Cellular mechanisms of insulin resistance in non-insulin-dependent (type II) diabetes

Cardiometabolic characteristics of people with metabolically healthy and unhealthy obesity

There is considerable heterogeneity in the cardiometabolic abnormalities associated with obesity. We evaluated multi-organ system metabolic function in 20 adults with metabolically healthy obesity (MHO; normal fasting glucose and triglycerides, oral glucose tolerance, intrahepatic triglyceride content, and whole-body insulin sensitivity), 20 adults with metabolically unhealthy obesity (MUO; prediabetes, hepatic steatosis, and whole-body insulin resistance), and 15 adults who were metabolically healthy lean. Compared with MUO, people with MHO had (1) altered skeletal muscle biology (decreased ceramide content and increased expression of genes involved in BCAA catabolism and mitochondrial structure/function); (2) altered adipose tissue biology (decreased expression of genes involved in inflammation and extracellular matrix remodeling and increased expression of genes involved in lipogenesis); (3) lower 24-h plasma glucose, insulin, non-esterified fatty acids, and triglycerides; (4) higher plasma adiponectin and lower plasma PAI-1 concentrations; and (5) decreased oxidative stress. These findings provide a framework of potential mechanisms responsible for MHO and the metabolic heterogeneity of obesity. This study was registered at ClinicalTrials.gov (NCT02706262).

The Impaired Bioenergetics of Diabetic Cardiac Microvascular Endothelial Cells

Diabetes causes hyperglycemia, which can create a stressful environment for cardiac microvascular endothelial cells (CMECs). To investigate the impact of diabetes on the cellular metabolism of CMECs, we assessed glycolysis by quantifying the extracellular acidification rate (ECAR), and mitochondrial oxidative phosphorylation (OXPHOS) by measuring cellular oxygen consumption rate (OCR), in isolated CMECs from wild-type (WT) hearts and diabetic hearts (db/db) using an extracellular flux analyzer. Diabetic CMECs exhibited a higher level of intracellular reactive oxygen species (ROS), and significantly reduced glycolytic reserve and non-glycolytic acidification, as compared to WT CMECs. In addition, OCR assay showed that diabetic CMECs had increased maximal respiration, and significantly reduced non-mitochondrial oxygen consumption and proton leak. Quantitative PCR (qPCR) showed no difference in copy number of mitochondrial DNA (mtDNA) between diabetic and WT CMECs. In addition, gene expression profiling analysis showed an overall decrease in the expression of essential genes related to β-oxidation (Sirt1, Acox1, Acox3, Hadha, and Hadhb), tricarboxylic acid cycle (TCA) (Idh-3a and Ogdh), and electron transport chain (ETC) (Sdhd and Uqcrq) in diabetic CMECs compared to WT CMECs. Western blot confirmed that the protein expression of Hadha, Acox1, and Uqcrq was decreased in diabetic CMECs. Although lectin staining demonstrated no significant difference in capillary density between the hearts of WT mice and db/db mice, diabetic CMECs showed a lower percentage of cell proliferation by Ki67 staining, and a higher percentage of cellular apoptosis by TUNEL staining, compared with WT CMECs. In conclusion, excessive ROS caused by hyperglycemia is associated with impaired glycolysis and mitochondrial function in diabetic CMECs, which in turn may reduce proliferation and promote CMEC apoptosis.

Anti-Insulin Receptor Antibodies in the Pathology and Therapy of Diabetes Mellitus

Diabetes mellitus (DM) is recognized as the most common and the world's fastest-growing chronic disease with severe complications leading to increased mortality. Many strategies exist for the management of DM and its control, including treatment with insulin and insulin analogs, oral hypoglycemic therapy such as insulin secretion stimulators and insulin sensitizers, and diet and physical training. Over the years, many types of drugs and molecules with an interesting pharmacological diversity have been developed and proposed for their anti-diabetic potential. Such molecules target diverse key receptors, enzymes, and regulatory/signaling proteins known to be directly or indirectly involved in the pathophysiology of DM. Among them, insulin receptor (IR) is undoubtedly the target of choice for its central role in insulin-mediated glucose homeostasis and its utilization by the major insulin-sensitive tissues such as skeletal muscles, adipose tissue, and the liver. In this review, we focus on the implication of antibodies targeting IR in the pathology of DM as well as the recent advances in the development of IR antibodies as promising anti-diabetic drugs. The challenge still entails development of more powerful, highly selective, and safer anti-diabetic drugs.

Catestatin peptide of chromogranin A as a potential new target for several risk factors management in the course of metabolic syndrome

Obesity, lipodystrophy, diabetes, and hypertension collectively constitute the main features of Metabolic Syndrome (MetS), together with insulin resistance (IR), which is considered as a defining element. MetS generally leads to the development of cardiovascular disease (CVD), which is a determinant cause of mortality and morbidity in humans and animals. Therefore, it is essential to implement and put in place adequate management strategies for the treatment of this disease. Catestatin is a bioactive peptide with 21 amino acids, which is derived through cleaving of the prohormone chromogranin A (CHGA/CgA) that is co-released with catecholamines from secretory vesicles and, which is responsible for hepatic/plasma lipids and insulin levels regulation, improves insulin sensitivity, reduces hypertension and attenuates obesity in murine models. In humans, there were few published studies, which showed that low levels of catestatin are significant risk factors for hypertension in adult patients. These accumulating evidence documents clearly that catestatin peptide (CST) is linked to inflammatory and metabolic syndrome diseases and can be a novel regulator of insulin and lipid levels, blood pressure, and cardiac function. The goal of this review is to provide an overview of the CST effects in metabolic syndrome given its role in metabolic regulation and thus, provide new insights into the use of CST as a diagnostic marker and therapeutic target.

Characterization of distinct subpopulations of hepatic macrophages in HFD/obese mice

The current dogma is that obesity-associated hepatic inflammation is due to increased Kupffer cell (KC) activation. However, recruited hepatic macrophages (RHMs) were recently shown to represent a sizable liver macrophage population in the context of obesity. Therefore, we assessed whether KCs and RHMs, or both, represent the major liver inflammatory cell type in obesity. We used a combination of in vivo macrophage tracking methodologies and adoptive transfer techniques in which KCs and RHMs are differentially labeled with fluorescent markers. With these approaches, the inflammatory phenotype of these distinct macrophage populations was determined under lean and obese conditions. In vivo macrophage tracking revealed an approximately sixfold higher number of RHMs in obese mice than in lean mice, whereas the number of KCs was comparable. In addition, RHMs comprised smaller size and immature, monocyte-derived cells compared with KCs. Furthermore, RHMs from obese mice were more inflamed and expressed higher levels of tumor necrosis factor-α and interleukin-6 than RHMs from lean mice. A comparison of the MCP-1/C-C chemokine receptor type 2 (CCR2) chemokine system between the two cell types showed that the ligand (MCP-1) is more highly expressed in KCs than in RHMs, whereas CCR2 expression is approximately fivefold greater in RHMs. We conclude that KCs can participate in obesity-induced inflammation by causing the recruitment of RHMs, which are distinct from KCs and are not precursors to KCs. These RHMs then enhance the severity of obesity-induced inflammation and hepatic insulin resistance.

