Binding, internalization, and lysosomal association of 125I-glucagon in isolated rat hepatocytes. A quantitative electron microscope autoradiographic study

Mechanisms of hyperinsulinemia and hyperglucagonemia after liver transplantation

These studies were undertaken to evaluate the mechanisms for changes in plasma insulin and glucagon levels observed post-liver transplantation. Two groups of pigs were studied: a control group (n = 8) underwent laparotomy and catheter placement in the carotid artery and portal and hepatic veins. Hepatic blood flow was measured by ultrasonic flow probes placed around the hepatic artery and portal vein. An experimental group (n = 8) underwent orthotopic liver transplantation and similar instrumentation. On Day 1 after surgery, an estimate of insulin and glucagon secretion and hepatic extraction was determined using arteriovenous difference techniques. Serum assays were performed for markers of hepatic and renal function. Plasma insulin levels of the transplanted pigs were higher in the carotid artery (4 +/- 1 microU/ml vs 7 +/- 1 microU/ml), but not in the hepatic vein (5 +/- 1 microU/ml vs 7 +/- 1 microU/ml) and in the portal vein (10 +/- 2 microU/ml vs 12 +/- 2 microU/ml). Arterial plasma C-peptide was significantly greater in the transplanted group (0.23 +/- 0.02 ng/ml vs 0.42 +/- 0.03 ng/ml); however, the molar ratio of C-peptide and insulin was not different between the two groups (3.6 +/- 0.9 vs 3.4 +/- 0.4). Plasma glucagon levels of the transplanted pigs were significantly elevated in the carotid artery (111 +/- 11 pg/ml vs 323 +/- 65 pg/ml), portal vein (221 +/- 27 pg/ml vs 495 +/- 69 pg/ml), and hepatic vein (142 +/- 15 pg/ml vs 395 +/- 58 pg/ml). The estimate of pancreatic secretion of insulin (115 +/- 28 microU/kg.min) vs 71 +/- 21 microU/kg.min) and glucagon (2.0 +/- 0.4 ng/kg.min vs 2.7 +/- 0.7 ng/kg.min) and the fractional hepatic extraction rate of insulin (35 +/- 8% vs 32 +/- 5%) were not different between the two groups. However, the hepatic fractional extraction rate of glucagon was significantly decreased in the transplanted group (25 +/- 5% vs 11 +/- 3%). Therefore, the hyperglucagonemia observed 24 hr following liver transplantation is partly due to reduced hepatic fractional extraction of glucagon while the hyperinsulinemia is mainly due to the nonhepatic clearance of insulin. We speculate that decreased renal function may contribute to the hyperinsulinemia, elevated C-peptide concentrations, and hyperglucagonemia.

Robert Feulgen Prize Lecture 1993. The journey of the insulin receptor into the cell: from cellular biology to pathophysiology

The data that we have reviewed indicate that insulin binds to a specific cell-surface receptor. The complex then becomes involved in a series of steps which lead the insulin-receptor complex to be internalized and rapidly delivered to endosomes. From this sorting station, the hormone is targeted to lysosomes to be degraded while the receptor is recycled back to the cell surface. This sequence of events presents two degrees of ligand specificity: (a) The first step is ligand-dependent and requires insulin-induced receptor phosphorylation of specific tyrosine residues. It consists in the surface redistribution of the receptor from microvilli where it preferentially localizes in its unoccupied form. (b) The second step is more general and consists in the association with clathrin-coated pits which represents the internalization gate common to many receptors. This sequence of events participates in the regulation of the biological action of the hormone and can thus be implicated in the pathophysiology of diabetes mellitus and various extreme insulin resistance syndromes, including type A extreme insulin resistance, leprechaunism, and Rabson-Mendehall syndrome. Alterations of the internalization process can result either from intrinsic abnormalities of the receptor or from more general alteration of the plasma membrane or of the cell metabolism. Type I diabetes is an example of the latter possibility, since general impairment of endocytosis could contribute to extracellular matrix accumulation and to an increase in blood cholesterol. Thus, better characterization of the molecular and cellular biology of the insulin receptor and of its journey inside the cell definitely leads to better understanding of disease states, including diabetes.

