Uptake of insulin by plasmalemma and Golgi subcellular fractions of rat liver

GTPases Rac1 and Ras Signaling from Endosomes

The endocytic compartment is not only the functional continuity of the plasma membrane but consists of a diverse collection of intracellular heterogeneous complex structures that transport, amplify, sustain, and/or sort signaling molecules. Over the years, it has become evident that early, late, and recycling endosomes represent an interconnected vesicular-tubular network able to form signaling platforms that dynamically and efficiently translate extracellular signals into biological outcome. Cell activation, differentiation, migration, death, and survival are some of the endpoints of endosomal signaling. Hence, to understand the role of the endosomal system in signal transduction in space and time, it is therefore necessary to dissect and identify the plethora of decoders that are operational in the different steps along the endocytic pathway. In this chapter, we focus on the regulation of spatiotemporal signaling in cells, considering endosomes as central platforms, in which several small GTPases proteins of the Ras superfamily, in particular Ras and Rac1, actively participate to control cellular processes like proliferation and cell mobility.

Insulin Signalling: The Inside Story

Insulin signalling begins with binding to its cell surface insulin receptor (IR), which is a tyrosine kinase. The insulin receptor kinase (IRK) is subsequently autophosphorylated and activated to tyrosine phosphorylate key cellular substrates that are essential for entraining the insulin response. Although IRK activation begins at the cell surface, it is maintained and augmented following internalization into the endosomal system (ENS). The peroxovanadium compounds (pVs) were discovered to activate the IRK in the absence of insulin and lead to a full insulin response. Thus, IRK activation is both necessary and sufficient for insulin signalling. Furthermore, this could be shown to occur with activation of only the endosomal IRK. The mechanism of pV action was shown to be the inhibition of IRK-associated phosphotyrosine phosphatases (PTPs). Our studies showed that the duration and intensity of insulin signalling are modulated within ENS by the recruitment of cellular substrates to ENS; intra-endosomal acidification, which promotes dissociation of insulin from the IRK; an endosomal acidic insulinase, which degrades intra-endosomal insulin; and IRK-associated PTPs, which dephosphorylate and, hence, deactivate the IRK. Therefore, the internalization of IRKs is central to insulin signalling and its regulation.

Spatial and Temporal Regulation of Receptor Tyrosine Kinase Activation and Intracellular Signal Transduction

Epidermal growth factor (EGF) and insulin receptor tyrosine kinases (RTKs) exemplify how receptor location is coupled to signal transduction. Extracellular binding of ligands to these RTKs triggers their concentration into vesicles that bud off from the cell surface to generate intracellular signaling endosomes. On the exposed cytosolic surface of these endosomes, RTK autophosphorylation selects the downstream signaling proteins and lipids to effect growth factor and polypeptide hormone action. This selection is followed by the recruitment of protein tyrosine phosphatases that inactivate the RTKs and deliver them by membrane fusion and fission to late endosomes. Coincidentally, proteinases inside the endosome cleave the EGF and insulin ligands. Subsequent inward budding of the endosomal membrane generates multivesicular endosomes. Fusion with lysosomes then results in RTK degradation and downregulation. Through the spatial positioning of RTKs in target cells for EGF and insulin action, the temporal extent of signaling, attenuation, and downregulation is regulated.

Global analysis of protein phosphorylation networks in insulin signaling by sequential enrichment of phosphoproteins and phosphopeptides

Although the important role of protein phosphorylation in insulin signaling networks is well recognized, its analysis in vivo has not been pursued in a systematic fashion through proteome-wide studies. Here we undertake a global analysis of insulin-induced changes in the rat liver cytoplasmic and endosomal phosphoproteome by sequential enrichment of phosphoproteins and phosphopeptides. After subcellular fractionation proteins were denatured and loaded onto iminodiacetic acid-modified Sepharose with immobilized Al³⁺ ions (IMAC-Al resin). Retained phosphoproteins were eluted with 50 mM phosphate and proteolytically digested. The digest was then loaded onto an IMAC-Al resin and phosphopeptides were eluted with 50 mM phosphate, and resolved by 2-dimensional liquid chromatography, which combined offline weak anion exchange and online reverse phase separations. The peptides were identified by tandem mass spectrometry, which also detected the phosphorylation sites. Non-phosphorylated peptides found in the flow-through of the IMAC-Al columns were also analyzed providing complementary information for protein identification. In this study we enriched phosphopeptides to ~85% purity and identified 1456 phosphopeptides from 604 liver phosphoproteins. Eighty-nine phosphosites including 45 novel ones in 83 proteins involved in vesicular transport, metabolism, cell motility and structure, gene expression and various signaling pathways were changed in response to insulin treatment. Together these findings could provide potential new markers for evaluating insulin action and resistance in obesity and diabetes.

Cellular signalling: Peptide hormones and growth factors

Peptide hormones and growth factors initiate signalling by binding to and activating their cell surface receptors. The activated receptors interact with and modulate the activity of cell surface enzymes and adaptor proteins which entrain a series of reactions leading to metabolic and proliferative signals. Rapid internalization of ligand-receptor complexes into the endosomal system both prolongs and augments events initiated at the cell surface. In addition endocytosis brings activated receptors into contact with a wider range of substrates giving rise to unique signalling events critical for modulating proliferation and apoptosis. Within the endosomal system, receptor function is regulated by lowering vacuolar pH, augmenting ligand proteolysis and promoting receptor kinase dephosphorylation. Ubiquitination-deubiquitination plays a key role in regulating receptor traffic through the endosomal system resulting in either recycling to the cell surface or degradation in multivesicular-lysosomal elements. From a clinical perspective there are several studies showing that manipulating endosomal processes may constitute a new therapeutic strategy.

