Ultrastructural localization of insulin receptors on adipocytes

Rapid insulin-mediated increase in microvascular glycocalyx accessibility in skeletal muscle may contribute to insulin-mediated glucose disposal in rats

It has been demonstrated that insulin-mediated recruitment of microvascular blood volume is associated with insulin sensitivity. We hypothesize that insulin rapidly stimulates penetration of red blood cells (RBC) and plasma into the glycocalyx and thereby promotes insulin-mediated glucose uptake by increasing intracapillary blood volume. Experiments were performed in rats; the role of the glycocalyx was assessed by enzymatic degradation using a bolus of hyaluronidase. First, the effect of insulin on glycocalyx accessibility was assessed by measuring the depth of penetration of RBCs into the glycocalyx in microvessels of the gastrocnemius muscle with Sidestream Dark-field imaging. Secondly, peripheral insulin sensitivity was determined using intravenous insulin tolerance tests (IVITT). In addition, in a smaller set of experiments, intravital microscopy of capillary hemodynamics in cremaster muscle and histological analysis of the distribution of fluorescently labeled 40 kDa dextrans (D40) in hindlimb muscle was used to evaluate insulin-mediated increases in capillary blood volume. Insulin increased glycocalyx penetration of RBCs by 0.34±0.44 µm (P<0.05) within 10 minutes, and this effect of insulin was greatly impaired in hyaluronidase treated rats. Further, hyaluronidase treated rats showed a 35±25% reduction in whole-body insulin-mediated glucose disposal compared to control rats. Insulin-mediated increases in capillary blood volume were reflected by a rapid increase in capillary tube hematocrit from 21.1±10.1% to 29.0±9.8% (P<0.05), and an increase in D40 intensity in individual capillaries of 134±138% compared to baseline at the end of the IVITT. These effects of insulin were virtually abolished in hyaluronidase treated animals. In conclusion, insulin rapidly increases glycocalyx accessibility for circulating blood in muscle, and this is associated with an increased blood volume in individual capillaries. Hyaluronidase treatment of the glycocalyx abolishes the effects of insulin on capillary blood volume and impairs insulin-mediated glucose disposal.

Historical perspectives in fat cell biology: the fat cell as a model for the investigation of hormonal and metabolic pathways

For many years, there was little interest in the biochemistry or physiology of adipose tissue. It is now well recognized that adipocytes play an important dynamic role in metabolic regulation. They are able to sense metabolic states via their ability to perceive a large number of nervous and hormonal signals. They are also able to produce hormones, called adipokines, that affect nutrient intake, metabolism and energy expenditure. The report by Rodbell in 1964 that intact fat cells can be obtained by collagenase digestion of adipose tissue revolutionized studies on the hormonal regulation and metabolism of the fat cell. In the context of the advent of systems biology in the field of cell biology, the present seems an appropriate time to look back at the global contribution of the fat cell to cell biology knowledge. This review focuses on the very early approaches that used the fat cell as a tool to discover and understand various cellular mechanisms. Attention essentially focuses on the early investigations revealing the major contribution of mature fat cells and also fat cells originating from adipose cell lines to the discovery of major events related to hormone action (hormone receptors and transduction pathways involved in hormonal signaling) and mechanisms involved in metabolite processing (hexose uptake and uptake, storage, and efflux of fatty acids). Dormant preadipocytes exist in the stroma-vascular fraction of the adipose tissue of rodents and humans; cell culture systems have proven to be valuable models for the study of the processes involved in the formation of new fat cells. Finally, more recent insights into adipocyte secretion, a completely new role with major metabolic impact, are also briefly summarized.

Cellular spelunking: exploring adipocyte caveolae

It has been known for decades that the adipocyte cell surface is particularly rich in small invaginations we now know to be caveolae. These structures are common to many cell types but are not ubiquitous. They have generated considerable curiosity, as manifested by the numerous publications on the topic that describe various, sometimes contradictory, caveolae functions. Here, we review the field from an "adipocentric" point of view and suggest that caveolae may have a function of particular use for the fat cell, namely the modulation of fatty acid flux across the plasma membrane. Other functions for adipocyte caveolae that have been postulated include participation in signal transduction and membrane trafficking pathways, and it will require further experimental scrutiny to resolve controversies surrounding these possible activities.

The effects of brefeldin A on the glucose transport system in rat adipocytes. Implications regarding the intracellular locus of insulin-sensitive Glut4

