Insulin-stimulated tyrosine phosphorylation of the insulin receptor in detergent extracts of human placental membranes. Comparison to epidermal growth factor-stimulated phosphorylation

Insulin receptor tyrosine kinase activity is unaltered in ob/ob and db/db mouse skeletal muscle membranes

Insulin binding and insulin receptor tyrosine kinase activity were examined in two rodent models with genetic insulin resistance using partially-purified skeletal muscle membrane preparations. Insulin binding activity was decreased about 50% in both 12-week (219 +/- 184 vs 1255 +/- 158 fmoles/mg, p less than 0.01) and 24-week old (2120 +/- 60 vs 1081 +/- 60 fmoles/mg, p less than 0.01) ob/ob mice. In contrast, insulin binding to membrane derived from 24-week old db/db mice was not significantly different from lean controls (1371 +/- 212 vs 1253 +/- 247 fmoles/mg). Insulin-associated tyrosine kinase activity of membranes from ob/ob skeletal muscle was decreased, compared to its normal lean littermate, when compared on a per mg of protein basis in both 12-week (37 +/- 3 vs 21 +/- 3 pmoles/min/mg, p less than 0.05) and 24-week old (71 +/- 5 vs 37 +/- 6 pmoles/min/mg, p less than 0.01) mice. However, no significant differences in kinase activities were observed when the data were normalized and compared on a per fmole of insulin-binding activity basis for the 12-week (12 +/- 1 vs 11 +/- 2) and 24-week (27 +/- 2 vs 20 +/- 3) age groups. Insulin receptor tyrosine kinase activity of db/db skeletal muscle membranes was not different than its normal lean littermate whether expressed on a protein (34 +/- 7 vs 30 +/- 3) or fmole of insulin-binding activity (21 +/- 4 vs 18 +/- 4) basis. These data suggest that insulin receptor tyrosine kinase is not associated with the insulin resistance observed in ob/ob and db/db mice and demonstrate differences in receptor regulation between both animal models.

Catecholamines and tumour promoting phorbolesters inhibit insulin receptor kinase and induce insulin resistance in isolated human adipocytes

The effect of the catecholamine isoprenaline (10(-5) mol/l) and of the tumour promoting phorbolester tetradecanoyl-beta-phorbol acetate (10(-9) mol/l) on insulin stimulated 3-O-methyl-glucose transport was studied in freshly isolated human adipocytes. Both substances reduced the maximal responsiveness of the glucose transport system to insulin by approximately 50%. To test if this is caused by inhibition of the insulin receptor kinase the receptor from phorbolester and isoprenaline treated cells was solubilized, partially purified and its kinase activity studied in vitro. Insulin stimulated 32P-incorporation into the beta-subunit of the insulin receptor of phorbolester or isoprenaline treated cells was reduced to 20-60% of the values found with receptor from control cells at insulin concentrations between 10(-10) mol/l and 10(-7) mol/l. This inhibition of kinase activity of receptor from phorbolester and isoprenaline treated cells was observed at nonsaturating adenosine triphosphate levels (5 mumol/l), and it could be overcome with higher concentrations of gamma-32P-adenosine triphosphate in the phosphorylation assay. A Lineweaver Burk analysis of the insulin stimulated receptor phosphorylation revealed that the Michaelis constant for adenosine triphosphate of the receptor kinase from phorbolester and isoprenaline treated cells was increased to greater than 100 mumol/l compared with less than 50 mumol/l for receptor from control cells. We conclude from the data that catecholamine and phorbolester treatment of human adipocytes modulates the kinase activity of the insulin receptor by increasing its Michaelis constant for adenosine-triphosphate, and propose that this modulation of receptor kinase is a mechanism that can contribute to the pathogenesis of insulin resistance in human fat cells.

