Covalent linking of photoreactive insulin to adipocytes produces a prolonged signal

Carbostyril-based beta-adrenergic agonists: evidence for long lasting or apparent irreversible receptor binding and activation of adenylate cyclase activity in vitro

The interaction of the carbostyril derivatives 5-[2-[[1-(4-aminophenyl)-2-methyl-prop-2-yl]amino]-1-hydroxyethyl]- 8-hydroxycarbostyril (carbo-amine) and 5-[2-[[3-[4-(bromoacetamido)phenyl]-2-methylprop-2-yl]amino]-1- hydroxyethyl]-8-hydroxycarbostyril (carbo-Br) with the rat reticulocyte beta-adrenoreceptor system has been partially characterized. In the absence of a guanine nucleotide, the concentration of carbo-amine, carbo-Br and (-)isoprenaline that inhibited (-)-[125I]iodocyanopindolol ([125I]CYP) binding by 50% (IC50) was 5.9 +/- 0.2, 3.3 +/- 0.3 and 49 +/- 3 nM, respectively. In the presence of a guanine nucleotide, the IC50 values were carbo-amine, 21 +/- 0.6 nM; carbo-Br, 7.6 +/- 0.3 nM and (-)isoprenaline, 813 +/- 66 nM. Preincubation of membranes with either of the carbostyril congeners followed by washing reduced specific [125I]CYP binding capacity without changing the KD value for the remaining receptors. The beta-antagonist nadolol largely prevented the receptor reduction induced by the carbostyril compounds. Incubation of membranes for 18 h at 25 degrees C resulted in an 11% recovery of the carbo-amine-induced receptor loss and no recovery of the receptors lost by preincubation with carbo-Br. However, the carbo-amine induced receptor loss could be largely reversed (80%) by membrane heating at 45 degrees C whereas little reversal (less than 10%) was observed with the carbo-Br pretreated membranes. The concentration of carbo-amine, carbo-Br and (-)isoprenaline that stimulated half-maximal cAMP formation in reticulocyte membranes was 17.8 +/- 3.1, 8.2 +/- 2.1 and 241 +/- 17 nM, respectively, and all 3 agonists produced the same maximal response. Initial cAMP formation stimulated by the carbostyril derivatives and (-)isoprenaline was blocked by concurrent addition of propranolol after 7 min of incubation with either of the two carbostyril derivatives did not affect further cAMP production whereas with (-)isoprenaline further cAMP production was blocked.(ABSTRACT TRUNCATED AT 250 WORDS)

Cellular mechanisms of insulin resistance in non-insulin-dependent (type II) diabetes

Recent studies have led to an enhanced understanding of cellular alterations that may play an important role in the pathophysiology of non-insulin-dependent diabetes mellitus (NIDDM). The insulin receptor links insulin binding at the cell surface to intracellular activation of insulin's effects. This transducer function involves the tyrosine kinase property of the beta-subunit of the receptor. It was found that adipocytes from subjects with NIDDM had a 50 to 80 percent reduction in insulin-stimulated receptor kinase activity compared with their non-diabetic counterparts. This defect was relatively specific for the diabetic state since no decrease was observed in insulin-resistant non-diabetic obese subjects. The reduction in kinase activity was accounted for by changes in the ratio of two pools of receptors, both of which bind insulin but only one of which is capable of tyrosine autophosphorylation and subsequent kinase activation; 43 percent of the receptors from non-diabetic subjects were capable of autophosphorylation compared with only 14 percent in the NIDDM group. A major component of cellular insulin resistance in NIDDM involves the glucose transport system. Exposure of cells to insulin normally results in enhanced glucose transport mediated by translocation of glucose transporters from a low-density microsomal intracellular pool to the plasma membrane. It was found that cells from NIDDM subjects had a marked depletion of glucose transporters in both plasma membranes and low-density microsomes, relative to obese non-diabetic control participants. Obese non-diabetic persons had a normal number of plasma membrane transporters but a reduced number of low-density microsome transporters in the basal state compared with lean control volunteers; insulin induced the translocation of relatively fewer transporters from the low-density microsome to the plasma membrane in the obese subgroups. In addition to the diminished number of glucose transporters, cells from both NIDDM and obese subjects had impaired functional activity of glucose carriers since decreased whole-cell glucose transport rates could not be entirely explained by the magnitude of the decrement in the number of plasma membrane transporters. Thus, impaired glucose transport is due to both a numerical and functional defect in glucose transporters. The cellular content of high-density microsomal transporters was the same in lean and obese control volunteers and NIDDM subjects, suggesting that transporter synthesis is normal and that cellular depletion results from increased protein turnover once transporters leave the high-density microsomal subfraction.(ABSTRACT TRUNCATED AT 400 WORDS)

