The effect of solubilization on the properties of the insulin receptor of human placental membranes

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

AIM

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Insulin antibody response to bovine insulin therapy: functional significance among insulin requiring young diabetics in India

The majority of young diabetics in India prefer to use low-cost bovine insulin for economic reasons. Therefore, the question of insulin antibody response to bovine insulin and its functional significance is still relevant in the Indian context. We assessed insulin antibody response in 52 young diabetics (type 1, n=25, malnutrition modulated form of diabetes, n=19 and fibrocalculous pancreatopathy (FCP) n=8) on bovine insulin therapy (mean duration 3.0+/-2.1 years) using an internationally standardised in-house radioligand assay. The functional significance of insulin antibody was assessed by calculating their affinity constant, maximum binding capacity and total insulin binding power by Scatchard analysis (type 1, n=14, malnutrition modulated form of diabetes, n=11). All the patients treated with bovine insulin showed high titers of insulin antibodies with S.D. score ranging from 5.1 to 42.0. No significant difference was observed in the mean S.D. score of insulin antibodies in the three diabetic groups. The mean daily insulin dose, maximum insulin binding capacity and total insulin binding power were significantly higher in type 1 when compared to the malnutrition modulated form of diabetes (36+/-8 vs. 26+/-11 IU/day, P<0.05; 9. 7+/-7.8 vs. 4.0+/-3.9 nmol/l, P=0.03 and 59+/-29 vs. 29+/-43, P=0.01, respectively). Insulin antibodies S.D. score and its affinity did not show significant relationship with daily insulin dose and glycemic control (HbAl) at admission. Only 24+/-7% variations in daily insulin requirement were accounted for by total insulin binding power. There was a significant inverse relationship between insulin antibody S.D. score and duration of insulin therapy (r=-0. 4172, P<0.0004). To conclude, insulin antibody response following bovine insulin therapy is not different among type 1, malnutrition modulated form of diabetes and FCP diabetes. The insulin antibody response to bovine insulin therapy does not contribute significantly to increase in daily insulin requirement in bovine insulin treated insulin requiring young diabetics.

Increased expression of low-affinity insulin receptor isoform and insulin/insulin-like growth factor-I hybrid receptors in term placenta from insulin-resistant women with gestational hypertension

Gestational hypertension is associated with insulin-resistance; insulin and insulin-like growth factor-1 (IGF-1), acting through their receptors, play a role in the growth of the feto-placental unit. Since both receptors are exposed to the maternal circulation, it has been suggested that maternal metabolic abnormalities might affect placental insulin (HIR) and IGF-1 (IGF-1R) receptors. To clarify this issue, we characterized HIR and IGF-1R in placenta at term from normal women, normoinsulinaemic women with gestational hypertension (NGH), and hyperinsulinaemic women with gestational hypertension (HGH). Insulin binding was decreased in HGH women (B/T 0.12 +/- 0.03) compared to control and NGH women (B/T 0.18 +/- 0.07, p < 0.036; and 0.22 +/- 0.5, p < 0.009 respectively). Receptor affinity was lower in HGH women (ED50 0.95 +/- 0.32 nmol/l) than control and NGH women (ED50 0.42 +/- 0.19 nmol/l, p < 0.01; and 0.40 +/- 0.1 nmol/l, p < 0.007, respectively), whereas low-affinity Ex11+ isoform was higher in HGH women (Ex11+ 50 +/- 7, %) than in control and NGH women (Ex11+ 34 +/- 9%, p < 0.001; and 39 +/- 4%, p < 0.01, respectively). Increased expression of Ex11+ isoform was correlated with ED50 (r = 0.71; p < 0.002) and insulinaemia (r = 0.70, p < 0.002). IGF-I binding was increased in HGH women (B/T 0.17 +/- 0.03) compared to control and NGH women (B/T 0.09 +/- 0.05, p < 0.002; and 0.11 +/- 0.03, p < 0.002, respectively). IGF-IR affinity was similar in the three groups. The percentage of insulin/IGF-I hybrid receptors was increased in HGH women (85 +/- 3%) compared to control and NGH women (68 +/- 7%, p < 0.001; and 63 +/- 9%, p < 0.001, respectively), and was positively correlated with insulinaemia (r = 0.62, p < 0.018), ED50 of insulin binding (r = 0.62, p < 0.05), and maximal IGF-I binding (r = 0.69, p < 0.004); whereas it was inversely correlated with maximal insulin binding (r = -0.69, p < 0.004). Results provide the first evidence for altered expression of insulin/IGF-I hybrids found in insulin-resistance states.

Identification and characterization of a tubulin binding protein in rat brain plasma membrane

Studies on the interaction of FITC-tubulin and 125I-tubulin with isolated plasma membrane of neural cells and with primary cultures of neuronal (N) and glial (G) cells of rat brain demonstrate the presence of specific, saturable, high affinity tubulin binding sites in these cells. The positive fluorescence of live unfixed primary cultures of N and G cells following incubation with FITC-tubulin indicate that the tubulin binding sites are located on the outer side of the plasma membrane. Such fluorescence was not observed with FITC-BSA, FITC-conalbumin or freshly dissociated cells from rat tissues or established cell lines. Binding of FITC-tubulin or 125I-tubulin is competed only by tubulin and not by other proteins. Scatchard analysis of the binding of 125I-tubulin to purified plasma membrane indicates very high affinity (Kd = 85 nM) with a Bmax of 7.4 pmol/mg protein. The putative tubulin receptor was partially purified by affinity chromatography on tubulin-sepharose column. Immunoprecipitation of the solubilized tubulin-receptor complex followed by SDS-PAGE analysis and autoradiography, revealed the presence of two components of molecular weights 70 and 45 kDa respectively, presumably representing the two nonidentical subunits of the putative receptor. In conjunction with several recent reports indicating the secretion of high molecular weight proteins from cultured neural cells and the ability of tubulin to modulate adenyl cyclase in synaptic membranes these findings suggest that the binding of exogenous tubulin to sites external to the plasma membrane may be involved in signal transduction.