Impaired hepatocyte glucose transport protein (GLUT2) internalization in chronic pancreatitis

Chronic pancreatitis (CP) is associated with impaired glucose tolerance and with reduced hepatic sensitivity to insulin. We have previously shown that in normal and sham-operated rats, insulin suppresses hepatic glucose production, and this suppression is associated with a decrease in the hepatocyte plasma membrane-bound quantity of the facilitative glucose transport protein GLUT2. The insulin-mediated reduction in membrane-bound GLUT2 is impaired in CP, and may play a role in the glucose intolerance associated with CP. To determine whether GLUT2 is actively internalized and whether this mechanism is disordered in CP, livers from fed and fasting rats in whom CP had been induced 2-3 months earlier by pancreatic duct oleic acid infusion, and in sham-operated (sham) rats, were fractionated to yield endosome (E)- and plasma membrane (PM)-enriched fractions. Forty-five minutes after duodenal intubation alone (fasting) or intubation plus duodenal feeding, livers were removed, homogenized and ultracentrifuged, and microsomal pellets were separated by sucrose density gradient ultracentrifugation. GLUT2 content of fractions was determined by Western blotting and scanning densitometry. The E:PM ratio of GLUT2 increased from 0.68 +/- 0.11 (mean +/- SEM) in fasting sham livers (n = 8) to 1.04 +/- 0.09 in fed sham livers (n = 8; p < 0.05). However, there was no change in the E:PM ratio of GLUT2 in CP livers after duodenal feeding (0.90 +/- 0.12 vs. 0.86 +/- 0.10; n = 8,8; p = NS). To test our findings using confocal laser scanning microscopy, liver specimens from fed and fasting CP and sham rats were minced, fixed in 4% paraformaldehyde, sectioned, and stained with rabbit antirat GLUT2 antibody followed by rhodamine-labeled secondary antibody. GLUT2 was quantified by mean pixel intensity in an 8 x 16-pixel area of PM and a 16 x 16-pixel area of cytosol (CYT) in each of 30 random cells/field (400x) in each of three rats per group. As in the fractionation study, duodenal feeding increased the CYT:PM ratio of GLUT2 from 0.75 +/- 0.01 in fasting sham liver to 0.86 +/- 0.01 in fed sham liver (p < 0.0001), while the CYT:PM ratio in CP remained unchanged. We conclude that feeding induces a shift in GLUT2 from the plasma membrane to the endosomal pool. The feeding-induced internalization of GLUT2 is absent in livers from rats with CP and may play a role in the glucose intolerance associated with CP.

The role of insulin dissociation from its endosomal receptor in insulin degradation

Mechanisms that terminate signals from activated receptors have potential to influence the magnitude and nature of cellular responses to insulin. The aims of this study were to determine in rat liver endosomes (the subcellular site of insulin signal termination) whether dissociation of insulin from its receptor was a pre-requisite for ligand degradation and whether the state of receptor phosphorylation influenced the dissociation and hence endosomal degradation of insulin and/or receptor recycling. Following in vivo administration of 125I-[A14]-insulin or analogues (native, X10 or H2, relative binding affinities 1:7:67) livers were removed and endosomes prepared. In the endosomal preparations a significantly greater percentage of both analogues were receptor-bound than native insulin with concomitantly less ligand degradation. When rats were injected with protein-tyrosine phosphatase inhibitors (peroxovanadium compounds bpV(phen) or bpV(pic)) before insulin, endosomal insulin receptor phosphotyrosine content, assessed by Western blotting, was increased as was receptor-bound 125I-[A14]-insulin, whilst insulin degradation was decreased. Peroxovanadiums also completely inhibited recycling of insulin receptors from endosomes. However, treatment of freshly isolated endosomes with acid phosphatase which completely dephosphorylated the insulin receptor, did not return the rate of insulin dissociation and degradation to control values, suggesting that peroxovanadium compounds elicit their effect on binding and degradation via a mechanism other than as protein-tyrosine phosphatase inhibitors. We conclude that promotion of sustained receptor binding decreases endosomal insulin degradation and extends the half-life of the activated endosomal receptor, which in turn would be expected to potentiate insulin signalling from this intracellular compartment.

ATP-dependent desensitization of insulin binding and tyrosine kinase activity of the insulin receptor kinase. The role of endosomal acidification

Incubating endosomes with ATP decreased binding of 125I-insulin but not 125I-labeled human growth hormone. Increasing ATP concentrations from 0.1 to 1 mM increased beta-subunit tyrosine phosphorylation and insulin receptor kinase (IRK) activity assayed after partial purification. At higher (5 mM) ATP concentrations beta-subunit tyrosine phosphorylation and IRK activity were markedly decreased. This was not observed with nonhydrolyzable analogs of ATP, nor with plasma membrane IRK, nor with endosomal epidermal growth factor receptor kinase autophosphorylation. The inhibition of endosomal IRK tyrosine phosphorylation and activity was completely reversed by bafilomycin A1, indicating a role for endosomal proton pump(s). The inhibition of IRK was not due to serine/threonine phosphorylation nor was it influenced by the inhibition of phosphotyrosyl phosphatase using bisperoxo(1,10-phenanthroline)oxovanadate anion. Prior phosphorylation of the beta-subunit with 1 mM ATP did not prevent the inhibition of IRK activity on incubating with 5 mM ATP. To evaluate conformational change we incubated endosomes with dithiothreitol (DTT) followed by SDS-polyacrylamide gel electrophoresis under nonreducing conditions. Without DTT the predominant species of IRK observed was alpha2 beta2. With DTT the alpha beta dimer predominated but on co-incubation with 5 mM ATP the alpha2 beta2 form predominated. Thus, ATP-dependent endosomal acidification contributes to the termination of transmembrane signaling by, among other processes, effecting a deactivating conformational change of the IRK.

Chloroquine extends the lifetime of the activated insulin receptor complex in endosomes

Insulin signal transduction, initiated by binding of insulin to its receptor at the plasma membrane, activates the intrinsic receptor tyrosine kinase and leads to internalization of the activated ligand-receptor complex into endosomes. This study addresses the role played by the activated insulin receptor within hepatic endosomes and provides evidence for its central role in insulin-stimulated events in vivo. Rats were treated with chloroquine, an acidotrophic agent that has been shown previously to inhibit endosomal insulin degradation, and then with insulin. Livers were removed and fractionated by density gradient centrifugation to obtain endosomal and plasma membrane preparations. Chloroquine treatment increased the amount of receptor-bound insulin in endosomes at 2 min after insulin injection by 93% as determined by exclusion from G-50 columns and by 90% as determined by polyethylene glycol precipitation (p < 0.02). Chloroquine treatment also increased the insulin receptor content of endosomes after insulin injection (integrated over 0-45 min) by 31% when compared with controls (p < 0.05). Similarly, chloroquine increased both insulin receptor phosphotyrosine content and its exogenous tyrosine kinase activity after insulin injection (64%; p < 0.01 and 96% and p < 0. 001, respectively). In vivo chloroquine treatment was without any observable effect on insulin binding to plasma membrane insulin receptors, nor did it augment insulin-stimulated receptor autophosphorylation or kinase activity in the plasma membrane. Concomitant with its effects on endosomal insulin receptors, chloroquine treatment augmented insulin-stimulated incorporation of glucose into glycogen in diaphragm (p < 0.001). These observations are consistent with the hypothesis that chloroquine-dependent inhibition of endosomal insulin receptor dissociation and subsequent degradation prolongs the half-life of the active endosomal receptor and potentiates insulin signaling from this compartment.

