Endocytosis in Chang liver cells. Quantitation by sucrose- 3 H uptake and inhibition by cytochalasin B

Sucrose induces vesicle accumulation and autophagy

It has been shown that the treatment of mammalian cells with sucrose leads to vacuole accumulation associated with lysosomes and upregulation of lysosomal enzyme expression and activity. Autophagy is an evolutionarily conserved homeostatic process by which cells deliver cytoplasmic material for degradation into lysosomes, thus it is probable that sucrose affects the autophagic activity. The role of sucrose in autophagy is unknown; however, another disaccharide, trehalose has been shown to induce autophagy. In the current study, we used mouse embryonic fibroblasts to investigate whether sucrose induces autophagy and whether vesicle formation is associated with autophagy. The results showed that sucrose induces autophagy while being accumulated within the endosomes/lysosomes. These vesicles were swollen and packed within the cytoplasm. Furthermore, trehalose and the trisaccharide raffinose, which are not hydrolyzed in mammalian cells, increased the rate of vesicles accumulation and LC3-II level (a protein marker of autophagy). However, fructose and maltose did not show the same effects. The correlation between the two processes, vesicle accumulation and autophagy induction, was confirmed by treatment of cells with sucrose plus invertase, or maltose plus acarbose-the α-glucosidase inhibitor-and by sucrose deprivation. Results also showed that vesicle accumulation was not affected by autophagy inhibition. Therefore, the data suggest that sucrose-induced autophagy through accumulation of sucrose-containing vesicles is caused by the absence of hydrolysis enzymes.

Technologies for investigating the physiological barriers to efficient lipid nanoparticle-siRNA delivery

Small interfering RNA (siRNA) therapeutics have advanced from bench to clinical trials in recent years, along with new tools developed to enable detection of siRNA delivered at the organ, cell, and subcellular levels. Preclinical models of siRNA delivery have benefitted from methodologies such as stem-loop quantitative polymerase chain reaction, histological in situ immunofluorescent staining, endosomal escape assay, and RNA-induced silencing complex loading assay. These technologies have accelerated the detection and optimization of siRNA platforms to overcome the challenges associated with delivering therapeutic oligonucleotides to the cytosol of specific target cells. This review focuses on the methodologies and their application in the biodistribution of siRNA delivered by lipid nanoparticles.

CT/fluorescence dual-modal nanoemulsion platform for investigating atherosclerotic plaques

Macrophages have become widely recognized as a key target for atherosclerosis imaging, since they contribute significantly to the progression of atherosclerosis. Dual-modal imaging contrast agents with unique X-ray computed tomography (CT) and optical imaging capabilities have great potential in disease diagnosis because of complementary combination of the high spatial resolution of CT with the high sensitivity of optical imaging. Here, a kind of quantum dots (QDs)-iodinated oil nanoemulsion of 80 nm was developed as a CT/fluorescence dual-modal contrast agent. Hydrophobic QDs were embedded in iodinated oil, which subsequently dispersed in water to form the oil-in-water nanoemulsion. The morphology and hydrodynamic size of the nanoemulsion were characterized, CT values and fluorescence properties were detected. Its cytotoxicity and affinity to three different cells were determined in vitro by MTT assay. In vitro Micro-CT and confocal microscopy cell imaging ability of the nanoemulsion were confirmed by co-incubating with murine macrophage cells and human liver cells. Then in vivo accumulation of this nanoemulsion in macrophages in atherosclerotic rabbits was investigated with clinic CT and fluorescence imaging. The results not only indicated the nanoemulsion could be served as a dual-modal contrast agent, but revealed it could specifically target to macrophages and visualize atherosclerotic plaques.

Association of αB-Crystallin, a Small Heat Shock Protein, with Actin: Role in Modulating Actin Filament Dynamics in Vivo

Disruption of cytoskeletal assembly is one of the early effects of any stress that can ultimately lead to cell death. Stabilization of cytoskeletal assembly, therefore, is a critical event that regulates cell survival under stress. alphaB-crystallin, a small heat shock protein, has been shown to associate with cytoskeletal proteins under normal and stress conditions. Earlier reports suggest that alphaB-crystallin could prevent stress-induced aggregation of actin in vitro. However, the molecular mechanisms by which alphaB-crystallin stabilizes actin filaments in vivo are not known. Using the H9C2 rat cardiomyoblast cell line as a model system, we show that upon heat stress, alphaB-crystallin preferentially partitions from the soluble cytosolic fraction to the insoluble cytoskeletal protein-rich fraction. Confocal microscopic analysis shows that alphaB-crystallin associates with actin filaments during heat stress and the extent of association increases with time. Further, immunoprecipitation experiments show that alphaB-crystallin interacts directly with actin. Treatment of heat-stressed H9C2 cells with the actin depolymerzing agent, cytochalasin B, failed to disorganize actin. We show that this association of alphaB-crystallin with actin is dependent on its phosphorylation status, as treatment of cells with MAPK inhibitors SB202190 or PD98059 results in abrogation of this association. Our results indicate that alphaB-crystallin regulates actin filament dynamics in vivo and protects cells from stress-induced death. Further, our studies suggest that the association of alphaB-crystallin with actin helps maintenance of pinocytosis, a physiological function essential for survival of cells.

Microfilament network is needed for the endocytosis of water channels and not for apical membrane insertion upon vasopressin action

In the current study, a novel role for the microfilaments in vasopressin-induced water transport in toad urinary bladders, a popular model for the mammalian collecting duct, was established. Vasopressin-induced water transport was not affected by cytochalasin D (CD, 20 microM) or latrunculin B (Lat B, 0.5-2 microM), microfilament-disrupting reagents, suggesting that the initial trafficking of vesicles containing water channels and insertion of membranes into the apical membrane are microfilament-independent. After the removal of vasopressin, bladders treated with CD or Lat B continued to transport water at least 2-3-fold greater than those that received the vehicle. Furthermore, the enhanced water transport was inhibited by HgCl2 (1 mM), a potent inhibitor of water channel-mediated water flow, suggesting that the enhanced water flow was through water channels. In addition, Lat B and CD inhibited vasopressin-induced endocytosis of horseradish peroxidase (HRP), a fluid endocytotic marker. These results suggested that although microfilaments are not needed for the initial trafficking of water channels to the apical side, the microfilament network is essential for the retrieval of water channels following their insertion into apical membranes.

