The interaction of soluble horseradish peroxidase with mouse peritoneal macrophages in vitro

Identification of specific apical membrane polypeptides associated with the antidiuretic hormone-elicited water permeability increase in the toad urinary bladder

Antidiuretic hormone (ADH) increases the water permeability of the toad urinary bladder. The increase occurs in the apical plasma membrane of granular cells that line the urinary surface of the bladder and is produced by the insertion of water permeability units that have been identified by freeze-fracture electron microscopy as intramembrane particle aggregates. Under water-impermeable conditions, particle aggregates reside in intracellular vesicles called "aggrephores." In response to ADH, the aggrephores fuse with the apical plasma membrane and render it water permeable. When ADH is removed, intramembrane particle aggregates and aggrephores are retrieved from the apical membrane, and it returns to a water-impermeable state. To identify proteins involved in the water permeability response, we used lactoperoxidase/glucose oxidase to 125I-label external apical membrane proteins to compare control and ADH-treated bladders. Several polypeptides were consistently labeled in ADH-treated bladders and not in paired controls. After demonstrating that lactoperoxidase behaves as a fluid-phase marker and is sequestered in aggrephore-like vesicles when ADH is withdrawn, we used the technique of Mellman et al. [Mellman, I.S., Steinman, R. M., Unkeless, J. C. & Cohn, Z. A. (1980) J. Cell Biol. 86, 712-722] to label proteins endocytosed when water permeability declines after ADH is withdrawn to test whether the membrane proteins labeled in ADH-treated bladders behaved like particle aggregates. The internalized membranes contained polypeptides of the same molecular weights (55,000, 17,000-14,000, and 7,000) as those labeled on the apical surface of ADH-treated but not control bladders. These polypeptides are evidently involved in the ADH-stimulated water permeability response and may be components of particle aggregates.

The cellular effects of estrogens on neuroendocrine tissues

Estrogen action on sensitive neurons in the rat diencephalon has been studied by morphologic techniques; evidence of estrogen action at every level is presented, including tracts, cells, circuitry and subcellular organelles. The demonstration in the arcuate nucleus of estrogen-induced synaptic remodelling, estrogen-induced postsynaptic membrane phenotypes, changes in intracellular membranes and rapid estrogen actions on neuronal endo-exocytosis indicates that cellular estrogen actions may underlie the neuronal control of reproduction.

Degradation of horseradish peroxidase after microinjection into mammalian cells

Horseradish peroxidase (HRP) has been microinjected into mammalian cells in tissue culture by the erythrocyte ghost-mediated technique. This protein was selected because it can be localized and quantified after injection by cytochemical and spectrophotometric methods. HRP labeled by reductive methylation retained full catalytic activity, was efficiently loaded into erythrocyte ghosts, and did not associate to a significant degree with ghost membranes. A combination of cytochemical staining and autoradiography established that HRP injected into rat L6 myoblasts, HE(39)L human diploid fibroblasts, or HeLa cells was intracellular and uniformly distributed throughout the cell, while cell lysis techniques showed that the catalytically active HRP was not membrane bound. Inactivation of labeled HRP after injection paralleled the disappearance of the 40-kDa polypeptide, and was always more rapid than its overall degradation. This difference was associated with a pool of water-insoluble radioactivity in the injected cells. This material was of smaller molecular size than the native protein: many labeled peptides were detected in the range of 10 to 38 kDa. By the use of inhibitors of autophagic proteolysis or lysosomal function it was established that HRP degradation was not subjected quantitatively to the same regulatory processes as the average endogenous protein labeled in the same cultures.

Peroxidase-induced enhancement of chemiluminescence by murine peritoneal macrophages

A number of substances have been shown to enhance the respiratory burst (RB) of macrophages. Many of these substances are not normally found in vivo. The present study suggests that a group of enzymes characterized as peroxidases have the ability to significantly enhance the RB and concomitant phagocytosis by murine peritoneal macrophages. Horseradish peroxidase (HRP), lactoperoxidase (LPO), and microperoxidase (MPO) can significantly augment these functions. Both resident and thioglycollate-induced macrophages exhibited enhanced chemiluminescence (CL) upon exposure to HRP, however, the effect was more pronounced with the latter. The increase in CL was correlated with an increase in production of superoxide, which was measured by reduction of cytochrome c. Horseradish peroxidase immobilized on an inert carrier, was capable of enhancing the RB suggesting that it does not have to enter the cell in order to function. Hemin, hematoheme and hematoporphyrin had little effect on macrophage stimulated CL. All of the peroxidases tested caused increased phagocytosis of opsonized zymosan. These studies indicate that peroxidases are capable of stimulating the RB, phagocytosis and possibly other macrophage functions.

Impaired polymorphonuclear leukocyte function in biliary atresia: role of bilirubin and bile acids

Our previous study suggested that impaired bactericidal activity of polymorphonuclear leukocytes in patients with biliary atresia might be due to decrease in superoxide-generating activity and that serum factors may play an important role. In the present study, analysis of the patients' serum revealed that, among 15 bile acids, glycine or taurine conjugated chenodeoxycholic acid and cholic acid were markedly elevated. To examine the effects of bilirubin and these bile acids and conjugated lithocholic acids on the leukocyte function, we measured cytolysis, phorbol myristate acetate-induced superoxide generation, and myeloperoxidase activity, using normal human polymorphonuclear leukocytes. Bilirubin ranging from 5 to 20 mumol/L was cytolytic and more potently inhibited the superoxide generation. The inhibition by bilirubin was also observed in the presence of 10% human serum or 2.0% human serum albumin, though the cell viability was almost completely preserved. On the other hand, conjugated chenodeoxycholic acids and cholic acids, ranging from 0.5 to 1.5 mmol/L or conjugated lithocholic acids, ranging from 0.02 to 0.05 mmol/L, did not inhibit the superoxide generating activity, though the conjugated chenodeoxycholic acids and lithocholic acids did induce cytolysis. Myeloperoxidase activity was little affected, except in the case of conjugated lithocholic acids. These results suggest that in patients with biliary atresia, bilirubin, probably the unconjugated form, more than bile acids might be one of the influential factors in the suppression of bactericidal activity of polymorphonuclear leukocytes, by inhibiting the superoxide-generating activity.

Aggregation of complement receptors on human neutrophils in the absence of ligand

C3bi receptors (CR3) on human polymorphonuclear leukocytes (PMN) bind ligand-coated particles and promote their ingestion. The binding activity of CR3 is not constitutive but is transiently enabled by phorbol esters (Wright, S. D., and B. D. Meyer, 1986, J. Immunol. 136:1759-1764). Our observations indicate that the capacity of CR3 to bind ligand is tightly correlated with the degree of ligand-independent aggregation of the receptor in the plane of the membrane. Fixed PMN were labeled with anti-CR3 monoclonal antibodies and streptavidin colloidal gold before viewing in the electron microscope either en face or in thin section. On unstimulated PMN, gold particles marking CR3 were dispersed randomly. Stimulation of PMN for 25 min with phorbol myristate acetate (PMA) dramatically enhances binding of C3bi-coated particles, and the CR3 on such stimulated cells was observed in clusters containing more than six gold particles. CR3 was not aggregated over coated pits. After 50 min in PMA, the binding activity of CR3 falls, and the distribution of CR3 was again observed to be disperse. If a hydrophilic phorbol ester was washed away after a 20-min stimulation, binding activity remains elevated for at least 50 min, and CR3 remained aggregated. Thus, clustering of CR3 was temporally correlated with its ability to bind ligand and initiate phagocytosis. Unlike CR3, Fc receptors and HLA did not exhibit changes in their aggregation state in response to PMA. Treating PMN with formyl-methionyl-leucyl-phenylalanine, which enhances expression of CR3 but not its function, did not lead to aggregation of CR3. These observations suggest that a clustered configuration is a precondition necessary for binding ligand and signaling phagocytosis.