LTB4 promotes insulin resistance in obese mice by acting on macrophages, hepatocytes and myocytes

Insulin resistance results from several pathophysiologic mechanisms, including chronic tissue inflammation and defective insulin signaling. We found that liver, muscle and adipose tissue exhibit higher levels of the chemotactic eicosanoid LTB4 in obese high-fat diet (HFD)-fed mice. Inhibition of the LTB4 receptor Ltb4r1, through either genetic or pharmacologic loss of function, led to an anti-inflammatory phenotype with protection from insulin resistance and hepatic steatosis. In vitro treatment with LTB4 directly enhanced macrophage chemotaxis, stimulated inflammatory pathways, reduced insulin-stimulated glucose uptake in L6 myocytes, and impaired insulin-mediated suppression of hepatic glucose output in primary mouse hepatocytes. This was accompanied by lower insulin-stimulated Akt phosphorylation and higher Irs-1/2 serine phosphorylation, and all of these events were dependent on Gαi and Jnk1, two downstream mediators of Ltb4r1 signaling. These observations elucidate a novel role of the LTB4-Ltb4r1 signaling pathway in hepatocyte and myocyte insulin resistance, and they show that in vivo inhibition of Ltb4r1 leads to robust insulin-sensitizing effects.

Estradiol signaling in the regulation of reproduction and energy balance

Our knowledge of membrane estrogenic signaling mechanisms and their interactions that regulate physiology and behavior has grown rapidly over the past three decades. The discovery of novel membrane estrogen receptors and their signaling mechanisms has started to reveal the complex timing and interactions of these various signaling mechanisms with classical genomic steroid actions within the nervous system to regulate physiology and behavior. The activation of the various estrogenic signaling mechanisms is site specific and differs across the estrous cycle acting through both classical genomic mechanisms and rapid membrane-initiated signaling to coordinate reproductive behavior and physiology. This review focuses on our current understanding of estrogenic signaling mechanisms to promote: (1) sexual receptivity within the arcuate nucleus of the hypothalamus, (2) estrogen positive feedback that stimulates de novo neuroprogesterone synthesis to trigger the luteinizing hormone surge important for ovulation and estrous cyclicity, and (3) alterations in energy balance.

Determinants of persistent obesity and hyperinsulinemia in a biracial cohort: a 15-year prospective study of schoolgirls

OBJECTIVE

To identify childhood-adolescent determinants of persistent hyperinsulinemia and obesity.

STUDY DESIGN

We conducted a 15-year prospective study of 296 African-American and 260 Caucasian girls.

RESULTS

Childhood insulin level (partial R2=40.4%) and 14-year change in body mass index (BMI; partial R2=20.2%) were major predictors for average insulin Z score during the 15-year follow-up. Waist circumference at age 19 years, 10-year mean percentage of calories from carbohydrates, 15-year change in insulin Z score, the interaction of race with 8-year change in waist, and 14-year change in glucose level were major predictors of a 14-year change in BMI, explaining 66.7% of variability. In girls with all 9 insulin measurements in 15 years persistently in the top 25% versus girls with all measures in the bottom 75%, variables predicting the persistent insulin category included waist circumference at age 11 years (odds ratio [OR], 1.25; 95% CI, 1.11-1.40; P=.0003), 14-year change in BMI (OR, 1.26; 95% CI, 1.01-1.57; P=.037), and 8-year change in waist circumference (OR, 1.16; 95% CI, 1.01-1.32; P=.038).

CONCLUSIONS

Childhood interventions to reduce occurrence of hyperinsulinemia and obesity in early adulthood should focus on childhood-adolescent hyperinsulinemia, obesity, central adiposity, and adolescent increases in these factors.

Polymorphisms within the Protein Tyrosine Phosphatase 1B (PTPN1) Gene Promoter: Functional Characterization and Association with Type 2 Diabetes and Related Metabolic Traits

Background: Protein tyrosine phosphatase 1B (PTPN1) dephosphorylates insulin receptors and attenuates insulin signaling. Polymorphisms in the coding sequence of PTPN1 have been variably associated with type 2 diabetes (T2D). We hypothesized that variations within the PTPN1 promoter might contribute to the development of T2D and related metabolic traits.

Methods: We screened 2.0 kb of PTPN1 promoter in 174 T2D patients and 412 controls using PCR and denaturing HPLC. Association analysis was performed between diabetes and related traits and single-nucleotide polymorphism genotypes. We functionally tested 2 variants (−1023C>A and −51delA) by measuring their influence on luciferase activity in HepG2 cells and performing the electrophoretic mobility shift assay (EMSA).

Results: One common (−1023C>A) and 6 rare (−51delA, −451A>G, −467T>C, −1045G>A, −1286-3bp-del, and −1291-9bp-del) variants were identified in the PTPN1 promoter. The −1023(C) allele had significant association with T2D that disappeared after we adjusted for established diabetes risk factors. The alleles of −1023C>A and −51delA variants did not show significant effects on the biochemical markers after adjustment for established diabetes risk factors in the nondiabetic and diabetic groups separately. The −51delA variant decreased luciferase gene expression in HepG2 cells by 2-fold. EMSA revealed a weaker binding of −51delA to specific protein family proteins compared with the A allele. The −1023C>A variant had no influence in either experiment.

Conclusions: The PTPN1 promoter variants −1023C>A and −51delA (which appears to be functional) were not associated with T2D or related traits in this study but must be investigated in a larger population to reveal any potential metabolic association.

The GLUT4 glucose transporter

Few physiological parameters are more tightly and acutely regulated in humans than blood glucose concentration. The major cellular mechanism that diminishes blood glucose when carbohydrates are ingested is insulin-stimulated glucose transport into skeletal muscle. Skeletal muscle both stores glucose as glycogen and oxidizes it to produce energy following the transport step. The principal glucose transporter protein that mediates this uptake is GLUT4, which plays a key role in regulating whole body glucose homeostasis. This review focuses on recent advances on the biology of GLUT4.

Interstitial glucose in skeletal muscle of diabetic patients during an oral glucose tolerance test

AIM

The presence of a transcapillary arterial-interstitial gradient for glucose (AIG(glu)) in skeletal muscle may be interpreted as a consequence of intact cellular glucose uptake. We hypothesized that the AIG(glu) decreases in Type 2 diabetes mellitus as a consequence of insulin resistance, whereas it remains intact in Type 1 diabetes.

METHODS

Glucose concentrations were measured in serum and interstitial space fluid of skeletal muscle during an oral glucose tolerance test (OGTT) in patients with Type 1 and Type 2 diabetes and in young and middle-aged healthy volunteers, using microdialysis.

RESULTS

The area under the curve for glucose in serum (AUC(SE)) was higher than in interstitial space fluid of skeletal muscle (AUC(MU)) in healthy young (AUC(SE) = 1147 +/- 332 vs. AUC(MU) = 633 +/- 257 mM/min/ml; P = 0.006), healthy middle-aged volunteers (AUC(SE) = 1406 +/- 186 vs. AUC(MU) = 1048 +/- 229 mM/min/ml; P = 0.001) and in Type 1 diabetic patients (AUC(SE) = 2273 +/- 486 vs. AUC(MU) = 1655 +/- 178 mM/min/ml; P = 0.003). In contrast, in Type 2 diabetic patients AUC(SE) (2908 +/- 1023 mM/min/ml) was not significantly different from AUC(MU) (2610 +/- 722 mM/min/ml; P = NS).

CONCLUSION

The present data indicate that AIG(glu) is compromised in Type 2 diabetes in contrast to Type 1 diabetes where it appears to be normal. Because no changes in muscle blood flow were detected, insulin resistance appears to be the main cause for the observed decreased AIG(glu) in skeletal muscle in Type 2 diabetic patients.