Effects of peroxovanadate and vanadate on insulin binding, degradation and sensitivity in rat adipocytes

The effects of vanadate and the stable peroxovanadate compound bpV(pic) on insulin binding and degradation were investigated in rat adipocytes under conditions of ongoing receptor cycling. Both bpV(pic) and vanadate increased 125I-insulin binding to intact cells through an increase in apparent receptor affinity. The maximal effect of bpV(pic) was to increase binding approximately 4-fold (EC50 0.06 +/- 0.01 mM), whereas vanadate increased binding approximately 2-fold (EC50 1.4 +/- 0.2 mM). Removal of cell surface insulin-receptor complexes with trypsin showed that the effects on binding exerted by bpV(pic) and vanadate were due to a similar increase in both cell surface binding and intracellular accumulation of radioactivity. Both bpV(pic) and vanadate inhibited the degradation of 125I-insulin in medium containing 1% bovine serum albumin. The ratio of degraded/intact intracellular 125I-insulin was also markedly reduced by these agents, suggesting that they inhibit intracellular insulin-degrading proteases. Similar to previous findings with vanadate, bpV(pic) stimulated glucose transport and, at low concentrations, enhanced insulin sensitivity. Taken together, these data demonstrate that both bpV(pic) and vanadate inhibit insulin degradation. In addition, they significantly enhance cell surface insulin binding in rat fat cells and this is associated with an improved insulin sensitivity.

Intracellular signal transduction: The role of endosomes

Polypeptide hormones, growth factors, and other biologically significant molecules are specifically internalized by target cells. Exposure of cells to these ligands results in the formation of ligand-receptor complexes on the cell surface and subsequent internalization of these complexes into the endosomal apparatus (endosomes, or ENs). The study of ENs has identified several important functions for this unique cellular organelle. These include the dissociation of ligand from receptor and receptor recycling to the cell surface and the degradation of some internalized ligands, as well as the delivery of others to lysosomes. More recently, it has become apparent that ENs fulfill another critical role, that of signal transduction. In this article, we review the evidence substantiating this role for ENs and propose three models by which ENs participate in cell signaling.

Selective activation of the rat hepatic endosomal insulin receptor kinase. Role for the endosome in insulin signaling

Insulin administration activates the insulin receptor kinase (IRK) in both plasma membrane (PM) and endosomes (ENs) raising the possibility of transmembrane signaling occurring in the endosomal compartment. Peroxovanadium compounds activate the IRK by inhibiting IR-associated phosphotyrosine phosphatase(s). Following the administration of the phosphotyrosine phosphatase inhibitor bisperoxo(1,10-phenanthroline)-oxovanadate (V) anion (bpV(phen)) activation of the hepatic IRK in ENs preceded that in PM by 5 min. When colchicine treatment preceded bpV(phen) administration IRK activation in ENs was unaffected but was totally abrogated in PM. Insulin receptor substrate-1 tyrosine phosphorylation followed the kinetics of IRK activation in ENs not PM and a hypoglycemic response similar to that achieved with a pharmacological dose of insulin ensued. These studies demonstrate that ENs constitute a site for IR-mediated signal transduction.

Histochemical and autoradiographic studies on elcatonin internalization and intracellular movement in osteoclasts

The binding sites and chronologic localization of elcatonin (eCT) in osteoclasts were examined by autoradiography using [125I]elcatonin (125I-eCT). In addition to the structural changes induced by calcitonin (CT) reported so far, changes were also observed in the structure of Golgi apparatus. These changes continued until 48-72 h after incubation with eCT. Developed silver grains of 125I-eCT were localized into multinucleated osteoclasts and mononuclear cells that were ultrastructurally defined as "preosteoclasts." The silver grains located on plasma membranes of those cells and were then internalized; they accumulated, especially in the Golgi apparatus, and remained for 48-72 h. A few silver grains were also detected in lysosomes and small vesicles. The decrease in the number of silver grains in the Golgi apparatus accompanied the recovery of osteoclast structures--Golgi apparatus and then ruffled borders. These findings suggest that (1) CT especially inhibits the sorting function of Golgi apparatus in osteoclasts, resulting in prolonged retention of CT in this organelle. (2) The CT in Golgi apparatus may keep its activity and cause the prolonged effect of CT on osteoclast activity.

Insulin induces rapid accumulation of insulin receptors and increases tyrosine kinase activity in the nucleus of cultured adipocytes

To better understand the mechanism by which insulin exerts effects on events at the cell nucleus, we have studied insulin receptors and tyrosine kinase activity in nuclei isolated by sucrose density gradient centrifugation following insulin treatment of differentiated 3T3-F442A cells. Insulin stimulated nuclear accumulation of insulin receptors by approximately threefold at 5 min. The half-maximal effect was observed with 1-10 nM insulin. Following insulin treatment, phosphotyrosine content associated with the nuclear insulin receptor was also increased by twofold at 5 min with a similar insulin concentration dependency. These nuclear insulin receptors differ from the membrane-associated insulin receptors in that they were not efficiently solubilized with 1% Triton X-100. During the same period of time, insulin stimulated nuclear tyrosine kinase activity toward the exogenous substrate poly Glu4:Tyr1 tenfold in a time-dependent manner reaching a maximum at 30 min. The insulin receptor substrate protein 1 (IRS-1) could not be detected in the nucleus by immunoblotting. However, a nuclear protein with M(r) approximately 220 kDa was tyrosine phosphorylated, and insulin further stimulated this process threefold > 30 mins. Surface labeling was performed to determine if the nuclear insulin receptors would emerge from the plasma membrane fraction. Using 125I-BPA-insulin with intact cells, the intensity of nuclear insulin receptor labeling was negligible and not increased throughout 30 min incubation at 37 degrees C. In contrast, there was an increase in labeled receptors in the microsomal fraction following insulin treatment. Taken together, these results indicate that insulin rapidly increases nuclear insulin receptor appearance and activates nuclear tyrosine kinase activity. The insulin-induced accumulation of nuclear insulin receptors cannot be accounted for by internalization of surface membrane receptors. These effects of insulin may play an important role in action of the hormone at the nuclear level.

