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

The cell biology of systemic insulin function

Tokarz et al. review the cell biology of insulin physiology throughout the body, from synthesis to the delivery, action, and final degradation of insulin.

Insulin is the paramount anabolic hormone, promoting carbon energy deposition in the body. Its synthesis, quality control, delivery, and action are exquisitely regulated by highly orchestrated intracellular mechanisms in different organs or “stations” of its bodily journey. In this Beyond the Cell review, we focus on these five stages of the journey of insulin through the body and the captivating cell biology that underlies the interaction of insulin with each organ. We first analyze insulin’s biosynthesis in and export from the β-cells of the pancreas. Next, we focus on its first pass and partial clearance in the liver with its temporality and periodicity linked to secretion. Continuing the journey, we briefly describe insulin’s action on the blood vasculature and its still-debated mechanisms of exit from the capillary beds. Once in the parenchymal interstitium of muscle and adipose tissue, insulin promotes glucose uptake into myofibers and adipocytes, and we elaborate on the intricate signaling and vesicle traffic mechanisms that underlie this fundamental function. Finally, we touch upon the renal degradation of insulin to end its action. Cellular discernment of insulin’s availability and action should prove critical to understanding its pivotal physiological functions and how their failure leads to diabetes.

Melanocortin receptors in rat liver cells: change of gene expression and intracellular localization during acute-phase response

MCRs are known to be expressed predominantly in the brain where they mediate metabolic and anti-inflammatory functions. Leptin plays an important role in appetite and energy regulation via signaling through melanocortin receptors (MCRs) in the brain. As serum levels of MCR ligands are elevated in a clinical situation [acute-phase response (APR)] to tissue damage, where the liver is responsible for the metabolic changes, we studied hepatic gene expression of MCRs in a model of muscle tissue damage induced by turpentine oil (TO) injection in rats. A significant increase in gene expression of all five MCRs (MC4R was the highest) in liver at the RNA and protein level was detected after TO injection. A similar pattern of increase was also found in the brain. Immunohistology showed MC4R in the cytoplasm, but also in the nucleus of parenchymal and non-parenchymal liver cells, whereas MC3R-positivity was mainly cytoplasmic. A time-dependent migration of MC4R protein from the cytoplasm into the nucleus was observed during APR, in parallel with an increase in α-MSH and leptin serum levels. An increase of MC4R was detected at the protein level in wild-type mice, while such an increase was not observed in IL-6ko mice during APR. Moreover, treatment of isolated liver cells with melanocortin agonists (α-MSH and THIQ) inhibited the endotoxin-induced upregulation of the acute-phase cytokine (IL-6, IL1β and TNF-α) gene expression in Kupffer cells and of chemokine gene expression in hepatocytes. MCRs are expressed not only in the brain, but also in liver cells and their gene expression in liver and brain tissue is upregulated during APR. Due to the presence of specific ligands in the serum, they may mediate metabolic changes and exert a protective effect on liver cells.

Autoantibodies to insulin receptors in man: immunological determinants and mechanism of action

The insulin receptor is a membrane glycoprotein of high Mr which binds insulin with high affinity and specificity and transmits some intracellular signal(s) that initiate(s) insulin action. Antibodies to the receptor have been identified in patients with a syndrome characterized by severe resistance to endogenous and exogenous insulin, varying degrees of glucose intolerance, and the skin lesion acanthosis nigricans. The syndrome is most common in non-Caucasian, middle-aged women, but occurs in patients of all races, both sexes, and spanning the ages of 12-62. Most patients have evidence of other autoimmune disease with increased erythrocyte sedimentation rate and gamma globulins, anti-DNA and anti-nuclear antibodies, leucopenia, and other signs and symptoms of autoimmune disease. Antibodies to the insulin receptor are detected by their ability to inhibit 125I-insulin binding or to immunoprecipitate solubilized insulin receptors. In vitro these antibodies acutely mimic most of insulin's metabolic effects. This insulin-like activity depends on antibody bivalence; monovalent Fab fragments block insulin binding and action but lack intrinsic activity. With prolonged exposure of cells to anti-receptor antibody the insulin-like effect is lost and a state of insulin resistance ensues. This is due to both a blockage of insulin binding and a form of post-receptor desensitization. The possible causation of anti-receptor antibodies in this condition is discussed.