Adipocyte insulin receptor. Generation of a cryptic domain of the alpha-subunit during internalization of hormone-receptor complexes

The dynamics of the internalization of photoaffinity-labelled insulin-receptor complexes was investigated in isolated rat adipocytes by using tryptic proteolysis to probe both the orientation and cellular location of the labelled complexes. In cells that were labelled at 16 degrees C and not prewarmed, 150 micrograms of trypsin/ml rapidly degraded the labelled 125 kDa insulin-receptor subunit into a major proteolytic fragment of 70 kDa and minor amounts of 90- and 50-kDa fragments. With milder trypsin treatment conditions (100 micrograms of trypsin/ml, 15 s at 37 degrees C), the 90 kDa peptide (different from the 90 kDa beta-subunit of the insulin receptor) appeared as a major intermediate proteolytic product, but this species was rapidly and completely converted into the 70- and 50-kDa fragments with continued exposure to trypsin, such that it did not accumulate to appreciable amounts in cells that were not prewarmed before trypsin exposure. By contrast, trypsin treatment of cells prewarmed to 37 degrees C for various times showed that: first, a proportion of the labelled 125 kDa receptors was internalized (became trypsin-insensitive); secondly, the 90 kDa tryptic peptide was formed in large amounts, with proportionate decreases occurring in the amounts of the 70- and 50-kDa tryptic peptides. The increased accumulation of the 90 kDa tryptic peptide from cells preincubated at 37 degrees C, but not at 16 degrees C, indicated that trypsin cleavage sites within the 90 kDa segment of the insulin-receptor alpha-subunit that were exposed at 16 degrees C were made inaccessible by incubation at 37 degrees C, a finding that is consistent with generation of a cryptic domain of the receptor subunit. The tryptic generation of the 90 kDa peptide at 37 degrees C was rapid, becoming half-maximal in 4.4 +/- 0.6 min and maximal in 15-20 min, preceded the intracellular accumulation of labelled receptors (half-maximal in 12.6 +/- 0.7 min and maximal in 30-40 min), was highly correlated with receptor internalization, and was not observed in cultured IM-9 lymphocytes, a cell line in which photolabelled insulin receptors are primarily lost by shedding into the incubation media. These results show that, in adipocytes incubated at 37 degrees C, rapid masking of a previously (at 16 degrees C) accessible domain of the insulin-receptor alpha-subunit occurs and that this dynamic process happens at an early stage in the internalization of insulin-receptor complexes.

Chymotrypsin substrate analogues inhibit endocytosis of insulin and insulin receptors in adipocytes

To explore the possible role of proteolytic step(s) in receptor-mediated endocytosis of insulin, the effects of inhibitors of various classes of proteases on the internalization process were studied in isolated rat adipocytes. Intracellular accumulation of receptor-bound 125I-insulin at 37 degrees C was quantitated after rapidly dissociating surface-bound insulin with an acidic buffer (pH 3.0). Of the 23 protease inhibitors tested, only chymotrypsin substrate analogues inhibited insulin internalization. Internalization was decreased 62-90% by five different chymotrypsin substrate analogues: N-acetyl-Tyr ethyl ester, N-acetyl-Phe ethyl ester, N-acetyl-Trp ethyl ester, benzoyl-Tyr ethyl ester, and benzoyl-Tyr amide. The effect of the substrate analogues in inhibiting insulin internalization was dose-dependent, reversible, and required the full structural complement of a chymotrypsin substrate analogue. Cell surface receptor number was unaltered at 12 degrees C. However, concomitant with their inhibition of insulin internalization at 37 degrees C, the chymotrypsin substrate analogues caused a marked increase (160-380%) in surface-bound insulin, indicating trapping of insulin-receptor complexes on the cell surface. Additionally, 1 mM N-acetyl-Tyr ethyl ester decreased overall insulin degradation by 15-20% and also prevented the chloroquine-mediated increase in intracellular insulin, further indicating that surface-bound insulin was prevented from reaching intracellular chloroquine-sensitive degradation sites. The internalization of insulin receptors that were photoaffinity labeled on the cell surface with B2(2-nitro-4-azidophenylacetyl)-des-PheB1-insulin was also inhibited 70-90% by the five chymotrypsin substrate analogues, as determined by the effects of the analogues on the accumulation of trypsin-insensitive (intracellular) 440-kD intact labeled receptors. In summary, these results show that chymotrypsin substrate analogues efficiently inhibit the internalization of insulin and insulin receptors in adipocytes and implicate a possible role for endogenous chymotrypsin-like enzyme(s) or related substances in receptor-mediated endocytosis of insulin.