PTP1B antisense-treated mice show regulation of genes involved in lipogenesis in liver and fat

Protein tyrosine phosphatases are important regulators of insulin signal transduction. Our studies have shown that in insulin resistant and diabetic ob/ob and db/db mice, reducing the levels of protein tyrosine phosphatase 1B (PTP1B) protein by treatment with a PTP1B antisense oligonucleotide resulted in improved insulin sensitivity and normalized plasma glucose levels. The mechanism by which PTP1B inhibition improves insulin sensitivity is not fully understood. We have used microarray analysis to compare gene expression changes in adipose tissue, liver and muscle of PTP1B antisense-treated ob/ob mice. Our results show that treatment with PTP1B antisense resulted in the downregulation of genes involved in lipogenesis in both fat and liver, and a downregulation of genes involved in adipocyte differentiation in fat, suggesting that PTP1B antisense acts through a different mechanism than thiazolidinedione (TZD) treatment. In summary, microarray results suggest that reduction of PTP1B may alleviate hyperglycemia and enhance insulin sensitivity by a different mechanism than TZD treatment.

Discovery and structure-activity relationship of oxalylarylaminobenzoic acids as inhibitors of protein tyrosine phosphatase 1B

Protein Tyrosine phosphatase 1B (PTP1B) has been implicated as a key negative regulator of both insulin and leptin signaling pathways. Using an NMR-based screening approach with 15N- and 13C-labeled PTP1B, we have identified 2,3-dimethylphenyloxalylaminobenzoic acid (1) as a general, reversible, and competitive PTPase inhibitor. Structure-based approach guided by X-ray crystallography facilitated the development of 1 into a novel series of potent and selective PTP1B inhibitors occupying both the catalytic site and a portion of the noncatalytic, second phosphotyrosine binding site. Interestingly, oral biovailability has been observed in rats for some compounds. Furthermore, we demonstrated in vivo plasma glucose lowering effects with compound 12d in ob/ob mice.

Rapid induction of insulin resistance in opioid mu-receptor knock-out mice

Role of opioid mu-receptor that has been showed to involve in the insulin-dependent diabetic rats in the induction of insulin resistance remains unclear. The present study is performed to clarify this point. The wild-type mice or opioid mu-receptor knockout mice were employed to induce insulin resistance by feeding with fructose-rich chow or by the repeated intraperitoneal injections of long-acting form of human insulin at 0.5 IU/kg three times daily. The basal plasma glucose concentration was not markedly changed in fructose-fed mice regardless the presence of opioid mu-receptor or not. However, the plasma glucose lowering activity of tolbutamide (10.0 mg/kg) disappeared rapidly in opioid mu-receptor knockout mice receiving fructose-rich chow as compared to that in wild type group. In opioid mu-receptor knockout mice, the elevation of plasma glucose concentration and the loss of plasma glucose lowering activity of tolbutamide were observed at 15 days after insulin injection. However, similar change was obtained at 21 days later of insulin injection in wild-type mice, showing that decrease of insulin action was more markedly in opioid mu-receptor knockout mice. Our results indicated that opioid mu-receptor is related to the delay of insulin resistance induced by fructose-fed method or insulin repeated injection.

PTP1B antisense oligonucleotide lowers PTP1B protein, normalizes blood glucose, and improves insulin sensitivity in diabetic mice

The role of protein-tyrosine phosphatase 1B (PTP1B) in diabetes was investigated using an antisense oligonucleotide in ob/ob and db/db mice. PTP1B antisense oligonucleotide treatment normalized plasma glucose levels, postprandial glucose excursion, and HbA(1C). Hyperinsulinemia was also reduced with improved insulin sensitivity. PTP1B protein and mRNA were reduced in liver and fat with no effect in skeletal muscle. Insulin signaling proteins, insulin receptor substrate 2 and phosphatidylinositol 3 (PI3)-kinase regulatory subunit p50alpha, were increased and PI3-kinase p85alpha expression was decreased in liver and fat. These changes in protein expression correlated with increased insulin-stimulated protein kinase B phosphorylation. The expression of liver gluconeogenic enzymes, phosphoenolpyruvate carboxykinase, and fructose-1,6-bisphosphatase was also down-regulated. These findings suggest that PTP1B modulates insulin signaling in liver and fat, and that therapeutic modalities targeting PTP1B inhibition may have clinical benefit in type 2 diabetes.

Measurements of interstitial muscle glycerol in normal and insulin-resistant subjects

The aim of this project was to study the regulation of interstitial glycerol levels in muscle in normal subjects, and to estimate interstitial muscle glycerol in obese subjects and patients with type 2 diabetes. In healthy lean subjects, microdialysis of forearm sc and muscle tissue were combined with arterial and deep venous catheterization, as well as blood flow registrations during oral glucose ingestion. In two other separate studies, obese (n = 9) vs. lean (n = 10) subjects and type 2 diabetes patients (n = 8) vs. weight-matched control subjects (n = 8) were investigated by means of muscle microdialysis during a euglycemic hyperinsulinemic clamp. Oral glucose ingestion suppressed the interstitial sc glycerol concentration by approximately 40% (P < 0.05), whereas no significant reduction of muscle interstitial glycerol was found. In contrast to the significant muscle interstitial-arterial (I-A) glycerol difference, the venous-arterial difference was small and varying throughout the oral glucose tolerance test. At steady-state hyperinsulinemia, obese subjects' interstitial muscle glycerol and I-A glycerol difference were both significantly higher than lean controls, whereas type 2 diabetes patient had interstitial muscle glycerol concentrations and I-A glycerol differences similar to those found in weight-matched controls. A significant and marked I-A glycerol difference exists in the absence of a significant venous-arterial difference, indicating that muscle glycerol cannot be taken as a marker of intramyocellular lipolysis because local turnover of muscle glycerol might be significant. The present data also suggest that, in contrast to sc tissue, muscle tissue lacks a clear antilipolytic effect of insulin. Moreover, the muscle interstitial glycerol concentration is elevated in obese patients but does not precipitate insulin resistance and type 2 diabetes.

Estimations of muscle interstitial insulin, glucose, and lactate in type 2 diabetic subjects

Previous measurement of insulin in human muscle has shown that interstitial muscle insulin and glucose concentrations are approximately 30-50% lower than in plasma during hyperinsulinemia in normal subjects. The aims of this study were to measure interstitial muscle insulin and glucose in patients with type 2 diabetes to evaluate whether transcapillary transport is part of the peripheral insulin resistance. Ten patients with type 2 diabetes and ten healthy controls matched for sex, age, and body mass index were investigated. Plasma and interstitial insulin, glucose, and lactate (measured by intramuscular in situ-calibrated microdialysis) in the medial quadriceps femoris muscle were analyzed during a hyperinsulinemic euglycemic clamp. Blood flow in the contralateral calf was measured by vein plethysmography. At steady-state clamping, at 60-120 min, the interstitial insulin concentration was significantly lower than arterial insulin in both groups (409 +/- 86 vs. 1,071 +/- 99 pmol/l, P < 0.05, in controls and 584 +/- 165 vs. 1, 253 +/- 82 pmol/l, P < 0.05, in diabetic subjects, respectively). Interstitial insulin concentrations did not differ significantly between diabetic subjects and controls. Leg blood flow was significantly higher in controls (8.1 +/- 1.2 vs. 4.4 +/- 0.7 ml. 100 g(-1).min(-1) in diabetics, P < 0.05). Calculated glucose uptake was less in diabetic patients compared with controls (7.0 +/- 1.2 vs. 10.8 +/- 1.2 micromol. 100 g(-1).min(-1), P < 0.05, respectively). Arterial and interstitial lactate concentrations were both higher in the control group (1.7 +/- 0.1 vs. 1.2 +/- 0.1, P < 0. 01, and 1.8 +/- 0.1 vs. 1.2 +/- 0.2 mmol/l, P < 0.05, in controls and diabetics, respectively). We conclude that, during hyperinsulinemia, muscle interstitial insulin and glucose concentrations did not differ between patients with type 2 diabetes and healthy controls despite a significantly lower leg blood flow in diabetic subjects. It is suggested that decreased glucose uptake in type 2 diabetes is caused by insulin resistance at the cellular level rather than by a deficient access of insulin and glucose surrounding the muscle cell.

