose uptake, glycolysis and flux through central carbon metabolism are not reduced. In this review, we compare and contrast the metabolic changes that occur with quiescence in different model systems.

Partial inactivation of Ankrd26 causes diabetes with enhanced insulin responsiveness of adipose tissue in mice

AIMS/HYPOTHESIS

ANKRD26 is a newly described gene located at 10p12 in humans, a locus that has been identified with some forms of hereditary obesity. Previous studies have shown that partial inactivation of Ankrd26 in mice causes hyperphagia, obesity and gigantism. Hypothesising that Ankrd26 mutant (MT) mice could develop diabetes, we sought to establish whether the observed phenotype could be (1) solely related to the development of obesity or (2) caused by a direct action of ankyrin repeat domain 26 (ANKRD26) in peripheral tissues.

METHODS

To test the hypothesis, we did a full metabolic characterisation of Ankrd26 MT mice that had free access to chow or were placed under two different energy-restricted dietary regimens.

RESULTS

Highly obese Ankrd26 MT mice developed an unusual form of diabetes in which white adipose tissue is insulin-sensitive, while other tissues are insulin-resistant. When obese MT mice were placed on a food-restricted diet, their weight and glucose homeostasis returned to normal. In addition, when young MT mice were placed on a pair-feeding diet with normal mice, they maintained normal body weight, but showed better glucose tolerance than normal mice, an increased responsiveness of white adipose tissue to insulin and enhanced phosphorylation of the insulin receptor.

CONCLUSIONS/INTERPRETATION

These findings show that the ANKRD26 protein has at least two functions in mice. One is to control the response of white adipose tissue to insulin; the other is to control appetite, which when Ankrd26 is mutated, leads to hyperphagia and diabetes in an obesity-dependent manner.

A novel integrative network approach to understand the interplay between cardiovascular disease and other complex disorders

There is accumulating evidence that the proteins encoded by the genes associated with a common disorder interact with each other, participate in similar pathways and share GO terms. It has been anticipated that the functional modules in a disease related functional linkage network can be integrated with bibliomics to reveal association with other complex disorders. In this study, the cardiovascular disease functional linkage network (CFN) containing 1536 nodes and 3345 interactions was constructed using proteins encoded by 234 genes associated with the disease. Integration of CFN with bibliomics showed that 227 out of 566 functional modules are significantly associated with one or more diseases. Analysis of functional modules revealed the possible regulatory roles of SP1 and CXCL12 in the pathogenesis of cardiovascular disease (CVD) and modulation of their activities may be considered as potential therapeutic tools. The integration of CFN with bibliomics also indicated significant relations of CVD with other complex disorders. In a stratified map the members of 227 functional modules and 58 diseases in 15 disease classes were combined. In this map, leprosy, listeria monocytogenes, myasthenia, hemorrhagic diathesis and Protein S deficiency, which were not previously reported to be associated with CVD, showed significant associations. Several cancers arising from epithelial cells were also found to be linked to other diseases through hub proteins, VEGFA and PTGS2.

Differential endocytosis and signaling dynamics of insulin receptor variants IR-A and IR-B

Insulin signaling comprises a complex cascade of events, playing a key role in the regulation of glucose metabolism and cellular growth. Impaired response to insulin is the hallmark of diabetes, whereas upregulated insulin activity occurs in many cancers. Two splice variants of the insulin receptor (IR) exist in mammals: IR-A, lacking exon 11, and full-length IR-B. Although considerable biochemical data exist on insulin binding and downstream signaling, little is known about the dynamics of the IR itself. We created functional IR transgenes fused with visible fluorescent proteins for use in combination with biotinamido-caproyl insulin and streptavidin quantum dots. Using confocal and structured illumination microscopy, we visualized the endocytosis of both isoforms in living and fixed cells and demonstrated a higher rate of endocytosis of IR-A than IR-B. These differences correlated with higher and sustained activation of IR-A in response to insulin and with distinctive ERK1/2 activation profiles and gene transcription regulation. In addition, cells expressing IR-B showed higher AKT phosphorylation after insulin stimulation than cells expressing IR-A. Taken together, these results suggest that IR signaling is dependent on localization; internalized IRs regulate mitogenic activity, whereas metabolic balance signaling occurs at the cell membrane.