Insulin activates glucose transport by recruiting Glut4 glucose transporters from an intracellular pool to plasma membrane (PM). To localize intracellular translocating Glut4, we studied the effects of brefeldin A (BFA), which disassembles Golgi and prevents trans-Golgi vesicular budding, on the glucose transport system. Isolated rat adipocytes were treated with and without both BFA (10 micrograms/ml) and insulin. BFA did not affect maximal rates of either 2-deoxyglucose or 3-O-methyl-glucose transport or the insulin:glucose transport dose-response curve but did increase basal transport by approximately 2-fold (p < 0.05). We also measured Glut4 in PM, low (LDM) and high density microsome subfractions. In basal cells, BFA increased PM Glut4 by 58% concomitant with a 18% decrease in LDM (p < 0.05). Insulin alone increased PM Glut4 by 3-fold concomitant with a 56% decrease in LDM. BFA did not affect insulin-induced changes in Glut4 levels in PM or LDM. Most intracellular Glut4 was localized to sub-PM vesicles by immunoelectron microscopy in basal cells, and BFA did not affect insulin-mediated recruitment of immunogold-labeled Glut4 to PM. In summary, 1) in basal cells, BFA led to a small increase in glucose transport activity and redistribution of a limited number of transporters from LDM to PM; 2) BFA did not affect insulin's ability to stimulate glucose transport or recruit normal numbers of LDM Glut4 to PM; and 3) insulin action is predominantly mediated by a BFA-insensitive pool of intracellular Glut4, which localizes to sub-PM vesicles. Thus, the major translocating pool of Glut4 in rat adipocytes does not involve trans-Golgi.

Regulated internalization of caveolae

Caveolae are specialized invaginations of the plasma membrane which have been proposed to play a role in diverse cellular processes such as endocytosis and signal transduction. We have developed an assay to determine the fraction of internal versus plasma membrane caveolae. The GPI-anchored protein, alkaline phosphatase, was clustered in caveolae after antibody-induced crosslinking at low temperature and then, after various treatments, the relative amount of alkaline phosphatase on the cell surface was determined. Using this assay we were able to show a time- and temperature-dependent decrease in cell-surface alkaline phosphatase activity which was dependent on antibody-induced clustering. The decrease in cell surface alkaline phosphatase activity was greatly accelerated by the phosphatase inhibitor, okadaic acid, but not by a protein kinase C activator. Internalization of clustered alkaline phosphatase in the presence or absence of okadaic acid was blocked by cytochalasin D and by the kinase inhibitor staurosporine. Electron microscopy confirmed that okadaic acid induced removal of caveolae from the cell surface. In the presence of hypertonic medium this was followed by the redistribution of groups of caveolae to the center of the cell close to the microtubule-organizing center. This process was reversible, blocked by cytochalasin D, and the centralization of the caveolar clusters was shown to be dependent on an intact microtubule network. Although the exact mechanism of internalization remains unknown, the results show that caveolae are dynamic structures which can be internalized into the cell. This process may be regulated by kinase activity and require an intact actin network.

Immunoelectron microscopic demonstration of insulin-stimulated translocation of glucose transporters to the plasma membrane of isolated rat adipocytes and masking of the carboxyl-terminal epitope of intracellular GLUT4

Polyclonal antibodies to the amino- or carboxyl-terminated peptide sequences of the GLUT4 transporter protein were used in immunoelectron microscopic studies to demonstrate the location and insulin-induced translocation of GLUT4 in intact isolated rat adipocytes. Labeling of untreated adipocytes with the amino-terminal antibody revealed 95% of GLUT4 was intracellular, associated with plasma membrane invaginations or vesicles contiguous with or within 75 nm of the cell membrane. Insulin treatment increased plasma membrane labeling approximately 13-fold, to 52% of the total transporters, and decreased intracellular labeling proportionately. In contrast, labeling of untreated adipocytes with the carboxyl-terminal antibody or with a monoclonal antibody (1F8) that binds to the carboxyl terminus of GLUT4 detected fewer transporters, only approximately 40% of which were intracellular. In insulin-treated cells, plasma membrane labeling increased approximately 20-fold, but the total number of labeled transporters also increased approximately 13-fold. The number of intracellular transporters was not changed. The insulin-induced increase in plasma membrane labeling was reversible. Thus, the vast majority of GLUT4 transporters in untreated adipocytes are intracellular in invaginations or vesicles attached or close to the plasma membrane. Insulin treatment causes translocation of transporters to the plasma membrane, which involves flow of transporters from invaginations to the cell surface and possible fusion of subplasma membrane vesicles with the plasma membrane. Differences in the labeling of intracellular transporters by peptide antibodies suggested the carboxyl-terminal epitope of intracellular transporters was masked. The unmasking of the carboxyl terminus during translocation to the plasma membrane may be part of the mechanism by which insulin stimulates glucose transport in rat adipocytes.

Ultrastructural evidence for the accumulation of insulin in nuclei of intact 3T3-L1 adipocytes by an insulin-receptor mediated process

Monomeric ferritin-labeled insulin (Fm-Ins), a biologically active, electron-dense marker of occupied insulin receptors, was used to characterize the internalization of insulin in 3T3-L1 adipocytes. Fm-Ins bound specifically to insulin receptors and was internalized in a time- and temperature-dependent manner. Fm-Ins was found in cytoplasmic vesicles within 5-10 min at 37 degrees C and subsequently was observed in multivesicular bodies and lysosomes. In addition, small amounts of Fm-Ins were associated with nuclei after 30 min. The number of Fm-Ins particles observed in nuclei continued to increase in a time-dependent manner until at least 90 min. In the nucleus, several Fm-Ins particles usually were found in the same general location--near nuclear pores, associated with the periphery of the condensed chromatin. Addition of a 250-fold excess of unlabeled insulin or incubation at 15 degrees C reduced the number of Fm-Ins particles found in nuclei after 90 min by 99% or 92%, respectively. Nuclear accumulation of unlabeled ferritin was only 2% of that found with Fm-Ins after 90 min at 37 degrees C. Biochemical experiments utilizing 125I-labeled insulin and subcellular fractionation indicated that intact 3T3-L1 adipocytes internalized insulin rapidly and that approximately equal to 3% of the internalized ligand accumulated in nuclei after 1 hr. These data provide biochemical and high-resolution ultrastructural evidence that 3T3-L1 adipocytes accumulate potentially significant amounts of insulin in nuclei by an insulin receptor-mediated process. The transport of insulin or the insulin-receptor complex to nuclei in this cell or in others may be directly involved in the long-term biological effects of insulin--in particular, in the control of DNA and RNA synthesis.

