Changes in fine structure of developing intestinal epithelium associated with pinocytosis.
Explore the source record for details and available documents.
Journal Article↗
SEARCH · Search PubMed
Results for “PINOCYTOSIS”
Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Receptor-mediated pinocytosis of IgG and immune complex in rat peritoneal macrophages: an electron microscopic study.
The uptake mechanism of homologous IgG and immune complex, and the participation of coated vesicles in this process were studied in rat peritoneal macrophages. Peroxidase-antiperoxidase (PAP) immune complex produced in rat, and purified rat IgG adsorbed to gold particles (IgG-Au) were used as ligands. Freshly collected peritoneal macrophages were preincubated with the ligands at 4 degrees C, washed, warmed up to 37 degrees C, maintained in a serum-free culture medium for 5 sec to 30 min and subsequently fixed for electron microscopy. In the IgG-Au experiments, acid phosphatase reaction was also applied to identify lysosomes, and ruthenium red to trace membranes exposed to the extracellular space. At the end of the preincubation period PAP and IgG were found randomly distributed on the external surface of the plasma membrane. After warming up the cells to 37 degrees C, the ligands bound to the plasma membrane showed a tendency to move towards deep labyrinthic invaginations of the cell surface from where they were internalized via coated pits and coated vesicles. In the initial period, these structures seemed to be the primary carriers of the ligands. In the period between 5 and 10 min, ligands were concentrated in vacuoles (endosomes) located in the deeper cytoplasm, while after 30 min, they were present in large lysosome-like or multivesicular bodies, which were found to be acid phosphatase positive.
Animals↗
Transcellular transport as a mechanism of blood-brain barrier disruption during stroke.
It is well-known that ischemia causes disruption of the blood-brain barrier (BBB), which leads to the formation of vasogenic brain edema. One major mechanism of BBB opening is enhanced pinocytotic vesicle formation that may be induced after transient focal ischemia by several mechanisms, including nitric oxide production, release of neurotransmitters, inflammatory mediators and hemodynamic alterations. In the present study we sought to characterize the extent of pinocytosis in cerebral endothelium during both ischemia/reperfusion (I/R) and elevated intravascular pressure. Transient focal ischemia was induced for 1 hour with 24 hours of reperfusion using the filament occlusion model in male Wistar rats, after which occluded middle cerebral arteries (MCAs) were dissected and mounted on glass cannulas in an arteriograph chamber. This system allowed control over intravascular pressure, measurement of lumen diameter and perfusion with various tracers (Lucifer Yellow and horseradish peroxidase) for measurement of transcellular transport and quantification of pinocytosis using transmission electron microscopy. I/R was found to increase vesicle formation by 166% basolaterally without a change in vesicle formation apically compared to non-ischemic control MCAs at 75 mmHg (p less than 0.01). Similarly, an acute increase in pressure to 200 mmHg caused a 78% increase in apical pinocytosis (p less than 0.05) and a non-significant 42% increase basolaterally. These results were confirmed by permeability measurements using Lucifer Yellow and demonstrate that both I/R and acute elevations in intravascular pressure enhance cerebral endothelial cell pinocytosis. The increase in basolateral pinocytosis during ischemia suggests enhanced efflux mechanisms that may be transporting substances from brain to blood. In addition, since the enhanced pinocytosis after an increase in pressure occurred in isolated arteries in vitro without the influence of metabolic or neuronal factors, these findings demonstrate that elevated intravascular pressure is a primary stimulus for pinocytosis in cerebral endothelial cells.
Animals↗
Factors influencing the endocytosis of immune complexes.