Effects of maturation and aging on the skeletal muscle glucose transport system

Insulin resistance in old, compared with young, humans and animals has been well documented. The resistance is due primarily to defects in skeletal muscle. In the present study, skeletal muscle sarcolemmal membranes were purified from five age groups of female Fischer rats ranging from 2 to 24 mo. Basal specific D-glucose transport was not significantly different among any of the groups. Maximum insulin-stimulated transport was progressively decreased from 96.4 +/- 5.0 pmol.mg-1.15 s-1 in the 2-mo-old animals to 70.8 +/- 8.9 pmol.mg-1.15 s-1 in the 24-mo-old animals. Most of the decrease occurred during maturation, and in fact there was no significant difference in maximum transport among the 8-, 16-, and 24-mo-old rats. The decrease in insulin-stimulated transport in the 24-mo-old animals was due to a reduction in the number of glucose transporters translocated into the sarcolemma membrane (9.8 +/- 0.6 vs. 7.8 +/- 0.6 pmol/mg protein). The intracellular or microsomal pool of glucose transporters was not significantly different between the 2- and 24-mo-old animals (8.8 +/- 0.6 vs. 8.5 +/- 0.9/mg protein). Western blotting revealed no differences in the cellular GLUT-4 contents between the 2- and 24-mo-old rats. The number of insulin receptors (2.3 +/- 0.4 vs. 2.1 +/- 0.5 pmol/mg protein) was not significantly different. Tyrosine kinase activity of the insulin receptor was, however, significantly reduced in the 24-mo-old compared with the 2-mo-old animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of hormones on dissociation of prolactin from the rabbit mammary gland prolactin receptor

Using microsomes prepared from rabbit mammary gland, the dissociation of prolactin (PRL) from its receptor was determined in the presence of peptide hormones or various concentrations of PRL. Among the hormones tested, PRL (ovine, mouse and bullfrog), human growth hormone and human placental lactogen each accelerated the dissociation of PRL in a manner proportional to their receptor-binding activities. Hormone-dependent dissociation was observed at higher concentrations than those at which the binding of PRL was completely inhibited by lactogenic hormones. In the concentration range 0.1 ng/ml-10 micrograms/ml, PRL increased the rate of dissociation in a logarithmic concentration-dependent manner. It was concluded that the dissociation of PRL from its receptor caused by lactogenic hormones is dependent on the hormone concentration. Arrhenius plot analysis revealed that PRL changed the frequency factor for the dissociation reaction. PRL in the medium inhibited the re-association of dissociated PRL. The data also suggested that PRL regulates the rate of dissociation by interacting with the PRL-receptor complex.

Abnormal insulin metabolism by specific organs from rats with spontaneous hypertension

Spontaneously hypertensive rats (SHR) have been shown to be both insulin resistant and hyperinsulinemic after oral glucose administration or infusion of exogenous insulin during an insulin suppression test. To determine if this hyperinsulinemia may be due to decreased removal of insulin, the metabolic clearance (k) of insulin was measured in isolated perfused liver, kidney, and hindlimb skeletal muscle from SHR and Wistar-Kyoto (WKY) control rats. The data indicate that the k for insulin removal by liver was similar in SHR and WKY rats, averaging 287 +/- 18 and 271 +/- 10 microliters.min-1.g-1 liver, respectively. In contrast, the k for insulin removal by hindlimbs from SHR was decreased 37% (P less than 0.001) compared with WKY rats (8.6 +/- 0.5 vs. 13.7 +/- 0.7 microliters.min-1.g-1 muscle), and this decrease was not accompanied by decreased binding of insulin to its receptor in plantaris muscle. Although the removal of insulin by glomerular filtration was similar in SHR and WKY rats (653 +/- 64 microliters/min vs. 665 +/- 90 microliters.min-1.kidney-1), total insulin removal by kidney was significantly lower (P less than 0.05) in SHR (710 +/- 78 microliters/min) compared with WKY rats (962 +/- 67 microliters/min), due to decreased peritubular clearance of insulin in SHR (56 +/- 73 vs. 297 +/- 59 microliters/min, P less than 0.05). These findings suggest that the decreased clearance of insulin in SHR rats was possibly not due to impaired hepatic removal of insulin but rather to decreased removal by skeletal muscle and kidneys.(ABSTRACT TRUNCATED AT 250 WORDS)

Human insulin receptor radioimmunoassay: applicability to insulin-resistant states