Effects of relaxin on the mouse mammary gland. III. The fat pad

The effects of relaxin on the growth of the mammary fat cells of ovariectomized virgin mice have been studied histologically and morphometrically. To characterize the effects of relaxin and investigate a possible synergism in promoting growth of the mammary fat pad, some animals were treated with other mammotrophic hormones, namely estrogen and insulin. The data obtained after 18-20 h of relaxin treatment suggest that this hormone induces hypertrophy and hyperplasia of adipose cells. The degree of hypertrophy is the same if relaxin is given either alone or after estrogen priming. The action of estrogen seems to be obligatory to obtain fat cell hyperplasia. Indeed, a de novo formation of fat cells occurs in all animals treated with estrogen, either alone or in association with relaxin or insulin. However, the maximum degree of adipose cell hyperplasia was attained only when relaxin followed a pretreatment with estrogen, thus suggesting a synergistic action of these two hormones in promoting the overall growth of the mammary fat pad. These findings follow the observation that relaxin stimulates the proliferation and differentiation of epithelial and myoepithelial cells of the duct system and strongly support the idea that relaxin may be regarded as a trophic hormone for the parenchymal and stromal components of the mammary gland.

Colloidal gold-labeled insulin complex. Characterization and binding to adipocytes

Biologically active insulin gold complex was used as an ultrastructural marker to study insulin binding sites, uptake, and internalization in isolated rat adipocytes. The preparation conditions for monodispersed particles, ca. 16 nm in diameter and loaded with approximately 100 insulin molecules, are reported. The complex is stable for at least six weeks. Single particles or small clusters were scattered across the cell membrane. The distribution of unbound receptors seemed to be independent of the extensive system of pre-existing surface connected vesicles in adipocytes. The uptake of particles took place predominantly via non-coated pinocytotic invaginations; clathrin-coated pits did not seem to be important for this process. Lysosome-like structures contained aggregates of 10-15 particles. These data suggest that insulin gold complex is a useful marker for the specific labeling of insulin binding sites.

Biological effects of sulphated insulin in adipocytes and hepatocytes

The binding affinity of sulphated insulin compared with unmodified, neutral insulin has been reported to be approximately four times lower in human and rat adipocytes but over twenty times lower in rat hepatocytes. In the present study the biological action of sulphated insulin was assessed in rat hepatocytes and human and rat adipocytes. To achieve half-maximal stimulation of fatty acid synthesis in rat hepatocytes about twenty one times higher concentrations of sulphated than neutral insulin were required (15.07 +/- 5.50 vs 0.71 +/- 0.34 nmol/l), this ratio being similar to the ratio of binding affinity in rat hepatocytes. In human adipocytes, half-maximal stimulation of initial rates of glucose uptake was observed at 11.6 +/- 5.1 vs 2.9 +/- 1.3 pmol/l for sulphated and neutral insulin respectively, and half-maximal inhibition of lipolysis at 31.0 +/- 13.5 vs 7.3 +/- 2.5 pmol/l respectively. These data are consistent with the four-fold lower binding affinity of sulphated insulin to human adipocytes. However, in rat adipocytes the biological potency of sulphated insulin was found to be much lower than anticipated from the binding data, half-maximal stimulation of initial rates of glucose uptake being observed at 757 +/- 299 vs 35 +/- 13 pmol/l respectively and half-maximal inhibition of lipolysis at 35.9 +/- 12.1 vs 1.5 +/- 0.5 pmol/l respectively. Thus, in rat adipocytes, approximately 22 times the concentration of sulphated insulin was required to achieve equivalent biological effect. A discrepancy between binding affinity and biological action with respect to sulphated insulin was identified in rat adipocytes but not human adipocytes nor rat hepatocytes suggesting differences in the binding-action linkage in these cells.