The MPS plays an important role in the removal of both particulate and soluble immunogenic material from the circulation. E.IgG adhere to mononuclear phagocytes if the Fc portion of the IgG can interact with the phagocyte's Fc receptors (Fc gamma R). Simultaneous sensitization with IgG and C3b via complement activation enhances the effectiveness of binding and ingestion. E.IgM adhere mainly via C3b to C3b receptors (C3bR) of macrophages. Unstimulated macrophages do not ingest E.C3b. Stimulated macrophages, on the other hand, can ingest E.C3b. If soluble material cannot adhere to the surface of macrophages, it will be endocytosed in vitro via fluid-phase pinocytosis at the concentration that is present in the medium. If the material adheres to the cell's surface via its chemical properties or via specific receptors, it will be selectively concentrated at the cell's surface and endocytosed by adsorptive pinocytosis. Ingestion of IC via Fc gamma R and C3b depends on the ability of the antibodies to interact with Fc gamma R and their capacity to activate the complement system. IC-bound C3b enhances the adsorptive pinocytosis of IC. Soluble AIgG are also pinocytosed more effectively when C3b is bound to AIgG. The degree of endocytosis varies with the level of C3b sensitization. The highly effective C3b-mediated pinocytosis can be abolished by treating with trypsin to inactivate C3bR. This observation illustrates that C3b-mediated pinocytosis can replace Fc-mediated pinocytosis in unstimulated macrophages. When macrophages are stimulated in vivo, Fc-mediated pinocytosis increases significantly. Under these conditions, the binding of C3b no longer stimulates; instead, it sterically interferes with Fc-Fc gamma R interaction. In vivo, E.IgG are removed mainly by splenic macrophages. C4-deficient guinea pigs clear E.IgG less effectively than guinea pigs with an intact complement system. On the other hand, soluble IC and AIgG are removed from the circulation mainly by hepatic Kupffer cells. Complement depletion does not seem to influence the clearance rates of these soluble IC or AIgG. The different results obtained in vitro and in vivo and the finding that different effector organs are responsible for the removal of sensitized erythrocytes and soluble Ic in vivo, suggest that more reliable techniques have to be developed to measure IC clearance in patients with a supposedly deficient or saturated MPS.
Animals↗
Fluid phase endocytosis and galactosyl receptor-mediated endocytosis employ different early endosomes.
Endocytosis may originate both in coated pits and in uncoated regions of the plasma membrane. In hepatocytes it has been shown that fluid phase endocytosis (here defined as 'pinocytosis') is unaffected by treatments that arrest coated pit-mediated endocytosis, indicating that pinocytosis is primarily a clathrin-independent process. In this study we have tried to determine possible connections between pinocytosis and clathrin-dependent endocytosis in rat hepatocytes by means of subcellular fractionation, electron microscopy, and by assessing the influence of inhibitors of clathrin-dependent endocytosis on pinocytosis. As marker for clathrin-dependent endocytosis was used asialoorosomucoid (AOM) labelled with [(125)I]tyramine cellobiose ([(125)I]TC). [(125)I]TC-labelled bovine serum albumin ([(125)I]TC-BSA) was found to be a useful marker for pinocytosis. Its uptake in the cells is not saturable, and any remnants of [(125)I]TC-BSA associated with the cell surface could be removed by incubating the cells with 0.3% pronase at 0 degrees C for 60 min. The data obtained by electron microscopy and by subcellular fractionation suggested that early after initiation of uptake (<15 min) [(125)I]TC-BSA and [(125)I]TC-AOM were present in different endocytic vesicles. The two probes probably join prior to their entrance in the lysosomal compartment. The relation between endocytosis via coated pits and pinocytosis was also studied with techniques that induced a selective density shift either in the clathrin-dependent pathway (by AOM-HRP) or in the pinocytic pathway (by allowing uptake of AuBSA). Both treatments indicated that the two probes ([(125)I]TC-AOM and [(125)I]TC-BSA) were early after uptake, at least partly, in separate endocytic compartments. The different distribution of the fluid phase marker and the ligand (internalised via coated pits) was not due to a difference in the rate at which they enter a later compartment, since a lowering of the incubation temperature to 18 degrees C, which should keep the probes in the early endosomes, did not affect their early density distribution. Incubation of cells in a hypertonic medium reduced uptake both of [(125)I]TC-AOM and [(125)I]TC-BSA; the uptake of [(125)I]TC-AOM was, however, reduced much more than that of the fluid phase marker. This finding supports the notion that the two probes enter the cells via different routes.