Phosphorylation of glycolytic and gluconeogenic enzymes by the insulin receptor kinase

Various glycolytic and gluconeogenic enzymes were tested as substrates for the insulin receptor kinase. Phosphofructokinase and phosphoglycerate mutase were found to be the best substrates. Phosphorylation of these enzymes was rapid, stimulated 2- to 6-fold by 10(-7) M insulin and occurred exclusively on tyrosine residues. Enolase, fructose 1,6-bisphosphatase, lactate dehydrogenases in decreasing order, were also subject to insulin-stimulated phosphorylation but to a smaller extent than that for phosphofructokinase or phosphoglycerate mutase. The phosphorylation of phosphofructokinase was studied most extensively since phosphofructokinase is known to catalyze a rate-limiting step in glycolysis. The apparent Km of the insulin receptor for phosphofructokinase was 0.1 microM, which is within the physiologic range of concentration of this enzyme in most cells. Tyrosine phosphorylation of phosphofructokinase paralleled autophosphorylation of the beta-subunit of the insulin receptor with respect to time course, insulin dose response (half maximal effect between 10(-9) and 10(-8) M insulin), and cation requirement (Mn2+ greater than Mg2+ much greater than Ca2+). Further study will be required to determine whether the tyrosine phosphorylation of phosphofructokinase plays a role in insulin-stimulated increases in glycolytic flux.

Dithiothreitol activation of the insulin receptor/kinase does not involve subunit dissociation of the native alpha 2 beta 2 insulin receptor subunit complex

The subunit composition of the dithiothreitol- (DTT) activated insulin receptor/kinase was examined by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis and gel filtration chromatography under denaturing (0.1% SDS) or nondenaturing (0.1% Triton X-100) conditions. Pretreatment of 32P-labeled insulin receptors with 50 mM DTT followed by gel filtration chromatography in 0.1% SDS demonstrated the dissociation of the alpha 2 beta 2 insulin receptor complex (Mr 400,000) into the monomeric 95,000 beta subunit. In contrast, pretreatment of the insulin receptors with 1-50 mM DTT followed by gel filtration chromatography in 0.1% Triton X-100 resulted in no apparent alteration in mobility compared to the untreated insulin receptors. Resolution of this complex by nonreducing SDS-polyacrylamide gel electrophoresis and autoradiography demonstrated the existence of the alpha 2 beta 2 heterotetrameric complex with essentially no alpha beta heterodimeric or free monomeric beta subunit species present. This suggests that the insulin receptor can reoxidize into the Mr 400,000 complex after the removal of DTT by gel filtration chromatography. Surprisingly, these apparently reoxidized insulin receptors were also observed to be functional with respect to insulin binding, albeit with a 50% decrease in affinity for insulin and insulin stimulation of the beta subunit autophosphorylation. To prevent reoxidation, the insulin receptors were pretreated with 50 mM DTT followed by incubation with excess N-ethylmaleimide prior to gel filtration chromatography in 0.1% Triton X-100. Under these conditions the insulin receptors migrated as the Mr 400,000 alpha 2 beta 2 complex.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin resistance in uremia: in vitro model in the rat liver using human serum to study mechanisms

We have previously demonstrated in a rat model of chronic uremia that the liver is resistant to insulin. To further investigate the mechanism(s) of insulin resistance in uremia, primary cultures of normal rat hepatocytes were incubated with varying dilutions (1/10 to 1/10,000) of sera from undialyzed end stage uremic and normal humans for 20 hours. We then studied insulin action, binding, and postbinding events. Dilutions of uremic sera as low as 1/1,000 rendered the hepatocytes resistant to maximal concentrations of insulin with regard to [14C]acetate incorporation into lipids. The dose response curve for insulin-stimulated [14C]aminoisobutyric acid uptake demonstrated a shift to the right in hepatocytes incubated with uremic sera when compared with those incubated with normal sera. The 125I-insulin binding sites and affinity, 125I-insulin internalization and degradation, insulin receptor structure, autophosphorylation of the insulin receptor, and its tyrosine-specific kinase activity were normal in the hepatocytes rendered resistant to insulin by uremic sera. However, these cells failed to generate the chemical mediator or second messenger of insulin action, as assessed by its ability to stimulate pyruvate dehydrogenase (PDH) in liver mitochondria from normal rats. We concluded that uremic sera renders normal rat hepatocytes resistant to insulin. Insulin resistance is a postinsulin receptor kinase defect possibly due to lack of the generation of the chemical mediator of insulin action. This in vitro cell model may be useful to further define the mechanism(s) and the serum factor(s) responsible for insulin resistance in uremia in the absence of complicating hormonal and substrate changes that occur in vivo.

