Insulin receptor variant forms and type 2 diabetes mellitus

Type 2 diabetes is a polygenic and heterogeneous disease resulting from interaction of genetic factors with environmental influences. Numerous candidate genes have been investigated, but no single major susceptibility gene for Type 2 diabetes has been identified. The insulin receptor was considered a plausible candidate gene. The insulin receptor exists in two isoforms differing by the absence (Ex11-) or presence (Ex11+) of 12 amino acids in the C-terminus of the alpha-subunit due to alternative splicing of exon 11.Ex11- binds insulin with two-fold higher affinity than Ex11+. This difference is paralleled by a decreased sensitivity for metabolic actions of insulin. Some, but not all, studies have reported that expression of the low-affinity Exll+ is increased in Type 2 diabetes, suggesting that alterations in abundance of the two isoforms mnight contribute to insulin resistance. Insulin and Type 1 insulin-like growth factor (IGF) receptors have been shown to form hybrid receptors in tissues co-expressing both molecules. Hybrid receptors bind IGF-I, but not insulin, with high affinity, and behave as IGF-I receptors rather than insulin receptors in terms of receptor autophosphorylation and hormone internalisation. It has been shown that the abundance of hybrid receptors is increased in skeletal muscle and fat from Type 2 diabetic patients, and is negatively correlated with in vivo insulin sensitivity. Mutations in the insulin receptor gene were identified in studies which examined an appropriately sized population of Type 2 diabetic patients. The prevalence of mutations in the insulin receptor gene ranged from 0.4 to 7.8%.     

Molecular basis of insulin action

Insulin is the main anabolic and anticatabolic hormone in mammals. The stimulatory effect of insulin on glucose uptake in muscle and adipose tissue is a consequence of the rapid translocation of GLUT4 glucose transporters from an intracellular site to the cell surface. The actions of insulin are initiated by hormone binding to its cell surface receptors. Insulin receptors are ligand-stimulated protein tyrosine kinases and phosphorylate a number of proteins, known as insulin receptor substrate proteins. Insulin resistance has been recognized as a main pathogenic factor in the development of type 2 diabetes, and has been associated with dyslipidemia, hypertension, endothelial dysfunction, inflammation and coagulative state. The current challenge is the study of impaired insulin signaling pathways leading to beta-cell dysfunction and its progression to type 2 diabetes, as well as control of chronic inflammation processes that may improve insulin action.     

Regulation of microvascular flow and metabolism: An overview

Skeletal muscle is an important site for insulin to regulate blood glucose levels. It is estimated that skeletal muscle is responsible for ~80% of insulin‐mediated glucose disposal in the post‐prandial period. The classical action of insulin to increase muscle glucose uptake involves insulin binding to insulin receptors on myocytes to stimulate glucose transporter 4 (GLUT 4) translocation to the cell surface membrane, enhancing glucose uptake. However, an additional role of insulin that is often under‐appreciated is its action to increase muscle perfusion thereby improving insulin and glucose delivery to myocytes. Either of these responses (myocyte and/or vascular) may be impaired in insulin resistance, and both impairments are apparent in type 2 diabetes, resulting in diminished glucose disposal by muscle. The aim of this review is to report on the growing body of literature suggesting that insulin‐mediated control of skeletal muscle perfusion is an important regulator of muscle glucose uptake and that impairment of microvascular insulin action has important physiological consequences early in the pathogenesis of insulin resistance. This work was discussed at the 2015 Australian Physiological Society Symposium “Physiological mechanisms controlling microvascular flow and muscle metabolism”.     