Platinum-shadowed, whole-mounts for transmission electron microscopy of cells cultured on plastic

A simple and convenient method is described whereby cells cultured on polystyrene plastic substrata may be critical point-dried, rotary-shadowed with platinum/carbon and finally released by means of propylene oxide for examination whole in the transmission electron microscope. The technique is particularly useful for the localization of colloidal gold probes, both intra- and extra-cellularly. The problem of premature melting of polystyrene at about 307 K after prolonged exposure to carbon dioxide is solved by limiting the residence time in the critical point-drier to 1 h. Details for cleaning and mounting the released films of cells on specimen grids are given. The technique expands the application of the 'whole-mount' approach of Hopkins and co-workers (Hopkins et al., 1981; Hopkins, 1985) to cells cultured directly on polystyrene substrata, that is, in conventional plastic culture vessels.

Uptake and axonal transport of horseradish peroxidase isoenzymes by different neuronal types

The uptake and transport of basic and acidic horseradish peroxidase isoenzymes was compared in the neuromuscular, visual and olfactory systems of Xenopus larvae and postmetamorphic frogs. The concentration (w/v) of the two preparations was corrected to compensate for their difference in enzymatic activity (unit/w), which was seven-fold higher in basic horseradish peroxidase. Uptake and transport of horseradish peroxidase isoenzymes could be demonstrated with 7% basic horseradish peroxidase, but not with equal amounts of 49% acidic horseradish peroxidase in all systems investigated: retrograde transport from terminals of retinal ganglion cells, isthmotectal neurons and spinal motoneurons, as well as anterograde transganglionic transport along olfactory neurons. A very weak labelling of the same neuronal pathways by acidic horseradish peroxidase was obtained only after increasing the amount injected by approximately two-fold. Basic horseradish peroxidase isoenzymes were also preferentially taken up and transported retrogradely by broken axons of the optic nerve. When tested, similar results were obtained in both larvae and frogs suggesting that preferential uptake and transport of basic horseradish peroxidase is a general feature of all neurons and of all developmental stages. Electron microscopical analysis of the outer layers of the optic tectum revealed that, in the same experimental conditions producing no retrotrade labelling of optic axons, acidic horseradish peroxidase was rarely found to enter nerve terminals. It appears that interactions between horseradish peroxidase and neuronal membranes occur during uptake and transport and that molecular charge plays an important role, beyond non-specific fluid-phase endocytosis. We suggest that differences between horseradish peroxidase isoenzymes as neuronal tracers reflect a process of adsorptive endocytosis related to general characteristics of neuronal membranes (regardless of age) and not to specific receptor-mediated interactions characteristic of neuronal specificity.

Subcellular distribution of Ca2+ pumping sites in human neutrophils

The distribution of nonmitochondrial Ca2+ pumping sites and the site of action of inositol 1,4,5-trisphosphate (Ins 1,4,5-P3) were studied in subcellular fractions of human neutrophils. In homogenates, two different Ca2+ pools could be observed: a mitochondrial Ca2+ pool and a nonmitochondrial, ATP-dependent, Ins 1,4,5-P3-responsive Ca2+ pool. When the homogenate was separated into microsomes, primary granules, and secondary granules, the nonmitochondrial Ca2+ pumping and the Ins 1,4,5-P3-induced Ca2+ release occurred only in the microsomal fraction. In a gradient developed to separate different microsomal organelles, maximal Ca2+ pumping activity occurred in fractions of low densities. Correlations between Ca2+ uptake and organelle markers were negative for the endoplasmic reticulum (r = -0.49) and positive for plasma membrane (r = 0.47), Golgi (r = 0.62), and endosomes (r = 0.96). Because the Ca2+ pumping organelles in these fractions were insensitive to micromolar vanadate and digitonin treatment, they are unlikely to be plasma membrane vesicles. We conclude first that microsomal fractions of human neutrophils contain organelles that lower the ambient free Ca2+ concentration and respond to Ins 1,4,5-P3. Second, granules are not involved in intracellular Ca2+ regulation in neutrophils. Third, nonendoplasmic reticulum organelles, such as endosomes, Golgi elements, or yet undefined specialized structures, play a major role in intracellular Ca2+ homeostasis in human neutrophils.

Effect of phagocytosis on receptor distribution and endocytic activity in macrophages

Phagocytosis requires the internalization of a significant fraction of the plasma membrane and results in the intracellular deposition of large particles. We evaluated the effect of phagocytosis on the cellular distribution of recycling receptors and uptake of ligand to determine whether phagocytosis affects receptor behavior. Phagocytosis of zymosan, latex particles, or IgG-coated red blood cells by rabbit alveolar macrophages did not decrease the number of cell surface receptors for transferrin, alpha 2-macroglobulin X protease complexes, maleylated proteins, or mannosylated proteins. The number of surface receptors for transferrin was also unaltered in J774 cells, a macrophage-like cell line. In both cell types extensive phagocytosis did not affect the rate of receptor-mediated endocytosis or the distribution of receptors between the endosome and the cell surface. However, fluid phase pinocytosis was reduced by phagocytosis. The major reduction appeared to be not in the rate of internalization but rather in the delivery of fluid to the lysosome. These results demonstrate that internalization of a significant amount of the plasma membrane during phagocytosis does not diminish the number of receptors on the cell surface and has no effect on receptor-mediated ligand uptake.

Control of membrane permeability in animal cells by divalent cations

The permeability of several cell lines, including HeLa, L929, 3T6 and 3T3, to various compounds is affected by the concentration of divalent cations in the culture medium. In the absence of Mg2+ ions but with 4-8 mM CaCl2 in the medium, HeLa and L929 cells become permeabilized, as measured by the entry of the aminoglycoside antibiotic hygromycin B. However, 3T3 and 3T6 cells become much more permeable when calcium and magnesium are both absent from the medium. Addition of Mg2+ above 2 mM abolishes the permeabilization induced by Ca2+. Basic pH favors permeabilization, whereas acidic pH inhibits the entry of hygromycin B. Increased entry of macromolecules, such as the toxin alpha-sarcin, horseradish peroxidase (HRP) and luciferase, is also observed under permeabilization conditions, suggesting that this method could be of general use, since it is not harmful to cells and is fully reversible. Exit of 86Rb+ ions and [3H]uridine-labelled nucleotides was also assayed. We did not observe increased release of these compounds from preloaded cells under various calcium concentrations. Finally, the effects of several inhibitors of endocytosis and other membrane functions on the permeabilization inhibitors of endocytosis and other membrane functions on the permeabilization process were also analysed. The entry of alpha-sarcin was not affected by nifedipine, dibucaine or mepacrine, but was partially inhibited by NH4Cl, amantadine and chloroquine.