Insulin resistance at the crossroads of metabolic syndrome: systemic analysis using microarrays

Recently, it has been suggested that insulin resistance is a better predictor of metabolic syndrome than obesity. Numerous studies have been conducted to identify insulin resistance susceptibility genes in various model systems. This review focuses on recent findings in microarray analyses, which have indicated that (i) in the liver, genes involved in lipid synthesis and gluconeogenesis are increased in an animal model of insulin resistance that leads into liver steatosis and hyperglycemia; (ii) in adipose tissues, genes involved in fatty acid synthesis and adipogenesis are down-regulated both in insulin-resistant humans and in animals; and (iii) in muscle, overall gene expression, including genes involved in fatty acid oxidation and biosynthesis, is either decreased or unresponsive compared to that of insulin-sensitive control human subjects or animals. Considering the multifaceted effects of insulin resistance in various tissues, aiming at multi-targets rather than a single target will be a more promising strategy for the prevention or treatment of insulin resistance.

Insulin and hyperandrogenism in women with polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is a very common endocrine disorder characterized by chronic anovulation, clinical and/or biochemical hyperandrogenism, and/or polycystic ovaries. But most experts consider that hyperandrogenism is the main characteristic of PCOS. Several theories propose different mechanisms to explain PCOS manifestations: (1) a primary enzymatic default in the ovarian and/or adrenal steroidogenesis; (2) an impairment in gonadotropin releasing hormone (GnRH) secretion that promotes luteal hormone (LH) secretion; or (3) alterations in insulin actions that lead to insulin resistance with compensatory hyperinsulinemia. However, in the past 20 years there has been growing evidence supporting that defects in insulin actions or in the insulin signalling pathways are central in the pathogenesis of the syndrome. Indeed, most women with PCOS are metabolically insulin resistant, in part due to genetic predisposition and in part secondary to obesity. But some women with typical PCOS do not display insulin resistance, which supports the hypothesis of a genetic predisposition specific to PCOS that would be revealed by the development of insulin resistance and compensatory hyperinsulinemia in most, but not all, women with PCOS. However, these hypotheses are not yet appropriately confirmed, and more research is still needed to unravel the true pathogenesis underlying this syndrome. The present review thus aims at discussing new concepts and findings regarding insulin actions in PCOS women and how it is related to hyperandrogenemia.

The double-stranded RNA-dependent protein kinase differentially regulates insulin receptor substrates 1 and 2 in HepG2 cells

The RNA-dependent protein kinase (PKR), initially known as a virus infection response protein, is found to differentially regulate two major players in the insulin signaling pathway, IRS1 and IRS2. PKR up-regulates the inhibitory phosphorylation of IRS1 and the expression of IRS2 at the transcriptional level.

Initially identified to be activated upon virus infection, the double-stranded RNA–dependent protein kinase (PKR) is best known for triggering cell defense responses by phosphorylating eIF-2α, thus suppressing RNA translation. We as well as others showed that the phosphorylation of PKR is down-regulated by insulin. In the present study, we further uncovered a novel function of PKR in regulating the IRS proteins. We found that PKR up-regulates the inhibitory phosphorylation of IRS1 at Ser312, which suppresses the tyrosine phosphorylation of IRS1. This effect of PKR on the phosphorylation of IRS1 is mediated by two other protein kinases, JNK and IKK. In contrast, PKR regulates IRS2, another major IRS family protein in the liver, at the transcriptional rather than the posttranslational level, and this effect is mediated by the transcription factor, FoxO1, which has been previously shown to be regulated by insulin and plays a significant role in glucose homeostasis and energy metabolism. In summary, we found for the first time that initially known as a virus infection response gene, PKR regulates the upstream central transmitters of insulin signaling, IRS1 and IRS2, through different mechanisms.

