Recycling of the glucose transporter, the insulin receptor, and insulin in rat adipocytes. Effect of acidtropic agents

Adipocyte cell size enlargement involves plasma membrane area increase

The adipocyte enlargement is associated with an increase in the cytoplasmic lipid content, but how the plasma membrane area follows this increase is poorly understood. We monitored single-cell membrane surface area fluctuations, which mirror the dynamics of exocytosis and endocytosis. We employed the patch-clamp technique to measure membrane capacitance (C(m)), a parameter linearly related to the plasma membrane area. Specifically, we studied whether insulin affects membrane area dynamics in adipocytes. A five-minute cell exposure to insulin increased resting C(m) by 12 ± 4%; in controls the change in C(m) was not different from zero. We measured cell diameter of isolated rat adipocytes microscopically. Twenty-four hour exposure of cells to insulin resulted in a significant increase in cell diameter by 5.1 ± 0.6%. We conclude that insulin induces membrane area increase, which may in chronic hyperinsulinemia promote the enlargement of plasma membrane area, acting in concert with other insulin-mediated metabolic effects on adipocytes.

The role of luteinizing hormone and prolactin in the regulation of insulin receptors in Leydig cells of the adult rat

The role of luteinizing hormone (LH), prolactin and their combination in the regulation of insulin receptors in Leydig cells was studied. Leydig cells were isolated from adult male Wistar rats and measurement of insulin binding and internalization was done by incubating the cells with a saturating concentration of 125I-insulin in the presence or absence of different doses of unlabeled LH/insulin. LH exposure (100 and 200 ng dose) caused a significant increase in Leydig cell surface and internalized insulin receptor concentrations. Prolactin at all doses was ineffective in inducing a significant change in insulin receptor concentration. Under basal condition, Leydig cell surface binding of 125I-insulin was greater than the internalization at 34 degrees C but at 4 degrees C, surface binding remained lower than that at 34 degrees C with negligible internalization. Internalization of insulin receptors was measured by incubating the cells at 4 degrees C for 16h and then rapidly incubating at 34 degrees C for various time intervals (60, 120, and 180 min). LH/PRL or LH + PRL did not induce any significant change in the internalization of 125I-insulin at 60 and 120 min. The rate of internalization was greater at 120 min in basal as well as LH/PRL exposed Leydig cells, compared to 60 min of incubation. Prolactin alone did not evoke any appreciable change in internalization of 125I-insulin compared to basal at all three time points tested. Total and acid soluble release of 125I-insulin recorded a significant increase in Leydig cells exposed to LH, which was marginally potentiated when prolactin was added along with LH. Monensin treatment of Leydig cells prevented the recycling of insulin receptors to the cell surface and thereby suppressed the surface binding and enhanced the internalized 125I-insulin. Under cycloheximide treatment, neither surface bound nor internalized 125I-insulin recorded a significant change compared to their respective basal values. It is concluded from the present study that LH has dose-dependent biphasic effects on insulin receptors in Leydig cells by modulating the internalization and intracellular processing of hormone-receptor complexes but prolactin has no such effects.

Water channel protein AQP3 is present in epithelia exposed to the environment of possible water loss

Aquaporins (AQPs) are membrane water channel proteins expressed in various tissues in the body. We surveyed the immunolocalization of AQP3, an isoform of the AQP family, in rat epithelial tissues. AQP3 was localized to many epithelial cells in the urinary, digestive, and respiratory tracts and in the skin. In the urinary tract, AQP3 was present at transitional epithelia. In the digestive tract, abundant AQP3 was found in the stratified epithelia in the upper part, from the oral cavity to the forestomach, and in the simple and stratified epithelia in the lower part, from the distal colon to the anal canal. In the respiratory tract, AQP3 was present in the pseudostratified ciliated epithelia from the nasal cavity to the intrapulmonary bronchi. In the skin, AQP3 was present in the epidermis. Interestingly, AQP3 was present at the basal aspects of the epithelia: in the basolateral membranes in the simple epithelia and in the multilayered epithelia at plasma membranes of the basal to intermediate cells. During development of the skin, AQP3 expression commenced late in fetal life. Because these AQP3-positive epithelia have a common feature, i.e., they are exposed to an environment of possible water loss, we propose that AQP3 could serve as a water channel to provide these epithelial cells with water from the subepithelial side to protect them against dehydration. (J Histochem Cytochem 47:1275-1286, 1999)