Autophosphorylation and kinase activity of insulin receptor in diabetic rats

Insulin resistance is observed in insulin-deficient diabetic states in spite of an increase in insulin binding to its target cells. To characterize this type of insulin resistance, autophosphorylation and kinase activity of the insulin receptor on liver was studied with streptozotocin (STZ)-induced and BB diabetic rats. Insulin binding capacity was increased in proportion to the severity of the diabetic state in the STZ rat. In the diabetic BB rat, the insulin binding capacity was also increased, and this was partially normalized by insulin treatment. By contrast, insulin-stimulated autophosphorylation of the beta-subunit of the insulin receptor was decreased in proportion to the severity of the diabetic state in the STZ rat. Peptide mapping by reverse-phase high-performance liquid chromatography revealed a decrease in labeling at all sites of autophosphorylation. Kinase activity of the insulin receptor to exogenous substrates was also decreased in proportion to the diabetic state. In the BB rat, autophosphorylation and kinase activity of the insulin receptor were both decreased in the diabetic state and partially normalized by insulin treatment. In addition to the beta-subunit of insulin receptor, a 170 kdalton phosphotyrosine-containing protein was also identified in the glycoprotein fraction of liver. Although the phosphorylation of this protein was not insulin dependent, it was decreased markedly in the diabetic state. This protein is immunologically distinct from the insulin receptor, but is rich in phosphotyrosine. Based on its size and phosphotyrosine content, this protein may be the epidermal growth factor receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation of placental-type alkaline phosphatase associated with human syncytiotrophoblast membranes using monoclonal antibodies

Placental-type alkaline phosphatase (PLAP) has been purified from human term syncytiotrophoblast microvillous plasma membranes by immunoaffinity chromatography using the immobilized monoclonal antibodies H317 and H315. The eluted enzymatically active PLAP had a molecular weight in the regions of 115K and 130K, with an occasional higher molecular weight form at 180K. On reduction to a 66K monomeric form, both enzymatic activity and immunoactivity against the PLAP-reactive monoclonal antibodies were lost. The monomeric form of syncytiotrophoblast microvillous plasma membrane- (StMPM-)associated PLAP displayed electrophoretic polymorphism on 2D-PAGE.

Purification and autophosphorylation of insulin receptors from rat skeletal muscle

Insulin receptors of rat skeletal muscle were purified by first extracting a plasma membrane-enriched pellet obtained from a muscle homogenate with Triton X-100, followed by WGA-Sepharose and insulin-Sepharose affinity chromatography. Routinely, 4-5 micrograms of purified receptor were obtained from 15 g of tissue. The purified receptors are composed of two major polypeptides with molecular weights of 130,000 and 95,000, respectively. The binding of [125I]insulin by the purified receptors was analyzed by a Scatchard plot. There are at least two binding components. The high-affinity component, with an apparent association constant (Ka) of 2.0 X 10(9) M-1, comprises 10% of the total insulin binding sites; while the low-affinity component, with a Ka value of 1.4 X 10(8) M-1, represents 90% of the binding sites. Assuming the insulin receptor to have a molecular weight of 300,000, the receptor binds 1.7 mol of insulin per mol at saturation. Insulin is capable of stimulating the autophosphorylation of the beta-subunit of the muscle insulin receptor (Mr 95,000) by 5-10-fold. The stoichiometry of this phosphorylation reaction was determined as 0.8 phosphate per insulin binding site after a 10 min incubation with 100 nM insulin. In a previous report, I showed that the insulin stimulation of glucose transport in diaphragms from neonatal rats was small, even although the diaphragms had normal levels of insulin receptors and glucose transporters (Wang, C. (1985). Proc. Natl. Acad. Sci. USA 82, 3621-3625). To determine whether or not receptor autophosphorylation might be related to this insensitivity to insulin, the level of receptor phosphorylation was quantitated in diaphragms from rats at different stages of development. Autophosphorylation remains unchanged from birth to 21 days of age, suggesting that the lower insulin-stimulated glucose uptake by diaphragms at early stages of postnatal development as compared to that by diaphragms of older rats, is not due to a difference in receptor kinase.

