Insulin receptor kinase in human skeletal muscle from obese subjects with and without noninsulin dependent diabetes

Purification and characterization of a feline hepatic insulin receptor

OBJECTIVE

To elucidate the functional characteristics of a highly purified soluble liver insulin receptor in cats.

SAMPLE POPULATION

Frozen livers from domestic cats were obtained commercially.

PROCEDURES

The feline hepatic insulin receptor was purified from Triton X-100 solubilized plasma membranes by the use of several chromatography matrices, including affinity chromatography on an insulin-Sepharose matrix.

RESULTS

The receptor, although not homogeneous, was purified 3,000-fold. Two silver-stained protein bands were identified following sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) with molecular weight of 134,000 and 97,000, which are similar to insulin receptors isolated from other animals. This isolated receptor had steady-state insulin binding by 40 minutes at 24 C. Optimal insulin binding occurred at pH 7.8 and with 150 mM NaCl. Under these conditions, a curvilinear Scatchard plot was obtained with the isolated receptor. Using a 2 binding-site model, the feline insulin receptor had a high-affinity low-capacity site with a dissociation constant (KD; nM) of 3 and a low-affinity high-capacity site with a K(D) of 1,180. The receptor also had tyrosine kinase activity toward an exogenous substrate that was stimulated by insulin and protamine.

CONCLUSIONS AND CLINICAL RELEVANCE

Many of the reported characteristics of the liver insulin receptor in cats are similar to those for the receptor isolated from other animals and tissues, although some differences exist. These similarities suggest that characterization of the feline insulin receptor is important to understanding insulin resistance in cats with diabetes as well as in humans with diabetes.

Pathophysiology and pharmacological treatment of insulin resistance

Diabetes mellitus type 2 is a world-wide growing health problem affecting more than 150 million people at the beginning of the new millennium. It is believed that this number will double in the next 25 yr. The pathophysiological hallmarks of type 2 diabetes mellitus consist of insulin resistance, pancreatic beta-cell dysfunction, and increased endogenous glucose production. To reduce the marked increase of cardiovascular mortality of type 2 diabetic subjects, optimal treatment aims at normalization of body weight, glycemia, blood pressure, and lipidemia. This review focuses on the pathophysiology and molecular pathogenesis of insulin resistance and on the capability of antihyperglycemic pharmacological agents to treat insulin resistance, i.e., a-glucosidase inhibitors, biguanides, thiazolidinediones, sulfonylureas, and insulin. Finally, a rational treatment approach is proposed based on the dynamic pathophysiological abnormalities of this highly heterogeneous and progressive disease.

Activation of insulin signal transduction pathway and anti-diabetic activity of small molecule insulin receptor activators

We recently described the identification of a non-peptidyl fungal metabolite (l-783,281, compound 1), which induced activation of human insulin receptor (IR) tyrosine kinase and mediated insulin-like effects in cells, as well as decreased blood glucose levels in murine models of Type 2 diabetes (Zhang, B., Salituro, G., Szalkowski, D., Li, Z., Zhang, Y., Royo, I., Vilella, D., Diez, M. T. , Pelaez, F., Ruby, C., Kendall, R. L., Mao, X., Griffin, P., Calaycay, J., Zierath, J. R., Heck, J. V., Smith, R. G. & Moller, D. E. (1999) Science 284, 974-977). Here we report the characterization of an active analog (compound 2) with enhanced IR kinase activation potency and selectivity over related receptors (insulin-like growth factor I receptor, epidermal growth factor receptor, and platelet-derived growth factor receptor). The IR activators stimulated tyrosine kinase activity of partially purified native IR and recombinant IR tyrosine kinase domain. Administration of the IR activators to mice was associated with increased IR tyrosine kinase activity in liver. In vivo oral treatment with compound 2 resulted in significant glucose lowering in several rodent models of diabetes. In db/db mice, oral administration of compound 2 elicited significant correction of hyperglycemia. In a streptozotocin-induced diabetic mouse model, compound 2 potentiated the glucose-lowering effect of insulin. In normal rats, compound 2 improved oral glucose tolerance with significant reduction in insulin release following glucose challenge. A structurally related inactive analog (compound 3) was not effective on insulin receptor activation or glucose lowering in db/db mice. Thus, small molecule IR activators exert insulin mimetic and sensitizing effects in cells and in animal models of diabetes. These results have implications for the future development of new therapies for diabetes mellitus.

Insulin resistance with aging: effects of diet and exercise

Insulin resistance, a reduction in the rate of glucose disposal elicited by a given insulin concentration, is present in individuals who are obese, and those with diabetes mellitus, and may develop with aging. Methods which are utilised to measure insulin sensitivity include the hyperinsulinaemic-euglycaemic and hyperglycaemic clamps and the intravenous glucose tolerance tests. Several hormones and regulatory factors affect insulin action and may contribute to the insulin resistance observed in obesity. In addition, abnormal free fatty acid metabolism plays an important role in insulin resistance and the abnormal carbohydrate metabolism seen in individuals who are obese or diabetic. Thus, the mechanisms underlying the development of insulin resistance are multifactorial, and also involve alterations of the insulin signalling pathway. Aging is associated with an increase in bodyweight and fat mass. Not only is abdominal fat associated with hyperinsulinaemia but visceral adiposity is correlated with insulin resistance as well. Modifications of the changes in body composition with aging by diet and exercise training could delay the onset of insulin resistance. Weight loss and aerobic and resistive exercise training result in losses of total body fat and abdominal fat. Several studies report that bodyweight loss increases insulin sensitivity and improves glucose tolerance. In addition, the insulin resistance observed in aged persons can be modified by physical training. Longitudinal studies indicate significant improvements in glucose metabolism with aerobic exercise training in middle-aged and older men and women. Moreover, the improvements in insulin sensitivity with resistive training are similar in magnitude to those achieved with aerobic exercise. The improvements in glucose metabolism after bodyweight loss and exercise training may in some cases be partially attributed to changes in body composition, including reductions in total and central body fat. Yet, additional changes in skeletal muscle, blood flow and other mechanisms likely interact to modify insulin resistance with exercise training. Lifestyle modifications including bodyweight loss and physical activity provide health benefits and functional gains and should be promoted to increase insulin sensitivity and prevent glucose intolerance and type 2 diabetes mellitus in older adults.

Protein tyrosine phosphatase-1B in diabetes

A role for protein tyrosine phosphatases in the negative regulation of insulin signaling and a putative involvement in the insulin resistance associated with type 2 diabetes have been postulated since their discovery. The recent demonstration that mice lacking the protein tyrosine phosphatase-1B (PTP-1B) have enhanced insulin sensitivity validates this. Furthermore, when fed a high fat diet, these mice maintained insulin sensitivity and were resistant to obesity, suggesting that inhibition of PTP-1B activity could be a novel way of treating type 2 diabetes and obesity. This commentary reviews our current knowledge of PTP-1B in insulin signaling and its role in diabetes and discusses the development of potent and selective PTP-1B inhibitors.

Elevated expression and activity of protein-tyrosine phosphatase 1B in skeletal muscle of insulin-resistant type II diabetic Goto-Kakizaki rats

We investigated the cellular mechanism(s) of insulin resistance associated with non-insulin dependent diabetes mellitus (NIDDM) using skeletal muscles isolated from non-obese, insulin resistant type II diabetic Goto-Kakizaki (GK) rats, a well known genetic rat model for type II diabetic humans. Relative to non-diabetic control rats (WKY), insulin-stimulated insulin receptor (IR) autophosphorylation and insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation were significantly inhibited in GK skeletal muscles. This may be due to increased dephosphorylation by a protein tyrosine phosphatase (PTPase). Therefore, we measured skeletal muscle total PTPase and PTPase 1B activities in the skeletal muscles isolated from control rats (WKY) and diabetic Goto-Kakizaki (GK) rats. PTPase activity was measured using a synthetic phosphopeptide, TRDIY(P)ETDY(P)Y(P)RK, as the substrate. Basal PTPase activity was 2-fold higher (P < 0.001) in skeletal muscle of GK rats when compared to WKY. Insulin infusion inhibited skeletal muscle PTPase activity in both control (26.20% of basal, P < 0.001) and GK (25.35% of basal, P < 0.001) rats. However, PTPase activity in skeletal muscle of insulin-stimulated GK rats was 200% higher than hormone-treated WKY controls (P < 0.001). Immunoprecipitation of PTPase 1B from skeletal muscle lysates and analysis of the enzyme activity in immunoprecipitates indicated that both basal and insulin-stimulated PTPase 1B activities were significantly higher (twofold, P < 0.001) in skeletal muscle of diabetic GK rats when compared to WKY controls. The increase in PTPase 1B activity in diabetic GK rats was associated with an increased expression of the PTPase 1B protein. We concluded that insulin resistance of GK rats is accompanied atleast by an abnormal regulation of PTPase 1B. Elevated PTPase 1B activity through enhanced tyrosine dephosphorylation of the insulin receptor and its substrates, may lead to impaired glucose tolerance and insulin resistance in GK rats.