Characterization of endocytic compartments using the horseradish peroxidase-diaminobenzidine density shift technique

We have employed a modification of the horseradish peroxidase (HRP)-diaminobenzidine density shift technique of Courtoy et al. (J. Cell Biol., 1984, 98:870-876) to examine the biochemical properties of the endosome. This organelle is involved in receptor recycling and the sorting of internalized receptor ligand complexes. Transferrin covalently bound to HRP was used to place peroxidase activity specifically within the endosome. The peroxidase-catalyzed polymerization of diaminobenzidine within these vesicles causes an increase in buoyant density, thus allowing them to be separated from other membranes. Using this technique we demonstrate that 125I-low density lipoprotein, 131I-epidermal growth factor, and Tf-HRP are internalized into the same endosome. We discovered that the diaminobenzidine reaction product "cross-links" the lumen of the vesicle, rendering vesicular components detergent insoluble. Furthermore, the reaction inactivates enzymatic activities associated with the endosome. Thus, the diaminobenzidine density shift procedure has limited usefulness in studies designed to isolate endosomal constituents. Nonetheless, we have found that the inactivation of enzymatic activities is confined to those endosomes that contain peroxidase. This selectivity allows us to define endosome-specific activities.

Effects of cholesterol and lipoproteins on endocytosis by a monocyte-like cell line

The human monocyte/macrophage-like cell line U937 is a cholesterol auxotroph. Incubation of these cells in the growth medium in which delipidated fetal calf serum has been substituted for fetal calf serum depletes cellular cholesterol and inhibits growth. The cholesterol requirement of these cells for growth can be satisfied by human low-density lipoprotein (LDL), and very-low-density lipoprotein (VLDL), but not by high-density lipoprotein (HDL). U937 cells can bind and degrade LDL via a high-affinity site and this recognition is altered by acetylation of LDL. This indicates that these cells express relatively high LDL receptor activity and low levels of the acetyl-LDL receptor. The cells were used to study the role of cholesterol in lectin-mediated and fluid-phase endocytosis. Growth of the cells in the medium containing delipidated fetal calf serum results in impairment of both concanavalin A-mediated endocytosis of horseradish peroxidase and concanavalin A-independent endocytosis of Lucifer Yellow. Supplementation of the medium with cholesterol prevents cellular cholesterol depletion, supports growth and stimulates Lucifer Yellow endocytosis but fails to restore horseradish peroxidase endocytosis. However, if the cells are incubated in the presence of no less than 40 micrograms LDL protein/ml to maintain normal cell cholesterol levels, concanavalin A-mediated endocytosis of horseradish peroxidase is activated. The effect of LDL is specific since neither VLDL nor HDL3 at the same protein concentration activates horseradish peroxidase uptake by the cells. Furthermore, the activation of endocytosis by LDL is not inhibited by the inclusion of heparin or acetylation of the LDL indicating that binding of LDL to the LDL receptor is not required for these effects. The mediation of activation of horseradish peroxidase endocytosis by the lectin is presumed to involve binding of LDL to concanavalin A associated with the cell surface which in turn stimulates horseradish peroxidase binding and uptake by adsorptive endocytosis. The rate of fluid endocytosis and endosome formation seems to depend on cellular cholesterol content presumably because cholesterol is involved in maintaining the appropriate plasma membrane structure and fluidity.

Kinetic analysis of the triggered exocytosis/endocytosis secretory cycle in cultured bovine adrenal medullary cells

Cultured bovine adrenal medullary cells are an excellent preparation for quantitative analysis of the secretory exocytosis/endocytosis cycle. In this paper we examine the kinetics of endocytosis after stimulation of secretion. Membrane retrieval was monitored by uptake of the fluid phase marker horseradish peroxidase. Horseradish peroxidase was found to be suitable because it can be washed off completely, assayed quantitatively, and its uptake increases linearly with concentration. If this marker is present during stimulation, the rate of uptake is initially slower than catecholamine secretion but faster at a later time, suggesting that the formation of endocytotic vesicles follows exocytosis. To monitor the time-dependent concentration of secretory vesicle-plasma membrane fusion product (omega-profiles), secretion was halted at various time intervals after stimulation and the excess membrane allowed to transform into endocytotic vesicles in the presence of horseradish peroxidase. By adding horseradish peroxidase at various times after inhibition of secretion, the time course of membrane retrieval could be measured directly. All our results are consistent with a two-step kinetic model in which exocytosis and membrane retrieval are consecutive events. The estimated volumes of the compartments involved are roughly equal. The rate of endocytosis is strongly temperature-dependent but unaffected by extracellular calcium in the range of 10(-8)-2.5 X 10(-3) M, suggesting that calcium is not required at the site of endocytotic membrane fusion. Membrane retrieval is also unaffected by Lanthanum (1 mM) but is slowed by hypertonic media.

Tritrichomonas foetus: ultrastructure and cytochemistry of endocytosis

Tritrichomonas foetus ingests horseradish peroxidase, native ferritin, cationized ferritin, and 0.08 micron latex beads by a process which involves the formation of pinocytic vesicles. These vesicles fuse with each other and with lysosomes forming large vacuoles. Biochemical determinations on the ingestion of horseradish peroxidase and morphometric analysis on the ingestion of cationized ferritin covered latex beads indicated that T. foetus has high endocytic activity. The process of ingestion of the various tracers used was analyzed by transmission electron microscopy of thin sections and freeze fracture replicas.

Plasmalemmal proteins of cultured vascular endothelial cells exhibit apical-basal polarity: analysis by surface-selective iodination

Vascular endothelium in vivo appears to function as a polarized epithelium. To determine whether cellular polarity exists at the level of the plasma membrane, we have examined cultured endothelial monolayers for evidence of differential distribution of externally disposed plasmalemmal proteins at apical and basal cell surfaces. Lactoperoxidase beads were used to selectively label the apical surfaces of confluent endothelial monolayers, the total surfaces of nonenzymatically resuspended cells, and the basal surfaces of monolayers inverted on poly-L-lysine-coated coverslips, while maintaining greater than 98% viability in all samples. Comparison of the SDS PAGE radioiodination patterns obtained for each surface revealed a number of specific bands markedly enriched on either apical or basal surface. This polarized distribution involved membrane-associated as well as integral membrane proteins and was observed in several strains of bovine aortic endothelial cells, as well as in both primary and passaged human umbilical vein endothelial cells. In contrast, two morphologically nonpolarized cell types, bovine aortic smooth muscle and mouse peritoneal macrophages, did not display differential localization of integral membrane proteins. Polarized distribution of integral membrane proteins was established before the formation of a confluent monolayer. When inverted (basal-side-up) monolayers were returned to culture, the apical-side-up pattern was reexpressed within a few days. These results demonstrate that cell surface-selective expression of plasmalemmal proteins is an intrinsic property of viable endothelial cells in vitro. This apical/basal asymmetry of membrane structure may provide a basis for polarized endothelial functions in vivo.