Insulin receptors: structure and function

The recent characterization of the human insulin receptor structure and its intrinsic tyrosine kinase activity represent major advances in our understanding of the mechanism of insulin action. It is reasonable to think that the insulin-induced autophosphorylation and activation of its receptor kinase represent an important event in the action of insulin on cell metabolism and growth. The fundamental research reviewed may be followed by the discovery of molecular receptor defects in clinical syndromes of insulin resistance.

Binding of IGF I and IGF I-stimulated phosphorylation in canine renal basolateral membranes

To characterize the interaction of the renal proximal tubular cell with insulin like growth factor I (IGF I), we measured binding of 125I-IGF I to proximal tubular basolateral membranes from dog kidney and induced IGF I-stimulated phosphorylation of basolateral membranes. Specific binding of 125I-IGF I to basolateral membranes was observed that was half-maximal at between 10(-9) and 10(-8) M IGF I. 125I-IGF I was affinity cross-linked to a 135,000 Mr protein in basolateral membranes that was distinct from the alpha-subunit of the insulin receptor and from the IGF II receptor. IGF I-stimulated phosphorylation of a 92,000 Mr protein was effected in detergent-solubilized membranes incubated with 100 microM [gamma-32P]ATP. The 32P-labeled protein was distinct from the beta-subunit of the insulin receptor, the 32P phosphorylation of which was stimulated by insulin. We conclude that specific receptors for IGF I are present in the basolateral membrane of the renal proximal tubular cell. Physiological actions of IGF I at this nephron site may occur through the binding of this peptide circulating in plasma, to specific basolateral membrane receptors, followed by IGF I stimulated phosphorylation.

Insulin receptors of chicken liver and brain. Characterization of alpha and beta subunit properties

Receptors on membranes of chicken liver and brain bound porcine 125I-insulin in a specific and temperature-dependent manner. Competition with unlabeled insulin derivatives exhibited typical insulin potency ratios, i.e. chicken greater than porcine insulin greater than human proinsulin (2.1/1/0.02). Apparent binding affinity was higher in brain with a 50% inhibition of tracer binding of 1.3 +/- 0.2 nM porcine insulin as compared to 2.8 +/- 0.3 nM in liver. The apparent molecular mass of the 125I-insulin cross-linked alpha subunit of the insulin receptor was 139 +/- 2 kDa for chicken liver and 127 +/- 2 kDa for chicken brain. These molecular masses were similar to those of rat liver and brain insulin receptors. Neuraminidase treatment of the cross-linked insulin receptor increased the mobility of the alpha subunit from liver but did not affect that from brain, suggesting a difference in the glycosylation of the chicken brain alpha subunit as previously described in the rat. Despite this change, both receptors could be purified on wheat germ agglutinin (WGA) chromatography after Triton solubilization. In the presence of CTP and vanadate (phosphatase inhibitors) insulin-stimulatable tyrosine-specific phosphorylation of exogenous substrates was demonstrated with chicken liver and brain receptors. The reaction was dependent on Mg2+ and Mn2+. As noted with other insulin receptors, the best artificial substrate for phosphorylation was poly(Glu,Tyr)4:1. In both chicken liver and brain the smallest effective insulin dose as well as maximal stimulation of phosphorylation of the substrate was similar to that seen with rat liver, and in all three tissues chicken insulin was more potent than porcine insulin. In chicken liver an active ATP hydrolytic activity copurified with the insulin receptors during WGA chromatography. Further purification using S-300 Sephacryl filtration or affinity (insulin-biotin-avidin) chromatography could dissociate the phosphorylation and the hydrolytic activities. Gel electrophoresis, under reducing conditions revealed beta subunits with apparent Mr of 97-99 kDa in chicken liver and brain, which were phosphorylated in the presence of insulin. Similar apparent molecular masses have been described for the beta subunit of rat liver receptors. These studies suggest that both chicken brain and liver insulin receptors exhibit coupling of alpha and beta subunits with fully active tyrosine kinase and that the structural difference of the brain insulin receptor is widespread and phylogenetically old.