Characterization of rat Glut4 glucose transporter expressed in the yeast Saccharomyces cerevisiae: comparison with Glut1 glucose transporter

Rat Glut4 glucose transporter was expressed in the yeast Saccharomyces cerevisiae, but was retained in an intracellular membranous compartment and did not contribute to glucose uptake by intact cells. A crude membrane fraction was prepared and reconstituted in liposome with the use of the freeze-thaw/sonication method. D-glucose-specific, cytochalasin B inhibitable glucose transport activity was observed. Kinetic analysis of D-glucose transport was performed by an integrated rate equation approach. The K(m) under zero-trans influx condition was 12 +/- 1 mM (mean +/- S.E., n = 3) and that under equilibrium exchange condition was 22 +/- 3 mM (n = 4). D-glucose transport was inhibited by 2-deoxy-D-glucose or 3-O-methyl-D-glucose, but not by D-allose, D-fructose or L-glucose. Cytochalasin B, phloretin and phlorizin inhibited D-glucose transport, but neither p-chloromercuribenzoic acid (pCMB) (0-0.1 mM) nor p-chloromercuribenzene sulfonic acid (pCMBS) (0-1.0 mM) inhibited this activity. High concentrations of HgCl2 were required to inhibit D-glucose transport (IC50, 370 microM). Comparing these properties to those of rat Glut1 we found two notable differences; (1) in Glut1, K(m) under zero-trans influx was significantly smaller than that under equilibrium exchange but in Glut4 less than two-fold difference was seen between these two K(m) values; and (2) Glut1 was inhibited with pCMB, pCMBS and low concentrations of HgCl2 (IC50, 3.5 microM), whereas Glut4 was almost insensitive to SH reagents. To examine the role of the exofacial cysteine, we replaced Met-455 of Glut4 (corresponding to Cys-429 of Glut1) with cysteine. The mutated Glut4 was inhibited by pCMB or pCMBS and the IC50 of HgCl2 decreased to 47 microM, whereas K(m), substrate specificity and the sensitivity to cytochalasin B were not significantly changed, indicating that the existence of exofacial cysteine contributed only to increase SH sensitivity in Glut4.

Transport of glucose across the blood-tissue barriers

In specialized parts of the body, free exchange of substances between blood and tissue cells is hindered by the presence of a barrier cell layer(s). Specialized milieu of the compartments provided by these "blood-tissue barriers" seems to be important for specific functions of the tissue cells guarded by the barriers. In blood-tissue barriers, such as the blood-brain barrier, blood-cerebrospinal fluid barrier, blood-nerve barrier, blood-retinal barrier, blood-aqueous barrier, blood-perilymph barrier, and placental barrier, endothelial or epithelial cells sealed by tight junctions, or a syncytial cell layer(s), serve as a structural basis of the barrier. A selective transport system localized in the cells of the barrier provides substances needed by the cells inside the barrier. GLUT1, an isoform of facilitated-diffusion glucose transporters, is abundant in cells of the barrier. GLUT1 is concentrated at the critical plasma membranes of cells of the barriers and thereby constitutes the major machinery for the transport of glucose across these barriers where transport occurs by a transcellular mechanism. In the barrier composed of double-epithelial layers, such as the epithelium of the ciliary body in the case of the blood-aqueous barrier, gap junctions appear to play an important role in addition to GLUT1 for the transfer of glucose across the barrier.

