Decreased tyrosine kinase activity in partially purified insulin receptors from muscle of young, non-obese first degree relatives of patients with type 2 (non-insulin-dependent) diabetes mellitus

Mechanisms of Insulin Action and Insulin Resistance

The 1921 discovery of insulin was a Big Bang from which a vast and expanding universe of research into insulin action and resistance has issued. In the intervening century, some discoveries have matured, coalescing into solid and fertile ground for clinical application; others remain incompletely investigated and scientifically controversial. Here, we attempt to synthesize this work to guide further mechanistic investigation and to inform the development of novel therapies for type 2 diabetes (T2D). The rational development of such therapies necessitates detailed knowledge of one of the key pathophysiological processes involved in T2D: insulin resistance. Understanding insulin resistance, in turn, requires knowledge of normal insulin action. In this review, both the physiology of insulin action and the pathophysiology of insulin resistance are described, focusing on three key insulin target tissues: skeletal muscle, liver, and white adipose tissue. We aim to develop an integrated physiological perspective, placing the intricate signaling effectors that carry out the cell-autonomous response to insulin in the context of the tissue-specific functions that generate the coordinated organismal response. First, in section II, the effectors and effects of direct, cell-autonomous insulin action in muscle, liver, and white adipose tissue are reviewed, beginning at the insulin receptor and working downstream. Section III considers the critical and underappreciated role of tissue crosstalk in whole body insulin action, especially the essential interaction between adipose lipolysis and hepatic gluconeogenesis. The pathophysiology of insulin resistance is then described in section IV. Special attention is given to which signaling pathways and functions become insulin resistant in the setting of chronic overnutrition, and an alternative explanation for the phenomenon of ‟selective hepatic insulin resistanceˮ is presented. Sections V, VI, and VII critically examine the evidence for and against several putative mediators of insulin resistance. Section V reviews work linking the bioactive lipids diacylglycerol, ceramide, and acylcarnitine to insulin resistance; section VI considers the impact of nutrient stresses in the endoplasmic reticulum and mitochondria on insulin resistance; and section VII discusses non-cell autonomous factors proposed to induce insulin resistance, including inflammatory mediators, branched-chain amino acids, adipokines, and hepatokines. Finally, in section VIII, we propose an integrated model of insulin resistance that links these mediators to final common pathways of metabolite-driven gluconeogenesis and ectopic lipid accumulation.

Vanadium compounds in medicine

Vanadium is a transition metal that, being ubiquitously distributed in soil, crude oil, water and air, also found roles in biological systems and is an essential element in most living beings. There are also several groups of organisms which accumulate vanadium, employing it in their biological processes. Vanadium being a biological relevant element, it is not surprising that many vanadium based therapeutic drugs have been proposed for the treatment of several types of diseases. Namely, vanadium compounds, in particular organic derivatives, have been proposed for the treatment of diabetes, of cancer and of diseases caused by parasites. In this work we review the medicinal applications proposed for vanadium compounds with particular emphasis on the more recent publications. In cells, partly due to the similarity of vanadate and phosphate, vanadium compounds activate numerous signaling pathways and transcription factors; this by itself potentiates application of vanadium-based therapeutics. Nevertheless, this non-specific bio-activity may also introduce several deleterious side effects as in addition, due to Fenton's type reactions or of the reaction with atmospheric O2, VCs may also generate reactive oxygen species, thereby introducing oxidative stress with consequences presently not well evaluated, particularly for long-term administration of vanadium to humans. Notwithstanding, the potential of vanadium compounds to treat type 2 diabetes is still an open question and therapies using vanadium compounds for e.g. antitumor and anti-parasitic related diseases remain promising.

Early diagnosis of pancreatic adenocarcinoma: role of stroma, surface proteases, and glucose-homeostatic agents

OBJECTIVES

New-onset diabetes in pancreatic adenocarcinoma is due to a combination of insulin resistance and decreased β-cell function. Its differentiation from the common type 2 diabetes is the prerequisite for early diagnosis of pancreatic adenocarcinoma. Little attention has been paid to pancreatic stroma and surface proteases.

METHODS

The activated fibroblasts selectively express fibroblast activation protein α, a structural homolog of the ubiquitously expressed dipeptidyl peptidase 4. Their role in pancreatic carcinogenesis is reviewed.