Increased phosphorylation of ribosomal protein S6 following microinjection of insulin receptor-kinase into Xenopus oocytes

The protein products of several transforming retroviruses as well as the receptors for several hormones and growth factors, including insulin, have been shown to possess a protein kinase activity in vitro specific for tyrosine residues in protein substrates, including themselves. In the case of pp60src and the insulin receptor, autophosphorylation activates the tyrosine kinase activity towards exogenous substrates. Experiments indicate that, in vivo, many of these viruses or growth factors induce an increase in cellular phosphotyrosine, as well as an increase in the phosphorylation of serine residues on proteins, including ribosomal protein S6. It seems likely that some of the effects of insulin might be mediated by phosphorylation of intracellular substrates by its receptor. As the beta subunit of the receptor is a transmembrane protein, such phosphorylation could occur either while the receptor is still in the membrane or after its internalization. In various cell systems, internalized receptors are degraded, reshuttled back to the plasmalemma or maintained in a separate compartment before reinsertion in the membrane; shuttling of the insulin receptor could provide the opportunity for it to phosphorylate various intracellular components as part of its mechanism of signal transduction. To approach directly the question of whether the receptor can elicit a signal while acting at an intracellular location, we have microinjected Xenopus oocytes with the insulin receptor kinase. The results indicate that an S6 protein-serine kinase is stimulated or an S6 protein-serine phosphatase inhibited by the activity of the insulin receptor, supporting the concept that the insulin receptor acting within the cell can elicit a biological response.

Stabilized complexes of epidermal growth factor and its receptor on the cell surface stimulate RNA synthesis but not mitogenesis

Treatment of mouse fibroblasts, prelabeled at 4 degrees C with 125I-labeled epidermal growth factor (EGF), with the lectin concanavalin A (Con A) stabilized the 125I-labeled EGF-receptor complex to dissociation and prevented receptor-mediated endocytosis; after 5 hr at 37 degrees C, approximately 50% of the 125I-labeled EGF initially bound at 4 degrees C remained cell associated, compared to less than 15% in control cells. The radioactivity lost from the Con A-treated cells was found as intact hormone in the medium, with almost no hormone degradation evident, whereas in control cells most of the medium radioactivity was in the form of low molecular weight degradation products. The trimolecular complex Con A-EGF-receptor was capable of stimulating RNA synthesis to levels greater than control (untreated) or EGF alone and maintained this stimulation for prolonged periods of time. However, there was no effect of Con A treatment on the stimulation of DNA synthesis induced by EGF prebound at 4 degrees C. Thus, maintaining the EGF-receptor complex at the cell surface allows enhanced stimulation of an acute biological response to EGF (RNA synthesis) but not stimulation of DNA synthesis. These data support the idea that processing subsequent to receptor binding is necessary to produce the mitogenic signal.