Immunolocalization of glucose transporter GLUT1 in the rat placental barrier: possible role of GLUT1 and the gap junction in the transport of glucose across the placental barrier

GLUT1 is an isoform of facilitated-diffusion glucose transporters and has been shown to be abundant in cells of blood-tissue barriers. Using antibodies against GLUT1, we investigated the immunohistochemical localization of GLUT1 in the rat placenta. Rat placenta is of the hemotrichorial type. Three cell layers (from the maternal blood side inward) cytotrophoblast and syncytiotrophoblasts I and II, lie between the maternal and fetal bloodstreams. GLUT1 was abundant along the invaginating plasma membrane facing the cytotrophoblast and the syncytiotrophoblast I. Also, the infolded basal plasma membrane of the syncytiotrophoblast II was rich in GLUT1. Apposing plasma membranes of syncytiotrophoblasts I and II, however, had only a small amount of GLUT1. Numerous gap junctions were seen between syncytiotrophoblasts I and II. Taking into account the localization of GLUT1 and the gap junctions, we suggest a possible major transport route of glucose across the placental barrier, as follows: glucose in the maternal blood passes freely through pores of the cytotrophoblast. Glucose is then transported into the cytoplasm of the syncytiotrophoblast I via GLUT1. Glucose enters the syncytiotrophoblast II through the gap junctions. Finally glucose leaves the syncytiotrophoblast II via GLUT1 and enters the fetal blood through pores of the endothelial cells.

Receptor-mediated processing of epidermal growth factor in the trophoblast of the human placenta

The processing of epidermal growth factor (EGF) and its receptor in human trophoblast during the first trimester and at term was studied using biotin-labeled EGF, an anti-EGF receptor monoclonal antibody and immunohistochemistry. In chorionic villi incubated with EGF-biotin the ligand was first bound to specific receptors on the syncytial surface, which are in contact with the maternal blood. After 2-5 min in the early gestation placenta, EGF-biotin was found at the basal plasma membrane of the syncytium accompanied by a pronounced EGF receptor immunostaining. In contrast, in the term placenta, immunostaining of EGF-biotin as well as EGF receptors was pronounced in the syncytioplasma within 30-60 min following EGF stimulation; in addition, EGF-biotin was found in some syncytial nuclei. These immunostaining reactions were enhanced after lysosomal blockage by chloroquine. The results reveal a transsyncytial, receptor-mediated transfer of EGF from the maternal blood to the cytotrophoblast, the proliferating part of the trophoblast, in the first trimester placenta. However, in the term placenta, the EGF signal seems to be directed primarily to the syncytium, thus probably influencing differentiated functions. In conclusion, the trophoblast examplifies three possible pathways of EGF processing: 1. transcytotic transfer, 2. direct intracellular signalling followed by lysosomal degradation, and 3. nuclear binding.

Regulation of the functional expression of hexose transporter GLUT-1 by glucose in murine fibroblasts: role of lysosomal degradation

The nature of the membrane compartments involved in the regulation by glucose of hexose transport is not well defined. The effect of inhibitors of lysosomal protein degradation on hexose transport (i.e., uptake of [3H]-2-deoxy-D-glucose) and hexose transporter protein GLUT-1 (i.e., immunoblotting with antipeptide serum) in glucose-fed and -deprived cultured murine fibroblasts (3T3-C2 cells) was studied. The acidotropic amines chloroquine (20 microM) and ammonium chloride (10 mM) cause accumulation (both approximately 4-fold) of GLUT-1 protein and a small increase (both approximately 25%) in hexose transport in glucose-fed fibroblasts (24 h). The endopeptidase inhibitor, leupeptin (100 microM) causes accumulation (approximately 4-fold) of GLUT-1 protein in glucose-fed fibroblasts (24 h) without changing hexose transport (less than or equal to 5%). These agents do not greatly alter the electrophoretic mobility of GLUT-1. Neither chloroquine nor leupeptin augment the glucose deprivation (24 h) induced increases in hexose transport (approximately 4-fold) and GLUT-1 content (approximately 7-fold). In contrast, chloroquine or leupeptin diminish the reversal by glucose refeeding of the glucose deprivation induced accumulation of GLUT-1 protein but fail to alter the return of hexose transport to control levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of erythrocyte/HepG2-type glucose transporter (GLUT1) in human placental villi