Identification of the guanine binding domain peptide of the GTP-binding site of glucagon

Glucagon, a peptide hormone synthesized and secreted by alpha islet cells, regulates glucose homeostasis by several mechanisms. Using [gamma 32P]8N3GTP, a proven photoaffinity probe for GTP, a specific nucleotide binding site on human glucagon was detected that showed preference for GTP. Half-maximal saturation of photoinsertion into the polypeptide hormone was at 8-12 microM with either [alpha 32P]8N3GTP or [gamma 32P]8N3GTP. GTP protected photolabeling with an apparent kd of 15 microM, whereas ATP was less effective as a protector, exhibiting an apparent kd of about 30 microM. Maximal protection by GTP and ATP was over 90%. UTP, CTP, GDP, ADP, GMP, AMP, guanosine, adenosine, guanine, and adenine were much less effective protectors, indicating that binding is specific for purine nucleoside triphosphates, particularly GTP. Mg2+ at 150 microM enhanced photoinsertion (twofold), whereas at 2-10 mM, it inhibited photoinsertion. Both Ca2+ and Zn2+ at 0.2 mM decreased photoinsertion about 45%. Purification of chymotryptic and tryptic digests of photolabeled glucagon by reverse-phase high performance liquid chromatography (HPLC) revealed that the N-terminal peptide, HSQGTF, was the only peptide region covalently photomodified by [32P]8N3GTP. GTP, if present during photolysis, greatly reduced both photoinsertion into glucagon and the amount of radiolabeled peptide recovered on HPLC. The specificity of binding to the N-terminal region is suggestive of a physiological role for a glucagon-GTP complex in the mechanism of action of this hormone.

Degradation of glucagon in isolated liver endosomes. ATP-dependence and partial characterization of degradation products

Endosomes have recently been identified as one major site of glucagon degradation in intact rat liver. In this study, a cell-free system has been used to assess the role of ATP-dependent acidification in endosomal glucagon degradation and identify the glucagon products generated. Percoll gradient fractionation of Golgi-endosomal fractions prepared 10-30 min after injection of [125I]iodoglucagon showed a time-dependent shift of the radioactivity towards high densities. Regardless of time, the radioactivity was less precipitable by trichloroacetic acid (Cl3Ac) at high densities than at low densities. Chloroquine treatment slightly increased the density shift of the radioactivity and decreased its Cl3Ac-precipitability throughout the gradient. Incubation of endosomal fractions containing [125I]iodoglucagon in 0.15 M-KCl at 30 degrees C resulted in a time- and pH-dependent generation of Cl3Ac-soluble radioactivity, with a maximum at pH 4 (t1/2, 7 min). At pH 5, 1,10-phenanthroline, bacitracin and p-chloromercuribenzoic acid partially inhibited [125I]iodoglucagon degradation. At pH 6-7, ATP stimulated [125I]iodoglucagon degradation by 5-10-fold and caused endosomal acidification as judged from Acridine Orange uptake. The effects of ATP were inhibited by chloroquine, monensin, N-ethylmaleimide and dansylcadaverine. Poly(ethylene glycol) (PEG) precipitation of the radioactivity associated with endosomes showed that lowering the pH below 5.5 caused dissociation of the glucagon-receptor complex, and that, regardless of incubation conditions, all degraded [125I]iodoglucagon diffused extraluminally. On h.p.l.c., at least three products less hydrophobic than [125I]iodoglucagon were identified in incubation mixtures along with monoiodotyrosine. Radiosequence analysis of the products revealed one major cleavage located C-terminally to Tyr-13 and two minor cleavages affecting Thr-5-Phe-6 and Phe-6-Thr-7 bonds. It is concluded that glucagon degradation in liver endosomes is functionally linked to ATP-dependent endosomal acidification and involves several cleavages in the glucagon sequence.

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.

The cell biology of the insulin receptor

Following initial binding to specific cell surface receptors insulin is internalized in target cells. The fate of the internalized insulin-receptor complexes and how the processes involved are regulated is reviewed. The implications of these events in the effects of insulin on its target cells and in the physiopathology of diabetes and insulin resistance states are also considered.

