Insulin Action Pathway Research Articles

Obesity, diabetes and vitamin D

Vitamin D deficiency and insufficiency are widespread globally with multiple factors playing a role in their development. Such as limited exposure to sunlight, poor nutrition, gastrointestinal diseases, and obesity, which affects approximately 39% of the world’s adult population. In addition to its canonical role in skeletal function, vitamin D modulates adipogenesis, insulin secretion, and action in diabetes. Vitamin D deficiency due to obesity is likely due to decreased bioavailability of vitamin D through synthesis in the skin and from dietary sources, which is associated with its deposition in body fat depots. While a direct effect on adipogenesis appears to be realized during critical periods of life (pre- and postnatal periods). Vitamin D directly regulates genes involved pathways of insulin secretion and action. The high worldwide prevalence of vitamin D deficiency may be associated with the high incidence of type 2 diabetes, and normalizing vitamin D levels in patients may be an effective strategy to slow the progression of type 2 diabetes and its complications.

Type 2 Diabetes: Molecular mechanism, Understanding and Prevention

Diabetes is a chronic condition that affects millions of people worldwide, with significant implications for individuals, families, and society at large. It is a leading cause of death, disability, and healthcare costs, with serious complications such as blindness, kidney disease, nerve damage, and cardiovascular disease. Furthermore, the prevalence of diabetes is increasing globally, highlighting the need for effective prevention and management strategies. This increase in prevalence is due to various factors such as changes in diet, sedentary lifestyles, and an aging population. Additionally, there are social determinants of health, such as poverty and inequality, that disproportionately affect vulnerable populations and contribute to the diabetes burden. This research aims to provide a comprehensive overview of the importance of diabetes by situating it in a larger context, defining the condition, and exploring its impact on day-to-day lives. Type 1 diabetes is an autoimmune disorder characterized by the destruction of pancreatic beta cells, resulting in a lack of insulin production. On the other hand, Type 2 diabetes is a metabolic disorder characterized by insulin resistance, where the body's cells become less responsive to insulin. The prevention of Type 1 diabetes involves reducing exposure to environmental triggers and maintaining a healthy lifestyle. In contrast, the prevention of Type 2 diabetes involves lifestyle modifications such as diet and exercise, as well as medication management. The molecular mechanisms underlying the pathogenesis of Type 1 and Type 2 diabetes involve complex interactions between genetic and environmental factors, leading to dysfunction in insulin secretion and insulin action pathways. Situating diabetes in a larger context also involves understanding the economic impact of the disease. The costs associated with diabetes are significant, including direct costs such as medical expenses and indirect costs such as loss of productivity and decreased quality of life. These costs place a considerable burden on healthcare systems, governments, and individuals.

Rectal Cancer and Diabetes Relationship: An Evidence-Based Overview for Healthcare Providers

Abstract As the third most frequently diagnosed cancer through the worldwide, colorectal cancer (CRC) is the fourth leading cause and account for around 8% of all cancer-related death. Diabetes mellitus (DM) is a complex metabolic disorder characterized by chronic hyperglycemia and inflammation due to deficiency in insulin secretion or dysregulation of the insulin action pathway, which further leads to dysfunction and failure of multiple organs. Many advances have been made in the diagnosis and management of rectal cancer. Although colorectal cancer survival is severely dependent on the stage of disease at diagnosis, it might also be influenced by several risk factors. The relationship between colorectal cancer and diabetes is a complex one and can raise problems in both diagnosis and the management of patients with both conditions. Metabolic pathways of the type II diabetes, glucose intolerance and obesity can be considered as a link to rectal cancer. This article provides not just an overview of the epidemiology, diagnosis and management of CRC and DM, but also highlights of CRC and DM relationship.

The Current State of Beta-Cell-Mass PET Imaging for Diabetes Research and Therapies.