RESULTS

Homodimers and heterodimers of both enzymes display high specificity for peptides and proteins with penultimate proline or alanine. Most glucose-homeostatic agents are candidate substrates of these enzymes. The biological activity of truncated substrates is decreased or absent.

CONCLUSIONS

The interactions of surface proteases with glucose-homeostatic agents may adequately explain the evolution of diabetes associated with pancreatic adenocarcinoma and differentiate it from the common type 2 diabetes.

Effect of physical training on insulin secretion and action in skeletal muscle and adipose tissue of first-degree relatives of type 2 diabetic patients

Physical training affects insulin secretion and action, but there is a paucity of data on the direct effects in skeletal muscle and adipose tissue and on the effect of training in first-degree relatives (FDR) of patients with type 2 diabetes. We studied insulin action at the whole body level and peripherally in skeletal muscle and adipose tissue as well as insulin-secretory capacity in seven FDR and eight control (CON) subjects before and after 12 wk of endurance training. Training improved physical fitness. Insulin-mediated glucose uptake (GU) increased (whole body and leg; P < 0.05) after training in CON but not in FDR, whereas glucose-mediated GU increased (P < 0.05) in both groups. Adipose tissue GU was not affected by training, but it was higher (abdominal, P < 0.05; femoral, P = 0.09) in FDR compared with CON. Training increased skeletal muscle lipolysis (P < 0.05), and it was markedly higher (P < 0.05) in subcutaneous abdominal than in femoral adipose tissue and quadriceps muscle with no difference between FDR and CON. Glucose-stimulated insulin secretion was lower in FDR compared with CON, but no effect of training was seen. Glucagon-like peptide-1 stimulated insulin secretion five- to sevenfold. We conclude that insulin-secretory capacity is lower in FDR than in CON and that there is dissociation between training-induced changes in insulin secretion and insulin-mediated GU. Maximal GU rates are similar between groups and increases with physical training.

The role of membrane glycoprotein plasma cell antigen 1/ectonucleotide pyrophosphatase phosphodiesterase 1 in the pathogenesis of insulin resistance and related abnormalities

Insulin resistance is a major feature of most patients with type 2 diabetes mellitus (T2D). A number of laboratories have observed that PC-1 (membrane [corrected] glycoprotein plasma cell antigen 1; also termed [corrected] ectonucleotide pyrophosphatase phosphodiesterase 1 or ENPP1) [corrected] is either overexpressed or overactive in muscle, adipose tissue, fibroblasts, and other tissues of insulin-resistant individuals, both nondiabetic and diabetic. Moreover, PC-1 (ENPP1) overexpression [corrected] in cultured cells in vitro and in transgenic mice in vivo, [corrected] impairs insulin stimulation of insulin receptor (IR) activation and downstream signaling. PC-1 binds to the connecting domain of the IR alpha-subunit that is located in residues 485-599. The connecting domain transmits insulin binding in the alpha-subunit to activation of tyrosine kinase activation in the beta-subunit. When PC-1 is overexpressed, it inhibits insulin [corrected]induced IR beta-subunit tyrosine kinase activity. In addition, a polymorphism of PC-1 (K121Q) in various ethnic populations is closely associated with insulin resistance, T2D, and cardio [corrected] and nephrovascular diseases. The product of this polymorphism has a 2- to 3-fold increased binding affinity for the IR and is more potent than the wild-type PC-1 protein (K121K) in inhibiting the IR. These data suggest therefore that PC-1 is a candidate protein that may play a role in human insulin resistance and T2D by its overexpression, its overactivity, or both.

Cinnamon extract (traditional herb) potentiates in vivo insulin-regulated glucose utilization via enhancing insulin signaling in rats

