Structure of the human erythrocyte insulin receptor

Metabolic Shades of S-D-Lactoylglutathione

S-D-lactoylglutathione (SDL) is an intermediate of the glutathione-dependent metabolism of methylglyoxal (MGO) by glyoxalases. MGO is an electrophilic compound that is inevitably produced in conjunction with glucose breakdown and is essentially metabolized via the glyoxalase route. In the last decades, MGO metabolism and its cytotoxic effects have been under active investigation, while almost nothing is known about SDL. This article seeks to fill the gap by presenting an overview of the chemistry, biochemistry, physiological role and clinical importance of SDL. The effects of intracellular SDL are investigated in three main directions: as a substrate for post-translational protein modifications, as a reservoir for mitochondrial reduced glutathione and as an energy currency. In essence, all three approaches point to one direction, namely, a metabolism-related regulatory role, enhancing the cellular defense against insults. It is also suggested that an increased plasma concentration of SDL or its metabolites may possibly serve as marker molecules in hemolytic states, particularly when the cause of hemolysis is a disturbance of the pay-off phase of the glycolytic chain. Finally, SDL could also represent a useful marker in such metabolic disorders as diabetes mellitus or ketotic states, in which its formation is expected to be enhanced. Despite the lack of clear-cut evidence underlying the clinical and experimental findings, the investigation of SDL metabolism is a promising field of research.

Ethnopharmacological, Phytochemical, Pharmacological, and Toxicological Review on Senna auriculata (L.) Roxb.: A Special Insight to Antidiabetic Property

Avartaki (Senna auriculata (L.) Roxb. syn. Cassia auriculata L.; Family- Fabaceae ) is a traditional medicinal plant, widely used for the treatment of various ailments in Ayurveda and Siddha system of medicine in India. Almost all the parts of the plant, such as flowers, leaves, seeds, barks, and roots have been reported for their medicinal uses. Traditionally, it has been used in the treatment of diabetes, asthma, rheumatism, dysentery, skin disease, and metabolic disorders. The principle phytochemicals in Senna auriculata (L.) Roxb. are alkaloids, anthraquinone, flavone glycosides, sugar, saponins, phenols, terpenoids, flavonoids, tannins, steroids, palmitic acid, linoleic acid, benzoic acid 2-hydroxyl methyl ester, 1-methyl butyl ester, resorcinol, α-tocopherol-β-D-mannosidase, epicatechin, ferulic acid, quercetin-3-O-rutinoside, quercetin, proanthocyanidin B1. The extracts from its different parts and their isolated compounds possess a wide range of pharmacological activities such as antidiabetic, antioxidant, anti-inflammatory, antihyperlipidemic, hepatoprotective, nephroprotective, cardioprotective, anti-atherosclerotic, anticancer, antimutagenic, antimicrobial, antiulcer, antipyretic, anthelmintic, immunomodulatory, antifertility, anti-venom, and anti-melanogenesis. The toxicological findings from preclinical studies ensured the safety of the plant, but comprehensive clinical studies are required for the safety and efficacy of the plant in humans. The current review article aimed to provide up-to-date information about Senna auriculata (L.) Roxb. covering its ethnomedicinal, phytochemical, pharmacological, and toxicological aspects with special emphasis on its clinical implications in diabetes.

Insulin inhibits human erythrocyte cAMP accumulation and ATP release: role of phosphodiesterase 3 and phosphoinositide 3-kinase

In non-erythroid cells, insulin stimulates a signal transduction pathway that results in the activation of phosphoinositide 3-kinase (PI3K) and subsequent phosphorylation of phosphodiesterase 3 (PDE3). Erythrocytes possess insulin receptors, PI3K and PDE3B. These cells release adenosine triphosphate (ATP) when exposed to reduced O(2) tension via a signaling pathway that requires activation of the G protein, Gi, as well as increases in cAMP. Although insulin inhibits ATP release from human erythrocytes in response to Gi activation by mastoparan 7 (Mas 7), no effect on cAMP was described. Here, we investigated the hypothesis that insulin activates PDE3 in human erythrocytes via a PI3K-mediated mechanism resulting in cAMP hydrolysis and inhibition of ATP release. Incubation of human erythrocytes with Mas 7 resulted in a 62 +/- 7% increase in cAMP (n = 9, P < 0.05) and a 306 +/- 69% increase in ATP release (n = 9, P < 0.05), both of which were attenuated by pre-treatment with insulin. Selective inhibitors of PDE3 (cilostazol) or PI3K (LY294002) rescued these effects of insulin. These results support the hypothesis that insulin activates PDE3 in erythrocytes via a PI3K-dependent mechanism. Once activated, PDE3 limits Mas 7-induced increases in intracellular cAMP. This effect of insulin leads, ultimately, to decreased ATP release in response to Mas 7. Activation of Gi is required for reduced O(2) tension-induced ATP release from erythrocytes and this ATP release has been shown to participate in the matching of O(2) supply with demand in skeletal muscle. Thus, pathological increases in circulating insulin could, via activation of PDE3 in erythrocytes, inhibit ATP release from these cells, depriving the peripheral circulation of one mechanism that could aid in the regulation of the delivery of O(2) to meet tissue metabolic need.

