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

Proteomics reveals novel protein associations with early endosomes in an epidermal growth factor-dependent manner

The epidermal growth factor receptor (EGFR) is a receptor tyrosine kinase that is an integral component of proliferative signaling. EGFRs on the cell surface become activated upon EGF binding and have an increased rate of endocytosis. Once in the cytoplasm, the EGF·EGFR complex is trafficked to the lysosome for degradation, and signaling is terminated. During trafficking, the EGFR kinase domain remains active, and the internalized EGFR can continue signaling to downstream effectors. Although effector activity varies based on the EGFR's endocytic location, it is not clear how this occurs. In an effort to identify proteins that uniquely associate with the internalized, liganded EGFR in the early endosome, we developed an early endosome isolation strategy to analyze their protein composition. Post-nuclear supernatant from HeLa cells stimulated with and without EGF were separated on an isotonic 17% Percoll gradient. The gradient was fractionated, and early endosomal fractions were pooled and immunoisolated with an EEA1 mAb. The isolated endosomes were validated by immunoblot using antibodies against organelle-specific marker proteins and transmission EM. These early endosomes were also subjected to LC-MS/MS for proteomic analysis. Five proteins were detected in endosomes in a ligand-dependent manner: EGFR, RUFY1, STOML2, PTPN23, and CCDC51. Knockdown of RUFY1 or PTPN23 by RNAi indicated that both proteins play a role in EGFR trafficking. These experiments indicate that endocytic trafficking of activated EGFR changes the protein composition, membrane trafficking, and signaling potential of the early endosome.

Insulin-dependent phosphorylation of DPP IV in liver. Evidence for a role of compartmentalized c-Src

Dipeptidyl peptidase IV (DPP IV, CD26, EC 3.4.14.5) serves as a model aimed at elucidating protein sorting signals. We identify here, by MS, several tyrosine-phosphorylated proteins in a rat liver Golgi/endosome (G/E) fraction including DPP IV. We show that a pool of DPP IV is tyrosine-phosphorylated. Maximal phosphorylation was observed after 2 min following intravenous insulin injection. DPP IV coimmunoprecipitated with the cellular tyrosine kinase Src (c-Src) with maximal association also observed after 2 min following insulin injection. DPP IV was found phosphorylated after incubation of nonsolubilized G/E membranes with [gamma-32P]ATP. The c-Src inhibitor PP2 inhibited DPP IV phosphorylation. Oriented proteolysis experiments indicate that a large pool of c-Src is protected in G/E fractions. Following injection of the protein-tyrosine phosphatase inhibitor bpV(phen), DPP IV levels markedly decreased by 40% both in plasma membrane and G/E fractions. In the fraction designated Lh, DPP IV levels decreased by 50% 15 min following insulin injection. Therefore, a pool of DPP IV is tyrosine-phosphorylated in an insulin-dependent manner. The results suggest the presence of a yet to be characterized signalling mechanism whereby DPP IV has access to c-Src-containing signalling platforms.

Cell itinerary and metabolic fate of proinsulin in rat liver: in vivo and in vitro studies

Proinsulin, the insulin precursor in pancreatic beta-cells, displays a slower hepatic clearance than insulin and exerts a more prolonged metabolic effect on liver in vivo. To elucidate the mechanisms underlying these differences, the cellular itinerary and processing of proinsulin and insulin in rat liver have been comparatively studied using cell fractionation. As [125I]-insulin, [125I]-proinsulin taken up into liver in vivo was internalized and accumulated in endosomes, in which it underwent dissociation from the insulin receptor and degradation in a pH- and ATP-dependent manner. However, relative to [125I]-insulin, [125I]-proinsulin showed a delayed and prolonged in vivo association with endosomes, a slower in vivo and cell-free endosomal processing, and a higher cell-free endosome-lysosome transfer. Endosomal extracts degraded to a lesser extent proinsulin than insulin at acidic pH; so did, and even proportionally less, at neutral pH, plasma membrane and cytosolic fractions. Proinsulin degradation products generated by soluble endosomal extracts were isolated by HPLC and characterized by mass spectrometry. Under conditions resulting in multiple cleavages in insulin, proinsulin was cleaved at eight bonds in the C peptide but only at the Phe24-Phe25 bond in the insulin moiety. As native insulin, native proinsulin induced a dose- and time-dependent endocytosis and tyrosine phosphorylation of the insulin receptor; but at an inframaximal dose, proinsulin effects on these processes were of longer duration. We conclude that a reduced proteolysis of proinsulin in endosomes, and probably also at the plasma membrane, accounts for its slower hepatic clearance and prolonged effects on insulin receptor endocytosis and tyrosine phosphorylation.

