Biochemical and morphological evidence that the insulin receptor is internalized with insulin in hepatocytes

Internalization and recycling of insulin receptors in hepatoma cells. Absence of regulation by receptor occupancy

Insulin receptors of Fao hepatoma cells were labelled with a 125I-labelled photoreactive insulin analogue or by surface iodination catalysed by lactoperoxidase. Cells were then incubated at 37 degrees C, and the cellular localization of the labelled receptors was assessed by limited exposure of intact cells to trypsin. The results show that: (1) photolabelled insulin-receptor complexes are internalized and recycled in Fao hepatoma cells; (2) the dynamics of photolabelled insulin receptors (internalization and recycling) is similar before and after down-regulation; (3) the unoccupied receptors labelled by surface iodination are internalized and recycled similarly to covalent insulin-receptor complexes; (4) insulin does not induce internalization of surface-iodinated insulin receptors. We conclude that internalization and recycling of insulin receptors are independent of receptor occupancy by insulin in Fao hepatoma cells.

Intracellular transformation of prolactin following internalization into rat liver

We investigated prolactin (PRL) degradation in rat liver lysosomes both in vivo and in vitro. In previous studies we showed that, in addition to the Golgi apparatus, PRL is internalized towards lysosomes and light, lysosome-like vesicles which we identified as 'prelysosomes'. Injected [125I]oPRL that localized in lysosomes and prelysosomes at times varying from 0 to 45 min showed significant differences from fresh and plasma membrane- (PM) or Golgi-bound hormone. First, it was more easily dissociable by 3 M MgCl2 than Golgi- but less than PM-bound [125I]oPRL. Second, it was only in lysosomal fractions that, as time following injection increased, a significant part of dissociable radioactivity became non-TAC-precipitable. When MgCl2-extracted [125I]oPRL was subjected to gel filtration on a Sephadex G-75 fine column, some of the radioactivity, and especially that extracted from prelysosomal or lysosomal fractions, eluted as a high molecular weight (HMW) entity, most co-migrated with fresh [125I]oPRL, and a little was found in small fragments. Only the central peak had any rebinding activity, which was comparable to that of fresh hormone. In an in vitro study we incubated [125I]hGH with lysosomal fractions for 16 h at 25 degrees C. After centrifugation, an aliquot of supernatant hormone was assayed for its binding capacity to standard receptor preparations and the rest subjected to gel filtration. Peak fractions were also tested in binding assay. [125I]hGH that had been in contact with prelysosomes lost almost all of its ability to bind to standard receptors and totally migrated in the HMW peak, at the void volume of the column.(ABSTRACT TRUNCATED AT 250 WORDS)

Spreading of non-transformed and transformed cells

Mechanisms of cellular reactions responsible for the spreading of non-transformed cultured tissue cells on the surface of various substrata and relationships of these reactions to the control of cell proliferation are reviewed; the special role of the membrane-cytoskeleton interactions leading to extension and attachment of pseudopods is stressed. Transition of cells from non-transformed to transformed phenotype is characterized by decreased spreading and by decreased dependence of proliferation on spreading. Manifestations of both of these spreading-associated changes are reviewed and their possible mechanisms are discussed. It is suggested that cell transition to transformed phenotype involves shift of an equilibrium between the reactions induced by the two groups of membrane-bound ligands: those attached and those not attached to the substratum.

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).

Immunoprecipitation of insulin receptors by antibodies against Class 1 antigens of the murine H-2 major histocompatibility complex

Insulin receptors from C57BL/6J mouse (H-2b) liver membranes were specifically labeled with 125I-photo-reactive insulin by UV irradiation. Membranes were solubilized and the capacity of various antibodies reacting with the major histocompatibility complex to immunoprecipitate insulin receptors was tested. About 5% of the labeled receptors were immunoprecipitated by a conventional mouse antiserum against H-2b histocompatibility antigens and by a monoclonal antibody against Class 1 antigens of the H-2b haplotype (Kb and Db). No immunoprecipitation was obtained with a monoclonal antibody against Class 2 antigens of I-Ab or against Class 1 antigens of the H-2k haplotype. Insulin receptors can thus be specifically immunoprecipitated by antibodies against class I histocompatibility antigens.