Binding and internalization of somatostatin, insulin, and glucagon by cultured rat islet cells

The pathways by which islet B, A, and D cells bind and internalize homologous (self) and heterologous (other) islet hormones were compared. [125I-Tyr]Somatostatin-14 (S-14), 125I-insulin, and 125I-glucagon were incubated with monolayer cultures of neonatal rat islet cells. Tissues were processed for quantitative electron microscopic autoradiography by the probability circle method coupled to morphometry. For all three radioligands and all three cell types surface labeling was rapidly followed by internalization of the radioligands into endocytotic vesicles. The further intracellular movement of the ligand occurred in a time- and temperature-related manner and depended on whether it was homologous or heterologous for the cell in question. Thus [125I-Tyr]S-14 in B and A cells, 125I-insulin in A and D cells, and 125I-glucagon in B and D cells were rapidly transferred from endocytotic vesicles to lysosomal structures. By contrast, [125I-Tyr]S-14 in D cells, 125I-insulin in B cells, and 125I-glucagon in A cells showed poor progression from endocytotic vesicles to downstream vesicular structures. We conclude that (a) each of the three radioligands is internalized by islet cells in a time- and temperature-dependent manner; (b) after initial internalization the further intracellular progression of the endocytosed radioligand occurs freely in cells heterologous for the radioligand but poorly in cells homologous for the radioligand; and (c) binding and endocytosis can be uncoupled from lysosomal degradation of ligand.

Quantitative analysis of internalization of glucagon by isolated hepatocytes

Biochemical methods have been used to quantitate total, acid-stable and acid-labile association of (mono[125I]iodoTyr10) glucagon with rat hepatocytes in suspension to evaluate internalization of glucagon and its receptors. Internalization is inhibited by low temperature, phenylarsine oxide, and by blocking receptor binding, consistent with receptor-mediated endocytosis. Approximately 30% of the total cell-associated hormone is internalized at 30 min of incubation. The rate declines until 90 min when the internalization of glucagon ceases, although the cells remain competent to internalize asialofetuin. From 90 min to 4 h, 27% of the maximum label internalized at 30 min remains within cells. The number of cell surface receptors decreases but the affinity of those remaining is unchanged. However, 1.7-2.7 surface receptors are lost to binding for each molecule of radiolabeled glucagon internalized. Uptake occurs according to a rate constant of 0.183 min-1 (t1/2 = 3.8 min). We conclude that (i) hepatocytes internalize a finite quantity of glucagon, implying the existence of undefined regulatory mechanisms; (ii) hormone is retained for greater than 2 h within cells and may play a physiological role within cells; and (iii) both occupied and unoccupied receptors become inaccessible to extracellular hormone as internalization proceeds; rapid recycling of receptors does not occur.

Fluorescent glucagon derivatives. II. The use of fluorescent glucagon derivatives for the study of receptor disposition in membranes

When monolayer cultured hepatocytes were incubated with 1 nM [125I]glucagon at 30 degrees C, equilibrium was reached after 10 min, whereas at 4 degrees C, equilibrium was reached after 60 min. At the higher temperature, 11.2% of the bound ligand was broken down after 60 min, at the lower temperature, the amount of degradation was negligible. At 30 degrees C, acid-washing did not remove specifically bound ligand; thus, it was assumed that the ligand was internalised at this temperature, since some of the specifically bound ligand could be washed off at lower temperatures. This was confirmed in experiments when monolayer cultures of hepatocytes were incubated with fluorescein-labelled derivatives of glucagon. The distribution of specific binding on the cell surface was studied at both 30 and 4 degrees C using video intensification microscopic techniques. In keeping with studies using radiolabelled glucagon, more fluorescence was detected following incubation at 4 degrees C than at 30 degrees C and it could be removed by washing the cells. Video intensification microscopy indicated that at the lower temperature, the bound ligand was distributed all over the cell surface. At the higher temperature, ligand-derived fluorescence could only be detected in mobile intracellular vesicles.