Traffic through the Golgi apparatus as studied by radioautography

The ability to radiolabel biological molecules, in conjunction with radioautographic or cell fractionation techniques, has brought about a revolution in our knowledge of dynamic cellular processes. This has been particularly true since the 1940's, when isotopes such as 35S and 14C became available, since these isotopes could be incorporated into a great variety of biologically important compounds. The first dynamic evidence for Golgi apparatus involvement in biosynthesis came from light microscope radioautographic studies by Jennings and Florey in the 1950's, in which label was localized to the supranuclear Golgi region of goblet cells soon after injection of 35S-sulfate. When the low energy isotope tritium became available, and when radioautography could be extended to the electron microscope level, a great improvement in spatial resolution was achieved. Studies using 3H-amino acids revealed that proteins were synthesized in the rough endoplasmic reticulum, migrated to the Golgi apparatus, and thence to secretion granules, lysosomes, or the plasma membrane. The work of Neutra and Leblond in the 1960's using 3H-glucose provided dramatic evidence that the Golgi apparatus was involved in glycosylation. Work with 3H-mannose (a core sugar in N-linked side chains), showed that this sugar was incorporated into glycoproteins in the rough endoplasmic reticulum, providing the first radioautographic evidence that glycosylation of proteins did not occur solely in the Golgi apparatus. Studies with the tritiated precursors of fucose, galactose, and sialic acid, on the other hand, showed that these terminal sugars are mainly added in the Golgi apparatus. With its limited spatial resolution, radioautography cannot discriminate between label in adjacent Golgi saccules. Nonetheless, in some cell types, radioautographic evidence (along with cytochemical and cell fractionation data) has indicated that the Golgi is subcompartmentalized in terms of glycosylation, with galactose and sialic acid being added to glycoproteins only within the trans-Golgi compartment. In the last ten years, radioautographic tracing of radioiodinated plasma membrane molecules has indicated a substantial recycling of such molecules to the Golgi apparatus.

Immunocytochemical demonstration of the binding and internalization of growth hormone in GERL of Chang hepatoma cells

The binding and internalization of endogenous growth hormone in Chang hepatoma cells were localized on the cell surface and in the Golgi-endoplasmic reticulum-lysosome (GERL) area by various indirect immunocytochemical labeling techniques, namely, peroxidase or colloidal gold conjugated to secondary antibody, and avidin-biotin complex methods. Rabbit antiserum and monoclonal antibodies raised against HPLC-purified porcine growth hormone were used in this study. In fixed material, antigen-antibody complexes were found to be homogeneously distributed along the cell membrane. Control groups showed negative binding on the cell surface. Trypsin treatment before immunolabeling removed antibody binding completely, but hyaluronidase was ineffective. Pretreatment of lectins did not block the recognition of primary antibody to antigen molecules on cell surface. Internalization of the antigen-antibody peroxidase or gold complexes was demonstrated in the cells, which were immunolabeled at 4 degrees C, and then reincubated for 0-30 min at 37 degrees C before fixation. After reincubation, the internalized ligand complexes were found in vesicles near the cell surface or in the GERL area near the Golgi apparatus which, however, did not label for peroxidase. These findings suggest that the trypsin-sensitive growth hormone, specifically bound and internalized into Chang hepatoma cells, is localized in the GERL instead of the Golgi apparatus and might be involved in the mechanism of tumor cell growth.

Degradation of insulin in isolated liver endosomes is functionally linked to ATP-dependent endosomal acidification

The degradation of insulin in isolated liver endosomes and the relationships of this process with ATP-dependent endosomal acidification have been studied. Incubation of endosomal fractions containing 125I-insulin in isotonic KCl at 30 degrees C resulted in a rapid loss of insulin integrity as judged from trichloroacetic acid precipitability, Sephadex G-50 chromatography, immunoreactivity and receptor binding ability, with a maximum at pH 5-6 (t1/2: 10, 10, 6 and 6 min, respectively). On a log/log plot, the amount of acid-soluble products generated was linearly related to the amount of insulin associated with endosomes (slope, 0.80). Upon incubation, virtually all acid-soluble products diffused out of endosomes as judged from their solubility in aqueous poly(ethyleneglycol). In permeabilized endosomes, intact insulin was also released in part extraluminally, but only when degradation was inhibited did this release increase with lowering pH. ATP shifted the pH for maximal insulin degradation to about 7.5-8.5 and caused endosomal acidification as judged from the uptake of acridine orange and the fluorescence of internalized fluorescein-labeled dextran and galactosylated bovine serum albumin (delta pH about 0.8-0.9). GTP, ITP and UTP exerted comparable effects but with lower potencies. The ability of ATP to alter the pH dependence of insulin degradation was maximal in the presence of Cl-, other anions being less effective (Br- greater than gluconate = SO4(2-) greater than NO3- = sucrose = mannitol) and/or inhibitory (NO3-). Na+, K+ and Li+ supported more effectively ATP-dependent insulin degradation than did choline. Divalent cations were required for the ATP effect (Mg2+ = Mn2+ greater than Co2+ greater than Ni2+ = Zn2 greater than Ca2+). Little or no effects of ATP occurred in the presence of proton ionophores such as monensin and carbonyl cyanide chlorophenylhydrazone, and inhibitors of the proton ATPase such as N-ethylmaleimide. The abilities of nucleotides, ions and inhibitors to support or inhibit ATP-dependent insulin degradation were well correlated with their abilities to affect ATP-dependent acidification. The acidotropic agents chloroquine and quinacrine caused a leftward shift in the pH dependence of insulin degradation and a decrease in maximal degradation; in the presence of ATP, chloroquine almost completely inhibited degradation at pH 5-9. It is concluded that ATP-dependent acidification, in part by enhancing the dissociation of the insulin-receptor complex, is required for optimum degradation of insulin within liver endosomes.