A radioimmunoassay of the human insulin receptor was developed employing a potent rabbit polyclonal antibody to the human insulin receptor and a highly purified human placental insulin receptor preparation. The receptor, obtained by sequential affinity chromatography with insulin receptor monoclonal antibody-agarose and wheat germ agglutinin-agarose, was radiolabeled with 125I-Bolton-Hunter reagent at specific activities of 2,100-3,300 Ci/mmol. Over 75% of this ligand was immunoprecipitable with the polyclonal antireceptor antibody and remained immunoprecipitable for greater than 45 days. The assay was sensitive to unlabeled receptor concentrations as low as 0.2 ng/0.5 ml; unlabeled insulin did not cross-react and unlabeled insulin-like growth factor (IGF)-I receptor cross-reacted weakly. The radioimmunoassay was applicable to the measurement of insulin receptors in tissues and cells that were extracted by solubilization in 1% Triton X-100; no purification of the extracted receptor was necessary. Of the three major target tissues for insulin action studied, liver had the highest concentration of receptors (47.6 ng/mg protein); fat and muscle had lower levels. Other studies with the radioimmunoassay indicated that insulin receptors were decreased both in monocytes from obese hyperinsulinemic subjects and in fibroblasts from patients with leprechaunism.

Effects of exercise training on the relationship between insulin binding and insulin-stimulated tyrosine kinase activity in rat skeletal muscle

The effect of exercise training on insulin binding and insulin receptor tyrosine kinase activity was studied using detergent solubilized wheat germ agglutinin (WGA)-agarose purified receptor preparations from rat biceps femoris (BF) and tensor fascia lata (TFL) muscles. Insulin receptor activity, as assessed by A14 [125I] insulin binding, was significantly elevated in BF of exercise-trained rats when compared with similar preparations from a sedentary control group. This increase in binding activity was due to change in Bmax not KD. In contrast, neither the Bmax nor the KD of insulin binding to TFL changed with exercise training. The structure of insulin receptors isolated from BF or TFL was unaltered by exercise training as determined by affinity labeling (alpha-subunit, molecular weight (mol wt) approximately 131 kilodaltons [kDa]) and electrophoretic mobility of the alpha- and beta-subunit. Furthermore, basal tyrosine kinase activity was not affected by exercise training in extracts from either BF or TFL. However, the insulin dependent increase in maximal tyrosine kinase activity (Vmax) of the BF, but not TFL, was enhanced by exercise training. Specifically, insulin stimulated phosphorylation of both the beta-subunit of the insulin receptor (auto phosphorylation) and of a synthetic peptide (exogenous phosphorylation) were increased over control values in BF from exercise-trained rats, whereas both measurements of tyrosine kinase activity of TFL from the two experimental groups were similar. In contrast, both insulin-stimulated autophosphorylation and tyrosine kinase activity were significantly decreased in BF of exercise-trained rats when normalized to insulin binding activity. This disassociation was only seen in BF from exercise-trained rats, and was not true of TFL. These data indicate that exercise training can lead to increases in insulin receptor number and tyrosine kinase activity, as well as modifying the relationship between these two variables. The changes noted are not observed in all exercising muscles, and their development seems to depend upon the fiber composition. These results emphasize the complex relationship that exists in the regulation of insulin action at the level of its receptor.

Delineation of atypical insulin receptors from classical insulin and type I insulin-like growth factor receptors in human placenta

Insulin-like growth factor (IGF)-binding sites copurifying with human placental insulin receptors during insulin-affinity chromatography consist of two immunologically distinct populations. One reacts with monoclonal antibody alpha IR-3, but not with antibodies to the insulin receptor, and represents Type I IGF receptors; the other reacts only with antibodies to the insulin receptor and is precipitated with a polyclonal receptor antibody (B-10) after labelling with 125I-multiplication-stimulating activity (MSA, rat IGF-II). The latter is a unique sub-population of atypical insulin receptors which differ from classical insulin receptors by their unusually high affinity for MSA (Ka = 2 x 10(9) M-1 compared with 5 x 10(7) M-1) and relative potencies for insulin, MSA and IGF-I (40:5:1 compared with 150:4:1). They represent 10-20% of the total insulin receptor population and account for 25-50% of the 125I-MSA binding activity in Triton-solubilized placental membranes. Although atypical and classical insulin receptors are distinct, their immunological properties are very similar, as are their binding properties in response to dithiothreitol, storage at -20 degrees C and neuraminidase digestion. We conclude that atypical insulin receptors with moderately high affinity for IGFs co-exist with classical insulin receptors and Type I IGF receptors in human placenta. They provide an explanation for the unusual IGF-II binding properties of human placental membranes and may have a specific role in placental growth and/or function.

Negative and positive site-site interactions, and their modulation by pH, insulin analogs, and monoclonal antibodies, are preserved in the purified insulin receptor

The kinetic properties of the insulin receptor were studied in solution after its purification to homogeneity. Dissociation of 125I-labeled insulin at a 1:50 dilution was not first order; unlabeled insulin at physiological concentrations accelerated the dissociation rate with a maximal effect at approximately 17 nM. At higher concentrations, the unlabeled insulin slowed the dissociation rate. Maximal acceleration was seen at pH 8.0. The ability to accelerate the dissociation rate was diminished with [LeuB24]insulin and suppressed with desoctapeptide, [LeuB25], [LeuB24,B25], desalanine-desasparagine, and desheptapeptide insulins, all of which slowed the dissociation at high concentrations. Monoclonal antibodies to the insulin receptor alpha subunit (MA-5, MA-10, MA-20, and MA-51) all competed for insulin binding to the purified receptor. MA-10 and MA-51 accelerated the dissociation of 125I-labeled insulin, while MA-5 and MA-20 slowed the off rate. Thus, all the aspects of both negatively and positively cooperative site-site interactions previously described in whole cells are present in solubilized purified receptors, demonstrating that these interactions represent intrinsic properties of the receptor molecule, most likely as a result of ligand-induced conformational changes.