Diabetes is a chronic metabolic disease affecting over 400 million people worldwide and one of the leading causes of death, especially in developing nations. The disease is characterized by chronic hyperglycemia, caused by defects in the insulin secretion or action pathway. Current diagnostic methods measure metabolic byproducts of the disease such as glucose level, glycated hemoglobin (HbA1c), insulin or C-peptide levels, which are indicators of the beta-cell function. However, they inaccurately reflect the disease progression and provide poor longitudinal information. Beta-cell mass has been suggested as an alternative approach to study disease progression in correlation to beta-cell function, as it behaves differently in the diabetes physiopathology. Study of the beta-cell mass, however, requires highly invasive and potentially harmful procedures such as pancreatic biopsies, making diagnosis and monitoring of the disease tedious. Nuclear medical imaging techniques using radiation emitting tracers have been suggested as strong non-invasive tools for beta-cell mass. A highly sensitive and high-resolution technique, such as positron emission tomography, provides an ideal solution for the visualization of beta-cell mass, which is particularly essential for better characterization of a disease such as diabetes, and for estimating treatment effects towards regeneration of the beta-cell mass. Development of novel, validated biomarkers that are aimed at beta-cell mass imaging are thus highly necessary and would contribute to invaluable breakthroughs in the field of diabetes research and therapies. This review aims to describe the various biomarkers and radioactive probes currently available for positron emission tomography imaging of beta-cell mass, as well as highlight the need for precise quantification and visualization of the beta-cell mass for designing new therapy strategies and monitoring changes in the beta-cell mass during the progression of diabetes.

The aetiology and molecular landscape of insulin resistance.

Insulin resistance, defined as a defect in insulin-mediated control of glucose metabolism in tissues - prominently in muscle, fat and liver - is one of the earliest manifestations of a constellation of human diseases that includes type 2 diabetes and cardiovascular disease. Thesediseases are typically associated with intertwined metabolic abnormalities, including obesity, hyperinsulinaemia, hyperglycaemia and hyperlipidaemia. Insulin resistance is caused by a combination of genetic and environmental factors. Recent genetic and biochemical studies suggest a key role for adipose tissue in the development of insulin resistance, potentially by releasing lipids and other circulating factors that promote insulin resistance in other organs. These extracellular factors perturb the intracellular concentration of a range of intermediates, including ceramide and other lipids, leading to defects in responsiveness of cells to insulin. Suchintermediates may cause insulin resistance by inhibiting one or more of the proximal components in the signalling cascade downstream of insulin (insulin receptor, insulin receptor substrate (IRS) proteins or AKT). However, there is now evidence to support the view that insulin resistance is a heterogeneous disorder that may variably arise in a range of metabolic tissues and that the mechanism for this effect likely involves a unified insulin resistance pathway that affects a distal step in the insulin action pathway that is more closely linked to the terminal biological response. Identifying these targets is of major importance, as it will reveal potential new targets for treatments of diseases associated with insulin resistance.

Insulin enhances neointimal hyperplasia following arterial injury through the PI3K/Akt pathway in type 1 diabetic rats.