Protein kinase inhibition differentially regulates organic cation transport

Previous studies showed that amantadine transport increased while tetraethylammonium (TEA) transport decreased in kidney tissue from diabetic rats. Changes in transport activity were reversed by exogenous insulin. We hypothesized that this difference in transport regulation is due to differential regulation of different transport systems. Native human embryonic kidney cortex cells (HEK293 cell line) and rat organic cation transporter (rOCT)-transfected cells were used to test the hypothesis. In support of differential regulation, short-term glucose starvation stimulated amantadine transport and inhibited TEA transport, but the effect was bicarbonate-modulated only for amantadine. cAMP analogues inhibited TEA transport while stimulating amantadine transport. This effect was additive to the effect of insulin, and the presence of bicarbonate affected the extent of the change. Our findings indicated that regulation of rOCT 1 and 2 was mediated by transmembrane adenylyl cyclase, and regulation of amantadine transport was mediated by soluble adenylyl cyclase, suggesting that intracellular microdomains of cAMP may be important in determining overall cellular transport for organic cations. Soluble adenylyl cyclase activity is known to be modulated by bicarbonate and lactate. These observations support our hypothesis and reconcile our previous studies demonstrating increased transport affinity for amantadine in the presence of bicarbonate and decreased transport affinity in the presence of lactate.

A dynamic analysis of IRS-PKR signaling in liver cells: a discrete modeling approach

A major challenge in systems biology is to develop a detailed dynamic understanding of the functions and behaviors in a particular cellular system, which depends on the elements and their inter-relationships in a specific network. Computational modeling plays an integral part in the study of network dynamics and uncovering the underlying mechanisms. Here we proposed a systematic approach that incorporates discrete dynamic modeling and experimental data to reconstruct a phenotype-specific network of cell signaling. A dynamic analysis of the insulin signaling system in liver cells provides a proof-of-concept application of the proposed methodology. Our group recently identified that double-stranded RNA-dependent protein kinase (PKR) plays an important role in the insulin signaling network. The dynamic behavior of the insulin signaling network is tuned by a variety of feedback pathways, many of which have the potential to cross talk with PKR. Given the complexity of insulin signaling, it is inefficient to experimentally test all possible interactions in the network to determine which pathways are functioning in our cell system. Our discrete dynamic model provides an in silico model framework that integrates potential interactions and assesses the contributions of the various interactions on the dynamic behavior of the signaling network. Simulations with the model generated testable hypothesis on the response of the network upon perturbation, which were experimentally evaluated to identify the pathways that function in our particular liver cell system. The modeling in combination with the experimental results enhanced our understanding of the insulin signaling dynamics and aided in generating a context-specific signaling network.

Zinc and diabetes--clinical links and molecular mechanisms

Zinc is an essential trace element crucial for the function of more than 300 enzymes and it is important for cellular processes like cell division and apoptosis. Hence, the concentration of zinc in the human body is tightly regulated and disturbances of zinc homeostasis have been associated with several diseases including diabetes mellitus, a disease characterized by high blood glucose concentrations as a consequence of decreased secretion or action of insulin. Zinc supplementation of animals and humans has been shown to ameliorate glycemic control in type 1 and 2 diabetes, the two major forms of diabetes mellitus, but the underlying molecular mechanisms have only slowly been elucidated. Zinc seems to exert insulin-like effects by supporting the signal transduction of insulin and by reducing the production of cytokines, which lead to beta-cell death during the inflammatory process in the pancreas in the course of the disease. Furthermore, zinc might play a role in the development of diabetes, since genetic polymorphisms in the gene of zinc transporter 8 and in metallothionein (MT)-encoding genes could be demonstrated to be associated with type 2 diabetes mellitus. The fact that antibodies against this zinc transporter have been detected in type 1 diabetic patients offers new diagnostic possibilities. This article reviews the influence of zinc on the diabetic state including the molecular mechanisms, the role of the zinc transporter 8 and MT for diabetes development and the resulting diagnostic and therapeutic options.

Improvement of high-fat-diet-induced metabolic syndrome by a compound from Balanophora polyandra Griff in mice

This study was to explore the effects of a compound (BPG) from Balanophora polyandra Griff on metabolic syndrome in mice. The animal models, developed obesity, dyslipidemia and insulin resistance, were induced by high-fat-diet in C57BL/6 mice, and were treated orally with 100 mg/kg/day BPG and 15 mg/kg/day rosiglitazone, respectively. The age-matched C57BL/6 mice fed with standard chow were used as normal control. The blood glucose, the value of serum triglyceride and the content of triglyceride in the skeletal muscle were determined by biochemical methods. The protein expression was evaluated by western blot. BPG administration decreased body weight gain, adiposity index, serum triglyceride levels, and triglyceride accumulation in skeletal muscle significantly. At the same time, BPG administration also exhibited extensive effects on insulin resistance by improving oral glucose tolerance test, insulin tolerance test and glucose infusion rate in hyperinsulinemic-euglycemic clamp test. Furthermore, in skeletal muscle, BPG reversed the defect expression of IRbeta, IRS-1 and PTP1B, and also decreased the expression of ACCbeta and increased the expression of p-AMPK in the high-fat-diet-induced mice. All the results suggest that BPG improves metabolic syndrome may by the enhancement of insulin sensitivity and fatty acid oxidation.

