Action of insulin analogues on cultured human fibroblasts reflects biological potency.
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Aminoisobutyric Acids↗
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Synthesis and properties of carbonylbis(methionyl)insulin, a proinsulin analogue which is convertible to insulin by cyanogen bromide cleavage.
The preparation and use of carbonylbis (L-methionine p-nitrophenyl ester) as a reversible cross-linking reagent for insulin are described. The reaction of 1 equiv of reagent with zinc insulin in dimethylformamide in the presence of triethylamine yields as one of the products NalphaA1, NepsilonB29-carbonylbis(methionyl)insulin, (CBM-insulin). The CBM-insulin was characterized by end group analysis and by the products formed on tryptic and chymotryptic cleavage. It possessed 91% of the immunological and 6.5% of the hormonal activity of insulin. Treatment of CBM-insulin with cyanogen bromide (CNBr) in 70% formic acid for 1 h resulted in nearly complete removal of the methionine bridge to yield insulin. A small amount of a side product was removed on DEAE-cellulose at pH 7.2 to give an overall recovery of insulin of 70-80%. Oxidative sulfitolyses of CBM-insulin gave the hexa(S-sulfonate) which was reduced with dithiothreitol to yield reduced CBM-insulin. The latter compound, containing 6 sulfhydryls, exhibited a pH-dependent circular dichroic spectrum. The form at pH 10 exhibited a spectrum typical of random coil which was converted to a form at pH 7.8 which was characterized by a negative extremum at 213 nm. The change in the spectrum at 213 nm with pH was characterized by an apparent pKa of 8.5. Studies on the reoxidation of reduced CBM-insulin were performed at pH values between 7.8 and 10 and at protein concentrations of 0.01-1 mg/ml. The best yields (ca. 85%) of the correctly paired disulfide bonds were obtained in reoxidations at pH 9.5-10 at protein concentration of 0.01-0.1 mg/ml. CBM-insulin, which had been isolated from reoxidation at high pH of the reduced CBM-insulin, was cleaved by CNBr to yield a fully active insulin in an overall yield of 60% from the reduced CBM-insulin.
Adipose Tissue↗
Synthesis and biological activity of seventeen analogues of human insulin.
We synthesized seventeen analogues of human insulin, applying the principle of stepwise, selective formation of the disulphide bonds. Most of these analogues only differ from human insulin in the replacement of a single amino acid in positions 2, 5, 6, 7, 8 and 11 of the A chain and 5, 7, 13 and 16 of the B-chain. The influence of these modifications on the physicochemical properties of the analogues is discussed. Eight analogues could be crystallized. All the analogues produce the same biological effects as insulin, but differ markedly in their potency. In isolated fat cells in vitro, [HisA8]insulin showed a relative potency of 2.46 in stimulating glucose oxidation (human insulin = 1), whereas [D-CysA6,A11]insulin had a potency of only 0.00027. Very low potency was observed when IleA2 or the half-cystines A6, A7, A11 or B7 were modified. Replacement of the invariant GlnA5 by alanine only reduced potency slightly. All the analogues are full agonists. The effects of the analogues on glucose oxidation and lipolysis are correlated, supporting the view that they are mediated by a common receptor on the fat-cell membrane. Hypoglycaemic potencies in the rat were similar to potencies in vitro. As expected, no correlation was demonstrable between antiserum binding--measured in the radioimmunoassay--and biological activity. Several results of this investigation are difficult to reconcile with the current view regarding the structure-activity relationship of insulin which appears to require further refinement.
Adipose Tissue↗
Effects of bicyclic leucine analogues on insulin release from the perfused rat pancreas and fluorescence of reduced pyridine nucleotides from perifused pancreatic islets.
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Animals↗
Binding and degradation of 125I-insulin by rat hepatocytes.
The binding and the velocity of degradation of 125I-insulin in the absence or presence of varying concentrations of native procline insulin were studied using isolated rat hepatocytes. At insulin concentrations ranging from 5 X 10(-11) to 10(-6) M, insulin degradation velocity showed a first order dependence on the total concentration of insulin bound at steady state. The overall reaction had an apparent rate constant of 0.030 +/- 0.011 min-1. Furthermore, the degradation of a given amount of 125I-insulin bound to cells was more rapid and extensive than the degradation of the same amount of insulin which had been newly exposed to fresh cells. Mid pretreatment of isolated hepatocytes with trypsin or chymotrypsin at concentrations of 5 to 20 mug/ml depressed to the same degree the amount of 125-I-insulin bound at steady state and the 125I-insulin degradation velocity. Peptide or protein hormones unrelated to insulin, including the oxidized A and B chains of insulin, failed to depress the amount of insulin bound or the velocity of insulin degradation when present at concentrations of 10-5 or 10-6 M. Over a wide range of concentrations, various synthetic insulin analogues and naturally occurring insulins depressed to the same degree the amount of 125I-insulin bound at steady state and the 125I-insulin degradation velocity. These observations suggest that insulin bound to hepatocyte plasma membranes is the substrate for insulin degradation by the liver.