Endocytic internalization in yeast and animal cells: similar and different

The internalization step of endocytosis has been the focus of several laboratories during the last forty years. Unlike some other budding events in the cell, many fundamental questions regarding the molecular machinery involved in the mechanism of budding itself still remain unsolved. Over the last few years the general picture of the field has quickly evolved from the originally simplistic view which postulated that clathrin polymerization is the major force driving budding at the plasma membrane. Refinement of the assays and molecular markers to measure endocytosis in animal cells has shown that other factors in addition to the clathrin coat are required and that endocytosis can also take place through clathrin-independent mechanisms. At the same time, recent introduction of genetic approaches to study endocytosis has accelerated the identification of molecules required for this process. The isolation of endocytosis mutants in budding yeast has been especially fruitful in this respect. Preliminary comparison of the results obtained in yeast and animal cells did not seem to coincide, but further progress in both systems now suggests that part of the divergence originally seen may be due to the particular experimental approaches used rather than fundamental differences in endocytic mechanisms. In this review we present a short historical overview on the advances made in yeast and animal cells regarding the study of endocytosis, underlining both emerging similarities and still interesting differences.

Manganese decreases glutamate uptake in cultured astrocytes

Recent data have shown an accumulation of manganese in the basal ganglia in patients with chronic hepatic encephalopathy (HE). Astrocytes and ammonia are critically involved in the pathogenesis of HE, and we have recently demonstrated that ammonia decreases glutamate uptake in cultured astrocytes. Since failure by astrocytes to take up glutamate may represent an important pathogenetic mechanism in HE, we, therefore, examined the effect of manganese on glutamate transport in these cells. Treatment of cultured astrocytes with 100 microM manganese for 2 days resulted in a 54% decrease in the uptake of D-aspartate, a nonmetabolizable analogue of glutamate. Kinetic analysis revealed a 28% decline in Vmax, with no change in the K(m). Treatment of cultures with 5 mM NH4 Cl inhibited D-aspartate uptake by 21%, and a combination of 5 mM NH4Cl with 100 microM manganese produced an additive effect on uptake inhibition. These results suggest a pathogenetic role for manganese in HE, possibly involving glutamate transport.

Cellular uptake properties of a 2'-amino/2'-O-methyl-modified chimeric hammerhead ribozyme targeted to the epidermal growth factor receptor mRNA

Catalytic RNA or ribozymes have important potential applications as molecular biological tools in the study of gene expression and as therapeutic inhibitors of disease-causing genes. Very little is known, however, about the cellular uptake mechanisms of exogenously delivered synthetic ribozymes. In this study, we have characterized the uptake properties of a synthetic, 2'-O-methyl-modified ribozyme containing U4/U7 amino groups within the catalytic core of the hammerhead motif. The cellular uptake of the internally [32P]-radiolabeled hammerhead ribozyme in U87-MG glioma cells was temperature, energy, and pH dependent and involved an active process that could be competed with cold ribozyme of the same chemistry and sequence, an all 2'-O-methyl-modified ribozyme of the same sequence, antisense PS-ODNs, and a variety of other polyanions (salmon sperm DNA, spermidine, dextran sulfate, and heparin). Subcellular distribution studies of fluorescently labeled ribozymes confirmed an extranuclear, punctate localization similar to that observed for an endosomal marker, dextran. Our study highlights that hammerhead ribozymes, despite exhibiting a defined secondary structure, enter cells by an endocytic mechanism that appears to be similar to that reported for a variety of antisense ODNs. These observations should facilitate the development of more efficient delivery systems.

The role of helper lipids in cationic liposome-mediated gene transfer

In the procedure for cationic liposome-mediated transfection, the cationic lipid is usually mixed with a "helper lipid" to increase its transfection potency. The importance of helper lipids, including dioleoylphosphatidylcholine (DOPC) and phosphatidylethanolamine (dioleoyl PE), DO was examined. Freeze-fracture electron microscopy of DNA:cationic complexes containing the pSV-beta-GAL plasmid DNA, the cationic lipid dioleoyl trimethylammonium propane, and these helper lipids showed that the most efficient mixtures were aggregates of ensheathed DNA and fused liposomes. PE-containing complexes aggregated rapidly when added to culture media containing polyanions, whereas PC-containing complexes did not. However, more granules of PC-containing complexes were formed on cell surfaces after the complexes were added to Chinese hamster ovary (CHO) cells in transfection media. Pronase treatment inhibited transfection, whereas dilute poly-L-lysine enhanced transfection, indicating that the attachment of DNA:liposome complexes to cell surfaces was mediated by electrostatic interaction. Fluorescence spectroscopy studies confirmed that more PC-containing complexes than PE-containing complexes were associated with CHO cells, and that more PC-containing complexes were located in a low pH environment (likely to be within endosomes) with time. Cytochalasin-B had a stronger inhibitory effect on PC-containing liposome-mediated than on PE-containing liposome-mediated transfection. Confocal microscopic recording of the fluorescently label lipid and DNA uptake process indicated that many granules of DNA:cationic liposome complexes were internalized as a whole, whereas some DNA aggregates were left out on the cell surfaces after liposomes of the complexes fused with the plasma membranes. For CHO cells, endocytosis seems to be the main uptake pathway of DNA:cationic liposome complexes. More PC-containing granules than PE-containing granules were formed on cell surfaces by cytoskeleton-directed membrane motion, after their respective DNA:liposome complexes attached to cell surfaces by electrostatic means. Formation of granules on the cell surface facilitated and/or triggered endocytosis. Fusion between cationic liposomes and the cell membrane played a secondary role in determining transfection efficiency.