Perifused precision-cut liver slice system for the study of hormone-regulated hepatic glucose metabolism

A nonrecirculatory perfusion system for precision-cut rat liver slices has been developed and utilized for investigating hormone-regulated hepatic glucose metabolism. In this system, slices are cultured in a highly controlled environment and exhibit excellent retention of viability as judged by their maintenance of intracellular potassium and glycogen contents. Using this system, the complex physiological phenomenon of hormone-regulated glycogenolysis was investigated at both extra- and intracellular sites. Specifically, the sensitive responses of intracellular cyclic AMP (cAMP) production, activation of cyclic AMP-dependent protein kinase, and production of glucose upon glucagon stimulation have been measured. The maximal responses observed for these parameters were either equal to or greater than those previously reported for either isolated hepatocytes or perfused livers, demonstrating the sensitivity of this technique. Upon dose-response examination of glucagon challenge, it was observed that high doses of glucagon (greater than 16 nM) stimulate glucose production by activating the cAMP-second messenger cascade. In contrast, low doses (less than 4 nM) stimulate this process without production of intracellular cAMP or activation of cAMP-dependent protein kinase, suggesting the operation of cAMP-independent messenger. Since this system permits measurements of parameters common to many cellular processes, this methodology is suitable for addressing both pharmacological and toxicological questions.

Insulin and alpha 2-macroglobulin-methylamine undergo endocytosis by different mechanisms in rat adipocytes: I. Comparison of cell surface events

This ultrastructural study compared the endocytosis of a peptide hormone, ferritin-labeled insulin (Fm-I) or gold-labeled insulin (Au-I), and a non-hormonal ligand, gold-labeled alpha-2-macroglobulin-methylamine (Au-alpha 2MGMA), by rat adipocytes. Quantitative analysis of the cell surface showed that coated pits occupied 0.4% of the adipocyte surface. This was one fifth to one tenth of that which has been reported on fibroblasts and hepatocytes, cell types in which receptor-mediated endocytosis has been extensively studied. In contrast, uncoated micropinocytotic invaginations were quite numerous and occupied 13.1% of the adipocyte cell surface. The frequency of micropinocytotic invaginations, 13.8 per micron 2 of plasma membrane, was 7-12 times greater than has been reported on fibroblasts. Therefore, the ultrastructure of the endocytic apparatus on rat adipocytes was different from more commonly studied cell types. At 4 degrees C, Au-alpha 2MGMA concentrated within coated pits to a density that was 52 times greater than that on the uncoated plasma membrane. Au-alpha 2MGMA was excluded from micropinocytotic invaginations by more than 93%; this exclusion was unrelated to the size of the Au-alpha 2MGMA particle. In contrast, at 4 degrees C, Fm-I did not concentrate within coated pits and occupied micropinocytotic invaginations in a random manner. At 37 degrees C, coated pits accounted for all of the endocytosis of Au-alpha 2MGMA, proving that these structures were functional despite their atypically low density. In contrast, greater than 99% of the endocytosis of Fm-I or Au-I occurred through micropinocytotic invaginations. These results demonstrated for the first time by a comparative, quantitative, ultrastructural method that insulin and Au-alpha 2MGMA undergo endocytosis by dissimilar mechanisms on rat adipocytes. Dissimilarities in the endocytosis of insulin and Au-alpha 2MGMA may be related to the different biological roles of these two molecules.

Hepatic glucagon metabolism. Correlation of hormone processing by isolated canine hepatocytes with glucagon metabolism in man and in the dog

We have found that canine and rat hepatocytes convert (125I)iodoTyr10-glucagon to a peptide metabolite lacking the NH2-terminal three residues of the hormone. The peptide is released into the cell incubation medium and its formation is unaffected by a variety of lysosomotropic or other agents. Use of specific radioimmunoassays and gel filtration demonstrated in both normal subjects and in chronic renal failure patients a plasma peptide having the properties of the hormone fragment identified by cell studies. Studies of the dog revealed a positive gradient of the fragment across the liver and no differential gradient of the fragment and glucagon across the kidney. We conclude that the glucagon fragment arises from the cell-mediated processing of the hormone on a superficial aspect of the hepatocyte, the glucagon fragment identified during experiments in vitro represents the cognate of a peptide formed during the hepatic metabolism of glucagon in vivo, and measurement of the fragment by COOH-terminal radioimmunoassays could lead to an understimulation of hepatic glucagon extraction.