Ultrastructural analysis of the organization and distribution of insulin receptors on the surface of 3T3-L1 adipocytes: rapid microaggregation and migration of occupied receptors

Monomeric ferritin-insulin and high-resolution electron microscopic analysis were used to study the organization, distribution, and movement of insulin receptors on differentiated 3T3-L1 adipocytes. Analysis of the binding to prefixed cells showed that insulin initially occupied single and paired receptors preferentially located on microvilli. The majority of receptors (60%) were found as single molecules and 30% were pairs. In 1 min at 37% C, 50% of the receptors on nonfixed cells were found on the intervillous plasma membrane and more than 70% of the total receptors had microaggregated. By 30 min only 7% of the receptors were single or paired molecules on microvilli. The majority were on the intervillous membrane, with 95% of those receptors in groups. The receptor groups on the intervillous plasma membrane could be found in both noncoated invaginations and coated pits. The concentration of occupied receptors in the noncoated invaginations and the coated pits was similar; however, ten times more noncoated invaginations than coated pits contained occupied insulin receptors. The observations in this study contrast with those reported on rat adipocytes using identical techniques (Jarett and Smith, 1977). Insulin receptors on adipocytes were initially grouped and randomly distributed over the entire cell surface and did not microaggregate into larger groups. Insulin receptors on rat adipocytes were found in noncoated invaginations but were excluded from the coated pits. The differences in the organization and behavior of the insulin receptor between rat and 3T3-L1 adipocytes suggest that the mechanisms regulating the initial organization of insulin receptors and the aggregation of occupied receptors may be controlled by tissue-specific processes. Since both of these cell types are equally insulin sensitive, the differences in the initial organization and distribution of the insulin receptors on the cell surface may not be related to the sensitivity or biological responsiveness of these cells to insulin but may affect other processes such as receptor regulation and internalization. On the other hand, the microaggregates of occupied receptors on both cell types may relate to biological responsiveness.

Insulin resistance in uremia. Characterization of insulin action, binding, and processing in isolated hepatocytes from chronic uremic rats

We have developed a model in the rat that leads to a predictable degree of severe uremia to study the role of the liver in the insulin-resistant state of uremia. The uremic animals were euglycemic and had increased serum immunoreactive insulin when compared with their pair-fed controls. Insulin action, binding, internalization, and degradation were characterized in freshly isolated hepatocytes from uremic animals, sham-operated pair-fed, and ad lib.-fed controls. The basal rate of aminoisobutyric acid (AIB) uptake was increased in hepatocytes from both uremic and pair-fed control rats. However, while hepatocytes from uremic animals were refractory to insulin with regard to AIB uptake, there was no significant difference in the absolute increment above basal AIB uptake by hepatocytes from pair-fed and fed ad lib. animals at any insulin concentration studied. 125I-Insulin binding at 24 degrees C was higher in hepatocytes from uremic rats at every insulin concentration studied when compared with fed ad lib. controls. The time course of 125I-insulin binding to the cell and to the fractions that were membrane bound or internalized were studied at 37 degrees C. An increase in membrane-bound 125I-insulin at 37 degrees C was present also in hepatocytes from uremic animals. The same fraction of membrane-bound 125I-insulin was internalized in hepatocytes from all groups of animals. Extracellular and receptor-mediated 125I-insulin degradation at the plasma membrane and after internalization was studied at 37 degrees C by gel chromatography. There was a delayed and decreased rate of 125I-insulin degradation in hepatocytes from uremic rats in the three compartments. We conclude: (a) In chronic uremia the liver is refractory to insulin with regard to AIB uptake. (b) Insulin resistance in uremic rat liver is not due to defects in insulin binding or internalization. (c) Despite the high level of circulating immunoreactive insulin, hepatocytes from uremic rats did not show the expected "down regulation" of their insulin receptors or an increased rate of insulin degradation. These studies further emphasize the primary role of postbinding events in the regulation of insulin binding and degradation. The mechanism as to how the coordinated steps of insulin metabolism in the liver are disrupted in a pathological state is presently unknown.

Ultrastructural basis for chloroquine-induced increase in intracellular insulin in adipocytes: alteration of lysosomal function

A quantitative morphological analysis of insulin uptake into adipocytes was undertaken to determine the structural basis for chloroquine-induced increases in intracellular insulin. Adipocytes were incubated with ferritin-labeled insulin in the presence or absence of 50 microM chloroquine at 37 degrees C for 2-90 min and the uptake of the hormone conjugate was determined quantitatively. Quantitative morphometry of cellular organelles also was performed. Chloroquine treatment of adipocytes incubated with 70 nM ferritin-labeled insulin resulted in: (i) a 120% increase in the number of lysosomes in the cytoplasm; (ii) a 75% increase in the average concentration of ferritin-labeled insulin in a lysosome; and (iii) a 25% increase in the percentage of lysosomes containing ferritin-labeled insulin. The cumulative result of these effects was a substantial increase in the amount of intact intracellular hormone within the lysosomes. These morphological data are consistent with biochemical data concerning chloroquine-induced accumulation of 125I-labeled insulin in adipocytes.