Cellular uptake of oligodeoxyribonucleoside methylphosphonates

The cellular uptake of oligodeoxyribonucleoside methylphosphonates has been evaluated using three radiolabeled oligomers. Oligomers I and II ([3H]-T8 and [3H]-T16, respectively) are nonionic methylphosphonate oligomers labeled with tritium on the phosphonate internucleotide linkage. EDA-III contains a single phosphodiester linkage, a [32P]-label and an ethylenediamine conjugate at the [32P]-5'-end. All three oligomers are stable in cells. At a 1 microM concentration, oligomer I is not taken up by human erythrocytes. The octanol/DPBS partition coefficients for oligomers I and II (1.5 x 10(-4) and 4.2 x 10(-4), respectively) further indicate that these molecules should not diffuse across cell membranes at appreciable rates. Oligomer I is taken up by HL-60 cells, although at a slower rate than the uptake of the fluid-phase marker sucrose. The cell-associated levels of oligomer II in K-562 cells following incubation of cells with the oligomer for 2 days is independent of concentration and nonsaturable, suggesting a mechanism of uptake independent of receptor. Finally, the initial uptake rate of EDA-III in mouse L cells is greater than the uptake of two oligodeoxyribonucleotides (T8, T16), reaching a plateau after 3 hours incubation with cells. These observations should aid in the elucidation of the mechanism by which this class of antisense agents enters the intracellular environment.

Fluid-phase endocytosis by in situ cardiac myocytes of rat atria

Fluid-phase endocytosis (FPE) associated with recycling of fused plasmalemma-secretory granules or membranes and/or membrane receptors by in situ cardiac myocytes was studied at 37 degrees C in vitro noncontracting adult rat atrial preparations. Measurements included 1) the volume (VS*) of the compartment consisting of presumptive endocytotic vesicles and the endosomes or lysosomes transiently in continuity with them (S*), which internalizes [14C]-sucrose but is inaccessible to simultaneously measured [methoxy-3H]inulin, 2) the kinetics of [14C]sucrose efflux from S*, and 3) morphometry to quantify interstitial space and non-heart muscle cells. Vs* (0.39 +/- 0.04 ml/g dry atrium for unstretched atria at 37 degrees C) was 1) variable over a 3.7-fold range under various experimental conditions, 2) significantly increased by neomycin or by lowering the temperature to 18 degrees C, and 3) significantly decreased by alpha 1-adrenergic stimulation. Analysis of sucrose efflux kinetics confirmed the presence of an intramyocytic sucrose-containing compartment. A smaller inulin-inaccessible sucrose space (S*) was also present in right ventricle. Thus, during FPE, vesicles and endosomes initially containing high (extracellular) Ca2+ and Cl- concentrations continually enter, circulate within, and undergo exocytosis from myocardial cells.

Cytochalasin B inhibits phosphoinositide hydrolysis in rat hippocampal slices

The effect of cytochalasin B on phosphoinositide (PI) hydrolysis was examined in rat hippocampal slices. Pretreatment of the slices with cytochalasin B caused a significant decrease in PI hydrolysis elicited by carbachol, norepinephrine, or by high K+. This effect was cytochalasin B dose- and time-dependent and was not mimicked by cytochalasin D, vinblastine, colchicine, or phloretin. In contrast, in [3H]inositol-prelabeled hippocampal membranes, cytochalasin B did not affect PI hydrolysis elicited by GTP gamma S and GTP gamma S plus carbachol. Similar result was obtained using the membranes prepared from the slices pretreated with cytochalasin B. The inhibitory effect of cytochalasin B on the carbachol-response was observed in SK-N-SH human neuroblastoma cells, but not in cultured rat astrocytes. These results indicate that cytochalasin B inhibits PI hydrolysis in neuron-specific manner and that its action may be an indirect cellular mechanism other than interaction with cytoskeleton elements.

Actin microfilaments play a critical role in endocytosis at the apical but not the basolateral surface of polarized epithelial cells

Treatment with cytochalasin D, a drug that acts by inducing the depolymerization of the actin cytoskeleton, selectively blocked endocytosis of membrane bound and fluid phase markers from the apical surface of polarized MDCK cells without affecting the uptake from the basolateral surface. Thus, in MDCK cell transformants that express the VSV G protein, cytochalasin blocked the internalization of an anti-G mAb bound to apical G molecules, but did not reduce the uptake of antibody bound to the basolateral surface. The selective effect of cytochalasin D on apical endocytosis was also demonstrated by the failure of the drug to reduce the uptake of 125I-labeled transferrin, which occurs by receptor-mediated endocytosis, via clathrin-coated pits, almost exclusively from the basolateral surface. The actin cytoskeleton appears to play a critical role in adsorptive as well as fluid phase apical endocytic events, since treatment with cytochalasin D prevented the apical uptake of cationized ferritin, that occurs after the marker binds to the cell surface, as well as uptake of Lucifer yellow, a fluorescent soluble dye. Moreover, the drug efficiently blocked infection of the cells with influenza virus, when the viral inoculum was applied to the apical surface. On the other hand, it did not inhibit the basolateral uptake of Lucifer yellow, nor did it prevent infection with VSV from the basolateral surface, or with influenza when this virus was applied to monolayers in which the formation of tight junctions had been prevented by depletion of calcium ions. EM demonstrated that cytochalasin D leads to an increase in the number of coated pits in the apical surface where it suppresses the pinching off of coated vesicles. In addition, in drug-treated cells cationized ferritin molecules that were bound to microvilli were not cleared from the microvillar surface, as is observed in untreated cells. These findings indicate that there is a fundamental difference in the process by which endocytic vesicles are formed at the two surfaces of polarized epithelial cells and that the integrity and/or the polymerization of actin filaments are required at the apical surface. Actin filaments in microvilli may be part of a mechanochemical motor that moves membrane components along the microvillar surface towards intermicrovillar spaces, or provides the force required for converting a membrane invagination or pit into an endocytic vesicle within the cytoplasm.

