Site of action of a polyanion inhibitor of phagosome-lysosome fusion in cultured macrophages

Effect of Monocerin, a Fungal Secondary Metabolite, on Endothelial Cells

This study reports the isolation and identification of the endophytic fungus Exserohilum rostratum through molecular and morphological analysis using optical and transmission electron microscopy (TEM), as well as the procurement of its secondary metabolite monocerin, an isocoumarin derivative. Considering the previously observed biological activities of monocerin, this study was performed on human umbilical vein endothelial cells (HUVECs) that are widely used as an in vitro model for several different purposes. Important parameters, such as cell viability, senescence-associated β-galactosidase, cellular proliferation by using 5(6)-carboxyfluorescein diacetate N-succinimidyl ester (CFSE), apoptosis analysis with annexin, cellular morphology through scanning electron microscopy (SEM), and laser confocal analysis were evaluated after exposing the cells to monocerin. After 24 h of exposure to monocerin at 1.25 mM, there was more than 80% of cell viability and a low percentage of cells in the early and late apoptosis and necrosis. Monocerin increased cell proliferation and did not induce cell senescence. Morphological analysis showed cellular integrity. The study demonstrates aspects of the mechanism of action of monocerin on endothelial cell proliferation, suggesting the possibility of its pharmaceutical application, such as in regenerative medicine.

Inhibition of vacuolar-type (H+)-ATPase by the cytostatic macrolide apicularen A and its role in apicularen A-induced apoptosis in RAW 264.7 cells

Apicularen A and the known vacuolar-type (H(+))-ATPase (V-ATPase) inhibitor bafilomycin A(1) induced apoptosis of RAW 264.7 cells, while apicularen B, an N-acetyl-glucosamine glycoside of apicularen A, was far less effective. Apicularen A inhibited vital staining with acridine orange of the intracellular organelles of RAW 264.7 cells, inhibited the ATP-dependent proton transport into inside-out microsome vesicles, and inhibited the bafilomycin A(1)-sensitive ATP hydrolysis. The IC(50) values of the proton transport were 0.58 nM for apicularen A, 13 nM for apicularen B, and 0.95 nM for bafilomycin A(1). Furthermore, apicularen A inhibited the bafilomycin A(1)-sensitive ATP hydrolysis more potently than apicularen B. F-ATPase and P-ATPase were not inhibited by apicularen A. We concluded that apicularen A inhibits V-ATPase, and thus induces apoptosis in RAW 264.7 cells.

Human targets of Pseudomonas aeruginosa pyocyanin

Pseudomonas aeruginosa produces copious amounts of the redoxactive tricyclic compound pyocyanin that kills competing microbes and mammalian cells, especially during cystic fibrosis lung infection. Cross-phylum susceptibility to pyocyanin suggests the existence of evolutionarily conserved physiological targets. We screened a Saccharomyces cerevisiae deletion library to identify presumptive pyocyanin targets with the expectation that similar targets would be conserved in humans. Fifty S. cerevisiae targets were provisionally identified, of which 60% have orthologous human counterparts. These targets encompassed major cellular pathways involved in the cell cycle, electron transport and respiration, epidermal cell growth, protein sorting, vesicle transport, and the vacuolar ATPase. Using cultured human lung epithelial cells, we showed that pyocyanin-mediated reactive oxygen intermediates inactivate human vacuolar ATPase, supporting the validity of the yeast screen. We discuss how the inactivation of V-ATPase may negatively impact the lung function of cystic fibrosis patients.

Vacuolar-type H(+)-ATPase inhibitory activity of synthetic analogues of the concanamycins: is the hydrogen bond network involving the lactone carbonyl, the hemiacetal hydroxy group, and the C-19 hydroxy group essential for the biological activity of the concanamycins?

Synthetic analogue of the concanamycins, which lacks the hydrogen bond network existing in the concanamycin structure, retains vacuolar-type H(+)-ATPase (V-ATPase) inhibitory activity and induces apoptosis to cancer cells that overexpressing epidermal growth factor receptors (EGFR).

Hygrolidin induces p21 expression and abrogates cell cycle progression at G1 and S phases

Hygrolidin family antibiotics showed selective cytotoxicity against both cyclin E- and cyclin A-overexpressing cells. Among them, hygrolidin was the most potent and inhibited growth of solid tumor-derived cell lines such as DLD-1 human colon cancer cells efficiently more than that of hematopoietic tumor cells and normal fibroblasts. FACS analysis revealed that hygrolidin increased cells in G1 and S phases in DLD-1 cells. While hygrolidin decreased amounts of cyclin-dependent kinase (cdk) 4, cyclin D, and cyclin B, it increased cyclin E and p21 levels. Hygrolidin-induced p21 bound to and inhibit cyclin A-cdk2 complex more strongly than cyclin E-cdk2 complex. Furthermore, hygrolidin was found to increase p21 mRNA in DLD-1 cells, but not in normal fibroblasts. Thus, hygrolidin inhibited tumor cell growth through induction of p21. In respect to p21 induction, inhibition of vacuolar-type (H+)-ATPase by hygrolidin was suggested to be involved.