Plasmalemmal vesicles (caveolae) of fibrous astrocytes of the cat optic nerve

Plasmalemmal vesicles (caveolae) are described in fibrous astrocytes of the cat optic nerve. In thin sections, astrocytic caveolae appear as flask-shaped invaginations of the plasma membrane with a 60-nm lumen that communicates with the extracellular space through a 35-nm stoma. Occasional bilobed caveolae occur. The caveolae extend approximately 85 nm into the astrocytic cytoplasm and are often embedded within a granular or filamentous ectoplasmic substance connected by 4-8-nm filament bridges to underlying bundles of 10-nm glial filaments. In freeze-fracture replicas, the caveolar stomata appear as dimples on the P face and as craters on the E face, often arranged in hexagonal or linear arrays and spaced at a center-to-center distance of 110-130 nm. The caveolar membrane is apparently particle-free. Fibrous astrocytes related to the connective-tissue septa of the optic nerve show different densities of caveolae on different areas of their plasma membranes. Plasma membranes apposing a basal lamina have few caveolae, whereas membranes not apposed to the basal lamina but to other astrocytic membranes have up to 17 caveolae/micrometers 2. Caveolae also occur on astrocytic plasma membranes apposed to myelin sheaths. Possible functions of the astrocytic caveolae are discussed in the light of plasmalemmal properties of other types of caveolae-bearing cells.

Surface redistribution of 125I-insulin in cultured human lymphocytes

The cultured human lymphocyte (IM-9) binds 125I-insulin by a receptor-mediated process; the receptor, in turn, is regulated by the ligand. In the present study we have examined quantitatively the morphologic events involved in 125I-insulin interaction with the surface of the lymphocyte. At 2 min of incubation of 15 degrees or 37 degrees C, the ligand localizes preferentially at the villous surface of the cell, whereas with longer periods of incubation, the ligand distributes indistinguishably between the villous and nonvillous surface. When rebinding is blocked, 125I-insulin localizes preferentially at the nonvillous surface of the cell. When the total cell surface is considered, there is little preferential association with coated pits; when only the nonvillous surface is considered, a preferential association with coated pits is found and is quantitatively increased in the absence of rebinding of the ligand. This cell has an abundant villous surface (approximately 55% of the total surface); and, as seen on freeze-fracture replicas, the plasma membrane of the villous surface contains a 60% greater density of intramembrane particles than the nonvillous surface. The data suggest an ordered pattern of insulin interaction with the cell surface (i.e., binding to villi followed by redistribution to the nonvillous portion of the cell containing coated pits). These events probably reflect the mechanism by which the cell segregates specific receptors and related proteins in the plane of the membrane so that they can be selectively removed.

Fluorescent low density lipoprotein for observation of dynamics of individual receptor complexes on cultured human fibroblasts

The visible wavelength excited fluorophore 3,3'-dioctadecylindocarbocyanine iodide (Dil[3]) was incorporated into human low density lipoprotein (LDL) to form the highly fluorescent LDL derivative dil(3)-LDL. Dil(3)-LDL binds to normal human fibroblasts and to human fibroblasts defective in LDL receptor internalization but does not bind to LDL receptor-negative human fibroblasts at 4 degrees C or 37 degrees C. It is internalized rapidly at 37 degrees C by normal fibroblasts and depresses the activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase) in a manner similar to that of LDL. It is prevented from binding to the LDL receptor by an excess of unlabeled LDL or by heparin sulfate. Identical distributions of dil(3)-LDL are observed on cells by either indirect immunofluorescence with fluorescein-labeled antibody or directly by dil(3) fluorescence. Upwards of 45 molecules of dil(3) are incorporated per molecule of LDL without affecting binding to the receptor. This labeling renders individual molecules visible by their fluorescence and enables the derivative to be used in dynamic studies of LDL-receptor motion on living fibroblasts by standard fluorescence techniques at low LDL receptor density. Observations with this derivative indicate that the LDL-receptor complex is immobilized on the surface of human fibroblasts but, when free of this linkage, undergoes a Brownian motion consistent with theory.

Biological activity of an angiotensin II--ferritin conjugate on rabbit aortic smooth muscle cells

Specific binding sites for [Asp1,Ile5]angiotensin II (angiotensin) have been demonstrated in homogenates and subcellular fractions of aortic medial smooth muscle cells, but the localization of the angiotensin receptor responsible for contraction has not been determined [Devynck, M. A., & Meyer, P. (1976) Am. J. Med. 61, 758-767]. To establish the location of this receptor, we have prepared a membrane-impermeable analogue of angiotensin by acylating its N-terminal amino group with the N-hydroxysuccinimide ester of succinylated ferritin. This angiotensin-ferritin conjugate possessed the same intrinsic activity as angiotensin but was approximately 200 times less potent in inducing contraction in rabbit aortic strips. The stability of the conjugate was investigated, and approximately 5% of the contractile activity of the angiotensin-ferritin conjugate was attributable to low molecular weight components that were present before or after exposure to aortic strips. The time required for aortic strips to reach a plateau of contraction in response to angiotensin-ferritin was significantly longer than that required by free angiotensin to produce the same level of contraction. With enzymatically dispersed aortic smooth muscle cells, however, the time taken to produce contractions by both angiotensin and angiotensin-ferritin was indistinguishable. [Sar1,-Ala8]angiotensin II, a competitive inhibitor of angiotensin, completely suppressed contractions induced by angiotensin or angiotensin-ferritin in aortic strips or dispersed aortic smooth muscle cells. These results suggest that angiotensin need not directly penetrate the plasma membrane to cause contraction and imply that the angiotensin receptor responsible for initiating contraction of aortic smooth muscle cells is located on the plasma membrane.