Erythrocyte insulin receptor tyrosine kinase activity is increased in glyburide-treated patients with type 2 diabetes in good glycaemic control

AIM

The goal of this study was to test the hypothesis that insulin receptor tyrosine kinase activity of isolated erythrocytes would be greater in glyburide-treated patients with type 2 diabetes in good glycaemic control (n = 13) than in untreated patients (n = 12) with significant fasting hyperglycaemia.

METHODS

The two groups were similar in age, sex distribution, and body mass index. By selection, glyburide-treated patients had significantly (p < 0.001) lower (mean +/- s.e.m.) fasting glucose (6.9+/-0.4 vs. 13.9+/-0.8 mmol/l) and HbA(IC) (7.4+/-0.2 vs. 11.8+/-0.9%) concentrations. In addition, insulin-stimulated tyrosine kinase activity was increased in erythrocytes from glyburide -treated patients (p < 0.01).

RESULTS

Although insulin receptor number was similar in solubilized erythrocytes from the two groups, tyrosine kinase activity per insulin receptor was significantly (p < 0.02) greater in erythrocytes from glyburide-treated patients with type 2 diabetes.

CONCLUSIONS

These findings are quite similar to previously published data in metformin-treated patients. As such, it is suggested that decreases in insulin receptor tyrosine kinase activity may contribute to the loss of insulin sensitivity in hyperglycaemic subjects (glucotoxicity), and that an improvement in glycaemic control, irrespective of how it is achieved, will help rectify this abnormality.

Regulation of MAP kinase pathway activity in vivo in human skeletal muscle

Insulin and exercise potently stimulate glucose metabolism and gene transcription in vivo in skeletal muscle. A single bout of exercise increases the rate of insulin-stimulated glucose uptake and metabolism in skeletal muscle in the postexercise period. The nature of the intracellular signaling mechanisms that control responses to exercise is not known. In mammalian tissues, numerous reports have established the existence of the mitogen-activated protein (MAP) kinase signaling pathway that is activated by a variety of growth factors and hormones. This study was undertaken to determine how a single bout of exercise and physiological hyperinsulinemia activate the MAP kinase pathway. The euglycemic-hyperinsulinemic clamp and cycle ergometer exercise techniques combined with percutaneous muscle biopsies were used to answer this question. In healthy subjects, within 30 min, insulin significantly increased MAP kinase [isoforms p42(MAPK) and p44(MAPK) (ERK1 and ERK2)] phosphorylation (141 +/- 2%, P < 0.05) and activity (177 +/- 5%, P < 0.05), and the activity of its upstream activator MEK1 (161 +/- 16%, P < 0.05). Insulin also increased the activity of the MAP kinase downstream substrate, the p90 ribosomal S6 kinase 2 (RSK2) almost twofold (198 +/- 45%, P < 0.05). In contrast, a single 30-min bout of moderate-intensity exercise had no effect on the MAP kinase pathway activation from MEK to RSK2 in muscle of healthy subjects. However, 60 min of exercise did increase extracellular signal-related kinase activity. Therefore, despite similar effects on glucose metabolism after 30 min, insulin and exercise regulate the MAP kinase pathway differently. Insulin more rapidly activates the MAP kinase pathway.

Obesity and cardiovascular disease. Pathogenetic role of the metabolic syndrome and therapeutic implications

Since obesity is a major risk factor for cardiovascular disease (CVD), the increasing prevalence and degree of obesity in all developed countries has the potential to significantly offset the current efforts to decrease CVD burden in our population. Obesity is pathogenetically related to several clinical and sub-clinical abnormalities that contribute to the development of atherosclerotic placks and their complication, leading to the onset of cardiovascular events. Obesity seems to interact with inheritable factors in determining the onset of insulin resistance, a metabolic abnormality that is responsible for altered glucose metabolism and predisposition to type 2 diabetes, but that also has a major role in the development of dyslipidemia, hypertension and many other sub-clinical abnormalities that contribute to the atherosclerotic process and onset of cardiovascular events. Inheritable factors seem to modulate the onset of type 2 diabetes, dyslipidemia, hypertension and various insulin resistance-related sub-clinical abnormalities, often in a clustering pattern that is commonly referred to as the "metabolic syndrome." Inheritable factors also are involved in the onset of CVD in a given population or individuals with various components of the metabolic syndrome. Intense research is currently undergoing to better understand the molecular mechanisms that could explain the relationship between environmental and inheritable factors that lead from obesity to atherosclerosis and cardiovascular event. The elucidation of these mechanisms will provide improved therapeutic strategies to reduce cardiovascular risk in the obese patients. However, effective therapeutic tools that control each of the known pathophysiological steps mediating CVD in obese patients are already available and should be used more aggressively. Patient education and coordinated approach of physicians, nurses and other health care providers in a multidisciplinary treatment of the obese patient is of fundamental importance to reduce CVD burden in our population.

Protein kinase C modulates insulin action in human skeletal muscle

There is good evidence from cell lines and rodents that elevated protein kinase C (PKC) overexpression/activity causes insulin resistance. Therefore, the present study determined the effects of PKC activation/inhibition on insulin-mediated glucose transport in incubated human skeletal muscle and primary adipocytes to discern a potential role for PKC in insulin action. Rectus abdominus muscle strips or adipocytes from obese, insulin-resistant, and insulin-sensitive patients were incubated in vitro under basal and insulin (100 nM)-stimulated conditions in the presence of GF 109203X (GF), a PKC inhibitor, or 12-deoxyphorbol 13-phenylacetate 20-acetate (dPPA), a PKC activator. PKC inhibition had no effect on basal glucose transport. GF increased (P < 0.05) insulin-stimulated 2-deoxyglucose (2-DOG) transport approximately twofold above basal. GF plus insulin also increased (P < 0.05) insulin receptor tyrosine phosphorylation 48% and phosphatidylinositol 3-kinase (PI 3-kinase) activity approximately 50% (P < 0.05) vs. insulin treatment alone. Similar results for GF on glucose uptake were observed in human primary adipocytes. Further support for the hypothesis that elevated PKC activity is related to insulin resistance comes from the finding that PKC activation by dPPA was associated with a 40% decrease (P < 0.05) in insulin-stimulated 2-DOG transport. Incubation of insulin-sensitive muscles with GF also resulted in enhanced insulin action ( approximately 3-fold above basal). These data demonstrate that certain PKC inhibitors augment insulin-mediated glucose uptake and suggest that PKC may modulate insulin action in human skeletal muscle.

Altered function of insulin receptor substrate-1-deficient mouse islets and cultured beta-cell lines

Insulin receptor substrate-1 (IRS-1) is pivotal in mediating the actions of insulin and growth factors in most tissues of the body, but its role in insulin-producing beta islet cells is unclear. Freshly isolated islets from IRS-1 knockout mice and SV40-transformed IRS-1-deficient beta-cell lines exhibit marked insulin secretory defects in response to glucose and arginine. Furthermore, insulin expression is reduced by about 2-fold in the IRS-1-null islets and beta-cell lines, and this defect can be partially restored by transfecting the cells with IRS-1. These data provide evidence for an important role of IRS-1 in islet function and provide a novel functional link between the insulin signaling and insulin secretion pathways. This article may have been published online in advance of the print edition. The date of publication is available from the JCI website, http://www.jci.org.

Role of PC-1 in the etiology of insulin resistance

Defects in insulin receptor tyrosine kinase activity have been demonstrated in tissues from insulin resistant subjects, but mutations in the insulin receptor gene are rare. Therefore, other molecules that are capable of modulating the insulin receptor most likely play a major role in insulin resistance. In cultured fibroblasts from an insulin resistant patient with Type 2 diabetes, we first identified membrane glycoprotein PC-1 as an inhibitor of the insulin receptor tyrosine kinase activity. PC-1 is overexpressed in fibroblasts from other insulin resistant subjects, both with and without Type 2 diabetes. PC-1 is a large class II exoprotein whose function is unknown. Studies in muscle and fat of insulin resistant subjects two primary tissues for insulin activation, reveal that elevated levels of PC-1 are inversely correlated with decreased insulin action both in vivo and in vitro. Transfection and expression of PC-1 in cultured cells demonstrate that overexpression of PC-1 produces impairments in insulin receptor tyrosine kinase activity, and the subsequent cellular responses to insulin. These studies indicate, therefore, that PC-1 is a major factor in the etiology of insulin resistance, and is a potential new therapeutic target for anti-diabetic therapy.