Combinations of palytoxin or 12-O-tetradecanoylphorbol-13-acetate and recombinant human insulin growth factor-I or insulin synergistically stimulate prostaglandin production in cultured rat liver cells and squirrel monkey aorta smooth muscle cells

In the presence of 12-O-tetradecanoylphorbol-13-acetate (TPA) or the non-TPA-type tumor promoter, palytoxin, recombinant human insulin growth factor-I (IGF-I) and insulin synergistically stimulate prostaglandin production in rat liver cells (the C-9 cell line). Combinations of palytoxin or TPA with recombinant human IGF-I or insulin also synergistically stimulate deesterification of cellular lipids in C-9 cells prelabelled with [3H]arachidonic acid. With both types of stimulations, prostaglandin production or deesterification, the synergistic response of the IGF-I and insulin is greater with palytoxin than with TPA. Production of prostaglandin E2 and F2 alpha by squirrel monkey smooth muscle cells incubated in the presence of TPA and insulin also is greater than the sum of the two effects taken independently.

Characterization of phosphatidylinositol kinase activity associated with the insulin receptor

Various lipids were tested as substrates for the insulin receptor kinase using either receptor partially purified from rat hepatoma cells by wheat-germ-agglutinin-Sepharose chromatography or receptor purified from human placenta by insulin-Sepharose affinity chromatography. Phosphatidylinositol was phosphorylated to phosphatidylinositol 4-phosphate by the partially purified insulin receptor. In contrast, phosphatidylinositol 4-phosphate and diacylglycerol were not phosphorylated. In some, but not all preparations of partially purified insulin receptor, the phosphatidylinositol kinase activity was stimulated by insulin (mean effect 33%). Phosphatidylinositol kinase activity was retained in insulin receptor purified to homogeneity. Insulin regulation of the phosphatidylinositol kinase was lost in the purified receptor; however, dithiothreitol stimulated both autophosphorylation of the purified receptor and phosphatidylinositol kinase activity in parallel about threefold. (Glu80Tyr20)n, a polymeric substrate specific to tyrosine kinases, inhibited the phosphatidylinositol kinase activity of the purified receptor by greater than 90% and inhibited receptor autophosphorylation by 67%. Immunoprecipitation by specific anti-receptor antibodies depleted by greater than 90% the phosphatidylinositol kinase activity in the supernatant of the purified receptor and the phosphatidylinositol kinase activity was recovered in the precipitate in parallel with receptor autophosphorylation activity. These characteristics of the phosphatidylinositol kinase activity of the purified insulin receptor and its metal ion preference paralleled those of the receptor tyrosine kinase activity and differed from bulk phosphatidylinositol kinase activity in cell extracts, which was not significantly inhibited by (Glu80Tyr20)n, stimulated by dithiothreitol or depleted by immunoprecipitation with anti-(insulin receptor) antibody. These results suggest that the insulin receptor is associated with a phosphatidylinositol kinase activity; however, this activity is not well regulated by insulin. This kinase appears to be distinct from the major phosphatidylinositol kinase(s) of cells. Its relationship to insulin action needs further study.

Concanavalin A- and calcium-dependent phosphorylation of a protein of 80 kDa in mouse lymphocytes rendered permeable to exogenously added [gamma-32P]ATP

The phosphorylation of a protein of 80 kDa in permeable mouse lymphocytes is shown to be dependent both on exogenously added calcium and on concanavalin A. Lymphocyte plasma membranes are rendered permeable to exogenously added [gamma-32P]ATP and other small molecules by treatment with 20 micrograms/ml alpha-lysophosphatidylcholine for 1 min on ice. Treated cells are permeable to Trypan blue dye and exhibit phosphatidylinositol turnover in response to concanavalin A stimulation. As determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autofluorography, maximal phosphorylation of this protein occurs 5 min after addition of 20 microM calcium and 4 micrograms/ml concanavalin A. Exogenously added cyclic nucleotide cofactors do not enhance the phosphorylation of this 80 kDa protein, nor do inhibitors of calcium or calmodulin-dependent kinases suppress it, although in each case, other proteins are affected. In contrast, an inhibitor of the calcium-activated, phospholipid-dependent protein kinase (protein kinase C), H-7, strongly suppresses the phosphorylation of the 80 kDa protein. The tumor-promoting phorbol ester, 12-O-tetradecanoylphorbol 13-acetate, a known activator of protein kinase C, significantly increases the phosphorylation of the 80 kDa protein. Finally, this protein is phosphorylated at a serine residue. These results taken together suggest that it is a substrate for protein kinase C. The possibility that it may also be an element of the concanavalin A signal transduction mechanism is discussed.