Binding and degradation of 125I-glucagon by highly purified rat liver plasma membranes

125I-glucagon binding and degradation were studied in highly purified plasma membranes from rat livers. Specific 125I-glucagon binding increased rapidly with time at 30 degrees C and reached a maximum between 30 and 120 min. At 120 min the labelled material present in the supernatants from incubation mixtures had extensively lost its ability to rebind to fresh membranes whatever the glucagon concentration. This impairment was not due to the release of a degradative activity into the incubation mixture, suggesting a membrane-mediated process. The presence of proteinase inhibitors (bacitracin/aprotinin) resulted both in an increase in specific 125I-glucagon binding to membranes and an improvement in the ability of the labelled material from the supernatant to rebind to fresh membranes. When analysed by Bio-Gel P-10 chromatography the loss in the ability of the labelled material in the supernatants to rebind to fresh membranes correlated with a decrease in the labelled material which eluted as 125I-glucagon from the column. Chromatographic analysis overestimated 125I-glucagon when compared to the radioreceptor assay. The labelled material extracted from membranes by Triton X-100 solubilization or dissociated from membranes after exposure to an excess of unlabelled glucagon mainly eluted as 125I-glucagon. However, a significant amount (20-30%) of the labelled material eluted in the low molecular weight region.

Characterization of glucagon receptors in Golgi fractions of rat liver: evidence for receptors that are uncoupled from adenylyl cyclase

Glucagon receptors have been identified and characterized in intermediate (Gi) and heavy (Gh) Golgi fractions from rat liver. At saturation, plasma membranes bound 3500 fmol of hormone/mg of membrane protein, while Gi and Gh bound 24 and 60 fmol of 125I-glucagon/mg of protein, respectively. Half-maximal saturation of binding to plasma membranes, Gi, and Gh occurred at approximately 4, 10, and 20 nM 125I-glucagon, respectively. Trichloroacetic acid precipitation of intact, but not degraded, glucagon was used to correct binding isotherms for hormone degradation. After such correction, half-maximal saturation of binding to plasma membranes, Gi, and Gh was observed in the presence of approximately 2, 7, and 14 nM hormone, respectively. After 90 min of dissociation in the absence of guanosine 5'-triphosphate (GTP), 86% of 125I-glucagon remained bound to plasma membranes, whereas only 42% remained bound to Golgi membranes. GTP significantly increased the fraction of 125I-glucagon released from plasma membranes but only slightly augmented the dissociation of hormone from Golgi fractions. 125I-Glucagon/receptor complexes solubilized from plasma membranes fractionated by gel filtration as high molecular weight (Kav = 0.16), GTP-sensitive complexes and lower molecular weight (Kav = 0.46), GTP-insensitive complexes. 125I-Glucagon complexes solubilized from Golgi membranes fractionated almost exclusively as the lower molecular weight species. The lower affinity of Golgi than plasma membrane receptors for hormone, the ability of glucagon to stimulate plasma membrane, but not Golgi membrane, adenylyl cyclase, and the near absence of high molecular weight, GTP-sensitive complexes in solubilized Golgi membranes demonstrate that plasma membrane contamination of Golgi fractions cannot account for the 125I-glucagon binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Cycle of VIP in the human transformed colonic epithelial cells (HT-29) in culture

The kinetics of VIP processing in different compartments (medium, plasma membrane and intracellular) by HT-29 cells was studied using direct biochemical methods and a homogeneous monoiodinated VIP. The compartmental radioactivity was characterized by HPLC fractionation and specific receptor binding. VIP once bound to the cell surface remains intact and is rapidly and maximally internalized (less than 10 min) at 37 degrees C. Then two different processes occur: (1) release of degradation products 125I and monoiodinated tyrosine in the medium; (2) VIP remains intact in the cells representing 67.2 +/- 4.7% of total radioactivity up to 90 min. The overall processing of VIP is time- and temperature-dependent and maximal internalization of VIP with minimal medium release is observed at 20 degrees C. Our results demonstrate a receptor mediated internalization of VIP and that at least two intracellular pathways may exist in the cycle of VIP. One is associated with a complete degradation of VIP detected in the extracellular medium and is optimal at 37 degrees C. The other results in the presence of intact intracellular VIP and is optimal at 20 degrees C.