Animals↗
[Insulin receptors (author's transl)].
The saturable insulin binding is linked to insulin degradation in two important target cells: The hepatocyte and the adipocyte. One of the consequences is that the changes in binding which are observed under various conditions cannot a priori be regarded as caused by either increased number of receptors or increased affinity of the binding site. This observation raises new questions. For instance, could the effect of insulin be mediated by a fragment of the molecule? No evidence which is available for the moment seems to rule out this hypothesis. The findings with insulin analogues, the kinetics of insulin binding and activation and the effect of mild trypsin treatment, would equally well support the hypothesis that the binding itself causes activation of hexose transport and that degradation secondary to binding mediates the activation.
Adipose Tissue↗
Insulin receptors in the heart muscle. Demonstration of specific binding sites and impairment of insulin binding in the plasma membrane of the obese hyperglycemic mouse.
The presence of insulin receptors in the heart muscle was investigated by measuring the binding of 125I-insulin to specific subcellular fractions of the rat and mouse myocardium. 125I-insulin bound to the plasma membrane fraction with a high degree of specificity and affinity. Insulin analogues competed with 125I-insulin in direct proportion to their biologic potency in vitro. Unlabeled insulin within the range of its concentrations in vivo inhibited 15 to 60 per cent of the 125I-insulin binding. The specific binding sites were finite in number and represented about 90 per cent of the total binding. The insulin-binding capacity of the plasma membrane fraction was twelve- to fifteenfold higher than that of the mitochondrial fraction. As in the liver, the binding was time- and temperature-dependent with a slower but higher binding achieved at a lower temperature. The binding sites appeared to be heterogeneous with respect to affinity. At 5 degrees C., the "higher-affinity" site had a K of about 2 times 10(9) M-1. No more than 10 per cent of the 125I-insulin was degraded by the heart plasma membranes after one hour at 30 degrees C. or twenty-two hours at 5 degrees C. Studies in the obese hyperglycemic (ob/ob) mouse revealed that the insulin binding is impaired in the heart muscle of this animal. Over a wide range of insulin concentrations, the plasma membrane fraction of ob/ob mice bound only 25 to 40 per cent as much insulin as did membranes of the thin littermates, suggesting that, as in the liver, the fat tissue, and the thymic lymphocyte, the number of insulin-binding sites is decreased in the heart of the ob/ob mouse. This defect selectively affected the plasma membrane fraction and could not be explained by differences in membrane purification or insulin-degrading activity. Heart and liver membranes of forty-hour fasted ob/ob mice bound two to three times as much insulin as did membranes of ob/ob mice fed ad libitum. These studies demonstrate and characterize the binding of insulin to heart muscle membranes; they extend to the heart muscle the insulin receptor defect also found in liver membranes and cells, in fat cell membranes, and in thymic lymphocytes of the ob/ob mouse.
Animals↗
The insulin receptor of the turkey erythrocyte: similarity to mammalian insulin receptors.
Avian erythrocytes possess insulin receptors which have binding properties that are virtually identical to those of the well studied mammalian insulin receptors. The affinity for porcine insulin was identical for the turkey and mammalian receptors over the entire range of insulin concentrations, as was the affinity of each of four insulin analogues which differed 300-fold in biological potency. Insulin induced acceleration of dissociation (i.e., the negatively cooperativite site-site interaction) was indistinguishable over a 10(6) range of insulin concentrations. Sharp pH dependence of binding was identical for turkey and mammalian receptors. The effects of temperature on association, dissociation and steady state binding were also identical. Thus, although birds and mammals have evolved separately for 300 million years there has been little change in the properties of the insulin receptor over this time period.
Allosteric Regulation↗
Retention and degradation of 125I-insulin by perfused livers from diabetic rats.