Although insulin is known to affect neointimal hyperplasia via distinct signaling pathways, how neointimal hyperplasia is affected in insulin‑deficient type1 diabetes remains unknown. The aim of the current study was to investigate two major signaling branches of insulin action regulating neointimal hyperplasia following arterial injury in type 1 diabetes with or without exogenous insulin administration. Rats were treated with vehicle (control group), streptozotocin(STZ) alone (STZ group; uncontrolled type1 diabetes) or STZ followed by insulin (STZ+I group; controlled type1 diabetes). Subsequently, a type 1 diabetic rat model of carotid artery balloon injury was established. Following this, the intima‑to‑media area ratios were examined for evidence of neointimal hyperplasia in the carotid arteries of the rats by performing hematoxylin‑eosin staining. Furthermore, the protein expression of extracellular signal‑regulated kinase (ERK), phosphorylated(p‑) ERK, protein kinaseB(Akt) and p‑Akt in the carotid arteries of the rats was determined via immunoblotting. Moreover, an invitro model of type1 diabetes was induced by incubation of primary vascular smooth muscle cells (VSMCs) with glucose and/or insulin. Cellular proliferation and signaling protein expression levels in VSMCs were determined by measuring the incorporation of tritiated thymidine and performing immunoblotting, respectively. The results demonstrated that compared with that in control rats, neointimal hyperplasia and expression of p‑Akt in uncontrolled type 1 diabetic rats were significantly decreased. This decrease was recovered in controlled type1 diabetes with insulin therapy. Furthermore, the difference in the expression of p‑ERK between groups was not significant. Additionally, the results of the cell experiments were consistent with those from the animal studies. In conclusion, the preferential signaling along the phosphatidylinositol3‑kinase/Akt pathway of insulin action in response to insulin restoration may contribute to neointimal hyperplasia. The present study provides a novel approach for the further treatment of neointimal hyperplasia in type1 diabetes.

1753-P: Insulin Sensitivity in Long-Lived Growth Hormone–Releasing Hormone Knockout Mice

Disruption of growth hormone releasing hormone results in extended longevity with increased insulin sensitivity in mice. Yet, less is known about tissue-specific insulin action and insulin signaling pathway in growth hormone releasing hormone knock-out mice (GHRH-/-). In this study, our hyperinsulinemic-euglycemic clamp data showed that glucose infusion rate to maintain euglycemia was dramatically elevated in GHRH-/- mice. Insulin-mediated hepatic glucose production was suppressed whereas glucose uptake in skeletal muscle and brown adipose tissue were significantly increased in GHRH-/- mice compared to WT littermates. Furthermore, growth hormone deficiency further enhanced insulin sensitivity in metabolically active tissues via activated PI3K-AKT and MAPK-ERK1/2 signaling cascades. Altered tissue-specific insulin response is likely to mediate the prolonged longevity and healthy-aging effects of growth hormone deficiency in mice. Disclosure F. Zhang: None. M. Icyuz: None. L. Sun: None. Funding National Institutes of Health (R01AG057734)

D-pinitol, a highly valuable product from carob pods: Health-promoting effects and metabolic pathways of this natural super-food ingredient and its derivatives

D-pinitol is a natural compound related to the important family of inositols. It can be found and isolated from many plants, being the active component of ayurvedic remedies such as Talisa patra ( Abies webbiana, A. pindrow ) or antidiabetic as Bougainvillea ( Bougainvillea spectabilis ). Although many synthetic and semi-synthetic methods have been reported for D-pinitol and its derivatives, through chemical and biochemical transformations, Ceratonia siliqua L. (Carob), a Mediterranean tree now in decline, known because of its environmental advantages, is the only raw material from which D-pinitol can be isolated in quantities enough for a viable commercial exploitation. Fortunately, the pharmacological interest in this compound has risen enormously in the last years owing to their established multifunctional properties through a variety of signalling pathways: ⅰ) anti-cancer, through inhibition of TNF-ᾳ and suppression of NF-ⱪB pathway; ⅱ) insulinomimetic and metabolic regulator in type 2 diabetes mellitus, via a post-receptor pathway of insulin action; ⅲ) antioxidant; ⅳ) hepatoprotective; ⅴ) immuno-modulator, balancing Th1/Th2 cytokines; ⅵ) osteoporosis preventive, through p38/JNK and NF-ⱪB pathways; ⅶ) anti-aging, via reduction of the insulin/IGF-1 signaling (IIS) pathway; ⅷ) improver of creatine retention; ⅸ) preventive and ameliorative of Alzheimer's disease through selective g-secretase modulation. Thus, the present review compress the literature reported to date in relation to the health-promoting effects and metabolic pathways of this naturally occurring super-food ingredient and its derivatives, providing an extensive guide for a future utilization of all of its potentialities, aiming a positive impact in the promotion and recovery of carob crops.