Ontogeny of insulin receptors in the rat hemochorial placenta

Binding of 125I-insulin to rat placental membranes was time and protein concentration dependent, reversible, and specific. Unlabeled porcine insulin competed for 125I-insulin binding with an IC50 of 65 nM, while IGF-I was much less potent with an IC50 of 2.12 mM. Specific binding of 125I-insulin decreased during the second half of gestation from Days 11 to 19. Scatchard analysis of the binding data for membranes prepared from Gestation Days 11 and 19 yielded typical curvilinear plots which showed a marked decrease in the number of binding sites in late gestation placenta. Beginning on Day 14, insulin binding was characterized with isolated labyrinth and basal zone portions of the hemochorial placenta. There was no evidence for differences in Kd values or the number of binding sites in these two functionally distinct portions of the rat placenta. Crosslinking of 125I-insulin followed by SDS-PAGE showed a single protein with a molecular weight of 130,000 from placental tissues on Gestation Days 11 and 19 and confirmed a gestational decrease in the number of insulin receptors. In solubilized, lectin-purified preparations from placenta and liver membranes, insulin stimulated the phosphorylation of a Mr 95,000 protein. 32P-incorporation into this 95,000 protein was stimulated fivefold by insulin in Day 11 placenta receptor, whereas no detectable 32P-incorporation was found in Day 19 placenta. Thus, while the alpha- and beta-subunits of insulin receptors in mid and late gestation placenta have molecular weights which are similar to receptors in maternal liver, data indicate the presence of a functional difference in insulin-stimulated kinase activities.

Optimisation of conditions for solubilisation of the bovine dopamine D2 receptor

Solubilisation of the dopamine D2 receptor from a membrane preparation of bovine corpus striatum using cholate and NaCl was independently optimised with regard to cholate (0.2%, wt/vol), NaCl (1.5 M), and membrane protein (4 mg/ml) concentrations. A maximum solubilisation yield of 58% was obtained and receptors were measured using a [3H]spiperone binding assay incorporating a polyethylene glycol precipitation step. Solubilisation was confirmed by ultracentrifugation studies, passage of the receptor through fine-pore filters, increased thermolability, and by retention of the prelabelled receptor on gel filtration. The soluble receptor showed saturability and reversibility of binding. Displacement of [3H]spiperone from the soluble receptor by competing compounds correlated closely with displacement from the membrane-bound receptors. [3H]Spiperone binding was found to be pH-dependent, with maximum binding occurring at pH 7.8. A comparison of solubilisation was made with six other agents both with and without added NaCl and it was concluded that the cholate/NaCl solubilisation system provides an efficient, inexpensive, and reliable method for the preparation of functional bovine dopamine D2 receptors.

Disulphide reduction alters the immunoreactivity and increases the affinity of insulin-like growth-factor-I receptors in human placenta

We previously identified two forms of the insulin-like growth-factor-I (IGF-I) receptor in human placenta: a lower-affinity form reactive with an autoantiserum (B-2) to the insulin receptor and a higher-affinity non-immunoreactive form [Jonas & Harrison (1985) J. Biol. Chem. 260, 2288-2294]. Evidence is now presented that the lower-affinity immunoreactive forms are convertible into higher-affinity non-immunoreactive forms via reduction of receptor disulphide bonds. Treatment of placental membranes with increasing concentrations of dithiothreitol (DTT): (1) converted native Mr-290 000 heterotetrameric IGF-I receptors into Mr-180 000 dimers (determined by chemical cross-linking of 125I-IGF-I with disuccinimidyl suberate); (2) increased 125I-IGF-I binding, owing to an increase in receptor affinity; and (3) abolished the reactivity of Triton-solubilized IGF-I receptors with antiserum B-2 and transformed the curvilinear plot of IGF-I binding to a linear form. In isolated complexes between receptor and B-2 antibody, DTT increased 125I-IGF-I binding and released a single class of higher affinity IGF-I receptors of Mr 180,000. Thus DTT-treated IGF-I receptors have similar properties to the higher-affinity non-immunoreactive forms of the native receptor, except that reduced dimeric forms are not detected by cross-linking of 125I-IGF-I to native membranes. Cleavage of the inter-dimeric disulphide bonds is therefore not a prerequisite for higher-affinity binding or loss of immunoreactivity. These observations suggest that the thiol redox state of the IGF-I receptor in vivo is an important determinant of receptor conformation and therefore of the biological responses to IGF-I.

Solubilization and characterization of a low-affinity histamine-binding site on human blood mononuclear cells

The extract of human peripheral blood lymphocytes and monocytes treated with Triton X-100, in direct- and competitive-binding studies, with 10(-6)-10(-2) M [14C]histamine contained a low-affinity binding site whose dissociation constant (Kd 1.8 X 10(-4) M) was commensurate with the concns of histamine (10(-6)-10(-3) M) that result from mast cell and basophil degranulation. Binding was enhanced by millimolar concns of divalent cations and by raising the incubation temp from 4 to 37 degrees C. It was inhibited by trypsin, EDTA, agents interacting with thiol groups, and by Triton X-100 concns greater than 0.2%. Thus a low-affinity histamine receptor that maintains its ligand-binding properties after solubilization from the cell surface was identified.