Checkpoints for vesicular traffic?

During interphase the transport of material between different intracellular organelles requires accurate regulation of fusiogenic domains. Recent studies on hepatic endosomes indicated that compartmentalized Cdk2 – cyclin E complexes act by braking fusion events. These Cdk2 complexes integrate tyrosine phosphorylation and dephosphory lation inputs, resulting in the control of the number of rounds of fusion at discrete domains. This leads to changes in the intracellular location of internalized receptors and ultimately their biological response.Key words: vesicular traffic, Cdk2, receptors tyrosine kinases.

Membrane-associated binding sites for estrogen contribute to growth regulation of human breast cancer cells

Membrane-associated binding sites for estrogen may mediate rapid effects of estradiol-17beta that contribute to proliferation of human breast cancers. After controlled homogenization and fractionation of MCF-7 breast cancer cells, the bulk of specific estradiol binding is found in nuclear fractions. However, a significant portion of specific, high-affinity estradiol-17beta binding-sites are also enriched in plasma membranes. These estradiol binding-sites co-purify with 5'-nucleotidase, a plasma membrane-marker enzyme, and are free from major contamination by cytosol or nuclei. Electrophoresis of membrane fractions allowed detection of a primary 67-kDa protein and a secondary 46-kDa protein recognized by estradiol-17beta and by a monoclonal antibody directed to the ligand-binding domain of the nuclear form of estrogen receptor. Estrogen-induced growth of MCF-7 breast cancer cells in vitro was blocked by treatment with the antibody to estrogen receptor and correlated closely with acute hormonal activation of mitogen-activated protein kinase and Akt kinase signaling. Estrogen-promoted growth of human breast cancer xenografts in nude mice was also significantly reduced by treatment in vivo with the estrogen receptor antibody. Thus, membrane-associated forms of estrogen receptor may play a role in promoting intracellular signaling for hormone-mediated proliferation and survival of breast cancers and offer a new target for antitumor therapy.

Characterization of Cdk2-cyclin E complexes in plasma membrane and endosomes of liver parenchyma. Insulin-dependent regulation

Rat liver parenchyma Golgi/endosomes fractions harbor a tyrosine-phosphorylated 34-kDa protein. Screening of Golgi, endosomes (ENs), plasmalemma (PM), and cytosolic (Cyt) fractions revealed the presence of the mitotic kinase Cdk2 in ENs, PM, and Cyt. The fluid phase endocytic marker horseradish peroxidase gained access to the endosomal Cdk2, confirming its localization. Cdk2 was shown to be associated to cyclin E and was active in ENs and PM fractions. The administration of a single dose of insulin (1.5 microgram/100 g, body weight) induced a time-dependent activation of the insulin receptor kinase in these structures. Insulin receptor-kinase activation was followed by the inhibition of immunoprecipitated Cdk2-cyclin E kinase activity in PM and the progressive disappearance of cyclin E. In marked contrast, no such effect was observed in ENs. The injection of a phosphotyrosyl phosphatase inhibitor (bpV(phen)) increased the levels of cyclin E in ENs and PM. A massive recruitment of p27(kip1) was observed in the Cdk2-cyclin E complexes isolated from PM and Cyt but not from ENs. In vitro, Cdk2-cyclin E complexes have the capacity to inhibit the formation of hybrid structures containing horseradish peroxidase and radioiodinated epidermal growth factor. Therefore, in the PM and ENs of adult rat liver, an active and regulated pool of the mitotic kinase Cdk2-cyclin E and some yet to be defined effectors are present. Cdk2 may contribute to the modulation of transport events and/or maintenance of the topology of endocytic elements.