Is insulin resistance the principal cause of type 2 diabetes?

The data presented from these recent studies raise serious doubt concerning the commonly held view that insulin resistance is the principal cause of type 2 diabetes: first of all they provide evidence that insulin resistance may not be the primary genetic factor for type 2 diabetes; secondly, they demonstrate that at least under certain circumstances insulin resistance is not essential for diabetes to occur, and then finally, they indicate that insulin resistance may not be the predominant factor determining the degree of hyperglycaemia. Although these studies suggest that the role of insulin resistance relative to that of beta-cell dysfunction in the pathogenesis of type 2 diabetes has been generally overestimated, one should not be left with the impression that insulin resistance is not important. It is certainly an important factor in determining the degree of hyperglycaemia or glucose intolerance present at a given level of beta-cell function. The improvement in glycaemic control after weight loss which lessens insulin resistance or after the administration of pharmacologic agents that improve insulin sensitivity clearly argue that insulin resistance is important in this regard. In addition to influencing the severity of glucose intolerance, insulin resistance is probably also important in determining the time of onset of diabetes. It may do this simply by altering the balance between the body's demand for insulin and the ability of the pancreas to provide insulin. It might adversely affect beta-cell function in addition to increasing the demand for insulin. This concept is schematically represented in figure 3. It is well established that beta-cell function normally deteriorates as a function of age [41]. Although the prevalence of type 2 diabetes increases as a function of age, this by itself obviously does not result in diabetes in the great majority of people. In such individuals their insulin sensitivity is sufficient to maintain the balance between the supply and demand for insulin above the threshold for developing diabetes. Theoretically one may postulate three other situations originating with a genetic beta-cell defect: some people may start off life with normal beta-cell function but experience a genetically determined accelerated deterioration; some people may start off life with reduced beta-cell function (e.g. less beta-cell s); still others may start off with reduced beta-cell function and have an accelerated rate of deterioration. In each of the above situations, at any given level of beta-cell function, the degree of insulin resistance present would alter the threshold for developing impaired glucose tolerance and ultimately type 2 diabetes; in other words, the greater the insulin resistance, the lower the threshold, the earlier the onset and the more severe the diabetes will be. It follows therefore that efforts to diminish insulin resistance and to preserve beta-cell function should both be beneficial. Weight loss and increased physical activity, both of which reduceinsulin resistance, have been shown to prevent progression of people with impaired glucose tolerance to diabetes. Whether this is simply due to shifting the balance between insulin requirements and insulin availability or whether it also involves an improvement in beta-cell function and/or prevention of its deterioration remains to be clarified. Furthermore, it is not known whether pharmacologic agents which improve insulin sensitivity have similar effects.

Characterization of selective resistance to insulin signaling in the vasculature of obese Zucker (fa/fa) rats

Both insulin resistance and hyperinsulinemia have been reported to be independent risk factors for cardiovascular diseases. However, little is known regarding insulin signaling in the vascular tissues in insulin-resistant states. In this report, insulin signaling on the phosphatidylinositol 3-kinase (PI 3-kinase) and mitogen-activated protein (MAP) kinase pathways were compared in vascular tissues of lean and obese Zucker (fa/fa) rats in both ex vivo and in vivo studies. Ex vivo, insulin-stimulated tyrosine phosphorylation of insulin receptor beta subunits (IRbeta) in the aorta and microvessels of obese rats was significantly decreased compared with lean rats, although the protein levels of IRbeta in the 2 groups were not different. Insulin-induced tyrosine phosphorylation of insulin receptor substrates 1 and 2 (IRS-1 and IRS-2) and their protein levels were decreased in the aorta of obese rats compared with lean rats. The association of p85 subunit to the IRS proteins and the IRS-associated PI 3-kinase activities stimulated by insulin in the aorta of obese rats were significantly decreased compared with the lean rats. In addition, insulin-stimulated serine phosphorylation of Akt, a downstream kinase of PI 3-kinase pathway, was also reduced significantly in isolated microvessels from obese rats compared with the lean rats. In euglycemic clamp studies, insulin infusion greatly increased tyrosine phosphorylation of IRbeta- and IRS-2-associated PI 3-kinase activity in the aorta of lean rats, but only slight increases were observed in obese rats. In contrast, insulin stimulated tyrosine phosphorylation of MAP kinase (ERK-1/2) equally in isolated microvessels of lean and obese rats, although basal tyrosine phosphorylation of ERK-1/2 was higher in the obese rats. To our knowledge, these data provided the first direct measurements of insulin signaling in the vascular tissues, and documented a selective resistance to PI 3-kinase (but not to MAP kinase pathway) in the vascular tissues of obese Zucker rats.

Carnitine increases glucose disposal in humans

OBJECTIVE

The goal of this work is to assess the effect of L-carnitine on glucose disposal, particularly on insulin sensitivity, in healthy volunteers.

METHODS

Fourteen healthy human volunteers were subjected to the intravenous glucose tolerance test (analyzed by means of the minimal model technique), together with indirect calorimetry and measurement of serum free fatty acids, after a bolus of glucose plus carnitine (C) or a bolus of glucose plus saline (P).

RESULTS

The minimal model demonstrated a significant increase in glucose disposal from plasma with carnitine: Glucose effectiveness passed from 2.7%/min to 3.8%/min. No significant changes were observed in the Insulin Sensitivity Index or in Insulin/C-Peptide secretion. Calorimetry showed a significant increase in respiratory quotient, resulting from a significant increase in carbohydrate oxidation rate during carnitine administration by an average of 0.0176+/-0.0118 g/min (p=0.015). Energy expenditure was not modified by treatment. A smaller decrease in plasma fatty acid concentrations was noted with carnitine plus glucose than after glucose alone.

CONCLUSIONS

From these data it appears that carnitine stimulates glucose disposal and oxidation in the healthy volunteer. Therefore, carnitine might be useful as an adjunct in the therapy of diabetes mellitus.

Marked impairment of protein tyrosine phosphatase 1B activity in adipose tissue of obese subjects with and without type 2 diabetes mellitus

Protein tyrosine phosphatases (PTPs) are required for the dephosphorylation of the insulin receptor (IR) and its initial cellular substrates, and it has recently been reported that PTP-1B may play a role in the pathogenesis of insulin resistance in obesity and type 2 diabetes mellitus (DM). We therefore determined the amount and activity of PTP-1B in abdominal adipose tissue obtained from lean nondiabetic subjects (lean control (LC)), obese nondiabetic subjects (obese control (OC)), and subjects with both type 2 DM (DM2) and obesity (obese diabetic (OD)). PTP-1B protein levels were 3-fold higher in OC than in LC (1444 +/- 195 U vs 500 +/- 146 U (mean +/- SEM), P < .015), while OD exhibited a 5.5-fold increase (2728 +/- 286 U, P < .01). PTP activity was assayed by measuring the dephosphorylating activity toward a phosphorus 32-labeled synthetic dodecapeptide. In contrast to the increased PTP-1B protein levels, PTP-1B activity per unit of PTP-1B protein was markedly reduced, by 71% and 88% in OC and OD, respectively. Non-PTP-1B tyrosine phosphatase activity was comparable in all three groups. Similar results were obtained when PTP-1B activity was measured against intact human IR. A significant correlation was found between body mass index (BMI) and PTP-1B level (r = 0.672, P < .02), whereas BMI and PTP-1B activity per unit of PTP-1B showed a strong inverse correlation (r = -0.801, P < .002). These data suggest that the insulin resistance of obesity and DM2 is characterized by the increased expression of a catalytically impaired PTP-1B in adipose tissue and that impaired PTP-1B activity may be pathogenic for insulin resistance in these conditions.

Insulin receptor autophosphorylation in cultured myoblasts correlates to glucose disposal in Pima Indians

In a previous study [Youngren, J. F., I. D. Goldfire, and R. E. Pratley. Am. J. Physiol. 273 (Endocrinol. Metab. 36): E276-E283, 1997] of skeletal muscle biopsies from insulin-resistant, nondiabetic Pima Indians, we demonstrated that diminished insulin receptor (IR) autophosphorylation correlated with in vivo insulin resistance. In the present study, to determine whether decreased IR function is a primary trait of muscle, and not secondary to an altered in vivo environment, we cultured myoblasts from 17 nondiabetic Pima Indians in whom insulin-stimulated glucose disposal (M) was measured during hyperinsulinemic-euglycemic glucose clamps. Myoblast IR autophosphorylation was determined by a highly sensitive ELISA. IR autophosphorylation directly correlated with M (r = 0.56, P = 0.02) and inversely correlated with the fasting plasma insulin (r = -0.58, P < 0.05). The relationship between M and IR autophosphorylation remained significant after M was adjusted for the effects of percent body fat (partial r = 0.53, P < 0.04). The relationship between insulin resistance and the capacity for myoblast IR autophosphorylation in nondiabetic Pima Indians suggests that variations in IR-signaling capacity may be intrinsic characteristics of muscle that contribute to the genetic component determining insulin action in this population.