Reversible reduction of insulin receptor affinity by ATP depletion in rat adipocytes

The effects of the metabolic inhibitor NaN3 on insulin receptors in isolated rat fat-cells were investigated. The agent reduced insulin binding in parallel to a decrease of the ATP content of cells. Both effects were observed in the same concentration range of NaN3, and were fully reversible. According to the binding curves the affinity rather than the number of receptors was reduced. Kinetic experiments revealed an increased dissociation rate of the insulin-receptor complex. The effects outlasted cell disruption, since the receptor affinity was still lowered in plasma membranes obtained from NaN3-treated cells. Thus an inhibition of insulin internalization could not account for the observed effects. It is suggested that the observed ATP-dependence of insulin receptor affinity reflects a reversible structural alteration of the receptor, or of some closely related membrane protein.

Effects of microtubule-disrupting agents on insulin binding and degradation in isolated cardiocytes from adult rat

Isolated muscle cells from adult rat heart have been used to study the effects of microtubule disruptive drugs on the maintenance of steady-state insulin binding to cardiac insulin receptors. Vinblastine, vincristine and podophyllotoxin significantly inhibited insulin binding (25-50%) in the presence of insulin (10(-8) mol/l). The effect of vinblastine was found to be time- and temperature-dependent and to be dependent on the amount of insulin bound to the cell. In the presence of cycloheximide (0.1 mmol/l) insulin binding decreased by 30%; this effect was found to be additive to the action of vinblastine. Treatment of cells with vinblastine significantly reduced the low-molecular mass material produced by receptor-mediated degradation of insulin. This effect was not additive to that of the lysosomotropic agent chloroquine. The results suggest involvement of microtubules in the intracellular transfer of insulin receptors from and to the plasma membrane.

Differences in degradation processes for insulin and its receptor in cultured foetal hepatocytes

Binding and degradation of 125I-labelled insulin were studied in cultured foetal hepatocytes after exposure to the protein-synthesis inhibitors tunicamycin and cycloheximide. Tunicamycin (1 microgram/ml) induced a steady decrease of insulin binding, which was decreased by 50% after 13 h. As the total number of binding sites per hepatocyte was 20000, the rate of the receptor degradation could not exceed 13 sites/min per hepatocyte. Cycloheximide (2.8 micrograms/ml) increased insulin binding by 30% within 6 h, an effect that persisted for up to 25 h. This drug had a specific inhibitory effect on the degradation of proteins prelabelled for 10 h with [14C]glucosamine, without affecting the degradation of total proteins. Chronic exposure to 10 nM-insulin neither decreased insulin binding nor modified the effect of the drugs. The absence of down-regulation of insulin receptors cannot be attributed to rapid receptor biosynthesis in foetal hepatocytes. Cellular insulin degradation, which is exclusively receptor-mediated, was determined by two different parameters. First, the rate of release of degraded insulin into the medium was 600 molecules/min per hepatocyte with 1 nM labelled hormone, and increased (preincubation with cycloheximide) or decreased (tunicamycin) as a function of the amount of cell-bound insulin. Secondly, the percentage of cell-bound insulin degraded was not changed by the presence of protein-synthesis inhibitors (25-30%). The stability of insulin degradation suggested that this process was dependent on long-life proteinase systems. Such differences in degradation rates and cycloheximide sensitivity imply that hormone- and receptor-degradation processes utilize distinct pathways.