Combined Use of Gene Expression Modeling and siRNA Screening Identifies Genes and Pathways Which Enhance the Activity of Cisplatin When Added at No Effect Levels to Non-Small Cell Lung Cancer Cells In Vitro.

Platinum-based combination chemotherapy is the standard treatment for advanced non-small cell lung cancer (NSCLC). While cisplatin is effective, its use is not curative and resistance often emerges. As a consequence of microenvironmental heterogeneity, many tumour cells are exposed to sub-lethal doses of cisplatin. Further, genomic heterogeneity and unique tumor cell sub-populations with reduced sensitivities to cisplatin play a role in its effectiveness within a site of tumor growth. Being exposed to sub-lethal doses will induce changes in gene expression that contribute to the tumour cell’s ability to survive and eventually contribute to the selective pressures leading to cisplatin resistance. Such changes in gene expression, therefore, may contribute to cytoprotective mechanisms. Here, we report on studies designed to uncover how tumour cells respond to sub-lethal doses of cisplatin. A microarray study revealed changes in gene expressions that occurred when A549 cells were exposed to a no-observed-effect level (NOEL) of cisplatin (e.g. the IC10). These data were integrated with results from a genome-wide siRNA screen looking for novel therapeutic targets that when inhibited transformed a NOEL of cisplatin into one that induced significant increases in lethality. Pathway analyses were performed to identify pathways that could be targeted to enhance cisplatin activity. We found that over 100 genes were differentially expressed when A549 cells were exposed to a NOEL of cisplatin. Pathways associated with apoptosis and DNA repair were activated. The siRNA screen revealed the importance of the hedgehog, cell cycle regulation, and insulin action pathways in A549 cell survival and response to cisplatin treatment. Results from both datasets suggest that RRM2B, CABYR, ALDH3A1, and FHL2 could be further explored as cisplatin-enhancing gene targets. Finally, pathways involved in repairing double-strand DNA breaks and INO80 chromatin remodeling were enriched in both datasets, warranting further research into combinations of cisplatin and therapeutics targeting these pathways.

Direction pathway analysis of large-scale proteomics data reveals novel features of the insulin action pathway

With the advancement of high-throughput techniques, large-scale profiling of biological systems with multiple experimental perturbations is becoming more prevalent. Pathway analysis incorporates prior biological knowledge to analyze genes/proteins in groups in a biological context. However, the hypotheses under investigation are often confined to a 1D space (i.e. up, down, either or mixed regulation). Here, we develop direction pathway analysis (DPA), which can be applied to test hypothesis in a high-dimensional space for identifying pathways that display distinct responses across multiple perturbations. Our DPA approach allows for the identification of pathways that display distinct responses across multiple perturbations. To demonstrate the utility and effectiveness, we evaluated DPA under various simulated scenarios and applied it to study insulin action in adipocytes. A major action of insulin in adipocytes is to regulate the movement of proteins from the interior to the cell surface membrane. Quantitative mass spectrometry-based proteomics was used to study this process on a large-scale. The combined dataset comprises four separate treatments. By applying DPA, we identified that several insulin responsive pathways in the plasma membrane trafficking are only partially dependent on the insulin-regulated kinase Akt. We subsequently validated our findings through targeted analysis of key proteins from these pathways using immunoblotting and live cell microscopy. Our results demonstrate that DPA can be applied to dissect pathway networks testing diverse hypotheses and integrating multiple experimental perturbations. The R package 'directPA' is distributed from CRAN under GNU General Public License (GPL)-3 and can be downloaded from: http://cran.r-project.org/web/packages/directPA/index.html jean.yang@sydney.edu.au Supplementary data are available at Bioinformatics online.