Insulin receptor internalization: molecular mechanisms and physiopathological implications

The initial interaction between insulin and its receptor on target cell surface is followed by a series of surface and intracellular steps which participate in the control of insulin action. Abnormalities of any of these steps could result in mishandling of the receptor leading to defective modulation of receptor number on the cell surface and to inappropriate cell sensitivity to the hormone. Thus, the identification of each of these steps as well as understanding the mechanisms governing them is obligatory to unravel some aspects of the pathogenesis of insulin resistance states. This was the goal of the studies we have carried out during recent years using combined molecular and cellular biology as well as biochemical techniques. These studies allowed us to propose the following ordered sequence of events: 1) insulin binds to receptors preferentially associated with microvilli on the cell surface; 2) insulin triggers receptor kinase activation and autophosphorylation which not only results in initiation of the various biological signals leading to insulin action but also in redistribution of the hormone-receptor complex in the plane of the membrane; 3) on the non-villous domain of the cell surface, insulin receptors anchor to clathrin-coated pits through specific "internalization sequences" present in their cytoplasmic juxtamembrane domain; 4) insulin-receptor complexes are internalized together with other receptors present in the same clathrin-coated pits through the formation of clathrin-coated vesicles; 5) the complexes are delivered to endosomes, the acidic pH of which induces the dissociation of insulin molecules from insulin receptors and their sorting in different directions; 6) insulin molecules are targetted to late endosomes and lysosomes where they are degraded; 7) receptors are recycled back to the cell surface in order to be reused.

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.

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.

Mullerian inhibiting substance binding and uptake

Mullerian inhibiting substance (MIS) is a 140,000 M(r) Sertoli cell derived glycoprotein with a critical regulatory role in the male fetus initiated presumably by ligand binding with receptor. To localize this binding species we performed time course incubations of cultured fetal rat lungs or control tissues with MIS, applied rabbit anti-MIS IgG, and fluorescein conjugated anti-rabbit IgG, and examined specimens with laser confocal microscopy. Punctate surface fluorescence followed by cytosolic and nuclear localization in lung consistent with specific adsorptive endocytosis was seen. Confocal imaging also detected MIS binding to the Mullerian duct in the urogenital ridge. Crosslinking of 125I-MIS with plasma membranes revealed a high molecular mass binder with signal displaceable by excess unlabeled ligand. These data support the hypothesis that a specific plasma membrane binding protein for MIS exists.

Kinetics of insulin internalization and processing in adipocytes: effects of insulin concentration

We have studied the effect of insulin concentration on the kinetics of insulin internalization and efflux in isolated rat adipocytes. To determine internalization rates adipocytes were incubated with 125I-insulin at 37 degrees C; and at frequent, early time points surface-bound and intracellular insulin were quantitated. Surface-bound and intracellular insulin were discriminated by the sensitivity of the former to rapid dissociation by a pH 3.0 buffer at 4 degrees C. From this data the endocytotic (internalization) rate constant (ke) was calculated for six insulin concentrations ranging from 0.3 to 100 ng/ml. Ke was found to decrease in an insulin concentration-dependent manner (P less than .001). Thus, values for ke were 0.121 +/- 0.006 min-1 versus 0.074 +/- 0.011 min-1 at 0.3 ng/ml and 100 ng/ml, respectively. The decrease in ke did not parallel insulin concentration-dependent changes in insulin receptor affinity indicating it was not the result of an inability of low affinity receptors to be internalized. The kinetics of insulin efflux were determined by loading various concentrations of 125I-insulin into the adipocyte interior, washing away surface-bound and extracellular insulin, and then monitoring the subsequent efflux of pre-loaded insulin into medium that contained the same concentration of insulin used in the loading step. The overall rate of efflux was independent of insulin concentration. In summary, these results show that at high insulin concentrations the efficiency of insulin internalization is impaired. In contrast, the rate of insulin efflux is unaffected.

Down-regulation of insulin receptors is related to insulin internalization

In the present study, we have tested the influence of inhibition of endocytosis by hypertonic medium on the regulation of cell surface insulin receptors. We show that active internalization of 125I-insulin is markedly inhibited by hypertonic media and that, in parallel, cell surface invaginations are significantly diminished. These two events are accompanied by a marked inhibition of cell surface insulin receptor down-regulation. These data provide further strong evidence that receptor-mediated endocytosis is the major mechanism by which insulin receptors are regulated at the surface of target cells.

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.