Kinetic properties of the insulin receptor tyrosine protein kinase: activation through an insulin-stimulated tyrosine-specific, intramolecular autophosphorylation

The insulin receptor is an insulin-activated, tyrosine-specific protein kinase. Previous studies have shown that autophosphorylation of tyrosine residues on the Mr 95,000 is associated with an activation of the protein kinase activity toward exogenous protein substrates. We have employed the highly purified insulin receptor, immobilized on insulin-Sepharose or eluted in an active form, to define the metal/ATP requirements for kinase activation, the relationship of receptor autophosphorylation to activation, and the kinetic properties of the autophosphorylated, activated receptor kinase. Prior incubation of the immobilized receptor with 2 mM ATP, 10 mM Mg (or 10 mM Mn), followed by removal of these reactants, served to abolish the upward curvilinearity in the rate of histone 2b (tyrosine) phosphorylation measured subsequently. This treatment also markedly increased the rate of histone 2b phosphorylation as compared to that observed with the unmodified, immobilized receptor, as estimated under conditions that per se minimized further activation. The extents of maximal activation of receptor histone 2b (tyrosine) kinase obtained on preincubation with MgATP or MnATP are identical; however, the affinity of the receptor for MnATP is approximately 10-fold higher than that for MgATP. The higher affinity of the receptor for MnATP is observed for both autophosphorylation/autoactivation and histone 2b tyrosine kinase activity (Km MnATP approximately 0.01 mM; Km MgATP approximately 0.1 mM). Autophosphorylation/autoactivation per se does not significantly alter the apparent affinity for MeATP (or protein substrate, as previously reported) but increases Vmax. Activation of receptor histone 2b (tyrosine) kinase is due to tyrosine-specific autophosphorylation of the Mr 95,000 (beta) subunit; thus the extent of total 32P incorporation into the beta subunit correlates precisely with the extent of kinase activation, both over time and at a wide variety of Me2+ ATP concentrations. Sequential treatment of the autophosphorylated receptor with elastase and trypsin yields a single, basically charged 32P-peptide, Mr less than 2000. The functional properties of the unphosphorylated and fully phosphorylated receptor were compared after elution from insulin-Sepharose. The insulin binding characteristics of the two forms of the receptor were indistinguishable; the kinase properties differed greatly; whereas the histone 2b activity of the unphosphorylated receptor was low in the basal state, and activated 10-fold by insulin, the fully autophosphorylated receptor exhibits maximal histone 2b kinase in the basal state and is unaffected by insulin addition.(ABSTRACT TRUNCATED AT 400 WORDS)

Urea treatment allows dithiothreitol to release the binding subunit of the insulin receptor from the cell membrane: implications for the structural organization of the insulin receptor

The sequence of the human insulin receptor has only one identifiable transmembrane region which is located in the beta subunit. The structure predicts that the alpha subunit, which binds insulin, is attached to the cell only by disulfide bonds to the beta subunit. However, treatment of membranes with dithiothreitol is ineffective at releasing the alpha subunit. If the receptor structure is unfolded with urea, dithiothreitol is able to release the alpha subunit. These data provided confirmatory evidence that the alpha subunit is not a transmembrane protein.

