Erythrocytes: a new cell type for the evaluation of insulin receptor defects in diabetic humans

Antidiabetic Action of Mcy Protein: Studies on Gene Expression and Competitive Binding to Insulin Receptors

Mcy protein, isolated from the fruits of Momordica cymbalaria, was shown to have antihyperglycemic, antihyperlipidemic activities along with renal as well as hepatoprotective activities in streptozotocin-induced diabetic rats. Mcy protein was shown to have insulin-like structure and/or function and/or insulin secretagogue activity. Hence, the present study was conducted to elucidate the molecular mechanism whereby Mcy protein elicits its therapeutic role and also to know whether the Mcy protein has any structural and functional similarity with insulin. Results of our experiments revealed that the Mcy protein is insulin-like protein. Furthermore, we assessed the effect of treatment with Mcy protein on the glucose transport (levels of glucose transporter, GLUT-2) and on the levels of key regulators of glucose and lipid metabolisms like hepatic glucokinase (GK) and sterol regulatory element-binding protein-1c (SREBP-1c). Our findings demonstrated that Mcy protein elevated the expressions of GK, SREBP-1c, and GLUT-2 that were decreased in diabetic animals. Insulin-receptor binding studies using rat erythrocytes demonstrated that mean specific binding of insulin with insulin receptors was significantly increased in Mcy-treated diabetic rats when compared to diabetic control rats. Scatchard analyses of insulin binding studies yielded curvilinear plots, and the number of receptor sites per cell was found to be 180 ± 21.1 in Mcy-treated diabetic animals and found to be significantly superior to those of diabetic control animals. Kinetic analyses also revealed an increase in the average receptor affinity of erythrocytes of Mcy-treated rats compared to diabetic control rats suggesting acute alteration in the number and affinity of insulin receptors on the membranes of erythrocytes.

Endoplasmic reticulum stress causes insulin resistance by inhibiting delivery of newly synthesized insulin receptors to the cell surface

Accumulation of unfolded proteins in the endoplasmic reticulum (ER) causes ER stress and activates a signaling network known as the unfolded protein response (UPR). Here we characterize how ER stress and the UPR inhibit insulin signaling. We find that ER stress inhibits insulin signaling by depleting the cell surface population of the insulin receptor. ER stress inhibits proteolytic maturation of insulin proreceptors by interfering with transport of newly synthesized insulin proreceptors from the ER to the plasma membrane. Activation of AKT, a major target of the insulin signaling pathway, by a cytosolic, membrane-bound chimera between the AP20187-inducible FV2E dimerization domain and the cytosolic protein tyrosine kinase domain of the insulin receptor was not affected by ER stress. Hence, signaling events in the UPR, such as activation of the JNK mitogen-activated protein (MAP) kinases or the pseudokinase TRB3 by the ER stress sensors IRE1α and PERK, do not contribute to inhibition of signal transduction in the insulin signaling pathway. Indeed, pharmacologic inhibition and genetic ablation of JNKs, as well as silencing of expression of TRB3, did not restore insulin sensitivity or rescue processing of newly synthesized insulin receptors in ER-stressed cells. [Media: see text].

In situ single molecule detection of insulin receptors on erythrocytes from a type 1 diabetes ketoacidosis patient by atomic force microscopy

Type 1 diabetes is an insulin-dependent metabolic disorder always associated with ketoacidosis and a high morbidity rate in teenagers. The in situ single molecule detection of insulin receptors on healthy and diseased erythrocytes is helpful to understand the pathomechanism of type 1 diabetes ketoacidosis (T1-DKA), which would also benefit the diagnosis and treatment of T1-DKA. Here, we demonstrated, for the first time, the single molecule interaction between insulin and insulin receptor on erythrocytes from a healthy volunteer and a T1-DKA patient using high sensitivity atomic force microscopy (AFM) in PBS solution. The single molecule force results demonstrated the decreased binding force and binding probability between insulin and insulin receptor on T1-DKA erythrocytes, implying the deficit of insulin receptor functions in T1-DKA. The binding kinetic parameters calculated from dynamic force spectroscopy indicated that the insulin-insulin receptor complexes on T1-DKA erythrocytes were less stable than those from healthy volunteer. Using high resolution AFM imaging, a decreased roughness was found both in intact T1-DKA erythrocytes and in the purified membrane of T1-DKA erythrocytes, and an increased stiffness was also found in T1-DKA erythrocytes. Moreover, AFM, which was used to investigate the single molecule interactions between insulin-insulin receptor, cell surface ultrastructure and stiffness in healthy and diseased erythrocytes, was expected to develop into a potential nanotool for pathomechanism studies of clinical samples at the nanoscale.