The syncytiotrophoblast covering the surface of the placental villi contains the machinery for the transfer of specific substances between maternal and fetal blood, and also serves as a barrier. Existence of a facilitated-diffusion transporter for glucose in the syncytiotrophoblast has been suggested. Using antibodies to erythrocyte/HepG2-type glucose transporter (GLUT1), one isoform of the facilitated-diffusion glucose transporters, we detected a 50 kD protein in human placenta at term. By use of immunohistochemistry, GLUT1 was found to be abundant in both the syncytiotrophoblast and cytotrophoblast. Endothelial cells of the fetal capillaries also showed positive staining for GLUT1. Electron-microscopic examination revealed that GLUT1 was concentrated at both the microvillous apical plasma membrane and the infolded basal plasma membrane of the syncytiotrophoblast. Plasma membrane of the cytotrophoblast was also positive for GLUT1. GLUT1 at the apical plasma membrane of the syncytiotrophoblast may function for the entry of glucose into its cytoplasm, while GLUT1 at the basal plasma membrane may be essential for the exit of glucose from the cytoplasm into the stroma of the placental villi. Thus, GLUT1 at the plasma membranes of syncytiotrophoblast and endothelial cells may play an important role in the transport of glucose across the placental barrier.

Pattern of glucose transporter (Glut 1) expression in embryonic brains is related to maturation of blood-brain barrier tightness

A constant supply of blood-borne glucose is vital to cerebral metabolism. Although transport of glucose into the nervous tissue, effectively separated from the blood by a functional barrier (the blood-brain barrier, BBB), is one of the essential properties of the cerebral endothelium, little is known about its metabolic regulation and developmental expression in the BBB. In this study we provide evidence by immunocytochemistry that the pattern of the brain endothelial glucose transporter in rat brains (BBB-GT), immunologically homologous with the human hepatoma (G2), human erythrocyte transporter (Glut 1), changes with BBB maturation. While the neuroepithelium at embryonic days 12 and 13 shows a high incidence of immuno-detectable BBB-GT, vascularisation of the cerebral anlage and subsequent development of vascular tightness, as evidenced by intravascularly applied horseradish peroxidase and fluorescinated dextrans, is accompanied by a significant reduction of BBB-GT expression in neuroepithelial cells and confinement of BBB-GT expression to the cerebral endothelium. Immunoblots and Northern blots of embryonic brain homogenates corroborate this change in BBB-GT expression in the brain anlage at the time of BBB maturation. However, low molecular weight glucose transporters, presumed to be of non-endothelial origin, are less dramatically reduced. The development of BBB tightness, therefore, seems to play a pivotal role in the pattern of BBB-GT expression during brain differentiation.

Immunohistochemical localization of Na(+)-dependent glucose transporter in rat jejunum

Glucose is actively absorbed via a Na(+)-dependent active glucose transporter (Na-GT) in the small intestine. We raised a polyclonal antibody against the peptide corresponding to amino acids 564-575 of rabbit intestinal Na-GT, and localized it immunohistochemically in the rat jejunum. By means of immunofluorescence staining, Na-GT was located at the brush border of the absorptive epithelial cells of the intestinal villi. Electron-microscopic examination showed that Na-GT was localized at the plasma membrane of the apical microvilli of these cells. Little Na-GT was found at the basolateral plasma membrane. Along the crypt-villus axis, all of the absorptive epithelial cells in the villus were positive for Na-GT. In addition to the brush border staining, the supranuclear positive staining, which was shown to be the Golgi apparatus by use of electron microscopy, was seen in cells located between the base to the middle of the villus. Cells in crypts exhibited little or no staining for Na-GT. Goblet cells scattered in the intestinal epithelium were negative for Na-GT staining. These observations show that Na-GT is specific to the apical plasma membrane of the absorptive epithelial cells, and that the onset of Na-GT synthesis may occur near the crypt-villus junction.

Radioimmunoassay for human insulin-like growth factor-I receptor: applicability to breast carcinoma specimens and cell lines

A radioimmunoassay for the human insulin-like growth factor-I (IGF-I) receptor was developed using a rabbit polyclonal antibody to the human IGF-I receptor and a highly purified IGF-I receptor. The purified receptor was radiolabeled with 125I-Bolton-Hunter reagent. Over 18% of the radiolabeled receptor was immunoprecipitated with the polyclonal antireceptor antibody. Purified IGF-I receptor concentrations as low as 5 ng/0.5 mL inhibited the radiolabeled IGF-I receptor binding. Purified insulin receptor weakly inhibited this binding, while the ligand IGF-I did not show inhibition. The radioimmunoassay was applicable to the measurements of IGF-I receptors in the Triton X-100 extracts of various tissues and cells. Breast cancer tissues and cells showed detectable IGF-I receptors, which correlated with IGF-I ligand binding. Receptor content was measurable in placenta and IM-9 cells, but receptor content was not measurable in liver and muscle extracts.