Animals↗
v-Src induces constitutive macropinocytosis in rat fibroblasts.
The role of v-Src as regulator of fluid-phase pinocytosis was investigated in Rat-1 cells expressing a stable (Rat-1/BB16) or a thermosensitive (Rat-1/tsLA29) v-Src protein. In the second cell line, this protein is inactive when cells are cultured at 40 degrees C but recovers its tyrosine kinase activity upon transfer to 34 degrees C, resulting into a transformed phenotype. The rate of fluid-phase pinocytosis of the tracer horseradish peroxidase was 2-fold higher in v-Src-transformed fibroblasts (Rat-1/BB16, Rat-1/tsLA29 cultured at 34 degrees C) as compared to non-transformed cells (Rat-1, Rat-1/tsLA29 kept at 40 degrees C). In contrast, receptor-mediated endocytosis of transferrin was poorly affected, suggesting that structures distinct from clathrin-coated pits are involved in pinocytosis stimulation. By light and electron microscopy, transformed cells frequently contained large peroxidase-labeled pinocytic vesicles located near to membrane ruffles, demonstrating that stimulation of pinocytosis corresponds to induction of constitutive macropinocytosis. Stimulation of pinocytosis occurred more than 8 hours after transfer to the permissive temperature, whereas transfer to the non-permissive temperature partially reversed the stimulation within 2 hours. Protein synthesis inhibition for 6 hours abrogated pinocytosis stimulation in transformed cells, indicating that constitutive macropinocytosis induced by v-Src depends on continuous synthesis of a short-lived regulatory machinery.
Amiloride↗
Activation of complement by soluble IgG aggregates and their subsequent interaction with macrophages.
The mononuclear phagocyte system (MPS) plays an important role in the removal of both particulate and soluble immunogenic material from the circulation. E.IgG adhere to mononuclear phagocytes if the Fc portion of the IgG can interact with the phagocytes' Fc receptors. Simultaneous sensitization with IgG and C3b enhances the effectiveness of binding and ingestion. If soluble material cannot adhere to the surface of macrophages, it will be endocytosed in vitro via fluid-phase pinocytosis at the concentration that is present in the medium. If the material adheres to the cell's surface via its chemical properties or via specific receptors, it will be selectively concentrated at the cell's surface and endocytosed by an adsorptive pinocytosis. Ingestion of IC via Fc gamma R and C3b depends on the ability of the antibodies to interact with Fc gamma R and their capacity to activate the complement system. IC-bound C3b enhances the adsorptive pinocytosis of IC. Soluble AIgG are also pinocytosed more efficiently when C3b is bound to AIgG. The degree of endocytosis varies with the level of C3b sensitization. The highly effective C3b-mediated pinocytosis can be abolished by treating the macrophages with trypsin to inactivate C3bR. This observation illustrates that C3-mediated pinocytosis can replace Fc-mediated pinocytosis in unstimulated macrophages. Soluble IC and AIgG are removed from the circulation mainly by hepatic Kupffer cells. It seems that the size, the Ag/Ab ratio, the capacity of the IC to bind C1, activate C1, and allow deposition of C3b together with the degree of phagocyte activation determine the degree of binding and subsequent degradation of soluble IC.(ABSTRACT TRUNCATED AT 250 WORDS)
Animals↗
Distinct endocytotic pathways in epidermal growth factor-stimulated human carcinoma A431 cells.