Novos mecanismos pelos quais o exercício físico melhora a resistência à insulina no músculo esquelético

O prejuízo no transporte de glicose estimulada por insulina no músculo constitui um defeito crucial para o estabelecimento da intolerância à glicose e do diabetes tipo 2. Por outro lado, é notório o conhecimento de que tanto o exercício aeróbio agudo quanto o crônico podem ter efeitos benéficos na ação da insulina em estados de resistência à insulina. No entanto, pouco se sabe sobre os efeitos moleculares pós-exercício sobre a sinalização da insulina no músculo esquelético. Assim, esta revisãoapresenta novos entendimentos sobre os mecanismos por meio dos quais o exercício agudo restaura a sensibilidade à insulina, com destaque ao importante papel que proteínas inflamatórias e a S-nitrosação possuem sobre a regulação de proteínas da via de sinalização da insulina no músculo esquelético.

Proteomic analysis reveals novel molecules involved in insulin signaling pathway

The binding of insulin to its receptor triggers a signaling cascade regulated by protein complexes via tyrosine phosphorylation events on a multitude of associated proteins. To search novel phosphotyrosine proteins or associated proteins involved in insulin signaling pathway, we employed a method in which Rat1 cells stably expressing the human insulin receptor were stimulated with or without insulin and sub-fractionated prior to enrichment of phosphotyrosine proteins by immunoprecipitation and analysis by LC-MS/MS. Bioinformatic analysis and manual confirmation of peptide phosphorylation site assignments led to identification of 35 phosphotyrosine sites derived from 31 protein groups. Over 50% of these proteins were reported for the first time as tyrosine phosphorylated, including gigaxonin, XIAP and CDK10. In addition, we also found that calcium/calmodulin-dependent protein serine kinase (CASK), a key protein in protein-targeting and vesicle transport in neurons, forms a complex with two unidentified phosphotyrosine proteins pp100 and pp95 in response to insulin-stimulation, though CASK is not itself tyrosine phosphorylated. Furthermore, insulin was able to decrease CASK nuclear location, as well as down-regulate the expression of CASK targeted genes. Our results imply CASK as a novel joint knot connecting CASK-mediated pathways with the insulin signaling. Our data provide a wealth of information potentially paving the way to identify new components in the insulin signaling network.

Lipid phosphatases as drug discovery targets for type 2 diabetes

The soaring incidence of type 2 diabetes has created pressure for new pharmaceutical strategies to treat this devastating disease. With much of the focus on overcoming insulin resistance, investigation has focused on finding ways to restore activation of the phosphatidylinositol 3'-kinase pathway, which is diminished in many patients with type 2 diabetes. Here we review the evidence that lipid phosphatases, specifically PTEN and SHIP2, attenuate this important insulin signalling pathway. Both in vivo and in vitro studies indicate their role in regulating whole-body energy metabolism, and possibly weight gain as well. The promise and challenges presented by this new class of drug discovery targets will also be discussed.

Allosteric activation of protein phosphatase 2C by D-chiro-inositol-galactosamine, a putative mediator mimetic of insulin action