Computer assisted analysis of ferritin-insulin receptor sites on adipocytes and the effects of cytochalasin B on groups of insulin receptor sites

A computerized quantitative technique was used to analyze the distribution of ferritininsulin receptor sites on rat adipocytes and the effects of cytochalasin B on groups of receptor sites. Computer analysis of separation distances between receptor sites established that insulin receptor sites on adipocytes did not have a random distribution but have a distinct tendency to exist in groups with a maximum separation distance between particles of 400 A. A peak in the distribution of separation distances occurred at 100-200 A. Cytochalasin B, but not cytochalasin D, treatment of adipocytes resulted in a decrease in the number of large groups of receptor sites and a corresponding increase in single and paired receptor sites without affecting the separation distance between the remaining grouped receptors. This suggested that when cytochalasin B disrupted the bond holding receptor sites together, it caused complete disruption. These observations provided additional information on the ultrastructural characteristics of the insulin receptor. Further application of these techniques to the analysis of insulin receptors may provide the necessary structural correlates to the biochemically observed differences in insulin action in other tissues and diseased states.

Insulin receptors: differences in structural organization on adipocyte and liver plasma membranes

Comparison was made of the distribution of the insulin receptor sites on adipocyte and liver plasma membranes by using ferritin-insulin. Two-thirds of the occupied insulin receptors on adipocytes occurred in groups of two or more whereas up to two-thirds of the receptors on liver occurred as single receptors. Ferritin-insulin did not cause aggregation of the receptor sites in either tissue. The naturally occurring groups of receptors on adipocyte membranes may play a role in the greater sensitivity of adipocytes to insulin.

Organelle relationships in cultured 3T3-L1 preadipocytes

In differentiating 3T3-L1 cells, lipid spheres, the endoplasmic reticulum (ER), microperoxisomes, and mitochondria form "constellations" that may reflect the interplay of lipid metabolizing enzymes in these organelles. ER cisternae are also situated very close to "rosettes,"plasmalemmal specializations found in mature adipocytes in vivo. As in hepatocytes and absorptive cells of the intestine, this spatial relationship of ER and plasmalemma suggests a role for rosettes in the uptake of exogenous lipid precursors. The morphological differentiation of 3T3-L1 preadipocytes includes the loss of "stress fibers" and the appearance of microfilament like structures that encase, in a complex manner, the cytosolic lipid spheres that appear during differentiation. Other features described for the first time in 3T3-L1 preadipocytes include: (a) the presence of an extensive acid phosphatase (AcPase) positive GERL from which coated vesicles apparently arise (these coated vesicles display AcPase activity and are much smaller and far more numerous than the coated vesicles that seem to arise from the plasmalemmal coated pits); (b) the abundance of AcPase-positive autophagic vacuoles; and (c) a high level of alpha-naphthyl-acetate-esterase activity which, by light microscopy cytochemistry, appears to be localized in the cytosol.

Surface structure changes of rat adipocytes during lipolysis stimulated by various lipolytic agents. A scanning electron microscopic study

A qualitative and quantitative electron microscopic study was performed on rat adipocytes during stimulation of lipolysis by various agents. Scanning electron microscopy of control cells revealed a spherical cell with a textured glycocalyx surface exhibiting small irregular projections. Globular surface evaginations or protrusions measuring 8-18 muM in diameter were seen on cell hemispheres, and there was an average of one protrusion for every two hemispheres examined. Distribution analysis showed that 60 percent of the hemispheres had no protrusions, and 25, 10, and 5 percent of the hemispheres had one, two or three protrusions, respectively. Thin-section and freeze- fracture electron microscopy of the protrusions showed a small triglyceride droplet surrounded by a thin cytoplasmic rim that was continuous with the main cytoplasmic matrix. The glycocalyx coating and plasma membrane extended from the cell surface onto, and over, the protrusion. Scanning microscopy of cells stimulated by lipolytic agents, including epinephrine, adrenocorticotropic hormone, theophylline, and dibutyryl cyclic AMP, revealed a dose-dependent increase in the number of protrusions per cell hemisphere. Maximal concentrations of lipolytic hormones cuase an average 2.5-fold increase in the number of protrusions per hemisphere without changing the average size of the protrusions. Only 40 percent of the stimulated cell hemispheres exhibited no protrusions; over 15 percent of the cells contained three or more; and a number of the protrusions were multilobulate. Insulin prevented the increase in the number of protrusions and the change in distribution caused by the lipolytic hormones but did not prevent the increase caused by theophylline and dibutryl cyclic AMP. The data suggest that the protrusions are a structural feature of the cell and may be related to the lypolytic pathway. These observations may help explain some of the discrepant biochemical data relating to hormonal stimulation of lipolysis.