Infectious entry pathway of adenovirus type 2

Internalization of the infectious fraction of human adenovirus type 2 into HeLa cells was followed by a quantitative internalization assay. Treatments known to selectively block receptor-mediated endocytosis reduced the internalization of infectious virus to an extent close to the reduction of endocytosis of transferrin. This suggests that one of the first steps in the infectious cycle of adenovirus type 2 is internalization by the coated-pit and -vesicle pathway.

Endocytic mechanisms utilized by chlamydiae and their influence on induction of productive infection

The microfilament-disrupting drug cytochalasin D and, initially, inoculation at 20 degrees C were used to differentiate between phagocytosis (sensitive to both treatments) and pinocytosis (resistant to both treatments) to assess whether chlamydial uptake into McCoy cells occurred by one or both mechanisms and whether each could contribute to productive infection. Both treatments suppressed the infectivity of Chlamydia trachomatis L2/434/Bu and C. psittaci GPIC (the guinea pig inclusion conjunctivitis strain) following static inoculation by only 50%, indicating that there was simultaneous operation of both phagocytosis and pinocytosis during uptake that led to productive infection. Measurement of the entry of organisms by two separate assays established that both strains predominantly used a cytochalasin D-resistant (pinocytic) mechanism, implying that phagocytic uptake was coupled to a higher frequency of productive infection. Integration of the data on infectivity and entry allowed the potential for an organism to infect a host cell to be quantified. This synthesis revealed that for both strains the infectivity potential following phagocytic entry was ca. 10-fold greater than that following pinocytic entry. However, both entry mechanisms were exploited more efficiently by strain L2/434/Bu than by strain GPIC (unless the latter was inoculated with centrifugation), indicating that intrinsic strain properties are more important for infectivity potential than the endocytic mechanism utilized.

The preendosomal compartment comprises distinct coated and noncoated endocytic vesicle populations

The transfer of molecules from the cell surface to the early endosomes is mediated by preendosomal vesicles. These vesicles, which have pinched off completely from the plasma membrane but not yet fused with endosomes, form the earliest compartment along the endocytic route. Using a new assay to distinguish between free and cell surface connected vesicle profiles, we have characterized the preedosomal compartment ultrastructurally. Our basic experimental setup was labeling of the entire cell surface at 4 degrees C with Con A-gold, warming of the cells to 37 degrees C to allow endocytosis, followed by replacing incubation medium with fixative, all within either 30 or 60 s. Then the fixed cells were incubated with anti-Con A-HRP to distinguish truly free (gold labeled) endocytic vesicles from surface-connected structures. Finally, analysis of thin (20-30 nm) serial sections and quantification of vesicle diameters were carried out. Based on this approach it is shown that the preendosomal compartment comprises both clathrin-coated and non-coated endocytic vesicles with approximately the same frequency but with distinct diameter distributions, the average noncoated vesicle being smaller (95 nm) than the average coated one (110 nm). In parallel experiments, using an anti-transferrin receptor gold-conjugate as a specific marker for clathrin-dependent endocytosis it is also shown that uncoating of coated vesicles plays only a minor role for the total frequency of noncoated vesicles. Furthermore, after perturbation of clathrin-dependent endocytosis by potassium depletion where uptake of transferrin is blocked, noncoated endocytic vesicles with Con A-gold, but not coated vesicles, exist already after 30 and 60 s. Finally, it is shown that the existence of small, free vesicles in the short-time experiments cannot be ascribed to recycling from the early endosomes.

Iodoacetate action on endocytic uptake of different fluid-phase markers by OK renal epithelial cells

When grown in monolayer culture, OK cells display endocytic uptake of soluble fluid-phase markers such as lucifer yellow (LY) and horseradish peroxidase (HRP). The response of this process to metabolic inhibitors was characterized in the present study. Inhibition of cell metabolism by cyanide produced a decrease in cell ATP content which was accompanied by a decrease in uptake of both LY and HRP, confirming the energy-dependence of fluid-phase endocytosis in OK cells. Use of iodoacetate also decreased cell ATP content but its action on endocytosis was unexpected. Cell uptake of HRP was decreased by iodoacetate, similar to the effect of cyanide, but there was a marked increase in LY uptake. Additional studies showed that cyanide did not change intracellular Na+ or intracellular K+ and did not interfere with the Na(+)-dependency of Pi uptake. In contrast, iodoacetate produced a marked increase in Na+, a decrease in K+, and abolished the Na(+)-dependency of Pi transport. The latter was due primarily to a 10-fold increase in Na(+)-independent uptake of Pi. These findings suggest, indirectly, that plasma membrane permeability to Na+, K+, Pi, and small molecules such as LY, may be increased by iodoacetate, possibly through its action as an alkylating agent. This mechanism may allow increased cell uptake of LY through a non-endocytic pathway, and may mask the inhibitory action of iodoacetate on endocytic uptake of LY. These additional effects complicate the use of iodoacetate to interrupt endocytosis.

Kinetics of hydrolysis of endocytosed substrates by mammalian cultured cells: early introduction of lysosomal enzymes into the endocytic pathway

The kinetics of exposure of endocytosed material to two lysosomal enzymes were determined for a number of cultured cell lines using fluorogenic substrates. Hydrolysis of endocytosed substrates for cathepsin B and acid phosphatase was observed to begin within 3-10 min of substrate addition and to proceed linearly for up to 60 min thereafter. Hydrolysis of the cathepsin B substrate was not affected by inhibition of protein synthesis with cycloheximide, indicating that the enzymes present in early endosomes are not exclusively newly synthesized. As had been observed previously for a cathepsin B substrate (Roederer, M., Bowser, R., and Murphy, R. F., J. Cell. Physiol., 131:200-209, 1987), hydrolysis of the acid phosphatase substrate was not blocked at temperatures below 20 degrees C. The results suggest that the endosome is the primary site of initial exposure of endocytosed material to hydrolytic enzymes.