Low O2-induced ATP release from erythrocytes of humans with type 2 diabetes is restored by physiological ratios of C-peptide and insulin

ATP release from erythrocytes in response to reduced oxygen (O2) tension stimulates local vasodilation, enabling these cells to direct perfusion to areas in skeletal muscle in need of O2. Erythrocytes of humans with type 2 diabetes do not release ATP in response to low O2. Both C-peptide and insulin individually inhibit low O2-induced ATP release from healthy human erythrocytes, yet when coadministered at physiological concentrations and ratios, no inhibition is seen. Here, we determined: that 1) erythrocytes of healthy humans and humans with type 2 diabetes possess a C-peptide receptor (GPR146), 2) the combination of C-peptide and insulin at physiological ratios rescues low O2-induced ATP release from erythrocytes of humans with type 2 diabetes, 3) residual C-peptide levels reported in humans with type 2 diabetes are not adequate to rescue low O2-induced ATP release in the presence of 1 nM insulin, and 4) the effects of C-peptide and insulin are neither altered by increased glucose levels nor explained by changes in erythrocyte deformability. These results suggest that the addition of C-peptide to the treatment regimen for type 2 diabetes could have beneficial effects on tissue oxygenation, which would help to ameliorate the concomitant peripheral vascular disease.

In vivo metabolic imaging of insulin with multiphoton fluorescence of human insulin-Au nanodots

Functional human insulin-Au nanodots (NDs) are synthesized for the in vivo imaging of insulin metabolism. Benefiting from its efficient red to near infrared fluorescence, deep tissue subcellular uptake of insulin-Au NDs can be clearly resolved through a least-invasive harmonic generation and two-photon fluorescence (TPF) microscope. In vivo investigations on mice ear and ex vivo assays on human fat tissues conclude that cells with rich insulin receptors have higher uptake of administrated insulin. Interestingly, the insulin-Au NDs can even permeate into lipid droplets (LDs) of adipocytes. Using this newly discovered metabolic phenomenon of insulin, it is found that enlarged adipocytes in type II diabetes mice have higher adjacent/LD concentration contrast with small-sized ones in wild type mice. For human clinical samples, the epicardial adipocytes of patients with diabetes and coronary artery disease (CAD) also show elevated adjacent/LD concentration contrast. As a result, human insulin-Au nanodots provide a new approach to explore subcellular insulin metabolism in model animals or patients with metabolic or cardiovascular diseases.

Coordinated regulation of insulin signaling by the protein tyrosine phosphatases PTP1B and TCPTP

The protein tyrosine phosphatase PTP1B is a negative regulator of insulin signaling and a therapeutic target for type 2 diabetes. Our previous studies have shown that the closely related tyrosine phosphatase TCPTP might also contribute to the regulation of insulin receptor (IR) signaling in vivo (S. Galic, M. Klingler-Hoffmann, M. T. Fodero-Tavoletti, M. A. Puryer, T. C. Meng, N. K. Tonks, and T. Tiganis, Mol. Cell. Biol. 23:2096-2108, 2003). Here we show that PTP1B and TCPTP function in a coordinated and temporally distinct manner to achieve an overall regulation of IR phosphorylation and signaling. Whereas insulin-induced phosphatidylinositol 3-kinase/Akt signaling was prolonged in both TCPTP-/- and PTP1B-/- immortalized mouse embryo fibroblasts (MEFs), mitogen-activated protein kinase ERK1/2 signaling was elevated only in PTP1B-null MEFs. By using phosphorylation-specific antibodies, we demonstrate that both IR beta-subunit Y1162/Y1163 and Y972 phosphorylation are elevated in PTP1B-/- MEFs, whereas Y972 phosphorylation was elevated and Y1162/Y1163 phosphorylation was sustained in TCPTP-/- MEFs, indicating that PTP1B and TCPTP differentially contribute to the regulation of IR phosphorylation and signaling. Consistent with this, suppression of TCPTP protein levels by RNA interference in PTP1B-/- MEFs resulted in no change in ERK1/2 signaling but caused prolonged Akt activation and Y1162/Y1163 phosphorylation. These results demonstrate that PTP1B and TCPTP are not redundant in insulin signaling and that they act to control both common as well as distinct insulin signaling pathways in the same cell.