Addition of EGF to human epidermoid carcinoma A431 cells increases the rate of fluid-phase pinocytosis 6-10-fold as measured by horseradish peroxidase uptake (Haigler, H.T., J. A. McKanna, and S. Cohen. 1979. J. Cell Biol. 83:82-90). We show here that in the absence of extracellular Na+ or in the presence of amiloride the stimulation of pinocytosis by EGF is substantially reduced. Amiloride had no effect on the endocytosis of EGF itself or of transferrin, demonstrating that the receptor-mediated endocytotic pathway operated normally under conditions that blocked stimulated pinocytosis. Amiloride blocked EGF-stimulated pinocytosis in both HCO3(-)-containing and HCO3(-)-free media. The EGF-stimulated pinocytotic activity can frequently be localized to areas of the cell where membrane spreading and ruffling are taking place. These results demonstrate that (a) EGF induces a distinct amiloride-sensitive endocytotic pathway on A431 cells; (b) occupied EGF receptors do not utilize this pathway for their own entry; (c) endocytosis of occupied EGF receptors is not in itself sufficient to stimulate pinocytosis.
Amiloride↗
Endocytosis and signal transduction: basic science update.
Endocytosis can be separated into the categories of phagocytosis and pinocytosis. Phagocytosis can be distinguished from pinocytosis primarily by the size of particle ingested and by its dependence on actin polymerization as a key step in particle ingestion. Several specific forms of pinocytosis have been identified that can be distinguished based on their dependence on clathrin or caveolin. Both clathrin- and caveolin-dependent pinocytosis appear to require the participation of dynamin to internalize the plasma membrane. Other, less well-characterized forms of pinocytosis have also been described. Although endocytosis has long been known to affect receptor density, recent studies have demonstrated that endocytosis through clathrin- and caveolin-dependent processes plays a key role in receptor-mediated signal transduction. In some cases, blockade of these processes attenuates, or even prevents, signal transduction from taking place. This information, coupled with a better understanding of endocytosis mechanisms, will help advance the field of cell biology as well as present new targets for drug development and disease treatment.
Actins↗
Altered visceral yolk sac function produced by a low-molecular-weight somatomedin inhibitor.
A fraction from diabetic rat serum containing a low-molecular-weight (800-1000) somatomedin inhibitor (SI) alters growth and development in both neurulation and early limb bud staged mouse embryos in vitro. Previous studies suggested that an accumulation of serum proteins and morphological changes of the visceral yolk sac (VYS) were produced following exposure to the SI in early limb bud staged conceptuses. The morphological changes, characterized by the presence of large endosomes in the endodermal cells, suggested that the SI altered histiotrophic nutrition, whereby proteins are pinocytosed by the endodermal VYS cells and degraded to constituent amino acids. Therefore, the effects of the SI on pinocytosis and protein degradation by the VYS were evaluated using the whole embryo culture system. Results showed that the SI reduced fluid phase pinocytosis as determined by the uptake of [U-14C]sucrose, but that accumulation of [3H]leucine-labeled hemoglobin ([3H]Hb) by the VYS was greater following exposure to the SI than in controls. In contrast, the accumulation of 3H-labeled amino acids in the embryo (produced from the degradation of [3H]Hb by the VYS) was reduced by the SI. The extent of amino acid reduction in embryonic accumulation is dependent upon the concentration of SI in the culture medium and correlates with the incidence of malformations produced by the SI, i.e., high rates of malformations occur with large reductions in embryonic 3H-labeled amino acid accumulation. The apparent paradox of high [3H]Hb accumulation in the presence of decreased pinocytosis appears to be the result of altered processing of the [3H]Hb in the endodermal cells. The altered processing decreases the "elimination" of the proteins from the VYS and results in the decrease in 3H-labeled amino acid present in the embryo proper. Therefore, the SI appears to alter two processes of VYS histiotrophic function. (1) decreased pinocytosis and (2) altered protein processing, ultimately resulting in a decreased availability of substrates for the embryo. During the early stages of embryogenesis in the human, the trophoblast cells of the placenta are responsible for the transport of nutrients from the maternal to embryonic systems. Since these cells show high phagocytic and pinocytotic activities, the SI may also disrupt these processes in the chorioallantoic placenta and contribute to diabetes-induced embryopathies.
Animals↗
Sources of amino acids for protein synthesis during early organogenesis in the rat. I. Relative contributions of free amino acids and of proteins.