Antibodies to insulin receptor tyrosine kinase stimulate its activity towards exogenous substrates without inducing receptor autophosphorylation

Anti-peptide antibodies directed against a highly-conserved sequence of the insulin receptor tyrosine kinase domain have been used to study the relationship between this specific region and kinase activation. Antibodies have been prepared by the injection into a rabbit of a synthetic peptide (P2) corresponding to residues 1110-1125 of the proreceptor. The peptide exhibits 88-95% sequence similarity with the corresponding sequence in the v-ros protein and in receptors for epidermal growth factor and for insulin-like growth factor 1. Two antibodies with different specificities could be separated from total antiserum obtained after immunization with P2. One antibody [anti-(P-Tyr)] cross-reacted with phosphotyrosine and immunoprecipitated solely autophosphorylated receptors. This antibody was shown to increase or decrease the receptor tyrosine kinase activity depending on its concentration. In all circumstances receptor autophosphorylation and substrate phosphorylation were modulated in a parallel fashion. The second antibody (anti-P2) failed to immunoprecipitate the insulin receptor, but was found to interact with both the peptide and the receptor by e.l.i.s.a. assay. Using a tyrosine co-polymer we found that anti-P2 activated the insulin receptor kinase leading to substrate phosphorylation at a level similar to that observed with insulin. This effect was additive to the hormonal effect. In contrast, receptor autophosphorylation was not modified by the anti-peptide. The differential effect of this anti-peptide further supports the idea that receptor autophosphorylation and kinase activity towards exogenous substrates might be independently regulated. Finally, our data suggest that conformational changes in the receptor tyrosine kinase domain may be sufficient for activation of its enzymic activity.

Proteolytic generation of constitutive tyrosine kinase activity of the human insulin receptor

Structural modification induced by partial digestion with trypsin has been shown to stimulate the tyrosine kinase activity of the insulin receptor both in solution and in intact cells [Tamura, Fujita-Yamaguchi & Larner (1983) J. Biol. Chem. 258, 14749-14752; Goren, White & Kahn (1987) Biochemistry 26, 2374-2382; Leef & Larner (1987) J. Biol. Chem. 262, 14837-14842]. Furthermore, experiments involving deletion of sequences encoding the extracellular domain of the insulin receptor suggest that it may function as a protooncogene in fibroblasts [Wang et al., (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 5725-5729]. To further understand the structural requirements that generate this activity, the major activated fragments generated in solution following trypsin digestion have been characterized here, one of which is shown to have a similar amino acid sequence to a transforming protein. Furthermore, treatment with trypsin of intact Chinese hamster ovary cells that overexpress the human insulin receptor stimulates both autophosphorylation of the receptor and 2-deoxyglucose uptake into the cells, but does not enhance receptor internalization. Unlike digestion in solution, no proteolysis or loss of activity of the activated insulin receptor beta-subunit could be detected using intact cells, even at high trypsin concentrations, despite the existence of extracellular sites that are readily cleaved by trypsin in the solubilized receptor. These studies provide further detail of a mechanism used during trypsinization of cells in culture which mimics activation of the insulin receptor and contributes to stimulation of growth.

Binding of antigen to Ia molecules on intact antigen presenting cells demonstrated by photoaffinity labeling

We used a photoaffinity labeling technique to investigate whether a molecular interaction occurs between antigen and Ia molecules on antigen presenting cells (APC) in the absence of T lymphocytes. M.12.4.1 B lymphoma cells (Iad), which are able to present bovine insulin to Iad lymph node primed T cells, were given radioiodinated bovine insulin derivatized with the photoreactive group (2-nitro-4-azidophenylacetyl) at Lys 29 of the B chain of the insulin molecule. Processing of insulin was allowed by incubating the APC with antigen for increasing periods of time at 37 degrees C or 4 degrees C. The covalent coupling of the processed photoreactive antigen to any neighboring cellular protein was thereafter induced by u.v. irradiation. Immunoprecipitation of membrane proteins by monoclonal antibodies showed that under these conditions, the alpha and beta subunits of the Ia molecules were selectively photolabeled. Labeling was time- and temp-dependent as was the internalization of insulin. The apparent mol. wts of the antigen-Ia molecule complexes were not significantly different from that of native Ia molecules radioiodinated by surface labeling, indicating that only a small fragment of the antigen was covalently coupled to Ia molecules. Similar experiments performed with human B lymphoma cells (526 cells) gave similar results. These observations therefore indicate: (1) that Ia molecules expressed by intact APC are able to bind antigens in the absence of T lymphocyte antigen receptor; and (2) that this association, at least for insulin, requires uptake and a proteolytic fragmentation of the antigen by the APC.

Radioautographic demonstration of receptors for epidermal growth factor in various cells of the oral cavity

Mouse iodinated epidermal growth factor (EGF) was localized by light and electron microscopic radioautography in basal cells of oral epithelium, papillary cells of the enamel organ, periodontal ligament fibroblasts, preodontoblast precursor cells, and preosteoblasts of the alveolar bone of 13-day-old Sprague-Dawley rats. The specificity of binding in these cells was suggested by an observed reduction of about 90% in the labeling when excess unlabeled EGF was injected along with the 125I-EGF. In contrast, fully differentiated cells, such as ameloblasts, odontoblasts, and osteoblasts, were only poorly labeled. Quantitative analysis of the light microscopic radioautographs revealed that the papillary cells had the highest level of labeling (5.5 grains per 100 micron 2 of cell area). The significance of the rather high labeling of the preosteoblasts of the alveolar bone and the fibroblasts of the periodontal ligament is unknown. However, the well-known effect of EGF in producing precocious eruption of teeth may be a consequence of an effect on these two cell types.