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.

Opposite translational control of GLUT1 and GLUT4 glucose transporter mRNAs in response to insulin. Role of mammalian target of rapamycin, protein kinase b, and phosphatidylinositol 3-kinase in GLUT1 mRNA translation

Prolonged exposure of 3T3-L1 adipocytes to insulin increases GLUT1 protein content while diminishing GLUT4. These changes arise in part from changes in mRNA transcription. Here we examined whether there are also specific effects of insulin on GLUT1 and GLUT4 mRNA translation. Insulin enhanced association of GLUT1 mRNA with polyribosomes and decreased association with monosomes, suggesting increased translation. Conversely, insulin arrested the majority of GLUT4 transcripts in monosomes. Insulin inactivates the translational suppressor eukaryotic initiation factor 4E-binding protein-1 (4E-BP1) through the mammalian target of rapamycin (mTOR). Hence, we examined the effect of rapamycin on GLUT1 mRNA translation and protein expression. Rapamycin abrogated the insulin-mediated increase in GLUT1 protein synthesis through partial inhibition of GLUT1 mRNA translation and partial inhibition of the rise in GLUT1 mRNA. 4E-BP1 inhibited GLUT1 mRNA translation in vitro. Because phosphatidylinositol 3-kinase (PI3K) and protein kinase B (PKB), in concert with mTOR, inactivate 4E-BP1, we explored their role in GLUT1 protein expression. Cotransfection of cytomegalovirus promoter-driven, hemagglutinin epitope-tagged GLUT1 with dominant inhibitory mutants of PI3K or PKB inhibited the insulin-elicited increase in hemagglutinin-tagged GLUT1 protein. These results unravel the opposite effects of insulin on GLUT1 and GLUT4 mRNA translation. Increased GLUT1 mRNA translation appears to occur via the PI3K/PKB/mTOR/4E-BP1 cascade.

Reversal of increased lymphocyte PC-1 activity in patients with type 2 diabetes treated with metformin

BACKGROUND

The plasma cell differentiation antigen (PC-1) is an inhibitor of insulin receptor tyrosine kinase activity, and has been implicated in the pathogenesis of insulin resistance in Type 2 diabetes. Metformin increases peripheral insulin sensitivity and, therefore, we have studied the effect of metformin treatment on lymphocyte PC-1 (ecto-alkaline phosphodiesterase I, APD) in patients with Type 2 diabetes.

METHODS

Basal, concanavalin A (Con A)-, and phorbol-12-myristate-13-acetate (PMA)-stimulated lymphocyte PC-1, aminopeptidase N (APN), and dipeptidylpeptidase IV (DPP IV) activities were determined in 16 patients with Type 2 diabetes before and after 3 months of metformin treatment.

RESULTS

Lymphocyte PC-1 in patients with Type 2 diabetes was increased significantly (p<0.001) over control; however, metformin treatment brought its activity in unstimulated and Con A-stimulated lymphocytes to the control level. PMA-stimulated PC-1 in patients with Type 2 diabetes was 17-times higher than in controls, and was reduced to near the control level by 3-month metformin treatment. In Type 2 diabetes, PMA-stimulated ecto-DPP IV was significantly (p<0. 005) increased over control, but was reduced after metformin treatment.

CONCLUSION

This study has shown an increased activity of lymphocyte PC-1 in Type 2 diabetes and its reversal by 3-month metformin treatment, corresponding to the improvement of insulin sensitivity. Data obtained are consistent with a role of PC-1 in insulin resistance and suggest a new mechanism of action for metformin via PC-1 inhibition.

Actions of the novel oral antidiabetic agent HQL-975 in insulin-resistant non-insulin-dependent diabetes mellitus model animals

The hypoglycemic effects of a novel oral antidiabetic agent HQL-975, were studied in normal rats, streptozotocin-induced diabetic (STZD) rats and genetically insulin-resistant non-insulin-dependent diabetes mellitus (NIDDM) model animals, KK-Ay mice and Zucker diabetic fatty (ZDF) rats. After the dietary administration of HQL-975 to KK-Ay mice, significant decreases in plasma glucose, insulin, triglyceride and non-esterified fatty acid levels were observed. The effective dosage of HQL-975 to decrease the plasma glucose level by 30% was 3.1 mg/kg per day. However, the plasma glucose level was not altered after the administration of HQL-975 in normal and STZD rats. The results suggest that HQL-975 is more effective against the abnormalities of glucose and lipid metabolism of insulin-resistant model animals than in that of normal and insulin-deficient diabetic animals. It is reported that ZDF rats indicate a severely diabetic state as a result of insulin resistance and further the presence of beta-cell insulin secretory defects. Here, HQL-975 (1-30 mg/kg per day for 7 days) was administered to ZDF rats; slight decreases in the plasma glucose (18%) and lipids (41%) levels were observed in the rats given 30 mg/kg. To clarify the action mechanism of HQL-975, we studied the effects of HQL-975 administration on the insulin action of target tissues in KK-Ay mice. After the dietary administration of HQL-975 (0.001, 0.003, 0.010% for 7 days) to KK-Ay mice, hepatic glycolytic and gluconeogenic key enzyme activities were measured. The glucose 6-phosphatase activity was decreased (20-40%) as compared with control. The results suggest that HQL-975 enhances the insulin action in hepatic enzyme regulation. To investigate the actions of HQL-975 in peripheral tissues such as muscle and adipose, an in vivo glucose uptake study using 3H-2-deoxyglucose was performed in KK-Ay mice treated with HQL-975 (0.010% for 7 days). The 2-deoxyglucose uptake of the basal state was not altered, but the insulin-stimulated 2-deoxyglucose uptake in muscle (41-191%) and adipose (46-88%) tissues was increased by the HQL-975 treatment as compared with control. These results suggest that HQL-975 also enhances the insulin action of peripheral tissues. Based on these findings, HQL-975 is expected to be useful for treatment of insulin-resistant patients with NIDDM.

The effect of low intensity bicycle exercise on the insulin-induced glucose uptake in obese patients with type 2 diabetes

OBJECTIVE

The present study was undertaken to reveal the effect of low intensity bicycle exercise on the insulin-induced glucose uptake in obese patients.

SUBJECTS AND METHODS

Seven obese men with Type 2 diabetes (OBDM) and seven healthy young men (HY) participated in this study. The glucose infusion rate (GIR) was determined by glucose clamp procedure at an insulin infusion rate of 40 mU m-2 min-1 (plasma insulin concentrations: 700-800 pmol l-1). Confirming stabilized GIR, a 30-min bicycle exercise was performed during the glucose clamp which was continued for 120 min after exercise.