Demonstration and partial characterization of insulin receptors in human platelets

Recently, evidence has been reported to suggest that human platelets like several other circulating blood cells may bind insulin. To examine whether human platelets contain specific insulin receptors, washed human platelets suspended in Hepes buffer were incubated at 24 degrees C with 125I-insulin in the presence and absence of unlabeled insulin and specific insulin binding was determined. Insulin binding by platelets increased progressively with time of incubation to reach a maximum at 3 h and was proportional to the number of platelets in the incubation mixture. Maximum insulin binding was observed at pH 8. Insulin degradation by platelets as assessed by TCA precipitability and reincubation studies was minimal. Scatchard analysis of the binding data and dissociation studies revealed evidence of negative cooperativity of the platelet insulin receptor. A high affinity dissociation constant of approximately equal to 3 X 10(9) M-1 was determined and the concentration of platelet insulin receptors was estimated as 25 binding sites/micron2 platelet surface area. Binding of 125I-insulin by platelets was inhibited by unlabeled porcine insulin and to a lesser extent by catfish insulin and porcine proinsulin but not by glucagon, prolactin, growth hormone, and thrombin. The findings indicate that human platelets contain specific insulin receptors. The significance of the platelet insulin receptor, particularly with respect to altered platelet function in diabetes mellitus, remains to be determined.

Effect of cytochalasin B and D on groups of insulin receptors and on insulin action in rat adipocytes. Possible evidence for a structural relationship of the insulin receptor to the glucose transport system

The possible physiological importance of the groups of insulin receptors on rat adipocytes and the relationship of these groups to insulin action were investigated. The effect of cytochalasin B and D on biological actions of insulin was measured and compared with the effect of these agents on the ultrastructural distribution of groups of insulin receptors. Cytochalasin B had no effect on epinephrine-stimulated lipolysis, insulin inhibition of epinephrine-stimulated lipolysis, or insulin stimulation of protein synthesis. Cytochalasin B, over a concentration range of 50 nM to 5 muM, progressively inhibited the basal glucose transport system, as measured by glucose oxidation, 2-deoxyglucose transport, and 3-O-methylglucose transport. Insulin was capable of fully stimulating remaining basal transport at submaximal concentrations of cytochalasin B. Insulin pretreatment of adipocytes partially protected the glucose transport system from inhibition by cytochalasin B. Cytochalasin B markedly altered the distribution pattern of insulin receptors, which caused an increase in the number of single receptor molecules by decreasing the number of larger groups. A significant correlation (r = 0.964; P < 0.001) was found between the percent increase in single receptors and the percent decrease in glucose transport. Ferritin-insulin pretreatment of adipocytes prevented disruption of the groups of insulin receptors by cytochalasin B. Cytochalasin D had no effect on the biological actions of insulin or on the groups of insulin receptors. These data suggest that the ability of insulin to affect adipocyte metabolism is independent of the hormone occupying adjacent, grouped receptor sites. The marked contrast in effects of cytochalasin B and D on groups of insulin receptors and glucose transport suggests that the microfilament system is not involved in insulin action or in holding the groups of insulin receptors together, as both agents are known disrupters of microfilaments and inhibitors of actin gelation. The correlation between the effects of cytochalasin B on insulin receptor distribution and glucose transport leads to the speculation that the glycoprotein molecules containing the insulin receptor are functionally linked with the glucose transport system.

Insulin binding sites on rat liver nuclear membranes: biochemical and immunofluorescent studies

Preliminary investigations (Horvat et al., '75) indicated the nucleus of rat liver as a site for specific binding of insulin. In this report these observations are confirmed. Nuclei from rat liver were isolated in a highly purified state as verified by interference contrast and electron microscopy and by chemical analysis. Extensive scanning of the preparations did not reveal the presence of structures resembling plasma membranes. The nuclear envelope was isolated by a modification of the method of Kay et al. ('72). Electron micrographs showed the presence of nuclear "ghosts" and few other recognizable nuclear elements, but no plasma membranes (60--80 A thick) were detected. The preparation was found to contain specific insulin binding activity. Specificity of the binding sites for insulin was demonstrated in competition studies with other polypeptide hormones and a synthetic insulin analog. Scatchard analysis of the binding data indicates the presence of a single class of high affinity receptors. In contrast to findings with plasma membranes the hormone-receptor complex is very stable and the kinetics of the dissociation of bound [125I]-insulin do not indicate negative cooperativity of the binding sites. Immunofluorescent labeling of intact, unfixed nuclei showed a specific fluorescent halo only around those nuclei that have been preincubated with insulin. All other controls were negative.

125I-insulin binding to cultured human lymphocytes. Initial localization and fate of hormone determined by quantitative electron microscopic autoradiography

Morphologic and biochemical studies indicate that the initial action of insulin is binding to a cell surface receptor. Whether further translocation of the hormone, or a product of the hormone, occurs is unclear and has not been investigated by direct means. To determine the fate of 125I-insulin bound to its receptor, we have examined the distribution of radioactivity by quantitative electron microscopic autoradiography. Cultured lymphocytes of the IM-9 cell line were incubated with 0.1 nM 125I-insulin at 15 degrees and 37 degreesC for incubation periods extending from 2 to 90 min. At 15 degreesC, grains localize to the plasma membane and there is no translocation as a function of time. At 37 degreesC, grains predominantly localize to the plasma membrane but there is a small shift in distribution to a distance of 300-700 nm from the plasma membrane. This small additional band component of irradiation extends to approximately to10--15% of the cell radius. When a morphometric analysis is applied to grains extending 300 nm and beyond from the plasma membrane, we find no preferential localization to any intracellular organelle. We interpret these data to indicate that in the cultured lymphocyte, labeled insulin initially localizes to the plasma membrane but as fuanction of time and increasing temperature there is a small but definite translocation of the hormone or a product of the hormone to a hihgly limited aea of the cell periphery.