Relationships between cell surface insulin binding and endocytosis in adipocytes

Chymotrypsin substrate analogues, such as N-acetyl-Tyr ethyl ester, have recently been demonstrated to inhibit the endocytic uptake of insulin in isolated rat adipocytes. In this study, the effects of N-acetyl-Tyr ethyl ester on cell surface insulin binding and dissociation were examined. Surface-bound 125I-insulin was distinguished from intracellular 125I-insulin by the sensitivity of the former to rapid dissociation with an acidic buffer (pH 3.0). Plateau levels of surface-bound insulin at 37 degrees C were increased 70% by inhibiting the internalization pathway. This increase was temperature and insulin concentration dependent. Thus differences in surface binding were small at 12 degrees C and also at high (100-200 ng/ml) insulin concentrations. Inhibition of internalization with N-acetyl-Tyr ethyl ester markedly slowed the loss of surface-bound insulin observed during dissociation studies. After 20-30 min of dissociation, the remaining levels of surface-bound insulin were three- to fourfold higher in treated adipocytes compared with control adipocytes. Added unlabeled insulin retained its ability to accelerate the dissociation of insulin in N-acetyl-Tyr ethyl ester-treated cells. These observations indicate that the internalization pathway is a quantitatively important factor in determining levels of surface binding at 37 degrees C and in determining the rate of deactivation of insulin binding.

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

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

Internalization of insulin receptors and HLA antigens in human hepatoma cells

Human HepG2 hepatoma cells express a high number of insulin receptors. Growing cells exhibit 70% of their insulin receptors on the plasma membrane. Moreover, cell-surface insulin receptors form molecular complexes with class I major histocompatibility antigens, as determined by co-immunoprecipitation of the receptors by anti-class I monoclonal antibodies. On exposure to saturating concentrations of insulin, the hormone is rapidly internalized into a Pronase-resistant compartment. Internalization of insulin is accompanied by a rapid (t1/2 = 2-3 min) redistribution of insulin receptors from the cell surface to an intracellular compartment. On removal of insulin from the medium, functional receptors recycle back to the plasma membrane, where they can bind insulin again. With chronic exposure of HepG2 cells to insulin, the initial redistribution of receptors is followed by a slow (t1/2 = 9 h) down-regulation of the receptors. Finally, notwithstanding their interaction at the cell surface, insulin receptors and class I major histocompatibility antigens are internalized at different rates and with independent regulation.

Adipocyte insulin receptor. Generation of a cryptic domain of the alpha-subunit during internalization of hormone-receptor complexes

The dynamics of the internalization of photoaffinity-labelled insulin-receptor complexes was investigated in isolated rat adipocytes by using tryptic proteolysis to probe both the orientation and cellular location of the labelled complexes. In cells that were labelled at 16 degrees C and not prewarmed, 150 micrograms of trypsin/ml rapidly degraded the labelled 125 kDa insulin-receptor subunit into a major proteolytic fragment of 70 kDa and minor amounts of 90- and 50-kDa fragments. With milder trypsin treatment conditions (100 micrograms of trypsin/ml, 15 s at 37 degrees C), the 90 kDa peptide (different from the 90 kDa beta-subunit of the insulin receptor) appeared as a major intermediate proteolytic product, but this species was rapidly and completely converted into the 70- and 50-kDa fragments with continued exposure to trypsin, such that it did not accumulate to appreciable amounts in cells that were not prewarmed before trypsin exposure. By contrast, trypsin treatment of cells prewarmed to 37 degrees C for various times showed that: first, a proportion of the labelled 125 kDa receptors was internalized (became trypsin-insensitive); secondly, the 90 kDa tryptic peptide was formed in large amounts, with proportionate decreases occurring in the amounts of the 70- and 50-kDa tryptic peptides. The increased accumulation of the 90 kDa tryptic peptide from cells preincubated at 37 degrees C, but not at 16 degrees C, indicated that trypsin cleavage sites within the 90 kDa segment of the insulin-receptor alpha-subunit that were exposed at 16 degrees C were made inaccessible by incubation at 37 degrees C, a finding that is consistent with generation of a cryptic domain of the receptor subunit. The tryptic generation of the 90 kDa peptide at 37 degrees C was rapid, becoming half-maximal in 4.4 +/- 0.6 min and maximal in 15-20 min, preceded the intracellular accumulation of labelled receptors (half-maximal in 12.6 +/- 0.7 min and maximal in 30-40 min), was highly correlated with receptor internalization, and was not observed in cultured IM-9 lymphocytes, a cell line in which photolabelled insulin receptors are primarily lost by shedding into the incubation media. These results show that, in adipocytes incubated at 37 degrees C, rapid masking of a previously (at 16 degrees C) accessible domain of the insulin-receptor alpha-subunit occurs and that this dynamic process happens at an early stage in the internalization of insulin-receptor complexes.