Characterization of insulin degradation products generated in liver endosomes: in vivo and in vitro studies

The degradation products generated from A14 and B26 125I-labelled insulins in liver endosomes in vivo and in vitro have been isolated by high-performance liquid chromatography and cleavages in the B chain have been identified by automated radiosequence analysis. In rats sacrificed various times after injection of each of the 125I-labelled insulins, two major degradation products slightly less hydrophobic than intact iodoinsulins were identified; these accounted, at 8 min. for about 45% (A14 125I-labelled insulin) and 15% (B26 125I-labelled insulin) of the total radioactivity recovered, respectively. The products generated from A14 125I-labelled insulin contained an intact A chain, whereas those generated from B26 125I-labelled insulin contained a B chain cleaved at the B16-B17 bond. With B26 125I-labelled insulin, two minor products, with cleavages at the B23-B24 and B24-B25 bonds, were also observed. In vivo chloroquine treatment did not alter the nature but caused a decrease in the amount of insulin degradation products associated with endosomes. When endosomal fractions isolated from iodoinsulin injected rats were incubated at 30 degrees C in isotonic KCl, a rapid degradation of iodoinsulin, maximal at pH 6, was observed. With A14 125I-labelled insulin, the two major degradation products identified in vivo were generated along with monoiodotyrosine, but with B26 125I-labelled insulin monoiodotyrosine was the main product formed. Addition of ATP, presumably by decreasing the endosomal pH, shifted the medium pH for maximal iodoinsulin degradation to about 7-8. These studies have allowed a direct identification of two previously suggested cleavage sites in the B chain. They have also shown that the degradation products generated in cell-free endosomes under conditions that promote endosomal acidification are similar to those identified in vivo.

Hepatocellular processing of endocytosed proteins

In conclusion, proteins of hepatobiliary transport utilize receptor-mediated endocytosis and intracellular vesicles and rely on functionally dynamic microtubules for their transport by hepatocytes. The many diverse transport pathways in hepatocytes reflect the many functions served by the uptake of various proteins from the blood. The mechanisms of sorting of ligands and their receptors in endosomes and the factors that regulate the intracellular transport pathways are not yet known. Future investigations in this area promise to yield many exciting discoveries about the hepatocellular processing of proteins.

The characterization, regulation, and function of insulin receptors on osteoblast-like clonal osteosarcoma cell line

The properties and regulation of insulin receptors on monolayers of cultured clonal osteoblastic rat osteosarcoma UMR-106 cells and human osteosarcoma U20S cells were studied. Confluent cultures of UMR-106 cells bound lactoperoxidase-labeled, HPLC-purified [125I]A-14-monoiodinated insulin in a reversible, saturable, and specific manner. Binding was related inversely to the incubation temperature. Prolonged period of steady-state binding was achieved at all temperatures studied. Competition curves demonstrated half-maximal inhibition of [125I]insulin binding at an unlabeled insulin concentration of about 1 nM. Scatchard analysis of the binding data was curvilinear, suggesting negative cooperativity, and revealed that UMR-106 osteoblasts contained about 87,000 receptor sites per cell according to a two-site model. Bound [125I]insulin dissociated from osteoblasts with a t1/2 of about 15 minutes at 22 degrees C. The dissociation curve was multiexponential, and the addition of native insulin accelerated the dissociation of intact but not degraded [125I]insulin. Preincubation with 125 nM insulin for 1 h induced 70% loss of binding sites and reduced total insulin bound by 30%. When monolayers were treated with the lysosomotropic agent chloroquine, a 40% increase in cell-associated radioactivity that could not be dissociable in fresh buffer was observed. The use of an energy depleter, sodium fluoride, completely inhibited the effects of chloroquine. Similar results were obtained for human osteosarcoma U20S cells except that the number of receptor sites was far less than that of UMR-106 cells. Insulin increased collagen synthesis at a half-maximal concentration of 1 nM. To conclude, cultured rat and human osteoblasts possess insulin receptors that exhibit kinetic properties and specificity similar to those of other insulin target cells. Receptor-bound insulin is internalized and degraded by a chloroquine-sensitive, energy-requiring reaction. Insulin receptor on bone cells modulates the synthesis of collagen and this role may be important in bone homeostasis.

Binding and internalization in vivo of [125I]hCG in Leydig cells of the rat

The present study was performed to demonstrate the binding, mode of uptake, pathway and fate of iodinated human chorionic gonadotropin ([125I]hCG) by Leydig cells in vivo using electron microscope radioautography. Following a single injection of [125I]hCG into the interstitial space of the testis, the animals were fixed by perfusion with glutaraldehyde at 20 minutes, 1, 3, 6 and 24 hours. The electron microscope radioautographs demonstrated a prominent and qualitatively similar binding of the labeled hCG on the microvillar processes of the Leydig cells at 20 minutes, 1, 3, and 6 hours. The specificity of the [125I]hCG binding was determined by injecting a 100-fold excess of unlabeled hormone concurrently with the labeled hormone. Under these conditions, the surface, including the microvillar processes of Leydig cells, was virtually unlabeled, indicating that the binding was specific and receptor-mediated. In animals injected with labeled hCG and sacrificed 20 minutes later, silver grains were also seen overlying the limiting membrane of large, uncoated surface invaginations and large subsurface vacuoles with an electron-lucent content referred to as endosomes. A radioautographic reaction was also seen within multivesicular bodies with a pale stained matrix. At 1 hour, silver grains appeared over dense multivesicular bodies and occasionally over secondary lysosomes, in addition to the structures mentioned above, while at 3 and 6 hours, an increasing number of secondary lysosomes became labeled. At 24 hours, binding of [125I]hCG to the microvillar processes of Leydig cells persisted but was diminished, although a few endosomes, multivesicular bodies and secondary lysosomes still showed a radioautographic reaction. No membranous tubules that were seen in close proximity to, or in continuity with, endosomes and multivesicular bodies were observed to be labeled at any time interval. Likewise, an attempt to correlate silver grains with small coated or uncoated pits, the stacks of saccules of the Golgi apparatus and other Golgi-related elements including GERL, proved unsuccessful, since these structures were mostly unlabeled. These in vivo experiments thus demonstrate the specific binding of [125I]hCG to the plasma membrane of Leydig cells predominantly on their microvillar processes, and the subsequent internalization of the labeled hCG to secondary lysosomes. In addition, binding and internalization of hCG persisted for long periods of time.