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

We have achieved significant progress in understanding the central role of the insulin receptor in an increasingly complicated web of intracellular signal transduction leading to the ultimate biological actions of insulin on glucose, lipid, and other metabolic pathways. The excitement for the future lies not only in clarifying these pathways but also returning to whole-body physiology to readdress basic mechanisms of insulin action in known and novel insulin-sensitive tissues. Hopefully, these new techniques and new perspectives will bring us closer to understanding the pathophysiology of type 2 diabetes mellitus.

Cellular mechanism of nutritionally induced insulin resistance: the desert rodent Psammomys obesus and other animals in which insulin resistance leads to detrimental outcome

Animal species with genetic or nutritionally induced insulin resistance, diabetes and obesity (diabesity) may be divided into two broad groups: those with resilient pancreatic beta-cells, e.g. ob/ob mice and fa/fa rats, capable of long-lasting compensatory insulin over-secretion, and those with labile beta-cells in which the secretion pressure leads to irreversible beta-cell degranulation, e.g. db/db mice, Macaca mulatta primates, ZDF diabetic rats. Prominent in this group is the Israeli desert gerbil Psammomys obesus (sand rat), which features low insulin receptor density in liver and muscle. On a diet of relatively high energy, the capacity of insulin to activate the receptor tyrosine kinase (TK) is reduced, in the face of hyperinsulinemia. With the following hyperglycemia, the rising insulin resistance imposes a vicious cycle of insulinemia and glycemia, accentuating the TK activation failure and the beta-cell failure. Among various factors affecting the insulin signaling pathway, multisite phosphorylation, including serine and threonine on the receptor beta-subunit, due to overexpression of certain protein kinase C isoforms, seems to be responsible for the inhibition of the critical step of TK phosphorylation activity. The compromised TK activation is reversible by diet restriction which restores to normal the glycemia and insulinemia. The beta-cell response to long-lasting stimulation and the receptor malfunction in diabesity have implications for a similar etiology in human insulin resistance syndrome and type 2 diabetes, particularly in populations emerging from a food scarce environment into nutritional affluence, inappropriate to the human metabolic capacity. It is suggested that the "thrifty gene" is characterized by a low threshold for insulin secretion and low capacity for insulin clearance. Thus, nutritionally-induced hyperinsulinemia is potentiated and becomes the primary phenotypic expression of the thrifty gene, linked to the insulin receptor signaling pathway malfunction.

Dephosphorylation increases insulin-stimulated receptor kinase activity in skeletal muscle of obese Zucker rats

Serine/threonine phosphorylation of insulin receptor has been implicated in the development of insulin resistance. To investigate whether dephosphorylation of serine/threonine residues of the insulin receptor may restore the decreased insulin-stimulated receptor tyrosine kinase activity in skeletal muscle of obese Zucker rats, insulin receptor tyrosine kinase activity was measured before and after alkaline phosphatase treatment. Compared to lean controls, insulin-stimulated glucose transport was depressed by 61% (p < 0.05) in obese Zucker rats. The insulin receptor and insulin receptor substrate-1 contents were decreased by 14% (p < 0.05) and 16% (p < 0.05), respectively, in skeletal muscle of obese Zucker rats. In vivo insulin-induced tyrosine phosphorylation of insulin receptor and insulin receptor substrate-1 was depressed by 82% (p < 0.05) and 86% (p < 0.05), respectively. In the meantime, in vitro insulin-stimulated receptor tyrosine kinase activity in obese rats was decreased by 39% (p < 0.05). Dephosphorylation of the insulin receptor by prior alkaline phosphatase treatment increased insulin-stimulated receptor tyrosine kinase activity in both lean and obese Zucker rats, but the increase was three times greater in obese Zucker rats (p < 0.05). These findings suggest that excessive serine/threonine phosphorylation of the insulin receptor in obese Zucker rats may be a cause for insulin resistance in skeletal muscle.

Tissue-specific knockout of the insulin receptor in pancreatic beta cells creates an insulin secretory defect similar to that in type 2 diabetes

Dysfunction of the pancreatic beta cell is an important defect in the pathogenesis of type 2 diabetes, although its exact relationship to the insulin resistance is unclear. To determine whether insulin signaling has a functional role in the beta cell we have used the Cre-loxP system to specifically inactivate the insulin receptor gene in the beta cells. The resultant mice exhibit a selective loss of insulin secretion in response to glucose and a progressive impairment of glucose tolerance. These data indicate an important functional role for the insulin receptor in glucose sensing by the pancreatic beta cell and suggest that defects in insulin signaling at the level of the beta cell may contribute to the observed alterations in insulin secretion in type 2 diabetes.

Complementary measures for promoting insulin sensitivity in skeletal muscle

Insulin resistance of skeletal muscle is fundamental to both syndrome X and its frequent sequel, type II diabetes. In these disorders, excessive exposure of muscle to free fatty acids (FFAs) and their metabolic derivatives appears to play a prominent role in the induction of insulin resistance. Recent evidence suggests that activation of novel isoforms of protein kinase C (PKC) by diacylglycerol may mediate at least part of the adverse impact of FFAs on muscle insulin sensitivity. Vitamin E and fish oil omega-3s, by promoting the activity of diacylglycerol kinase and inhibiting that of phosphatidate phosphohydrolase, should reduce diacylglycerol levels, thus accounting for their documented favorable impact on insulin sensitivity. Thiazolidinediones such as troglitazone, on the other hand, appear to intervene in the signaling pathway whereby PKC down-regulates insulin function. The insulin-sensitizing activity of chromium picolinate may be attributable, at least in part, to increased expression of insulin receptors. In combination with lifestyle modifications which reduce FFA exposure--weight loss, very-low-fat eating, excessive training--these measures can be expected to work in a complementary way to promote increased numbers of insulin receptors that are more functionally competent. As these measures appear to be safe and well-tolerated, they may have utility for the prevention of diabetes as well as its therapy. When they do not prove sufficient to achieve optimal glycemic control, excessive hepatic glucose output and impaired cell response to glucose can be addressed with metformin and sulfonylureas, respectively. The prospects for a rational medical management of type II diabetes, obviating the need for injectible insulin, have never been brighter.

A muscle-specific insulin receptor knockout exhibits features of the metabolic syndrome of NIDDM without altering glucose tolerance

Skeletal muscle insulin resistance is among the earliest detectable defects in humans with type 2 diabetes mellitus. To determine the contribution of muscle insulin resistance to the metabolic phenotype of diabetes, we used the Cre-loxP system to disrupt the insulin receptor gene in mouse skeletal muscle. The muscle-specific insulin receptor knockout mice exhibit a muscle-specific > 95% reduction in receptor content and early signaling events. These mice display elevated fat mass, serum triglycerides, and free fatty acids, but blood glucose, serum insulin, and glucose tolerance are normal. Thus, insulin resistance in muscle contributes to the altered fat metabolism associated with type 2 diabetes, but tissues other than muscle appear to be more involved in insulin-regulated glucose disposal than previously recognized.

A high fat diet impairs stimulation of glucose transport in muscle. Functional evaluation of potential mechanisms

A high fat diet causes resistance of skeletal muscle glucose transport to insulin and contractions. We tested the hypothesis that fat feeding causes a change in plasma membrane composition that interferes with functioning of glucose transporters and/or insulin receptors. Epitrochlearis muscles of rats fed a high (50% of calories) fat diet for 8 weeks showed approximately 50% decreases in insulin- and contraction-stimulated 3-O-methylglucose transport. Similar decreases in stimulated glucose transport activity occurred in muscles of wild-type mice with 4 weeks of fat feeding. In contrast, GLUT1 overexpressing muscles of transgenic mice fed a high fat diet showed no decreases in their high rates of glucose transport, providing evidence against impaired glucose transporter function. Insulin-stimulated system A amino acid transport, insulin receptor (IR) tyrosine kinase activity, and insulin-stimulated IR and IRS-1 tyrosine phosphorylation were all normal in muscles of rats fed the high fat diet for 8 weeks. However, after 30 weeks on the high fat diet, there was a significant reduction in insulin-stimulated tyrosine phosphorylation in muscle. The increases in GLUT4 at the cell surface induced by insulin or muscle contractions, measured with the 3H-labeled 2-N-4-(1-azi-2,2, 2-trifluoroethyl)-benzoyl-1,3-bis-(D-mannose-4-yloxy)-2-propyla min e photolabel, were 26-36% smaller in muscles of the 8-week high fat-fed rats as compared with control rats. Our findings provide evidence that (a) impairment of muscle glucose transport by 8 weeks of high fat feeding is not due to plasma membrane composition-related reductions in glucose transporter or insulin receptor function, (b) a defect in insulin receptor signaling is a late event, not a primary cause, of the muscle insulin resistance induced by fat feeding, and (c) impaired GLUT4 translocation to the cell surface plays a major role in the decrease in stimulated glucose transport.