Protein kinase activity of the insulin receptor

The insulin receptor is an integral membrane glycoprotein (Mr approximately 300,000) composed of two alpha-subunits (Mr approximately 130,000) and two beta-subunits (Mr approximately 95,000) linked by disulphide bonds. This oligomeric structure divides the receptor into two functional domains such that alpha-subunits bind insulin and beta-subunits possess tyrosine kinase activity. The amino acid sequence deduced from cDNA of the single polypeptide chain precursor of human placental insulin receptor revealed that alpha- and beta-subunits consist of 735 and 620 residues, respectively. The alpha-subunit is hydrophilic, disulphide-bonded, glycosylated and probably extracellular. The beta-subunit consists of a short extracellular region which links the alpha-subunit through disulphide bridges, a hydrophobic transmembrane region and a longer cytoplasmic region which is structurally homologous with other tyrosine kinases like the src oncogene product and EGF receptor kinases. The cellular function of insulin receptors is dual: transmembrane signalling and endocytosis of hormone. The binding of insulin to its receptor on the cell membrane induces transfer of signal from extracellular to cytoplasmic receptor domains leading to activation of cell metabolism and growth. In addition, hormone-receptor complexes are internalized leading to intracellular proteolysis of insulin, whereas receptors are recycled to the membrane. These phenomena are kinetically well-characterized, but their molecular mechanisms remain obscure. Insulin receptor in different tissues and animal species are homologous in their structure and function, but show also significant differences regarding size of alpha-subunits, binding kinetics, insulin specificity and receptor-mediated degradation. We suggest that this heterogeneity of receptors may be linked to the diversity in insulin effects on metabolism and growth in various cell types. The purified insulin receptor phosphorylates its own beta-subunit and exogenous protein and peptide substrates on tyrosine residues, a reaction which is insulin-sensitive, Mn2+-dependent and specific for ATP. Tyrosine phosphorylation of the beta-subunit activates receptor kinase activity, and dephosphorylation with alkaline phosphatase deactivates the kinase. In intact cells or impure receptor preparations, a serine kinase is also activated by insulin. The cellular role of two kinase activities associated with the insulin receptor is not known, but we propose that the tyrosine- and serine-specific kinases mediate insulin actions on metabolism and growth either through dual-signalling or sequential pathways.(ABSTRACT TRUNCATED AT 400 WORDS)

The nonclassical insulin binding of insulin receptors from rat liver is due to the presence of two interacting alpha-subunits in the receptor complex

The binding characteristics of the insulin receptor tetramer (alpha 2 beta 2) and dimer (alpha beta) were examined. Unlabelled insulin enhanced the dilution-induced dissociation only of the receptor tetramer-bound 125I-insulin. Furthermore, when both the receptor forms had been preincubated with anti-receptor-antibodies (B9-antiserum), insulin binding only to the receptor tetramer but not to the dimer was inhibited. However, both oligomers are not immunologically distinct since more than 80% of the two forms were immunoprecipitated by the antiserum. These results suggest that both insulin and anti-receptor-antibodies induce cooperative interactions between the two linked alpha-subunits of the receptor tetramer leading to a decrease in insulin binding of this receptor form.

Purification and characterization of calcitonin receptors in rat kidney membranes by covalent cross-linking techniques

We have characterized the binding parameters of renal receptors (Scatchard analysis revealed the presence of two binding sites: site I, Ka1 = 1.29 X 10(9) M-1, number of binding sites = 9.9 X 10(6)/micrograms protein; site II, Ka2 = 0.93 X 10(8) M-1, number of binding sites = 4.27 X 10(8)/micrograms protein) and studied the effect of solubilization. The high-affinity sites are preserved during affinity chromatography and the process results in a 6080-fold purification of those sites. The lower-affinity sites are also preserved but the overall purification factor is about 40% lower than that obtained using molecular sieving. The purification of the renal calcitonin receptor by molecular sieving (Sephacryl S-200) is accompanied by total loss of the high-affinity site; however, the low-affinity site is enriched over 1642-fold. Binding parameters were obtained for the purified fractions. Synthetic salmon calcitonin was also bound to renal membranes using the bifunctional reagent disuccinimidyl suberate and photo-affinity cross-linking using hydroxysuccinimidyl azidobenzonate reagent. Cross-linked receptor eluted in the same volume as solubilized membranes specifically binding salmon calcitonin (S-200 chromatography). Sodium dodecyl sulfate polyacrylamide gel electrophoresis analysis of purified fractions showed several protein bands with apparent molecular masses ranging from 18 000 Da to 100 000 Da in the presence or absence of a reducing agent (2-mercaptoethanol). Autoradiography of polyacrylamide gels of cross-linked calcitonin receptor showed only three protein bands specifically binding salmon calcitonin. Their molecular masses were 70 000 Da, 40 000 Da and 33 000 Da respectively. The 40 000-Da molecule represents a major band (47% total binding species). This suggests that these three proteins are the principal components of the calcitonin receptor and that S-S bonds are not involved in the assembly of the receptor subunits.