Cinnamon has been shown to potentiate the insulin effect through upregulation of the glucose uptake in cultured adipocytes. In the present study, we evaluated the effect of the cinnamon extract on the insulin action in awaked rats by the euglycemic clamp and further analyzed possible changes in insulin signaling occurred in skeletal muscle. The rats were divided into saline and cinnamon extract (30 and 300 mg/kg BW-doses: C30 and C300) oral administration groups. After 3-weeks, cinnamon extract treated rats showed a significantly higher glucose infusion rate (GIR) at 3 mU/kg per min insulin infusions compared with controls (118 and 146% of controls for C30 and C300, respectively). At 30 mU/kg per min insulin infusions, the GIR in C300 rats was increased 17% over controls. There were no significant differences in insulin receptor (IR)-beta, IR substrate (IRS)-1, and phosphatidylinositol (PI) 3-kinase protein content between C300 rats and controls. However, the skeletal muscle insulin-stimulated IR-beta and the IRS-1 tyrosine phosphorylation levels in C300 rats were 18 and 33% higher, respectively, added to 41% higher IRS-1/PI 3-kinase association. These results suggest that the cinnamon extract would improve insulin action via increasing glucose uptake in vivo, at least in part through enhancing the insulin-signaling pathway in skeletal muscle.

Insulin Resistant States and Insulin Signaling

The incidence of type 2 diabetes mellitus (T2D) and obesity is increasing rapidly worldwide, reaching epidemic proportions. Insulin resistance is a key feature in both conditions and plays an important pathophysiological role. Over the last two decades a central role in the origin of insulin resistance has emerged for defects in the intracellular insulin signaling cascade leading to glucose uptake. Herein, we will 1) review insulin signaling pathways leading to glucose uptake, 2) review mouse models of insulin resistance that demonstrate the pathophysiologic importance of specific defects of these pathways and 3) discuss the molecular basis for insulin resistance in some human disease states known to be associated with insulin resistance. Finally, we will briefly mention some novel treatment targets for T2D stemming from this knowledge.

Role of 17beta-estradiol administration on insulin sensitivity in the rat: implications for the insulin receptor

The role of 17beta-estradiol in the early steps of insulin action is only partially known, although its effect on glucose homeostasis has been reported. In this paper, we attempt to prove the influence of 17beta-estradiol on the insulin receptor of ovariectomized rats treated with different hormonal doses. Our results show that high doses of estradiol impair insulin sensitivity while low doses improve it. We think that these results are the consequence of changes at a molecular level, because high doses of estradiol produced lower expression of the insulin receptor gene, lower content of this receptor in target tissues, and lower phosphorylation of insulin receptor in these tissues. However, low doses of estradiol seem to produce just the opposite. The possible existence of consensus response elements in the insulin receptor gene promoter to estradiol could be controlling the expression of this gene, this control being dose and timing dependent. Moreover, we cannot discard a possible effect of estradiol on the activity of protein tyrosine phosphatases, and therefore, on the activity of the insulin receptor. These new findings improve knowledge about the possible risk for insulin resistance in women taking oral contraceptives or receiving hormonal replacement therapy around the menopause, but could also open the door towards the possible utilization of 17beta-estradiol in some diabetes cases.

Impaired ('diabetic') insulin signaling and action occur in fat cells long before glucose intolerance--is insulin resistance initiated in the adipose tissue?

This review postulates and presents recent evidence that insulin resistance is initiated in the adipose tissue and also suggests that the adipose tissue may play a pivotal role in the induction of insulin resistance in the muscles and the liver. Marked impairments in insulin's intracellular signaling cascade are present in fat cells from type 2 diabetic patients, including reduced IRS-1 gene and protein expression, impaired insulin-stimulated PI3-kinase and PKB/Akt activities. In contrast, upstream insulin signaling in skeletal muscle from diabetic subjects only shows modest impairments and PKB/Akt activation in vivo by insulin appears normal. However, insulin-stimulated glucose transport and glycogen synthesis are markedly reduced. Similar marked impairments in insulin signaling, including reduced IRS-1 expression, impaired insulin-stimulated PI3-kinase and PKB/Akt activities are also seen in some (approximately 30%) normoglycemic individuals with genetic predisposition for type 2 diabetes. In addition, GLUT4 expression is markedly reduced in these cells, similar to what is seen in diabetic cells. The individuals with reduced cellular expression of IRS-1 and GLUT4 are also markedly insulin resistant and exhibit several characteristics of the Insulin Resistance Syndrome.Thus, a 'diabetic' pattern is seen in the fat cells also in normoglycemic subjects and this is associated with a marked insulin resistance in vivo. It is proposed that insulin resistance and/or its effectors is initiated in fat cells and that this may secondarily encompass other target tissues for insulin, including the impaired glucose transport in the muscles.