Immunologically relevant peptide antigen exists on the presenting cell in a manner accessible to macromolecules in solution

Although studies of the association of antigen with APC have been complicated by antigen-processing requirements, recent studies have suggested that immunologically relevant antigen should be present on the APC surface. Nevertheless, blocking of antigen presentation with antibody to the antigen has not been demonstrable in most systems. To study this problem we developed a system using avidin to block presentation of amino-terminal biotinylated synthetic peptide 132-146 of sperm whale myoglobin (B132) to a murine T cell clone specific for this site in association with I-Ed. greater than 95% specific inhibition was observed with doses of B132 equipotent to unmodified peptide. Specific blocking could be observed: (a) after pulsing APC with antigen, washing, and incubating for a chase period of 8-16 h before addition of avidin and T cells to assure adequate time for intracellular trafficking and maximal display of antigen on the cell surface, or (b) when monensin is present during the antigen pulse to inhibit such traffic. Therefore, the inhibition appeared to be occurring at the cell surface unless dissociation and reassociation were constantly occurring. To distinguish these, B10.GD APC (I-Ed-negative) were pulsed with antigen and cocultured with B10.D2 APC (I-Ed-positive). No detectable antigen presentation resulted. Thus, minimal dissociation and reassociation between antigen and APC occurs and, consequently, blocking by extracellular solution-phase binding of avidin to antigen is unlikely. Taken together, these data suggest that the blocking is occurring at the cell surface. Thus, under physiologic conditions, immunologically relevant antigen necessary for T cell activation appears to be present on the APC surface and is freely accessible to macromolecules the size of avidin. These findings hold specific implications for models of antigen presentation for T cell recognition.

Transepithelial water flow regulates apical membrane retrieval in antidiuretic hormone-stimulated toad urinary bladder

Antidiuretic hormone (ADH) increases the osmotic water permeability (Posm) of toad urinary bladder. This increase is believed to be produced by fusion of intracellular vesicles called aggrephores with the granular cell apical plasma membrane. Aggrephores contain intramembrane particle aggregates postulated to be water channels. ADH-stimulated Posm is decreased by osmotic gradient exposure, which is termed flux inhibition. We studied flux inhibition by exposing ADH-stimulated bladders to various osmotic gradients. Osmotic water flow was initially proportional to the applied osmotic gradient, but Posm decreased with time. Ultrastructural and quantitative studies of endocytosis demonstrate that apical membrane retrieval was a direct function of the transepithelial osmotic gradient. Posm remained unchanged when apical membrane retrieval was blocked by incubation of bladders at 2 degrees C, or under low water-flow conditions. These effects were reversed by increases in temperature or the applied osmotic gradient. We conclude that apical membrane retrieval causes the phenomenon of flux inhibition.

Insulin-stimulated fluid-phase pinocytosis and internalization of the insulin receptor: differences between the U-937 monocyte and rat adipocyte

U-937 monocytes, a human cell line, respond acutely to insulin by internalization of the insulin receptor and acceleration of fluid-phase pinocytosis. In the present studies, both processes were shown to require energy and both were dependent on the number of insulin receptors. Monocytes with a reduced number of insulin receptors, ie, down-regulated by a 16-hour insulin treatment, had a markedly reduced response to insulin-stimulation of pinocytosis and a decrease in the amount of insulin receptors internalized. This latter feature resulted, however, from the reduction in the cellular content of insulin receptors. The proportion of receptor internalized during a 30-minute acute treatment with insulin (eg, 59% of the cell surface receptors) was slightly greater than the proportion internalized in control cells. Therefore, down-regulation does not selectively destroy receptors that cycle, leaving only a subpopulation of receptors anchored in the membrane. Apparently, there is only one population of insulin receptors, all of which are equally competent with respect to internalization. Although these results suggest a close relationship between pinocytosis and receptor internalization, it was possible to separate the two systems. The addition of poly-L-lysine produced a marked stimulation of fluid-phase pinocytosis in the absence of any increase in insulin receptor internalization. Thus, movement of the receptor into the internal pool requires more than an increase in the rate of pinocytosis. Rat adipocytes were also studied, and the results differed in several aspects from those of U-937 monocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

1-Naphthylisothiocyanate-induced permeability of hepatic tight junctions to proteins

We have studied the early action of 1-naphthylisothiocyanate (ANIT) in relation to its effect on the permeability barrier formed by hepatic tight junctions. Materials having different Mr values [inulin (5000), horseradish peroxidase (HRP) (40,000), ovalbumin (also 40,000) and pig gamma-globulin (IgG) (160,000)] were individually pulsed, within 1 min, into perfused rat livers operating under single-pass conditions. In untreated rats, a small peak of HRP and ovalbumin and a comparatively larger peak of inulin were observed in the bile at 7 min. In rats treated with ANIT, with increasing duration of ANIT treatment the inulin peak increased proportionally, whereas the HRP and ovalbumin peaks remained unchanged until after 10 h of ANIT exposure; gamma-globulin was not detected in the 7 min bile sample until after 14 h of ANIT treatment. Bile flow in all rats remained approximately the same until after 14 h of ANIT pretreatment, when substantial bile-flow reduction was observed. Phenobarbitone pretreatment increased the effect of ANIT and massively elevated the first HRP peak; it also shortened the time (to 4 h) at which the increase in permeability to this protein was observed. In contrast, the first HRP peak was virtually abolished in rats that had received the mixed-function-oxidase inhibitor SKF 525A. These experiments suggest that (i) ANIT progressively increased the permeability of the junctional barrier before the reduction in bile flow, (ii) the ANIT-increased permeability change seems to be inversely dependent upon the Mr of the infused proteins, and (iii) metabolites of ANIT were involved in the development of the junctional permeability change.

Interferon-gamma depresses binding of ligand by C3b and C3bi receptors on cultured human monocytes, an effect reversed by fibronectin

Cultivation of human monocytes with recombinant IFN-gamma causes a 5-10-fold depression in their binding of EC3b or EC3bi. This effect is observed within 18 h and is expressed for 5 d in the presence of 100 U/ml IFN-gamma. The capacity of IFN-gamma-treated phagocytes to bind EC3b and EC3bi is fully restored if the phagocytes are allowed to spread for 45 min on surfaces coated with Fn. IFN-gamma-treated cells express normal levels of cell surface C3b and C3bi receptors as measured with monoclonal anti-receptor antibodies, and spreading on Fn does not alter receptor number. We conclude that cultivation with IFN-gamma causes a change in the nature of these receptors that prevents them from interacting with ligand. Immunoelectron microscopy shows that C3bi receptors are expressed on the apical surface of the IFN-gamma-treated MO and that these receptors exhibit normal capacity to migrate in the plane of the membrane. Thus, the nature of the change caused by IFN-gamma is not related to changes in receptor number, location, or mobility. While spreading of IFN-gamma-treated cells on Fn enables C3 receptors to bind ligand, it does not enable them to promote phagocytosis. Treatment of cells with PMA alone does not affect binding or phagocytosis, but treatment of cells with both Fn and PMA enables cells to phagocytose EC3b and EC3bi. These data indicate that the binding and signaling activities of C3 receptors are separately regulated. Fn enables receptors to bind ligand and PMA enables them to signal phagocytosis.