Induction of EGF-dependent apoptosis by vacuolar-type H(+)-ATPase inhibitors in A431 cells overexpressing the EGF receptor

The stimulation of human tumor cells overexpressing epidermal growth factor receptor (EGFR) with EGF enhances tumor development and malignancy. Therefore, compounds that modulate the EGF-mediated signal inducing apoptosis in EGFR-overexpressing cells would represent a new class of antitumor drug and might be useful in the treatment of a subset of human tumors. In the course of screening for compounds that induce apoptosis in EGFR-overexpressing human epidermal carcinoma A431 cells from secondary metabolites of microorganisms, we found that vacuolar-type H(+)-ATPase (V-ATPase) inhibitors, such as concanamycin B and destruxin E, induced apoptosis only when the cells were stimulated with EGF. The EGF-dependent apoptosis by V-ATPase inhibitors was not observed in other types of human tumor cells which do not overexpress EGFR. The apoptosis in A431 cells was inhibited by anti-FasL antibody which neutralized the cytotoxic effect of FasL, indicating that the Fas/FasL system was involved. The expression of cell surface FasL was upregulated by stimulation with EGF and increased further by V-ATPase inhibitors. Moreover, EGF inhibited cytotoxic Fas antibody-induced apoptosis, whereas V-ATPase inhibitors disrupted the protective effect of EGF on apoptosis in A431 cells. Taken together, these results suggested that V-ATPase inhibitors induced EGF-dependent apoptosis in A431 cells, possibly through both the enhancement of EGF-induced cell surface expression of FasL and the disruption of an EGF-induced survival signal.

Acidic endomembrane organelles are required for mouse postimplantation development

Vacuolar-type H(+)-ATPase (V-ATPase) plays a major role in endomembrane and plasma membrane proton transport in eukaryotes. We found that the acidic compartments generated by V-ATPase are present from the one-cell stage of mouse preimplantation embryos. Upon differentiation of trophoblasts and the inner cell mass at the blastocyst stage, these compartments exhibited a polarized perinuclear distribution. PL16(-/-) embryos, lacking the V-ATPase 16-kDa proteolipid (c subunit), developed to the blastocyst stage and were implanted in the uterine epithelium, but died shortly thereafter. This mutant showed severe defects in development of the embryonic and extraembryonic tissues at a stage that coincided with rapid cell proliferation. When cultured in vitro, PL16(-/-) blastocysts could hatch and become attached to the surface of a culture dish, but the inner cell mass grew significantly slower and most cells failed to survive for more than 4 days. PL16(-/-) cells showed impaired endocytosis as well as organellar acidification. The Golgi complex became swollen and vacuolated, possibly due to the absence of the luminal acidic pH. These results clearly indicate that acidic compartments are essential for development after implantation.

Proteophosphoglycan secreted by Leishmania mexicana amastigotes causes vacuole formation in macrophages

The amastigote form of Leishmania mexicana parasites colonizes macrophage phagolysosomes and induces the enlargement of these compartments to form huge parasitophorous vacuoles. We report here that a purified secreted amastigote product, proteophosphoglycan, is a macromolecule which causes vacuolization of peritoneal macrophages in vitro. Secretion of this glycoconjugate by intracellular parasites may contribute to the expansion of phagolysosomal compartments in infected cells.

Inhibition of mitogen-induced DNA synthesis by bafilomycin A1 in Swiss 3T3 fibroblasts

Quiescent cells (in G0) can be stimulated to enter the cell cycle and proceed to DNA synthesis in S-phase by a wide range of growth factors and mitogens. Activation of cell-surface growth factor receptors with intrinsic protein tyrosine kinase activity initiates autophosphorylation of the receptors and subsequent activation of signal transduction cascades. After activation the receptors undergo ligand-induced internalization to endosomes, which become acidified by the action of a vacuolar H(+)-ATPase (V-ATPase). The extent to which vesicular acidification plays a role in mitogenic signalling by receptors with intrinsic tyrosine kinase activity remains unknown. Here we have shown that bafilomycin A1, a specific inhibitor of V-ATPase, inhibits endosome acidification and mitogen-induced DNA synthesis in Swiss 3T3 fibroblasts. Addition of bafilomycin A1 at successively later times during G1 progressively decreased the inhibition of DNA synthesis such that no inhibition was observed when bafilomycin A1 was added at the onset of S-phase. Bafilomycin A1 also induced a dramatic but reversible change in the morphology of Swiss 3T3 cells. However, the rapid activation of c-fos mRNA accumulation by epidermal growth factor and insulin was unaffected by bafilomycin A1. Together, the results suggest that activation of the V-ATPase plays an important role in the mitogenic signalling pathways that occur during the G1 phase of the cell cycle but is not required for the initial epidermal growth factor and insulin-evoked signalling events that lead to c-fos mRNA expression.

Heparin inhibits the accumulation of re-esterified cholesterol in macrophages loaded with acetylated low-density lipoprotein

Heparin enhances the endocytosis of low density lipoprotein (LDL) in macrophages via a formation of complex with LDL. The direct effect of heparin on the metabolism of cholesterol in macrophages has not been elucidated. We therefore evaluated the effects of heparin on the accumulation and reesterification of cholesterol in cultured macrophages. We used acetylated LDL (acetyl-LDL), which lacks an affinity for heparin. Rat peritoneal macrophages induced with thioglycollate were incubated with 100 micrograms of acetyl-LDL for 14 h. Heparin significantly inhibited the accumulation of total and esterified cholesterol but did not affect the binding of 125I-labeled acetyl-LDL to macrophages or its cellular degradation. Heparin at concentration above 5 micrograms/ml inhibited the incorporation of [3H]oleate into cholesteryl oleate in macrophages. Heparin significantly inhibited the acyl CoA:cholesterol acyl transferase (ACAT) activity of macrophages by 68%. Data suggest that heparin inhibits the accumulation and reesterification of cholesterol in macrophages loaded with acetyl-LDL. Heparin-like proteoglycans may thus protect the macrophages against the excessive accumulation of esterified cholesterol.