Rosiglitazone maleate/metformin hydrochloride: a new formulation therapy for type 2 diabetes

Many patients with type 2 diabetes require treatment with more than one antihyperglycemic drug to achieve optimal glycemic control. The thiazolidinediones are a novel class of oral antihyperglycemic drugs that improve glycemic control primarily by increasing peripheral insulin resistance and sensitizing the skeletal muscle, liver and adipose tissue to the actions of insulin, in addition to improving beta-cell function. One of the many features of the thiazolidinedione class of drugs is their synergism with other antihyperglycemic drugs that have a different mechanism of action. The combination of metformin hydrochloride, a biguanide that enhances glucose uptake in peripheral tissues and reduces hepatic gluconeogenesis, with rosiglitazone maleate, one of the newly available members of the thiazolidinedione family, offers a rational therapeutic approach to the treatment of type 2 diabetes. In patients whose type 2 diabetes is inadequately controlled with metformin monotherapy, the addition of rosiglitazone significantly improves glycemic control, insulin sensitivity and beta-cell function, compared with either drug alone. In addition, this combination therapy has beneficial effects on other cardiovascular risk factors. Rosiglitazone maleate/metformin hydrochloride combination therapy is well tolerated in patients with type 2 diabetes and has a favorable safety profile. This review summarizes the available evidence on the clinical efficacy and safety of rosiglitazone maleate and metformin hydrochloride combination therapy in patients with type 2 diabetes.     

Clinical use of somatomedin-1: yes or no?

Insulin-like growth factor-I (IGF-I) is a polypeptide of 70 amino acids. The circulatory form of IGF-I is synthesised in the liver. The metabolic activity of IGF-I is regulated by 6 IGF binding proteins. the most important being IGF binding protein-3. IGF-I acts via its own receptor, which resembles that of insulin. It has been demonstrated that the effects of pituitary growth hormone (GH, somatropin) on protein metabolism, including growth and effects on nervous and renal tissue, are mediated by IGF-I, whereas these 2 hormones are antagonistic in their effects on insulin and some aspects of lipid metabolism. The biosynthesis of recombinant human IGF-I (somatomedin-1) in 1986 enabled the initiation of clinical trials. The most important involves replacement therapy in primary IGF-I deficiency, such as Laron syndrome (primary GH resistance or insensitivity), and in patients who have developed antibodies to human GH. In Laron syndrome, which is characterised by dwarfism, somatomedin-1 stimulates growth and increases muscle and bone mass, as well as normalising blood chemistry. In diabetes mellitus types 1 and 2 (insulin-dependent and non-insulin-dependent), somatomedin-1 increases the sensitivity to insulin and improves glucose utilisation, and may contribute to healing of injured nerve tissue and improve renal function in humans. Adverse effects of somatomedin-1 appear to be related to overdosage. In conclusion, somatomedin-1 is an important hormone which has clinical roles as replacement therapy and other pharmacological therapies.     

The antihyperglycaemic effect of metformin: therapeutic and cellular mechanisms

Metformin is regarded as an antihyperglycaemic agent because it lowers blood glucose concentrations in type 2 (non-insulin-dependent) diabetes without causing overt hypoglycaemia. Its clinical efficacy requires the presence of insulin and involves several therapeutic effects. Of these effects, some are mediated via increased insulin action, and some are not directly insulin dependent. Metformin acts on the liver to suppress gluconeogenesis mainly by potentiating the effect of insulin, reducing hepatic extraction of certain substrates (e.g. lactate) and opposing the effects of glucagon. In addition, metformin can reduce the overall rate of glycogenolysis and decrease the activity of hepatic glucose-6-phosphatase. Insulin-stimulated glucose uptake into skeletal muscle is enhanced by metformin. This has been attributed in part to increased movement of insulin-sensitive glucose transporters into the cell membrane. Metformin also appears to increase the functional properties of insulin- and glucose-sensitive transporters. The increased cellular uptake of glucose is associated with increased glycogen synthase activity and glycogen storage. Other effects involved in the blood glucose-lowering effect of metformin include an insulin-independent suppression of fatty acid oxidation and a reduction in hypertriglyceridaemia. These effects reduce the energy supply for gluconeogenesis and serve to balance the glucose-fatty acid (Randle) cycle. Increased glucose turnover, particularly in the splanchnic bed, may also contribute to the blood glucose-lowering capability of metformin. Metformin improves insulin sensitivity by increasing insulin-mediated insulin receptor tyrosine kinase activity, which activates post-receptor insulin signalling pathways. Some other effects of metformin may result from changes in membrane fluidity in hyperglycaemic states. Metformin therefore improves hepatic and peripheral sensitivity to insulin, with both direct and indirect effects on liver and muscle. It also exerts effects that are independent of insulin but cannot substitute for this hormone. These effects collectively reduce insulin resistance and glucotoxicity in type 2 diabetes.     