Rat conceptuses on the 10th day of gestation were cultured for 27 h in whole rat serum. An addition of either [3H]leucine or [3H]leucine-labelled rat serum proteins was made once during the culture period, and the acid-soluble and acid-insoluble radioactivities of embryo and visceral yolk sac measured at harvesting. The extent of radiolabel incorporation into embryonic and yolk-sac proteins increased linearly with the duration of exposure of the conceptus to the radiolabelled leucine or radiolabelled serum proteins, indicating roughly constant rates of incorporation, per unit mass of tissue, throughout the culture period. The incorporation rates, expressed as clearances, were 0.73 and 0.78 microliter/mg tissue protein/h for embryo and yolk sac, respectively, when the source was [3H]leucine; and 1.8 and 1.3 microliters/mg tissue protein/h, for embryo and yolk sac, respectively, when the source was [3H]leucine-labelled serum proteins. It is estimated, from the known leucine and protein concentrations in serum, that protein contributed over 99 per cent of the leucine supplied to the conceptus for its protein synthesis. In parallel experiments, measurements were made on cultures conducted in the presence of an antiserum against rat visceral yolk sac (100 micrograms/ml). Antiserum profoundly inhibited incorporation of radioactivity into embryo and yolk-sac proteins, when the source was 3H-labelled protein, a result consistent with the known ability of the antiserum to inhibit pinocytosis in the yolk sac. Antiserum also decreased incorporation from [3H]leucine in the yolk sac, suggesting that a proportion of the free leucine entering the yolk sac does so by pinocytosis. The failure of antiserum to affect incorporation of [3H]leucine into the embryo probably indicates that leucine can enter the embryo without the mediation of yolk-sac pinocytosis. The primacy of protein, as a source of amino acids for the organogenesis-stage embryo, is consistent with the serious effects, in terms of embryonic death and malformation, that result from the interruption of amino acid supply when either pinocytosis or lysosomal proteolysis in the yolk sac is inhibited.
Amino Acids↗
Response of endocytosis to altered fatty acyl composition of macrophage phospholipids.
Mouse peritoneal macrophages incubated in serumless medium containing a 19:0 or trans-18:1 fatty acid complexed to bovine serum albumin incorporate the exogenous fatty acid supplement into cellular phospholipids. Within 8 hr, 25% of the total phospholipid fatty acids are derived from the supplement, with cell viability remaining greater than 95%. The incorporation of either of these supplements increases the saturated/unsaturated fatty acid ratio in the phospholipids 2-fold over that of cells cultured in serum and effects striking changes in endocytic activities. The levels of both fluid-phase pinocytosis and receptor-mediated phagocytosis are decreased at all temperatures examined between 15 degrees and 37 degrees. The increased degree of saturation of cell phospholipids correlates with decreased endocytic rates for both processes and with increased activation energies (Eact) for phagocytosis. The Eact values for phagocytosis, which range from 54 to 90 kcal/mol, depend on the supplementation conditions used. Although the levels of pinocytosis are depressed, the Eact values for pinocytosis (17--25 kcal/mol) are not strikingly affected by saturated fatty acid enrichment. These observations suggest that the degree of lipid fluidity of macrophage membranes influences both phagocytosis and pinocytosis in macrophages.
Acetates↗
Colchicine effects on lysosomal enzyme induction and intracellular degradation in the cultivated macrophage.