Adiposity and Insulin Resistance in Humans: The Role of the Different Tissue and Cellular Lipid Depots

Human adiposity has long been associated with insulin resistance and increased cardiovascular risk, and abdominal adiposity is considered particularly adverse. Intra-abdominal fat is associated with insulin resistance, possibly mediated by greater lipolytic activity, lower adiponectin levels, resistance to leptin, and increased inflammatory cytokines, although the latter contribution is less clear. Liver lipid is also closely associated with, and likely to be an important contributor to, insulin resistance, but it may also be in part the consequence of the lipogenic pathway of insulin action being up-regulated by hyperinsulinemia and unimpaired signaling. Again, intramyocellular triglyceride is associated with muscle insulin resistance, but anomalies include higher intramyocellular triglyceride in insulin-sensitive athletes and women (vs men). Such issues could be explained if the "culprits" were active lipid moieties such as diacylglycerol and ceramide species, dependent more on lipid metabolism and partitioning than triglyceride amount. Subcutaneous fat, especially gluteofemoral, appears metabolically protective, illustrated by insulin resistance and dyslipidemia in patients with lipodystrophy. However, some studies suggest that deep sc abdominal fat may have adverse properties. Pericardial and perivascular fat relate to atheromatous disease, but not clearly to insulin resistance. There has been recent interest in recognizable brown adipose tissue in adult humans and its possible augmentation by a hormone, irisin, from exercising muscle. Brown adipose tissue is metabolically active, oxidizes fatty acids, and generates heat but, because of its small and variable quantities, its metabolic importance in humans under usual living conditions is still unclear. Further understanding of specific roles of different lipid depots may help new approaches to control obesity and its metabolic sequelae.

Double-Stranded RNA-Activated Protein Kinase Is a Key Modulator of Insulin Sensitivity in Physiological Conditions and in Obesity in Mice

The molecular integration of nutrient- and pathogen-sensing pathways has become of great interest in understanding the mechanisms of insulin resistance in obesity. The double-stranded RNA-dependent protein kinase (PKR) is one candidate molecule that may provide cross talk between inflammatory and metabolic signaling. The present study was performed to determine, first, the role of PKR in modulating insulin action and glucose metabolism in physiological situations, and second, the role of PKR in insulin resistance in obese mice. We used Pkr(-/-) and Pkr(+/+) mice to investigate the role of PKR in modulating insulin sensitivity, glucose metabolism, and insulin signaling in liver, muscle, and adipose tissue in response to a high-fat diet. Our data show that in lean Pkr(-/-) mice, there is an improvement in insulin sensitivity, and in glucose tolerance, and a reduction in fasting blood glucose, probably related to a decrease in protein phosphatase 2A activity and a parallel increase in insulin-induced thymoma viral oncogene-1 (Akt) phosphorylation. PKR is activated in tissues of obese mice and can induce insulin resistance by directly binding to and inducing insulin receptor substrate (IRS)-1 serine307 phosphorylation or indirectly through modulation of c-Jun N-terminal kinase and inhibitor of κB kinase β. Pkr(-/-) mice were protected from high-fat diet-induced insulin resistance and glucose intolerance and showed improved insulin signaling associated with a reduction in c-Jun N-terminal kinase and inhibitor of κB kinase β phosphorylation in insulin-sensitive tissues. PKR may have a role in insulin sensitivity under normal physiological conditions, probably by modulating protein phosphatase 2A activity and serine-threonine kinase phosphorylation, and certainly, this kinase may represent a central mechanism for the integration of pathogen response and innate immunity with insulin action and metabolic pathways that are critical in obesity.