Bacterial endosymbiont-derived lipopolysaccharides and a protein on symbiosome membranes in newly infected amoebae and their roles in lysosome-symbiosome fusion

Experimental results are presented to support the view that symbiont-derived lipopolysaccharides are involved in the prevention of lysosome-symbiosome fusion in xD amoebae harboring bacterial endosymbionts. Monoclonal antibodies against lipopolysaccharides and a 96-kDa protein present on symbiosome membranes of amoebae were used to monitor the appearance of the membrane-specific components in newly infected amoebae with endosymbionts from xD amoebae. The lipopolysaccharides and protein appeared on the newly forming symbiosome membranes within 3 to 7 days, as detected by indirect immunofluorescence staining with monoclonal antibodies. The lysosome-symbiosome fusion was followed by double staining of two antigens with different monoclonal antibodies applied to the same amoeba. Antilipopolysaccharide monoclonal antibodies were detected by staining with a fluorescein isothiocyanate-conjugated secondary antibody, and a biotinylated anti-lysosomal protein monoclonal antibody was detected by staining with Texas Red-conjugated streptavidin. In xD amoebae injected with an antilipopolysaccharide antibody, lysosomes fused with some of the symbiosomes that did not fuse with lysosomes in noninjected cells.

Lysosomal membrane proteins of Amoeba proteus, as studied with monoclonal antibodies

Monoclonal antibodies were prepared against lysosomal membrane proteins of amoebae and used to follow lysosome-phagosome fusion after induced phagocytosis. The specificity of antibodies was checked by indirect immunofluorescence microscopy, immunoelectron microscopy, and localization of the antigen in subcellular fractions. The antibody-recognized proteins started to appear on the membranes of phagolysosomes about 5 min after phagocytosis as detected by indirect immunofluorescence, and the intensity of fluorescence increased for up to 1 h. Results of injection experiments in which purified antibodies had been injected into living cells and probed by indirect fluorescence indicated that the antigens were located on the cytoplasmic side of the lysosomal membranes. Lysosomes fuse with phagosomes on the one hand but not with non-fusible vesicles such as symbiosomes on the other. The results support the view that a membrane component(s) of non-fusible vesicles somehow prevents lysosomes from fusing with them.

Interaction of a high-affinity heparin subfraction with low-density lipoprotein stimulates cholesteryl ester accumulation in mouse macrophages

A high-affinity heparin subfraction accounting for 8% of whole heparin from bovine lung was isolated by low-density lipoprotein (LDL)-affinity chromatography. When compared to whole heparin, the high-affinity subfraction was relatively higher in molecular weight (11,000 vs. 17,000) and contained more iduronyl sulfate as hexuronic acid (76% vs. 86%), N-sulfate ester (0.75 vs. 0.96 mol/mol hexosamine), and O-sulfate ester (1.51 vs. 1.68 mol/mol hexosamine). Although both heparin preparations formed insoluble complexes with LDL quantitatively in the presence of 30 mM Ca2+, the concentrations of NaCl required for 50% reduction in maximal insoluble complex formation was markedly higher with high-affinity subfraction (0.55 M vs. 0.04 M). When compared to complex of 125I-LDL and whole heparin (H-125I-LDL), complex of 125I-LDL and high-affinity heparin subfraction (HAH-125I-LDL) produced marked increase in the degradation of lipoproteins by macrophages (7-fold vs. 1.4-fold over native LDL, after 5 h incubation) as well as cellular cholesteryl ester synthesis (16.7-fold vs. 2.2-fold over native LDL, after 18 h incubation) and content (36-fold vs. 2.7-fold over native LDL, after 48 h incubation). After a 5 h incubation, macrophages accumulated 2.3-fold more cell-associated radioactivity from HAH-125I-LDL complex than from [125I]acetyl-LDL. While unlabeled HAH-LDL complex produced a dose-dependent inhibition of the degradation of labeled complex, native unlabeled LDL did not elicit any effect even at a 20-fold excess concentration. Unlabeled particulate LDL aggregate competed for 33% of degradation of labeled complex; however, cytochalasin D, known inhibitor of phagocytosis, did not effectively inhibit the degradation of labeled complex. Unlabeled acetyl-LDL produced a partial (33%) inhibition of the degradation of labeled complex. These results indicate that (1) the interaction of high-affinity heparin subfraction with LDL leads to scavenger receptor mediated endocytosis of the lipoprotein, and stimulation of cholesteryl ester synthesis and accumulation in the macrophages; and (2) with respect to macrophage recognition and uptake, HAH-LDL complex was similar but not identical to acetyl-LDL. These observations may have implications for atherogenesis, because both mast cells and endothelial cells can synthesize heparin in the arterial wall.