Insulin-induced receptor regulation in early gestation and term human placental cell cultures

Human placentae of different gestational ages have been used to investigate the binding and degradation of insulin as well as the regulation of insulin membrane receptors. Bacitracin was found to be an effective inhibitor of insulin degradation in human early gestation and term placental cell cultures. In the presence of bacitracin, [125I]-insulin bound rapidly and reversibly: maximal binding occurred at 4 degrees C, with a sharp pH optimum at 7.5, and exhibited a high degree of specificity. The extent of binding was proportional to cell protein and [125I]-insulin concentrations. Term (38 to 41 weeks; n = 25) placental cell cultures possessed receptors for insulin that were increased three-fold compared to early gestation (8 to 18 weeks; n = 17). This was due to an increase in receptor number with no significant alteration in affinity. A decrease in insulin binding in both early gestation and term placental cells was related to both the insulin and bacitracin concentrations present during 12 to 20 h of preincubation at 37 degrees C. The receptor loss was due to a decrease in the number of receptors per mg cell protein with no apparent change in their affinity. We conclude that our in vitro system, which utilizes human placental cells in monolayer culture, will permit a more direct study of the metabolic effects of insulin in both early gestation and term placentae.

The insulin receptor kinase

The insulin receptor appears as a tetrameric glycoprotein consisting of two Mr 130,000 subunits (alpha), and two Mr 95,000 subunits (beta) in a disulfide-linked complex. Insulin bound to its specific cell surface receptors in its target cells leads to a complex array of molecular events resulting in insulin effects. It is now generally believed that protein phosphorylation-dephosphorylation reactions represent an important mechanism by which a variety of extracellular stimuli regulate cellular functions. Insulin mediates such reactions, but it is not known whether these are the biochemical link between the binding of insulin to its receptor and its final cellular effects. In search of initial post-binding events which might play a role in insulin action, we looked for phosphorylation of insulin receptors. We show that the insulin receptor displays two functional domains, an insulin binding alpha-subunit, and an insulin responsive protein kinase contained in the beta-subunit. We envisage the insulin receptor as an integrated system for transmembrane signal transmission in which hormone binding to the alpha-subunit leads to activation of the beta-subunit via conformational changes.

Post-translational processing and activation of insulin and EGF proreceptors

We have investigated the role of glycosylation on the post-translational processing of the insulin, and EGF proreceptor polypeptides. Following translation of the insulin proreceptor, by 3T3-L1 adipocytes, about 1.5 h are required for its conversion into active receptor; an additional 1.5 h are needed for the active receptor to reach the plasma membrane. During this 3-hour period the proreceptor undergoes a complex series of processing events, glycosylation being an essential processing step. Thus, treatment of 3T3-L1 adipocytes with tunicamycin caused the loss of cellular insulin binding activity and the accumulation of an inactive aglyco-proreceptor. Similarly, it was demonstrated in human A431 epidermoid carcinoma cells that the initial EGF-proreceptor (160 kDa) translation product undergoes a slow (t 1/2 = 30 min) processing step by which ligand (EGF) binding activity was acquired. It was shown that N-linked core oligosaccharide addition is essential for this critical processing step and the acquisition of EGF binding activity. This was found not to require the conversion of high mannose chains to complex chains which have been capped with fucose and sialic acid. Possible explanations for this activation in terms of translocation of intermediates and/or formation of disulfide bonds are discussed. To investigate post-translational processing of normal insulin proreceptor and the role of glycosylation in active receptor formation, metabolic labeling experiments were conducted. The first 35S-methionine-labeled intermediate detected is a 190 kDa polypeptide (proreceptor) which is rapidly (t 1/2 = 15 min) processed into a 210 kDa species. Both polypeptides contain N-linked core oligosaccharide chains, but in the latter case these chains appear to contain terminal N-acetylglucosamine. The 210 kDa precursor is converted slowly (t 1/2 = 2 h) by proteolytic processing into a 125 kDa (alpha') and 83 kDa (beta') species. Immediately prior to insertion into the plasma membrane, 3 h after its synthesis, the alpha' and beta' precursors are converted to mature receptor comprised of alpha-(135 kDa) and beta-(95 kDa) subunits. The 125 kDa alpha'- and 83 kDa beta'-subunit precursors are endoglycosidase H-sensitive and their oligosaccharide chains do not contain terminal sialic acid. Just prior to insertion into the plasma membrane the alpha' and beta' precursors are sialylated, apparently in the Golgi apparatus, giving rise to the 135 kDa alpha and 95 kDa beta receptor subunits and become Endo H-resistant and neuraminidase-sensitive. A proposed sequence of post-translational processing events for the insulin proreceptor is shown in Figure 10.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of epidermal growth factor-induced phosphorylation by trifluoperazine