The retention of degradation of insulin by isolated perfused liver have been examined. Noncyclically perfused livers from streptozotocin-diabetic rats retained 25% and degraded 10% of 125I-insulin administered as a 1-min pulse. On gel filtration (Sephadex G50F), the degradation products released into the vascular effluent eluted in the salt peak. During the 45-min interval after the end of the 125I-insulin infusion, 0.19% of the total dose was excreted in the bile. 60-90% of this material consisted of iodinated, low-molecular-weight degradation products. Inclusion of native insulin with the 125I-insulin in the pulse depressed both the retention and degradation of iodinated material; however, this reflected increased retention and degradation of the total insulin dose (125I-insulin plus native hormone). The log of the total amounts of insulin retained and degraded were linearly related to the log of the total amount of insulin infused at concentrations between 12.7 nM and 2.84 muM. Increasing the amount of native insulin in the infused pulse also depressed the total amount of iodinated material found in the bile and led to the appearance in the bile of intermediate-sized degradation products that did not simultaneously appear in the vascular effluent. Addition of high concentrations of glucagon to the infused 125I-insulin had no effect on the retention or degradation of the labeled hormone, or on the apparent size and amount of iodinated degradation products found in the vascular effluent or in the bile. Preinfusion of concanavalin A inhibited both 125I-insulin retention and degradation. A greater depression by concanavalin A of degradation than binding was also observed with isolated hepatocytes. In contrast to 125I-insulin, the retention and degradation of two iodinated insulin analogues of relative low biological potency, proinsulin and desalanyl-desasparaginyl insulin, were small. The amount of radioactivity appearing in the bile after infusion of these analogues was almost negligible. However, degradation products of these analogues that appeared in the bile and in the vascular effluent was qualitatively similar to those found after the infusion of 125I-insulin. Our findings suggest that the rapid initial uptake of 125I-insulin after its infusion into noncyclically perfused liver, as well as its subsequent degradation, behaves in a qualitatively similar fashion to the binding of 125I-insulin and its degradation by isolated rat hepatocytes. This uptake and the subsequent phase of degradation may be attributable to binding of insulin at specific recognition sites, preliminary to its transfer to a degradative site(s) presumed to be located inside the cell.
Animals↗
Gross structural changes in isolated liver cell plasma membranes upon binding of insulin.
The addition of 10(-9) M insulin to a suspension of rat liver plasma membranes increases the overall lipid microviscosity, eta, by about 10--20%. The effect is confined to physiological concentrations of the hormone and is highly specific. The specificity was demonstrated in experiments where insulin analogues were added to liver plasma membranes and where insulin was added to human erythrocyte membranes. In both of these experiments practically no change in eta was detected. Upon in vitro enrichment of the membrane cholesterol, eta exceeded the level mediated by insulin binding, and the addition of 10(-9) M insulin to the cholesterol-enriched membranes did not further increase eta. Concomitant to the increase in eta upon insulin binding, the overall degree of exposure of the membrane protein, presumably to both sides of the membrane, is substantially increased. This effect is in line with the notion of vertical displacement of membrane proteins induced by changes in eta. The observed structural modulation can account for the effect of insulin on unrelated membrane responses, as well as for the negative cooperativity of insulin binding.
Animals↗
The NSILA-s receptor in liver plasma membranes. Characterization and comparison with the insulin receptor.
NSILA-s (nonsuppressible insulin-like activity, soluble in acid ethanol) is a serum peptide that has insulin-like and growth-promoting activities. We have demonstrated previously that liver plasma membranes possess separate receptors for NSILA-s and insulin and have characterized the insulin receptor in detail. In the present study we have characterized the properties and specificity of the NSILA-s receptor and compared them to those of the insulin receptor in the same tissue. Both 125I-NSILA-s and 125I-insulin bind rapidly and reversibly to their receptors in liver membranes; maximal NSILA-s binding occurs at 20 degrees while maximal insulin binding is seen at 1-4 degrees. The pH optimum for NSILA-s binding is broad (6.0 to 8.0), in contrast to the very sharp pH optimum (7.5 to 8.0) for insulin binding. Both receptors exhibit a high degree of specificity. With the insulin receptor, NSILA-s and insulin analogues compete for binding in proportion to their insulin-like potency: insulin greater than proinsulin greater than NSILA-s. With the NSILA-s receptor, NSILA-s is most potent and the order is reversed: NSILA-s greater than proinsulin greater than insulin. Furthermore, six preparations of NSILA-s which varied 70-fold in biological activity competed for 125I-NSILA-s binding in order of their potencies. NSILA-s which had been inactivated biologically by reduction and aminoethylation and growth hormone were less than 1/100,000 as potent as the most purified NSILA-s preparation. Purified preparations of fibroblast growth factor, epidermal growth factor, nerve growth factor, and somatomedins B and C were less than 1% as effective as NSILA-s in competing for the 125I-NSILA-s suggesting that these factors act through other receptors. In contrast, somatomedin A was 10% as active as NSILA-s and multiplication-stimulating activity was fully as active as NSILA-s in competing for the NSILA-s receptor. Analysis of the data suggests that there are approximately 50 times more insulin receptors than NSILA-s receptors per liver cell, while the apparent affinity of NSILA-s receptors is somewhat higher than that of the insulin receptor.