Membrane glycoprotein PC-1 and insulin resistance

Peripheral resistance to insulin is a major component of non-insulin dependent diabetes mellitus. Defects in insulin receptor tyrosine kinase activity have been demonstrated in several tissues from insulin resistant subjects, but mutations in the insulin receptor gene occur in only a small fraction of cases. Therefore, other molecules that are capable of modulating the function of the insulin receptor are likely candidates in the search for the cellular mechanisms of insulin resistance. We have isolated an inhibitor of insulin receptor tyrosine kinase activity from cultured fibroblasts of an insulin resistant NIDDM patient and identified it as membrane glycoprotein PC-1. Subsequently we have demonstrated that expression of PC-1 is elevated in fibroblasts from other insulin resistant subjects, both with and without NIDDM. Studies in muscle, the primary site for insulin-mediated glucose disposal, have shown that the levels of PC-1 in this tissue are inversely correlated to insulin action both in vivo and in vitro. Transfection of PC-1 into cultured cells has confirmed that overexpression of PC-1 can produce impairments in insulin receptor tyrosine kinase activity and the subsequent cellular responses to insulin. Preliminary data suggests a direct interaction between PC-1 and the insulin receptor. However, the mechanisms whereby PC-1 inhibits insulin receptor signaling remain to be determined.

Protein-tyrosine phosphatase-1B acts as a negative regulator of insulin signal transduction

Insulin signaling involves a dynamic cascade of protein tyrosine phosphorylation and dephosphorylation. Most of our understanding of this process comes from studies focusing on tyrosine kinases, which are signal activators. Our knowledge of the role of protein-tyrosine phosphatases (PTPases), signal attenuators, in regulating insulin signal transduction remains rather limited. Protein-tyrosine phosphatase 1B (PTP-1B), the prototypical PTPase, is ubiquitously and abundantly expressed. Work from several laboratories, including our own, has implicated PTP-1B as a negative regulator of insulin action and as a potentially important mediator in the pathogenesis of insulin-resistance and non-insulin dependent diabetes mellitus (NIDDM).

Minimal influence of blood flow on interstitial glucose and lactate-normal and insulin-resistant muscle

To study the regulation of the interstitial glucose concentration in skeletal muscle, nine control subjects and nine older and overweight non-insulin-dependent diabetes mellitus (NIDDM) subjects with extreme insulin resistance were investigated with microdialysis in the medial femoral muscle before and during a euglycemic insulin clamp. After an overnight fast, arterial plasma glucose concentration was 4.9 +/- 0.1 and 8.5 +/- 0.6 mmol/l (P < 0.001), respectively. The arterial-interstitial concentration ([a-i]) differences of glucose and lactate were 0.43 +/- 0.16 (P < 0.05) and -0.13 +/- 0.05 mmol/l, respectively, in normal subjects. In NIDDM subjects, [a-i] differences for glucose and lactate were nonsignificant. Muscle blood flow was similar in controls and NIDDM subjects. During the glucose clamp, the glucose [a-i] differences increased and the lactate [a-i] differences decreased significantly in both groups. The glucose 170 infusion rate was 8.0 +/- 0.77 vs. 3.2 +/- 0.51 mg.kg-1.min-1 (P < 0.001), and blood flow was 9.9 +/- 1.6 vs. 6.7 +/- 0.9 ml.100 g-1.min-1 (P < 0.05) in controls and NIDDM subjects, respectively. These results show that 1) the capillary wall is rate limiting for muscle glucose uptake and lactate release in control subjects but not in postabsorptive hyperglycemic insulin-resistant subjects, 2) vasodilation during insulin infusion does not prevent the increase in [a-i] difference of glucose in normal subjects, and 3) in severely insulin-resistant muscle, the [a-i] difference of glucose is not extended despite lack of vasodilation.

Role of exercise training in the prevention and treatment of insulin resistance and non-insulin-dependent diabetes mellitus

Recent epidemiological studies indicate that individuals who maintain a physically active lifestyle are much less likely to develop impaired glucose tolerance and non-insulin-dependent diabetes mellitus (NIDDM). Moreover, it was found that the protective effect of physical activity was strongest for individuals at highest risk of developing NIDDM. Reducing the risk of insulin resistance and NIDDM by regularly performed exercise is also supported by several aging studies. It has been found that older individuals who vigorously train on a regular basis exhibit a greater glucose tolerance and a lower insulin response to a glucose challenge than sedentary individuals of similar age and weight. While the evidence is substantial that aerobic exercise training can reduce the risk of impaired glucose tolerance and NIDDM, the evidence that exercise training is beneficial in the treatment of NIDDM is not particularly strong. Many of the early studies investigating the effects of exercise training on NIDDM could not demonstrate improvements in fasting plasma glucose and insulin levels, or glucose tolerance. The adequacy of the training programmes in many of these studies, however, is questionable. More recent studies using prolonged, vigorous exercise-training protocols have produced more favourable results. There are several important adaptations to exercise training that may be beneficial in the prevention and treatment of insulin resistance, impaired glucose tolerance and NIDDM. An increase in abdominal fat accumulation and loss of muscle mass are highly associated with the development of insulin resistance. Exercise training results in preferential loss of fat from the central regions of the body and should therefore contribute significantly in preventing or alleviating insulin resistance due to its development. Likewise, exercise training can prevent muscle atrophy and stimulate muscle development. Several months of weight training has been found to significantly lower the insulin response to a glucose challenge without affecting glucose tolerance, and to increase the rate of glucose clearance during a euglycaemic clamp. Muscle glucose uptake is equal to the product of the arteriovenous glucose difference and the rate of glucose delivery or muscle blood flow. While it has been known for many years that insulin will accelerate blood glucose extraction by insulin-sensitive peripheral tissues, recent evidence suggests that it can also acutely vasodilate skeletal muscle and increase muscle blood flow in a dose-dependent manner. A reduced ability of insulin to stimulate muscle blood flow is a characteristic of insulin-resistant obese individuals and individuals with NIDDM. Exercise training, however, has been found to help alleviate this problem, and substantially improve the control of insulin over blood glucose. Improvements in insulin resistance and glucose tolerance with exercise training are highly related to an increased skeletal muscle insulin action. This increased insulin action is associated with an increase in the insulin-regulatable glucose transporters, GLUT4, and enzymes responsible for the phosphorylation, storage and oxidation of glucose. Changes in muscle morphology may also be important following training. With exercise training there is an increase in the conversion of fast twitch glycolytic IIb fibres to fast twitch oxidative IIa fibres, as well as an increase in capillary density. IIa fibres have a greater capillary density and are more insulin-sensitive and -responsive than IIb fibres. Evidence has been provided that morphological changes in muscle, particularly the capillary density of the muscle, are associated with changes in fasting insulin levels and glucose tolerance. Furthermore, significant correlations between glucose clearance, muscle capillary density and fibre type have been found in humans during a euglycaemic clamp. Exercise training may also improve control over hepatic glucose production by increasin

Decreased muscle insulin receptor kinase correlates with insulin resistance in normoglycemic Pima Indians

Defects in insulin receptor tyrosine kinase activity are present in insulin-resistant non-insulin-dependent diabetes mellitus patients and certain nondiabetic individuals, both lean and obese. However, the relationship between insulin receptor function, insulin action, and obesity is unclear. To address this issue, we have employed a new and highly sensitive enzyme-linked immunosorbent assay to measure in vitro insulin-stimulated autophosphorylation of immunocaptured muscle insulin receptors in a group of 25 normoglycemic Pima Indians. Insulin action, determined during two-step euglycemic insulin clamps, varied widely in these subjects. Maximal in vitro insulin stimulation of insulin receptor autophosphorylation strongly correlated with both low (Mlow)- and high (Mhigh)-dose insulin-stimulated glucose disposal (r = 0.62 and 0.51, P < 0.002 and 0.011, respectively). Insulin receptor autophosphorylation was inversely related to percent body fat (r = -0.52, P < 0.009). After control for percent body fat, receptor autophosphorylation remained correlated with Mlow (partial r = 0.49, P < 0.025). These data therefore suggest that defects in insulin receptor function are major contributors to insulin resistance in both lean and obese normoglycemic Pima Indians.