Site-specific phosphorylation of protein phosphatase 1 regulatory subunit 12A stimulated or suppressed by insulin

Protein phosphatase 1 (PP1) is one of the major phosphatases responsible for protein dephosphorylation in eukaryotes. So far, only few specific phosphorylation sites of PP1 regulatory subunit 12A (PPP1R12A) have been shown to regulate the PP1 activity. The effect of insulin on PPP1R12A phosphorylation is largely unknown. Utilizing a mass spectrometry based phosphorylation identification and quantification approach, we identified 21 PPP1R12A phosphorylation sites (7 novel sites, including Ser20, Thr22, Thr453, Ser478, Thr671, Ser678, and Ser680) and quantified 16 of them under basal and insulin stimulated conditions in hamster ovary cells overexpressing the insulin receptor (CHO/IR), an insulin sensitive cell model. Insulin stimulated the phosphorylation of PPP1R12A significantly at Ser477, Ser478, Ser507, Ser668, and Ser695, while simultaneously suppressing the phosphorylation of PPP1R12A at Ser509 (more than 2-fold increase or decrease compared to basal). Our data demonstrate that PPP1R12A undergoes insulin stimulated/suppressed phosphorylation, suggesting that PPP1R12A phosphorylation may play a role in insulin signal transduction. The novel PPP1R12A phosphorylation sites as well as the new insulin-responsive phosphorylation sites of PPP1R12A in CHO/IR cells provide targets for investigation of the regulation of PPP1R12A and the PPP1R12A-PP1cδ complex in insulin action and other signaling pathways in other cell models, animal models, and humans.

GLUT12: a second insulin-responsive glucose transporters as an emerging target for type 2 diabetes.

GLUT12: a second insulin-responsive glucose transporters as an emerging target for type 2 diabetes.

Lipopolysaccharide, high glucose and saturated fatty acids induce endoplasmic reticulum stress in cultured primary human adipocytes: Salicylate alleviates this stress

Recent findings indicate that endoplasmic reticulum (ER) stress is significantly increased in adipose tissue of obese human subjects and is critical to the initiation and integration of pathways of inflammation and insulin action. But the factors inducing ER stress in human adipose tissue are unknown. The common factors increased in obesity and linked to insulin resistance are hyperglycaemia, hyperlipidemia and also endotoxemia. Therefore, our aims were to investigate: (1) the role of lipopolysaccharide (LPS), high glucose (HG) and saturated fatty acids (SFA) as inducers of ER stress in primary human adipocytes and (2) whether salicylate, a known anti-inflammatory compound, can alleviate this effect. Components of the ER stress pathways were studied in human abdominal subcutaneous (AbSc) adipose tissue (AT) from obese and lean. Following the culture and differentiation of primary human preadipocytes, these adipocytes were treated with LPS, HG, tunicamycin (Tun) and SFA either alone or in combination with sodium salicylate (Sal). Markers of ER stress were significantly increased in AbSc AT of obese. Differentiated human adipocytes treated with LPS, Tun, HG and SFA showed significant activation of eukaryotic translation initiation factor 2α (eIF2α) and activating transcription factor 6 (ATF6) and their down-stream targets. Sal alleviated this effect and activated AktSer473 phosphorylation. This study presents important evidence that: (1) there is increased ER stress in adipose tissue of obese individuals, (2) LPS, hyperglycaemia and saturated fatty acids induce significant ER stress in primary human adipocytes and (3) this induction is alleviated by salicylate.

P2-23 Peri-implantation and late gestation maternal undernutrition differentially alter pathways of insulin and IGF-1 action in fetal sheep skeletal muscle

P2-23 Peri-implantation and late gestation maternal undernutrition differentially alter pathways of insulin and IGF-1 action in fetal sheep skeletal muscle

Endothelin‐1 impairs glucose transporter trafficking via a membrane‐based mechanism