The effects of colchicine on lysosomal fusion and lysosomal enzyme induction in the cultivated mouse peritoneal macrophage have been examined. Colchicine (10- minus 6 M), but not lumicolchicine, inhibited lysosomal enzyme induction by both phagocytic and pinocytic stimuli. In addition, the drug significantly retarded pinocytic uptake of [3-H] sucrose and transport of the amino acids [3-H] alpha aminoisobutyric acid and L-[3-H] leucine. In contrast, lumicolchicine had no effect on pinocytosis or amino acid transport. Thus, a role for intact microtubules in lysosomal enzyme induction, pinocytosis, and amino acid uptake in these cells is suggested. That colchicine inhibited lysosomal enzyme induction by phagocytic stimuli under conditions in which pinocytosis contributed little to the enzyme rise indicated that inhibition of pinocytosis was unlikely to account for colchicine effects on lysosomal enzyme induction. Effects of colchicine on degradation of phagocytized and pinocytized substrates were examined to determine if intact microtubules are required for fusion among lysosomes, pinosomes, and phagosomes. Colchicine did not alter the rate of intracellular digestion of radiolabeled bacteria by the cultivated macrophage. Similarly, it had no effect on enzymatic hydrolysis of intracellular [3-H] sucrose resulting from uptake of exogenous invertase. The finding that colchicine had no effect on the functional consequences of fusion of lysosomes with endosomes suggests that intact microtubules are not required for fusion among these constituents of the vacuolar apparatus.
Acid Phosphatase↗
Membrane structure of corneal epithelium: freeze-fracture observation.
Cytological details of the corneal epithelium were studied with special emphasis on the structure of the cell membrane. Normal corneas of various animals were fixed in a 4% solution of glutaraldehyde and divided into two portions. One-half was frozen with Freon 22 and freeze-fractured in frontal and sagittal directions using a Balzer instrument. The other half was examined by transmission electron microscopy. The general appearance of the split cell membrane of the corneal epithelium was similar to that of other membranes. Besides the characteristic structure of the fractured cell membrane at desmosomes, gap junctions and hemidesmosomes, the cell membrane demonstrated many "bumps and hollows" type configurations measuring 70-80 nm in diameter. This structure is known to be commonly associated with micropinocytotic vesicles and cellular pores. Careful examination of transmission electron micrographs of the corneas of the animals in this study revealed that the number of micropinocytotic vesicles was far greater than previously noted. Pinocytosis was most abundant in the basal cell and decreased toward the superficial layers. Cluster distribution of pinocytosis was frequently demonstrated in the basal cell. In addition, abundant pinocytosis was frequently demonstrated in the basal cell. Also, abundant pinocytosis was present in the cell membrane surrounding nerve fibers which occasionally reached the superficial layer. It was found that numerous nerve fibers pierce through the cytoplasm of the basal cell, in addition to running through the normal intercellular spaces. These findings may be of certain significance in the explanation of transport of fluid through the epithelium and innervation of the cornea.
Animals↗
Inhibition of growth of Dictyostelium discoideum amoebae by bisphosphonate drugs is dependent on cellular uptake.
PURPOSE: The aim of the study was to determine whether bisphosphonates are internalised by Dictyostelium amoebae and whether cellular uptake is required for their growth-inhibitory effects. Bisphosphonates inhibit growth of amoebae of the slime mould Dictyostelium discoideum, by mechanisms that appear to be similar to those that cause inhibition of osteoclastic bone resorption. METHODS: Cell-free extracts prepared from amoebae that had been incubated with bisphosphonates were analysed by 31P-n.m.r, spectroscopy or ion-exchange f.p.l.c., to identify the presence of bisphosphonates or bisphosphonate metabolites respectively. The growth-inhibitory effect of bisphosphonates towards Dictyostelium amoebae was also examined under conditions in which pinocytosis was inhibited. RESULTS: All of the bisphosphonates studied were internalised by Dictyostelium amoebae, probably by fluid-phase pinocytosis, and could be detected in cell-free extracts. Amoebae that were prevented from internalising bisphosphonates by pinocytosis were markedly resistant to the growth-inhibitory effects of these compounds. In addition, bisphosphonates encapsulated within liposomes were more potent growth inhibitors of Dictyostelium owing to enhanced intracellular delivery of bisphosphonates. CONCLUSIONS: All bisphosphonates inhibit Dictyostelium growth by intracellular mechanisms following internalisation of bisphosphonates by fluid-phase pinocytosis. It is therefore likely that bisphosphonates also affect osteoclasts by interacting with intracellular, rather than extracellular, processes.