Attachment role of gonococcal pili. Optimum conditions and quantitation of adherence of isolated pili to human cells in vitro

Gonoccocal pili facilitate attachment of virulent Neisseria gonorrhoeae to human cells. To characterize this attachment function, purified gonococcal pili isolated from four strains possessing antigenically distinct pili were radiolabeled with 125I and used to measure the attachment of pili to various human cells in vitro. Human buccal and cervical-vaginal mucosal epithealial cells, fallopian tube mucosa, and sperm bound pili in greater numbers per micrometer2 of surface area (1--10) than fetal tonsil fibroblasts, HeLa M cells, erythrocytes, or polymorphonuclear leukocytes. This cell specificity of attachment suggests a greater density of membrane pili binding sites on cells similar or identical to cells from natural sites of infection. The pili binding sites were quantitated as 1 X 10(4) per cervical-vaginal squamous cell. Pili of all antigenic types attached equally to a given cell type, implying that the attachment moiety of each pilus was similar. Attachement of gonoccocal pili to human cells occurred quickly with saturation of presumed receptor sites within 20--60 min. Attachment was temperature dependent (37 degrees greater than 20 degrees greater than 4 degrees C), and pH dependent (3.5 less than 4.5 less than 5.5 less than 7.5). Attachment was inhibited by antibody to pili (homologous pili Ab greater than heterologous Ab). The extent of possible protection against gonococcal infection due to inhibition of pili-mediated attachment might prove limited as a result of the considerable antigenic heterogeneity among pili and the observation that blockage of pili attachment is maximal only with antibody to pili of the infecting strain.

Alterations in insulin binding accompanying differentiation of 3T3-L1 preadipocytes

Expression of the adipocyte phenotype by differentiating 3T3-L1 preadipocytes occurs upon exposure of the cells to insulin. Differentiation-linked changes in 125I-labeled insulin binding to 3T3-L1 cells were monitored and compared with those in nondifferentiating 3T3-C2 controls treated similarly. Without chronic insulin treatment, 3T3-L1 cells failed to express the adipocyte phenotype but maintained a level of 25,000-35,000 insulin-binding sites per cell. Treatment of 3T3-L1 cells with insulin resulted in an initial suppression of insulin binding followed by a 12-fold increase that paralleled the appearance of differentiated cells. A maximum of 170,000 insulin-binding sites per cell was attained for a population in which greater than 75% of the cells had differentiated. The increase of insulin receptor level appears to be differentiation-dependent and is not a general response of cells to the culture conditions. 3T3-C2 cells maintained in the presence of insulin for 30 days exhibited the undifferentiated phenotype and suppressed levels of insulin binding (35,000 sites per cell). The binding capacity of 3T3-L1 cells for epidermal growth factor remained unchanged between 25,000 and 40;000 sites per cell and was independent of the state of differentiation. Thus, induction by insulin in receptor-specific changes. Insulin receptors increase in number but epidermal growth factor receptors remain constant.

Electron microscopic localization of adenylate cyclase activity of white and brown adipose tissue of the rat and chicken

Brown adipose tissue of newborn rats and chicken embryos and white adipose tissue of adult rats were studied. Adenylate cyclase (EC 4.6.1.1.) activity stimulated by 0.1 mmol/l noradrenaline was demonstrated using an electron microscopic histochemical method. The reaction product was visualized as a cobalt salt in the plasmalemma of the adipocytes. The adipocytes of the brown adipose tissue of the newborn rats showed most intense reaction in the outer surfaces of their plasmalemma. Alloxan totally inhibited the enzymatic reaction. The histochemical reaction used in the present study probably demonstrated the hormonal receptor sites in the plasmalemmas of the adipocytes which are stimulated by noradrenaline.

Receptor mobility and its cooperative restriction by ligands

The functional significance of membrane fluidity and receptor mobility in lymphoid cells has been studied in the recent literature. Although far from clarified, the role of membrane fluidity in achieving control over cell activity is probably important; it allows cooperative interactions over long distances. Here, the emphasis is put on the phenomenon of restriction of receptor mobility by ligands such as Concanavalin A, a phenomenon discovered in recent years using morphological techniques. We discuss in some detail our own approach for studying this phenomenon. This consists of quantitatively measuring the active sites on cell-bound lectin molecules by subsequent fixation of horse-radish peroxidase. This study has shown a cooperative binding of Concanavalin A to cells which corresponds to a modification of the membrane, leading to the recruitment of new receptors. The existence of a post-binding event, that we have called micro-redistribution, has been shown at 4 degrees C, through the use of peroxidase binding to cell-bound lectin. A cooperative restriction of receptor microredistribution is observed when the cooperative recruitment of receptors induced by increasing concentrations of Concanavalin A occurs. Both phenomena were shown to be modulated by drugs such as colchicine and cytochalasin B. The characteristics of this modulation suggest that density and distribution of receptors are dependent upon the state of a multimeric submembrane structure which is still functional at 4 degrees C.