Sequential phosphorylation of protein band 3 by Syk and Lyn tyrosine kinases in intact human erythrocytes: identification of primary and secondary phosphorylation sites

Treatment of intact human erythrocytes with pervanadate induces Tyr (Y)-phosphorylation of the transmembrane protein band 3; in parallel, the activity of the immunoprecipitated tyrosine kinases Syk and Lyn is increased. When erythrocytes are incubated with pervanadate together with PP1, a specific inhibitor of Src kinases, including Lyn, the Y-phosphorylation of band 3 is only partially reduced. Indeed, the PP1-resistant phosphorylation of band 3 precedes and is a prerequisite for its coimmunoprecipitation with Lyn, which interacts with the phosphoprotein via the SH2 domain of the enzyme, as proven by binding competition experiments. Upon recruitment to primarily phosphorylated band 3, Lyn catalyzes the secondary phosphorylation of the transmembrane protein. These data are consistent with the view that band 3 is phosphorylated in intact erythrocytes by both PP1-resistant (most likely Syk) and PP1-inhibited (most likely Lyn) tyrosine kinases according to a sequential phosphorylation process. Similar radiolabeled peptide maps are obtained by tryptic digestion of32P-band 3 isolated from either pervanadate-treated erythrocytes or red cell membranes incubated with exogenous Syk and Lyn. It has also been demonstrated by means of mass spectrometry that the primary phosphorylation of band 3 occurs at Y8 and Y21, while the secondary phosphorylation affects Y359 and Y904.

Sequential phosphorylation of protein band 3 by Syk and Lyn tyrosine kinases in intact human erythrocytes: identification of primary and secondary phosphorylation sites

Treatment of intact human erythrocytes with pervanadate induces Tyr (Y)-phosphorylation of the transmembrane protein band 3; in parallel, the activity of the immunoprecipitated tyrosine kinases Syk and Lyn is increased. When erythrocytes are incubated with pervanadate together with PP1, a specific inhibitor of Src kinases, including Lyn, the Y-phosphorylation of band 3 is only partially reduced. Indeed, the PP1-resistant phosphorylation of band 3 precedes and is a prerequisite for its coimmunoprecipitation with Lyn, which interacts with the phosphoprotein via the SH2 domain of the enzyme, as proven by binding competition experiments. Upon recruitment to primarily phosphorylated band 3, Lyn catalyzes the secondary phosphorylation of the transmembrane protein. These data are consistent with the view that band 3 is phosphorylated in intact erythrocytes by both PP1-resistant (most likely Syk) and PP1-inhibited (most likely Lyn) tyrosine kinases according to a sequential phosphorylation process. Similar radiolabeled peptide maps are obtained by tryptic digestion of32P-band 3 isolated from either pervanadate-treated erythrocytes or red cell membranes incubated with exogenous Syk and Lyn. It has also been demonstrated by means of mass spectrometry that the primary phosphorylation of band 3 occurs at Y8 and Y21, while the secondary phosphorylation affects Y359 and Y904.

Insulin binding to erythrocytes of nonpregnant women: a reevaluation, underlining the importance of body weights even in nonobese subjects

Insulin binding to erythrocytes was measured in 18 healthy, non-obese women in the follicular phase and in 6 women in the mid-luteal phase of the same menstrual cycle. The presence of 55 nM and 220 nM monoclonal anti-IGF I receptor antibody (alpha-IR3) reduced only the number of low affinity binding sites for insulin by 20% and 33%, respectively. Women with relative body weights 110-119% had a lower number of high affinity receptors and an increased high affinity compared to women with relative body weights 91-109%. In women with relative body weights greater than or equal to 100%, maximum specific binding and high affinity constants increased and the receptor numbers decreased from the follicular to the luteal phase, whereas in women with relative body weights less than 100% the parameter changes were reverted. The data indicate: (1) erythrocytes contain two different classes of binding sites for insulin, (2) IGF I receptors might contribute to low-affinity binding of insulin to erythrocytes and (3) the relative body weight must be considered even for 'non-obese' control groups used in insulin binding studies of various clinical conditions.