Animals↗
The relation of polypeptide hormone structure and flexibility to receptor binding: the relevance of X-ray studies on insulins, glucagon and human placental lactogen.
Thr relevance of the crystal structure of the polypeptide hormones, insulin, glucagon and human placental lactogen to conformation and flexibility in solution and to receptor binding is considered. X-ray studies for crystal forms of glucagon, human placental lactogen and three insulin derivatives (A1 acetyl insulin, A1-t-butoxy carbonyl insulin and A1 2,2-dimethyl-3-formyl-L-thiazolidine-4-carbonyl insulin) are reported. Neither glucagon nor human placental lactogen are as ordered as insulin in the crystal form. Glucagon crystals undergo distinct transformations on changing the pH of the mother liquor from pH 9.5 to pH 6, indicating that the glucagon molecule is flexible in the crystal, as it is in solution. On the other hand all insulin analogues have a similar three dimensional structure to that of native insulin. Three dimensional difference Fourier studies of two insulin derivatives at 3 A resolution indicate the position of the modifying groups and define the small conformational changes which have occurred. The in vitro biological activity and receptor binding decrease with the increasing size of the group added to A1. The correlation of the structure analysis with the biological data strongly implicate a region close to A1 in receptor binding. Insulin appears to bind to the receptor in a specific conformation similar to that observed in the crystal structure and in solution; amino acid residues which are separated in the primary structure but brought into close juxtaposition in the tertiary structure are important for full potency.
Animals↗
Insulin action on adipocytes. Evidence that the anti-lipolytic and lipogenic effects of insulin are mediated by the same receptor.
1. The dose-response relationships of insulin stimulation of lipogenesis and inhibition of lipolysis were studied simultaneously by using rat adipocytes to determine whether these different effects of insulin are mediated through the same or different sets of receptors. 2. The sensitivity (defined as the concentration of insulin required to produce a half-maximal effect) of the stimulated lipogenic response to insulin was not significantly different from the sensitivity of the anti-lipolytic response to insulin. The addition of different adrenaline and glucose concentrations did not alter the half-maximal concentration of insulin required to inhibit lipolysis. 3. The specificities of the lipogenic and antilipolytic responses were studied by using insulin analogues. The sensitivities of the lipogenic and anti-lipolytic responses were the same for five chemically modified insulins and hagfish insulin, which have potencies compared with bovine insulin of between 3 and 90%. 4. Starving rats for 48h significantly increased the sensitivities of both the antilipolytic and lipogenic responses to insulin, but the changes in the sensitivities of both lipogenesis and anti-lipolysis returned to that of fed rats. 5. We conclude that insulin stimulates lipogenesis and inhibits lipolysis over the same concentration range. These observations provide powerful evidence that the different effects of insulin are mediated through the same set of receptors.
Adipose Tissue↗
Insulin receptors of skeletal muscle: specific insulin binding sites and demonstration of decreased numbers of sites in obese rats.
A membrane preparation was obtained from rat striated muscle. The preparation used has been shown to contain plasma membranes by electron microscopy as well as by enrichment in specific activity of both a plasma membrane enzyme "marker" (5'-nucleotidase) and cell surface 125I-incorporated radioactivity. The characteristics of 125I-insulin binding to this striated muscle preparation were studied, and it was found that 125I-insulin readily and specifically binds to this membrane preparation. The binding reaction was time, pH, and temperature dependent with optimal steady-state binding conditions occurring at 20 degrees C and at pH 7.6. Under these conditions (20 degrees C, pH 7.6) skeletal muscle plasma membranes displayed little ability to degrade insulin. Binding of 125I-insulin was readily inhibited at physiologic concentrations of unlabeled insulin and the specificity of this receptor for insulin was demonstrated by finding that high concentrations of glucagon, b-LH, b-FSH, p-PRL, hCG, TSH, and HGH were without effect on 125I-insulin binding and that insulin analogues inhibited binding in proportion to their biologic activity. When membranes from older, fatter rats were compared to membranes from younger, lean animals, 5'-nucleotidase specific activity and insulin degrading activity were found to be comparable. On the other hand, insulin binding to membrane receptors was decreased 30%-40% in the older, fatter animals. Thus, these studies indicate that (1) specific insulin receptors exist in skeletal muscle plasma membranes, and (2) membranes from older, fatter rats have fewer receptors than those from younger, lean animals.
Age Factors↗
Insulin receptors in cultured human fibroblasts.