Insulin-stimulated glucose disposal in skeletal muscle proceeds predominantly through a nonoxidative pathway with glycogen synthase as a rate-limiting enzyme, yet the mechanisms for insulin activation of glycogen synthase are not understood despite years of investigation. Isolation of putative insulin second messengers from beef liver yielded a pseudo-disaccharide consisting of pinitol (3-O-methyl-d-chiro-inositol) beta-1,4 linked to galactosamine chelated with Mn(2+) (called INS2). Here we show that chemically synthesized INS2 has biological activity that significantly enhances insulin reduction of hyperglycemia in streptozotocin diabetic rats. We used computer modeling to dock INS2 onto the known three-dimensional crystal structure of protein phosphatase 2C (PP2C). Modeling and FlexX/CScore energy minimization predicted a unique favorable site on PP2C for INS2 in a surface cleft adjacent to the catalytic center. Binding of INS2 is predicted to involve formation of multiple H-bonds, including one with residue Asp163. Wild-type PP2C activity assayed with a phosphopeptide substrate was potently stimulated in a dose-dependent manner by INS2. In contrast, the D163A mutant of PP2C was not activated by INS2. The D163A mutant and wild-type PP2C in the absence of INS2 had the same Mn(2+)-dependent phosphatase activity with p-nitrophenyl phosphate as a substrate, showing that this mutation did not disrupt the catalytic site. We propose that INS2 allosterically activates PP2C, fulfilling the role of a putative mediator mimetic of insulin signaling to promote protein dephosphorylation and metabolic responses.

IRS-1 and Vascular Complications in Diabetes Mellitus

The expected explosive increase in the number of patients with diabetes mellitus will increase the stress on health care. Treatment is focused on preventing vascular complications associated with the disorder. In order to develop better treatment regimens, the field of research has made a great effort in understanding this disorder. This chapter summarizes the current views on the insulin signaling pathway with emphasis on intracellular signaling events associated with insulin resistance, which lead to the prothrombotic condition in the vasculature of patience with diabetes mellitus.

Leucine culture reveals that ATP synthase functions as a fuel sensor in pancreatic beta-cells

Our goal was to investigate whether leucine culture affects beta-cell glucose sensing. One-day culture of rat islets with 10 mM leucine had no effect on glucose-induced insulin secretion. One-week leucine culture decreased the threshold for glucose-induced insulin secretion and increased maximal insulin secretion at 30 mM glucose. Glucose-induced cytosolic free Ca(2+) was increased at 1 week but not at 1 day of leucine culture. Without glucose, ATP content was not different with or without leucine culture for 1 week. With 20 mM glucose, ATP content was higher by 1.5-fold in islets cultured for 1 week with leucine than those without leucine. Microarray experiments indicated that culture of RINm5F cells with leucine increased expression of ATP synthase beta subunit 3.2-fold, which was confirmed by real time reverse transcription-PCR analysis (3.0- +/- 0.4-fold) in rat islets at 1 week but not after 1 day with leucine culture. Down-regulation of ATP synthase beta subunit by siRNA decreased INS1 cell ATP content and insulin secretion with 20 mM glucose. Overexpression of ATP synthase beta subunit in INS1 cell increased insulin secretion in the presence of 5 and 20 mM glucose. In conclusion, one-week leucine culture of rat islets up-regulated ATP synthase and increased ATP content, which resulted in elevated [Ca(2+)] levels and more insulin exocytosis by glucose. Depletion of ATP synthase beta subunit with siRNA produced opposite effects. These data reveal the fuel-sensing role of mitochondrial ATP synthase in the control of ATP production from glucose and the control of glucose-induced insulin secretion.

Insulin resistance and the pathogenesis of nonalcoholic fatty liver disease

The prevalence of obesity has reached epidemic proportions in most of the western world. Current estimates suggest that 22.5%of the population of the United States suffers from obesity and is at risk for development of obesity-related complications, including hypertension, coronary artery disease, diabetes, hyperlipidemia,increased predisposition for various cancers, and nonalcoholic fatty liver disease. Fatty liver disease is currently the most common abnormality observed in hepatology practice. Since it was first reported in the 1980s in obese diabetic females, our understanding of nonalcoholic fatty liver (NAFL) and nonalcoholic steatohepatitis (NASH) has undergone significant metamorphosis. It is now universally accepted that insulin resistance and subsequent hyperinsulinemia are key factors that lead to both NAFL and NASH.This article reviews the role of insulin resistance in the genesis of these conditions.