Red blood cell insulin receptors in health and disease

CONTENTS

Structure and characteristics of erythrocyte insulin receptor. Red blood cell age and insulin receptors. Insulin receptors in human disease states. Obesity. Chronic renal failure. Acanthosis nigricans. Miscellaneous disease states. Insulin receptors in children. Insulin receptors in women during pregnancy. Insulin binding and other hormones. Comparison of biosynthetic insulin, pancreatic human insulin and porcine insulin binding to erythrocytes. Effect of exercise on insulin binding to red blood cells of normal human volunteers. Miscellaneous insulin binding studies. Insulin internalization and degradation. Insulin and erythrocyte metabolism. Summary and conclusion.

Insulin resistance and hypertension

The author has reviewed the development of the concept that insulin resistance is related not only to the hypertensive state but potentially to the initiation and maintenance of high blood pressure. Potential sequelae of insulin resistance and hyperinsulinemia, as they apply to atherogenesis, are also discussed. The impact of present antihypertensive pharmacologic therapy on insulin resistance is addressed, as are future directions in pharmacologic and nonpharmacologic management of hypertension. In addition, the author speculates on possible mechanisms leading to insulin resistance in hypertension.

Insulin receptor binding and metabolic effects of insulin in human subcutaneous adipose tissue in untreated non-insulin dependent diabetes mellitus

Insulin action at the target tissue level in non-insulin dependent diabetes mellitus was investigated using human adipose tissue. Specific adipocyte receptor binding of insulin and the effects of the hormone on glucose oxidation and lipolysis were determined in subcutaneous adipose tissue. The study included 25 patients with untreated non-insulin dependent diabetes mellitus and 38 healthy control subjects matched for age, sex and body weight. Insulin stimulated adipose tissue glucose oxidation in a dose-dependent way in the control subjects. On the other hand, a marked inhibition of this insulin effect was observed in the diabetics. A weak stimulation was observed only at high unphysiological hormone concentrations [greater than or equal to 0.7 nmol/l] and the maximal insulin response was 6 times lower than that in the control subjects. However, neither specific insulin receptor binding nor the antilipolytic effect of insulin were inhibited in diabetes. Similar results with insulin binding and the metabolic effects of insulin were obtained in non-obese normoinsulinemic diabetics as compared to moderately obese hyperinsulinemic diabetics. It is concluded that adipose tissue insulin resistance in non-insulin dependent diabetes mellitus only involves glucose metabolism and not antilipolysis. Furthermore, it may solely be due to postreceptor defects in insulin action and seems not to be influenced by obesity or oversecretion of insulin.

Changes in insulin receptor functions of the erythrocyte by treatment of non-insulin-dependent diabetes mellitus (NIDDM) patients with glibenclamide and diet control

The insulin binding of erythrocytes from: (i) fifteen age-matched normal subjects, (ii) ten untreated NIDDM patients and (iii) fifteen treated (glibenclamide + hypocaloric diet) NIDDM patients (all males) has been studied. A significant decrease in specific insulin binding was observed in group (ii) which improved in cases controlled after treatment (group iii). Scatchard analysis of the results suggested that changes in insulin binding were due to alteration in the number of insulin receptors on erythrocytes. The number of insulin receptors/cell was 471 in normals, 160 in diabetics and 282 in treated diabetic subjects. No significant change in the binding affinity was observed in the three groups (1.0 X 10(8), 1.2 X 10(8) and 1.1 X 10(8) M-1 in normal subjects, untreated diabetics and treated diabetics, respectively).

Insulin effects on human red blood cells

The effect of insulin on human red blood cells was investigated, both on intact cells and on isolated plasma membranes, testing the responsiveness of membrane-bound enzymes--such as (Na+-K+)-ATPase and 5'-nucleotidase--as well as the ouabain binding and ionic fluxes. It appears that insulin stimulates Na-pumping mechanisms increasing (Na+-K+)-ATPase activity through an enhanced availability of pumping sites, as can be inferred from the increased ouabain binding. The apparent unresponsiveness of fluorescence polarization parameters, following insulin treatment of isolated plasma membranes and intact cells, rules out--at present--an involvement of membrane lipid fluidity in the mechanism of action of insulin on human erythrocytes.