Insulin degradation: progress and potential

Insulin degradation is a regulated process that plays a role in controlling insulin action by removing and inactivating the hormone. Abnormalities in insulin clearance and degradation are present in various pathological conditions including type 2 diabetes and obesity and may be important in producing clinical problems. The uptake, processing, and degradation of insulin by cells is a complex process with multiple intracellular pathways. Most evidence supports IDE as the primary degradative mechanism, but other systems (PDI, lysosomes, and other enzymes) undoubtedly contribute to insulin metabolism. Recent studies support a multifunctional role for IDE, as an intracellular binding, regulatory, and degradative protein. IDE increases proteasome and steroid hormone receptor activity, and this activation is reversed by insulin. This raises the possibility of a direct intracellular interaction of insulin with IDE that could modulate protein and fat metabolism. The recent findings would place intracellular insulin-IDE interaction into the insulin signal transduction pathway for mediating the intermediate effects of insulin on fat and protein turnover.

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.

Membrane transport in rat liver endocytic pathways: preparation, biochemical properties and functional roles of hepatic endosomes

The endocytic compartment has emerged as a major regulator of the uptake and processing of circulating ligands, and has been extensively studied during the last decade. In this work, the polypeptides of the three endosomal fractions: compartment of uncoupling receptors and ligands (CURL), multivesicular bodies (MVB) and receptor recycling compartment (RRC), isolated from livers of estradiol-treated rats, were analyzed by two-dimensional gel electrophoresis. Silver-stained gels revealed that although the three endosomal fractions shared a generally similar pattern of approximately 120 components, qualitative and quantitative differences between the three endocytic fractions could be demonstrated. The polypeptide composition of the bile was also studied and compared with ligands and proteins identified in the different endosomal fractions. One- and two-dimensional gel electrophoresis and Western blotting were used to investigate the protein composition of the three isolated endocytic fractions and 39 proteins were identified. The distribution of identified receptors, ligands and structural proteins among the three endosomal fractions was in agreement with their expected functionalities and with the different endocytic pathways in the hepatocyte.

Identification and distribution of proteins in isolated endosomal fractions of rat liver: involvement in endocytosis, recycling and transcytosis

1. The polypeptides of the three endosomal fractions isolated from livers of oestradiol-treated rats were analysed by two-dimensional gel electrophoresis. Silver-stained gels revealed that although the three endosomal fractions shared a generally similar pattern of approx. 120 components, qualitative and quantitative differences between the three endocytic fractions could be demonstrated. 2. The 'early' endosomes [compartment of uncoupling of receptors and ligands (CURL)] comprised the most complex fraction and contained most of the polypeptides found in the 'late' endosomes [multivesicular bodies (MVBs)] and the receptor recycling compartment (RRC). When CURL was analysed by two-dimensional gel electrophoresis after partition with Triton X-114, it showed the largest number of integral membrane polypeptides. 3. Some of the major receptors (polymeric immunoglobulin receptor, transferrin receptor, low-density lipoprotein receptor, asialoglycoprotein receptor, beta1-integrin, mannose 6-phosphate receptor, epidermal growth factor receptor and AGp110) and internalized ligands (IgA, IgG, albumin, haptoglobin, transferrin and alpha2-macroglobulin) were further studied by Western blotting. 4. The distribution of the identified receptors and ligands among the three endosomal fractions was in agreement with their expected functionalities. 5. The polypeptide composition of the bile was also examined and compared with ligands and proteins identified in the different endocytic fractions. 6. Finally, an electron microscopy study confirms the distinctive physical and ultrastructural features of the three isolated endosomal fractions.

Endosomes, receptor tyrosine kinase internalization and signal transduction

Upon the binding of insulin or epidermal growth factor to their cognate receptors on the liver parenchymal plasmalemma, signal transduction and receptor internalization are near co-incident. Indeed, the rapidity and extent of ligand mediated receptor internalization into endosomes in liver as well as other organs predicts that signal transduction is regulated at this intracellular locus. Although internalization has been thought as a mechanism to attenuate ligand mediated signal transduction responses, detailed studies of internalized receptors in isolated liver endosomes suggest an alternative scenario whereby selective signal transduction pathways can be accessed at this locus.