We studied the effects of a series of drugs on A431, a cell line with well-characterized growth factor requirements and epidermal growth factor (EGF) receptors. The major [32PO4]-labeled protein immunoprecipitated with anti-phosphotyrosine antibodies from EGF-stimulated A431 cells was the EGF receptor. Both the quantity of [32PO4]-labeled EGF receptor immunoprecipitated and the phosphotyrosine content of total [32PO4]-labeled proteins were reduced by the addition during EGF stimulation of trifluoperazine (TFP). TFP had little effect on the binding, internalization, and processing of [125I]-EGF. In addition to the effects on phosphorylation, TFP inhibited cell growth both in the presence and absence of EGF. Morphologically, TFP blocked EGF-induced ruffling. TFP did not alter the EGF-stimulated phosphatidylinositol turnover. In an in vitro experiment using A431 cell membranes, TFP did not inhibit phosphorylation of the EGF receptor.

Normal hepatic insulin receptor autophosphorylation in nonketotic diabetes mellitus

Insulin receptor autophosphorylation is the earliest recognizable event in insulin action subsequent to insulin binding. To determine if the postbinding hepatic insulin resistance of nonketotic diabetes mellitus could reside in an inability of insulin to stimulate insulin receptor autophosphorylation, we evaluated the ability of insulin to stimulate 32P incorporation into the beta subunit of lectin-purified rat liver plasma membrane insulin receptors. The data indicate that both the absolute plasma membrane insulin receptor autophosphorylation in response to insulin as well as the insulin dose-response relationship for autophosphorylation are normal in diabetic animals when expressed per microgram of protein or per unit of binding activity. The previous data from our laboratory indicates that hepatic insulin resistance in non-ketotic streptozotocin-induced diabetes mellitus is present despite normal to increased insulin binding, is selective, is reversible with insulin treatment and involves an inability of insulin to stimulate the release of the putative mediator of insulin action. We conclude, therefore, that the hepatic insulin resistance of nonketotic diabetes mellitus resides distal to insulin receptor binding and autophosphorylation and is reflected in metabolic events at or near the plasma membrane which may include the generation or release of the putative mediator of insulin action.

Use of tyrosine-containing polymers to characterize the substrate specificity of insulin and other hormone-stimulated tyrosine kinases

Synthetic copolymers containing tyrosine residues were used to characterize the substrate specificity of the insulin receptor kinase and compare it to tyrosine kinases stimulated by epidermal growth factor, insulin-like growth factor-1 and phorbol ester. In partially purified receptor preparations from eight different tissues insulin best stimulated (highest V) phosphorylation of a random copolymer composed of glutamic and tyrosine residues at a 4:1 ratio (Glu/Tyr, 4:1). The insulin-stimulated phosphorylation of this polymer was highly significant also in receptor preparations from fresh human monocytes, where insulin binding and autophosphorylation were difficult to detect. Other tyrosine-containing polymers Ala/Glu/Lys/Tyr (6:2:5:1) and Glu/Ala/Tyr (6:3:1) were also phosphorylated by the insulin-stimulated kinase but to a lower extent. A tyrosine kinase stimulated by insulin-like growth factor-1, and one stimulated by phorbol ester also best phosphorylated the polymer Glu/Tyr (4:1). The three kinases differed only in their capability to phosphorylate Glu/Ala/Tyr (6:3:1) or Ala/Glu/Lys/Tyr (6:2:5:1). Glu/Tyr (4:1) was a poor substrate for the epidermal growth factor receptor kinase which best phosphorylated the polymer Glu/Ala/Tyr (6:3:1). Three additional polymers: Glu/Tyr (1:1), Glu/Ala/Tyr (1:1:1), and Lys/Tyr (1:1) failed to serve as substrates for all four tyrosine kinases tested. Taken together these findings suggest that. Hormone-sensitive tyrosine kinases have similar yet distinct substrate specificity and are likely to phosphorylate their native substrates on tyrosines adjacent to acidic (glutamic) residues. Tyrosine-containing polymer substrates are highly sensitive and convenient tools to study (hormone-sensitive) tyrosine kinases whose native substrates are unknown or present at low concentrations.