Erythrocyte/HepG2-type glucose transporter is concentrated in cells of blood-tissue barriers

In search of possible diverse roles of glucose transporters (GT's), we examined whether any GT's are present in blood-tissue barriers where selective flow of glucose from blood to tissue cells is critically important. We found in rat that the erythrocyte/HepG2-type GT is localized in all the limiting plasma membranes known to serve as blood-tissue barriers, whether the barriers are endothelial type (brain, iris, inner retina, peripheral nerve) or epithelial type (choroid plexus, ciliary body, outer retina, peripheral nerve, placenta), except for plasma membranes in testis and thymus where no appreciable amount of the GT was found. The erythrocyte/HepG2-type GT may play a vital role for the entry of glucose into these firmly guarded tissues.

Different effects of two proteinase inhibitors on insulin-induced cellular responses in rat adipocytes

Among various proteinase inhibitors, N-acetyl-L-tyrosine ethyl ester (ATEE), a chymotrypsin substrate analog, and N alpha-p-tosyl-L-lysine chloromethyl ketone (TLCK), a trypsin inhibitor, showed significant inhibitory effects on insulin stimulated glucose transport in rat adipocytes. ATEE did not affect insulin binding, but inhibited insulin internalization. In intact adipocytes, ATEE inhibited tyrosine phosphorylation of the beta-subunit of the insulin receptor, a 170 kDa protein and a 60 kDa protein at almost the same concentration (ID50 = 0.24 +/- 0.05 mM, n = 4, mean +/- S.E.), but in a plasma membrane fraction, ATEE did not appreciably inhibit the tyrosine phosphorylation of the beta-subunit of the insulin receptor, TLCK did not inhibit insulin binding. At 0.25 mM, TLCK did not inhibit insulin internalization, but inhibited 70% of the insulin-stimulated glucose transport (ID50 = 0.19 +/- 0.02 mM, n = 7). TLCK inhibited insulin internalization at more than 0.25 mM. TLCK did not inhibit the tyrosine phosphorylation of the beta-subunit of the insulin receptor in intact cells or in the plasma membrane fraction. In intact cells, TLCK inhibited the phosphorylation of the 60 kDa protein and simultaneously it stimulated the phosphorylation of the 170 kDa protein more than 3-fold. These results indicate that there are at least two sites in the insulin-induced signal transduction pathway where proteinase inhibitors act to suppress the insulin signal transduction. A major ATEE site is very close to phosphorylation of the beta-subunit of the insulin receptor. On the other hand, TLCK inhibits a step(s) in the signal transduction pathway after the insulin receptor but before the glucose transporter.

Isolation and characterization of Chinese hamster ovary cell mutants defective in glucose transport

Cultured Chinese hamster ovary (CHO) cells possess an insulin-sensitive facilitated diffusion system for glucose transport. Mutant clones of CHO cells defective in glucose transport were obtained by repeating the selection procedure, which involved mutagenesis with ethyl methanesulfonate, radiation suicide with tritiated 2-deoxy-D-glucose, the polyester replica technique and in situ autoradiographic assaying for glucose accumulation. On the first selection, we obtained mutants exhibiting about half the glucose uptake activity of parental CHO-K1 cells and half the amount of a glucose transporter, the amount of which was determined by immunoblotting with an antibody to the human erythrocyte glucose transporter. The second selection, starting from one of the mutants obtained in the first-step selection, yielded a strain, GTS-31, in which both glucose uptake activity and the quantity of the glucose transporter were 10-20% of the levels in CHO-K1 cells, whereas the responsiveness of glucose transport to insulin, and the activities of leucine uptake and several glycolytic enzymes remained unchanged. GTS-31 cells grew slower than CHO-K1 cells at both 33 and 40 degrees C, and in a medium containing a low concentration of glucose (0.1 mM), the mutant cells lost the ability to form colonies. All the three spontaneous GTS-31 cell revertants, which were isolated by growing the mutant cells in medium containing 0.1 mM glucose, exhibited about half the glucose uptake activity and about half the amount of glucose transporter, as compared to in CHO-K1 cells, these characteristics being similar to those of the first-step mutant. These results indicate that the decrease in glucose uptake activity in strain GTS-31 is due to a mutation which induces a reduction in the amount of the glucose transporter, providing genetic evidence that the glucose transporter functions as a major route for glucose entry into CHO-K1 cells.