RESULTS

Average GIR in OBDM for last 30 min prior to exercise were significantly lower than HY (28.3 +/- 1.7, 47.4 +/- 1.8 mumol kg-1 min-1 respectively, P < 0.05). GIR abruptly increased during exercise and gradually decreased after exercise to the nadir almost at the time from 30 to 60 min in recovery period in both groups. GIR in OBDM, however, gradually increased significantly over pre-exercise levels (P < 0.05), following exercise and reached the same levels compared to HY after 80 min of recovery period.

CONCLUSION

These results indicated that in obese Type 2 diabetes, 30 min of low intensity bicycle exercise significantly enhances the lower level of insulin-induced glucose uptake shortly after exercise and might be useful for the treatment of post-prandial hyperglycemia.

Long-term (12 months) treatment with an anti-oxidant drug (silymarin) is effective on hyperinsulinemia, exogenous insulin need and malondialdehyde levels in cirrhotic diabetic patients

BACKGROUND/AIMS

Several studies have demonstrated that diabetic patients with cirrhosis require insulin treatment because of insulin resistance. As chronic alcoholic liver damage is partly due to the lipoperoxidation of hepatic cell membranes, anti-oxidizing agents may be useful in treating or preventing damage due to free radicals. The aim of this study was to ascertain whether long-term treatment with silymarin is effective in reducing lipoperoxidation and insulin resistance in diabetic patients with cirrhosis.

METHODS

A 12-month open, controlled study was conducted in two well-matched groups of insulin-treated diabetics with alcoholic cirrhosis. One group (n=30) received 600 mg silymarin per day plus standard therapy, while the control group (n=30) received standard therapy alone. The efficacy parameters, measured regularly during the study, included fasting blood glucose levels, mean daily blood glucose levels, daily glucosuria levels, glycosylated hemoglobin (HbA1c) and malondialdehyde levels.

RESULTS

There was a significant decrease (p<0.01) in fasting blood glucose levels, mean daily blood glucose levels, daily glucosuria and HbA1c levels already after 4 months of treatment in the silymarin group. In addition, there was a significant decrease (p<0.01) in fasting insulin levels and mean exogenous insulin requirements in the treated group, while the untreated group showed a significant increase (p<0.05) in fasting insulin levels and a stabilized insulin need. These findings are consistent with the significant decrease (p<0.01) in basal and glucagon-stimulated C-peptide levels in the treated group and the significant increase in both parameters in the control group. Another interesting finding was the significant decrease (p<0.01) in malondialdehyde/levels observed in the treated group.

CONCLUSIONS

These results show that treatment with silymarin may reduce the lipoperoxidation of cell membranes and insulin resistance, significantly decreasing endogenous insulin overproduction and the need for exogenous insulin administration.

Development of insulin resistance in 3T3-L1 adipocytes

Insulin resistance is a manifestation of both diabetes mellitus and obesity. However, the mechanism is still not clearly identified. Herein, we describe a procedure that allows us to evaluate the development of insulin resistance in 3T3-L1 adipocytes. Under these conditions, we show that the concentration of insulin required for 50% desensitization of glucose transport activity is 100 pM; maximal desensitization could be achieved with 1 nM. This demonstrates for the first time that 3T3-L1 adipocytes develop insulin resistance in response to physiologically relevant concentrations of insulin. Glucose (or glucosamine), in addition to insulin, was required to establish desensitization. The expression of GLUT4 protein decreased by 50% with exposure to 10 nM insulin. The dose-dependent loss of GLUT4 was similar to the dose dependence for insulin-resistant transport activity. Translocation in the presence of acute insulin was apparent, but the extent of recruitment directly reflected the decrease in GLUT4 protein. GLUT4 mRNA also declined, but the ED50 was approximately 5 nM. Together, these data suggest that the loss of GLUT4 protein likely underlies the cause of desensitization. However, the loss of GLUT4 protein did not correlate with the loss in GLUT4 mRNA suggesting post-translational control of GLUT4 expression.

Insulin-stimulated glucose disposal: does adipose play a role?

Although muscle is thought to be a primary assimilator of glucose, adipose may provide a substantial amount of substrate for gluconeogenesis, even in the fed state.

Pioglitazone attenuates the inhibitory effect of phorbol ester on epidermal growth factor receptor autophosphorylation and tyrosine kinase activity

A new anti-diabetic drug, pioglitazone, was tested as to whether it could ameliorate the decreased kinase activity of epidermal growth factor (EGF) receptor induced by phorbol ester (PMA) in A431 cells. The treatment of A431 cells with PMA decreased the tyrosine kinase activity of EGF receptors to 37% of normal in autophosphorylation and to 24% in tyrosine kinase activity toward Glu/Tyre synthetic polymers. Co-incubation of the cells with pioglitazone and PMA improved the receptor tyrosine kinase activity to 81% of control. Pioglitazone treatment alone did not change the kinase activity of EGF receptors. Pioglitazone did not decrease the PMA-activated protein kinase C activity and did not affect the protein tyrosine phosphatases activity in A431 cells. These results suggest that pioglitazone may act as a specific antagonist to the inhibitory effect by protein kinase C on the EGF receptor tyrosine kinase.

Insulin sensitization in diabetic rat liver by an antihyperglycemic agent

This study aimed to demonstrate directly that the thiazolidinedione pioglitazone acts as an insulin sensitizer. We tested the hypothesis that pioglitazone treatment of diabetic rats alters liver function such that responsiveness of selected genes to subsequent insulin regulation is enhanced. Although flux through gluconeogenic/glycolytic pathways involves regulation of many enzymes, we presently report the effects of insulin on expression of two key enzymes in these metabolic pathways, ie, phosphoenolpyruvate carboxykinase (PEPCK) and glucokinase (GK). Rats were either studied as nondiabetic controls or injected with streptozotocin as a model for insulin-deficient diabetes. Diabetic animals were treated without or with pioglitazone and subsequently examined for acute responses to insulin. Pioglitazone treatment of diabetic animals significantly enhanced the effects of insulin to reverse elevated blood glucose. Although the mean level of liver mRNA transcripts encoding PEPCK was increased to nearly 300% in diabetic animals as compared with nondiabetic controls (100%), it was significantly lower in pioglitazone-treated diabetic rats (119% of control) than in diabetic rats without pioglitazone (223% of control) after insulin treatment. By contrast, mRNA transcripts encoding GK were not detectable in diabetic animals, but were increased markedly by insulin treatment in all animal groups. Insulin-enhanced expression of GK was significantly greater in liver from animals that were treated earlier with pioglitazone (291% of control) than in liver from those that were untreated (214% of control). An amplified acute response of liver to insulin thus established pioglitazone as an insulin sensitizer. Our findings further showed that such sensitization can be developed even in the insulin-deficient state.(ABSTRACT TRUNCATED AT 250 WORDS)

The genetics and pathophysiology of type II and gestational diabetes

The development of both type II diabetes and gestational diabetes is probably governed by a complex and variable interaction of genes and environment. Molecular genetics has so far failed to identify discrete gene mutations accounting for metabolic changes in NIDDM. Both beta cell dysfunction and insulin resistance are operative in the manifestation of these disorders. Specific and sensitive immunoradiometric assays found fasting hyperproinsulinemia and first-phase hypoinsulinemia early in the natural history of the disorder. A lack of specificity of early radioimmunoassays for insulin resulted in measuring not only insulin but also proinsulins, leading to overestimation of insulin and misleading conclusions about its role in diabetes. The major causes of insulin resistance are the genetic deficiency of glycogen synthase activation, compounded by additional defects due to metabolic disorders, receptor downregulation, and glucose transporter abnormalities, all contributing to the impairment in muscle glucose uptake. The liver is also resistant to insulin in NIDDM, reflected in persistent hepatic glucose production despite hyperglycemia. Insulin resistance is present in many nondiabetics, but in itself is insufficient to cause type II diabetes. Gestational diabetes is closely related to NIDDM, and the combination of insulin resistance and impaired insulin secretion is of importance in its pathogenesis.