Modulation of red cell metabolism by states of decreased activation: comparison between states

Marked decline of red cell metabolism has been described during the acute state of decreased activation associated with the stylized mental technique of transcendental meditation (TM) in long-term meditators (5-10 years regular elicitation, TM instructors). It is not known whether unstylized rest is accompanied by a similar effect and it is not known what effector(s) may contribute to red cell metabolic changes in these states. In the present study ordinary, unstylized rest was found to be accompanied by small increase of red cell glycolytic rate. Apparently, either repeated elicitation of TM behavior or some special feature of this practice become associated with new mechanisms of metabolic control than those previously in operation. Although the data of this study do not permit isolation of the precise psychological determinants of this effect, the range of possible physiological effectors can be delimited. Blood pH, PCO2, PO2, and phosphate can be eliminated as significant for red cell metabolic control during both TM and rest, and based upon related studies, several known hormones such as insulin, T3, T4, arginine vasopressin, oxytocin, prolactin and growth hormone can also be eliminated as responsible effector(s).

Platelet insulin receptor determination in non-insulin dependent diabetes mellitus

The platelet membrane insulin receptors of healthy and non-insulin dependent (type 2) diabetic patients were studied. Receptor number and affinity proved to be decreased in type 2 diabetes mellitus. The changes in platelet insulin receptor characteristics are in good correlation with the alterations reported in other tissues or cells. The possible role of these phenomena in the pathogenesis of disturbed platelet function in diabetics needs further investigation.

Distribution of insulin receptors in human erythrocyte membranes. Insulin binding to sealed right-side-out and inside-out human erythrocyte vesicles

Analyses of insulin binding to human erythrocytes and to resealed right-side-out and inside-out erythrocyte membrane vesicles have revealed that high affinity insulin binding receptors are present on both sides of the erythrocyte membranes. Insulin binding to human erythrocytes was examined with the use of a binding assay designed to minimize the potential errors arising from the low binding capacity of this cell type and from non-specific binding in the assay. Scatchard analysis of equilibrium binding to the cells revealed a class of high affinity sites with a dissociation constant (Kd) of (1.5 +/- 0.5) X 10(-8) M and a maximum binding capacity of 50 +/- 5 sites per cell. Interestingly, both resealed right-side-out and inside-out membrane vesicles exhibited nearly identical specific sites for insulin binding. At the high affinity binding sites, for both right-side-out and inside-out vesicles, the dissociation constant (Kd) was (1.5 +/- 0.5) X 10(-8) M, and the maximum binding capacity was 17 +/- 3 sites per cell equivalent. These findings suggest that insulin receptors are present on both sides of the plasma membrane and are consistent with the participation of the erythrocyte insulin receptors in an endocytic/recycling pathway which mediates receptor-ligand internalization/externalization.

Developmental changes in insulin receptors of pig red blood cells

Scatchard analysis of the insulin binding to pig reticulocytes, fetal red cells, and adult erythrocytes showed the maximum number of high-affinity binding sites per cell to be 274, 147, and 29, respectively. All three cell types displayed a practically identical dissociation constant of approximately 1.22 X 10(-8) M at the high-affinity region. A long-term in vitro incubation of the fetal red cells and reticulocytes under tissue culture conditions was accompanied by a significant loss of insulin-binding capacity without any appreciable alteration of the dissociation constant. The isolation and characterization of insulin-receptor complexes from these cell types were carried out to establish whether the difference in insulin-binding capacity was due to the difference in the amount of the same species or due to different species of insulin receptors. Membrane proteins were extracted with Triton X-102 and fractionated by DEAE-Sephacel ion-exchange column chromatography. Each peak sample was complexed with 125I-insulin, and the complexes were covalently crosslinked and then applied to a Sepharose CL-6B column. A 95,000-Da complex was obtained from adult pig erythrocyte membranes; 220,000- and and 95,000-Da complex was obtained from adult pig erythrocyte membranes; 220,000- and 95,000-Da complexes from pig reticulocyte membranes; and greater than 600,000-, 220,000-, and 95,000-Da complexes from pig fetal cell membranes. Upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis under a nonreducing condition, the 95,000-Da complex was dissociated into a 53,000-Da component; the greater than 600,000-Da complex into greater than 320,000-, 130,000-, and 53,000-Da components; and the 220,000-Da complex was dissociated into 220,000-, 130,000-, and 53,000-Da components. These findings strongly suggest that the decrease in insulin binding during the developmental changes of red blood cells is due to a disappearance of high-molecular-weight insulin receptors rather than a decrease in the amount of the smaller receptor molecules.