Insulin signal transduction in skeletal muscle from glucose-intolerant relatives of type 2 diabetic patients [corrected]

To determine whether defects in the insulin signal transduction cascade are present in skeletal muscle from prediabetic individuals, we excised biopsies from eight glucose-intolerant male first-degree relatives of patients with type 2 diabetes (IGT relatives) and nine matched control subjects before and during a euglycemic-hyperinsulinemic clamp. IGT relatives were insulin-resistant in oxidative and nonoxidative pathways for glucose metabolism. In vivo insulin infusion increased skeletal muscle insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation (P = 0.01) and phosphatidylinositide 3-kinase (PI 3-kinase) activity (phosphotyrosine and IRS-1 associated) in control subjects (P < 0.02) but not in IGT relatives (NS). The incremental increase in insulin action on IRS-1 tyrosine phosphorylation was lower in IGT relatives versus control subjects (P < 0.05). The incremental defects in signal transduction noted for IRS-1 and PI 3-kinase may be attributed to elevated basal phosphorylation/activity of these parameters, because absolute phosphorylation/activity under insulin-stimulated conditions was similar between IGT relatives and control subjects. Insulin increased Akt serine phosphorylation in control subjects and IGT relatives, with a tendency for reduced phosphorylation in IGT relatives (P = 0.12). In conclusion, aberrant phosphorylation/activity of IRS-1, PI 3-kinase, and Akt is observed in skeletal muscle from relatives of patients with type 2 diabetes with IGT. However, the elevated basal activity of these signaling intermediates and the lack of a strong correlation between these parameters to glucose metabolism suggests that other defects of insulin signal transduction and/or downstream components of glucose metabolism may play a greater role in the development of insulin resistance in skeletal muscle from relatives of patients with type 2 diabetes.

Skeletal muscle insulin resistance in normoglycemic subjects with a strong family history of type 2 diabetes is associated with decreased insulin-stimulated insulin receptor substrate-1 tyrosine phosphorylation

Normoglycemic subjects with a strong family history of type 2 diabetes are insulin resistant, but the mechanism of insulin resistance in skeletal muscle of such individuals is unknown. The present study was undertaken to determine whether abnormalities in insulin-signaling events are present in normoglycemic, nonobese subjects with a strong family history of type 2 diabetes. Hyperinsulinemic-euglycemic clamps with percutaneous muscle biopsies were performed in eight normoglycemic relatives of type 2 diabetic patients (FH(+)) and eight control subjects who had no family history of diabetes (FH(-)), with each group matched for age, sex, body composition, and ethnicity. The FH(+) group had decreased insulin-stimulated glucose disposal (6.64 +/- 0.52 vs. 8.45 +/- 0.54 mg. kg(-1) fat-free mass. min(-1); P < 0.05 vs. FH(-)). In skeletal muscle, the FH(+) and FH(-) groups had equivalent insulin stimulation of insulin receptor tyrosine phosphorylation. In contrast, the FH(+) group had decreased insulin stimulation of insulin receptor substrate (IRS)-1 tyrosine phosphorylation (0.522 +/- 0.077 vs. 1.328 +/- 0.115 density units; P < 0.01) and association of PI 3-kinase activity with IRS-1 (0.299 +/- 0.053 vs. 0.466 +/- 0.098 activity units; P < 0.05). PI 3-kinase activity was correlated with the glucose disposal rate (r = 0.567, P = 0.02). In five subjects with sufficient biopsy material for further study, phosphorylation of Akt was 0.266 +/- 0.061 vs. 0.404 +/- 0.078 density units (P < 0.10) and glycogen synthase activity was 0.31 +/- 0.06 vs. 0.50 +/- 0.12 ng. min(-1). mg(-1) (P < 0.10) for FH(+) and FH(-) subjects, respectively. Therefore, despite normal insulin receptor phosphorylation, postreceptor signaling was reduced and was correlated with glucose disposal in muscle of individuals with a strong genetic background for type 2 diabetes.

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.

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.