The effects of acetylated low-density lipoproteins on fluid-phase pinocytosis by macrophages

A simple method has been set up to measure the rate of fluid-phase pinocytosis in resident mouse peritoneal macrophages in culture. The method uses 125I-labelled polyvinylpyrrolidone as a nondegradable marker of fluid-phase pinocytosis. The accumulation of 125I-labelled polyvinylpyrrolidone by the cells was directly proportional to its concentration in the culture medium up to at least 200 micrograms/ml. The estimates of the rate of fluid-phase pinocytosis were reproducible within each experiment (coefficient of variation 8.5%) but varied between individual experiments. Fluid-phase pinocytosis was undetectable at 4 degrees C and reduced greatly at 37 degrees C by metabolic inhibitors and 1 mM ZnSO4. High concentrations of human acetylated low-density lipoproteins, which are taken up rapidly by macrophages, decreased the rate of fluid-phase pinocytosis by up to about 70%. The inhibition was seen after only 2 h of incubation. Unmodified low-density lipoproteins, which are taken up only slowly by macrophages, did not usually inhibit fluid-phase pinocytosis (in fact, they sometimes increased it). Modified low-density lipoprotein uptake, leading to massive lipid accumulation in macrophages in the arterial wall, has been postulated to be involved in the pathogenesis of atherosclerosis. This study raises the possibility that the rate of fluid-phase pinocytosis in these lipid-laden arterial macrophages may be reduced.

Utilization of medium-chain triglycerides and octanoic acid by cultured human fibroblasts and lymphoblasts

Cultured human fibroblasts and lymphoblasts were incubated with emulsions containing 14C-trioctanoin or 14C-tripalmitin. Both cell types were able to hydrolyse the medium-chain triglyceride but not the long-chain triglyceride to the corresponding fatty acids. At the end of a 3 days incubation period, 25-30% of the initial amount of 10 nmol/ml trioctanoin were present as triglyceride. The observed hydrolysis seems to be mediated by an esterase secreted into the culture medium, as was shown by the use of cell-conditioned medium. CO2 production from octanoic acid was below 2 nmol per mg protein and day, demonstrating that these cells have a low capacity to use this substrate for their energy metabolism.

Mechanism of sucrose uptake by isolated rat hepatocytes

The transport of molecules by nonspecific endocytosis has been described in many cell types, but it has not been characterized in hepatocytes. Because of its central role in the clearance of solutes from portal blood, endocytosis might represent a significant mode of cellular transport. We investigated the mechanism of sucrose uptake in an isolated hepatocyte system. Liver cells were isolated by perfusion and collagenization of rat liver, followed by differential centrifugation. Hepatocytes were then incubated with 14C-sucrose and harvested by spinning through oil in microfuge tubes. Radioactivity was standardized against DNA content. We found that sucrose uptake is concentration-dependent from 5 microM to 100 mM and follows first-order kinetics. Washout studies indicate that exocytosis is responsible for the dynamic equilibrium reached. Arrhenius analysis of temperature dependence yields a linear plot (Ea = 14.2 Kcal/mol). In addition, sucrose uptake is independent of cellular ATP levels. We conclude that sucrose is transported by fluid-phase micropinocytosis in isolated hepatocytes and that this transport mechanism may be important in the uptake of diverse molecules into liver cells.

Kinetics and temperature dependence of exposure of endocytosed material to proteolytic enzymes and low pH: evidence for a maturation model for the formation of lysosomes

The temperature dependence of acidification of internalized dextran by Swiss 3T3 cells was determined using dual fluorescence flow cytometry. Essentially no acidification was observed at 11 degrees C; acidification was limited to pH 6-6.5 at temperatures between 13 degrees C and 17 degrees C. In contrast, a rapid drop to pH 6-6.5 followed by acidification to pH 5-5.5 was observed at temperatures above 19 degrees C. These results confirm the biphasic nature of the acidification process (J. Cell Biol. (1984) 98: 1757-1762). The timing of exposure of material internalized by fluid-phase endocytosis to lysosomal enzymes was determined for Swiss 3T3 cells by using a fluorogenic substrate specific for Cathepsin B. Hydrolysis of the substrate, as measured by both fluorometry and flow cytometry, began within minutes of its addition to cells at 37 degrees C, and was inhibited by coincubation with leupeptin, a competitive inhibitor of the enzyme, or by weak bases, which raise the pH of acidic compartments. At temperatures between 13 degrees (and 21 degrees C, the rate of hydrolysis was reduced to 31-44% of that at 37 degrees C. Thus, in contrast to previous reports, exposure of endocytosed material to at least one lysosomal enzyme is not inhibited below 20 degrees C; the reduction in hydrolysis rate may be explained by the temperature effects on the efficiency of the enzyme. The results for acidification and proteolysis are consistent with, but do not prove, a maturation model for the formation of lysosomes. We suggest that at lower temperatures, part of the maturation involving recycling and/or concentration of the contents of the endosome is inhibited. This causes the endosome to remain as a mildly acidic, low-density organelle containing lysosomal enzymes.

Photolabeling of the adipocyte hexose carrier with an aryl azide derivative of maltose

A nitrophenyl azide derivative of maltose, N-(4-azido-2-nitrophenyl)-maltosylamine (NAP-maltosylamine), was synthesized as a potential photoaffinity label for the hexose carrier of the rat adipocyte. This derivative inhibited 3-O-methylglucose uptake with a Ki of 1.3 mM in the dark, while that of maltose was 10.0 mM. Carbon-14-labeled maltose and NAP-maltosylamine entered adipocytes via the hexose carrier, the latter in a concentrative fashion. Photolysis of NAP-[14C]maltosylamine in the presence of an adipocyte low density microsomal membrane fraction labeled several electrophoretic bands. Among these are a 45 kDa band which showed features expected of the hexose carrier: its labeling was decreased 40% by D- but not L-glucose and pretreatment of intact adipocytes with insulin decreased labeling of the 45 kDa band by 10-40%, as predicted by the translocation theory of insulin-stimulated transport activation. These studies show the suitability of using carbon-1-modified sugar photoaffinity labels as probes for the hexose carrier and possibly of its regulation in rat adipocytes.