Variant forms of glucokinase gene in Japanese patients with late-onset type 2 diabetes

We have applied the technique of single-strand conformation polymorphism analysis to detect mutations of the glucokinase gene in 50 Japanese patients with late-onset type 2 diabetes and in 50 normal Japanese subjects. Out of the 50 patients with late-onset type 2 diabetes, we observed three kinds of variant patterns: one in exon 1b, one in exon 4, and one in exon 5. The incidence of these patterns was one in exon 1b, two in exon 4 and one in exon 5. Direct sequencing of exon 1b and exon 5 revealed mutations in intron areas at the 12th nucleotide downstream from the 5' splice points in two cases. Direct sequencing of exon 4 revealed a heterozygous silent mutation, CCC[Pro]-->CCG[Pro] at codon 145. In contrast, 50 normal Japanese subjects showed no variant patterns in any exons. Our results showed that although 8% (4 out of 50) of Japanese patients with late-onset type 2 diabetes have variant forms of the glucokinase gene, none is expected to cause apparent qualitative changes in glucokinase. We think that the frequency of mutations of the glucokinase gene which could cause qualitative change is very low in Japanese patients with late-onset type 2 diabetes.

Effect of metformin on insulin-stimulated glucose transport in isolated skeletal muscle obtained from patients with NIDDM

Metformin has been demonstrated to lower blood glucose in vivo by a mechanism which increases peripheral glucose uptake. Furthermore, the therapeutic concentration of metformin has been estimated to be in the order of 0.01 mmol/l. We investigated the effect of metformin on insulin-stimulated 3-0-methylglucose transport in isolated skeletal muscle obtained from seven patients with non-insulin-dependent diabetes mellitus (NIDDM) and from eight healthy subjects. Whole body insulin-mediated glucose utilization was decreased by 45% (p < 0.05) in the diabetic subjects when studied at 8 mmol/l glucose, compared to the healthy subjects studied at 5 mmol/l glucose. Metformin, at concentrations of 0.1 and 0.01 mmol/l, had no effect on basal or insulin-stimulated (100 microU/ml) glucose transport in muscle strips from either of the groups. However, the two control subjects and three patients with NIDDM which displayed a low rate of insulin-mediated glucose utilization (< 20 mumol.kg-1.min-1), as well as in vitro insulin resistance, demonstrated increased insulin-stimulated glucose transport in the presence of metformin at 0.1 mmol/l (p < 0.05). In conclusion, the concentration of metformin resulting in a potentiating effect on insulin-stimulated glucose transport in insulin-resistant human skeletal muscle is 10-fold higher than the therapeutic concentrations administered to patients with NIDDM. Thus, it is conceivable that the hypoglycaemic effect of metformin in vivo may be due to an accumulation of the drug in the extracellular space of skeletal muscle, or to an effect of the drug distal to the glucose transport step.

Pioglitazone promotes insulin-induced activation of phosphoinositide 3-kinase in 3T3-L1 adipocytes by inhibiting a negative control mechanism

Activation of phosphoinositide 3-kinase (PI 3-kinase) is an early event in insulin signal transduction that is blocked completely in adipocytes from insulin-resistant KKAy mice. Treatment of KKAy mice with pioglitazone, an anti-diabetic thiazolidinedione, partially restores insulin-dependent changes in PI 3-kinase. The mechanism of this effect of pioglitazone was investigated using murine 3T3-L1 cells as an experimental model. Insulin and insulin-like growth factor I (IGF-I) each elicited rapid (within 2 min) and large (2-5-fold) increases in PI 3-kinase activity that could be immunoprecipitated using anti-phosphotyrosine (pY) antibodies. Maximal insulin-induced activity of PI 3-kinase in pY-immunoprecipitates was similar in 3T3-L1 adipocytes and mouse adipocytes, but the kinetics of activation differed. Insulin- and IGF-I-induced changes in PI 3-kinase were each half-maximal at 3-5 nM of hormone and were not additive. Increases in both insulin-induced and IGF-I-induced pY-immunoprecipitable PI 3-kinase activity were observed when 3T3-L1 fibroblasts became confluent and when they adopted the adipocyte phenotype. Pioglitazone (10 microM), administered either acutely or chronically to either 3T3-L1 adipocytes or 3T3-L1 fibroblasts, did not greatly alter the kinetics, magnitude or sensitivity of changes in PI 3-kinase elicited by either insulin or IGF-I. In contrast, the attenuation by isoproterenol of insulin-induced changes in PI 3-kinase was prevented in cells pretreated with pioglitazone. This effect of pioglitazone did not involve inhibition of isoproterenol-elicited accumulation of cyclic AMP. Pioglitazone also prevented attenuation of insulin induced changes in PI 3-kinase by cell penetrating analogs of cyclic AMP. Pioglitazone, therefore, has no direct effect on insulin-stimulated PI 3-kinase activity, but interferes with a cyclic AMP-dependent mechanism that normally antagonizes this action of insulin. These data support the proposition that the facilitation of insulin action by pioglitazone involves, at least in part, an inhibition of a negative control mechanism.

Pioglitazone promotes insulin-induced activation of phosphoinositide 3-kinase in 3T3-L1 adipocytes by inhibiting a negative control mechanism

Activation of phosphoinositide 3-kinase (PI 3-kinase) is an early event in insulin signal transduction that is blocked completely in adipocytes from insulin-resistant KKAy mice. Treatment of KKAy mice with pioglitazone, an anti-diabetic thiazolidinedione, partially restores insulin-dependent changes in PI 3-kinase. The mechanism of this effect of pioglitazone was investigated, using murine 3T3-L1 cells as an experimental model. Insulin and insulin-like growth factor I (IGF-I) each elicited rapid (within 2 min) and large (2- to 5-fold) increases in PI 3-kinase activity that could be immunoprecipitated using anti-phosphotyrosine (pY) antibodies. Maximal insulin-induced activity of PI 3-kinase in pY-immunoprecipitates was similar in 3T3-L1 adipocytes and mouse adipocytes, but the kinetics of activation differed. Insulin- and IGF-I-induced changes in PI 3-kinase were each half-maximal at 3-5 nM of hormone and were not additive. Increases in both insulin-induced and IGF-I-induced pY-immunoprecipitable PI 3-kinase activity were observed when 3T3-L1 fibroblasts became confluent and when they adopted the adipocyte phenotype. Pioglitazone (10 microM), administered either acutely or chronically to either 3T3-L1 adipocytes or 3T3-L1 fibroblasts, did not alter greatly the kinetics, magnitude or sensitivity of changes in PI 3-kinase elicited by either insulin or IGF-I. In contrast, the attenuation by isoproterenol of insulin-induced changes in PI 3-kinase was prevented in cells pretreated with pioglitazone. This effect of pioglitazone did not involve inhibition of isoproterenol-elicited accumulation of cyclic AMP. Pioglitazone also prevented attenuation of insulin induced changes in PI 3-kinase by cell penetrating analogs of cyclic AMP. Pioglitazone, therefore, has no direct effect on insulin-stimulated PI 3-kinase activity, but interferes with a cyclic AMP-dependent mechanism that normally antagonizes this action of insulin. These data support the proposition that the facilitation of insulin action by pioglitazone involves, at least in part, an inhibition of a negative control mechanism.