Inhibition of down regulation by chloroquine in cultured lymphocytes (RPMI-1788 line)

We studied the effect of chronic exposure to insulin on insulin receptors of cultured lymphocytes (RPMI-1788 line). The cells treated with insulin (2 micrograms/ml) at 37 degrees C for 12 h, showed a 36.5% decrease in the number of binding sites. Solubilized extract from the cells treated with insulin showed a 35.9% decrease of binding capacity, suggesting that insulin exposure induced the loss of total (cell surface and intracellular) receptors. Insulin-induced loss of receptors was blocked by chloroquine, suggesting that receptor loss is mediated by a chloroquine sensitive pathway. Bacitracin, which inhibited the insulin degradation, had no effect on insulin-induced receptor loss in this cell line. We also found that vitamin K5, one of the insulin mimickers, induced a 31.5% loss of insulin receptors. Therefore, the post-receptor process appeared to mediate down regulation. In cultured lymphocytes, insulin exposure caused a significant loss of total receptors, suggesting that insulin-induced receptor loss may be due to receptor degradation. Insulin-induced receptor loss is mediated by a chloroquine sensitive pathway, and is related to the post-binding process stimulated by vitamin K5.

Regulation of insulin receptor kinase by multisite phosphorylation

The regulation of the insulin receptor kinase by phosphorylation and dephosphorylation has been examined. Under in vitro conditions, the tyrosine kinase activity of the insulin receptor toward histone is markedly activated when the receptor either undergoes autophosphorylation or is phosphorylated by a purified preparation of src tyrosine kinase on tyrosine residues of its beta subunit. The elevated kinase activity of the phosphorylated insulin receptor is readily reversed when the receptor is dephosphorylated with alkaline phosphatase. Analysis of tryptic digests of phosphorylated insulin receptor using reverse-phase high pressure liquid chromatography suggests that phosphorylation of a specific tyrosine site on the receptor beta subunit may be involved in the mechanism of the receptor kinase activation. Further studies indicate that tyrosine phosphorylation-mediated increase in insulin receptor activity also occurs in intact cells. Thus, when the histone kinase activities of insulin receptor from control and insulin-treated H-35 hepatoma cells are assayed in vitro following the purification of the receptors under conditions which preserve the phosphorylation state of the receptors, the insulin receptors extracted from insulin-treated cells exhibit histone kinase activities 100% higher than those from control cells. The elevated receptor kinase activity from insulin-treated cells appears to result from the increase in phosphotyrosine content of the receptor. Taken together, these results indicate that tyrosine phosphorylation of the insulin receptor beta subunit exerts a major stimulatory effect on the kinase activity of the receptor. Insulin receptor partially purified by specific immunoprecipitation from detergent extracts of control and isoproterenol-treated cells have similar basal but diminished insulin-stimulated beta subunit autophosphorylation activities when incubated with [gamma-32 P]ATP. Similarly, the ability of insulin to stimulate the receptor beta subunit phosphorylation in intact isoproterenol-treated adipocytes is greatly attenuated, whereas, the basal phosphorylation of the insulin receptor is slightly increased by the beta-catecholamine. These data indicate that in rat adipocytes, a cyclic AMP-mediated mechanism, possibly through serine and threonine phosphorylation of the receptor or its regulatory components, may uncouple the receptor tyrosine kinase activity from activation by insulin. Treatment of 32P-labeled H-35 hepatoma cells with phorbol myristate acetate (PMA) results in a marked increase in serine phosphorylation of the insulin receptor beta subunit.(ABSTRACT TRUNCATED AT 400 WORDS)

Structure of covalent insulin-receptor complexes (I-S-S-R) in isolated rat adipocytes and human placental membranes

The structure of naturally-formed covalent disulphide-linked complexes between insulin and its receptor was examined by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. To prevent destabilization of disulphide bonds at alkaline pH the standard discontinuous electrophoresis conditions were changed to a continuous buffer system at pH 7.0. 125I-insulin was first bound to either rat adipocytes or human placental membranes for 10 min at 37 degrees C. After washing, non-dissociable radioactivity was extracted from cells or membranes in Triton X-100 and immunoprecipitated with an antiserum (B-2) to the insulin receptor. Electrophoresis of the immune precipitate revealed the two smaller of the three reported species of native insulin receptor (Mr values approx. 350 000, 290 000 and 260 000); in addition, a species of Mr 200 000 was also frequently observed in adipocytes. When non-dissociable 125I-insulin was chemically crosslinked to adipocytes or placental membranes, prior to solubilization and immunoprecipitation, all three species of the native receptor were labelled; after reduction, only a single species of Mr 130000 was observed. These findings indicate that disulphide exchange of insulin occurs with the Mr 130000 (alpha) binding subunit within partially reduced species of the native, oligomeric receptor. The degree of disulphide binding of insulin could therefore depend on the relative abundance of partially reduced receptor species and on the redox state of the cell membrane.

Substrate specificities of insulin and epidermal growth factor receptor kinases

The abilities of insulin and EGF stimulated protein kinases to phosphorylate a series of exogenous substrates were compared using wheat germ lectin purified preparations of solubilized rat liver membranes. Three different kinds of substrates were found: substrates phosphorylated primarily by insulin stimulated kinase, substrates phosphorylated primarily by EGF stimulated kinase and substrates phosphorylated by both kinases to a similar extent. These results indicate that the insulin and the EGF receptor kinase have different, but overlapping, substrate specificities. In vivo, phosphorylation of cellular proteins by various hormone receptor kinases may be part of the signal transmission process for actions of the hormones. Different substrate specificities of kinases of different hormone receptors may therefore represent an important mechanism to preserve the specificity of the hormonal signal at the post receptor level.