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 genetic basis of type 2 diabetes mellitus: impaired insulin secretion versus impaired insulin sensitivity

Despite the fact that it is the prevalent view that insulin resistance is the main genetic factor predisposing to development of type 2 diabetes, review of several lines of evidence in the literature indicates a lack of overwhelming support for this concept. In fact, the literature better supports the case of impaired insulin secretion being the initial and main genetic factor predisposing to type 2 diabetes, especially 1) the studies in people at high risk to subsequently develop type 2 diabetes (discordant monozygotic twins and women with previous gestational diabetes), 2) the studies demonstrating compete alleviation of insulin resistance with weight loss, and 3) the studies finding that people with type 2 diabetes or IGT can have impaired insulin secretion and no insulin resistance compared with well matched NGT subjects. The fact that insulin resistance may be largely an acquired problem in no way lessens its importance in the pathogenesis of type 2 diabetes. Life style changes (exercise, weight reduction) and pharmacological agents (e.g., biguanides and thiazolidendiones) that reduce insulin resistance or increase insulin sensitivity clearly have major beneficial effects (122, 144-146, 153-155).

Insulin resistance versus insulin deficiency in non-insulin-dependent diabetes mellitus: problems and prospects

A definitive assessment of the relative roles of insulin resistance and insulin deficiency in the etiology of NIDDM is hampered by several problems. 1) Due to better methodology, data on insulin resistance are generally more accurate and consistent than data on insulin deficiency. 2) In source data, case-control studies are prone to selection bias, while epidemiological associations, whether cross-sectional or longitudinal, are liable to misinterpretation. 3) Insulin secretion and action are physiologically interconnected at multiple levels, so that an initial defect in either is likely to lead with time to a deficit in the companion function. The fact that both insulin resistance and impaired insulin release have been found to precede and predict NIDDM in prospective studies may be in part a reflection of just such relatedness. 4) Direct genetic analysis is effective in rarer forms of glucose intolerance (MODY, mitochondrial mutations, etc.) but encounters serious difficulties with typical late-onset NIDDM. Despite these uncertainties, the weight of current evidence supports the view that insulin resistance is very important in the etiology of typical NIDDM for the following reasons: 1) it is found in the majority of patients with the manifest disease; 2) it is only partially reversible by any form of treatment (117); 3) it can be traced back through earlier stages of IGT and high-risk conditions; and 4) it predicts subsequent development of the disease with remarkable consistency in both prediabetic and normoglycemic states. Of conceptual importance is also the fact that the key cellular mechanisms of skeletal muscle insulin resistance (defective stimulation of glucose transport, phosphorylation, and storage into glycogen) have been confirmed in NIDDM subjects by a variety of in vivo techniques [ranging from catheter balance (118) to multiple tracer kinetics (119) to 13C nuclear magnetic resonance spectroscopy (120)], and have been detected also in normoglycemic NIDDM offspring (121). If insulin resistance is a characteristic finding in many cases of NIDDM, insulin-sensitive NIDDM does exist. On the other hand, given the tight homeostatic control of plasma glucose levels in humans, beta-cell dysfunction, relative or absolute, is a sine qua non for the development of diabetes. If insulin deficiency must be present whereas insulin resistance may be present, is this proof that the former is etiologically primary to the latter? If so, do we have convincing evidence that the primacy of insulin deficiency is genetic in nature? The answer to both questions is negative on several accounts. The defect in insulin secretion in overt NIDDM is functionally severe but anatomically modest: beta-cell mass is reduced by 20-40% in patients with long-standing NIDDM (122). Moreover, the insulin secretory deficit is progressively worse with more severe hyperglycemia (123) and recovers considerably upon improving glycemic control (124). These observations indicate that part of the insulin deficiency is acquired (through glucose toxicity, lipotoxicity, or both). In addition, although insulin deficiency is necessary for diabetes, it may not always be sufficient to cause NIDDM. In fact, subtle defects in the beta-cell response to glucose may be widespread in the population (108, 125) and only cause frank hyperglycemia when obesity/insulin resistance stress the secretory machinery. Conceivably, there could be beta-cell dysfunction without NIDDM just as there is insulin resistance without diabetes. Incidentally, any defect in insulin secretion, whether in normoglycemic or hyperglycemic persons, could be due to other factors than primary beta-cell dysfunction: amyloid deposits in the pancreas (126), changes in insulin secretagogues (amylin, GLP-1, GIP, galanin) (127-130), early intrauterine malnutrition (131). Finally, the predictive power of early changes in insulin secretion for the development of typical NIDDM is generally lower than that of insulin

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.