Guanine nucleotide binding regulatory proteins in liver from obese humans with and without type II diabetes: evidence for altered "cross-talk" between the insulin receptor and Gi-proteins

A novel pathway for physiological "cross-talk" between the insulin receptor and the regulatory Gi-protein has been demonstrated. We tested the hypothesis that a coupling defect between Gi and the insulin receptor is present in the liver of obese patients with and without type II diabetes. Insulin 1 x 10(-9) M (approximately ED50) and 1 x 10(-7) M (Max) inhibited pertussis toxin-catalyzed ADP ribosylation of Gi in human liver plasma membranes from lean and obese nondiabetic patients. However, 1 x 10(-7) M insulin was without effect in membranes from patients with type II diabetes. This coupling defect was not intrinsic to Gi, since Mg2+ and GTP gamma S inhibited pertussis toxin-catalyzed ADP ribosylation in both diabetic and nondiabetic patients. Binding of insulin of the alpha-subunit and activation of the tyrosine kinase intrinsic to the beta-subunit of the insulin receptor are not responsible for the coupling defect. 125I insulin binding is the same in obese patients with or without diabetes. Tyrosine kinase of the insulin receptor is decreased in diabetes. However, a monoclonal antibody to the insulin receptor (MA-20) at equimolar concentrations with insulin equally inhibits pertussis toxin-catalyzed ADP ribosylation of Gi without activating tyrosine kinase or insulin receptor autophosphorylation. Immunodetection of G-proteins suggested that Gi3 alpha was normal in diabetes and Gi1-2 alpha was decreased by 40% in the diabetic group as compared to the obese nondiabetic group but was normal when compared to the lean non diabetic group. We conclude that the novel pathway of insulin signaling involving the regulatory Gi proteins via biochemical mechanisms not directly involving the tyrosine kinase of the insulin receptor is altered in obese type II diabetes and offers a new target for the search of the mechanism(s) of insulin resistance.

Skeletal muscle protein tyrosine phosphatase activity and tyrosine phosphatase 1B protein content are associated with insulin action and resistance

Particulate and cytosolic protein tyrosine phosphatase (PTPase) activity was measured in skeletal muscle from 15 insulin-sensitive subjects and 5 insulin-resistant nondiabetic subjects, as well as 18 subjects with non-insulin-dependent diabetes mellitus (NIDDM). Approximately 90% of total PTPase activity resided in the particulate fraction. In comparison with lean nondiabetic subjects, particulate PTPase activity was reduced 21% (P < 0.05) and 22% (P < 0.005) in obese nondiabetic and NIDDM subjects, respectively. PTPase1B protein levels were likewise decreased by 38% in NIDDM subjects (P < 0.05). During hyperinsulinemic glucose clamps, glucose disposal rates (GDR) increased approximately sixfold in lean control and twofold in NIDDM subjects, while particulate PTPase activity did not change. However, a strong positive correlation (r = 0.64, P < 0.001) existed between particulate PTPase activity and insulin-stimulated GDR. In five obese NIDDM subjects, weight loss of approximately 10% body wt resulted in a significant and corresponding increase in both particulate PTPase activity and insulin-stimulated GDR. These findings indicate that skeletal muscle particulate PTPase activity and PTPase1B protein content reflect in vivo insulin sensitivity and are reduced in insulin resistant states. We conclude that skeletal muscle PTPase activity is involved in the chronic, but not acute regulation of insulin action, and that the decreased enzyme activity may have a role in the insulin resistance of obesity and NIDDM.

Mechanisms of cellular insulin resistance in human pregnancy

OBJECTIVES

The cellular mechanism(s) of insulin resistance developed during pregnancy were studied by investigating the functionality of insulin receptors and glucose transport.

STUDY DESIGN

Abdominal adipose tissue was obtained from eight lean pregnant and nine control subjects, matched for insulin resistance by intravenous glucose tolerance testing. Insulin receptor binding and glucose transport were measured in freshly isolated adipocytes. Receptor kinase activity was measured on partially purified receptors. Data were analyzed by Student t test.

RESULTS

High-affinity insulin receptors were reduced in cells from pregnant compared with normal controls (2.0 +/- 0.4 vs 5.8 +/- 1.3 x 10(4) sites per cell, p < 0.05). Kinase activity of insulin receptors was unaltered in pregnancy. Adipocytes from pregnant subjects displayed a threefold decrease in insulin sensitivity for glucose transport (median effective concentration 324 +/- 93 vs 93 +/- 14 pmol/L, p < 0.025) and a reduction in maximal insulin-stimulated glucose transport (1.58 +/- 0.15 vs 2.33 +/- 0.24 pmol/10(5) cells/10 seconds, p < 0.025).

CONCLUSIONS

These results show that adipocytes from pregnant subjects exhibit decreased insulin receptor number and an impaired insulin sensitivity in the absence of functional alterations of receptor kinase activity.

Conventional and enantioselective determination of a new blood glucose-lowering agent in biological fluids using liquid-liquid extraction and high-performance liquid chromatography

Two analytical methods are described for the determination of 2-(4-tert.-butylphenoxy)-7-(4-chlorophenyl)heptanoic acid sodium salt (I) in animal models (beagle dog and rat). Method 1 is conventional reversed-phase high-performance liquid chromatography on an octadecylsilane column with an eluent of acetonitrile-0.02 M potassium buffer (pH 3) (65:35, v/v). Method 2 is used for the enantioselective determination of I. This method uses a chiral column (Chiralcel OJ) with an eluent of n-hexane-2-propanol (95:5, v/v) containing 3 ml/l trifluoracetic acid. The analytical procedure has a recovery of more than 90%; within-run precision of less than 5.1%, and between-run precision of less than 4.3%.

Hepatic insulin resistance in KKA(y) mice and its amelioration by pioglitazone do not involve alterations in phospholipase C activity

It has been proposed that an abnormality in the regulation of cytosolic-free Ca2+ may be the cause of some forms of insulin resistance. In support of this proposition, it was reported that phospholipase C-catalyzed hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) by liver plasma membranes from obese patients with non-insulin-dependent diabetes mellitus (NIDDM) was abnormally augmented (Thakker et al., J. Biol. Chem. 264, 7169-7175). The objective of this investigation was to determine if a novel antidiabetic agent, pioglitazone, ameliorated hepatic insulin resistance in KKA(y) mice and to identify any alterations in PIP2-phospholipase C activity of liver plasma membranes that may accompany changes in insulin sensitivity. Treatment of KKA(y) mice for 4 days with pioglitazone (20 mg/kg per day) decreased blood glucose and insulin and improved a variety of indices of hepatic insulin resistance, but did not alter the rate of PIP2 hydrolysis by liver plasma membranes. Acute treatment of isolated liver plasma membranes with pioglitazone (1-100 microM) also failed to alter PIP2-phospholipase C activity. Furthermore, the specific activity, Ca(2+)-requirement, pH-dependence and sensitivity to guanosine 5'-thiotriphosphate of the PIP2-phospholipase C in KKA(y) liver membranes were indistinguishable from those of C57BL/6J (normal) mice. Among C57BL/6J and KKA(y) mice fed either a control or pioglitazone-supplemented diet, there was no correlation between PIP2-phospholipase C activity in isolated liver membranes and either glucose or insulin concentrations in the circulation. These data indicate that an alteration in PIP2-phospholipase C activity of liver plasma membranes is neither a cause nor an obligatory consequence of insulin resistance in KKAy mice or its amelioration by pioglitazone. Alterations of liver membrane phospholipase C activity in NIDDM, therefore, may reflect diabetic pathology other than the insulin resistance associated with this disease.