Retroendocytosis of insulin in a cultured kidney epithelial cell line

It has been generally accepted that in renal tubular epithelium endocytosed proteohormones are transported to lysosomes where they undergo complete hydrolysis. En route, as endosomal pH falls, the proteohormone uncouples from the endocytosed membrane binding site, which recycles to the cell surface. However, studies in other tissues have uncovered alternate intracellular pathways for proteins. One such pathway is retroendocytosis (endocytosis then exocytosis). To determine whether a retroendocytotic pathway exists for insulin in renal epithelium, a study was carried out with confluent monolayers of a proximal-like opossum kidney cell line that exhibits receptor-mediated endocytosis of insulin. Cells were preloaded with 125I-labeled insulin (4 X 10(-10) M) for 30 min, surface-bound insulin was then removed by acid washing, and over the next 60 min the release of intracellular radioactivity into the medium was monitored. At 37 degrees C, control cells released on average 7-15% of the intracellular radioactivity as intact insulin [trichloroacetic acid (TCA)-precipitable radioactivity] and approximately 62% as TCA-soluble degradation products. In the presence of 0.1 mM chloroquine (an acidotropic agent) the release of intact insulin increased approximately twofold while degradation fell by nearly one-half. With Sephadex G-50 chromatography we found that the released radioactivity included insulin-size material that increased in the presence of chloroquine. High-performance liquid chromatography revealed that 53 (controls) and 81% (chloroquine treatment) of this latter material consisted of intact insulin. We conclude that, in addition to a major degradative pathway, cultured kidney epithelial cells exhibit a retroendocytotic pathway for insulin. Chloroquine inhibits degradation and appears to divert insulin from the degradative into the retroendocytotic pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism for increased insulin-stimulated glucose metabolism in adipocytes from 13-week-old obese Zucker rats

A study was made on the mechanism of increased glucose metabolism in enlarged adipocytes from 13-week-old obese Zucker rats showing hyperinsulinaemia and hyperglycaemia. Glucose metabolism was assessed by measuring CO2 production from glucose and the concentration of glucose transporters was estimated by immunoblotting. In comparing adipocytes from obese rats with those from lean rats, the basal rates of glucose oxidation at 0.02 mmol/l glucose increased 2.6-fold per unit cellular surface area and the transporters in the plasma membrane increased 1.4-fold per protein, while that in low-density microsome was 67% of the value in lean rats. The increase of glucose oxidation rates observed in basal cells from obese rats could be partly explained by translocation of the transporters from the intracellular site to the plasma membrane. In the presence of insulin, as the basal rates of glucose oxidation increased in obese rats, maximally insulin-stimulated oxidation increased 4-fold in lean rats and 1.7-fold in obese rats. Thus, the rates of insulin-stimulated oxidation on a per unit cellular surface area as well as the transporters on a per protein basis in the plasma membrane became almost similar in cells from both groups of rats. Since protein content per cell increased with cell enlargement, increased glucose metabolism per cell which was observed in adipocytes from the obese rats was mainly due to an increase of glucose transporters accompanied by a similar degree of cellular protein increase.

Identification of a novel gene encoding an insulin-responsive glucose transporter protein

Insulin, rapidly and independently of new protein synthesis, stimulates glucose transport in sensitive target tissues. A cDNA has been cloned from a skeletal muscle library that encodes a novel glucose transporter protein exhibiting the following properties of an insulin-regulated hexose carrier protein: it is expressed exclusively in adipose tissue, skeletal muscle and heart, the principal organs with insulin-responsive glucose transport; RNA transcribed from the muscle cDNA, when expressed in Xenopus oocytes, encodes a protein capable of cytochalasin B inhibitable 2-deoxyglucose transport; and treatment of isolated rat adipocytes with insulin effects a redistribution of "muscle" transports from low density microsomes to the plasma membrane to an extent comparable to the activation of glucose transport.