Degradation of acetylcholine receptors in muscle cells: effect of leupeptin on turnover rate, intracellular pool sizes, and receptor properties

The cellular mechanisms of degradation of a transmembrane protein, the acetylcholine receptor (AChR), have been examined in a mouse muscle cell line, BC3H-1. The halftime of degradation of cell surface receptors labeled with [125I] alpha-Bungarotoxin ([125I] alpha-BuTx) is 11-16 h. Leupeptin, a lysosomal protease inhibitor, slows the degradation rate two- to sixfold, depending on the concentration of inhibitor used. The inhibition is reversible since the normal degradation rate is regained within 20 h after removal of the inhibitor. Cells incubated with leupeptin accumulate AChR. Little change in the number of surface AChR occurs but the amount of intracellular AChR increases two- to threefold. Accumulated AChR are unable to bind [125I] alpha-BuTx if excess, unlabeled alpha-BuTx is present in the culture medium during leupeptin treatment. Thus, leupeptin causes the accumulation of a surface-derived receptor population not previously described in these cells. Subcellular fractionation studies utilizing Percoll and metrizamide gradient centrifugation in addition to molecular exclusion chromatography suggest that the accumulated AChR reside in a compartment with lysosomal characteristics. In contrast, the subcellular component containing another intracellular pool of AChR not derived from the surface is clearly separated from lysosomes on Percoll gradients. The sedimentation properties of AChR solubilized from the plasma membrane and the lysosomal fraction have been compared. The plasma membrane AChR exhibits a sedimentation coefficient of 9S in sucrose gradients containing Triton, whereas the AChR derived from the lysosomal fraction exists in part in a high molecular weight form. The large aggregate and the organelle in which it resides may represent important intermediates in the degradative pathway of this membrane protein.

Coated vesicles participate in the receptor-mediated endocytosis of insulin

We have purified coated vesicles from rat liver by differential ultracentrifugation. Electron micrographs of these preparations reveal only the polyhedral structures typical of coated vesicles. SDS PAGE of the coated vesicle preparation followed by Coomassie Blue staining of proteins reveals a protein composition also typical of coated vesicles. We determined that these rat liver coated vesicles possess a latent insulin binding capability. That is, little if any specific binding of 125I-insulin to coated vesicles is observed in the absence of detergent. However, coated vesicles treated with the detergent octyl glucoside exhibit a substantial specific 125I-insulin binding capacity. We visualized the insulin binding structure of coated vesicles by cross-linking 125I-insulin to detergent-solubilized coated vesicles using the bifunctional reagent disuccinimidyl suberate followed by electrophoresis and autoradiography. The receptor structure thus identified is identical to that of the high-affinity insulin receptor present in a variety of tissues. We isolated liver coated vesicles from rats which had received injections of 125I-insulin in the hepatic portal vein. We found that insulin administered in this fashion was rapidly and specifically taken up by liver coated vesicles. Taken together, these data are compatible with a functional role for coated vesicles in the receptor-mediated endocytosis of insulin.

Receptor-mediated endocytosis of insulin: role of microvilli, coated pits, and coated vesicles

When 125I-labeled insulin (125I-insulin) is incubated with 3T3-L1 adipocytes and cells processed for electron microscopic autoradiography, the ligand initially localizes preferentially to microvilli and coated pits. As a function of time and temperature, this initial preferential localization to microvilli is lost, and the ligand is internalized by the cell. Serial sections of apparent coated vesicles near the cell surface indicate that about half of these structures are true vesicles and, therefore, intermediates in this receptor-mediated endocytotic process. With time, 125I-insulin localizes to larger intracellular membrane-bounded structures. When cells are incubated with another ligand, cationic ferritin, that is taken up by adsorptive endocytosis, essentially the same structures are involved as for the endocytosis of 125I-insulin. The data suggest that specificity for receptor-mediated endocytosis is conferred by the specific ligand receptor and possibly by ligand-induced receptor mobility in the plane of the plasma membrane. Other structures such as coated pits, coated vesicles, larger vesicles, and secondary lysosomes are common for different ligands.