Development of a novel polygenic model of NIDDM in mice heterozygous for IR and IRS-1 null alleles

NIDDM is a polygenic disease characterized by insulin resistance in muscle, fat, and liver, followed by a failure of pancreatic beta cells to adequately compensate for this resistance despite increased insulin secretion. Mice double heterozygous for null alleles in the insulin receptor and insulin receptor substrate-1 genes exhibit the expected approximately 50% reduction in expression of these two proteins, but a synergism at a level of insulin resistance with 5- to 50-fold elevated plasma insulin levels and comparable levels of beta cell hyperplasia. At 4-6 months of age, 40% of these double heterozygotes become overtly diabetic. This NIDDM mouse model in which diabetes arises in an age-dependent manner from the interaction between two genetically determined, subclinical defects in the insulin signaling cascade demonstrates the role of epistatic interactions in the pathogenesis of common diseases with non-Mendelian genetics.

Increased expression of insulin/insulin-like growth factor-I hybrid receptors in skeletal muscle of noninsulin-dependent diabetes mellitus subjects

Insulin receptors (IR) and IGF-I receptors (IGF-IR) have been shown to form hybrid receptors in tissues coexpressing both molecules. To date there is no information about the distribution of hybrids in tissues of normal or diabetic subjects. We developed a microwell-based immunoassay to quantitate hybrids in small human tissues samples. Microwells were coated with MA-20 anti-IR antibody or alpha-IGF-IR-PA antibody directed against the IGF-IR alpha-subunit, and incubated with skeletal muscle extracts of patients with noninsulin-dependent diabetes mellitus (NIDDM) and normal controls. Immobilized receptors were incubated with 125I-insulin or 125I-IGF-I in the presence or absence of the two unlabeled ligands. Hybrids were quantified as the fraction of 125I-IGF-I binding immunoadsorbed with MA-20 and expressed as percentage of total IGF-IR (type I+hybrids) immobilized with alpha-IGF-IR-PA. The immunoassay was validated using Western blotting analysis. Relative abundance of hybrids detected in NIDDM patients was higher than in controls. The percentage of hybrids was negatively correlated with IR number and in vivo insulin sensitivity measured by an insulin tolerance test, whereas the percentage was positively correlated with insulinemia. Insulin binding affinity was lower in NIDDM patients than in controls, and was correlated with the percentage of hybrids. Maximal IGF-I binding was significantly higher in muscle from NIDDM patients compared to controls and was positively correlated with the percentage of hybrid receptors whereas IGF-I binding affinity did not differ between the two groups. These results raise the possibility that alterations in expression of hybrid receptors may contribute to decreased insulin sensitivity, and to increased sensitivity to IGF-I. Because IGF-I has been proposed as a hypoglycemic agent in NIDDM, these results are relevant to the development of new approaches to the treatment of insulin resistance of NIDDM.

PC-1 content in skeletal muscle of non-obese, non-diabetic subjects: relationship to insulin receptor tyrosine kinase and whole body insulin sensitivity

Insulin sensitivity varies widely in non-obese, non-diabetic subjects, and we have previously reported that in vivo insulin action correlates with in vitro insulin stimulated insulin receptor tyrosine-kinase activity in skeletal muscle. Plasma membrane glyco-protein PC-1 content is elevated in fibroblasts of insulin-resistant subjects, and expression of PC-1 cDNA in cultured cells reduces both insulin receptor tyrosine-kinase activity and the biological actions of insulin. In the present study we investigated non-obese, non-diabetic subjects and found a significant negative correlation between muscle PC-1 content and both in vivo insulin action as measured by the intravenous insulin tolerance test (r = -0.51, p = 0.035) and the sensitivity (ED50) of in vitro insulin stimulation of insulin receptor tyrosine-kinase activity (r = 0.66, p = 0.027). These studies indicate, therefore, that increased muscle PC-1 content is associated with reduced insulin action both in vivo and in vitro. Moreover, they suggest a possible role for PC-1 in regulating insulin receptor function in human skeletal muscle.

Tumor necrosis factor-alpha- and hyperglycemia-induced insulin resistance. Evidence for different mechanisms and different effects on insulin signaling

Inhibition of insulin receptor signaling by high glucose levels and by TNF-alpha was recently observed in different cell systems. The aim of the present study was to characterize the mechanism of TNF-alpha-induced insulin receptor inhibition and to compare the consequences of TNF-alpha- and hyperglycemia-induced insulin receptor inhibition for signal transduction downstream from the IR. TNF-alpha (0.5-10 nM) and high glucose (25 mM) showed similar rapid kinetics of inhibition (5-10 min, > 50%) of insulin receptor autophosphorylation in NIH3T3 cells overexpressing the human insulin receptor. TNF-alpha effects were completely prevented by the phosphotyrosine phosphatase (PTPase) inhibitors orthovanadate (40 microM) and phenylarsenoxide (35 microM), but they were unaffected by the protein kinase C (PKC) inhibitor H7 (0.1 mM), the phosphatidylinositol-3 kinase inhibitor wortmannin (5 microM), and the thiazolidindione troglitazone (CS045) (2 microgram/ml). In contrast, glucose effects were prevented by PKC inhibitors and CS045 but unaffected by PTPase inhibitors and wortmannin. To assess effects on downstream signaling, tyrosine phosphorylation of the following substrate proteins of the insulin receptor was determined: insulin receptor substrate-1, the coupling protein Shc, focal adhesion kinase (FAK125), and unidentified proteins of 130 kD, 60 kD. Hyperglycemia (25 mM glucose) and TNF-alpha showed analogous (> 50% inhibition) effects on tyrosine phosphorylation of insulin receptor substrate-1, Shc, p60, and p44, whereas opposite effects were observed for tyrosine phosphorylation of FAK125, which is dephosphorylated after insulin stimulation. Whereas TNF-alpha did not prevent insulin-induced dephosphorylation of FAK125, 25 mM glucose blocked this insulin effect completely. In summary, the data suggest that TNF-alpha and high glucose modulate insulin receptor-signaling through different mechanisms: (a) TNF-alpha modulates insulin receptor signals by PTPase activation, whereas glucose acts through activation of PKC. (b) Differences in modulation of the insulin receptor signaling cascade are found with TNF-alpha and high glucose: Hyperglycemia-induced insulin receptor inhibition blocks both insulin receptor-dependent tyrosine phosphorylation and dephosphorylation of insulin receptor substrate proteins. In contrast, TNF-alpha blocks only substrate phosphorylation, and it does not block insulin-induced substrate dephosphorylation. The different effects on FAK125 regulation allow the speculation that long-term cell effects related to FAK125 activity might develop in a different way in hyperglycemia- and TNF-alpha-dependent insulin resistance.

Effects of exercise training on abdominal obesity and related metabolic complications

Excessive deposition of visceral adipose tissue is known to predispose to cardiovascular diseases. Considerable epidemiological and experimental evidence suggests that many physiological factors are involved in the aetiology of premature atherosclerosis associated with visceral obesity. Insulin resistance is frequently associated with abdominal obesity, and probably plays an important role in the pathophysiology of hypertriglyceridaemia, low levels of plasma high-density lipoprotein (HDL)-cholesterol, hypertension and reduced fibrinolytic activity. Exercise training may counteract the aberrant metabolic profile associated with abdominal obesity both directly and as a consequence of body fat loss. Exercise may increase insulin sensitivity, favourably alter the plasma lipoprotein profile and improve fibrinolytic activity. Changes in the activity of insulin-sensitive glucose transporters and of skeletal muscle lipoprotein lipase are some of the possible explanations for the increased insulin sensitivity and improved blood lipid profile associated with regular exercise. This review presents physical training as a relevant nonpharmacological tool in the treatment of abdominal obesity and associated metabolic disorders. The impact of regular exercise on the different aspects of the insulin resistance syndrome is discussed. The roles of gender, age and the state of insulin resistance on the metabolic effect of physical training are also considered.

Diverse signaling pathways in the cellular actions of insulin

Insulin is one of the most important regulators of glucose and lipid homeostasis. Many of its cellular actions are mediated by changes in protein phosphorylation. The consequences of these phosphorylation events extend from a series of different short-term metabolic actions to longer-term effects of the hormone on cellular growth and differentiation. Although the insulin receptor itself is a tyrosine kinase that is activated upon hormone binding, the ensuing changes in phosphorylation occur predominantly on serine and threonine residues. Moreover, insulin can simultaneously stimulate the phosphorylation of some proteins and the dephosphorylation of others. These paradoxical effects of insulin suggest that separate signal transduction pathways may emanate from the receptor itself to produce the pleiotropic actions of the hormone.