Adenine Nucleotides↗
Effect of hypo-osmotic incubation on membrane recycling.
Incubation of alveolar macrophages in hypo-osmotic media causes a time-and temperature-dependent increase in the number of surface receptors for three different ligands. Exposure of cells to solutions of 210 mOsM or less, at 37 degrees C but not at 0 degree C, resulted in an increase in the number of surface receptors for diferric transferrin, alpha-macroglobulin-protease complexes, and mannose-terminated glycoproteins. Upon media dilution at 37 degrees C, surface receptor number reached a maximum within 5 min and returned to near-normal values by 30 min. The increase in surface receptor number was the result of a decrease in the rate of internalization of receptors, either occupied or unoccupied. The rate of receptor exteriorization was unaltered by hypo-osmotic incubation of cells. The rate of fluid-phase pinocytosis was also inhibited upon incubation in hypo-osmotic solution. In experiments in which both receptor-mediated endocytosis and fluid phase pinocytosis were measured on the same samples, inhibition of both processes occurred with the same kinetics and to a similar extent. The rate of receptor-mediated endocytosis recovered to normal rates after 60 min in hypo-osmotic solutions, whereas the rate of fluid phase pinocytosis did not recover to the same extent.
Animals↗
Lysosomal movements during heterophagy and autophagy: with special reference to nematolysosome and wrapping lysosome.
Recent studies on lysosomal movements during heterophagy and autophagy performed in our laboratory for the past several years were reviewed; methods for the investigation of lysosomes and the cytoskeleton in these studies mainly involved electron microscopic cytochemistry. Lysosomal movements during heterophagy were observed in cultured rat alveolar macrophages taking up horseradish peroxidase (HRP) and rat peroxidase-antiperoxidase (PAP) by fluid-phase pinocytosis and adsorptive pinocytosis, respectively. A characteristic lysosomal change which was induced by the pinocytosis was the appearance of long, threadlike lysosomes (nematolysosomes) in the cytoplasm. The effects of actin filament destabilizer and antimicrotubular drug on lysosomal changes revealed that the appearance of nematolysosomes was dependent on the presence of both actin filaments and microtubules. The close morphological relationship between lysosomes and cytoskeletal elements, such as actin filaments and microtubules in the alveolar macrophages, supports the participation of the cytoskeletal system in the regulatory mechanism of lysosomal movements. In the study of the lysosomal wrapping mechanism (LWM), which is one type of lysosomal movement that occurs during autophagy, it was found that the occurrence of LWM was dependent on energy--namely, the supply of ATP--and on the presence of actin filaments. However, deconstruction of microtubules induced or favored the occurrence of LWM. It is conceivable that the LWM is also related to the cytoskeletal system. We conclude that intracellular dynamics of lysosomes during heterophagy and autophagy are largely a consequence of complicated modulation by the cytoskeletal system.
Animals↗
Phosphoinositide 3-kinase as an upstream regulator of the small GTP-binding protein Rac in the insulin signaling of membrane ruffling.
Membrane ruffling and the closely linked response of fluid-phase pinocytosis were investigated in Chinese hamster ovary cells that stably overexpress the human insulin receptor and a mutant 85-kDa subunit of phosphoinositide (PI) 3-kinase (delta p85) that lacks a binding site for the catalytic 110-kDa subunit of this enzyme. Both membrane ruffling and pinocytosis induced by insulin were markedly impaired in these cells. Microinjection of Rac, a Ras-related small GTP-binding protein, induced membrane ruffling in human epidermoid carcinoma KB cells, and this effect of Rac was not blocked by coinjection of delta p85 or by exposure of cells to wortmannin, a specific PI 3-kinase inhibitor. These results suggest that PI 3-kinase is essential not only for insulin-stimulated membrane ruffling but also for pinocytosis, and that PI 3-kinase possibly functions upstream of Rac in the signal transduction pathway.
Animals↗