Isolation, characterization, and regulation of the prolactin receptor

The prolactin, or lactogenic hormone, receptor has been purified (approximately 80%) from lactating mouse liver and human term placenta by the nondenaturing zwitterionic detergent 3-[(3-cholamidopropyl)-dimethylammonio]-1-propane sulfonate and a prolactin affinity column. The isolated "core-binding unit" has a molecular weight of 37,000 +/- 2,000 daltons. It retains the specificity for lactogenic hormones and binds prolactin with an affinity (Ka = 2 to 6 X 10(9) M-1) similar to that of the receptor as it occurs in its membranous environment (Ka = 3 to 5 X 10(9) M-1). Whether this "core-binding unit" exists on the cell surface in a cryptic or active form is influenced greatly by its association with other membrane proteins and the concentration of phosphatidylcholine within its local membranous environment.

Receptor-mediated endocytosis of glucagon in isolated mouse hepatocytes

The binding of glucagon to the cell surface and the pathway of intracellular transport of the hormone in isolated mouse hepatocytes were studied by autoradiography, colloidal gold-labeled glucagon (Au-glucagon), and biochemical methods. In cells incubated with 1251-glucagon at 4 degrees C, the label was mainly localized to the plasma membrane even after 60 min of incubation. At 20 degrees C, the labeled ligand was internalized by the cells and the amount of internalized ligand increased with time of incubation. At 37 degrees C, the ligand was rapidly internalized and found to be associated with coated or uncoated vesicles. Au-glucagon experiments revealed clearly the process of internalization of glucagon. Au-glucagon bound to the plasma membrane was transported to coated regions and then internalized into vesicles via coated pits. Biochemical results supported these findings from autoradiography and Au-glucagon experiments. Thus, glucagon is internalized by hepatocytes via receptor-mediated endocytosis.

Glucagon degradation by human mononuclear cells

Degradation of 125I-iodoglucagon by human mononuclear cell preparations including one containing 18%-27% monocytes, one consisting of 97% pure monocytes and one consisting of 98% lymphocytes was examined. Intact cells were incubated with 125I-iodoglucagon and degradation assessed by measuring an increase in trichloroacetic acid soluble products or in non-immunoprecipitable products. The preparation consisting of intact lymphocytes did not degrade glucagon. Glucagon was degraded by preparations containing monocytes and this degradation increased with time. No difference between monocyte degradation as measured by trichloroacetic acid or immunoprecipitation was found. Degradation by intact monocytes and by mononuclear homogenates increased sixfold from 4 degrees C to 37 degrees C. Subcellular fractionation demonstrated that the majority of the neutral glucagon degrading activity was in the 100,000 g supernatant (cytosol). Kinetic analyses gave Km values of 1.1 x 10(-5) mol/l, 7.5 x 10(-6) mol/l, and 1.2 x 10(-5) mol/l for glucagon degradation by intact mononuclear cells, homogenates, and cytosol, respectively. Inhibitor studies indicated a sulphydryl dependent enzyme was involved in glucagon degradation by both intact cells and cytosol. The monocyte appeared to be the cell responsible for degradation of glucagon by mononuclear cell preparations. The degradation of glucagon under physiological conditions by intact monocytes was mediated by a neutral proteolytic enzyme, primarily localized in the cytosol.

Effect of temperature on insulin binding and action in cultured human fibroblasts

Insulin stimulation of glycogen synthase activity and insulin binding were measured in fibroblast monolayers at 24, 32, and 37 degrees C. Insulin stimulation of %I glycogen activity increased with increasing temperature. Maximum response was greater at 37 degrees C than at 32 degrees C, and half maximal stimulation required at 2.0 nM insulin at 37 degrees C vs. 10 nM at 32 degrees C. Insulin stimulation of glycogen synthase was greater and somewhat faster at 37 degrees C than at 32 degrees C. No insulin effect was observed at 24 degrees C. 125I-insulin binding to monolayers became maximal in 15 min at 37 degrees C, 60 min at 32 degrees C, and 120 min at 24 degrees C. However, insulin binding decreased with increasing temperature, and this decline was due to decreased numbers of receptors. Insulin binding and stimulation of glycogen synthase were comparable at 32 degrees C, with half maxima at 10 nM, indicating no evidence of "spare" receptors. The data indicate that temperature effects on insulin binding and action in fibroblasts are not directly related. The results also suggest that a rate limiting step(s) of insulin action is temperature sensitive, and that this step is not insulin binding.