Metabolism of insulin in erythrocytes from renal failure patients on maintenance hemodialysis

Glucose tolerance does not improve to the normal level after dialysis; however, our studies showed that the insulin receptor binding to erythrocytes of nondiabetic patients with chronic renal failure (CRF) on hemodialysis was more than that in normal subjects. To understand this apparent anomaly in insulin receptor action and glucose metabolism, we investigated insulin degradation-a postreceptor event of insulin binding-in erythrocytes from CRF patients and compared it with that of normal subjects. We studied insulin degradation by erythrocytes from each of eight CRF patients and five normal subjects. The average hyperbolic insulin degradation curve for the CRF patients showed lower activity and a right-handed shift compared to the curve for the normal subjects. The average maximum degradation of insulin in the CRF patients was significantly lower than that of normal subjects. The number of erythrocytes required to produce 50% of maximum insulin degradation was significantly greater in these patients than that in the normal subjects. Furthermore, a linear correlation was observed between the duration of dialysis and maximum percent of insulin degradation in the CRF patients. Clinical implications of these findings are unclear at the present time. However, the insulin-degrading activity in erythrocytes may be reflective of that in other body tissues.

Characterization of insulin binding to the erythroleukemia cell line K 562

The K 562 is a transformed human erythroid stemcell and is used as a target cell for NK-T-cells. In this study the presence of insulin receptors in K 562 is established. The best binding and negative cooperativity was found in the two Hepes containing buffers whereas no cooperativity was obtained in the Krebs-Ringer buffer. The calculated affinity constants and receptor number per cell varied according to the buffer. Preincubation with insulin caused a down-regulation of the insulin binding capacity. 10 ng/ml caused a lowering of the affinity, with an unchanged number of receptors. 100 ng/ml caused a decrease in receptor number with unchanged affinity. These results were found in both Hepes and Krebs-Ringer phosphate buffer. IGF-I shows cross-reactivity with the insulin receptor, with a potency of 12 and 100 times less than insulin in Krebs-Ringer phosphate buffer and G-buffer respectively. However, no specific IGF-I receptors were found. The presence of receptors on K 562 cells suggests a biological role for insulin. The different results in the different buffers, indicate that a buffer containing Hepes and/or Tris, is required to expose negative cooperativity and make the receptors more accessible to insulin.

Metabolic control in a state of decreased activation: modulation of red cell metabolism

Very little is known in depth of the biochemical and physiological changes induced at the cellular level by human behavioral states. For study of the physiology of behavior at this level, the erythrocyte may be useful, because it is readily available and its metabolism and metabolic control are comparatively well understood. In this report we describe a marked decline of red cell glycolytic rate induced by the transcendental meditation technique (TM). This decline was significantly correlated with decreased plasma lactate concentration and with relaxation as indicated by electrodermal response. The occurrence of sleep was not correlated with the metabolic changes. The observed lack of variation of blood pH, blood gases, glucose, and hematocrit in this behavior implies that the decrease of erythrocyte metabolism is not an epiphenomenon of respiratory change or substrate availability. Based upon further measurements indicating persisting alteration of the red blood cell, we suggest the possibility of attachment of a humoral agent(s) to the cell in the mechanism of this effect. This behavioral effect is unique, and the effector(s) responsible may increase our understanding of metabolic control of the erythrocyte and of TM.

The effect of high-protein and high-carbohydrate diets on [125I]iodoinsulin binding in skeletal muscle plasma membranes and isolated hepatocytes of rainbow trout (Salmo gairdneri)

[125I]iodoinsulin-binding studies in the presence of a concentration range of bovine insulin were conducted to establish specific insulin-binding levels in skeletal muscle plasma membranes and isolated hepatocytes of rainbow trout (Salmo gairdneri) reared on control, high-protein or high-carbohydrate diets. Negative co-operativity was observed and receptor concentrations and apparent dissociation constants established for each preparation. No differences of specific binding attributed to diet were detected in skeletal muscle plasma membrane preparations; however, the receptor concentration of isolated hepatocytes from high-carbohydrate-reared trout was increased. This contrasted to comparable mammalian studies.

Modulation of insulin receptors in cultured adipocytes as studied by a latex minibead probe

In monolayer cultures, adipocytes transform into spindle-shaped cells, morphologically similar to buccal submucosa-derived fibroblasts. Insulin receptors were searched for on the surface of these cells using a visual probe which consisted of latex minibeads covalently bound to insulin. Adipocyte-derived cells showed clusters of insulin receptors not observed on the surface of fibroblasts derived from submucosa. The finding indicates that despite their fibroblastic morphology and the loss of their lipid inclusions in culture, these adipocytes do not lose their state of differentiation and therefore should be considered lipid-depleted adipocytes rather than fibroblasts. Moreover, quantitative analysis demonstrated a significant increase in the number of insulin receptors in cultured cells as compared to isolated but not cultured cells. This may reflect the low concentration of insulin in the culture medium as compared to the in vivo environment and indicates that the cells are subject to regulatory mechanisms of adipocytes.