Changes in lysosome shape and distribution correlated with changes in cytoplasmic pH

Lysosomes labeled by uptake of extracellular horseradish peroxidase display remarkable changes in shape and cellular distribution when cytoplasmic pH is experimentally altered. Normally, lysosomes in macrophages and fibroblasts cluster around the cell center. However, when the cytoplasmic pH is lowered to approximately pH 6.5 by applying acetate or by various other means, lysosomes promptly move outward and accumulate in tight clusters at the very edge of the cell, particularly in regions that are actively ruffling before acidification but become quiescent. This movement follows the distribution of microtubules in these cells, and does not occur if microtubules are depolymerized with nocodazole before acidification. Subsequent removal of acetate or the other stimuli to acidification results in prompt resumption of ruffling activity and return of lysosomes into a tight cluster at the cell center. This is correlated with a rebound alkalinization of the cytoplasm. Correspondingly, direct application of weak bases also causes hyperruffling and unusually complete withdrawal of lysosomes to the cell center. Thus, lysosomes appear to be acted upon by microtubule-based motors of both the anterograde (kinesin) type as well as the retrograde (dynein) type, or else they possess bidirectional motors that are reversed by changes in cytoplasmic pH. During the outward movements induced by acidification, lysosomes also appear to be smaller and more predominantly vesicular than normal, while during inward movements they appear to be more confluent and elongated than normal, often becoming even more tubular than in phorbol-treated macrophages (Phaire-Washington, L., S. C. Silverstein, and E. Wang. 1980. J. Cell Biol. 86:641-655). These size and shape changes suggest that cytoplasmic pH also affects the fusion/fission properties of lysosomes. Combined with pH effects on their movement, the net result during recovery from acidification is a stretching of lysosomes into tubular forms along microtubules.

Modulation of macrophage lysosomal pH by Mycobacterium tuberculosis-derived proteins

Macrophage lysosomal pH was significantly (greater than 1 pH unit) increased in a reversible, concentration-dependent manner characterized by a saturable and cyclic kinetics after exposure to culture filtrate protein extract derived from Mycobacterium tuberculosis. Lysosomal alkalinization peaked 30 min after administration of culture filtrate protein extract to cells of the macrophagelike cell line J774A.1. The alkalinization was reversible, and a second peak was observed approximately 60 min after incubation. Maximum lysosomal alkalinization increased as a function of culture filtrate protein extract concentration, reaching an apparent saturation level around 700 to 1,000 micrograms/ml, although the time course for this process was not significantly dependent on antigen concentration. The alkalinizing agent(s) was heat labile and produced a similar effect in cells which had a different lysosomal enzyme composition. Our observations are consistent with the presence of one or more mycobacterial antigens which have a pH-dependent affinity for lysosomal structures essential for lysosomal acidification and which are able to inhibit this lysosomal acidification.

Gamma interferon reverses inhibition of leukocyte bactericidal activity by a 25-kilodalton fraction from Mycobacterium tuberculosis

In this study we examined the effects of Mycobacterium tuberculosis cell extracts on the phagocytic activity of polymorphonuclear leukocytes and cultured peripheral blood monocytes. M. tuberculosis cell extracts were fractionated on Sephacryl S-200 columns, and a 25-kilodalton glycolipoprotein was shown to inhibit the intracellular killing ability of these leukocytes but had no effect on their phagocytic potential. This same fraction inhibited fusion of phagosomes with lysosomes, as assessed by noting the transfer of acridine orange from lysosomes to phagosomes. This fraction was shown to have a maximal inhibitory effect when it was in the form of an intact carbohydrate-lipid-protein complex. Gamma interferon (IFN-gamma), but not IFN-alpha, reversed the inhibitory effect of the mycobacterial component on bactericidal activity and on fusion of phagosomes and lysosomes. Thus, this 25-kilodalton fraction of M. tuberculosis cell extract may be important in protecting organisms against phagocytic degradation, an effect which can be reversed by IFN-gamma.

Inhibition of phagosome-lysosome fusion in macrophages by certain mycobacteria can be explained by inhibition of lysosomal movements observed after phagocytosis

We have investigated the mechanism of the inhibition of phagosome-lysosome (P-L) fusion in macrophages known to occur after infection by Mycobacterium tuberculosis and by the mouse pathogen Mycobacterium microti. We have used an M. microti infection and have studied, first, the saltatory movements of periphagosomal secondary lysosomes by means of visual phase-contrast microscopy (a similar use of the method having been previously supported by computer analyses). The movements became slow or static after ingestion of live but not of heat-killed M. microti. They were unaffected by a fusiogenic mycobacterium M. lepraemurium. Second, we studied the behavior of a normally fusiogenic unrelated organism, Saccharomyces cerevisiae, after its phagocytosis by cells already containing live M. microti ingested 18 h previously. We observed, using a fluorescent assay of fusion, that many of these yeast phagosomes now also failed to fuse with the lysosomes; in contrast, when the host M. microti had been heat killed the yeast phagosomes fused normally. These observations were extended by ultrastructural quantitative analyses of P-L fusion, which confirmed the nonfusion of phagosomes of live M. microti and, more particularly, the change to nonfusion from the normal fusion behavior of the separate phagosomes of accompanying yeasts. Third, we have assembled evidence against the likelihood that these M. microti-induced phenomena are nonspecific, i.e., secondary to a general depression of activity of heavily infected host cells. The evidence includes the feasibility of adjusting the degree of infection so as to facilitate visual assessment of organelle movements without the presence of detectable damage to the cells studied; the absence of lysosomal stasis after comparable infection with another mycobacterium of comparable virulence (M. lepraemurium); and the reversibility of the stasis. We conclude that inhibition of lysosome saltatory movements (and consequently its secondary effect on the associated yeasts) is a significant, specifically induced phenomenon. From these observations and considerations, therefore, in conjunction with the analogous inhibition of lysosomal movements in normal macrophages by some chemical inhibitors of P-L fusion, and our suggestion that this association is causally related, we now suggest that M. microti-induced focal lysosomal stasis is also the main means by which the inhibition of P-L fusion is brought about by this organism. This concept is strengthened by the observations on S. cerevisiae, which provide strong evidence that stasis can cause suppression of fusion.