In order to study human insulin resistance, we have first characterized the interaction of insulin with specific insulin receptors in cultures of normal human fibroblasts. 125 I-insulin bound rapidly to human fibroblasts in suspension at 15 degrees, achieving steady state between one and three hours. Insulin was not degraded during the binding assays. In competitive binding experiments, 2 ng/ml. (3.3 X 10(-10) M) of unlabeled insulin reduced 125 I-insulin binding by 50 per cent. Insulin analogues competed for binding in proportion to their biologic potencies. A curvilinear Scatchard plot was obtained, suggesting the existence of negatively cooperative site-site interactions among the insulin receptors. This was confirmed directly by studies of the dissociation kinetics. The high affinity, specificity, and negative cooperativity of the fibroblast insulin receptor closely resembles the properties of other human insulin receptors. The cultuted human fibroblast should prove a useful tissue for the study of insulin-resistant states in man.
Cells, Cultured↗
Binding and degradation of insulin by human peripheral granulocytes. Demonstration of specific receptors with high affinity.
The interaction of insulin with human circulating granulocytes was studied with the use of 125I-insulin. Human granulocytes, isolated from blood by the Böyum technique, showed high insulin-degrading activity in vitro which almost obscured the presence of specific, high affinity binding sites. Degradation, measured by trichloroacetic acid precipitation and by binding to well characterized insulin receptors on cultured human lymphocytes (IM-9 line), was due to extracellular as well as cell-bound enzymes. Degradation was enhanced by Ca2+ and thiols and inhibited by various protease inhibitors and sulfhydryl-blocking reagents. Phenylmethylsulfonyl fluoride (5 X 10(-4) M), a serine protease inhibitor, was the most potent and inhibited 125I-insulin degradation by 80 to 90%. Tert-butyl hydroperoxide (2 X 10(-3) M), a glutathione-oxidizing reagent, inhibited degradation by 35 to 50%, possibly due to an effect on a glutathione-insulin transhydrogenase. Neither of the inhibitors affected cell viability. In the presence of inhibitors of degradation, binding sites for insulin with high affinity were detected, which by multiple criteria were true insulin receptors. Binding to these sites was rapid, saturable, and reversible with about 1000 sites/cell. The Hill coefficient for binding was 0.7, and the Scatchard plot of B/F versus B was curvilinear, due to site-site interactions of the negative cooperative type; the latter were demonstrated directly by kinetic studies. As shown previously for all other insulin receptors, binding was highly pH-dependent, and insulin analogues had affinities for these sites that closely correlated with their biological potencies.
Binding Sites↗
The mobile receptor hypothesis and "cooperativity" of hormone binding. Application to insulin.
The mobile receptor hypothesis has been proposed to describe the process by which hormone receptor binding initiates a biological response; it states that receptors, which can diffuse independently in the plane of the membrane, reversibly associate with effectors to regulate their activity. The affinity for effector is greater when the receptor is occupied by hormone. A mathematical expression of the mobile receptor hypothesis is used to show that: (1) The predicted kinetics of hormone receptor binding may be indistinguishable from "negative cooperativity." (2) Receptor occupancy and biological response may be coupled in a non-linear fashion. By choosing specific parameters, most of the existing data on insulin binding and biological responses can be explained in terms of the mobile receptor hypothesis. Thus, the following are easily explained: (1) A single homogeneous receptor may appear kinetically to be composed of two classes (of high and low affinity) of receptors. (2) Occupancy of the apparent class of high affinity receptors is related linearly to the biological response. (3) The same receptor in different tissues may appear to have different affinity. (4) The binding of different biologically active insulin analogues may exhibit different degrees of "cooperativity." These considerations may also be pertinent to interpretations of other hormone-receptor systems and of various ligand-macromolecule interactions.
Binding Sites↗
Effect of a 5'-substituted adenosine analogue (744c96) on insulin and glucagon release in isolated rat islets of Langerhans.
The effect of adenosine-5'-(N-ethylcarboxamide) (744-96), a highly vasoactive adenosine analogue, was investigated on insulin and glucagon release from isolated rat islets of Langerhans. The compound inhibited glucose-induced insulin release in a dose-dependent manner (concentration range 10(-8) to 10(-5 M). No effect of 744-96 was observed on basal or on arginine-induced glucagon release. By contrast, 744-96 (5 microgram/kg i.v.) produced a significant increase in plasma glucagon and glucose levels in normal as well as in reserpinized rats. The observed failure to promote glucagon release from isolated islets may be attributable to the isolation procedure, to a release of glucagon in vivo from non-pancreatic stores, or to a non-specific effect of 744-96 on glucagon secretion.
Adenosine↗