Insulin receptor kinase is hyperresponsive in adipocytes of young obese Zucker rats

Thirty-day-old obese Zucker rats have hyperresponsive adipose tissue, whereas their skeletal muscle normally responds to insulin in vitro. To further substantiate the role of insulin receptor tyrosine kinase in insulin action, we have studied the kinase activity of receptors obtained from adipocytes and skeletal muscle of these young obese Zucker rats. Insulin receptors, partially purified by wheat germ agglutinin agarose chromatography from plasma membranes of isolated adipocytes or from skeletal muscles, were studied in a cell-free system for auto-phosphorylation and for their ability to phosphorylate a synthetic glutamate-tyrosine copolymer. For an identical amount of receptors, the insulin stimulatory action on its beta-subunit receptor phosphorylation was markedly augmented in preparations from hyperresponsive adipocytes of obese animals compared with lean rats. Basal phosphorylation of adipocyte insulin receptors was nearly identical in lean and obese animals. Similarly the capacity of adipocyte insulin receptors to catalyze the phosphorylation of the synthetic substrate in response to insulin was increased. By contrast, the kinase activity of insulin receptors prepared from normally insulin-responsive skeletal muscle was similar in preparations of lean and obese rats. These results show that a state of hyperresponsiveness to insulin is correlated with a parallel increase of insulin receptor kinase activity suggesting an important role for this activity in insulin action.

Cell-surface insulin receptor cycling and its implication in the glycogenic response in cultured foetal hepatocytes

The insulin-receptor cycle was investigated in cultured foetal rat hepatocytes by determining the variations in insulin-binding sites at the cell surface after short exposure to the hormone. Binding of 125I-insulin was measured at 4 degrees C after dissociation of prebound native insulin. Two protocols were used: exchange binding assay and binding after acid treatment; both gave the same results. Cell-surface 125I-insulin-receptor binding decreased sharply (by 40%) during the first 5 min of 10 nM-insulin exposure (t1/2 = 2 min) and remained practically constant thereafter; subsequent removal of the hormone restored the initial binding within 10 min. This fall-rise sequence corresponded to variations in the number of insulin receptors at the cell surface, with no detectable change in receptor affinity. The reversible translocation of insulin receptors from the cell surface to a compartment not accessible to insulin at 4 degrees C was hormone-concentration- and temperature-dependent. SDS/polyacrylamide-gel electrophoresis after cross-linking of bound 125I-insulin to cell-surface proteins with disuccinimidyl suberate showed that these variations were not associated with changes in Mr of binding components, in particular for the major labelled band of Mr 130,000. The insulin-receptor cycle could be repeated after intermittent exposure to insulin. Continuous or intermittent exposure to the hormone gave a similar glycogenic response, contrary to the partial effect of a unique short (5-20 min) exposure. A relationship could be established between the repetitive character of the rapid insulin-receptor cycle and the maximal expression of the biological effect in cultured foetal hepatocytes.

Chymotrypsin substrate analogues inhibit endocytosis of insulin and insulin receptors in adipocytes

To explore the possible role of proteolytic step(s) in receptor-mediated endocytosis of insulin, the effects of inhibitors of various classes of proteases on the internalization process were studied in isolated rat adipocytes. Intracellular accumulation of receptor-bound 125I-insulin at 37 degrees C was quantitated after rapidly dissociating surface-bound insulin with an acidic buffer (pH 3.0). Of the 23 protease inhibitors tested, only chymotrypsin substrate analogues inhibited insulin internalization. Internalization was decreased 62-90% by five different chymotrypsin substrate analogues: N-acetyl-Tyr ethyl ester, N-acetyl-Phe ethyl ester, N-acetyl-Trp ethyl ester, benzoyl-Tyr ethyl ester, and benzoyl-Tyr amide. The effect of the substrate analogues in inhibiting insulin internalization was dose-dependent, reversible, and required the full structural complement of a chymotrypsin substrate analogue. Cell surface receptor number was unaltered at 12 degrees C. However, concomitant with their inhibition of insulin internalization at 37 degrees C, the chymotrypsin substrate analogues caused a marked increase (160-380%) in surface-bound insulin, indicating trapping of insulin-receptor complexes on the cell surface. Additionally, 1 mM N-acetyl-Tyr ethyl ester decreased overall insulin degradation by 15-20% and also prevented the chloroquine-mediated increase in intracellular insulin, further indicating that surface-bound insulin was prevented from reaching intracellular chloroquine-sensitive degradation sites. The internalization of insulin receptors that were photoaffinity labeled on the cell surface with B2(2-nitro-4-azidophenylacetyl)-des-PheB1-insulin was also inhibited 70-90% by the five chymotrypsin substrate analogues, as determined by the effects of the analogues on the accumulation of trypsin-insensitive (intracellular) 440-kD intact labeled receptors. In summary, these results show that chymotrypsin substrate analogues efficiently inhibit the internalization of insulin and insulin receptors in adipocytes and implicate a possible role for endogenous chymotrypsin-like enzyme(s) or related substances in receptor-mediated endocytosis of insulin.