Receptor-mediated endocytosis of enterokinase by rat liver. Preliminary characterisation of low-density endosomes

The endocytosis of enterokinase by rat hepatocytes has been studied both in a perfused liver system and in the intact, anaesthetised animal. 10 min after administration of the enzyme, only 70% of the activity was cleared by the perfused liver, whereas clearance was total in the intact animal. In both cases, about 85% of the internalised enzyme co-purified with the smooth microsomes and virtually all (more than 90%) of the catalytic activity was latent and could only be detected in the presence of detergent. After 10 min, 22.5% of the activity remained with the sinusoidal plasma membrane in the case of the perfused liver, while for the intact animal this figure was only 10%, confirming the more efficient clearance of enterokinase in the intact animal. Further subcellular fractionation showed that in the anaesthetised animal 8% of the internalised enzyme was associated with a low-density Golgi-like endosomal compartment (prepared from the mitochondrial pellet), whereas the corresponding value for the perfused liver was only 2.5%. Enterokinase specific activity was also up to 50-times greater in the low-density endosomes prepared from the intact animal. A second low-density Golgi-like compartment (purified from the smooth microsomes) also contained latent enterokinase, which together with the endosomes derived from the mitochondria accounted for 20% of the total enterokinase internalised by the liver 10 min after its administration to the intact animal. The passage of enterokinase through these two low-density compartments was shown not to be synchronous with its passage through the peripheral (sinusoidal membrane) and internal endosomes (smooth microsomes). There were qualitative differences in marker enzymes and polypeptide composition between the mitochondria and microsome-derived low-density endosomes. The sub-fractionation of low-density fractions on shallow sucrose gradients revealed a complex enzyme and polypeptide heterogeneity both between and within fractions. There was an apparent density-dependent separation of enterokinase from galactosyltransferase and the asialoglycoprotein receptor which was coincident with marked changes in the polypeptide composition of the endosomal membranes, particularly in the 30-45 kDa range.

Ligand-induced changes in the subcellular distribution of insulin receptors in rat liver: effects of colchicine

The in vivo effects of colchicine on the subcellular distribution of insulin receptors have been studied in insulin-injected rats and in control animals. Colchicine (0.1 mg/100 g or 10 mg/100 g body weight, i.v.) did not affect the ability of plasma membranes and Golgi fractions of control rats to bind insulin. As previously reported (Desbuquois et al., 1982), the injection of native insulin (8 nmol, i.v.) caused a 50% decrease in the insulin binding activity of plasma membranes and a concomitant 50% increase in insulin binding to Golgi fractions. These changes occurred at 4 and 40 min after insulin injection but were no longer detectable at 3 h. Colchicine treatment did not affect the initial changes in the distribution of insulin receptors induced by insulin; however, in rats treated with the low dose of colchicine, insulin binding to plasma membranes at 3 h was not fully restored. Colchicine treatment did not alter the amount of acid-extractable insulin associated with Golgi fractions of insulin-injected rats. The time course of uptake of 125I-insulin was similar in plasma membranes, microsomal fraction and Golgi fractions of colchicine-treated (0.1 mg/100 g) and of untreated rats. These results suggest that colchicine does not interfere with the endocytosis of insulin receptors induced by their ligand and has little effect, if any, on the reinsertion of internalized receptors in the plasma membrane.

Fate of injected 125I-labeled cholera toxin taken up by rat liver in vivo. Generation of the active A1 peptide in the endosomal compartment

Subcellular fractionation techniques have been used to assess the localization of injected 125I-labeled cholera toxin (125I-CT) taken up by rat liver in vivo, and to determine whether internalization of the toxin is required for the generation of the active A1 peptide. The uptake of injected 125I-CT into the liver is maximal at 5 min (about 10% injected dose/g). At this time the radioactivity is for the most part recovered in the microsomal (P) fraction, but later on it progressively associates with the mitochondrial-lysosomal (ML) and supernatant fractions. The radioactivity is enriched 7-fold in plasma membranes at 5-15 min, and 15-60-fold in Golgi-endosome (GE) fractions at 15-60 min. On analytical sucrose gradients the radioactivity associated with the P fraction is progressively displaced from the region of 5'-nucleotidase (a plasma membrane marker) to that of galactosyltransferase (a Golgi marker). On Percoll gradients, however, it is displaced towards acid phosphatase (a lysosomal marker). Density-shift experiments, using Triton WR 1339, suggest that some radioactivity associated with the P fraction (at 30 min) and all the radioactivity present in the ML fraction (at 2 h) is intrinsic to acid-phosphatase-containing structures, presumably lysosomes. Comparable experiments using 3,3'-diaminobenzidine cytochemistry indicate that the radioactivity present in GE fractions is separable from galactosyltransferase, and thus is presumably associated with endosomes. The fate of injected 125I-labeled cholera toxin B subunit differs from that of the whole toxin by a more rapid uptake (and/or clearance) of the ligand into subcellular fractions, and a greater accumulation of ligand in the ML fraction. Analysis of GE fractions by SDS/polyacrylamide gel electrophoresis shows that, up to 10 min after injection of 125I-CT, about 80% of the radioactivity is recovered as A subunit and 20% as B subunit, similarly to control toxin. Later on there is a time-dependent decrease in the amount of A subunit and, at least with the intermediate GE fraction, a concomitant appearance of A1 peptide (about 15% of the total at 60 min). In contrast the radioactivity associated with plasma membranes remains indistinguishable from unused toxin. It is concluded that, upon interaction with hepatocytes, 125I-CT (both subunits A and B) sequentially associates with plasma membranes, endosomes and lysosomes, and that endosomes may represent the major subcellular site at which the A1 peptide is generated.