Evidence that pinocytosis in lymphoid cells has a low capacity

In contrast to adherent cells, human B and T lymphoblasts, marmoset monkey T lymphoblasts, and mouse T lymphoblasts do not form monolayers and have a poor ability to pinocytose. After a 10-min incubation of lymphoblasts at 37 degrees C, the level of internalized medium reached a plateau. During this time, lymphoblasts pinocytosed 3-4 femtoliters (1 fl = 10(-15) l) of medium per cell as calculated by the quantity of the entrapped pinocytic marker 5(6)-carboxyfluorescein. The levels of pinocytosed liquid did not increase during a subsequent 90-min incubation of cells at 37 degrees C. Adherent HeLa cells took up 27 fl of medium per cell per hour. Other types of adherent cells were reported by others to pinocytose 20 to 90 fl of medium per cell per hour. The process of pinocytosis in lymphoblasts appeared to be reversible since cells which were pre-loaded with carboxyfluorescein and then incubated at 37 degrees C in fresh medium lost the marker almost completely within 40 min. Similar results were obtained with horseradish peroxidase as the pinocytic marker. Further evidence that lymphoblasts have a low capacity for pinocytic internalization relative to adherent cells was obtained from the observation that Namalwa lymphoblasts were approximately 100 times more resistant to the cytotoxic action of the protein toxin gelonin than the adherent HeLa cells. Gelonin is a ribosome-inactivating toxin which is not capable of binding to cells, and its only mode for internalization appears to be pinocytosis. Ribosomes in cell lysates of the two lines were equally sensitive to gelonin. It is speculated that the poor pinocytic ability of lymphoid cells may reflect a fundamental difference between adherent and non-adherent cells and that this may impede the targeting of drugs into lymphoid cells.

Effects of leupeptin on endocytosis and membrane recycling in rat visceral yolk-sac endoderm

The effect of exposure to leupeptin (25 micrograms/ml for 24 h) on the endocytotic activity and the membrane flow of apical cell membranes was studied in endodermal cells of cultured rat visceral yolk sacs by applying a double-labelling method using concanavalin-A ferritin (Con-A Fer) and horseradish peroxidase (HRP). Control and leupeptin-treated yolk sacs were labelled with Con-A Fer at 4 degrees C and then incubated with HRP for 5, 15 or 60 min at 37 degrees C. In controls, HRP reaction product was detected after 5 min in many of the apical vacuoles as well as a few lysosomes; after 15 min, reaction product was observed in all apical vacuoles and in lysosomes of various sizes. These HRP-positive structures usually contained a variable amount of membrane-bound Fer. After 60 min, all apical vacuoles and almost all lysosomes exhibited HRP reactions, but only some of these structures contained Fer particles. At this time, many apical canaliculi (which are involved in membrane recycling) exhibited positive HRP reactions and sometimes also contained Fer particles. In leupeptin-treated cells, HRP reaction product and variable amounts of membrane-bound Fer particles were found in apical vacuoles after 5 min; after 15 min, both labels were also observed in some small lysosomes, and after 60 min, they were found in all apical vacuoles as well as some small and middle-sized lysosomes. Significantly fewer labelled apical vacuoles, lysosomes and apical canaliculi were present after leupeptin treatment than in controls at corresponding times. At all times examined, the giant lysosomes found in leupeptin-treated cells did not exhibit any labeling.(ABSTRACT TRUNCATED AT 250 WORDS)

External ATP permeabilizes transformed cells to macromolecules

External ATP under certain ionic conditions render transformed cells permeable to the translation inhibitor hygromycin B. With this method the protein toxin alpha-sarcin selectively penetrates into 3T6 cells, as compared to 3T3 cells. This entry is enhanced by ATP synthesis blockers such as CCCP. Other proteins, such as horseradish peroxidase and luciferase, also pass selectively in 3T6 cells under permeabilization conditions.

Unusual binding sites for horseradish peroxidase on the surface of cultured and isolated mammalian cells. Suppression of binding by certain nucleotides and glycoproteins, and a role for calcium

Binding sites for horseradish peroxidase (HRP), with unusual properties, were detected on the surface of cultured and isolated cells after the cells (on cover slips) had been quickly dried, fixed in cold methanol, and post-fixed in a paraformaldehyde solution. The reaction for surface-bound HRP was suppressed by micromolar concentrations of glycoproteins such as invertase, equine luteinizing hormone (eLH) or human chorionic gonadotropin (hCG). The reaction was also suppressed by 20 mM CDP, UDP, GTP, NAD, and ribose 5-phosphate. Two to six times higher concentrations of GMP, fructose 1-phosphate, galactose 6-phosphate, mannose 6-phosphate, fructose 6-phosphate, and glucose 6-phosphate were required to suppress the binding reaction. AMP, ATP, heparin, mannan, and eight non-phosphorylated sugars showed relatively low competing potencies but fucoidin and alpha-lactalbumin were strong inhibitors. No addition of Ca2+ was required for the binding of HRP to the cell surface. However, calcium-depleted, inactive HRP did not compete with the binding of native (calcium-containing) HRP whereas H2O2-inactivated HRP suppressed the binding. GTP, NAD, ribose 5-phosphate, and EGTA accelerated the release of previously-bound HRP from the cell surface whereas glycoproteins (invertase, eLH, and hCG) did not do so. Addition of Ca2+ to GTP, NAD, ribose 5-phosphate or to EGTA prevented the accelerated release of HRP from the cell surface. It is suggested that calcium, present either in the surface membrane or in HRP itself, is involved in the binding of HRP to the cell surface and in the inhibition of binding by GTP, NAD, and ribose 5-phosphate. It is also suggested that alpha-lactalbumin, GTP, UDP, and CDP compete with the binding of HRP to a glycosyltransferase on the cell surface.

Uptake and destruction of 125I-CSF-1 by peritoneal exudate macrophages

The binding and uptake of the colony-stimulating factor CSF-1 by peritoneal exudate macrophages (PEM) from lipopolysaccharide insensitive C3H/HeJ mice was examined at 2 degrees C, and at 37 degrees C. At 2 degrees C, 125I-CSF-1 was bound irreversibly to the cell surface. At 37 degrees C, 90% of the cell surface associated 125I-CSF-1 was rapidly internalized and subsequently degraded and the remaining 10% dissociated as intact 125I-CSF-1. Thus classical equilibrium or steady state methods could not be used to quantitatively analyze ligand-cell interactions at either temperature, and alternative approaches were developed. At 2 degrees C, the equilibrium constant (Kd less than or equal to 10(-13) M) was derived from estimates of the rate constants for the binding (kon congruent to 8 x 10(5) M-1 s-1) and dissociation (koff less than or equal to 2 x 10(-7) s-1) reactions. At 37 degrees C, the processes of dissociation and internalization of bound ligand were kinetically competitive, and the data was formally treated as a system of competing first order reactions, yielding first order rate constants for dissociation, koff = 0.7 min-1 (t1/2 = 10 min) and internalization, kin = 0.07 min-1 (t 1/2 = 1 min). Approximately 15 min after internalization, low-molecular weight 125I-labeled degradation products began to appear in the medium. Release of this degraded 125I-CSF-1 was kinetically first order over three half-lives (Kd = 4.3 x 10(-2) min-1, t1/2 = 16 min). Thus CSF-1 binds to a single class of receptors on PEM, is internalized with a single rate limiting step, and is rapidly destroyed without segregation into more slowly degrading intracellular compartments.