Internalization and release of insulin from hepatocytes

Degradation of internalized insulin was studied after binding at 25 degrees C and 37 degrees C to isolated hepatocytes. The cells were washed to avoid extracellular insulin contamination. Degradation of both, intracellular and extracellular 125I-insulin, was measured with TCA and insulin antibody. In these conditions binding at 25 degrees C and 37 degrees C was equal but both intra and extracellular degradation were greater at 37 degrees C than at 25 degrees C. At both temperatures, intracellular degradation was greater than extracellular degradation with accumulation of degraded and non-degraded intracellular insulin. To study in what state hepatocytes release internalized insulin into the medium, 125I-insulin association was performed at an intermediate temperature (30 degrees C). Extracellular insulin contamination (whether associated or not) was avoided by three methods: 1) washing; 2) treatment with insulin degrading enzyme(s) and washing; 3) treatment with insulin degrading enzyme(s) then with trypsin and washing. Kinetics of radioactivity released from the cells was identical in the three conditions and the radioactivity was released throughout the experiments. Complete degradation of the released insulin was observed by gel filtration when the previous binding was 0.4 ng insulin/10(6) cells. When the dose of associated insulin increased (25 ng/10(6) cells) 3.5% of non-degraded insulin was liberated and when the dose was 14,300 ng/10(6) cells, the insulin released was 44.3%. In one experiment during the first 30 min, the insulin released was 52.88% and in the last 45 min 39.59%. To study the biologic behavior of the insulin released from cells, a group of mice were injected with this insulin (8.4 mU/mouse) and blood glucose was measured. The released insulin behaved as intact insulin as far as blood glucose responses were concerned. We may conclude that liver cells have the ability to internalize insulin and release biologically active insulin after accumulation.

Co-localization of a glucose transporter and the insulin receptor in microsomes of insulin-treated rat adipocytes

Microsomal vesicles prepared from rat adipocytes were immuno-adsorbed to formaldehyde-fixed Staphylococcus aureus cells (Pansorbin) coated with anti-human-erythrocyte-glucose-transporter IgG. More than 75% of the glucose transporter detected was precipitated. The glucose transporter was about 10-fold enriched by the adsorption procedure. On insulin treatment, the insulin receptor in plasma membranes was internalized and the receptor in the microsome fraction increased 5-fold. Thirty-five % of the insulin receptor in the microsome fraction was recovered with the glucose-transporter-containing vesicles. These observations indicate that on insulin treatment a considerable portion of the microsome vesicles containing the insulin receptor fuses or becomes tightly associated with ones containing the glucose transporter.

Insulin processing in primary endosomes is not responsible for insulin resistance observed in parametrial adipocytes from lactating rats

The fate of [125I insulin and the insulin receptor after internalization was characterized in parametrial adipocytes from virgin rats. Parallel experiments were carried out on parametrial adipocytes from 2-4-day lactating rats, which are insulin resistant. Similar results were obtained in adipocytes from either group of animals. Insulin caused 10% of the plasma membrane insulin receptor to be translocated to a compartment resistant to extracellular trypsin. The intracellularly located insulin receptor rapidly recycled to the plasma membrane at 37 degrees C. An endosomal compartment involved in both the endocytosis and subsequent recycling of [125I]insulin and the insulin receptor to the plasma membrane was identified on sucrose density floatation gradients. [125I]Insulin internalized at 37 degrees C accumulated in a fraction of modal density 1.12 g/ml. Crosslinking experiments revealed the presence of intact [125I]insulin-insulin receptor complexes in endosomes. After a pulse with [125I]insulin, 55-60% of the 125I radioactivity recovered in the endosome compartment was intact [125I]insulin. The remainder was composed of low molecular weight degradation products. Endosomal 125I radioactivity was rapidly retroendocytosed to the medium with a mean half-life of 6 min. These results suggest: (1) [125I]insulin and the insulin receptor are internalized by parametrial adipocytes into an early endosomal compartment (primary endosomes), from which the receptor, intact [125I]insulin, and [125I]tyrosine are returned to the cell surface; and (2) the damping of the insulin signal observed in parametrial adipocytes from lactating rats is not expressed at the level of altered endocytotic processing of [125I]insulin and the insulin receptor.