Internalization and recycling of 125I-photoreactive insulin-receptor complexes in hepatocytes in primary culture

When the insulin receptor is tagged with a 125I-photoreactive insulin analogue that can be covalently coupled to it by UV irradiation, the fate of this labeled receptor can be followed both morphologically and biochemically. In the present study we have applied this tool to trace the pathway followed by 125I-photoreactive insulin-receptor complex in hepatocytes in primary culture. As determined by quantitative electron microscopic autoradiography, the internalized labeled material first associates with clear vesicles, second is found in multivesicular bodies, third associates with dense bodies and fourth returns to the cell surface via clear vesicles. This recycling process is inhibited by lysosomotropic agents, i.e. NH4Cl or chloroquine. These data confirm, in another cell system, our previous observations carried out in freshly isolated rat hepatocytes and demonstrate the feasibility and complementarity of both freshly isolated hepatocytes and hepatocytes in primary culture to study internalization and recycling of the insulin receptor.

Tunicamycin sensitivity of prolactin, insulin and epidermal growth factor receptors in rat liver plasmalemma

We have used the glycosylation inhibitor tunicamycin to assess the stability of the receptors for prolactin, insulin and epidermal growth factor (EGF) in rat liver cell membrane. Direct binding studies on liver plasmalemma fractions which were isolated from tunicamycin-treated rats revealed a rapid loss of prolactin receptors (t1/2 approximately 35 min) with a more prolonged half-life for insulin (10 h) and EGF receptors (8 h). The rates of receptor loss were similar to the respective half-lives of the receptors as documented by others using cultured cells. The respective ligands for each receptor were lost more rapidly from liver, i.e. prolactin, t1/2 approximately 10 min, insulin, t1/2 approximately 5 min and EGF, t1/2 approximately 17 min. Previous studies have shown ligand loss in vivo to be receptor mediated. Thus, receptors and their ligands do not turn over synchronously in vivo. These studies also point to a major role for N-linked oligosaccharide side chains in the functional insertion of prolactin, insulin and EGF receptors into the hepatocyte cell surface in vivo.

Increased phosphorylation of ribosomal protein S6 following microinjection of insulin receptor-kinase into Xenopus oocytes

The protein products of several transforming retroviruses as well as the receptors for several hormones and growth factors, including insulin, have been shown to possess a protein kinase activity in vitro specific for tyrosine residues in protein substrates, including themselves. In the case of pp60src and the insulin receptor, autophosphorylation activates the tyrosine kinase activity towards exogenous substrates. Experiments indicate that, in vivo, many of these viruses or growth factors induce an increase in cellular phosphotyrosine, as well as an increase in the phosphorylation of serine residues on proteins, including ribosomal protein S6. It seems likely that some of the effects of insulin might be mediated by phosphorylation of intracellular substrates by its receptor. As the beta subunit of the receptor is a transmembrane protein, such phosphorylation could occur either while the receptor is still in the membrane or after its internalization. In various cell systems, internalized receptors are degraded, reshuttled back to the plasmalemma or maintained in a separate compartment before reinsertion in the membrane; shuttling of the insulin receptor could provide the opportunity for it to phosphorylate various intracellular components as part of its mechanism of signal transduction. To approach directly the question of whether the receptor can elicit a signal while acting at an intracellular location, we have microinjected Xenopus oocytes with the insulin receptor kinase. The results indicate that an S6 protein-serine kinase is stimulated or an S6 protein-serine phosphatase inhibited by the activity of the insulin receptor, supporting the concept that the insulin receptor acting within the cell can elicit a biological response.