Internalized proteins directed into accumulative compartments of mosquito oocytes by the specific ligand, vitellogenin

We have investigated the internalization pathways for a specific protein, vitellogenin, and a non-specific protein, horseradish peroxidase, in the mosquito oocyte in vivo. The internalized proteins were localized by electron microscopical immunocytochemistry or autoradiography; the relationship of their destination compartments with lysosomes was monitored by visualization of acid phosphatase. Proteins internalized by the oocyte follow either a specific accumulative route or a lysosomal degradative route. Via coated vesicles, both proteins enter the same compartment, the endosome, where they dissociate from membrane-binding sites. The route to their final destination depends on the presence of the specific ligand. In its absence, the degradative route is followed, and the endosome with non-specific protein fuses with lysosomes. In the presence of the specific ligand, the accumulative route is followed, and both specific and non-specific proteins are delivered into an accumulative compartment, the transitional yolk body. During the transformation of the transitional yolk body into the final storage compartment, a mature yolk body, vitellogenin undergoes crystallization, whereas the non-specific protein is concentrated in small vesicular extensions of the compartmental membrane. These vesicles are separated from the yolk bodies and apparently deliver the non-specific protein into the lysosomal system. We concluded that any protein bound to the membrane would be internalized by the oocyte, but only binding of the specific ligand to its receptor serves as a transmembrane signal stimulating the formation of accumulative compartments.

Studies on the mechanism of membrane fusion: site-specific mutagenesis of the hemagglutinin of influenza virus

Oligonucleotide-directed mutagenesis of a cDNA encoding the hemagglutinin of influenza virus has been used to introduce single base changes into the sequence that codes for the conserved apolar "fusion peptide" at the amino-terminus of the HA2 subunit. The mutant sequences replaced the wild-type gene in SV40-HA recombinant virus vectors, and the altered HA proteins were expressed in simian cells. Three mutants have been constructed that introduce single, nonconservative amino acid changes in the fusion peptide, and three fusion phenotypes were observed: substitution of glutamic acid for the glycine residue at the amino-terminus of HA2 abolished all fusion activity; substitution of glutamic acid for the glycine residue at position 4 in HA2 raised the threshold pH and decreased the efficiency of fusion; and, finally, extension of the hydrophobic stretch by replacement of the glutamic acid at position 11 with glycine yielded a mutant protein that induced fusion of erythrocytes with cells with the same efficiency and pH profile as the wild-type protein. However, the ability of this mutant to induce polykaryon formation was greatly impaired. Nevertheless, all the mutant proteins underwent a pH-dependent conformational change and bound to liposomes. These results are discussed in terms of the mechanism of HA-induced membrane fusion.

Effects of gold sodium thiomalate on functional correlates of human monocyte maturation

The mechanism of action of gold salts in the treatment of rheumatoid arthritis is unknown. Effects of gold on monocyte-macrophage function could be due to inhibition of maturation and differentiation. We found that 3 markers of monocyte differentiation, loss of peroxidase activity, spontaneous synthesis of C2, and spontaneous cytotoxicity for chicken erythrocytes, were all inhibited by gold treatment. This was not a general toxic effect since phorbol myristate acetate could still induce gold-treated monocytes to lyse chicken erythrocytes. Also, phorbol myristate acetate-stimulated superoxide production, a monocyte function not requiring further differentiation, was not inhibited by incubation with gold. Lymphokine-stimulated cytotoxicity for nucleated target cells, another function of monocytes, was inhibited only partially for certain target cells and not at all for others. These data suggest that gold has the capacity to selectively inhibit some monocyte functions which are associated with macrophage differentiation.

Growth hormone effect on accumulation of arterial basement membrane-like material studied on rabbit aortic myomedial cell cultures

The effect of human growth hormone on arterial basement membrane-like (BM) material was studied. BM-like material was obtained from the cell layer of cultured aortic myomedial cells using a sonication-differential centrifugation technique. After the addition of small amounts of growth hormone (1 ng/ml) to the cultures, we observed a 26% increased incorporation of amino acids into BM-like material (2p less than 0.005). However, further increase in the incorporation was not observed using either 3 ng or 10 ng growth hormone per ml. Growth hormone inhibited removal/degradation of BM-like material by 16% (2p less than 0.01). However, pinocytosis rate and activity of major lysosomal enzymes: cathepsin D, acid phosphatase and beta-N-acetyl-glucosaminidase were unchanged. Incorporation of glycosaminoglycans as evaluated by [35SO4]-labelling was reduced by 8% when cells were exposed to growth hormone (2p less than 0.01). The present study demonstrates an effect of growth hormone on the turnover and composition of BM-like material in cultured arterial myomedial cells.

Quantification of endocytosis-derived membrane traffic

The main data covered by this article have been summarized in Table I. A fairly uniform picture is obtained for endocytosis-derived membrane transfer and compartmentation. This may be due to the limited amount of information and the resulting low resolution. Data on mainly three cell types are presented: macrophages, fibroblasts and amoebae. The data vary as much for one cell type as between different cells. Therefore, no possible differences related to cell function emerge. More detailed data, for more cell types, may change the picture. The values for cell surface area, although significantly different in absolute terms (column S in Table I), are rather similar when related to cell diameter, all being about 3-fold in excess of the surface area of the smooth sphere of comparable volume (column xi in Table I). The rate of plasma membrane internalization for macrophages and amoebae both professional phagocytes, is about 2 cell surface area equivalents per h or more. This may be somewhat higher than for fibroblasts (column PM/h in Table I). The average residence time for membrane on the cell surface, therefore, is about 30 min. A most interesting finding seems to be the rather uniform values obtained for the average size (volume weighted) of primary pinosomes, being about 0.3 micron in diameter (column phi-Internalization in Table I). Due to their rapid increase in size as a result of fusion (cf. Fig. 2), it has not been feasible to directly measure the size of primary pinosomes by morphometric means. The values in Table I, give no information on the size distributions of primary pinosomes and on whether these consist of one or more size classes. The steady-state average diameter of pinosomes is noticeably larger than that of primary pinosomes (column phi-pinosomes in Table I; cf. Table II for Acanthamoebae). The corresponding decrease in surface-to-volume ratio can make about 50% of pinosomal membrane available for recycling directly from this membrane compartment. Membrane recycling from the pinosomal compartment occurs after an average residence time of about 3 min for macrophages and 4-6 min for fibroblasts (column tau-pinosomes in Table I). The relative pool size of intracellular membranes participating in shuttling to and from the cell surface is significantly different for animal cells and amoebae (column rho in Table I). For macrophages, fibroblasts, CHO cells, and mast cells, this intracellular membrane pool amounts to about 10-20% the plasma membrane area, compared to 150-200% in the case of amoebae.(ABSTRACT TRUNCATED AT 400 WORDS)