Effect of Triton X-100 on insulin and epidermal growth factor receptor binding and autophosphorylation in Golgi fractions and partially purified receptors from rat liver

Triton X-100 strongly affects the receptor binding and autophosphorylation of insulin and epidermal growth factor (EGF) in rat liver Golgi fractions and partially purified microsomal receptors. At concentration 0.05% Triton X-100 decreased the insulin receptor binding by 15% and the EGF receptor binding by 70% as compared to controls. In contrast, 0.05% Triton X-100 increased insulin-stimulated receptor autophosphorylation by more than 370% as compared to 87% in the control. Similarly, the same concentration of Triton X-100 increased the EGF-stimulated receptor phosphorylation by 65% as compared to 14% in the control. EGF receptor binding was more sensitive to the treatment of Triton. At Triton concentrations 0.2% or more, the EGF receptor binding was totally abolished while the insulin receptor binding was decreased by 50%. On the other hand, the activity of ligand-stimulated receptor phosphorylation of both insulin and EGF receptors was only slightly decreased in the presence of 0.2% Triton.

Sedimentation characteristics of subcellular vesicles associated with internalized insulin and those bound with intracellular glucose transport activity

Comparative studies were made on the sedimentation characteristics of microsomal vesicles associated with internalized [125I]iodoinsulin and those bound with intracellular glucose transport activity. Upon linear sucrose density gradient centrifugation, the internalized hormone formed a peak slightly, but significantly, on the higher density side of the peak of intracellular glucose transport activity. After a long centrifugation, the peak of 125I activity became lower and broader than that of glucose transport activity. Internalized 125I activity was also found in the medium-density microsomal fraction, which had little glucose transport activity. Accumulation of 125I activity in the medium-density fraction and that in the low-density fraction were both completed in approximately 10 min. Under basal conditions, little, if any, insulin binding activity was detectable in either the medium- or low-density microsomal fractions; in contrast, some glucose transport activity was always present in the low-density fraction. These results indicate that the subcellular distribution of internalized insulin and of intracellular glucose transport activity are different, suggesting that the pathways of intracellular processing of the insulin receptor and the glucose transport mechanism are different.

ATP and other nucleoside triphosphates inhibit the binding of insulin to its receptor

ATP, in a dose-dependent manner, inhibited the binding of 125I-insulin to its receptor in rat liver and human placental membranes. With rat liver plasma membranes an effect of ATP was detected at concentrations between 1.0 and 2.5 mmol/L, and maximal effects were seen at 10.0 mmol/L where binding was decreased by approximately 40%. The effect of ATP was one half-maximal within 10 minutes and maximal within 60 minutes. Scatchard analysis indicated that ATP was acting primarily to change the binding affinity of the insulin receptor. The effect of ATP was mimicked by CTP, GTP, and UTP, but not by ADP, 5'-AMP, 3'-AMP, 3'5'-cyclic AMP and adenosine. The ATP analog AMP-PNP had a potency approximately 10% that of ATP. The effect of ATP was not significantly influenced by inhibitors of phosphoprotein kinases and phosphoprotein phosphatases. In human placental membranes, ATP had a similar effect in inhibiting 125I-insulin binding to its receptor. Moreover, ATP was active in inhibiting insulin binding to purified human placental insulin receptors at 0.01 mmol/L, a concentration 1/100 of that needed for inhibiting binding to intact membranes. These studies indicate, therefore, that ATP and other nucleoside triphosphates influence the ability of the insulin receptor to bind insulin.

The somatomedin-C receptor of human placenta

The binding and physical characteristics of the human placenta somatomedin-C (Sm-C) receptor were investigated and compared with those of the placental insulin receptor. These receptors were found to have many similar properties, but several distinctions exist between them. The two receptors have similar binding properties. Although distinct receptors, as evidenced by specificity studies, the Sm-C and insulin receptors possess similar affinities (10 X 10(9) L/mol and 3 X 10(9) L/mol, respectively) and receptor concentrations (0.4 pm/mg and 0.2 pm/mg) in placental membranes. There is also a low-affinity, high-capacity receptor for each hormone. The intact receptors are indistinguishable in size by three methods. The Stokes radius is 7.2 nm by gel filtration, the sedimentation coefficient is 11S, and the SDS polyacrylamide gel apparent molecular weight is approximately 350,000 daltons. The two receptors differ in their pH optimum for ligand binding, their isoelectric points, and their subunit size. The pH optimum for Sm-C binding is 8.2, and for insulin binding the optimum is 7.9. The isoelectric point for the Sm-C receptor is 4.6, whereas the isoelectric point for the insulin receptor is 4.2. Although the whole receptor and the half receptors for Sm-C and insulin cannot be distinguished by size on SDS gels, the binding subunit for the Sm-C receptor appears to be 8000 daltons smaller than that of the insulin receptor (134,000 v 142,000 daltons). We conclude from these studies that the insulin and Sm-C receptors of human placenta are similar in structure but are distinct entities.