A new paradigm for diabetes and obesity: the hepatic insulin sensitizing substance (HISS) hypothesis

The glucose disposal effect of insulin after a meal is accounted for in approximately equal measure by the direct action of insulin and the action of HISS (hepatic insulin sensitizing substance) released from the liver and acting on skeletal muscle to stimulate glucose storage as glycogen. The ability of insulin to cause HISS release is determined by hepatic parasympathetic nerves. Eliminating the parasympathetic signal by surgical denervation of the liver or by blockade of hepatic muscarinic receptors, hepatic nitric oxide synthase, or hepatic cyclooxygenase results in insulin resistance that can be accounted for by the absence of HISS action and is referred to as HISS-dependent insulin resistance (HDIR). Animal models in which the insulin resistance has been shown to be HDIR includes the spontaneously hypertensive rat, sucrose fed rats, animals with liver disease, adult offspring of fetal alcohol exposure, acute stress, and ageing. We suggest that HDIR accounts for the major metabolic disturbances in type 2 diabetes, including the postprandial hyperglycemia that results in the majority of pathologies related to diabetes. The observation of meal-induced insulin sensitization (MIS) and the role of HISS allows for consideration of a new paradigm relating meal processing, diabetes, obesity, and insulin resistance. New diagnostic approaches and therapeutic targets are described.     

Cardiovascular disease and PPARdelta: targeting the risk factors

Metabolism, in part, is regulated by the peroxisome proliferator-activated receptors (PPARs). The PPARs act as nutritional lipid sensors and three mammalian PPAR subtypes designated PPARalpha (NR1C1), PPARgamma (NR1C3) and PPARdelta (NR1C2) have been identified. This subgroup of nuclear hormone receptors binds DNA and controls gene expression at the nexus of pathways that regulate lipid and glucose homeostasis, energy storage and expenditure in an organ-specific manner. Recent evidence has demonstrated activation of PPARdelta in the major mass peripheral tissue (ie, adipose and skeletal muscle). It enhances glucose tolerance, insulin-stimulated glucose disposal, lipid catabolism, energy expenditure, cholesterol efflux and oxygen consumption. These effects positively influence the blood-lipid profile. Furthermore, PPARdelta activation produces a predominant type I/slow twitch/oxidative muscle fiber phenotype that leads to increased endurance, insulin sensitivity and resistance to obesity. PPARdelta has rapidly emerged as a potential target in the battle against dyslipidemia, insulin insensitivity, type II diabetes and obesity, with therapeutic efficacy in the treatment of cardiovascular disease risk factors. GW-501516 is currently undergoing phase II safety and efficacy trials in human volunteers for the treatment of dyslipidemia. The outcome of these clinical trials are eagerly awaited against a background of conflicting reports about cancer risks in genetically predisposed animal models. This review focuses on the potential pharmacological utility of selective PPARdelta agonists in the context of risk factors associated with metabolic and cardiovascular disease.     

Phosphatase and tensin homolog deleted on chromosome 10 suppression is an important process in peroxisome proliferator-activated receptor-gamma signaling in adipocytes and myotubes