Inhibitory effect of anoxia on 125I-insulin binding by rat hepatocytes

We have examined the possibility that 125I-insulin binding by isolated rat hepatocytes is modulated by cellular ATP levels. To avoid complications due to ATP-dependent internalization of bound insulin, 125I-insulin binding was determined at 10 degrees C; at this temperature, equilibrium binding was achieved after incubation for 4-6 h. When hepatocytes were incubated at 37 degrees C under anaerobic conditions, ATP levels and 125I-insulin binding were both lowered by about 65%. Anoxia inhibited the association of 125I-insulin with the hepatocyte receptor; the dissociation of insulin from hepatocytes was not affected. Cellular ATP levels and 125I-insulin binding were both restored when anaerobic cells were incubated further at 37 degrees C under aerobic conditions. When anaerobic cells were incubated in air at 10 degrees C during the insulin binding assay, 125I-insulin binding recovered completely, but ATP levels were unaffected. The inhibitory effect of anoxia on 125I-insulin binding was not due to any effect on 125I-insulin degradation or on cell viability. We conclude (1) that the ability of hepatocytes to bind insulin can be modulated on a short-term basis in response to the metabolic status of the cell, and (2) that modulation of the liver cell insulin receptor is not a function of cellular ATP levels.

Intracellular pathway followed by the insulin receptor covalently coupled to 125I-photoreactive insulin during internalization and recycling

After it interacts with a specific receptor on the cell surface, insulin is internalized in its target cell by an adsorptive endocytotic process and eventually degraded in lysosomes. It was also recently shown that the initial surface interaction between the hormone and its receptor is followed by an internalization of the receptor, which later is recycled back to the cell surface. In the present study the insulin receptor was tagged with a 125I-photoreactive insulin analogue that can be covalently coupled to the insulin receptor by ultraviolet irradiation. Using this tool we could trace by quantitative electron microscope autoradiography the intracellular pathway followed by this labeled receptor. The quantitative analysis of the intracellular distribution of the labeled material as a function of incubation time at 37 degrees C supports the following sequence of events: association first with clear vesicles, second with multivesicular bodies, third with dense bodies, and fourth, a return to the cell surface via clear vesicles. This insulin receptor recycling process is inhibited by monensin but unaffected by cycloheximide.

Internalization of the neonatal brain insulin receptor

Using 10-15 day neonatal rabbit brain cells, we studied the internalization (n = 6) and intracellular degradation (n = 8) of specifically bound 125I-insulin. In addition we investigated the association between the internalization of the specifically bound 125I-insulin and the metabolic effects of insulin such as glucose (n = 13) and amino-acid (leucine) uptake (n = 6). Phenylarsine oxide (10 microM), an agent that inhibits the internalization of the insulin receptor (n = 6) decreased the specifically bound 125I-insulin in the intact and trypsin-resistant (inside) part of the brain cells by 50% (p less than 0.05). On the other hand chloroquine (100 microM), a lysosomotropic agent that interferes with the intracellular degradation of the insulin receptor (n = 8) increased two-fold the 125I-insulin specifically bound to the intact and trypsin resistant part of the cells (p less than 0.05). Both these agents did not alter the time-dependent basal glucose uptake by the brain cells. Glucose alone regulated its own uptake (n = 4) whereas 1 X 10(-6) M insulin did not augment the glucose uptake (n = 11+13) above basal. Similarly leucine regulated the leucine uptake (n = 4) but insulin did not alter this basal uptake by the brain cells (n = 6). In summary we observed no associated glucose or leucine uptake along with the presence of internalization and intracellular degradation of specifically bound 125I-insulin in the brain cells.

Insulin receptor internalization and recycling: mechanism and significance

Using a 125I-photoreactive insulin analogue that can be covalently coupled to its receptor we have shown that in rat hepatocytes the insulin receptor is concomitantly internalized with the labeled hormone and afterwards is progressively recycled back to the cell surface. In the course of the internalization process the insulin-receptor complex associates with clear vesicles and later on with lysosomes from which it is recycled through clear vesicles. On the basis of these observations it is suggested that modulation of the rates of internalization and of recycling of the insulin receptor can regulate the number of available surface insulin receptors. This hypothesis is supported by the results of experiments showing that monensin, an inhibitor of receptor recycling enhances insulin induced loss of its own surface receptors (down regulation) in U-937 monocytes.