Photolabeling of erythrocyte and adipocyte hexose transporters using a benzophenone derivative of bis(D-mannose)

The benzophenone derivative of 1,3-bis(D-mannos-4-yloxy)-2-propylamine (BB-BMPA) has been tested as an exofacial photoaffinity label for the sugar transport systems of human erythrocytes and rat adipocytes. The half-maximal inhibition constants for the reagent are 971 microM in erythrocytes and 536 microM in basal and 254 microM in insulin-treated adipocytes. The photolabelling of erythrocyte membranes is very specific for the 50 kDa transporter peptide and is completely displaced by D-glucose. The exofacial photoaffinity labelling of adipocytes also shows labelling of a 50 kDa transporter peptide, which is displaced by cytochalasin B, but extensive nonspecific labelling of a 75 kDa plasma membrane peptide occurs. The transporter is labelled in insulin-treated cells but not in basal cells which indicates that this in situ labelling technique selectively reveals only those transporters that visit and are active in the plasma membrane during the labelling period. This also indicates that in basal cells transporters do not turn over rapidly. Subcellular redistribution of transporters after the labelling period has been studied. Following incubation and washing at 37 degrees C in the presence of insulin, 30% of the transporters photolabelled at the plasma membrane are internalised and are found in the light microsome fraction of the cell. The proportion of transporter that is observed to be internalised is much greater than can be accounted for by a contamination of the light microsome fraction by plasma membrane. The labelled 50 kDa transporter peptide in the light microsomes is enriched when compared with the carry-over of the 75 kDa nonspecifically labelled plasma membrane peptide. Thus we have obtained direct evidence for transporter translocation.

Giant proteoliposomes prepared by freezing-thawing without use of detergent: reconstitution of biomembranes usually inaccessible to patch-clamp pipette microelectrode

When sarcoplasmic reticulum membrane vesicles or synaptosomes were mixed with sonicated phospholipid vesicles and subjected to freezing-thawing, giant vesicles of up to 50 microns in diameter were formed. When the biomembrane vesicles were labeled with a covalently binding fluorescent dye, the resultant giant vesicles were fluorescent, thereby suggesting that the freezing-thawing process induces fusion of phospholipid and biomembrane vesicles. When membranes of the giant proteoliposomes thus prepared were studied using the patch-clamp technique, potassium channels of the biomembranes were detectable. The present method of the giant proteoliposome preparation is simple and rapid, and provides a system suitable for the study of ion channels of various biomembranes usually inaccessible to a patch-pipette microelectrode.

Analysis of intracellular receptor/ligand sorting in endosomes

After binding to specific cell surface receptors, many extracellular ligand molecules are internalized via the process termed receptor-mediated endocytosis. Within the cell, in endosomes, a sorting process occurs: receptors and ligands are directed along various intracellular pathways. The extent of this intracellular separation of receptors from ligands has been shown experimentally to vary with receptor and ligand properties such as binding affinity and valency. In this paper, we propose and analyze a simple model mechanism for the sorting process based on binding and dissociation kinetics along with diffusive molecular transport. We show that the outcome of the sorting process can be directly linked to measurable parameters such as the intrinsic rate constants for the binding to, dissociation from, and crosslinking of receptors by ligands. We further show that this mechanism is able to account for the wide range of reported experimental observations. Manipulation of ligand and receptor properties guided by the results presented here may enable the outcome of the sorting process to be controlled.

Effects of gluconeogenic hormones on insulin binding in intact human red blood cells

The effects of gluconeogenic hormones, adrenaline and cortisol, on insulin binding were studied in intact human red blood cells. Insulin binding was significantly decreased when red blood cells were preincubated with 1.0 microgram . ml-1 adrenaline or cortisol respectively. The Scatchard plot suggested that this was due to a decrease in surface receptor concentration. Furthermore, it showed that adrenaline also increased insulin receptor affinity. The negative co-operativity affinity profile demonstrated that adrenaline caused a rise in only the upper limit average affinity, Ki, of the insulin receptor.