Simulation model of defective insulin receptors as byproducts of receptor recycling

Our simulation model assumes that the defective insulin-binding receptors in non-insulin-dependent diabetes (NIDDM) patients result from functional receptor recycling. The model is a short program written in MS DOS 5.0 Qbasic. MODEL DESIGN: Receptors with intracellular portions damaged in the process of recycling are considered defective since they bind insulin but do mediate insulin effects, or recycle. Their occurrence depends on the average activation rate of functional receptors. The insulin-binding receptors (defective and functional) are objects of slow and time-dependent turnover defined by the turnover rate. Recycled receptors rejoin functional receptors or enter the pool of defective receptors. The waste in the functional receptors' pool is covered by a limited amount of newly synthesized receptors. The defective receptors often accumulate in cases of increased activation of functional receptors. SIMULATION RESULTS: The insulin-binding receptor quantity is determined, in the model, only by the number of newly synthesized receptors, reflecting the intensity of insulin stimulation. Synthesis is increased following variable insulin stimulations and decreased after sustained, intensive insulin stimulation. The number of functional receptors inversely reflects the average activation rate of functional receptors compared with the insulin-binding receptors turnover rate. High activation rates can diminish the proportion of functional receptors to less than 5% of that of all insulin-binding receptors. The model predicts that cells bearing only functional receptors show progressively shortened half-lives of receptors, reflecting the receptor activation intensity. On the other hand, cells bearing both defective and functional receptors show stable receptors' half-lives (20-36% of the defective receptors' half-life). Simulation results suggest that reduced functional receptor proportions in NIDDM patients might reflect the imbalance between the activation of functional receptors and the slow catabolism of defective receptors.

Insulin receptor gene in hypertension

The first molecular genetic association with human essential hypertension (HT) involved the insulin receptor gene (INSR). This highly significant result in Caucasians was for an insertion/deletion polymorphism in intron 9. A polymorphism in exon 8 showed a weak association, but a microsatellite in intron 2 proved negative for HT, although has shown an association with plasma insulin in Japanese. A similar spectrum of genetic associations for variants spanning INSR has been noted for insulin-dependent diabetic patients with rapidly-progressing renal disease, a subgroup having a strong family history of essential HT. Association with HT has also been found for an INSR variant in CHinese. Insulin resistance secondary to an INSR 'defect', or other causes, would increase insulin, which has cardiovascular effects, and insulin can raise angiotensinogen. Also, insulin is co-secreted with amylin, which can increase renin secretion. In the spontaneously HT rat there is evidence for reduced down-regulation of INSR expression in response to NaCl-loading, consistent with a promoter effect. When combined with observations of insulin resistance in essential HT patients and their pre-HT offspring, the possibility of dys-regulation of INSR merits attention in disease etiology in a proportion of essential HT patients.

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.

Insulin resistance in relatives of NIDDM patients: the role of physical fitness and muscle metabolism

First degree relatives of patients with non-insulin-dependent diabetes mellitus (NIDDM) are often reported to be insulin resistant. To examine the possible role of reduced physical fitness in this condition 21 first degree relatives of NIDDM patients and 22 control subjects without any history of diabetes were examined employing a 150-min hyperinsulinaemic (0.6 mU insulin. kg-1.min-1) euglycaemic clamp combined with the isotope dilution technique (3-(3)H-glucose, Hot GINF), the forearm technique and indirect calorimetry. During hyperinsulinaemia glucose disposal (Rd) and forearm glucose extraction were significantly diminished in the relatives (p < 0.01 and p < 0.05), but glucose oxidation and the suppressive effect on hepatic glucose production were normal. Arteriovenous differences across the forearm of the gluconeogenic precursors lactate, alanine and glycerol as well as the increments in forearm blood flow during hyperinsulinaemia were similar in the two groups. Maximal oxygen uptake (VO2 max) was lower in the relatives than in the control subjects (36.8 +/- 1.9 vs 42.1 +/- 2.0 ml.kg-1.min-1; p = 0.03). There was a highly significant correlation between Rd and VO2 max in both relatives and control subjects (r = 0.68 and 0.66, respectively; both p < 0.001). Comparison of the linear regression analyses of insulin-stimulated Rd on VO2 max in the two groups showed no significant differences between the slopes (0.10 +/- 0.03 vs 0.09 +/- 0.02) or the intercepts. In stepwise multiple linear regression analyses with insulin-stimulated Rd as the dependent variable VO2 max significantly determined the level of Rd (p < 0.01), whereas forearm blood flow and anthropometric data did not. In conclusion, the insulin resistance in healthy first degree relatives of patients with NIDDM is associated with a diminished physical work capacity. Whether, this finding is ascribable to environmental or genetic factors (e.g. differences in muscle fibre types, capillary density etc) remains to be determined.