The endosomal compartment of rat hepatocytes. Its characterization in the course of [125I]insulin internalization

When freshly isolated hepatocytes are incubated with [125I]insulin in the presence of the microtubule-disrupting agent colchicine, internalization of the labelled hormone is not significantly altered. However, the drug limits the endocytosis of the labelled material to a peripheral band of cytoplasm extending 1 micron beyond the plasma membrane. Both in the presence and absence of colchicine, internalized [125I]insulin preferentially associates with clear vesicles (endosomes) and lysosome-like structures, but the relative amount of labelled material associated with clear vesicles is higher in the presence of the drug than in its absence. An inverse pattern is observed for the lysosome-like structures. As demonstrated by cytochemical methods, clear vesicles do not contain the lysosomal enzyme aryl sulfatase. Moreover, colchicine induces an increase of the clear vesicle diameter without affecting their frequency, while it perturbs multivesicular bodies and dense bodies in an opposite way by increasing their frequency without affecting their size. By reducing and/or delaying the fusion between internalized endocytotic vesicles and lysosomes, colchicine allows better characterization of the endosomal compartment of isolated rat hepatocytes and allows it to be distinguished from other compartments, such as multivesicular bodies and the Golgi apparatus.

Insulin receptor tyrosine kinase is defective in skeletal muscle of insulin-resistant obese mice

Obese syndromes of genetic origin or experimentally induced are characterized by resistance to insulin both in vivo (association of hyperglycaemia and hyperinsulinaemia) and in vitro. Thus, skeletal muscle of obese mice, which is the most important target organ for the action of insulin, displays a reduced response to insulin. This hormonal resistance cannot be explained by the moderate decrease in the number of insulin receptors found in obese animals. In fact, it is generally believed that a biochemical event occurring very early after binding of insulin to its receptor, which is the first step in insulin action, is defective in obesity. One of the earliest post-binding events so far recognized, and which is thought to have a key role in cellular signalling by the insulin receptor, is the insulin-stimulated phosphorylation of its receptor. In an effort to localize the defect responsible for the insulin resistance in obesity, we have studied the insulin receptor protein kinase activity and we show here that insulin receptors from skeletal muscles of insulin-resistant obese mice have an altered kinase activity for phosphorylation of both the receptor itself and of exogeneous substrates.

Light and electron microscopic analysis of insulin binding sites on neurons in dissociated brain cell cultures

The distribution of insulin binding sites on primary cultured neurons and glia from the fetal rat was examined by the immunoperoxidase method using a specific insulin receptor antiserum. Light and electron microscopic analysis revealed a homogenous distribution of insulin binding sites on selective neuron-like cells of the dissociated cell culture system. To determine the influence of medium insulin on the distribution of insulin binding sites, dissociated cell cultures were maintained in the presence or absence of porcine insulin for varying time periods. We observed a significant increase in the number of insulin stained neuron-like cells maintained in insulin free defined medium compared to neuron-like cells maintained in insulin supplemented defined medium. Further, we examined the distribution of insulin binding sites after incubation with the antibody, which has agonistic properties in peripheral tissues, for varying time periods prior to fixation. Under these conditions, the light microscopic analysis revealed a heterogeneous (patchy) distribution of immunoreactive insulin binding sites, suggesting that the ligand receptor complex migrates. These results demonstrate the presence and distribution of insulin binding sites on neurons maintained in vitro, and provide morphological evidence to support a functional role for insulin in CNS tissues.

The preparation and characterization of mono-iodinated photoreactive analogs of insulin

Photoreactive derivatives of insulin (B29-(p-azidobenzoyl-insulin) iodinated primarily in either the B26 or A14 tyrosine of insulin were prepared by lactoperoxidase catalyzed iodination followed by separation on reverse-phase high-performance liquid chromatography. The binding affinities and photoaffinity labeling characteristics of these derivatives were studied in isolated rat adipocytes. Under nonreducing conditions, three forms of the insulin receptor were labeled equally by the B26-derivative, the A14-derivative, and the mixture of the iodinated derivatives. When dithiothreitol was used to reduce the radiolabeled receptors, the radioactivity associated with the binding subunit was much less than that in the intact receptor and the magnitude of the decrease was proportional to the amount of iodine in the A chain of the photoderivatives. Use of the photoreactive derivative iodinated primarily in the B26 position resulted in greater labeling of insulin receptor subunits since most of the radioactivity (80%) remained associated with the receptor upon reduction.

Internalization of insulin: structures involved and significance

The binding of insulin to its receptor is followed by aggregation of hormone-receptor complexes and their internalization into the cell. Internalized hormone is concentrated in Golgi-enriched not lysosomal endocytotic structures which, in rat liver, contain lipoprotein particles and can be resolved by centrifugation techniques into three different entities. Recent work has shown that the bulk of endocytotic structures can be resolved from biochemically defined (i.e., galactosyltransferase-containing) Golgi elements. The endosomal apparatus or endosomes appear to function as a sorting center wherein internalized hormone-receptor complexes are concentrated and dissociated prior to directing hormone to lysosomes and receptor back to the cell surface for reutilization. Endosomes are heterogeneous and different functions might be subserved by different endosomal structures. Since an insulin stimulable receptor kinase activity can be identified in endosomes certain aspects of insulin action might be initiated herein.