Defects in oxygen supply to skeletal muscle of prediabetic ZDF rats

In humans, prediabetes is characterized by marked increases in plasma insulin and near normal blood glucose levels as well as microvascular dysfunction of unknown origin. Using the extensor digitorum longus muscle of 7-wk inbred male Zucker diabetic fatty rats fed a high-fat diet as a model of prediabetes, we tested the hypothesis that hyperinsulinemia contributes to impaired O(2) delivery in skeletal muscle. Using in vivo video microscopy, we determined that the total O(2) supply to capillaries in the extensor digitorum longus muscle of prediabetic rats was reduced to 64% of controls with a lower O(2) supply rate per capillary and higher O(2) extraction resulting in a decreased O(2) saturation at the venous end of the capillary network. These findings suggest a lower average tissue Po(2) in prediabetic animals. In addition, we determined that insulin, at concentrations measured in humans and Zucker diabetic fatty rats with prediabetes, inhibited the O(2)-dependent release of ATP from rat red blood cells (RBCs). This inability to release ATP could contribute to the impaired O(2) delivery observed in rats with prediabetes, especially in light of the finding that the endothelium-dependent relaxation of resistance arteries from these animals is not different from controls and is not altered by insulin. Computational modeling confirmed a significant 8.3-mmHg decrease in average tissue Po(2) as well as an increase in the heterogeneity of tissue Po(2), implicating a failure of a regulatory system for O(2) supply. The finding that insulin attenuates the O(2)-dependent release of ATP from RBCs suggests that this defect in RBC physiology could contribute to a failure in the regulation of O(2) supply to meet the demand in skeletal muscle in prediabetes.

Effect of tetrahydrocurcumin on insulin receptor status in type 2 diabetic rats: studies on insulin binding to erythrocytes

Curcumin is the most active component of turmeric. It is believed that curcumin is a potent antioxidant and anti-inflammatory agent. Tetrahydrocurcumin (THC) is one of the major metabolites of curcumin, and exhibits many of the same physiological and pharmacological activities as curcumin and, in some systems, may exert greater antioxidant activity than curcumin. Using circulating erythrocytes as the cellular mode, the insulin-binding effect of THC and curcumin was investigated. Streptozotocin (STZ)-nicotinamide-induced male Wistar rats were used as the experimental models. THC (80 mg/kg body weight) was administered orally for 45 days. The effect of THC on blood glucose, plasma insulin and insulin binding to its receptor on the cell membrane of erythrocytes were studied. Mean specific binding of insulin was significantly lowered in diabetic rats with a decrease in plasma insulin. This was due to a significant decrease in mean insulin receptors. Erythrocytes from diabetic rats showed a decreased ability for insulin-receptor binding when compared with THC-treated diabetic rats. Scatchard analysis demonstrated that the decrease in insulin binding was accounted for by a decrease in insulin receptor sites per cell, with erythrocytes of diabetic rats having less insulin receptor sites per cell than THC-treated rats. High affinity (K d1), low affinity (K d2) and kinetic analyses revealed an increase in the average receptor affinity of erythrocytes from THC-treated rats compared with those of diabetic rats. These results suggest that acute alteration of the insulin receptor on the membranes of erythrocytes occurred in diabetic rats. Treatment with THC significantly improved specific insulin binding to the receptors, with receptor numbers and affinity binding reaching near-normal levels. Our study suggests the mechanism by which THC increases the number of total cellular insulin binding sites resulting in a significant increase in plasma insulin. The effect of THC is more prominent than that of curcumin.