Time-dependence of biological activity induced by covalent insulin-receptor complexes in rat adipocytes

Lipogenesis in isolated adipocyte preparations is stimulated when photosensitive insulin derivatives are attached covalently to specific receptors. This response was compared quantitatively with that to reversibly associated insulin, and it was shown that both covalent and reversible insulin-receptor complexes behave very similarly. The extent of stimulation of lipogenesis was studied as a function of time. Cells were incubated in buffer for various times before addition to vials containing 0 (basal) or 10 ng of monocomponent insulin/ml (maximal) and [U-3H]glucose. After 60 min, the toluene-soluble [3H]lipids were measured. The maximal stimulation induced by reversibly bound insulin was virtually constant over a period of 4 h. In contrast, adipocytes to which N alpha B2-(2-nitro-4-azidophenylacetyl)-des-PheB1-insulin had been covalently attached at the start of the experiment showed a loss of stimulation with time when incubated at 37 degrees C. This loss was decreased in the presence of lysosomotropic agents such as chloroquine at concentrations (approx. 200 microM) that had very little or no effect on the basal and maximal lipogenesis rates. A simple method was used to transform the measured rate of loss of stimulation into a rate of loss of effective units. A half-time of 80 min was calculated for the effective covalent insulin-receptor units in adipocytes at 37 degrees C at pH 7.4. This is very close to values reported by others for the internalization of covalent complexes in these cells, suggesting that this may be the causative event for the deactivation of the insulin-receptor unit. The inhibitory effect of chloroquine on the deactivation may indicate that the insulin-receptor complex can function even after internalization.

Structural and functional characteristics of insulin receptors in rat neuroblastoma cells

Insulin receptors were detected in a variety of rat neuroblastoma and glioma cell lines. The binding of 125I-insulin to B103 neuroblastoma cells had characteristics typical of insulin receptors in other tissues, including high affinity for insulin, low affinity for insulin-like growth factor I (IGF-I), and curvilinear Scatchard plots. Using photoaffinity labeling procedures and sodium dodecyl sulfate (SDS) gel electrophoresis to analyze the subunit structure of insulin receptors in B103 cells, the predominantly labeled protein had an apparent molecular weight of 125K and the mobility of this protein was shifted after removal of sialic acid residues. On the basis of size and susceptibility to neuraminidase, the insulin binding subunit in neuroblastoma cells was identical to the alpha-subunit of insulin receptors in adipocytes and different from the 115K subunit found in brain. The presence of an "adipocyte" form of the insulin receptor in clonal cells derived from brain is probably a consequence of transformation and results from more extensive oligosaccharide processing of the 115K receptor expressed in normal brain cells. The fully glycosylated receptors in neuroblastoma cells were capable of exerting functions typical of insulin receptors in adipocytes such as internalization of insulin and stimulation of glucose transport.

Biological action and fate of photoaffinity-labelled insulin-receptor complexes

Covalent linking of two photoactivatable insulin derivatives, B2-(2-nitro,4-azidophenylacetyl)-des-PheB1-insulin and B29-(2-nitro,4-azidophenylacetyl)-insulin to viable rat adipocytes gives a system, which contains a fixed stoichiometry between hormone and receptor. The biological signal of prolonged lipogenesis has been used to study several aspects of insulin binding and action: the role of the site of the crosslink between insulin and receptor, recognition of bound photoinsulin by anti-insulin antibodies, the half-life of the biologically active complex, the pH-dependence of the biological signal, and the possible role of internalization. Furthermore, the effect of trypsin on the insulin receptor, as well as the insulin-receptor complex, has been investigated and a refined model of the receptor is presented.

Photoaffinity labelling of MSH receptors on Anolis melanophores: effects of catecholamines, calcium and forskolin

Photoaffinity labelling of MSH receptors on Anolis melanophores was used as a tool for studying the effects of catecholamines, calcium and forskolin on hormone-receptor interaction and receptor-adenylate cyclase coupling. Covalent attachment of photoreactive alpha-MSH to its receptor was suppressed in calcium-free buffer but was hardly influenced by catecholamines or forskolin. The longlasting signal generated by the covalent MSH-receptor complex was readily and reversibly abolished by adrenaline, noradrenaline, dopamine or clonidine or by the absence of calcium. The suppression of pigment dispersion by catecholamines was blocked by the simultaneous presence of yohimbine but not prazosin, indicating that the catecholamines antagonize the alpha-MSH signal by inhibitory action on the adenylate cyclase system through an alpha-2 receptor. Forskolin, which stimulates melanophores by direct action on the catalytic unit of the adenylate cyclase and at about the same speed as alpha-MSH, produced a slower and weaker response in the presence of noradrenaline. If MSH receptors were covalently labelled and then exposed to noradrenaline, the characteristics of the forskolin-induced response were identical to those of unlabelled cells that had not been exposed to noradrenaline. This may point to a partial restoration of receptor-adenylate cyclase coupling by forskolin. The results show that the longlasting stimulation of Anolis melanophores by photoaffinity labelling proceeds via a permanently stimulated adenylate-cyclase system whose coupling to the receptor depends on calcium and is abolished by alpha-2 receptor agonists. Calcium is also essential for hormone-receptor binding.