Transtubular transport of proteins in rabbit proximal tubules

The purpose of the present experiments was to study possible different pathways of intracellular transport of proteins after luminal and basolateral uptake in isolated rabbit proximal tubules. Tubules were exposed to cationized ferritin (CF) in the perfusion fluid and horseradish peroxidase (HRP) in the bath simultaneously or to HRP in the bath alone for 30 min. The peritubular fluid (bath) and perfusion fluid were then exchanged and the tubules either fixed immediately or allowed to function during chase-periods for 10, 20, 30, or 60 min before fixation to follow the migration of the proteins through the cells. The proteins were to a large extent found separated in different vacuoles and lysosomes at all time periods studied, indicating separate pathways after uptake via the luminal and basolateral membranes respectively. About 0.5% of the CF taken up by the cells was transported through the cells and became located in the intercellular spaces. HRP was transported from the peritubular fluid to the apical cytoplasm of the tubules indicated by a gradual accumulation of small HRP-containing vesicles, first in the basal part of the cells and then in the apical cytoplasm. In tubules perfused with both CF and HRP in the perfusate, the CF and HRP were found together in apical vacuoles and lysosomes. After perfusion with HRP alone, this tracer was found in similar large vacuoles and lysosomes in the apical cytoplasm, in contrast to the small HRP-filled vacuoles seen after uptake from the bath.

Endocytic pathways and time sequence of lysosomal transfer of macromolecules in cultured mouse peritoneal macrophages. Double-labeling experiments with horseradish peroxidase and ferritin

A double-labeling protocol was used to study endocytic pathways and lysosomal transfer of exogenous macromolecules in cultured mouse peritoneal macrophages. After pulse-chase labeling of lysosomes with horseradish peroxidase (visualized cytochemically), the cells were exposed to native, anionic ferritin for 0-45 min at 37 degrees C and then analysed by transmission electron microscopy. The results show that ferritin binds to the plasma membrane, accumulates in coated pits, and is rapidly taken up in small, smooth-surfaced endocytic vesicles. The latter carry the ferritin molecules directly to lysosomes, recognized by their peroxidase labeling, or fuse with each other to form larger endocytic vacuoles (endosomes) which in turn fuse with and empty their content into lysosomes. The first signs of transfer of ferritin into the lysosomes were seen after 5-10 min of exposure and after 25-30 min most of the lysosomes were labeled. Union of ferritin-labeled and other lysosomes was also noted, suggesting that the contents of the lysosomes were spread within the lysosomal compartment by fusion-fission processes. It is concluded that a multiplicity of structures is involved in the uptake and intracellular transport of exogenous macromolecules in macrophages and that the time sequence of lysosomal transfer of the interiorized material is highly variable.

Possible involvement of transglutaminase in endocytosis and antigen presentation

Experiments were carried out to determine as to whether or not internalization of antigen is necessary for subsequent antigen presentation by accessory cells using monoamines which are known as transglutaminase (TGase) inhibitors. It was found that endocytosis for immune complexes via Fc receptors such as sheep erythrocytes coated with IgG class antibody (EA) was different from receptor-independent endocytosis for soluble protein such as horse radish peroxidase (HRP) in the sensitivity to monoamines; methylamine inhibited the receptor-dependent endocytosis of immune complexes at a concentration of over 20 mM and the receptor-independent endocytosis of HRP at 2 mM, while dansylcadaverine (DC) inhibited both at a concentration of 100 microM. It was noteworthy that antigen-specific T cell proliferation to splenic adherent cells pulsed with DNP9.6-ovalbumin (DNP9.6-OVA) was blocked strongly by DC as well, but weakly by methylamine. These results suggest the possibility that antigen presentation requires internalization of antigen by a mechanism such as receptor-dependent endocytosis for the subsequent reexpression of antigen on membranes. Furthermore, it was confirmed that TGase activity is high in peritoneal exudate and spleen adherent cells, both of which have accessory cell activities for lymphocytes, suggesting the possibility that TGase might be involved intimately in receptor-dependent endocytosis and subsequent antigen presentation.

Transcytosis and paracellular movements of horseradish peroxidase across liver parenchymal tissue from blood to bile. Effects of alpha-naphthylisothiocyanate and colchicine

The pathways for the entry of horseradish peroxidase (HRP) into bile have been investigated using the isolated perfused rat liver operating under one-pass conditions. Following a 1 min one-pass infusion of HRP, two peaks of HRP activity were noted in the bile. The first, at 5-7 min post-infusion, correlated with the biliary secretion of the [3H]methoxyinulin which was infused simultaneously with the HRP. The second peak of HRP activity occurred at 20-25 min, and correlated with the biliary secretion of 125I-IgA, which was also infused simultaneously with the HRP. If the isolated livers were perfused with a medium containing 2.5 microM-colchicine, the biliary secretion of IgA and the second secretion peak of HRP were inhibited by 60%. If rats were pretreated for 12h with alpha-naphthylisothiocyanate (25mg/100g body wt.) prior to liver isolation, the biliary secretion of [3H]methoxyinulin and the first secretion peak of HRP were increased. Taken together, these results suggest that HRP enters the bile via two routes. The faster route, which was increased by alpha-naphthylisothiocyanate and correlated with [3H]methoxyinulin entry into bile, was probably paracellular, involving diffusion across tight junctions. The slower route, which was inhibited by colchicine and correlated with the secretion of IgA, was probably due to transcytosis, possibly within IgA and other transport vesicles.

An efficient method for introducing macromolecules into living cells

The hemagglutinin (HA) of influenza virus was used to obtain efficient and rapid bulk delivery of antibodies and horseradish peroxidase (HRP) into the cytoplasm of living tissue culture cells. By exploiting HA's efficient cell surface expression, its high affinity for erythrocytes, and its acid-dependent membrane fusion activity, a novel delivery method was developed. The approach is unique in that the mediator of both binding and fusion (the HA) is present on the surfaces of the target cells. A recently developed 3T3 cell line which permanently expresses HA, Madin-Darby canine kidney cells infected with influenza virus, and CV-1 cells infected with a simian virus 40 vector carrying the HA gene were used as recipient cells. Protein-loaded erythrocytes were bound to the HA on the cell surface and a brief drop in pH to 5.0 was used to trigger HA's fusion activity and hence delivery. About 3 to 8 erythrocytes fused per 3T3 and CV-1 cell, respectively, and 75-95% of the cells received IgG or HRP. Quantitative analysis showed that 1.8 X 10(8) molecules of HRP and 1.4 X 10(7) IgG molecules were delivered per CV-1 cell and 6.2 X 10(7) HRP molecules per 3T3 cell. Cell viability, as judged by methionine incorporation into protein and cell growth and division, was not impaired. Electron and fluorescence microscopy showed that the fused erythrocyte membranes remained as discrete domains in the cell's plasma membrane. The method is simple, reliable, and nonlytic. The ability to simultaneously and rapidly deliver impermeable substances into large numbers of cells will permit biochemical analysis of the fate and effect of a variety of delivered molecules.