Antidiabetic agent pioglitazone enhances adipocyte differentiation of 3T3-F442A cells

Adipocytes play an important role in normal physiology as a major site for systemic energy homeostasis. In disorders such as diabetes, adipocyte function is markedly altered. In this study, we investigated the effect of pioglitazone, a novel antidiabetic agent known to lower plasma glucose in animal models of diabetes mellitus, on cellular differentiation and expression of adipose-specific genes. Treatment of confluent 3T3-F442A preadipocyte cultures for 7 days with pioglitazone (Pio; 1 microM) and insulin (Ins; 0.17 microM) resulted in > 95% cell differentiation into lipid-accumulating adipocytes in comparison with 60-80% cell differentiation by treatment with either agent alone. Analysis of triglyceride accumulation showed increases of triglyceride content over time above untreated preadipocytes by treatment of the cells with Ins, Pio, and especially with Ins + Pio. Basal glucose transport, as measured by cellular uptake of 2-deoxy-D-[14C]glucose, was likewise enhanced in a time-dependent manner by treatment of preadipocytes with Ins, Pio, or Ins + Pio, such that a synergistic effect resulted from the combined treatment with both agents. It was further determined that RNA transcript abundance for genes encoding glucose transporters GLUT-1 and GLUT-4, as well as the adipose-specific genes encoding adipsin and aP2, were increased by the Ins, Pio, or Ins + Pio treatment. Taken together, these findings indicate that pioglitazone is a potent adipogenic agent. By promoting differentiation, this agent may move cells into a state active for glucose uptake, storage, and metabolism.

Dyslipidaemias, insulin resistance and atherosclerosis

Recent studies have demonstrated that insulin resistance, a proportionate decrease in insulin action at all insulin concentrations, is associated with clustering of many cardiovascular risk factors, particularly hyperlipidaemias. Already epidemiological studies have indicated that high insulin levels are related to low high-density lipoprotein (HDL) cholesterol and high total and very-low-density lipoprotein (VLDL) triglyceride levels. Recent studies based on the direct quantification of insulin resistance by the euglycaemic clamp method have verified these findings. In contrast, insulin resistance seems not to be associated with high low-density lipoprotein (LDL) cholesterol. Abnormalities in HDL and VLDL levels may contribute to accelerated atherosclerosis, but there is evidence that insulin resistance is also directly associated with asymptomatic atherosclerosis. Thus, prevention of atherosclerosis should not be targeted only to the lowering of LDL cholesterol, but also to the reduction of the degree of insulin resistance either with diet, weight reduction, regular exercise or drug therapy.

Hyperglycemic athymic nude mice: factors affecting in vitro insulin secretion

The strain of athymic nude male mice (ANM) developed at the University of Southern California (USC) exhibits spontaneous hyperglycemia and relative hypoinsulinemia in vivo. To investigate factors that influence insulin secretion in this animal model of non-insulin-dependent diabetes mellitus, we utilized the isolated perfused mouse pancreas of the ANM-USC and control BALB/c mice. We compared in vitro glucose-induced insulin secretion in ANM-USC and control mice, inhibition of secretion by somatostatin, and variability of insulin secretion over the two-year period it took to complete these experiments. Glucose-induced insulin secretion from the isolated pancreas was biphasic in both ANM-USC and controls. Insulin secretion was quantitatively equal to or greater than control mice, depending on the phase of secretion analyzed and the source of the control mice. In contrast to pancreases of control mice, insulin secretion from ANM-USC pancreases was relatively resistant to inhibition of insulin secretion by somatostatin. Variability in insulin secretion over the two years in which these experiments were performed was greater from pancreases of control than that observed from pancreases of the ANM-USC. The hyperglycemic ANM-USC mouse does not demonstrate diminished insulin secretion in vitro yet is relatively hypoinsulinemic in vivo. Thus circulating factors other than somatostatin might contribute to the insulinopenic stage in this animal model.

Insulin resistance versus insulin secretion in the hypertension of obesity

We measured the degree of association between obesity, blood pressure, insulin resistance, and insulin secretion in 72 male and female obese hypertensive, obese nonhypertensive, and normal weight control subjects. Baseline weight, body mass index, percent body fat, waist/hip ratio, and systolic and diastolic blood pressures were obtained. Insulin sensitivity was assessed according to Bergman's minimal model. Twelve-hour urinary c-peptide was measured after a standard liquid meal. Insulin action was inversely associated with blood pressure status, obesity status, and age. Meal-stimulated c-peptide excretion significantly correlated with systolic blood pressure and percent fat but not with body mass index or age. Multivariate regression analysis indicated that, of the measures of body composition, percent fat and waist/hip ratio had the strongest correlation with insulin action either alone or in combination with c-peptide excretion. Obese hypertensive patients had an index of insulin action (10(-4).min-1/[microunits/ml]) of 1.34 +/- 0.19, which was significantly (p less than 0.003) lower than in the obese nonhypertensive patients (index, 2.26 +/- 0.10) or the nonobese subjects (index, 5.41 +/- 0.26, p less than 0.001). Meal-stimulated c-peptide excretion (nmol/kg lean body mass) was increased only in the obese hypertensive group (0.32 +/- 0.01) and was significantly higher (p less than 0.001) than in the obese nonhypertensive (0.16 +/- 0.01) or the nonobese subjects (0.14 +/- 0.01). These results support the hypothesis that abnormalities in blood pressure regulation, insulin-stimulated glucose uptake, and insulin secretion coexist.

Insulin-receptor binding in skeletal muscle of trout

Two hundred rainbow trout (Oncorhynchus mykiss) age 0 +, weight range 11.3 - 11.5 g, were distributed randomly in two groups and maintained for five weeks on either 10% dextrin, or 20% dextrin diet. The fish were sampled 3-5 h and 18-20 h after the last feeding and insulin binding to partially purified insulin receptors in white and red skeletal muscles and to liver plasma membranes was assessed. Plasma insulin, plasma glucose, and liver glycogen content were analyzed in the same fish.Fish fed a diet with higher carbohydrate content (HC) had elevated insulin and glucose levels in peripheral blood, but lower liver glycogen contents compared to the fish fed a diet with lower carbohydrate content (LC). No growth retardation was observed in the fish from HC group.Three to five hours after the last feeding, insulin-receptor binding in white skeletal muscles was higher in HC group of fish, mostly because of an increase in number of high affinity binding sites. Eighteen to twenty hours after the last feeding this difference disappeared. In contrast, the specific binding of insulin to the liver plasma membranes appeared to be lower in the HC group of fish. The lower insulin binding to the liver plasma membranes observed 3-5 h after feeding, could be attributed to the lower quantity of binding sites, while the same phenomenon 18 h after feeding was likely a result of affinity changes. We conclude that higher glycemic levels observed in trout fed a HC diet as compared to LC group of fish, are not a consequence of impaired binding of insulin to its receptors in skeletal muscles.