Studies of liver insulin receptors in non-obese and obese human subjects

The insulin-binding isotherms and the structural composition of human liver insulin receptors were examined by using plasma membranes that were prepared from liver biopsies of nine non-obese and 10 obese subjects undergoing elective surgery. The insulin-binding characteristics of liver membranes from non-obese subjects were quite similar to those previously described in rat liver membranes. However, when the membranes from obese subjects were compared with the non-obese group, insulin-binding activity was reduced by 50% (P less than 0.01). The reduction in obesity resulted primarily from a decrease in total receptor number, although a small decrease in receptor affinity was also observed. Insulin binding was not correlated with sex or with the fasting plasma insulin level. The insulin-binding sites of liver membranes were affinity-labeled with 125I-insulin and the cross-linking reagent, disuccinimidyl suberate. The liver membranes from both the non-obese and the obese group had heterogenous (nonreduced) insulin-binding species of 300,000, 260,000, and 150,000 mol wt, which were again comparable to the findings reported in rat liver. Sulfhydryl reduction demonstrated a major sub-unit of 125,000 and a minor component of 40,000-45,000 in both groups. These results indicate a close similarity between the hepatic insulin receptor of man and the more intensely studied rat hepatic receptor. Obesity in human subjects is associated with a loss of hepatic insulin receptors. This alteration may contribute to the insulin resistance reported in this organ as well as to obesity-mediated glucose tolerance.

Solubilization and partial purification of the high-affinity gonadoliberin receptor from the bovine pituitary gland

The high-affinity gonadoliberin (GnRH) receptor contained in a membrane preparation from frozen bovine anterior pituitary glands has been solubilized in Triton X-100 and the binding properties of the solubilized product have been examined. The radioreceptor-binding assay, using the GnRH agonist [D-Ser(t-Bu)6] des-Gly10GnRH N-ethylamide (GnRH-A) as radioligand, demonstrated that the kinetics of association and dissociation, the binding constants, as well as the specificity of receptor were not altered in the solubilized receptor preparations. Affinity chromatography on a concanavalin A-Sepharose column, with elution of adsorbed material using a solution of alpha-methyl-D-mannoside, allowed a 33-fold purification of the receptor. The Ka of the receptor thus purified was of the same order as that of the starting material, although slightly higher values were found. Only about one-half of the total receptor activity applied to the column was retained in spite of several recyclings. The other half was found in the nonadsorbed fraction. It is postulated that the detergent-solubilized fraction contains two forms of the GnRH receptor. The nonadsorbed fraction probably contains a partially or totally deglycosylated form. It is possible that the detergent-solubilization process somewhat alters the physicochemical properties of a part of the GnRH receptor molecules. Electrophoretic analysis of the purified receptor preparations, with a subsequent GnRH-binding assay, suggests that the apparent molecular mass of the high-affinity GnRH receptor, or of its monomeric form, is approximately 60,000 Da.

Pathogenetic mechanisms of hereditary diabetes mellitus

Novel laboratory techniques such as the determination of proinsulin, insulin, C-peptide and insulin receptors have allowed further subclassification of diabetes mellitus today as representing a symptom rather than a disease. Numerous pathogenetic mechanisms of diabetes have been identified, many of these being genetically determined and this may extend possibilities for genetic counselling. As a geneticist's view, the pathogenetic concept of diabetes presented here is mostly confined to genetic mechanisms, leaving aside other influences on the development of diabetes such as hormones, viruses, nutrition and drugs. Mendelian inherited diabetic disorders are related to different pathogenetic principles where possible, being speculative in some cases.

Inhibition of insulin receptor binding by phorbol esters

Phorbol esters inhibit the binding of insulin to its receptors on U-937 monocyte-like and HL-60 promyelocytic leukemia human cell lines. Within 20-30 min, exposure of these cells to 12-O-tetradecanoylphorbol 13-acetate (TPA) at 37 degrees C results in a 50% reduction of the specific binding of 125I-insulin. Half-maximal inhibition occurs at 1 nM TPA. Other tumor-promoting phorbol esters also inhibit 125I-insulin binding in a dose-dependent manner which parallels their known promoting activity in vivo. TPA does not alter the degradation of the hormone nor does it induce any shedding of its receptors in the medium. The effect of phorbol esters is dependent on temperature and cell type. It is less prominent at 22 degrees C than at 37 degrees C. It is reversible within 2 h at 37 degrees C. TPA reduces the binding of insulin predominantly by increasing its dissociation rate. This effect results in an accelerated turnover of the hormone on its receptors.

Insulin receptors on cultured murine lymphoid tumor cell lines

Eight murine lymphoid tumor cell lines have been examined for the presence of high-affinity insulin receptors. The eight cell lines included two Abelson murine leukemia virus-transformed pre-B cell lines, three plasmacytoma cell lines, and three spontaneous T-cell lymphomas from AKR mice. All of the cell lines in the B-cell series had high-affinity insulin binding sites. The apparent equilibrium association constant (Ka) for the high-affinity binding sites on these cells was 1.3-3.3 X 10(9) M-1. Two of the T-cell lymphomas had high-affinity receptor levels so low as to be undetectable in the whole cell binding assay under the conditions used for assaying the other cell lines, although in binding assays performed at very high cell densities, these two cell lines did appear to have a small number of high-affinity insulin binding sites. These results indicate that the growth stimulus provided by the tumor virus in neoplastic transformation of the AKR thymic lymphocytes differs from that provided by lectins in blast transformation of lymphocytes in that the neoplastic transforming event does not always result in the emergence of large numbers of high-affinity insulin receptors. In addition, the existence of cell lines such as the T-cell lymphomas that have nearly exclusively low-affinity binding sites suggests that the low-affinity sites may represent a distinct receptor that is not freely interconvertible with the high-affinity receptor.