Effect of N-benzoyl-D-phenylalanine and metformin on insulin receptors in neonatal streptozotocin-induced diabetic rats: studies on insulin binding to erythrocytes

In the present study, we focused on the insulin-receptor binding in circulating erythrocytes of N-benzoyl-D-phenylalanine (NBDP) and metformin in neonatal streptozotocin (nSTZ)-induced male Wistar rats. We measured blood levels of glucose and plasma insulin and the binding of insulin to cell-membrane ER receptors in NBDP and metformin-treated diabetic rats. The mean specific binding of insulin to ER was significantly lower in diabetic control rats (DC) (53.0 +/- 3.1%) than in NBDP (62.0 +/- 3.1%), metformin (66.0 +/- 3.3%) and NBDP and metformin combination-treated (72.0 +/- 4.2%) diabetic rats, resulting in a significant decrease in plasma insulin. Scatchard plot analysis demonstrated that the decrease in insulin binding was accounted for by a lower number of insulin receptor sites per cell in DC rats when compared with NBDP and metformin-treated rats. High-affinity (Kd1), low-affinity (Kd2), and kinetic analysis revealed an increase in the average receptor affinity in ER from NBDP and metformin-treated diabetic rats having NBDP 2.0 +/- 0.10 x 10(-10) M(-1) (Kd1); 12.0 +/- 0.85 x 10(-8) M(-1) (Kd2), Metformin 2.1 +/- 0.15 x 10(-10) M(-1) (Kd1); 15.0 +/- 0.80 x 10(-8) M(-1) (Kd2), NBDP and metformin 2.7 +/- 0.10 x 10(-10) M(-1) (Kd1); 20.0 +/- 1.2 x 10(-8) M(-1) (Kd2) compared with 0.9 +/- 0.06 x 10(-10) M(-1) (Kd1); 6.0 +/- 0.30 x 10(-8) M(-1) (Kd2) in DC rats. The results suggest an acute alteration in the number of insulin receptors on ER membranes in nSTZ induced diabetic control rats. Treatment with NBDP along with metformin significantly improved specific insulin binding, with receptor number and affinity binding reaching almost normal non-diabetic levels. The data presented here show that NBDP along with metformin increase total ER membrane insulin binding sites with a concomitant significant increase in plasma insulin.

The effect of malnutrition on insulin binding to rat erythrocytes

Insulin binding to erythrocyte receptors was compared in malnourished and control rats. Percentage specific insulin binding to malnourished rat erythrocytes was significantly lower than to control erythrocytes (P less than 0.001). The low insulin binding in the malnourished rat erythrocytes was accompanied by low insulin receptor affinity (P = 0.035).

Decreased insulin binding in Asian Indian women with gestational diabetes mellitus

Insulin binding to erythrocyte insulin receptors was studied in 10 women with gestational diabetes and compared with 10 matched, normal, pregnant women and 10 normal, non-pregnant controls, with no family history of diabetes. Pregnant women had higher mean fasting and post-glucose plasma immunoreactive insulin (IRI) compared to non-pregnant controls (p less than 0.001). Women with gestational diabetes had higher mean fasting and post-glucose plasma glucose levels and a lower mean specific binding of insulin when compared with the other two groups (p less than 0.001). The decreased insulin binding was significant only at lower insulin concentrations (0.2-2 ng/ml) when compared with those of normal pregnant women (p less than 0.01), suggesting decreased receptor affinity with no change in receptor number. In addition, an increased mean ED50 value for 50% inhibition of maximal binding and a lower mean average affinity constant Ke (empty site) obtained in gestational diabetes in comparison to the other two groups also suggested de decreased affinity of the receptor. The finding that pregnancy with normal glucose tolerance was not accompanied by changes in insulin binding against decreased insulin binding and affinity observed in gestational diabetes suggested a pathogenetic role for impaired insulin binding as one of the factors responsible for insulin resistance and hyperglycemia in gestational diabetes.

Reduced insulin removal and erythrocyte insulin binding in obese children

To study the relationship between childhood obesity, weight loss, hyperinsulinaemia and the erythrocyte insulin receptor, we measured the plasma concentrations of immunoreactive insulin (IRI) and C-peptide and the binding of 125I-insulin to erythrocytes in 12 obese children with a mean age +/- SD of 11.4 +/- 2.5 years and a mean relative weight score +/- SD of 4.8 +/- 1.4 and 12 age-matched normal-weight children. Eight obese children were re-evaluated after 1 year's participation in a weight reduction programme. The obese children had higher fasting plasma concentrations of IRI (P less than 0.01) and C-peptide (P less than 0.05) and a lower C-peptide to IRI molar ratio (P less than 0.01) than the normal-weight children. The obese children had in addition a reduced erythrocyte insulin binding (P less than 0.05 or less) over the physiological range of circulating insulin concentration. There was a negative correlation (r = -0.60; P less than 0.01) between the insulin tracer binding and the relative weight. The weight reduction programme resulted in a decrease of 1.0 SD (P less than 0.05) in the mean relative weight score. At the end of the therapy the obese children had lower fasting blood glucose levels (P less than 0.05) and lower plasma IRI concentrations at 90 min (P less than 0.05) after an oral glucose load than at the onset of therapy. There were no significant differences between the insulin binding characteristics at the commencement and at the end of the treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of alcohol on glucose tolerance in normal and noninsulin-dependent diabetic subjects