"Pull" and "push" in neurite elongation: observations on the effects of different concentrations of cytochalasin B and taxol

Neurite elongation involves two distinct cytoskeletal functions the "push" of anterograde transport of the cytoskeleton and associated organelles to the neurite tip, and the "pull" exerted by protrusion and generation of tensions in the growth cone. We investigated the roles of these two activities in neurite elongation via the drugs taxol and cytochalasin B (CB), which act on the key cytoskeletal components, microtubules and actin filaments, respectively. When neurons are treated with concentrations of CB below 0.2 micrograms/ml, neurite elongation, growth cone protrusion, and neurite tension are all inhibited in a similar concentration dependent manner. Protrusive activity and tensions are absent at CB concentrations above 0.3 micrograms/ml, yet neurite elongation continues at a plateau level. Thus, "pull" does modulate, but it is not required for neurite elongation. Surprisingly, the inhibitory effects of taxol on neurite elongation are removed by the addition of CB at levels that substantially disrupt the actin filaments of neurites. The neurites extended by taxol-CB neurons are unbranched and curiously unattached to the substratum. When CB is added to taxol-treated neurons, neurite extension begins rapidly, even if protein synthesis is severely reduced. We propose that taxol inhibits microtubule transport in neurites, and this inhibition of "push" is reversed by the disruptive effects of CB on the cytoplasmic matrix, allowing taxol-induced microtubule bundles to be transported distally.

A role for the cytoskeleton in renal vitamin D metabolism

Conversion of circulating 25-hydroxyvitamin D3 (25(OH)D3) to its active metabolite 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) occurs in the renal tubule mitochondrion. Recent reports have implicated the cytoskeleton in certain other steroid metabolizing cells as a mediator of a rate-limiting mitochondrial transport step. Whilst the activity of the renal converting enzyme, a typical steroid hydroxylase, is known to be regulated closely by a number of well studied factors, no information is available to indicate whether an analogous transport step is relevant to the regulation of vitamin D metabolism. Cytochalasin B and vinblastine were used as chemical antagonists of the microfilamentous and microtubular elements of the cytoskeleton. Both agents inhibited the conversion of 25(OH)D3 to 1,25(OH)2D3 by isolated vitamin D-deficient chick renal tubules in a dose-dependent manner. At the concentrations required to inhibit 25(OH)D3-1 alpha-hydroxylase activity in whole cells, these agents inhibited neither isolated mitochondrial 1,25(OH)2D3 production, nor 24,25(OH)2D3 synthesis by vitamin D-replete tubules. The cytoskeletal antagonists were found to increase the content of labelled 1,25(OH)2D3 and 25(OH)D3 in a mitochondrial fraction prepared by Percoll fractionation of tubule cells pre-exposed to the antagonists and labelled 25(OH)D3 substrate. The data suggest that disruption of the cytoskeleton may result in inhibition of transport of newly synthesised 1,25(OH)2D3 out of the mitochondrion and through the cell, and accumulating 1,25(OH)2D3 may oppose its further synthesis. This is consistent with a transport process mediated by the cytoskeleton being involved in the regulation of renal vitamin D metabolism.

Changes occurring in plasma membranes and intercellular junctions during the process of carcinogenesis and in squamous cell carcinoma

A 0.5% mineral-oil solution of 9.10-dimethyl-1.2-benzanthracene (DMBA) was applied to artificial cecal pouches in the lower lips of rats. Ultrastructural studies were made of plasma membranes and intercellular junctions during the process of malignant transformation in the oral mucosal epithelium and after squamous cell carcinoma had been induced by the carcinogen. After the administration of DMBA, the inner leaflet of the membranes where the microfilaments are attached showed high electron density and intramembranous particles on the P-face of basal cells decreased to about half that of controls. However, on the E-face the number of intramembranous particles increased by approximately 10% compared with controls. Though the normal size range for intramembranous particles was 9-12 nm, the administration of DMBA caused aggregations of from three to six particles on the P-face. In squamous cell carcinomas, only the outer leaflet of the membranes showed high electron density; the number of intramembranous particles was 30% higher on the P-face and approximately three times higher on the E-face compared with controls and the morphology of the intramembranous particles, which formed irregular aggregates of from five to 20 particles, was specific. In animals treated with DMBA, the number of gap junctions decreased by between 50% and 70%, although no structural changes occurred. In squamous cell carcinomas, the area of gap junctions was about 50% lower and the number of gap junctions about 40% lower than in controls. Changes in the number and area of desmosomes were similar to those of gap junctions both in the DMBA-treated animals and in squamous cell carcinomas.

Degradation of surface-labeled hepatoma membrane polypeptides: effect of inhibitors

When their membrane proteins were labeled with 125I by lactoperoxidase, dividing hepatoma cells lost radioactivity to the medium in a biphasic manner (T1/2 = 16-26 h, greater than 40 h). Lysosomotropic weak bases, chloroquine, and NH4Cl inhibited the rapid phase by 59%. More than 50% of the radioactivity which accumulates in the media from dividing cells during the first 4 h after labeling was trichloroacetic acid-soluble, and was identified as iodotyrosine. Iodotyrosine release from labeled membrane proteins was 60-71% inhibited by lysosomotropic agents chloroquine and NH4Cl as well as the sodium-proton ionophore, monensin. The inhibitory effect of NH4Cl and monensin was reversible. Inhibitors of microtubule and microfilament function and transglutamination had no effect on release of iodotyrosine to the medium, but trypsin-like protease inhibitors, p-aminobenzamidine, tosyl-L-lysine/chloromethylketone, and phenylmethylsulfonyl fluoride, as well as the cathepsin B inhibitor, leupeptin, inhibited by 21-24%. Iodotyrosine release showed a biphasic Arrhenius plot with an activation energy of 17 kcal/mol above but 27 kcal/mol below 20 degrees C. These results indicate that cell membrane polypeptides require a temperature-limiting event as well as passage through an ion-sensitive compartment prior to their complete degradation to constituent amino acids. In contrast to other lysosomal-mediated events, however, iodinated membrane proteins of dividing cells are degraded in a manner insensitive to agents which disrupt the cytoskeleton.