Receptors for insulin and CCK in the acinar pancreas: relationship to hormone action

These studies, therefore, allow a model of how CCK and insulin regulate the acinar pancreas in a coordinated manner (Fig. 27). CCK, after its secretion by gut cells, interacts with a specific receptor on the cell surface and then increases intracellular free Ca2+. Ca2+, in turn (1) interacts with the secretory granules leading to zymogen release, (2) stimulates protein synthesis, and (3) increases glucose transport. The model is supported on the finding of specific high affinity CCK receptors on acini and by the localization of CCK to the plasma membrane in EM autoradiographs. Insulin, secreted from the pancreatic islets, also interacts with a specific receptor on the cell surface. Either via a messenger generated by this reaction, or via insulin's subsequent direct interaction with intracellular organelles, such as the Golgi-endoplasmic reticulum, protein synthesis is initiated and glucose transport is increased. Then a series of events is initiated to increase cell growth, amylase content, and sensitivity to CCK. These studies, therefore, indicate that the control of acinar cell function is a product of cooperative intrahormonal interactions.

Binding and internalization of 125I-glucagon in hepatocytes of intact mouse liver. An autoradiographic study

Localization of labeled material in hepatocytes of intact mouse liver injected via the portal vein with 125I-glucagon was studied. At 3 minutes after pulse injection of the labeled glucagon, most grains were localized associated with the plasma membrane. At 10 min after the injection, the grains were distributed throughout the cytoplasm and did not appear localized exclusively in any specific cell organelles. At 20 min after the injection, the labeled material appeared decreased, and was not yet localized exclusively in any specific cell organelles. Thus, in hepatocytes of intact liver, labeled glucagon is internalized more rapidly than in freshly isolated hepatocytes (Barazzone et al. 1980), and there appear to be no cell organelles in which internalized glucagon is preferentially localized.

Proteolytic fragmentation of rat prolactin by the rat ventral prostate gland

Homogenates of rat ventral prostate diminished the immunoreactivity of rat prolactin (PRL) in a fashion dependent on time and tissue concentration, suggestive of hormone degradation. Direct evidence of PRL proteolysis was demonstrated by subjecting ventral prostate homogenates and cell fractions that were incubated with 125I-labeled rat PRL to SDS-PAGE and radioautography. Heat-labile PRL proteolysis predominated in the crude homogenate, the 3,300g pellet and the 100,000g (100 K g) supernatant of rat ventral prostate. The degradation of PRL by the 100 K g supernatant led to the formation of two stable peptide fragments weighing approximately 16,000 and 12,000 daltons. PRL proteolysis was negligible in 100 K g supernatants of the liver, salivary gland, testis, epididymis, vas deferens, seminal vesicle, and dorsolateral prostates of male rats. On the other hand, 100 K g supernatants of rat spleen, lung, and kidney did degrade rat PRL, although the patterns of peptide fragment formation differed from that of ventral prostate. Enzyme inhibitor analysis suggested that PRL degradation by the 100 K g supernatants of rat ventral prostate was due to sulfhydryl and serine proteases and not due to metalloenzyme or aspartate proteases. The functional significance of PRL fragmentation by rat ventral prostate cytosol remains to be determined.

Receptor mediated endocytosis of polypeptide hormones: mechanism and significance

Polypeptide hormones bind to specific receptors on the surface of cells. Under certain conditions they localize to specific microdomains of the membrane, i.e., microvilli and coated pits. At physiologic temperature the ligand is internalized by a process of adsorptive endocytosis. This process involves several intracellular membrane bounded structures including coated vesicles, non-coated vesicles and lysosomal structure. These events provide a simple and general mechanism for removal of the ligand from the cell surface in order to terminate its signal. Linked to this process is a mechanism for surface receptor regulation. Thus, the concentration of hormone receptors on the cell surface is a function of the synthetic rate, internalization rate and the rate of recycling of surface membrane.