Differential kinetics and sensitivity to chloroquine of receptor-mediated insulin and prolactin endocytosis in liver parenchymal cells

Systemically injected [125I]prolactin or [125I]insulin was accumulated and cleared from rat liver at different rates. Quantitative subcellular fractionation indicated a predominant accumulation of [125I]insulin in liver microsomes while [125I]prolactin was found in both the light-mitochondrial and microsomal fractions. The acidotropic agent chloroquine diminished the rate and extent of loss of each ligand from liver homogenates. In chloroquine treated rats, radiolabeled insulin accumulated in both the light-mitochondrial and the microsomal fractions. Subfraction of microsomes on discontinuous sucrose gradients revealed "early' endosomes in which ligand uptake was maximal at 2-5 min. In contrast, comparable subfraction of the of light mitochondrial fraction revealed "late' endosomes in which ligand uptake was maximal at 10-20 min. Chloroquine-treated rats showed a more marked enhancement of insulin compared to prolactin uptake in the "early' endosomes. It is suggested that "early' endosomes found in the Golgi-intermediate and -heavy fractions floated from parent microsomes may selectively degrade insulin but not prolactin. This could account for the apparently different kinetics of insulin and prolactin uptake into liver parenchyma.

Differential and analytical subfractionation of rat liver components internalizing insulin and prolactin

Receptor-mediated endocytosis of 125I-insulin and 125I-prolactin into liver parenchymal cells has been studied by quantitative subcellular fractionation. Differential centrifugation yielded three particulate fractions, N (nuclear), ML (large granule), and P (microsomes), and a final supernatant (S). Quantitative differences in the extent and rates of accumulation of 125I-insulin and 125I-prolactin into the fractions were observed. The acidotropic agent chloroquine and the microtubule disrupting agent colchicine were administered separately to rats. The agents increased significantly the T 1/2 of hormone clearance from the liver and augmented the accumulation of both ligands in the low-speed ML fraction. However, differences in the rates of accumulation of insulin and prolactin into all cell fractions were still maintained. Analytical centrifugation of each of the particulate fractions was carried out in order to determine if different endocytic components were specific to insulin or prolactin internalization. This was not the case. An "early" endosomal component of density 1.11 was identified in microsomes. A "late" endosome of density 1.10 was identified in the large granule (ML) fraction. Both endosomal components appeared to accumulate insulin and prolactin but at different rates. Marker enzyme analysis identified the presumed plasma membrane component in microsomes (density approximately 1.155). This component showed a significant difference in the rate of loss of 125I-insulin (T 1/2 approximately 4.1 min) as compared to that of 125I-prolactin (T 1/2 approximately 12.7 min). A further difference in the handling of the ligands was observed in early endosomes.(ABSTRACT TRUNCATED AT 250 WORDS)

Endocytosis of mannose-6-phosphate binding sites by mouse T-lymphoma cells

The endocytosis and intracellular transport of mannose-6-phosphate conjugated to bovine serum albumin (Man-6-P:BSA) by mouse T-lymphoma cells were investigated in detail using several methods of analysis, both morphological and biochemical. Man-6-P:BSA was labeled with fluorescein or 125I and used to locate both surface and intracellular Man-6-P binding sites by light or electron microscopy, respectively. Incubation of cells with either fluorescent- or 125I-labeled Man-6-P:BSA at 0 degree C revealed a uniform distribution of the Man-6-P binding sites over the cell surface. Competition experiments indicate that the Man-6-P:BSA binding sites on the cell surface are the same receptors that can recognize lysosomal hydrolases. After as little as 1 min incubation at 37 degrees C, endocytosis of Man-6-P binding sites was clearly observed to occur through regions of the plasma membrane and via vesicles that also bound anticlathrin antibody. After a 5-15-min incubation of cells at 37 degrees C, the internalized ligand was detected first in the cis region of the Golgi apparatus and then in the Golgi stacks using both autoradiography and immunocytochemistry to visualize the ligand. The appearance of Man-6-P:BSA in the Golgi region after 15-30 min was confirmed by subcellular fractionation, which demonstrated an accumulation of Man-6-P:BSA in light membrane fractions that corresponded with the Golgi fractions. After a 30-min incubation at 37 degrees C, the internalized Man-6-P binding sites were localized primarily in lysosomal structures whose membrane but not lumen co-stained for acid phosphatase. These results demonstrate a temporal participation of clathrin-containing coated vesicles during the initial endocytosis of Man-6-P binding sites and that one step in the Man-6-P:BSA transport pathway between plasma membrane and the lysosomal structure can involve a transit through the Golgi stacks.

Acute in vivo stimulation of low-Km cyclic AMP phosphodiesterase activity by insulin in rat-liver Golgi fractions

A low-Km phosphodiesterase activity, which is acutely stimulated by insulin in vivo, has been identified in plasma membranes and Golgi fractions prepared from rat liver homogenates in isotonic sucrose. Within seconds after insulin injection (25 micrograms/100 g body weight) cAMP phosphodiesterase activity increases by 30-60% in Golgi fractions and by 25% in plasma membranes; activity in crude particulate and microsomal fractions is unaffected. The increase in activity is short-lived in the light and intermediate Golgi fractions, but persists for at least 10 min in the heavy Golgi fraction. It precedes the translocation of insulin and insulin receptors to these fractions, which is maximal at 5 min. The doses of insulin required for half-maximal and maximal activation are, respectively, 7.5 micrograms/100 g and 25 micrograms/100 g body weight. Golgi-associated cAMP phosphodiesterase activity shows non-linear kinetics; a high-affinity component (Vmax, 13 pmol min-1 mg protein-1; Km, 0.35 microM) is detectable. Insulin treatment increases the Vmax 60-70%, but does not affect the Km. Unlike the low-Km cAMP phosphodiesterase associated with crude particulate fractions, the Golgi-associated activity is not easily extractable by solutions of low or high ionic strength. On analytical sucrose density gradients, low-Km cAMP phosphodiesterase associated with the total particulate fraction equilibrates at lower densities than endoplasmic reticulum and lysosomal markers, but at a higher densities than plasma membrane, Golgi markers and insulin receptors. Insulin treatment increases the specific activity of the enzyme by 20-60% at densities below 1.12 g cm-3, and by 20-40% in the density interval 1.23-1.25 g cm-3. Such treatment also causes a slight, but significant shift in the distribution of phosphodiesterase towards lower densities. It is suggested that Golgi elements or physically similar subcellular structures are a major site of localization of insulin-sensitive cAMP phosphodiesterase in rat liver. However, internalization of the insulin-receptor complex is probably not required for enzyme activation.