Endothelin-1 (ET-1) disrupts insulin-regulated glucose transporter GLUT4 trafficking. Since the negative consequence of chronic ET-1 exposure appears to be independent of signal disturbance along the insulin receptor substrate-1/phosphatidylinositol (PI) 3-kinase (PI3K)/Akt-2 pathway of insulin action, we tested if ET-1 altered GLUT4 regulation engaged by osmotic shock, a PI3K-independent stimulus that mimics insulin action. Regulation of GLUT4 by hyperosmotic stress was impaired by ET-1. Because of the mutual disruption of both insulin- and hyperosmolarity-stimulated GLUT4 translocation, we tested whether shared signaling and/or key phosphatidylinositol 4,5-bisphosphate (PIP2)-regulated cytoskeletal events of GLUT4 trafficking were targets of ET-1. Both insulin and hyperosmotic stress signaling to Cbl were impaired by ET-1. Also, plasma membrane PIP2 and cortical actin levels were reduced in cells exposed to ET-1. Exogenous PIP2, but not PI 3,4,5-bisphosphate, restored actin structure, Cbl activation, and GLUT4 translocation. These data show that ET-1-induced PIP2/actin disruption impairs GLUT4 trafficking elicited by insulin and hyperosmolarity. In addition to showing for the first time the important role of PIP2-regulated cytoskeletal events in GLUT4 regulation by stimuli other than insulin, these studies reveal a novel function of PIP2/actin structure in signal transduction.

Insulin resistance and clinical aspects of non-alcoholic steatohepatitis (NASH)

Non-alcoholic steatohepatitis (NASH) is one of the most common liver disorders. This is highly prevalent in obese and diabetic subjects. Persons with central obesity are at particular risk. Other clinical predictors are age more than 40-50 years and hyperlipidemias, but none of these factors is invariable for causation of NASH. Other reported associations are, celiac disease, Wilson's Disease and few other metabolic diseases. Drugs, particularly amiodarone, tamoxifen, nucleoside analogues and methotrxate have also been linked to NASH. The disease is evenly distributed in both sexes but advanced disease is more common in women. Ethnic variation exists and African Americans are less affected than Hispanic Americans. Specific clinical features of NASH are infrequent. Patients usually come to clinical attention by elevated liver enzymes found on routine evaluation but on history, about two third of patients will admit to have mild fatigue and about half will report right upper quadrant pain. Rarely, patient may present with a complication of cirrhosis. Physical examination may reveal hepatomegaly and splenomegaly. Research in last few years has stressed that development of steatosis, stetohepatitis, fibrosis with subsequent cirrhosis are most probably the result of insulin resistance. Therefore, clinical features may reflect existence of insulin resistance. Obesity, particularly central obesity is most important of these. Patients may have sleep apnea syndrome. Hypertension and manifestations of diabetes mellitus like polyuria, polydypsia, and neurological deficits may occur. Patients may have varying combination of obesity, diabetes, hyperlipidemia, hypertension and impaired fibrinolysis (syndrome X). Children with insulin resistance may show acanthosis nigricance. Patients with polycystic ovary syndrome, which consists of insulin resistance, diabetes, obesity, hirsutism, oligo or polymenorrha and hyperlipidemia may have NASH. Other rare manifestations of insulin resistance, which can be seen in patients of NASH are lipomatosis, lipoatrophy/lipodystrophy and panniculitis. Most other rare conditions known to cause NASH like peroxisomal diseases, mitochondialpathies, Weber-Christian disease, Mauriac syndrome, Madelung's lipomatosis and abetaliopprotenemia also have insulin resistance. This is believed that primary defect underlying insulin resistance is impairment in postreceptor pathways (through tyrosine kinase activity) of insulin action. Primary defect in insulin receptors appear uncommon. This results in down regulation of insulin receptor substance 1 (IRS-1) signaling by excess free fatty acids. In muscle, activated IRS-1 promotes translocation of glucose transporter protein 4 (GLUT4) to cell membrane. As a result, monocyte glucose uptake by GLUT4 increases glucose disposal from blood and reduced need for insulin. PKC-0 is a likely candidate as serine kinase in muscle regulated by fatty acids that can impair the activation of IRS-1. Insulin resistance is usually evaluated by fasting insulin levels, Quantitative Insulin Check Index (QUICKI) and Homeostasis Model Assessment of Insulin Resistance (HOMA), C-peptid/insulin ratio oral glucose tolerance test and hyper insulinemic euglycemic clamp. The clamp technique is considered the gold standard.