Microvascular recruitment is an early insulin effect that regulates skeletal muscle glucose uptake in vivo

Insulin increases glucose disposal into muscle. In addition, in vivo insulin elicits distinct nitric oxide synthase-dependent vascular responses to increase total skeletal muscle blood flow and to recruit muscle capillaries (by relaxing resistance and terminal arterioles, respectively). In the current study, we compared the temporal sequence of vascular and metabolic responses to a 30-min physiological infusion of insulin (3 mU. min(-1). kg(-1), euglycemic clamp) or saline in rat skeletal muscle in vivo. We used contrast-enhanced ultrasound to continuously quantify microvascular volume. Insulin recruited microvasculature within 5-10 min (P < 0.05), and this preceded both activation of insulin-signaling pathways and increases in glucose disposal in muscle, as well as changes in total leg blood flow. Moreover, l-NAME (N(omega)-nitro-l-arginine-methyl ester), a specific inhibitor of nitric oxide synthase, blocked this early microvascular recruitment (P < 0.05) and at least partially inhibited early increases in muscle glucose uptake (P < 0.05). We conclude that insulin rapidly recruits skeletal muscle capillaries in vivo by a nitric oxide-dependent action, and the increase in capillary recruitment may contribute to the subsequent glucose uptake.

Crystal structure of the Apo, unactivated insulin-like growth factor-1 receptor kinase. Implication for inhibitor specificity

The x-ray structure of the unactivated kinase domain of insulin-like growth factor-1 receptor (IGFRK-0P) is reported here at 2.7 A resolution. IGFRK-0P is composed of two lobes connected by a hinge region. The N-terminal lobe of the kinase is a twisted beta-sheet flanked by a single helix, and the C-terminal lobe comprises eight alpha-helices and four short beta-strands. The ATP binding pocket and the catalytic center reside at the interface of the two lobes. Despite the overall similarity to other receptor tyrosine kinases, three notable conformational modifications are observed: 1) this kinase adopts a more closed structure, with its two lobes rotated further toward each other; 2) the conformation of the proximal end of the activation loop (residues 1121-1129) is different; 3) the orientation of the nucleotide-binding loop is altered. Collectively, these alterations lead to a different ATP-binding pocket that might impact on inhibitor designs for IGFRK-0P. Two molecules of IGFRK-0P are seen in the asymmetric unit; they are associated as a dimer with their ATP binding clefts facing each other. The ordered N terminus of one monomer approaches the active site of the other, suggesting that the juxtamembrane region of one molecule could come into close proximity to the active site of the other.

The metabolic abnormalities associated with non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is a common disorder in the Western hemisphere. It encompasses two histological lesions: fatty liver and steatohepatitis. A large body of literature indicates that insulin resistance is a key pathophysiological abnormality in patients with NAFLD. Insulin resistance results from a complex interplay between the major targets of insulin action, i.e. muscle, adipose tissue and liver, versus the ability of the pancreatic islet beta cells to compensate for insulin resistance by increasing insulin production. The metabolic and clinical profile associated with insulin resistance is thus defined by the factors that produce and maintain insulin resistance and the effects of decreased insulin sensitivity on various insulin-dependent pathways. The major metabolic defects associated with insulin resistance are increased peripheral lipolysis, increased hepatic glucose output due to increased gluconeogenesis and increased lipid oxidation. This is associated with an oxidative stress in the liver that may be compounded by additional pathophysiological abnormalities. While much work remains to be done, the current understanding of the pathogenesis of NAFLD provides direction for both future investigation and development of therapeutic trials.

Vias de Sinalização da Insulina

A insulina é um hormônio anabólico com efeitos metabólicos potentes. Os eventos que ocorrem após a ligação da insulina são específicos e estritamente regulados. Definir as etapas que levam à especificidade deste sinal representa um desafio para as pesquisas bioquímicas, todavia podem resultar no desenvolvimento de novas abordagens terapêuticas para pacientes que sofrem de estados de resistência à insulina, inclusive o diabetes tipo 2. O receptor de insulina pertence a uma família de receptores de fatores de crescimento que têm atividade tirosina quinase intrínseca. Após a ligação da insulina o receptor sofre autofosforilação em múltiplos resíduos de tirosina. Isto resulta na ativação da quinase do receptor e conseqüente fosforilação em tirosina de um a família de substratos do receptor de insulina (IRS). De forma similar a outros fatores de crescimento, a insulina usa fosforilação e interações proteína-proteína como ferramentas essenciais para transmitir o sinal. Estas interações proteína-proteína são fundamentais para transmitir o sinal do receptor em direção ao efeito celular final, tais como translocação de vesículas contendo transportadores de glicose (GLUT4) do pool intracelular para a membrana plasmática, ativação da síntese de glicogênio e de proteínas, e transcrição de genes específicos.