Decreased skeletal muscle phosphotyrosine phosphatase (PTPase) activity towards insulin receptors in insulin-resistant Zucker rats measured by delayed Europium fluorescence

In order to measure the phosphotyrosine phosphatase (PTPase) activity in small muscle biopsies, a sandwich-immunofluorescence assay was developed using the phosphorylated human insulin receptor as a substrate, a C-terminal insulin receptor antibody as catching antibody and Europium-labelled anti-phosphotyrosine as detecting antibody. Soluble and particulate muscle fractions were prepared from soleus muscle of obese, diabetic (fa/fa) Zucker rats and their lean littermates (Fa/-). In the soluble muscle fractions of the obese (fa/fa) rats PTPase activity was significantly reduced compared to control (Fa/-) rats (45.2 +/- 2.6% vs 61.3 +/- 4.7%, p < 0.02). This reduction was completely prevented by 24 days of metformin treatment which decreased plasma glucose and plasma insulin levels. In particulate muscle fractions, however, no difference in PTPase activity was found among any groups of rats examined. These results show that the alterations in soluble PTPase activity in the insulin-resistant, diabetic Zucker rat vary with the abnormality in glucose homeostasis.

Long-term effects of fluoxetine on glycemic control in obese patients with non-insulin-dependent diabetes mellitus or glucose intolerance: influence on muscle glycogen synthase and insulin receptor kinase activity

Fluoxetine (F) is a specific serotonin-reuptake inhibitor that has been shown to promote weight loss and improve glycemic control in obese diabetic patients. To study its long-term metabolic effect, 40 obese patients with non-insulin -dependent diabetes mellitus (NIDDM) or impaired glucose tolerance (IGT) were included in a 12-month, randomized, placebo controlled study. Patients were assigned to receive either 60 mg F or placebo (P) daily in conjunction with a 5.0-MJ/d diet (> 50% carbohydrate). Both groups showed a significant weight loss, with a nadir after 6 months without group differences (mean +/- SD: F, 10.1 +/- 10.0 kg; P, 9.4 +/- 11.5 kg). Fifteen patients from the F group and 14 from the P group completed the 12-month study without weight loss differences. Glycemic regulation improved along with the weight loss, but with a larger decline in plasma C-peptide and fasting glucose levels on the F group (P < .05). Total skeletal muscle glycogen synthase (GS) activity increased by 31% in the F group (P < .01) and by 17% in the P group (nonsignificant) after 6 months of treatment, but was still less than the activity in normal-weight controls (aged 28.0 +/- 6.3 years; body mass index, 23.5 +/- 2.2). After adjustment for fasting glucose, insulin, weight loss, and diabetic state, a positive effect of F remained on the total GS activity, which accounted for 27% of the variation (P < .05). The waist to hip ratio was reduced in P subjects as compared with F subjects (P < .05). Fat-free mass (FFM) tended to be more reduced in the F group as compared with P subjects (4.9 v 1.9 kg), although the difference did not reach statistical significance. In conclusion, F seems to improve insulin sensitivity beyond the effect mediated through weight loss by a possible effect on GS activity in skeletal muscle tissue.

Increased abundance of specific skeletal muscle protein-tyrosine phosphatases in a genetic model of insulin-resistant obesity and diabetes mellitus

Resistance to the biological action of insulin in its target tissues is a cardinal feature of non-insulin-dependent diabetes mellitus. Protein-tyrosine phosphatases (PTPases) have been postulated to play a key role in the regulation of the insulin action pathway, especially in skeletal muscle, the major site of insulin-mediated glucose disposal in vivo. To evaluate whether changes in the activity and/or abundance of candidate skeletal muscle PTPases is associated with severe resistance to insulin in an animal model, we measured PTPase enzyme activity and PTPase protein level by immunoblotting in subcellular fractions of skeletal muscle in lean (+/?), insulin-resistant obese (fa/fa), and diabetic (ZDF/Drt-fa/fa) Zucker rats. Using a phosphotyrosylmyelin basic protein substrate, the solubilized-particulate fraction PTPase activity was increased by 65% and 74% (P < .05) and in vitro dephosphorylation of a recombinant rat insulin receptor kinase domain was increased by 104% and 114% in obese and diabetic animals, respectively (P < .01). These changes in PTPase activity were associated with an increase in specific immunoreactivity of leukocyte common antigen-related PTPase ([LAR] by 42% and 50%), PTPase 1B (by 61% and 69%), and the SHZ domain containing PTPase (SH-PTP2) (by 44% and 48%) in the solubilized-particulate fraction of obese and diabetic animals, respectively (P < .05). In diabetic muscle, increased SH-PTP2 abundance was also associated with a shift of SH-PTP2 to a plasma membrane component, which may have important consequences for the activation of this enzyme in the insulin-resistant state. These results provide evidence that specific PTPases play a role in the insulin resistance of this genetic model of obesity and non-insulin-dependent diabetes.

Who would have thought it? An operation proves to be the most effective therapy for adult-onset diabetes mellitus

OBJECTIVE

This report documents that the gastric bypass operation provides long-term control for obesity and diabetes.

SUMMARY BACKGROUND DATA

Obesity and diabetes, both notoriously resistant to medical therapy, continue to be two of our most common and serious diseases.

METHODS

Over the last 14 years, 608 morbidly obese patients underwent gastric bypass, an operation that restricts caloric intake by (1) reducing the functional stomach to approximately 30 mL, (2) delaying gastric emptying with a c. 0.8 to 1.0 cm gastric outlet, and (3) excluding foregut with a 40 to 60 cm Roux-en-Y gastrojejunostomy. Even though many of the patients were seriously ill, the operation was performed with a perioperative mortality and complication rate of 1.5% and 8.5%, respectively. Seventeen of the 608 patients (< 3%) were lost to follow-up.

RESULTS

Gastric bypass provides durable weight control. Weights fell from a preoperative mean of 304.4 lb (range, 198 to 615 lb) to 192.2 lb (range, 104 to 466) by 1 year and were maintained at 205.4 lb (range, 107 to 512 lb) at 5 years, 206.5 lb (130 to 388 lb) at 10 years, and 204.7 lb (158 to 270 lb) at 14 years. The operation provides long-term control of non-insulin-dependent diabetes mellitus (NIDDM). In those patients with adequate follow-up, 121 of 146 patients (82.9%) with NIDDM and 150 of 152 patients (98.7%) with glucose impairment maintained normal levels of plasma glucose, glycosylated hemoglobin, and insulin. These antidiabetic effects appear to be due primarily to a reduction in caloric intake, suggesting that insulin resistance is a secondary protective effect rather than the initial lesion. In addition to the control of weight and NIDDM, gastric bypass also corrected or alleviated a number of other comorbidities of obesity, including hypertension, sleep apnea, cardiopulmonary failure, arthritis, and infertility. Gastric bypass is now established as an effective and safe therapy for morbid obesity and its associated morbidities. No other therapy has produced such durable and complete control of diabetes mellitus.

Insulin resistance in skeletal muscle of the male Otsuka Long-Evans Tokushima Fatty rat, a new model of NIDDM

The Otsuka Long-Evans Tokushima Fatty rat is a new inbred obese strain with a late onset and chronic course of spontaneous hyperglycaemia in the male, and is considered to be a model of non-insulin-dependent diabetes mellitus [1, 2]. Fat distribution analysis showed a typical accumulation of intra-abdominal visceral fat in Otsuka Long-Evans Tokushima Fatty rats compared with a control strain, Long-Evans Tokushima Otsuka rats. To examine the insulin sensitivity of Otsuka Long-Evans Tokushima Fatty rats, we performed euglycaemic hyperinsulinaemic clamp experiments in vivo in rats under anaesthesia on this strain and on Long-Evans Tokushima Otsuka rats. The Otsuka Long-Evans Tokushima Fatty rats showed lower values for the glucose infusion rate (60% of the control at 12 weeks old and 20-30% of the control at 18, 24, 30 and 39 weeks old) than age-matched controls, indicating the development of insulin resistance with age. Hindlimb perfusion experiments in vitro also showed a 45% decrease of insulin-stimulated glucose uptake in Otsuka Long-Evens Tokushima Fatty rats in the diabetic stage. These results indicate that insulin resistance exists in the skeletal muscle of Otsuka Long-Evans Tokushima Fatty rats. To obtain information on the mechanism of insulin resistance in the skeletal muscle of Otsuka Long-Evans Tokushima Fatty rats, the insulin binding, autophosphorylation and tyrosine kinase activity of their partially-purified insulin receptors in vitro were compared with those from control rats. The results showed no marked differences in these insulin receptor functions between diabetic and control rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Excessive insulin receptor serine phosphorylation in cultured fibroblasts and in skeletal muscle. A potential mechanism for insulin resistance in the polycystic ovary syndrome