Study on membrane recycling in the rat visceral yolk-sac endoderm using concanavalin-A conjugates

The internalization and intracellular movements of apical-cell-membrane material were investigated in the endodermal cells of cultured visceral yolk-sacs of rats (whole-embryo culture; explanted at 10.5 days of gestation and cultured for 24 h) using horseradish peroxidase- and ferritin-labelled concanavalin A (Con-A HRP, Con-A Fer). When visceral yolk-sac endoderm was exposed to Con-A HRP or Con-A Fer for 5 min at 4 degrees C, the apical cell membranes containing a well-developed fuzzy coat were heavily labelled, whereas apical vacuoles, lysosomes and apical canaliculi were not. Incubation of Con-A-labelled endoderm for 5-60 min at 20 degrees and 37 degrees C in Con-A-free serum resulted in a temperature-dependent internalization of membrane-bound lectin into coated vesicles, apical vacuoles and lysosomes, and the apical cell membranes were cleared of the heavy labelling. With increasing incubation time, the number of labelled vacuolar structures and the intensity of their labelling decreased gradually, whereas the number of labelled apical canaliculi increased. Thus, after 30 and 60 min at 37 degrees C, most of the apical canaliculi contained high concentrations of the markers. It was possible to observe labelled apical canaliculi that were in continuity with labelled apical vacuoles and lysosomes as well as with the apical cell membrane. These findings in rat endodermal cells indicate that constituents of the apical cell membrane are internalized in apical vacuoles and lysosomes, and are then brought back to the apical cell membrane by the apical canaliculi, which concentrate and store this membrane material.

The reaction of horseradish peroxidase with hydroperoxides derived from Triton X-100

All of the commercially available Triton X-100 examined gave Compound I upon reaction with horseradish peroxidase, followed by its gradual transition into Compound II. Titration of horseradish peroxidase with Triton X-100 to form Compound I indicated that 1% (v/v) aqueous solutions of the detergent contained 0.4 to 3.2 microM equivalent peroxide but iodometric titration revealed 1.1 to 5.0 microM peroxide, suggesting the occurrence of different types of peroxides, reactive and unreactive with the peroxidase. The rate constant for Compound I formation was 1.5 X 10(7) M-1 S-1 at pH 7.4 at 25 degrees C, and for conversion into Compound II apparent first-order rate constants were 5.2 X 10(-3) to 1.7 X 10(-2) S-1. These results indicate that the Triton peroxides are as highly reactive as hydrogen peroxide. The amount of Triton peroxides increased as aqueous solutions of the detergent were allowed to stand, but the peroxides were destroyed by treatment with sodium borohydride. Although freshly prepared aqueous solutions of sodium cholate, sodium dodecyl sulfate, Tween 20 (polyoxyethylene sorbitan monolaurate), and Emasol 1130 (an equivalent of Tween 20) did not contain any detectable amount of peroxide, aged solutions of sodium dodecyl sulfate and Emasol 1130 contained peroxides. These observations suggest the need for appropriate precautions when biologically active substances vulnerable to attack by peroxides are incubated with Triton X-100 either for their solubilization from biomembranes or for other processing.

Murine epidermal Langerhans cells mature into potent immunostimulatory dendritic cells in vitro

Murine epidermal Langerhans cells (LC) have been studied in tissue culture and compared to spleen dendritic cells (DC). LC comprised 3% of the starting cell suspensions and were distinguished from keratinocytes by cytology and reactivity with anti-Ia and anti-Mac-1 monoclonal antibodies. The LC were nonadherent, had a low buoyant density, did not proliferate, and could be enriched to 10-50% purity. LC continued to exhibit Ia and Mac-1 antigens for 4 d in culture. However, LC rapidly lost Birbeck granules, Fc receptors, F4/80 antigen, and cytochemical reactivity for nonspecific esterase and membrane ATPase. As a result, the ultrastructure and phenotype of cultured LC became remarkably similar to lymphoid DC. Stimulatory capacity for T cell proliferative responses (oxidative mitogenesis and the mixed leukocyte reaction) was monitored daily. Initially, stimulatory capacity was very weak, even though LC expressed substantial levels of Ia antigens. After 2-3 d in culture, LC had become 3-10 times more potent than spleen DC. 30 LC could induce significant responses in cultures of 3 X 10(5) responding T cells. Removal of Ia+ LC at the start of culture ablated the development of stimulatory activity, but exposure to 1,500 rad of ionizing irradiation did not. Mixing experiments showed that contaminating Ia- epidermal cells did not alter the function of Ia+ stimulators. Therefore, LC seem to be immunologically immature, but acquire many of the features of spleen DC during culture. We suggest that functioning lymphoid DC may, in general, be derived from less mature precursors located in nonlymphoid tissues.

Phorbol esters and horseradish peroxidase stimulate pinocytosis and redirect the flow of pinocytosed fluid in macrophages

Lucifer Yellow CH (LY) is an excellent probe for fluid-phase pinocytosis. It accumulates within the macrophage vacuolar system, is not degraded, and is not toxic at concentrations of 6.0 mg/ml. Its uptake is inhibited at 0 degree C. Thioglycollate-elicited mouse peritoneal macrophages were found to exhibit curvilinear uptake kinetics of LY. Upon addition of LY to the medium, there was a brief period of very rapid cellular accumulation of the dye (1,400 ng of LY/mg protein per h at 1 mg/ml LY). This rate of accumulation most closely approximates the rate of fluid influx by pinocytosis. Within 60 min, the rate of LY accumulation slowed to a steady-state rate of 250 ng/mg protein per h which then continued for up to 18 h. Pulse-chase experiments revealed that the reduced rate of accumulation under steady-state conditions was due to efflux of LY. Only 20% of LY taken into the cells was retained; the remainder was released back into the medium. Efflux has two components, rapid and slow; each can be characterized kinetically as a first-order reaction. The kinetics are similar to those described by Besterman et al. (Besterman, J. M., J. A. Airhart, R. C. Woodworth, and R. B. Low, 1981, J. Cell Biol. 91:716-727) who interpret fluid-phase pinocytosis as involving at least two compartments, one small, rapidly turning over compartment and another apparently larger one which fills and empties slowly. To search for processes that control intracellular fluid traffic, we studied pinocytosis after treatment of macrophages with horseradish peroxidase (HRP) or with the tumor promoter phorbol myristate acetate (PMA). HRP, often used as a marker for fluid-phase pinocytosis, was observed to stimulate the rate of LY accumulation in macrophages. PMA caused an immediate four- to sevenfold increase in the rate of LY accumulation. Both HRP and PMA increased LY accumulation by stimulating influx and reducing the percentage of internalized fluid that is rapidly recycled. A greater proportion of endocytosed fluid passes into the slowly emptying compartment (presumed lysosomes). These experiments demonstrate that because of the considerable efflux by cells, measurement of marker accumulation inaccurately estimates the rate of fluid pinocytosis. Moreover, pinocytic flow of water and solutes through cytoplasm is subject to regulation at points beyond the formation of pinosomes.