Reduced insulin endocytosis in serum-transformed fibroblasts demonstrated by flow cytometry

Neoplastic transformation often results in the loss of growth control and concomitant changes in cell surface properties. The changes in endocytosis of a variety of probes after serum or anchorage transformation were measured for mouse fibroblasts by flow cytofluorometry. No major differences in dextran (fluid phase) or histone (nonspecific-adsorptive) endocytosis were observed among four cell lines with different growth properties. However, decreased receptor-mediated internalization of alpha 2-macroglobulin was observed for cell lines transformed to either serum or anchorage independence. Furthermore, increased wheat germ agglutinin and decreased insulin endocytosis were observed, but only in serum transformants. The changes specific to serum transformants were not accounted for by changes in binding of wheat germ agglutinin or insulin. The possible implications of these observations regarding serum transformation and the insulin requirement for growth in serum-free medium are discussed.

The effects of sucrose loading on lysosomal hydrolases

The addition of 88 mM sucrose to the culture medium of human skin fibroblasts from normal subjects caused remarkable increase in the intracellular lysosomal hydrolase activities. The mechanism of this induction by sucrose loading was carefully studied with several fibroblast strains of different inherited lysosomal storage disorders. In single lysosomal hydrolase defect such as GM1-gangliosidosis, mannosidosis and Sandhoff disease, no induction of the deficient hydrolase was found with 88 mM sucrose loading. In contrast, sucrose loading caused normalization of intracellular lysosomal hydrolase activities in I-cell disease fibroblasts and cytoplasmic inclusion materials disappeared. Subsequent investigations reveal that I-cell disease cells are classified into three subgroups by the degree of hydrolase induction by sucrose loading; a high responding, an intermediate responding and a no-response group. The heterogeneity may be based upon different induction by sucrose loading of the enzyme, probably the residual phosphotransferase which is involved in the processing steps of lysosomal enzyme molecules. With the addition of mannose-6-phosphate and 10 mM NH4Cl to cultured skin fibroblasts, it was shown that sucrose loading caused increased synthesis of lysosomal enzyme proteins. The result of the test with 2,4-dinitrophenol suggests that sucrose is indeed pinocytosed by cultured human skin fibroblasts and localized in lysosomes and that this event is the essential factor to trigger the induction of lysosomal hydrolases. Simultaneous loading of both invertase and sucrose in cultured cells caused no induction of alpha-mannosidase activity. This result indicates that invertase is also pinocytosed, reaches the lysosomes and hydrolyzes sucrose in the lysosomes. Lysosomal overloading with sucrose resulted in induction of lysosomal hydrolases and invertase blocked the induction of alpha-mannosidase activity. However, some induction still exists in beta-galactosidase and alpha-fucosidase activity. Thus it is very likely that the induction of lysosomal hydrolases demands a complicated process. In this article, we investigated the effects of sucrose on the lysosomal hydrolases in cultured human skin fibroblasts of several inherited lysosomal storage disorders and normal subjects and discuss the possible mechanism of the induction of lysosomal hydrolase activities by sucrose loading.

Dependence of fluid-phase pinocytosis in arterial smooth-muscle cells on temperature, cellular ATP concentration and the cytoskeletal system

125I-labelled poly(vinylpyrrolidone) was used as a marker of fluid-phase pinocytosis in cultured pig arterial smooth-muscle cells. The rate of pinocytosis was temperature-dependent. A decrease in cellular ATP concentrations as a result of inhibition of either glycolysis or oxidative phosphorylation was associated with a similar decrease in pinocytosis. A microfibrillar-disruptive agent, cytochalasin B, caused a concentration-dependent stimulation of pinocytosis, whereas the microtubular-disruptive agents colchicine and vinblastine decreased pinocytosis to approximately half of control values at all concentrations used. These results indicate that fluid-phase pinocytosis in smooth-muscle cells is dependent on a continuing supply of energy and the integrity of the microtubules. Furthermore, microfilaments appear to exert a certain degree of constraint on pinocytosis, possibly by restricting invagination of the plasma membrane.

Isolation and characterization of macrophage phagosomes containing infectious and heat-inactivated Chlamydia psittaci: two phagosomes with different intracellular behaviors

Infectious Chlamydia psittaci enters macrophages via a cytochalasin B-insensitive pathway in which chlamydia-containing phagosomes do not fuse with lysosomes; heat-inactivated C. psittaci enters macrophages via a route in which phagosomes do fuse with lysosomes. In an attempt to explain these differences, phagosomes containing infectious and heated chlamydiae were isolated from mouse macrophages by a procedure developed to isolate L-cell chlamydial phagosomes by rate zonal centrifugation. Macrophage phagosomes acted similarly to L-cell phagosomes on dextran and discontinuous sucrose gradients and exhibited similar detergent sensitivities. Total proteins of the two phagosomes were compared with each other, L-cell proteins, and surface-labeled proteins from macrophages. Both macrophage phagosome membranes had at least nine proteins with equal sodium dodecyl sulfate-polyacrylamide gel electrophoresis mobilities; some were the same as L-cell phagosome proteins. Each phagosome had at least one protein not seen in the other. Only two phagosome proteins had mobilities equal to macrophage plasma membrane proteins. Macrophage phagosomes containing infectious and heat-inactivated C. psittaci, although created by different entry mechanisms and destined for different intracellular fates, exhibited only a few differences in their proteins.