Photoaffinity labelling of the insulin receptor in intact rat hepatocytes, mouse soleus muscle, and cultured human lymphocytes

Using the photoreactive, biologically active insulin analogue, B2-(2 nitro, 4-azidophenylacetyl)des-PheB1 insulin, which can be covalently bound to receptor molecules upon photolysis, the insulin receptor has been studied in three different types of cells or tissues: isolated rat hepatocytes, intact murine soleus muscle and cultured human lymphocytes. When compared with native insulin, this analogue displayed a slightly reduced binding affinity. Accordingly, the biological potency of the photoreactive analogue was decreased by approximately 30% compared with native insulin when tested for its ability to stimulate amino acid transport in hepatocytes, and deoxyglucose uptake in soleus muscles. It was as effective as insulin, however, at maximally stimulating concentrations and therefore is a full insulin agonist. This photoprobe was used to specifically label the insulin receptor in the three tissues: after ultra-violet irradiation, sodium dodecyl sulphate-polyacrylamide gel analysis of extracts under reducing conditions revealed that most of the radioactivity was associated with a 130,000 dalton band. In isolated hepatocytes, two bands at 125,000 and 23,000 daltons were also specifically labelled. In three different cell types from three different animal species, the 130,000 dalton band appeared to be the major subunit of the insulin receptor.

Weak bases and ionophores rapidly and reversibly raise the pH of endocytic vesicles in cultured mouse fibroblasts

It has been shown that endocytic vesicles in BALB/c 3T3 cells have a pH of 5.0 (Tycko and Maxfield, Cell, 28:643-651). In this paper, a method for measuring the effect of various agents, including weak bases and ionophores, on the pH of endocytic vesicles is presented. The method is based on the increase in fluorescein fluorescence with 490-nm excitation as the pH is raised above 5.0. Intensities of cells were measured using a microscope spectrofluorometer after internalization of fluorescein-labeled alpha 2-macroglobulin by receptor-mediated endocytosis. The increase in endocytic vesicle pH was determined from the increase in fluorescence after addition of various concentrations of the test agents. The following agents increased endocytic vesicle pH above 6.0 at the indicated concentrations: monensin (6 microM), FCCP (10 microM), chloroquine (140 microM), ammonia (5 mM), methylamine (10 mM). The ability of many of these agents to raise endocytic vesicle pH may account for many of their effects on receptor-mediated endocytosis. Dansylcadaverine caused no effect on vesicle pH at 1 mM. The observed increases in vesicle pH were rapid (1-2 min) and could be reversed by removal of the perturbant. This reversibility indicates that the vesicles themselves contain a mechanism for acidification. The increase in vesicle pH due to these treatments can be observed visually using an SIT video camera. Using this method, it is shown that endocytic vesicles become acidic at very early times (i.e., within 5-7 min of continuous uptake at 37 degrees C).

Internalized insulin receptors are recycled to the cell surface in rat hepatocytes

We have followed the fate of cell surface insulin receptors in isolated rat hepatocytes by both a biochemical and a morphological approach. Hepatocytes were labeled with the photoreactive and biologically active 125I-labeled insulin analogue, [2-nitro-4-azidophenylacetylB2]des-PheB1-insulin, under conditions that allow for minimal internalization (2 hr at 15 degrees C). Analysis of the cell-associated radioactivity by NaDodSO4/polyacrylamide gel electrophoresis under reducing conditions followed by autoradiography revealed the specific labeling of a major insulin receptor subunit with Mr 130,000 and a minor degradation product with Mr 125,000. When the cells were exposed at 15 degrees C to trypsin at the end of the association period, these two bands were no longer observed, indicating that the labeled receptors were at the cell surface. This trypsin sensitivity of the receptor disappeared within 30-60 min of incubation of the cells at 37 degrees C, reflecting the internalization of the hormone-receptor complexes. Over the subsequent 4 hr of incubation, this was followed by a progressive reappearance of the receptor complexes at the cell surface, as indicated by the recovery of trypsin sensitivity of the labeled insulin receptors. An identical (both chronologically and quantitatively) journey of the insulin receptors was observed when the labeled material was studied by quantitative electron microscopic autoradiography. Thus, when the cells were incubated at 37 degrees C there was a rapid decrease (30-60 min) in the percentage of autoradiographic grains associated with the plasma membrane, followed by a progressive increase in this percentage over the subsequent 4 hr of incubation. In conclusion, using a biochemical and morphological approach to trace the photoaffinity-labeled insulin receptor, we have shown that the internalized hormone-receptor complex is recycled back to the cell surface.