Proteolysis of glucagon within hepatic endosomes by membrane-associated cathepsins B and D

The acidic glucagon-degrading activity of hepatic endosomes has been attributed to membrane-bound forms of cathepsins B and D. Endosomal lysates processed full-length nonradiolabeled glucagon to 32 different peptides that were identified by amino acid analysis and full-length sequencing. These indicated C-terminal carboxypeptidase, endopeptidase as well as N-terminal tripeptidyl-aminopeptidase activities in endosomes. Glucagon proteolysis was inhibited 95% by E-64 and pepstatin A, inhibitors of cathepsins B and D, respectively. This was confirmed by the pH 6-dependent chemical cross-linking of [125I]iodoglucagon to a polypeptide of 30 kDa, which was immunodepleted by polyclonal anti-cathepsin B antibody, and the removal of greater than 80% of glucagon-degrading activity by polyclonal antibodies to cathepsins B and D. By similar criteria, insulin-degrading enzyme was ruled out as a candidate enzyme for endosomal proteolysis of glucagon. Lysosomal contamination was unlikely since all forms of cathepsin B in endosomes, i.e. the major 45-kDa inactive precursor as well as the lesser amounts of the 32- and 28-kDa active forms, were tightly bound to endosomal membranes. Furthermore the mature 29-kDa single-chain and 22-kDa heavy-chain forms of cathepsin L were undetectable in endosomes, although high levels of the 37-kDa proform were observed. Membrane association of the cathepsins B and D was not to the mannose 6-phosphate receptor since association was unaffected by mannose 6-phosphate and/or EDTA, thereby indicating a distinct endosomal receptor. Hence, a pool of active cathepsins B and D as well as a poorly defined tripeptidyl aminopeptidase is maintained in endosomes by selective membrane retention. These hydrolases degrade glucagon internalized into liver parenchyma early in endocytosis.

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.

Nuclear translocation of prolactin: collaboration of tyrosine kinase and protein kinase C activation in rat Nb2 node lymphoma cells

Recent evidence has suggested that prolactin (PRL), internalized by lactogen-dependent Nb2 lymphoma cells, is actively translocated to the nucleus where it binds to PRL receptors. Moreover, the mitogenic action of PRL in these cells has been separately linked to protein tyrosyl phosphorylation and activation of protein kinase C (PKC). Therefore, the coupling of PRL internalization and nuclear translocation to the activation of these signal transduction pathways was investigated. Results from control experiments indicated that 30% of internalized and 5% total cell-associated 125I-rat PRL could be recovered within nuclei obtained from Nb2 cells previously incubated with the radiolabel for 3 h at 37 degrees C. Furthermore, internalized PRL was found to be intact and not associated with any carrier proteins. Addition of tyrosine kinase (TK) antagonists, genistein or tyrphostin, significantly reduced cell surface binding, internalization, and nuclear translocation of 125I-rat PRL. In contrast, neither the level of cell-associated nor internalized hormone differed between cells treated with the PKC antagonists, staurosporine or calphostin C, and control cultures. Instead, PKC inhibition significantly reduced nuclear PRL translocation. The inhibitory effects of the TK and PKC antagonists on PRL internalization and nuclear translocation in intact Nb2 cells were verified by immunofluorescence microscopy in parallel experiments. In other experiments, each of the kinase inhibitors blocked PRL-stimulated Nb2 cell proliferation in a concentration-dependent manner. It is concluded that activated TK and PKC collaborate in the process of PRL internalization and translocation to the nucleus. TK activation may participate in PRL receptor binding or hormone internalization while activation of PKC appears to be required for its nuclear targeting. Since TK and PKC activation are required for lactogen-stimulated Nb2 cell proliferation, we suggest that a component of the mitogenic pathway in these cells is a direct nuclear interaction of PRL.