C-peptide stimulates glucose transport in isolated human skeletal muscle independent of insulin receptor and tyrosine kinase activation

We have previously demonstrated that C-peptide stimulates glucose transport in skeletal muscle from non-diabetic subjects in a dose-dependent manner. To further elucidate the mechanism by which C-peptide activates glucose transport, we investigated the influence of human recombinant C-peptide on receptor and post-receptor events involved in the glucose transport process. Human skeletal muscle specimens were obtained from the vastus lateralis by means of an open biopsy procedure. Stimulation of isolated muscle strips from healthy control subjects with supra-physiological concentrations of insulin (6,000 pmol/l) and C-peptide (2,500 pmol/l), did not further augment the twofold increase in the rate of 3-o-methylglucose transport induced by either stimulus alone. C-peptide did not displace 125I-insulin binding from partially purified receptors, nor did it activate receptor tyrosine kinase activity. Tyrosine-labelled 125I-C-peptide did not bind specifically to crude membranes prepared from skeletal muscle, or to any serum protein other than albumin. The beta-adrenergic receptor stimulation with isoproterenol inhibited insulin- but not C-peptide-mediated 3-o-methylglucose transport by 63 +/- 18% (p < 0.01), whereas the cyclic AMP analogue, Bt2cAMP, abolished the insulin- and C-peptide-stimulated 3-o-methylglucose transport. C-peptide (600 pmol/l) increased 3-o-methylglucose transport 1.8 +/- 0.2-fold in skeletal muscle specimens from patients with insulin-dependent diabetes mellitus. In conclusion, C-peptide stimulates glucose transport by a mechanism independent of insulin receptor and tyrosine kinase activation. In contrast to the effect on insulin-stimulated glucose transport, catecholamines do not appear to have a counter regulatory action on C-peptide-mediated glucose transport.

Mutant insulin receptors in syndromes of insulin resistance

To date, mutations of the insulin receptor remain the only well-established causes of severe insulin resistance. There is a broad correlation between the extent of impairment of signal transduction seen when the mutant receptors are expressed in vitro with the severity of the clinical phenotype. Thus leprechaunism, Rabson-Mendenhall syndrome and Type A insulin resistance appear to represent points on a continuum of severity of receptor dysfunction, rather than completely distinct syndromes. In other syndromes of insulin resistance, insulin receptor abnormalities remain the exception. However, functional studies of expressed naturally occurring insulin receptor mutations have acted as experiments of nature and greatly aided attempts to dissect the structure-function relationships of the receptor. The next few years will no doubt begin to reveal the contributions made by defects in the post-receptor signalling cascade to the syndromes of insulin resistance in man.

Insulin receptor tyrosine-kinase activity is altered in both muscle and adipose tissue from non-obese normoglycaemic insulin-resistant subjects

We performed i.v. insulin tolerance test in 30 non-obese (BMI < 30 male and < 28 female) non-diabetic (by oral glucose tolerance test) subjects and subdivided them into three groups of 10 subjects each, according to their insulin sensitivity (K(itt) values). Then we compared the tyrosine-kinase activity of immunopurified insulin receptors (using 32P-ATP and poly-glu-tyr (4:1) from both muscle and adipose tissue in 7 of the most insulin-sensitive and 7 of the most insulin-resistant subjects. No difference was observed between the two groups in the basal (no insulin) receptor tyrosine-kinase activity from both tissues. In contrast, tyrosine-kinase activity response to insulin was significantly higher (p < 0.05 by 2-way ANOVA test) in receptors from both tissues of insulin-sensitive subjects. In addition, a decreased tyrosine-kinase sensitivity to insulin was observed in muscle, but not adipose, tissue of insulin-resistant subjects (insulin ED50 being 0.87 +/- 0.05 nmol/l vs 2.03 +/- 0.07, p < 0.05 in insulin-sensitive and -resistant subjects). Insulin ED50 of muscle receptor tyrosine-kinase significantly (p = 0.001) correlated to both K(itt) values (r = -0.79) and plasma insulin values at 120 min during OGTT (r = + 0.80). Insulin receptor content, as assessed by radioimmunoassay, was similar in both muscle (7.9 +/- 1.3 and 9.2 +/- 1.9 ng/mg protein) and adipose tissue (8.2 +/- 1.3 and 7.5 +/- 1.4) of insulin-sensitive and -resistant subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased glucose effectiveness in normoglycemic but insulin-resistant relatives of patients with non-insulin-dependent diabetes mellitus. A novel compensatory mechanism