The effect of proteoglycans, collagen and lysyl oxidase on the metabolism of low density lipoprotein by macrophages

To study the interactions of lipoproteins, connective tissue components and cells, mouse peritoneal macrophages were incubated in the presence of human low density lipoproteins (LDL) that had been complexed with pig aortic proteoglycans (PG) or incubated in the presence of soluble collagen and/or lysyl oxidase, which catalyses the formation of cross-linkages in collagen and elastin by oxidising epsilon-amino groups of lysine residues to aldehydes. Soluble and insoluble PG-LDL complexes increased the incorporation of [3H]oleate into cellular cholesteryl esters (CE) 1.6- and 2.8-fold, respectively, while LDL incubated with collagen and lysyl oxidase had no effect compared to control LDL. As judged on the basis of incubations with fucoidin, spermine and 125I-labelled lipoproteins, the mechanism of internalisation of the PG-LDL complexes is different from that of acetylated LDL or dextran sulphate-LDL complexes. The formation of PG-LDL complexes in the arterial intima may lead to an increased uptake of lipoproteins by intimal macrophages during the early phase of atherogenesis.

Movements and other distinguishing features of small vesicles identified by darkfield microscopy in living macrophages

Perinuclear vesicles (estimated diameter less than 0.15 micron), too small to be seen in living mouse macrophages by direct phase-contrast microscopy, could be detected by darkfield microscopy thanks to their rapid non-saltatory movements at 37 degrees C, contrasting with the slower saltations of accompanying phase-visible larger vesicles (0.25-0.5 micron, presumed secondary lysosomes). The movements of these 'small visicles' also differed from those of the 'larger visicles' in their responses to changes in temperature, and to chemical agents known to inhibit both the saltations of secondary lysosomes and the latter's fusion with phagosomes. Thus the 'larger vesicles' stopped moving at 25 degrees C, the small ones did not; both stopped at 18 degrees C. The 'small vesicles' continued to move actively after cell uptake of the polyanion poly-D-glutamic acid, while the saltations of the 'larger vesicles' were markedly slowed; both sets of vesicles stopped after uptake of ammonium chloride. Degranulation of the small vesicles paralleled that of the larger, while simultaneously observed preformed pinosomes (labelled with fluorescent wheat germ agglutinin (WGA) appeared to be unaffected. On the basis also of refractivity, location and speed the 'small vesicles' are considered not to be pinosomes, but probably to be lysosomes. The question of whether they are a subgroup of small immature secondary lysosomes or primary lysosomes (0.05-0.08 micron) is discussed. The broad spectrum of movement inhibited by ammonia in macrophages raises the possibility that this weak base inhibits movements of all lysosomes. Further characterization of these 'small vesicles' requires their relation to be defined to the small particles in other cell types (especially in axoplasm) which have been detected by video-enhanced microscopy.

Lipoprotein-heparin-fibronectin-denatured collagen complexes enhance cholesteryl ester accumulation in macrophages

The sequestration of low-density lipoprotein (LDL) by components of the vascular extracellular matrix has long been recognized as a contributing factor to lipid accumulation during atherogenesis. The effects, however, that components of the extracellular matrix might have on LDL catabolism by scavenger cells have been little investigated. For these purposes we have prepared insoluble complexes of LDL, heparin, fibronectin, and denatured collagen (gelatin) and examined their effects on lipid accumulation, LDL uptake and degradation, and cholesteryl ester synthesis in mouse peritoneal macrophages. The results of these experiments have demonstrated that the cholesteryl ester content of macrophages incubated with a particular suspension of LDL, heparin, fibronectin, and collagen complexes is four- to fivefold that of cells incubated with LDL alone. The uptake of complexes containing 125I-LDL is rapid; however, in contrast to either endocytosed 125I-LDL or 125I-acetyl LDL, the degradation of complex-derived LDL is impaired. In addition, the uptake of complex-derived LDL stimulates the incorporation of [14C]oleic acid into cholesteryl oleate, however, the stimulation was a small fraction of that observed in cells incubated with acetyl LDL. Ultrastructurally, macrophages incubated with LDL, heparin, fibronectin, and collagen complexes did not contain many lipid droplets, but rather their cytoplasm is filled with phagosomes containing material similar in appearance to LDL-matrix complexes. These results indicate that components of the extracellular matrix can alter the catabolism of LDL by scavenger cells, suggesting that they may play a role in cellular lipid accumulation in the atherosclerotic lesion.

Effects of exogenous amines on mammalian cells, with particular reference to membrane flow