Peroxisome proliferator-activated receptor-gamma (PPARgamma) activation enhances insulin sensitivity in type 2 diabetes mellitus. However, downstream mediators of PPARgamma activation in adipocytes and myotubes, the most important cell types involved in glucose homeostasis, remained unclear. Here we show by using two synthetic PPARgamma agonists (rosiglitazone and KR-62776, a novel PPARgamma agonist) that phosphatase and tensin homolog deleted on chromosome 10 (PTEN) is a key downstream mediator of PPARgamma signaling. The PPARgamma agonists down-regulated PTEN expression, resulting in glucose uptake increase in differentiated 3T3-L1 adipocytes and C2C12 skeletal muscle cells. In both cells, PTEN knockdown increased glucose uptake, whereas overexpression abolished the agonist-induced effects. The effects of PPARgamma agonists on PTEN expression and glucose uptake disappeared by pretreatment with a PPARgamma antagonist or by knockdown of PPARgamma expression. In vivo treatment of the agonists to C57BL/6J-ob/ob mice resulted in the reduction of PTEN level in both adipose and skeletal muscle tissues and decreased plasma glucose levels. Thus, these results suggest that PTEN suppression is a key mechanism of the PPARgamma-mediated glucose uptake stimulation in insulin-sensitive cells such as adipocytes and skeletal muscle cells, thereby restoring glucose homeostasis in type 2 diabetes.     

Active role for the vasculature in the delivery of insulin to skeletal muscle

1. In the 80+ years since insulin's discovery, an enormous amount of literature has accumulated relating to its actions on body fat, glucose and protein metabolism. In particular, skeletal muscle has been extensively studied because of its major role as a site of insulin-mediated glucose disposal. Liver and adipose tissue are two other extensively studied sites of insulin action. Much less investigation has been directed towards delineating insulin's actions on cells other than myocytes, adipocytes and hepatocytes. 2. Over the past 5-10 years it has become increasingly evident that insulin exerts important actions on vascular cells. Here, we review evidence that insulin's action within muscle may be very much regulated by its ability to transit the vasculature to access the interstitial fluid (and hence the myocyte insulin receptor). Surprisingly little is known regarding the regulation of vascular events that first bring insulin to the capillary endothelium within muscle, whence presumably it transits from the vascular to the interstitial space. Recent studies suggest that insulin can increase blood flow and also influence the distribution of blood flow within skeletal muscle, potentially therefore regulating its own delivery to the capillary endothelium. Beyond insulin's ability to access the vascular lumen within skeletal muscle microvasculature lies the issue of its passing the endothelial barrier. Even less is known about the processes involved in insulin's actual transit across the endothelium. Available data do not clearly indicate whether this is a saturable, receptor-mediated process or a passive-diffusion pathway. Also, whether insulin in any manner regulates its own transit across the endothelium or its clearance via the lymphatic system is entirely unknown. 3. The aim of the present review is to identify areas where knowledge is deficient and highlight hypotheses which may lead to a better understanding of the coordinated relationship between insulin's vascular actions within muscle and its metabolic actions in that tissue. Even so, there is now sufficient evidence to indicate that insulin's vascular action within skeletal muscle is a major regulatory locus for its insulin mediated glucose disposal.     

Vascular effects of insulin

Insulin as a vascular hormone, apart from its effect on intermediary metabolism, has been considered to play an important role in cardiovascular regulation and pathophysiology of cardiovascular diseases such as essential hypertension, congestive cardiac failure and atherosclerosis. Insulin induces pressor effects by mechanisms of increased sympathetic activity, renal sodium retention and proliferation of vascular smooth muscle cells. On the other hand, accumulating evidence indicates that insulin decreases vascular resistance and increases organ blood flow especially in skeletal muscle tissue, indicating that insulin is a vasodilator. Several mechanisms underlying insulin-induced vasodilation have been proposed. Insulin enhances calcium efflux from vascular smooth muscle cells by activating the plasma membrane Ca(2+)-ATPase and causes hyperpolarization by stimulating Na+, K(+)-ATPase and sodium/potassium pump. Insulin also stimulates nitric oxide (NO) synthase and increases release of NO from vascular endothelium to cause vasodilation. An increase in cyclic AMP levels is induced by insulin, via activation of insulin receptors, beta-adrenoceptors and calcitonin gene-related peptide receptors. However, main cause of mechanisms mediating the vasodilation remain obscure. Hypertension is associated with insulin resistance and hyperinsulinemia. Insulin resistance may contribute to hypertension by sympathetic overactivity, endothelium dysfunction and decreased vasodilator action of insulin. Therefore, insulin must be considered a vasoactive peptide and more investigations are needed to better understand the full significance of the hemodynamic effect of insulin.     