20 normoglycemic first degree relatives of non-insulin-dependent diabetes mellitus (NIDDM) patients were compared with 20 matched subjects without any family history of diabetes using the intravenous glucose tolerance test with minimal model analysis of glucose disappearance and insulin kinetics. Intravenous glucose tolerance index (Kg) was similar in both groups (1.60 +/- 0.14 vs 1.59 +/- 0.18, x 10(-2) min-1, NS). However, insulin sensitivity (Si) was reduced (3.49 +/- 0.43 vs 4.80 +/- 0.61, x 10(-4) min-1 per mU/liter, P = 0.05), whereas glucose effectiveness (Sg) was increased (1.93 +/- 0.14 vs 1.52 +/- 0.16, x 10(-2) min-1, P < 0.05) in the relatives. Despite insulin resistance neither fasting plasma insulin concentration (7.63 +/- 0.48 vs 6.88 +/- 0.45, mU/liter, NS) nor first phase insulin responsiveness (Phi1) (3.56 +/- 0.53 vs 4.13 +/- 0.62, mU/liter min-1 per mg/dl, NS) were increased in the relatives. Phi1 was reduced for the degree of insulin resistance in the relatives so that the Phi1 x Si index was lower in the relatives (11.5 +/- 2.2 vs 16.7 +/- 2.0, x 10(-4) min-2 per mg/dl, P < 0.05). Importantly, glucose effectiveness correlated with Kg and with basal glucose oxidation but not with total glucose transporter 4 (GLUT4) content in a basal muscle biopsy. In conclusion we confirm the presence of insulin resistance in first degree relatives of NIDDM patients. However, insulin secretion was altered and reduced for the degree of insulin resistance in the relatives, whereas glucose effectiveness was increased. We hypothesize that increased glucose effectiveness maintains glucose tolerance within normal limits in these "normoinsulinemic" relatives of NIDDM patients.

Modulation of insulin receptor signalling: significance of altered receptor isoform patterns and mechanism of hyperglycaemia-induced receptor modulation

Insulin resistance of the skeletal muscle plays a key role in the development of the metabolic endocrine syndrome and its further progression to non-insulin dependent diabetes (NIDDM). Available data suggest that insulin resistance is caused by an impaired signal from the insulin receptor to the glucose transport system and to glycogen synthase. The impaired response of the insulin receptor tyrosine kinase which is found in NIDDM appears to contribute to the pathogenesis of the signalling defect. The reduced kinase activation is not caused by mutations within the insulin receptor gene. We investigated two potential mechanisms that might be relevant for the abnormal function of the insulin receptor in NIDDM, i.e. changes in the expression of the receptor isoforms and the effect of hyperglycaemia on insulin receptor tyrosine kinase activity. The insulin receptor is expressed in two different isoforms (HIR-A and HIR-B). We found that HIR-B expression in the skeletal muscle is increased in NIDDM. However, the characterisation of the functional properties of HIR-A and HIR-B revealed no difference in their tyrosine kinase activity in vivo. The increased expression of HIR-B might represent a compensatory event. In contrast, hyperglycaemia might directly inhibit insulin-receptor function. We have found that in rat-1 fibroblasts which overexpressing human insulin receptor an inhibition of the tyrosine kinase activity of the receptor may be induced by high glucose levels. This appears to be mediated through activation of certain protein kinase C isoforms which form stable complexes with the insulin receptor and modulate the tyrosine kinase activity of the insulin receptor through serine phosphorylation of the receptor beta subunit.(ABSTRACT TRUNCATED AT 250 WORDS)