Receptor-linked degradation of 125I-insulin is mediated by internalization in isolated rat hepatocytes

When hepatocytes were freshly isolated from rat liver and incubated for various periods of time at 37 degrees C, the media from the incubation, when completely separated from the cells, actively degraded 125I-insulin. THis soluble protease activity was strongly inhibited by bacitracin but was unaffected by the lysosomatropic agent ammonium chloride (NH4Cl). When hepatocytes were incubated with 125I-insulin at 37 degrees C in the presence or absence of 8 mM NH4Cl the ligand initially bound to the plasma membrane and was subsequently internalized as a function of time. When hepatocytes were incubated at 37 degrees C for 30 minutes with 125I-insulin in the presence of bacitracin and NH4Cl or bacitracin alone and the cells were washed, diluted, and the cell-bound radioactivity allowed to dissociate, the percent intact 125I-insulin in the cell pellet and in the incubation media was greater in the presence of NH4Cl at each time point of incubation. Under these same conditions a higher proportion of the cell-associated radioactivity was internalized and a higher proportion was associated with lysosomes. The data suggest that receptor-mediated internalization is required for insulin degradation by the cell, and that this process, at least in part, involves lysosomal enzymes. Furthermore, the data demonstrate that internalization is not blocked by the presence of bacitracin or NH4Cl in the incubation media, but that degradation is inhibited.

Glucagon degradation in isolated rat hepatocytes: effect of ammonium chloride and chloroquine

It has recently been shown that, when [125I]glucagon is incubated with isolated rat hepatocytes at 37 degrees, the radiolabeled material is progressively internalized by the cell and is found to associate preferentially with lysosome-like structures. To assess the role of this process in the degradation of the hormone, the degradation of [125I]glucagon by isolated rat hepatocytes was examined both in incubation media and in cell extracts, after exposure of the radiolabelled hormone to hepatocytes in the absence and in the presence of lysosomotropic agents NH4Cl (8 mmoles/l) or chloroquine (10 micromoles/l); bacitracin (0.8 mg/ml, i.e. 0.6 mmoles/l) was present in all experimental conditions to minimize extracellular degradation. Neither NH4Cl nor chloroquine altered the time course and steady-state binding of [125I]glucagon, or the degradation of the hormone in incubation media. However, both agents partially inhibited the degradation of cell-associated [125I]glucagon in steady-state conditions. In dissociation experiments, NH4Cl, and even more so chloroquine, decreased the rate and the extent of release of radiolabelled material from the cells. Moreover, after 60 min dissociation, the presence of either agent resulted in less degradation of both cell-associated [125I]glucagon and that released into the medium. These results suggest that lysosomes are involved in the intracellular degradation of glucagon.

Hepatic metabolism of glucagon in the dog: contribution of the liver to overall metabolic disposal of glucagon

The hepatic extraction (HE) of glucagon (G) and insulin (I) was measured in 27 dogs, using peripheral infusion of the hormones following elimination of endogenous secretion by pancreatectomy (Px) or somatostatin (S) infusion. HE(G) was 22.5 +/- 1.7%, and HE(I) was 45.1 +/- 3%. HE(G) in seven Px dogs was 27.9 +/- 4.2%, not significantly different from the value of 20.6 +/- 1.6% in 20 S-infused dogs, with corresponding values for HE(I) being 44.9 +/- 6 and 46.0 +/- 3.6%, respectively, suggesting that S does not affect HE of either hormone. HE of endogenous G (22.1 +/- 2.8%) was similar to that of exogenously infused G (19.1 +/- 1.9). HE(G) was nonsaturable in the physiologic and pathophysiologic range of plasma G levels, but there was evidence of saturability in the pharmacologic range. Comparison of simultaneously measured parameters of I and G metabolism indicated independence of the metabolic processes of these two islet hormones, despite distinct similarities in their overall patterns of metabolic disposal. Metabolic clearance rates (MCR) for G and I were 12.6 +/- 0.8 and 19.5 +/- 1.0 ml . kg-1 . min-1, while simultaneously measured hepatic HE rates were 4.2 +/- 0.3 and 8.1 +/- 0.6 ml . kg-1 . min-1, respectively. MCR(G) was independent of arterial G levels. Half-life of infused G and I was 5.5 +/- 0.5 and 4.1 +/- 0.3 min, respectively. The liver accounted for 34.7 +/- 2.4% of the MCR(G) and 42.0 +/- 2.9% of MCR(I). The liver is thus an important site for G removal. However, HE(G) varies widely in different animals, and it is therefore not possible to predict portal vein G concentrations or G secretion rates from G levels in peripheral vessels.