Attogram detection limit for aqueous dye samples by laser-induced fluorescence

A modified flow cytometer has been used to detect attogram quantities of aqueous rhodamine 6G by laser-induced fluorescence analysis. A detection limit of 28 attograms (35,000 molecules) was obtained, nearly two orders of magnitude better than earlier measurements. The detection limit in concentration units was 1.4 x 10(-13) mole per liter. During the 1-second measurement period, the total volume sampled was 0.42 microliter. On average, only half a rhodamine 6G molecule was present in the 6-picoliter probed volume.

Ultrastructural localization of interferon receptors on the surfaces of cultured cells and erythrocytes

The bindings sites for interferon (IFN) on the limiting cell membranes of human and mouse fibroblasts and erythrocytes were revealed by an indirect immunoferritin technique. Mouse IFN-beta and human IFN-beta of high specific activity were used with the corresponding purified antibodies. Species-specific IFN binding was demonstrated by ferritin deposition on human erythrocytes and fibroblast membranes treated with human IFN and on mouse erythrocytes and fibroblast membranes treated with mouse IFN, but not on human erythrocytes or fibroblast membranes treated with mouse IFN. IFN binding sites on fibroblasts were located on regions of membranes between microvilli, whereas diphtheria toxin receptors were demonstrated mainly on microvilli. IFN binding altered the diphtheria toxin after IFN treatment. This reduced toxicity correlated with a decrease in the quantity of receptors for diphtheria toxin on the cell membrane. Thus, the species-specific binding of IFN appears to depend on membrane receptors in discrete regions of the limiting membrane which are present not only on functionally responsive fibroblasts but also on erythrocytes.

Insulin binding in cultured Chinese hamster kidney epithelial cells: the effect of serum in the medium

[125I] Insulin (porcine) binding to an epithelial cell line established from a Chinese hamster kidney, CHK-AC E-100, showed an optimum at pH 8.0 and reached a maximum after 2.5 h incubation at 25 degrees C. Dissociation of bound [125I] insulin was facilitated by the addition of unlabeled insulin in the dilution buffer. Porcine insulin effectively competed for [125I] insulin binding to the cultured cells and was 30 and 90 times as potent as guinea pig insulin and porcine proinsulin in causing 50% inhibition of [125I] insulin binding; glucagon was completely ineffective. Scatchard analysis of the binding data yielded a curvilinear plot and a capacity of 0.6 ng/10(6) cells; the average affinity of the empty receptor, Ke, was calculated to be 1.78 X 10(6) M-1 and that of the filled receptor, Kf, 0.57 X 10(8) m-1, Substitution of fetal bovine serum (FBS) in the culture medium with bovine calf, bovine newborn, of bovine calf serum altered insulin binding characteristics in the cells and reduced cell growth. Insulin binding characteristics of cells grown in hormone-supplemented medium containing 0 to 0.1% FBS were similar to those of cells grown in minimum essential medium (MEM) containing 2 to 5% FBS. The data indicated that the established Chinese hamster kidney epithelial cell line CHK-AC E-100 possessed specific insulin receptors and the characteristics of the receptors could be manipulated by changing the serum in culture medium.

Red blood cell insulin binding studies in Reye's syndrome survivors and families

RBC insulin binding was examined in Reye's survivors and families of affected patients to determine whether their previously reported hyperinsulinemic responses to oral glucose are accompanied by alterations in insulin binding and could contribute to the hypercatabolism seen in this disorder. The mean (125I)-insulin binding to 3 X 10(9) RBC's was 5.7 +/- SEM 0.4 percent in survivors compared to 6.6 +/- 0.3 in siblings (p less than .05) and 6.6 +/- 0.4 in control children (p = .05). Sex and maturity differences were found with higher binding values in men than women as well as higher values in men than boys. Receptor numbers in survivors were comparable to control values. Average affinities varied widely. Plasma insulin levels were low in the fathers (9 +/- SEM 1.4 uU/ml compared to 18.3 +/- 1.8 for control men and 20 +/- 4.5 for mothers of affected patients). The acute syndrome is accompanied by hypercatabolism in the presence of increased plasma insulin levels and familial clustering of cases and recurrences are known to occur. Reduction in insulin binding may play a role in the acute disease if such is shared by more traditionally hormone-responsive cells.