Localization of putative gonadotrophin releasing hormone receptor protein in the anterior pituitary

Specific binding of a fully biologically active 125I-gonadotrophin releasing hormone (GnRH) to isolated anterior pituitary cells is time dependent, saturable and the concentration dependent binding curves exhibit positive cooperativity. Binding to intact or solubilized plasma membranes and an affinity purified GnRH receptor protein reveals in all instances multiple high affinity binding sites. Thus, GnRH receptor protein appears to be an intrinsic constituent of the cell membrane, and perhaps, other membranous organelles. To investigate the latter, the binding of 125I-GnRH to various subcellular fractions was studied and its affinity and time requirements determined. GnRH binding to plasma membranes and secretory granules was to multiple high affinity sites, while that to nuclei and microsomes was to a single high affinity site. Binding was 1.83 +/- 0.07, 0.78 +/- 0.04, 0.31 +/- 0.03 and 0.27 +/- 0.03 fmol micrograms-1 protein for isolated plasma membranes, secretory granules, microsomes and nuclei, respectively, after 30 min incubation with 10(-9) M GnRH. The magnitude of binding to microsomes did not change during the incubation period. It did not show any decrease (p greater than 0.05) in isolated nuclei and plasma membranes, except for the 24 h time period, when a significant drop (p less than 0.001) was seen. Binding to the secretory granule fraction culminated at 15 min and then decreased (p less than 0.001) steadily to a non-detectable level at 24 h. Thus GnRH receptor protein or its portion may be an integral part of some membranous particles in the anterior pituitary cells.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo uptake of insulin into hepatic Golgi fractions: application of the diaminobenzidine-shift protocol

The hypothesis that insulin is internalized into the hepatic Golgi apparatus was tested by the diaminobenzidine-shift protocol of Courtoy et al. (1984, J. Cell Biol. 98, 870). Highly purified Golgi fractions were isolated after the coinjection of [125I]insulin and the synthetic ligand, galactose-bovine serum albumin-horseradish peroxidase. Golgi fractions were subsequently reacted in the presence or absence of diaminobenzidine, then subjected to Percoll gradient centrifugation. For incubations carried out in the absence of diaminobenzidine, [125I]insulin-containing components were found at a low density (peak density congruent to 1.042) identical to that of the Golgi marker enzyme galactosyltransferase. However after incubations carried out in the presence of diaminobenzidine, the majority of [125I]insulin-containing components was shifted to a higher density of greater than 1.06 while that of galactosyltransferase remained unchanged (peak congruent to 1.042). These observations indicate that the majority of internalized insulin is not located in galactosyltransferase-containing Golgi components.

Internalization of insulin and its receptor in the isolated rat adipose cell. Time-course and insulin concentration dependency

The time-course and insulin concentration dependency of internalization of insulin and its receptor have been examined in isolated rat adipose cells at 37 degrees C. The internalization of insulin was assessed by examining the subcellular distribution of cell-associated [125I]insulin among plasma membrane, and high-density (endoplasmic reticulum-enriched) and low-density (Golgi-enriched) microsomal membrane fractions prepared by differential ultracentrifugation. The distribution of receptors was measured by the steady-state exchange binding of fresh [125I]insulin to these same membrane fractions. At 37 degrees C, insulin binding to intact cells is accompanied initially by the rapid appearance of intact insulin in the plasma membrane fraction, and subsequently, by its rapid appearance in both the high-density and low-density microsomal membrane fractions. An apparent steady-state distribution of insulin per mg of membrane protein among these subcellular fractions is achieved within 30 min in a ratio of 1:1.54:0.80, respectively. Concomitantly, insulin binding to intact cells is associated with the rapid disappearance of approx. 30% of the insulin receptors initially present in the plasma membrane fraction and appearance of 20-30% of those lost in the low-density microsomal membrane fraction. However, the number of receptors in the high-density microsomal membrane fraction does not change. This redistribution of receptors also appears to reach a steady-state within 30 min. Both processes are insulin concentration-dependent, correlating with receptor occupancy in the intact cell, and are partially inhibited at 16 degrees C. While the steady-state subcellular distributions of insulin and its receptor do not correlate with that of acid phosphatase, chloroquine markedly increases the levels of insulin associated with all three membrane fractions in apparent proportion to the distribution of this lysosomal marker enzyme activity, without more than marginally potentiating insulin's effects on the distribution of receptors. These results demonstrate that insulin, initially bound to the plasma membrane of the isolated rat adipose cell, is rapidly translocated by a receptor-mediated process into at least two intracellular compartments associated with the cell's high- and low-density microsomes. Furthermore, insulin simultaneously induces the translocation of its own receptor from the plasma membrane into the latter compartment. These translocations appear to represent the internalization and partial dissociation of the insulin-receptor complex through insulin-induced receptor cycling.

Sedimentation characteristics of subcellular vesicles associated with internalized insulin and those bound with intracellular glucose transport activity

Comparative studies were made on the sedimentation characteristics of microsomal vesicles associated with internalized [125I]iodoinsulin and those bound with intracellular glucose transport activity. Upon linear sucrose density gradient centrifugation, the internalized hormone formed a peak slightly, but significantly, on the higher density side of the peak of intracellular glucose transport activity. After a long centrifugation, the peak of 125I activity became lower and broader than that of glucose transport activity. Internalized 125I activity was also found in the medium-density microsomal fraction, which had little glucose transport activity. Accumulation of 125I activity in the medium-density fraction and that in the low-density fraction were both completed in approximately 10 min. Under basal conditions, little, if any, insulin binding activity was detectable in either the medium- or low-density microsomal fractions; in contrast, some glucose transport activity was always present in the low-density fraction. These results indicate that the subcellular distribution of internalized insulin and of intracellular glucose transport activity are different, suggesting that the pathways of intracellular processing of the insulin receptor and the glucose transport mechanism are different.