Oral glucose tolerance tests were conducted in 10 noninsulin-dependent diabetic and 14 healthy control subjects with a 75-g glucose load. The tests were repeated 1 week later with 43 g of ethanol mixed with the glucose. Blood samples were analyzed for ethanol, glucose, insulin, C-peptide, and glucagon levels. The blood ethanol peak was nearly equal in diabetic and control subjects (mean +/- SEM values of 55 +/- 8 and 48 +/- 6 mg/dl 45 min after ethanol ingestion). Ethanol did not affect glucose tolerance in either of the study groups. Mean +/- SEM values of the sum of the increment above the baseline glucose level were 659 +/- 48 vs. 675 +/- 76 mg/dl with or without ethanol in diabetics and 227 +/- 35 vs. 244 +/- 36 mg/dl in control subjects. The plasma insulin and C-peptide responses to glucose were delayed in diabetic patients compared to controls but were not affected by ethanol. In vitro, ethanol, at a concentration of 100 mg/dl or greater, significantly decreased insulin binding to erythrocytes in a dose-related manner. Scatchard analysis of competitive insulin binding to erythrocytes indicated that ethanol reduced insulin binding affinity (1.6 +/- 0.5 vs. 4.2 +/- 0.8 x 10(8)/M), but not binding capacity (4.5 +/- 2.4 vs. 4.4 +/- 1.7 nM, with and without ethanol, respectively).

Rapid improvement in insulin binding to erythrocyte insulin receptors in non-insulin-dependent diabetes mellitus during therapy

Insulin binding to erythrocyte receptors was studied in 36 newly diagnosed male subjects with NIDDM, treated with diet alone (Group I; n = 10) or diet + glibenclamide (Group II; n = 12) or diet + glibenclamide + metformin (Group III; n = 14). Fourteen matched non-diabetic subjects were also studied as controls. Initially, mean (+/- SD) specific insulin binding was lower in NIDDM patients than in controls (p less than 0.001), due to decreased receptor number and affinity. Control of diabetes with short-term therapy (10 +/- 2 days) resulted in significantly increased specific insulin binding in Groups II and III (p less than 0.001). A marginal increase was observed in Group I (p less than 0.01). The improved insulin binding observed in Group II and III patients after short-term therapy was maintained even after long-term therapy (9 +/- 1 months). Analysis of the insulin binding data by Scatchard plots and average affinity profiles indicated increased receptor number and affinity after short-term therapy. However, changes in affinity were reversed with long-term therapy in Groups II and III and the predominant effect appeared to be an increase in the number of binding sites.

The effect of dexamethasone on B cell mass and function in partial pancreatctomized rats

We have now examined islet mass and B-cell secretory function 19 days following a 40–50% pancreatectomy (Px). Plasma glucose and insulin values in Px were equal to those of sham-operated control rats both in the fed state and following an in traperitoneal glucose tolerance test. Glucose potentiation of arginine-induced insulin secretion remained fully preserved when assessed with the invitro perfused pancreas. On the other hand, 5 days of dexamethasone caused mild hyperglycemia in Px, but not in the control group. Moreover, in vitro insulin secretion from the dexamethasone-treated Px rats tended to be modestly lower than the dexamethasone-treated controls. Islet mass returned to a value not significantly lower than that of shams, with a further 30% increase noted in both dexamethasone-treated groups. In contrast, pancreatic insulin content in both Px groups was only about 40% of comparable controls. These data show almost complete islet regeneration within 3 weeks of the 40–50% pancreatectomy. Islet function, on the other hands, was characterized by limited reserve capacity which coexisted with and may have contributed to the development of mild hyperglycemia in the presence of dexamethasone-induced insulin resistance.

Insulin receptor binding to blood cells: an outdated concept for clinical studies on insulin resistance?

Clinical data on insulin binding to blood cells contribute to our present knowledge of insulin resistance. Binding data have been frequently presented to indicate alterations in insulin action. It is now evident that insulin binding data are of limited value for our understanding of insulin resistance. This is mainly due to the tissue-specific binding of insulin as well as the "pleiotypic" nature of insulin action. As insulin action is determined at (a) the "pre-" receptor level, (b) the receptor, and/or (c) different postbinding sites, it is unlikely that receptor data alone can explain defective insulin action. As knowledge of the molecular biology of insulin receptor morphology and function as well as of the action of insulin on intermediary metabolism increases, clinicians should not be further overloaded with binding data. Nevertheless, binding data on blood cells may be still of some value in investigating patients with severe insulin resistance due to genetic disorders of the insulin receptor or insulin receptor autoantibodies.