Activated T lymphocytes in Type 2 diabetes: implications from in vitro studies

The number of subjects with Type 2 diabetes (DM2) has risen significantly in the last ten years. Obtaining sufficient human tissue to study this disease process as well as many other diseases is generally difficult. T lymphocytes offer a unique opportunity for these studies. Although resting human peripheral T-lymphocytes are devoid of insulin receptors, these receptors emerge upon activation of cells by specific antigens or mitogens. Concomitant with the insulin receptors, two other growth factor receptors (IGF-1 and IL-2) also emerge on the T lymphocyte cell surface along with intracellular signal transduction mechanisms and insulin degrading enzyme (IDE). After binding to its receptor, insulin has been shown to exert its classical effects on carbohydrate metabolism in the stimulated T- cell; thereby, validating the use of activated T-lymphocytes for studying the pathogenesis of metabolic and immune disorders and the mechanism(s) by which insulin exerts its effects. In activated T-lymphocytes, insulin stimulates glucose uptake, glucose oxidation, pyruvate flux and pyruvate dehydrogenase activity, amino acid transport, lipid metabolism and protein synthesis. Through its ability to enhance nutrient uptake and raise the levels of intermediary cellular metabolism, insulin is believed to maintain the allo-activated state of lymphocytes, enhance T-Lymphocyte responsiveness, and support or possibly promote the actions of immuno-derived regulatory growth and differential factors. Since insulin enhances energy requirements and protein synthesis necessary for appropriate T-cell functions, defects in insulin action may lead to inappropriate immunoresponses in various metabolic states such as in diabetes. Studies from our lab have found insulin binding, processing, and responsiveness in phytohemagglutinin(PHA)-activated T-cells are reflective of the donor's glycemic status and ambient insulin levels in subjects with Type 1 and Type 2 diabetes (DM2) and other insulin resistant states. Our studies show that patients with diabetic ketoacidosis and hyperglycemia have increased proinflammatory cytokines and activated CD4+ and CD8+ T lymphocytes. The diabetic state, where effective insulin concentrations are low and both glucose and free fatty acids are high, provides an environment of oxidative stress and activation of the inflammatory pathways. The mechanisms underlying insulin action, in general, or in the CD4+ and CD8+ T-lymphocytes, in particular, have not been clearly elucidated. Due to the accessibility of obtaining these cells from patients, activated T-lymphocytes offer the potential of studying diabetes and other disease in human subjects.     

Relationship between non-genomic actions of estrogens and insulin resistace

Numerous experimental and clinical data show that the physiological actions of insulin and sexual steroids interact in target tissues for these hormones. In the other hand, sexual steroids has effects on peripheral tissues, and since the skeletal muscle is the main responsible for peripheral glucose uptake, it would be possible that the sexual steroids induce directly in the muscle a decrease of the sensibility of this tissue to insulin action. Some of the biological actions of the estrogens are too fast like to be compatible with this classical mechanism of action, and this mechanism has been called not classical, non-genomic or rapid actions of the estrogens. Moreover, some experiments have shown that low concentrations of estradiol, induce an increase in the rate of IRS-1 phosphorylation, promotes the association between IRS-1 and the subunit of PI3-k, p85alpha, causes a decrease in the rate of IRS-1 serine phosphorylation and increases the rate of Akt phosphorylation. Therefore, the evidences suggest the existence of a narrow interrelation between the estrogens and insulin sensitivity, but relatively few studies have tried to resolve the molecular base of this relation in insulin-dependent tissues. The resolution of these unknown questions would be able to have a great long-term therapeutic repercussion. In this sense, we should not forget that insulin resistance is the underlying cause of several associated pathologies to the female aging, as Type 2 diabetes, cardio-circulatory pathology or neurodegenerative disease.     