Physiological functions of endosomal proteolysis

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Acute effects of ethanol on hepatic endocytosis and processing of insulin in perfused rat liver

This study utilizes the perfused rat liver combined with subcellular fractionation and compartmental analysis to investigate the effects of ethanol on hepatic uptake, endocytosis, and processing of insulin. At 4 min after the start of a 2-min pulse of radiolabeled insulin, increasing ethanol concentrations progressively inhibited insulin uptake by the liver (57% at 50 mM ethanol). Subcellular fractionation of the perfused livers showed a progressive shift in distribution from a predominantly endosomal location (control) to a bimodal distribution between endosomes and plasma membrane. This could be largely accounted for by a specific reduction in the endosome-associated insulin. Binding studies showed no changes in the binding properties of the plasma membrane insulin receptor. Compartmental analysis of the perfusate efflux curves confirmed the lack of effect of ethanol on the binding constants but showed a significant decrease in the endocytic rate constant (50%) together with an increase in the retroendocytic rate constant (33%). Simulation studies with the compartmental model showed that these changes could account for the observed decrease in uptake by the liver. No changes were found in the subsequent endocytic degradation of insulin.

Identification of prolactin receptors in hepatic nuclei

Prolactin is a trophic hormone which may act directly at the hepatocyte nucleus. In this study, specific prolactin binding sites were sought in purified rat liver nuclei. Saturable and specific, high affinity 125I-prolactin binding sites were demonstrated to be on or within the nucleus. Prolactin binding was competitively inhibited by rat and ovine prolactins but not by rat growth hormone. Using immunogold electron microscopy, we detected prolactin receptors throughout the nucleus, in association with heterochromatin. Furthermore, endogenous immunoreactive prolactin was demonstrated to be within hepatic nuclei. We conclude that rat liver nuclei possess prolactin binding sites which likely participate in hormone-directed growth processes.

Fate of injected glucagon taken up by rat liver in vivo. Degradation of internalized ligand in the endosomal compartment

The uptake and processing of glucagon into liver endosomes were studied in vivo by subcellular fractionation. After injection of [[125I]iodo-Tyr10]glucagon and [[125I]iodo-Tyr13]glucagon to rats, the uptake of radioactivity into the liver was maximum at 2 min (6% of the dose/g of tissue). On differential centrifugation, the radioactivity in the homogenate was recovered mainly in the nuclear (N), microsomal (P) and supernatant (S) fractions, with maxima at 5, 10 and 40 min, respectively; recovery of radioactivity in the mitochondrial-lysosomal (ML) fraction did not exceed 6% and was maximal at 20 min. On density-gradient centrifugation, the radioactivity associated first (2-10 min) with plasma membranes and then (10-40 min) with Golgi-endosomal (GE) fractions, with 2-5-fold and 20-150-fold enrichments respectively. Subfractionation of the GE fractions showed that, unlike the Golgi marker galactosyltransferase, the radioactivity was density-shifted by diaminobenzidine cytochemistry. Subfractionation of the ML fraction isolated at 40 min showed that more than half of the radioactivity was recovered at lower densities than the lysosomal marker acid phosphatase. Throughout the time of study, the [125I]iodoglucagon associated with the P, PM and GE fractions remained at least 80-90% trichloroacetic acid (TCA)-precipitable, whereas that associated with other fractions, especially the S fraction, became progressively TCA-soluble. On gel filtration and h.p.l.c., the small amount of degraded [125I]iodoglucagon associated with GE fractions was found to consist of monoiodotyrosine. Chloroquine treatment of [125I]iodoglucagon-injected rats caused a moderate but significant increase in the late recovery of radioactivity in the ML, P and GE fractions, but had little effect on the association of the ML radioactivity with acid-phosphatase-containing structures. Chloroquine treatment also led to a paradoxical decrease in the TCA-precipitability of the radioactivity associated with the P and GE fractions. Upon h.p.l.c. analysis of GE extracts of chloroquine-treated rats, at least four degradation products less hydrophobic than intact [125I]iodoglucagon were identified. Radio-sequence analysis of four of these products revealed three cleavages, affecting bonds Ser2-Gln3, Thr5-Phe6 and Phe6-Thr7. When GE fractions containing internalized [125I]iodoglucagon were incubated in iso-osmotic KCl at 30 degrees C, a rapid generation of TCA-soluble products was observed, with a maximum at pH 4. We conclude that endosomes are a major site at which internalized glucagon is degraded, endosomal acidification being required for optimum degradation.