We investigated the cellular mechanisms of the unique disorder of insulin action found in the polycystic ovary syndrome (PCOS). Approximately 50% of PCOS women (PCOS-Ser) had a significant increase in insulin-independent beta-subunit [32P]phosphate incorporation (3.7-fold, P < 0.05 vs other groups) in skin fibroblast insulin receptors that was present in serine residues while insulin-induced tyrosine phosphorylation was decreased (both P < 0.05 vs other groups). PCOS skeletal muscle insulin receptors had the same abnormal phosphorylation pattern. The remaining PCOS women (PCOS-n1) had basal and insulin-stimulated receptor autophosphorylation similar to control. Phosphorylation of the artificial substrate poly GLU4:TYR1 by the PCOS-Ser insulin receptors was significantly decreased (P < 0.05) compared to control and PCOS-n1 receptors. The factor responsible for excessive serine phosphorylation appeared to be extrinsic to the receptor since no insulin receptor gene mutations were identified, immunoprecipitation before autophosphorylation corrected the phosphorylation defect and control insulin receptors mixed with lectin eluates from affected PCOS fibroblasts displayed increased serine phosphorylation. Our findings suggest that increased insulin receptor serine phosphorylation decreases its protein tyrosine kinase activity and is one mechanism for the post-binding defect in insulin action characteristic of PCOS.

Mechanism of insulin receptor kinase inhibition in non-insulin-dependent diabetes mellitus patients. Phosphorylation of serine 1327 or threonine 1348 is unaltered

The tyrosine kinase activity of insulin receptor isolated from the skeletal muscle of NIDDM patients has previously been found to be decreased compared with the activity of receptor from nondiabetic subjects but the mechanism underlying this defect is unknown. Phosphorylation of receptor serine/threonine residues has been proposed to exert an inhibitory influence on receptor tyrosine kinase activity and Ser 1327 and Thr 1348 have been identified as specific sites of phosphorylation in the insulin receptor COOH terminal domain. To address the potential negative regulatory role of phosphorylation of these residues in vivo, we assessed the extent of phosphorylation of each site in insulin receptor isolated from the skeletal muscle of 12 NIDDM patients and 13 nondiabetic, control subjects. Phosphorylation of Ser 1327 and Thr 1348 was determined using antibodies that specifically recognize insulin receptor phosphorylated at these sites. In addition, a phosphotyrosine-specific antibody was used to monitor receptor tyrosine phosphorylation. The extent of insulin-induced tyrosine autophosphorylation was decreased in receptor isolated from diabetic versus nondiabetic muscle, thus confirming earlier reports. In contrast, there was no significant difference in the extent of phosphorylation of either Ser 1327 or Thr 1348 in receptor isolated from diabetic or nondiabetic muscle as assessed by immunoprecipitation (Ser 1327: 5.6 +/- 1.6% diabetics vs. 4.7 +/- 2.0% control; Thr 1348: 3.8 +/- 1.0% diabetics vs. 3.2 +/- 1.2% control). Moreover, within each group there was no correlation between the level of tyrosine kinase activity and the extent of serine/threonine phosphorylation. It is concluded that the stoichiometry of serine/threonine phosphorylation of insulin receptor in vivo is low, and that increased phosphorylation of Ser 1327 or Thr 1348 is not responsible for the decreased insulin receptor tyrosine kinase activity observed in the skeletal muscle of NIDDM patients.

Protein kinase C is increased in the liver of humans and rats with non-insulin-dependent diabetes mellitus: an alteration not due to hyperglycemia

We tested the hypothesis that liver protein kinase C (PKC) is increased in non-insulin-dependent diabetes mellitus (NIDDM). To this end we examined the distribution of PKC isozymes in liver biopsies from obese individuals with and without NIDDM and in lean controls. PKC isozymes alpha, beta, epsilon and zeta were detected by immunoblotting in both the cytosol and membrane fractions. Isozymes gamma and delta were not detected. There was a significant increase in immunodetectable PKC-alpha (twofold), -epsilon (threefold), and -zeta (twofold) in the membrane fraction isolated from obese subjects with NIDDM compared with the lean controls. In obese subjects without NIDDM, the amount of membrane PKC isozymes was not different from the other two groups. We next sought an animal model where this observation could be studied further. The Zucker diabetic fatty rat offered such a model system. Immunodetectable membrane PKC-alpha, -beta, -epsilon, and -zeta were significantly increased when compared with both the lean and obese controls. The increase in immunodetectable PKC protein correlated with a 40% elevation in the activity of PKC at the membrane. Normalization of circulating glucose in the rat model by either insulin or phlorizin treatment did not result in a reduction in membrane PKC isozyme protein or kinase activity. Further, phlorizin treatment did not improve insulin receptor autophosphorylation nor did the treatment lower liver diacylglycerol. We conclude that liver PKC is increased in NIDDM, a change that is not secondary to hyperglycemia. It is possible that PKC-mediated phosphorylation of some component in the insulin signaling cascade contributes to the insulin resistance observed in NIDDM.

Insulin receptor phosphorylation, insulin receptor substrate-1 phosphorylation, and phosphatidylinositol 3-kinase activity are decreased in intact skeletal muscle strips from obese subjects

To determine whether the impaired insulin-stimulated glucose uptake in obese individuals is associated with altered insulin receptor signaling, we measured both glucose uptake and early steps in the insulin action pathway in intact strips of human skeletal muscle. Biopsies of rectus abdominus muscle were taken from eight obese and eight control subjects undergoing elective surgery (body mass index 52.9 +/- 3.6 vs 25.7 +/- 0.9). Insulin-stimulated 2-deoxyglucose uptake was 53% lower in muscle strips from obese subjects. Additional muscle strips were incubated in the basal state or with 10(-7) M insulin for 2, 15, or 30 min. In the lean subjects, tyrosine phosphorylation of the insulin receptor and insulin receptor substrate-1 (IRS-1), measured by immunoblotting with anti-phosphotyrosine antibodies, was significantly increased by insulin at all time points. In the skeletal muscle from the obese subjects, insulin was less effective in stimulating tyrosine phosphorylation (maximum receptor and IRS-1 phosphorylation decreased by 35 and 38%, respectively). Insulin stimulation of IRS-1 immunoprecipitable phosphatidylinositol 3-kinase (PI 3-kinase) activity also was markedly lower in obese subjects compared with controls (10- vs 35-fold above basal, respectively). In addition, the obese subjects had a lower abundance of the insulin receptor, IRS-1, and the p85 subunit of PI 3-kinase (55, 54, and 64% of nonobese, respectively). We conclude that impaired insulin-stimulated glucose uptake in skeletal muscle from severely obese subjects is accompanied by a deficiency in insulin receptor signaling, which may contribute to decreased insulin action.

Purification, identification and subcellular distribution of three predominant protein-tyrosine phosphatase enzymes in skeletal muscle tissue

Protein-tyrosine phosphatases (PTPases) play a key role in the regulation of insulin action. In order to identify PTPases in skeletal muscle, the major site of insulin-mediated glucose disposal in vivo, we purified PTPases from rat muscle tissue fractions by a series of column chromatographic techniques. PTPase activities were assayed by measuring the dephosphorylation of a rat insulin receptor kinase domain, derivatized lysozyme and p-nitrophenylphosphate, and the enzymes were further characterized by immunoblotting. Of the total PTPase activity in muscle homogenates, 51-64% was localized to the solubilized particulate fraction, with the specific PTPase activity 3.3-fold and 5.6-fold higher in the particulate fraction towards RCM-lysozyme or the insulin receptor, respectively. The major peak (> 75%) of PTPase activity in the particulate fraction was purified further to 700-fold; 75% of this activity passed through a Blue-3GA column and revealed immunoreactivity for both LAR and SH-PTP2. PTPase activity retained on the Blue-3GA column contained PTPase1B. The major peak (> 70%) from muscle cytosol was further purified to 1500-fold. After the Blue-3GA step, immunoblotting revealed both SH-PTP2 and PTPase1B in the cytosol fraction, but LAR was absent from this fraction. LRP (RPTP-alpha) was not detected by blotting the PTPase activities from the purified particulate or cytosol fractions. Immunodepletion studies demonstrated that LAR, SH-PTP2 and PTPase1B were quantitatively major PTPase activities in the initial muscle homogenate, together accounting for over 70% of the total activity towards RCM-lysozyme. These studies provide insight into the relative abundance and subcellular distribution of specific PTPases in muscle tissue that are involved in the regulation of reversible tyrosine phosphorylation in this tissue.