We have reviewed the evidence that amines accumulate in intracellular vesicles of low pH, such as lysosomes and endosomes. There is consequent elevation of intravesicular pH, and inhibition of receptor-ligand dissociation often results from this pH change. We have argued that the capacity for fusion of such vesicles is also reduced by the high pH. We suggest that the variety of effects of amines on membrane flow and macromolecular transport we describe are at least partly due to such reduced fusion (Figs. 1 and 2). We propose that an internal low pH may facilitate heterologous vesicle-vesicle and vesicle-plasma membrane fusion. There is some evidence that clathrin can accelerate phospholipid vesicle fusion in vitro at low pH (Blumenthal et al., 1983) but no direct evidence on the role of intravesicular pH. This idea is consistent not only with the preceding discussion, but also with the fact that the intracellular membrane-bound compartments least involved in fusion events (e.g. mitochondria) are of neutral or alkaline internal pH. Membrane fusion is certainly required for the formation of vesicles at the periphery of the Golgi apparatus, and possibly earlier in the transport and processing of biosynthetic products in the Golgi (Bergeron et al., 1982). Thus the accumulation of amines in the Golgi may be responsible for several effects on the flow of macromolecules along their translocation pathways. The status of the plasma membrane in this view is complex. It might be argued that the pH dictating the fusion step in endocytosis is that of the extracellular fluid, in which case the inhibitory effects of amines on this process are not explained. However, the rapidity of acidification of the newly formed endocytic vesicles allows the possibility that plasma membrane invaginations might temporarily sequester areas which are of lower pH than that of the bulk extracellular fluid even before fusion, since the proton pumping enzyme(s) are probably present on the plasma membrane. Were this the case, then an acid pH could again be a factor determining membrane fusion at the plasma membrane. The inhibition of endocytosis by weak bases thus may again reflect elevation of pH in a sequestered compartment. From the data on the dependence of response on the concentration of amines, we anticipate that most responses involving membrane flow will be biphasic, with inhibitory effects at low amine concentration, giving way to stimulatory ones at higher concentrations. We suggest that the reported dichotomy between different amines in intracellular membrane fusion systems (D'Arcy Hart, 1982) may result from this concentration dependence.(ABSTRACT TRUNCATED AT 400 WORDS)

pH in the endosome. Measurements during pinocytosis and receptor-mediated endocytosis

By fluorescence spectroscopy, the average pH within endocytic compartments was determined during endocytosis of fluorescein conjugates by macrophages and hepatocytes. In mouse macrophages and hepatocytes fluorescein conjugates taken up either in the fluid phase or by binding to cell surface receptors were rapidly transferred to an acidic compartment (pH 5-5.5). The half-time for this process was generally less than 4 min. The pH within yeast-containing phagosomes was also rapidly reduced to similar levels, following a unique and transient increase. In each case, the acid endosomal compartments involved probably do not contain lysosomal enzymes. When fluorescein conjugates of asialoglycoproteins were internalised by hepatocytes at 20 degrees C, no proteolysis occurred within the acidic endosome until the temperature was raised. Fluorescein conjugates of concanavalin A (conA) and polylysine were relatively more slowly internalised by macrophages. The half-times for uptake, estimated by fluorescence change, were comparable with the turnover time for bulk plasma membrane. The relatively high average pH experienced by these conjugates indicated that a small proportion of these non-specific cell-surface labels was always in contact with the extracellular medium.

Acidification of phagosomes is initiated before lysosomal enzyme activity is detected

We have measured changes of pH in a protein's microenvironment consequent on its binding to the cell surface and incorporation into pinosomes. Changes of pH were measured from single, living cells and selected regions of cells by the fluorescence ratio technique using a photon-counting microspectrofluorimeter. The chemotactic agent and pinocytosis inducer, ribonuclease, labeled with fluorescein (FTC-RNase), adsorbed to the surface of Amoeba proteus, and was pinocytosed by cells in culture media at pH 7.0. The FTC-RNase entered an apparently acidic microenvironment, pH approximately 6.1, upon binding to the surface of amoebae. Once enclosed within pinosomes, this protein's microenvironment became steadily more acidic, reaching a minimum of pH approximately 5.6 in less than 10 min. FTC-RNase pinocytosed by the giant amoeba, Chaos carolinensis, entered pinosomes whose pH was correlated with their cytoplasmic location during the initial 30-40 min after pinocytosis. The majority of pinosomes containing FTC-RNase clustered in the tail ectoplasm of C. carolinensis during this interval and had a pH of approximately 6.5; those released into endoplasm and carried into the tip of cells had a pH below 5.0. As pinosomes became distributed at random in C. carolinensis (1-2 h after initial pinocytosis), differences in pH between tip and tail pinosomes vanished. We have also measured the pH within single phagosomes of A. proteus. Phagosomal pH dropped steadily to approximately 5.4 within 5 min after particle ingestion in 70% of the cells measured, and reached this level of acidity within 10 min in 90% of the cells measured. By contrast, stain for the lysosomal enzyme, acid phosphatase, was evident within only 20% of 5-min-old phagosomes visualized by light microscopy, and within only 40% of 10-min-old phagosomes. A microfluorimetric assay was used to simultaneously record changes in pH, and the initial deposition of lysosomal esterases, within phagosomes of single, living Amoeba proteus. Near complete acidification of the phagosome was recorded from some cells before phagosomal fusion was evident by this microfluorimetric assay. From other cells, however, continued acidification of phagosomes was recorded after lysosomal fusion was initiated. We conclude that acidification of phagosomes by A. proteus is initiated but not necessarily completed prior to phagosome-lysosome formation, and that the two events are closely linked in time. Initial acidification of endosomes is a property intrinsic to the plasma membrane which envelops particles at the cell surface, rather than the result of lysosomal fusion with phagosomes.