Nutrient-induced insulin resistance in human skeletal muscle

Nutrient excess is associated with reduced insulin sensitivity (insulin resistance) and plays a central role in the pathogenesis of type 2 diabetes. Recently, free fatty acids as well as amino acids were shown to induce insulin resistance by decreasing glucose transport/phosphorylation with subsequent impairment of glycogen synthesis in human skeletal muscle. These results do not support the traditional concept of direct substrate competition with glucose for mitochondrial oxidation but indicate that the cellular mechanisms of such lipotoxicity and "proteotoxicity" might primarily affect the insulin signaling cascade. The signaling pathways involved in nutrient dependent modulation of insulin action include protein kinase C isoforms and IkappaB kinase. Therefore, pharmacological modulation of these enzymes might represent a promising target for future treatment of insulin resistance. Finally, hyperglycemia which occurs late in the insulin resistance syndrome further augments insulin resistance by mechanisms summarized as glucose toxicity. Chronic hyperglycemia might lead to inhibition of lipid oxidation and thereby to accumulation of intracellular lipid metabolites. Therefore, glucotoxicity might be in part indirectly caused by lipotoxicity (glucolipotoxicity). In conclusion, different nutrients affect common metabolic pathways and thereby induce insulin resistance in humans.     

Growth hormone, IGF-I and insulin and their abuse in sport

There is widespread anecdotal evidence that growth hormone (GH) is used by athletes for its anabolic and lipolytic properties. Although there is little evidence that GH improves performance in young healthy adults, randomized controlled studies carried out so far are inadequately designed to demonstrate this, not least because GH is often abused in combination with anabolic steroids and insulin. Some of the anabolic actions of GH are mediated through the generation of insulin-like growth factor-I (IGF-I), and it is believed that this is also being abused. Athletes are exposing themselves to potential harm by self-administering large doses of GH, IGF-I and insulin. The effects of excess GH are exemplified by acromegaly. IGF-I may mediate and cause some of these changes, but in addition, IGF-I may lead to profound hypoglycaemia, as indeed can insulin. Although GH is on the World Anti-doping Agency list of banned substances, the detection of abuse with GH is challenging. Two approaches have been developed to detect GH abuse. The first is based on an assessment of the effect of exogenous recombinant human GH on pituitary GH isoforms and the second is based on the measurement of markers of GH action. As a result, GH abuse can be detected with reasonable sensitivity and specificity. Testing for IGF-I and insulin is in its infancy, but the measurement of markers of GH action may also detect IGF-I usage, while urine mass spectroscopy has begun to identify the use of insulin analogues.     

IS INSULIN RESISTANCE LINKED TO HYPERTENSION?

1. The volume of work reporting insulin resistance in multiple forms of chronic hypertension has generated tremendous interest in whether this abnormality is an important factor in causing hypertension. Insulin resistance, however, is an imprecise term used interchangeably to describe widely disparate types of impairment in insulin action throughout the body and the type of insulin resistance has major ramifications regarding its potential for inducing long-term increases in blood pressure (BP). 2. Hepatic insulin resistance (impaired insulin-mediated suppression of hepatic glucose output) is the primary cause of fasting hyperinsulinaemia and is a cardinal feature of obesity hypertension. Evidence from chronic insulin infusion studies in rats suggests hyperinsulinaemia can increase BP under some conditions; however, conflicting evidence in humans and dogs leaves in question whether hyperinsulinaemia is a factor in hypertension induced by obesity. 3. Peripheral insulin resistance (impaired insulin-mediated glucose uptake, primarily of an acute glucose load in skeletal muscle) also present in obesity hypertension, but now reported in lean essential hypertension as well, is linked most notably to impaired insulin-mediated skeletal muscle vasodilation. This derangement has also been proposed as a mechanism through which insulin resistance can cause hypertension. 4. The present review will discuss the lack of experimental or theoretical support for that hypothesis and will suggest that a direct link between insulin resistance and BP control may not be the best way to envision a role for insulin resistance in cardiovascular morbidity and mortality.