Preparation and characterization of photoreactive derivatives of GnRH. Persistent activation of function by photoaffinity labeling

A photoreactive derivative of the highly potent gonadotropin releasing hormone (GnRH) agonist, D-Lys6-GnRH(1-9)-ethylamide, was prepared by selective modification of the epsilon-amino group with 2-nitro-4-azidophenyl sulfenyl chloride (2,4-NAPS C1). The modified peptide [D-Lys(NAPS)]6-GnRH-(1-9)-ethylamide was found to be a full agonist of LH release from rat pituitary cells with a relative potency 23 compared to GnRH. Covalent attachment of the photoreactive analog to rat pituitary cells resulted in prolonged activation of LH secretion which could not be inhibited by a potent GnRH antagonist. Persistent stimulation of pituitary gonadotrophs caused by covalently bound hormone led to desensitization of the LH releasing mechanism.

Biological potency of covalently linked insulin-receptor in rat adipocytes. Comparison with the potency of reversible complexes

Irradiation of photoreactive insulin derivatives in the presence of isolated rat adipocytes produces a prolonged stimulation of lipogenesis in the cells even after exogenous and reversibly bound derivative has been removed by extensive washing. The quantitative nature of this response has now been studied using 125I-B2(2-nitro-4-azidophenylacetyl)des-PheB1-insulin. This derivative possesses nearly full biological potency and binding affinity prior to irradiation. After covalent linkage to adipocytes the efficacy of the derivative is reduced to 25 +/- 4% of the reversibly bound derivative, viz. 4-times as much needs to be covalently associated as reversibly bound to induce the same level of stimulation of lipogenesis. This reduced relative molar potency is due to a reduced ability of specific covalent insulin-receptor complexes to trigger a response.

Biological activity of covalently-linked insulin-receptor complexes in rat adipocytes. Effect of pH

The relative molar potencies of covalently and reversibly-bound insulin-receptor complexes were studied as a function of pH. The insulin derivatives used were 125I-B2 (2-nitro-4-azidophenylacetyl)des-PheB1-insulin and 125I-B29(2-nitro-4-azidophenylacetyl)des-PheB1-insulin. The potencies of both types of reversible complexes were effectively identical and constant between pH 7 and 8. The relative potency of the covalent B2-complex increased from 25 to 75%, and of the covalent B29 complex from 30 to nearly 100%. This indicates that the covalently linked partners in the complex are able to flex about the cross-linkages. Variations in the potency are due to variations in the number of correctly associated, reversibly or covalently bound insulin-receptor complexes. The form of the pH dependance suggests that an ionizable group, possibly an amino group, must be deprotonated to allow effective interaction.

Insulin receptors in isolated human adipocytes. Characterization by photoaffinity labeling and evidence for internalization and cellular processing