Chemical inhibitors of phagosome-lysosome fusion in cultured macrophages also inhibit saltatory lysosomal movements. A combined microscopic and computer study

The effects on lysosomal movements produced by the weak base ammonium chloride and by a representative polyanion poly-D-glutamic acid (PGA), previously reported to inhibit phagosome-lysosome (P-L) fusion, have been studied in cultured mouse macrophages using direct visual phase-contrast microscopy, a previously described (1, 3, 7) fluorescence assay of fusion, and computer analysis techniques. Treatment of the macrophages with 5-10 mM NH4Cl for 0.5-2 h or with 100 micrograms PGA/ml for 5 d caused a striking inhibition of saltatory lysosomal movements, as well as the expected inhibition of P-L fusion. Two other anionic fusion inhibitors tested, dextran sulphate and suramin, inhibited movements similarly. Removal of the NH4Cl from the cell medium reversed the lysosomal stasis and restored P-L fusion. Computer analyses of changes in lysosomal positions in treated and untreated macrophages during 2, 10, and 30-s intervals, using data from photomicrographs, computer graphics, and quantitative nearest-neighbour techniques developed for this purpose, supported the qualitative visual observation of the inhibition of lysosomal movements by the fusion inhibitors NH4Cl and PGA. Over the chosen intervals, from 80 to 96% of the lysosomes could be paired within 1 micron of each other in the NH4Cl- and PGA-treated cells in comparison with 50-70% in normal cells. The differences between the drug-treated and normal cells were highly significant. In an analogous system, the lysosomal stasis induced by hypertonic sucrose was examined and it was observed that P-L fusion too was inhibited. Both effects were reversible. We conclude that inhibition of P-L fusion and of lysosomal movement are associated. We suggest a causal relationship between these changes, namely, that the lysosomotropic inhibitors of fusion under study produce their effects largely, though perhaps not exclusively, by reducing saltatory lysosomal motion and consequently periphagosomal assembly, rather than directly and independently on P-L contact or on the fusion process itself. The possibility is raised that microtubules may be involved in the effector mechanism of these modulations.

The mode of action of suramin on the filarial worm Brugia pahangi

The mode of action of suramin upon Brugia pahangi has been investigated in vivo and in vitro. The drug was without effect on the glycolytic activity of worms in vitro at 2 X 10(-4) M. The lack of effect was correlated with the failure of [14C]suramin to penetrate the worms in vitro. Suramin bound to the surface of worms in vitro presumably by virtue of its polyanionic nature. B. pahangi adults ingested [14C]suramin in vivo but no reduction in the rate of lactate production, of glucose utilization or in the rates of uptake of [14C]glucose, [14C]leucine or [14C]adenosine was observed in worms recovered from jirds between weeks 1 and 5 following 4 daily doses of suramin at 50 mg/kg given intraperitoneally. Worm death occurred between weeks 5 and 7 but this delayed drug effect was not the result of a progressive accumulation of suramin in the worms. Ultrastructural changes were observed in the intestinal epithelium of worms from suramin-treated jirds and parallel observations on worms exposed to Trypan blue in vivo suggest that both polyanionic compounds are restricted to the intestinal lumen of the worms. The evidence presented is consistent with the concept that, in B. pahangi, suramin acts at the surface of the intestinal epithelium and not by primarily inhibiting glucose catabolism or inhibiting phagosome and lysosome fusion as previously demonstrated for bloodstream trypanosomes and mammalian macrophages, respectively.

Phagosome-lysosome fusion inhibited by algal symbionts of Hydra viridis

Certain species of Chlorella live within the digestive cells of the fresh water cnidarian Hydra viridis. When introduced into the hydra gut, these symbiotic algae are phagocytized by digestive cells but avoid host digestion and persist at relatively constant numbers within host cells. In contrast, heat-killed symbionts are rapidly degraded after phagocytosis. Live symbionts appear to persist because host lysosomes fail to fuse with phagosomes containing live symbionts. Neither acid phosphatase nor ferritin was delivered via lysosomes into phagosomes containing live symbionts, whereas these lysosomal markers were found in 50% of the vacuoles containing heat-killed symbionts 1 h after phagocytosis. Treatment of symbiotic algae before phagocytosis with polycationic polypeptides abolishes algal persistence and perturbs the ability of these algae to control the release of photosynthate in vitro. Similarly, inhibition of photosynthesis and hence of the release of photosynthetic products as a result of prolonged darkness and 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) treatment also abolishes persistence. Symbiotic algae are not only protected from host digestive attack but are also selectively transported within host cells, moving from the apical site of phagocytosis to a basal position of permanent residence. This process too is disrupted by polycationic polypeptides, DCMU and darkness. Both algal persistence and transport may, therefore, be a function of the release of products from living, photosynthesizing symbionts. Vinblastine treatment of host animals blocked the movement of algae within host cells but did not perturb algal persistence: algal persistence and the transport of algae may be initiated by the same signal, but they are not interdependent processes.

Intralysosomal accumulation of polyanions. I. Fusion of pinocytic and phagocytic vacuoles with secondary lysosomes

The long-term exposure of macrophages to low concentrations of a number of polyanions leads to their accumulation in high concentration within secondary lysosomes. This was associated with enlargement of the lysosomes, the presence of membranous whorls, and intense toluidine blue staining of the organelles at pH 1.0. After the ingestion of a particulate load by these cells, newly formed phagocytic vacuoles failed to fuse with polyanion-laden lysosomes. The lack of fusion was evident in both fluorescence and electron micrographic studies which followed the transfer of acridine orange or Thorotrast from 2 degrees lysosomes to phagosomes. Agents that inhibited phagosome-lysosome (P-L) fusion included molecules containing high densities of sulfate, sulfonate, or carboxylate residues. Dextran sulfate (DS) in microgram/ml quantities was an excellent inhibitor, whereas nonsulfated dextran (D) was without effect at 1,000-fold higher concentrations. In contrast to their effects on P-L fusion, polyanions failed to influence the fusion of pinocytic vesicles with 2 degrees lysosomes. The uptake, intravacuolar distribution, and intralysosomal digestion of fluid-phase pinocytic markers were unaltered in lysosomes containing either D or DS. Furthermore, subcellular fractionation studies showed that the fluid-phase pinocytic marker HRP was efficiently transferred from pinosomes to large, dense 2 degrees lysosomes containing DS.