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.

A two-dimensional electrophoretic analysis of the proteins and glycoproteins of liver plasma membrane domains and endosomes. Implications for endocytosis and transcytosis

1. Polypeptides of liver plasma membrane fractions enriched in three surface domains of hepatocytes, blood-sinusoidal, lateral and bile canalicular, were analysed by isoelectric focusing (IEF) and non-equilibrium pH gel electrophoresis (NEPHGE) across a wide pH range, followed by SDS/PAGE. The overall Coomassie Blue-stained polypeptide patterns in the fractions were different. lateral plasma membrane fractions contained a characteristically higher number of polypeptides focusing at the basic pH range, whereas few basic polypeptides were present in sinusoidal plasma membrane fractions. The glycoproteins in these plasma membrane fractions stained by a lectin overlay technique with radio-iodinated concanavalin A, wheat-germ agglutinin and a slug lectin, were also different. 2. The polypeptides and glycoproteins of 'early' and 'late' endosome fractions were also compared by two-dimensional electrophoresis. Their composition was shown by Coomassie Blue staining, lectin overlay staining and in membranes metabolically labelled with [35S]methionine to be generally similar. The glycoproteins of sinusoidal plasma membranes and early and late endosomes were generally similar, but major differences in polypeptides of molecular mass 20-50 kDa, pI 7.5-8.5, in plasma membranes and endosomes were demonstrated, with a specific population of basic (pI 8-9) low-molecular-mass polypeptides being present at highest levels in 'late' endosomal fractions (shown by Coomassie Blue staining). 3. Analysis of the distribution of three specific membrane glycoproteins identified by using immunoblotting techniques showed that the asialoglycoprotein and the divalent-cation-sensitive mannose 6-phosphate receptors were present in sinusoidal plasma membrane and in early and late endocytic fractions: they were not detected in canalicular plasma membrane fractions. In contrast, 5'-nucleotidase was detected in all fractions examined. The role of the endocytic compartment in regulating trafficking pathways between the plasma membrane domains of the hepatocyte is discussed.

Selective degradation of insulin within rat liver endosomes

To characterize the role of the endosome in the degradation of insulin in liver, we employed a cell-free system in which the degradation of internalized 125I-insulin within isolated intact endosomes was evaluated. Incubation of endosomes containing internalized 125I-insulin in the cell-free system resulted in a rapid generation of TCA soluble radiolabeled products (t1/2, 6 min). Sephadex G-50 chromatography of radioactivity extracted from endosomes during the incubation showed a time dependent increase in material eluting as radioiodotyrosine. The apparent Vmax of the insulin degrading activity was 4 ng insulin degraded.min-1.mg cell fraction protein-1 and the apparent Km was 60 ng insulin.mg cell fraction protein-1. The endosomal protease(s) was insulin-specific since neither internalized 125I-epidermal growth factor (EGF) nor 125I-prolactin was degraded within isolated endosomes as assessed by TCA precipitation and Sephadex G-50 chromatography. Significant inhibition of degradation was observed after inclusion of p-chloromercuribenzoic acid (PCMB), 1,10-phenanthroline, bacitracin, or 0.1% Triton X-100 into the system. Maximal insulin degradation required the addition of ATP to the cell-free system that resulted in acidification as measured by acridine orange accumulation. Endosomal insulin degradation was inhibited markedly in the presence of pH dissipating agents such as nigericin, monensin, and chloroquine or the proton translocase inhibitors N-ethylmaleimide (NEM) and dicyclohexylcarbodiimide (DCCD). Polyethylene glycol (PEG) precipitation of insulin-receptor complexes revealed that endosomal degradation augmented the dissociation of insulin from its receptor and that dissociated insulin was serving as substrate to the endosomal protease(s). The results suggest that as insulin is internalized it rapidly but incompletely dissociates from its receptor. Dissociated insulin is then degraded by an insulin specific protease(s) leading to further dissociation and degradation.