We photolabeled and characterized insulin receptors in isolated adipocytes from normal human subjects and then studied the cellular fate of the labeled insulin-receptor complexes at physiologic temperatures. The biologically active photosensitive insulin derivative, B2(2-nitro-4-azidophenylacetyl)des-PheB1-insulin (NAPA-DP-insulin) was used to photoaffinity label the insulin receptors, and the specifically labeled cellular proteins were identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and autoradiography. At saturating concentrations, the binding of 125I-NAPA-DP-insulin to the isolated adipocytes at 16 degrees C was rapid (half-maximal in approximately 1 min and maximal in approximately 10 min) and approximately 25% of the specifically bound ligand was covalently linked to the cells by a 3-min exposure to long-wave (366 nm) ultraviolet light. Analysis of the photolabeled cellular proteins by PAGE in the absence of disulfide reductants revealed the specific labeling of a major protein band of Mr 330,000 and two less intense bands of Mr 295,000 and 260,000. Upon reduction of disulfide bonds with dithiothreitol, all three unreduced forms of the insulin receptor were converted into a major labeled Mr-125,000 band and a less intensely labeled Mr-90,000 band. The labeling of the Mr-125,000 receptor subunit was saturable and native porcine insulin effectively inhibited (half-maximal inhibition at 12 ng/ml) the photolabeling of this binding subunit by NAPA-DP insulin. When intact adipocytes photolabeled at 16 degrees C (a temperature that inhibits endocytosis) were immediately trypsinized, all of the labeled receptor bands were converted into small molecular weight tryptic fragments, indicating that at 16 degrees C all of the labeled insulin-receptor complexes remained on the cell surface. However, when the photolabeled cells were further incubated at 37 degrees C and then trypsinized, a proportion of the labeled receptors became trypsin insensitive, indicating that this fraction has been translocated to the cell interior and thus was inaccessible to the trypsin in the incubation medium. The intracellular translocation of the labeled receptors was observed within 2 min, became half-maximal by 10 min, and maximal by approximately 30 min of incubation at 37 degrees C. Cellular processing of the internalized insulin-receptor complexes also occurred, since incubation at 37 degrees C (but not 16 degrees C) resulted in the generation of a Mr-115,000 component from the labeled receptors. Inclusion of chloroquine, a drug with lysosomotropic properties, in the incubation media caused a time-dependent increase (maximal increase of 50% above control by 2 h at 37 degrees C) in the intracellular pool of labeled receptors. In contrast to these findings in human adipocytes, no appreciable internalization of insulin-receptor complexes and no chloroquine effect was observed in cultures human IM-9 lymphocytes during a 1-h incubation at 37 degrees C. We concluded that in isolated human adipocytes: (a) the subunit structure of insulin receptors is the same as that reported for several other tissues, (b) insulin-receptor complexes are rapidly internalized and processed at physiologic temperatures, and (c) the cellular processing of insulin-receptor complexes occurs at one or more chloroquine-sensitive intracellular site(s).

Covalent linking of photoreactive gonadotropin-releasing hormone to gonadotropes produces a prolonged signal

A bioactive, photoreactive derivative of gonadotropin-releasing hormone (GnRH; gonadoliberin), [azidobenzoyl-D-Lys6]GnRH, was shown to bind covalently to dispersed pituitary cells after irradiation. Approximately 7% of the total cell-associated radioactivity was covalently bound to the receptors. Photolysis of cultured pituitary cells in the presence of the photoreactive derivative resulted in persistent activation of luteinizing hormone (LH; lutropin) release. This persistent response was time dependent and concentration dependent. No increase in the basal rate of LH release was observed with cells incubated in the presence of photoreactive GnRH analog and maintained in the dark or with hormone derivatives that lack the photoreactive azido group. These results suggest that only the covalently bound cell surface receptors account for the persistent activation of LH release after photolysis.

Photoaffinity labelling of peptide hormone receptors

Photoreactive peptide derivatives for the labelling of hormone receptors are usually prepared by inserting a chemically stable aryl azide or nitroaryl azide into a specific site of the molecule, such as an alpha or omega amino or carboxyl group, or into the side-chain of an Arg, Cys, His, Trp or Tyr. With p-azidophenylalanine (Pap), a more or less isosteric replacement of Tyr or Phe can be achieved when other alterations would impair the biological activity of the hormone. Reversible attachment of photoreactive groups via S-S linkage to, e.g., SH-Trp makes it possible to release the covalently bound hormone from the receptor. This is an advantage when photolabelling is used for the isolation of receptors. Photoaffinity labelling of intact cell systems may reveal insights into dynamic aspects, such as receptor inactivation and turnover or the temporal involvement of second messengers, and may be particularly useful for the study of peptide action on target cells occurring only in small numbers. The Xenopus MSH-melanophore system which is stimulated irreversibly upon UV-irradiation of photoreactive alpha-MSH represents such an example.