Poly(L-lysine) has different membrane transport and drug-carrier properties when complexed with heparin

Methotrexate (MTX) conjugated to a Mr 3000 poly(L-Lys) markedly inhibits the growth of Pro-3 MtxRII5-3 Chinese hamster ovarian cells, a mutant line known to be drug resistant because of defective MTX transport. In these cells, membrane transport of [3H]MTX-poly(Lys) is sharply decreased by addition of 0.5- to 2.5-fold heparin but remains at 15-20% of control in 2.5- to 50-fold heparin excess. Heparin addition at first markedly inhibits but, at high concentration, restores the growth inhibitory effect of MTX-poly(Lys). In excess heparin, MTX-poly(Lys) is transported as a heparin complex. Because reduced transport (15-20%) is sufficient to cause a 90% inhibition of cell growth, MTX-poly(Lys) apparently gains pharmacologic potency when compared to heparin. This gain can be related to a greater inhibitory effect on dihydrofolate reductase and to a different mode of transport. The inhibitory effect of MTX-poly(Lys) on dihydrofolate reductase in vitro is increased nearly 100-fold in the presence of excess heparin but remains less than that of free MTX. Unlike that of MTX-poly(Lys), the transport of MTX-poly(Lys)-heparin has the characteristics and efficiency of a receptor-mediated process. It proceeds by endocytosis but is not, as in the case of uncomplexed conjugate, followed by the intracellular generation of pharmacologically active breakdown products that would account for cytotoxicity. These observations raise the possibility that at least part of the MTX-poly(Lys)-heparin reaches cellular dihydrofolate reductase in the form of macromolecular complexes that escape from entrapment in endocytotic structures. Our data illustrate a way to overcome drug resistance by taking advantage of the specific uptake of a macromolecular drug carrier. They offer a method of drug delivery in which heparin improves selectivity and decreases the unwanted toxicity inherent to polycationic carriers.

Histological and ultrastructural studies of the interaction of Toxoplasma gondii Tachyzoites with mouse omentum in experimental infection

Mouse omentum was studied after intraperitoneal challenge with tachyzoites of Toxoplasma gondii. Parasites inhabit omental histiocytes, fibroblasts, mesothelial cells, and free peritoneal macrophages. Recently infected cells showed enhanced metabolic and functional activity. Villous projections of the parasitophorous vacuole wall appeared, usually opposite the anterior pole of the parasite. In mesothelial cells, projections formed terminal swellings not observed in other infected cells. Activation of host cells was followed by reduction of the density of the cytoplasmic matrix, autophagosome formation, and intracellular edema, indicating the damage. The wall of the parasitophorous vacuole loses the supporting host cell endoplasmic reticulum that was attached to the vacuole just after entrance of the parasite into the cell. Then lysis of the parasitophorous vacuole and complete cell destruction occurs. The growth of parasites in undamaged cells does not coincide with the inflammatory response. Inflammation of the peritoneum develops only after the start of mass destruction of infected cells. Thus tachyzoites of Toxoplasma exert significant pathogenic effects by their ability to activate the host cell, causing lysis of the parasitophorous vacuole and subsequent destruction of the entire cell.

Temporal changes of lysosome and phagosome pH during phagolysosome formation in macrophages: studies by fluorescence spectroscopy

Intravascular pH was measured within the lysosomes and newly formed phagosomes in cultured mouse peritoneal macrophages. The kinetics of pH change in both vacuolar systems was quantitatively determined within a large cell population by fluorescence spectroscopy. Additionally, pH changes within individual phagosomes were followed semiquantitatively using indicator dyes. Two novel findings were made. Firstly, the pH in new phagosomes was transiently driven alkaline (higher than physiological) even when the external medium was buffered at pH 6.5. Secondly, perturbations of phagosome-lysosome fusion had little effect upon phagosomal pH changes, even though the compounds used markedly altered the pH of the lysosomes in resting and phagocytosing cells.

Ammonia inhibits phagosome-lysosome fusion in macrophages

When foreign bodies, including many microorganisms, are ingested by cultured macrophages, they become enclosed in phagosomes, with which lysosomes usually fuse and then discharge their enzymes and other contents into the resulting phagolysosomes. Such fusion is, however, diminished or absent after the phagocytosis of some pathogens, notably Mycobacterium tuberculosis and Toxoplasma gondii. Assuming that the nonfusion is due to active inhibition by the intrapoagosomal microbe, identification of an inhibitor should clarify the lysosomal control mechanism. It has been suggested that strongly acidic sulphatides present in virulent tuberculosis, which, like other substances with polyanionic structural features, can themselves block phagosome-lysosome fusion (P-LF), may contribute to the negative lysosome response to ingested tubercle bacilli. We report here another possibility, based on inhibition of fusion of yeast-containing phagosomes by filtrates from cultures of tubercle bacilli on traditional-type defined media; we show that the ammonia content of such filtrates is sufficient to account for their effect. This inhibition of fusion seems to be an hitherto unrecognized intracellular consequence of added ammonia, in striking contrast to the enhancement produced by some lipophilic amines.