Role of Insulin-Induced Reactive Oxygen Species in the Insulin Signaling Pathway

Oxidants, including hydrogen peroxide (H2O2), have been recognized for years to mimic insulin action on glucose transport in adipose cells. Early studies also demonstrated the complementary finding that H2O2 was elaborated during treatment of cells with insulin, suggesting that cellular H2O2 generation was integral to insulin signaling. Recently, reactive oxygen species elicited by various hormones and growth factors have been shown to affect signal transduction pathways in various cell types. We recently reported that insulin-stimulated H2O2 modulates proximal and distal insulin signaling, at least in part through the oxidative inhibition of protein tyrosine phosphatases (PTPases) that negatively regulate the insulin action pathway. Nox4, a homologue in the family of NADPH oxidase catalytic subunits, was found to be prominently expressed in insulin-sensitive cells. By various molecular approaches, Nox4 was shown to mediate insulin-stimulated H2O2 generation and impact the insulin signaling cascade. Overexpression of Nox4 also significantly reversed the inhibition of insulin-stimulated receptor tyrosine phosphorylation by PTP1B, a widely expressed PTPase implicated in the negative regulation of insulin signaling, by inhibiting its catalytic activity. These recent studies have provided insight into Nox4 as a novel molecular link between insulin-stimulated reactive oxygen species and mechanisms involved in their modulation of insulin signal transduction.

Integration of multiple downstream signals determines the net effect of insulin on MAP kinase vs. PI 3′-kinase activation: potential role of insulin-stimulated H 2O 2

Cellular insulin stimulation generates a burst of H 2O 2 that modulates protein-tyrosine phosphorylation in the insulin action pathway, in part by the inhibition of redox-sensitive protein-tyrosine phosphatases [J. Biol. Chem. 276 (2001) 21938]. Blocking the insulin-induced rise in H 2O 2 with the NADPH oxidase inhibitor diphenyleneiodonium (DPI) strongly attenuated the activation of phosphatidylinositol 3′ (PI 3′)-kinase, Akt and GLUT4 translocation by insulin in 3T3-L1 adipocytes; however, under identical conditions, we observed a paradoxical increase in the activation of p42/p44 mitogen-activated protein (MAP) kinase. DPI inhibited the insulin-stimulated tyrosine phosphorylation of the insulin receptor and IRS-1/2, and also reduced the association of Grb2 with IRS-1, suggesting that the effect of DPI on MAP kinase activation occurred downstream of the IR and IRS proteins. DPI increased the insulin-stimulated phosphorylation of p42/p44 MAP kinase with no change in basal, and increased insulin-stimulated MAP kinase kinase (MEK) activity by a similar degree. DPI enhanced basal Grb2–Sos binding and reduced the effect of insulin to potentiate the dissociation of the Grb2–Sos complex, suggesting that the effect of DPI was mediated upstream of Raf-1. Cell treatment with dibutyryl cAMP significantly reduced the enhancement of MAP kinase activation in the presence of DPI. However, forskolin, acting in a PKA-independent manner, increased the insulin stimulation of MAP kinase and MEK, but fully abrogated the effect of DPI to enhance these insulin responses. PLCγ inhibition with U73122 blocked the insulin stimulation of MAP kinase and MEK as well as the enhancing effect of DPI on these responses. PKC activation strongly stimulated MAP kinase and MEK activation, even in the presence of U73122, consistent with PKC acting downstream of PLCγ. These data show that the insulin-stimulated oxidant signal differentially affects the two major downstream components of the insulin signaling pathway, PI 3′-kinase and MAP kinase, and cross-talk between insulin action, PLCγ and, to a lesser extent, PKA modulates the net cellular effects of insulin-stimulated cellular H 2O 2.