Endocytosis in absorptive cells of cultured human small-intestinal tissue: effect of cytochalasin B and D

The effect of cytochalasin B (CB) and cytochalasin D (CD) on the endocytotic uptake of horseradish peroxidase (HRP) by intestinal absorptive cells was investigated by morphometric methods. The results showed that CD inhibited endocytosis considerably, and without any detrimental side-effects. CB had hardly any effect on the endocytosis of HRP, but caused a significant decrease in the number of apical vesicles and tubules involved in the transport of cell-coat glycoproteins from the Golgi apparatus to the brush border. Electron-microscopic autoradiographic analysis of the effect of CD showed that although endocytosis is inhibited significantly by the drug, the amount of radiolabelled cell-coat material entering the lysosome-like bodies was unaltered compared with control cultures. These observations support our hypothesis that the cell-coat glycoproteins of the absorptive cells enter the lysosome-like bodies by a crinophagic rather than by an exocytotic-endocytotic mechanism.

Inhibition by glucocorticoids of endocytosis in a macrophage-like cell line

A macrophage-like cell line (P388D1) has been used to demonstrate that glucocorticoids inhibit the fluid-phase endocytosis of fluorescein-labeled dextran (FITC-dextran). Initial experiments demonstrated that the interaction of FITC-dextran with cells had all the features of fluid-phase uptake, ie, the amount taken up was proportional to the concentration in the medium, the uptake proceeded continuously with time and was blocked at 4 degrees C. Dexamethasone (10(-7) M) had no effect on endocytosis until 11 hours after addition of the steroid, when it inhibited the uptake of FITC-dextran by 35%. The amount of inhibition increased with longer exposure times to the hormone up to 50% after 22 hours. Although this effect on endocytosis was observed prior to any effect on growth of the cells, endocytosis as well as cell proliferation were inhibited in a dose dependent fashion. A preliminary survey of selected steroids has established that the inhibition of endocytosis was restricted to steroids of the glucocorticoid class. The key experiments were also performed using horseradish peroxidase instead of FITC-dextran with, essentially, identical results.

Exocytosis of pinocytosed fluid in cultured cells: kinetic evidence for rapid turnover and compartmentation

The uptake and fate of pinocytosed fluid were investigated in monolayers of pulmonary alveolar macrophages and fetal lung fibroblasts using the fluid-phase marker, [14C]sucrose. Initial experiments revealed that cellular accumulation of chromatographically repurified [14C]sucrose was not linear with incubation time. Deviation from linearity was shown to be due to constant exocytosis of accumulating marker. Chromatographic analysis revealed that the cells were unable to metabolize sucrose and were releasing it intact by a process that was temperature-sensitive but not dependent on extracellular calcium and magnesium. A detailed analysis of the kinetics of exocytosis was undertaken by preloading cells with [14C]sucrose for various lengths of time and then monitoring the appearance of radioactivity into isotope-free medium. Results indicated that modeling the process of fluid-phase pinocytosis and subsequent exocytosis required at least two intracellular compartments in series, one compartment being of small size and turning over very rapidly (t1/2 = 5 min in macrophages, 6--8 min in fibroblasts) and the other compartment being apparently larger in size and turning over very slowly (t1/2 = 180 min in macrophages, 430--620 min in fibroblasts). Computer-simulation based on this model confirmed that the kinetics of efflux faithfully reflected the kinetics of influx and that the rate of efflux completely accounted for the deviation from linearity of accumulation kinetics. Moreover, the sizes of the compartments and magnitude of the intercompartment fluxes were such that the majority of fluid internalized in pinocytic vesicles was rapidly returned to the extracellular space via exocytosis. This result provides direct experimental evidence for a process previously thought necessary based solely on morphological and theoretical considerations. Furthermore, the turnover of pinocytosed fluid was so dynamic that accumulation deviated from linearity even within the first few minutes of incubation. We were able to show that the kinetics of exocytosis allowed calculation of the actual pinocytic rate, a rate that was nearly 50% greater than the apparent initial rate obtained from the slope of the uptake curve over the first 10 min.

Effects of cytochalasin B on actin and myosin association with particle binding sites in mouse macrophages: implications with regard to the mechanism of action of the cytochalasins

The intracellular distribution of F-actin and myosin has been examined in mouse peritoneal macrophages by immunofluorescence microscopy. In resting, adherent cells, F-actin was distributed in a fine networklike pattern throughout the cytoplasm. Myosin, in contrast, was distributed in a punctate pattern. After treatment with cytochalasin B (CB), both proteins showed a coarse punctate pattern consistent with a condensation of protein around specific foci. After CB-pretreated cells were exposed to opsonized zymosan particles, immunofluorescent staining for F-actin and myosin showed an increased staining under particle binding sites. Transmission electron microscope (TEM) examination of whole-cell mounts of such preparations revealed a dense zone of filaments beneath the relatively electron-translucent zymosan particles. At sites where particles had detached during processing, these filament-rich areas were more clearly delineated. At such sites dense arrays of filaments that appeared more or less randomly oriented were apparent. The filaments could be decorated with heavy meromyosin, suggesting that they were composed, in part, of F-actin and were therefore identical to the structures giving rise to the immunofluorescence patterns. After viewing CB-treated preparations by whole-mount TEM, we examined the cells by scanning electron microscopy (SEM). Direct SEM comparison of the filament-rich zones seen by TEM showed that these structures resulted from the formation of short lamellipodial protrusions below the site of particle binding. Electron micrographs of thin-sectioned material established that these lamellipodial protrusions were densely packed with microfilaments that were in part associated with the cytoplasmic surface of the plasma membrane. The formation of particle-associated lamellipodia did not appear to represent merely a slower rate of ingestion in the presence of CB, because they formed within minutes of particle contact with the cell membrane and were not followed by particle ingestion even after a 1-h or longer incubation. Furthermore, their formation required cellular energy. These results suggest that cytochalasin B blocks phagocytosis of large particles by affecting the distances over which any putative actomyosin-mediated forces are generated.