Proteins associated with somatogenic and lactogenic receptors in microsomal membranes and intact rat hepatocytes

Lactogenic and somatogenic receptors present in rat liver have been examined by cross-linking with a derivative of human somatotropin (AP-hGH1) followed by SDS-polyacrylamide gel electrophoresis. AP-hGH1, which has a content of 2.2 azidophenacyl groups per molecule, mainly linked to half Cys-182 and half Cys-189, exerted a specificity similar to that of the native hormone (hGH), with an ability of 46% with respect to hGH to compete with the radiolabelled hormone for the binding sites of microsomal preparations. Photolysis of the 125I-labelled derivative bound to the lactogenic receptors present in either microsomal membranes or Triton X-100 solubilized preparations gave rise to a 63 kDa species. In addition, 30% of the covalent complexes formed in microsomal membranes belonged to a species with a molecular mass of 70 kDa. Incubation of viable rat hepatocytes with the radiolabelled derivative at either 0 degrees C for 3 h or 15 degrees C for 1.5 h and subjection to irradiation, yielded covalent complexes of molecular masses estimated at 130, 73, 63, 45 and 35 kDa. Experiments performed in the presence of 1 mM NaCN, gave rise to the previous species in a similar yield as that obtained in the absence of cyanide. The 130 kDa complex is related to the somatogenic binding sites, since it was not visualized in the presence of unlabelled bovine somatotropin, while the 70-73, 63, 45 and 35 kDa bands disappeared when the incubations were performed in the presence of unlabelled ovine prolactin.

Characterization of hepatic lactogen receptor. Subcellular distribution and characterization of N-linked carbohydrate chains

The types of carbohydrate chains present in a rat liver lactogenic hormone-binding receptor species with an Mr of 82,000, and in its hormone-binding subunits with Mr values of 40,000 and 35,000, were characterized using carbohydrate-chain-cleaving enzymes and affinity cross-linking. The subcellular distribution of lactogenic hormone-binding species was studied in organelle-enriched fractions. The monomeric Mr-40,000 and Mr-35,000 species contain N-linked tri- or tetra-antennary complex and high-mannose chains respectively. The Mr-82,000 species exists in two forms, where the Mr-40,000 and Mr-35,000 subunits are each combined with unglycosylated and, with the technique used, unlabelled subunit(s). Studies with organelle-enriched fractions revealed that the Mr-35,000 species was found in an endoplasmic reticulum-enriched fraction. The Mr-40,000 species was the predominant monomeric binding species in Golgi/endosome- and plasma membrane-enriched fractions. It is suggested that the Mr-35,000 species is a precursor to the Mr-40,000 species. In lysosome/endosome- or lysosome-enriched fractions, a broad distribution in Mr (35,000-40,000) was characteristic of the hormone-binding species. The Mr-82,000 species was only found in a Golgi/endosome-enriched fraction. Labelling of endosome lactogen receptor by injection of 125I-labelled ovine prolactin in vivo and cross-linking yielded only the Mr-40,000 species. Thus, the Mr-40,000 and Mr-35,000 lactogenic hormone-binding species each appear to be combined with the unglycosylated receptor subunit(s) in the Golgi complex to form Mr-82,000 heterodimeric complexes.

Reconstitution of the Golgi apparatus after microinjection of rat liver Golgi fragments into Xenopus oocytes