Insulin effect in vitro on human erythrocyte plasma membrane

The effect of porcine insulin has been tested in vitro on human erythrocyte plasma membrane (Na+-K+) and Mg2+-ATPase activities as well as on membrane fluidity. The results indicate that the hormonal treatment significantly inhibits (Na+-K+)-ATPase activity, and at the same time decreases membrane fluidity.

Hematologic alterations in diabetes mellitus

Several hematologic abnormalities have been defined in patients with diabetes mellitus, despite the lack of classic hematologic pathologic findings in this condition. Studies of the erythrocyte and the formation of hemoglobin A1c have provided a means of documenting glycemia and a model reaction for diabetic sequelae through postsynthetic protein modification. Oxygen affinity has been noted to be abnormal in the diabetic erythrocyte, concomitant with a decreased concentration of inorganic phosphorus, glycosylation of the 2,3-diphosphoglycerate binding site or preexisting vascular disease. Red cell membrane viscosity has also been documented to be increased in the hyperglycemic subject. Abnormalities in the polymorphonuclear leukocyte have been described, involving the properties of adherence, random migration, chemotaxis, phagocytosis and killing. Certain metabolic abnormalities are also present in this cell type. The lymphocyte has been shown to have abnormal metabolic properties, mitogen responses and cell surface properties in diabetes both in animals and human subjects. Certain subpopulations of lymphocytes appear to be especially vulnerable to changes concomitant with diabetes mellitus. In vitro abnormalities of platelet behavior have been widely studied, although the in vivo significance of these findings remains controversial. Studies of the fluid phase of coagulation have suggested the existence of a hypercoagulable state in hyperglycemic subjects. The clinical significance of most of these findings remains to be defined. Nevertheless, the observation that many of the abnormalities described are reversible when hyperglycemia is corrected has given impetus to the development of improved systems of glucose "control" for diabetic patients.

Mechanisms of insulin resistance in man. Assessment using the insulin dose-response curve in conjunction with insulin-receptor binding

During the past few years it has become increasingly apparent that insulin resistance may be a more frequent cause of carbohydrate intolerance or contributing factor in carbohydrate intolerance than was hitherto appreciated. Abnormal insulin action may result from prereceptor, receptor or postreceptor defects. These may be manifested by an increase in the concentration of insulin necessary for a half-maximal effect (decreased sensitivity) or a decrease in the maximal response to insulin (decreased responsiveness), or both. Alterations in sensitivity and responsiveness to insulin can be distinguished only by evaluating insulin dose-response curves. When used in conjunction with measurements of insulin binding to its receptor, the characteristics of these curves can provide insight into the mechanism or mechanisms responsible for insulin resistance.

Evidence for two insulin receptor populations on human erythrocytes

Human erythrocytes specifically bind 125I-insulin in a manner similar to cells in which insulin exerts a physiological response. In addition, erythrocytes are of practical value for correlating in vitro insulin binding with in vivo carbohydrate intolerance. The competitive binding of labelled and unlabelled insulin to erythrocyte receptor3 is typically curvilinear when plotted according to Scatchard. The curvilinear nature of the Scatchard plot describing insulin binding to membrane receptors, although originally attributed to heterogeneous sites, has been more recently interpreted as negative cooperativity between homogenous sites. Evidence reported here, however, suggests that there are two populations of insulin receptors on erythrocytes. Specific concentrations of concanavalin A (Con A), a lectin which mimics insulin activity, are shown here to inhibit one population of receptors leaving another population unaffected.

A membrane abnormality in lymphocytes from diabetic subjects

The fluorescence properties of the hydrophobic probe 1,6-diphenyl1-1,3,5-hexatriene incorporated in the lymphocytes of 30 diabetic patients and 21 normal control subjects were studied. The mean value of the probe polarization was 0.314 for the control group and 0.294 for the patient group. The difference was significant at p < 0.001. The decreased polarization was correlated with the level of plasma glucose in the patients (p < 0.01). Nanosecond fluorescence results obtained from the lymphocytes of 7 patients and 5 controls indicated that there was no significant difference in the probe lifetimes between the two groups of subjects and suggest that the decreased polarization of the probe in the patient group resulted from a more fluid lipid environment of cell membranes.