Insulin signaling coordinately regulates cardiac size, metabolism, and contractile protein isoform expression

To investigate the role of insulin signaling on postnatal cardiac development, physiology, and cardiac metabolism, we generated mice with a cardiomyocyte-selective insulin receptor knockout (CIRKO) using cre/loxP recombination. Hearts of CIRKO mice were reduced in size by 20-30% due to reduced cardiomyocyte size and had persistent expression of the fetal beta-myosin heavy chain isoform. In CIRKO hearts, glucose transporter 1 (GLUT1) expression was reduced by about 50%, but there was a twofold increase in GLUT4 expression as well as increased rates of cardiac glucose uptake in vivo and increased glycolysis in isolated working hearts. Fatty acid oxidation rates were diminished as a result of reduced expression of enzymes that catalyze mitochondrial beta-oxidation. Although basal rates of glucose oxidation were reduced, insulin unexpectedly stimulated glucose oxidation and glycogenolysis in CIRKO hearts. Cardiac performance in vivo and in isolated hearts was mildly impaired. Thus, insulin signaling plays an important developmental role in regulating postnatal cardiac size, myosin isoform expression, and the switching of cardiac substrate utilization from glucose to fatty acids. Insulin may also modulate cardiac myocyte metabolism through paracrine mechanisms by activating insulin receptors in other cell types within the heart.

Hydrogen peroxide generated during cellular insulin stimulation is integral to activation of the distal insulin signaling cascade in 3T3-L1 adipocytes

In a variety of cell types, insulin stimulation elicits the rapid production of H(2)O(2), which causes the oxidative inhibition of protein-tyrosine phosphatases and enhances the tyrosine phosphorylation of proteins in the early insulin action cascade (Mahadev, K., Zilbering, A., Zhu, L., and Goldstein, B. J. (2001) J. Biol. Chem. 276, 21938-21942). In the present work, we explored the potential role of insulin-induced H(2)O(2) generation on downstream insulin signaling using diphenyleneiodonium (DPI), an inhibitor of cellular NADPH oxidase that blocks insulin-stimulated cellular H(2)O(2) production. DPI completely inhibited the activation of phosphatidylinositol (PI) 3'-kinase activity by insulin and reduced the insulin-induced activation of the serine kinase Akt by up to 49%; these activities were restored when H(2)O(2) was added back to cells that had been pretreated with DPI. Interestingly, the H(2)O(2)-induced activation of Akt was entirely mediated by upstream stimulation of PI 3'-kinase activity, since treatment of 3T3-L1 adipocytes with the PI 3'-kinase inhibitors wortmannin or LY294002 completely blocked the subsequent activation of Akt by exogenous H(2)O(2). Preventing oxidant generation with DPI also blocked insulin-stimulated glucose uptake and GLUT4 translocation to the plasma membrane, providing further evidence for an oxidant signal in the regulation of the distal insulin-signaling cascade. Finally, in contrast to the cellular mechanism of H(2)O(2) generation by other growth factors, such as platelet-derived growth factor, we also found that insulin-stimulated cellular production of H(2)O(2) may occur through a unique pathway, independent of cellular PI 3'-kinase activity. Overall, these data provide insight into the physiological role of insulin-dependent H(2)O(2) generation, which is not only involved in the regulation of tyrosine phosphorylation events in the early insulin signaling cascade but also has important effects on the regulation of downstream insulin signaling, involving the activation of PI 3'-kinase, Akt, and ultimately cellular glucose transport in response to insulin.

Insulin signalling and the regulation of glucose and lipid metabolism

The epidemic of type 2 diabetes and impaired glucose tolerance is one of the main causes of morbidity and mortality worldwide. In both disorders, tissues such as muscle, fat and liver become less responsive or resistant to insulin. This state is also linked to other common health problems, such as obesity, polycystic ovarian disease, hyperlipidaemia, hypertension and atherosclerosis. The pathophysiology of insulin resistance involves a complex network of signalling pathways, activated by the insulin receptor, which regulates intermediary metabolism and its organization in cells. But recent studies have shown that numerous other hormones and signalling events attenuate insulin action, and are important in type 2 diabetes.