We have studied the reconstitution of the Golgi apparatus in vivo using an heterologous membrane transplant system. Endogenous glycopeptides of rat hepatic Golgi fragments were radiolabeled in vitro with [3H]sialic acid using detergent-free conditions. The Golgi fragments consisting of dispersed vesicles and tubules with intraluminal lipoprotein-like particles were then microinjected into Xenopus oocytes and their fate studied by light (LM) and electron microscope (EM) radioautography. 3 h after microinjection, radiolabel was observed by LM radioautography over yolk platelet-free cytoplasmic regions near the injection site. EM radioautography revealed label over Golgi stacked saccules containing the hepatic marker of intraluminal lipoprotein-like particles. At 14 h after injection, LM radioautographs revealed label in the superficial cortex of the oocytes between the yolk platelets and at the oocyte surface. EM radioautography identified the labeled structures as the stacked saccules of the Golgi apparatus, the oocyte cortical granules, and the plasmalemma, indicating that a proportion of microinjected material was transferred to the surface via the secretion pathway of the oocyte. The efficiency of transport was low, however, as biochemical studies failed to show extensive secretion of radiolabel into the extracellular medium by 14 h with approximately half the microinjected radiolabeled constituents degraded. Vinblastine (50 microM) administered to oocytes led to the formation of tubulin paracrystals. Although microinjected Golgi fragments were able to effect the formation of stacked saccules in vinblastine-treated oocytes, negligible transfer of heterologous material to the oocyte surface could be detected by radioautography. The data demonstrate that dispersed fragments of the rat liver Golgi complex (i.e., unstacked vesicles and tubules) reconstitute into stacked saccules when microinjected into Xenopus cytoplasm. After the formation of stacked saccules, reconstituted Golgi fragments transport constituents into a portion of the exocytic pathway of the host cell by a microtubule-regulated process.

Receptor-mediated endocytosis and degradation of bovine growth hormone in rat liver

Receptor-mediated endocytosis of radiolabelled bovine growth hormone (125I-bGH) via somatogenic receptors in the liver was studied following in vivo intraportal injection. At different times after injection, subcellular membrane fractions involved in binding (plasma membranes), endocytosis (endocytic vesicles) and degradation (lysosomes) of peptide hormones were isolated by sucrose density gradient centrifugation. These fractions were evaluated for the time-course accumulation of radiolabelled bGH and for the presence of internalized 125I-bGH-receptor complexes. These uptake studies indicate that after initial plasma membrane association of 125I-bGH, the ligand is transported in two successive endocytic compartments prior to arrival in lysosomes. The molecular weight of the somatogenic binders of male and female rat livers involved in internalization of 125I-bGH was determined to 95,000, 64,000, 55,000, 43,000 and 35,000, assuming a 1:1 binding of the hormone to the binder. These binders were seen in both endosomes and lysosomes, which suggests that growth hormone is transported to the lysosomes in a complex with its receptor. Binding and uptake of 125I-bGH was also compared in male and female rat livers, and endocytosis of 125I-bGH was compared to that of radiolabelled ovine prolactin (125I-oPrl). The specific uptake of 125I-bGH appeared not to be sexually differentiated in contrast to that of 125I-oPrl which showed a 35-fold higher uptake in female rat liver. Degradation of 125I-bGH was studied under in vitro binding assay conditions. A distinct 15,000 Da fragment was generated by plasma membrane, endosomal and lysosomal fractions. Based on protease inhibitor studies, a non-trypsin-like serine protease is suggested to be involved in the degradation of bGH. The 15,000 Da proteolytic fragment of GH can be affinity cross-linked to somatogenic binders of similar molecular weights as those involved in the binding of intact GH.

Uptake and intracellular fate of [14C]sucrose-insulin in perfused rat livers

Insulin was covalently linked to [14C]sucrose by means of cyanuric chloride to provide a label that would remain entrapped within the vacuolar system. The uptake of the conjugate by the perfused rat liver was rapid (half-life = 2.9 min), competitively inhibited by native insulin, and abolished by alkali denaturation. As assessed by its distribution on self-generating gradients of colloidal silica-povidone, label in lysosome-enriched samples of liver taken at different times after the addition of the conjugate moved progressively during 15 min from the plasma membrane into an intermediate peak and then to dense lysosomal fractions. After 30-60 min, the label had equilibrated throughout the lysosomal-vacuolar system. The initial movement from the plasma membrane to the intermediate peak occurred between 2 and 5 min. Because label in the peak could be physically separated from the lysosomal marker, beta-acetylglucosaminidase, by dispersing the sample through the gradient mixture before centrifugation rather than layering it, we concluded that the intermediate particles in question were not lysosomal in nature. On gel-filtration chromatography, label extracted from the intermediate peak did not move with insulin but rather as a broad band of lower molecular weight products, suggesting that insulin is subject to early proteolytic attack within a nonlysosomal compartment.

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.

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.