Immunoelectron microscopic localization of acidic intracellular compartments in hepatoma cells

Albumin-based nanoparticles as contrast medium for MRI: vascular imaging, tissue and cell interactions, and pharmacokinetics of second-generation nanoparticles

This multidisciplinary study examined the pharmacokinetics of nanoparticles based on albumin-DTPA-gadolinium chelates, testing the hypothesis that these nanoparticles create a stronger vessel signal than conventional gadolinium-based contrast agents and exploring if they are safe for clinical use. Nanoparticles based on human serum albumin, bearing gadolinium and designed for use in magnetic resonance imaging, were used to generate magnet resonance images (MRI) of the vascular system in rats ("blood pool imaging"). At the low nanoparticle doses used for radionuclide imaging, nanoparticle-associated metals were cleared from the blood into the liver during the first 4 h after nanoparticle application. At the higher doses required for MRI, the liver became saturated and kidney and spleen acted as additional sinks for the metals, and accounted for most processing of the nanoparticles. The multiple components of the nanoparticles were cleared independently of one another. Albumin was detected in liver, spleen, and kidneys for up to 2 days after intravenous injection. Gadolinium was retained in the liver, kidneys, and spleen in significant concentrations for much longer. Gadolinium was present as significant fractions of initial dose for longer than 2 weeks after application, and gadolinium clearance was only complete after 6 weeks. Our analysis could not account quantitatively for the full dose of gadolinium that was applied, but numerous organs were found to contain gadolinium in the collagen of their connective tissues. Multiple lines of evidence indicated intracellular processing opening the DTPA chelates and leading to gadolinium long-term storage, in particular inside lysosomes. Turnover of the stored gadolinium was found to occur in soluble form in the kidneys, the liver, and the colon for up to 3 weeks after application. Gadolinium overload poses a significant hazard due to the high toxicity of free gadolinium ions. We discuss the relevance of our findings to gadolinium-deposition diseases.

Protein Production and Secretion in Exocrine Cells

Acinar cells of exocrine glands are highly specialized for producing, storing, and discharging secretory proteins for use on surfaces that represent interfaces between the organism and the surrounding environment. These functions are achieved through the secretory pathway that includes a series of functionally distinct intracellular compartments — the endoplasmic reticulum, subcompartments of the Golgi complex, and the secretion granule in which exportable macromolecules are stored at high concentrations. Most secretion occurs by granule exocytosis in response to external hormonal or neural stimuli. Although these processes have been traced in a variety of morphological and biochemical studies, very Utile is known about the mechanisms involved in facilitating and maintaining secretory storage, orchestrating discharge at the apical cell surface, and in ensuring conservation and re-internalization of the granule membrane. Recent studies initiated on cell fractions obtained from the rat parotid gland have provided significant insight into the protein storage conditions that prevail in the granule interior and the components of the granule membrane that are likely to be involved in general secretory function such as exocytosis.

Acidification and protein traffic

Acidification of some organelles, including the Golgi complex, lysosomes, secretory granules, and synaptic vesicles, is important for many of their biochemical functions. In addition, acidic pH in some compartments is also required for the efficient sorting and trafficking of proteins and lipids along the biosynthetic and endocytic pathways. Despite considerable study, however, our understanding of how pH modulates membrane traffic remains limited. In large part, this is due to the diversity of methods to perturb and monitor pH, as well as to the difficulties in isolating individual transport steps within the complex pathways of membrane traffic. This review summarizes old and recent evidence for the role of acidification at various steps of biosynthetic and endocytic transport in mammalian cells. We describe the mechanisms by which organelle pH is regulated and maintained, as well as how organelle pH is monitored and quantitated. General principles that emerge from these studies as well as future directions of interest are discussed.

Mechanisms of pH regulation in the regulated secretory pathway

A precise pH gradient between organelles of the regulated secretory pathway is required for sorting and processing of prohormones. We studied pH regulation in live endocrine cells by targeting biotin-based pH indicators to cellular organelles expressing avidin-chimera proteins. In AtT-20 cells, we found that steady-state pH decreased from the endoplasmic reticulum (ER) (pH(ER) = 7.4 +/- 0.2, mean +/- S.D.) to Golgi (pH(G) = 6.2 +/- 0.4) to mature secretory granules (MSGs) (pH(MSG) = 5.5 +/- 0.4). Golgi and MSGs required active H(+) v-ATPases for acidification. ER, Golgi, and MSG steady-state pH values were also dependent upon the different H(+) leak rates across each membrane. However, neither steady-state pH(MSG) nor rates of passive H(+) leak were affected by Cl(-)-free solutions or valinomycin, indicating that MSG membrane potential was small and not a determinant of pH(MSG). Therefore, our data do not support earlier suggestions that organelle acidification is primarily regulated by Cl(-) conductances. Measurements of H(+) leak rates, buffer capacities, and estimates of surface areas and volumes of these organelles were applied to a mathematical model to determine the H(+) permeability (P(H+)) of each organelle membrane. We found that P(H+) decreased progressively from ER to Golgi to MSGs, and proper acidification of Golgi and MSGs required gradual decreases in P(H+) and successive increases in the active H(+) pump density.

Primaquine interferes with membrane recycling from endosomes to the plasma membrane through a direct interaction with endosomes which does not involve neutralisation of endosomal pH nor osmotic swelling of endosomes

The anti-malaria drug primaquine is a weak base which accumulates in endosomes in a protonated form and consequently neutralises the endosomal pH. Bafilomycin A1 prevents endosome acidification by inhibiting the vacuolar proton pump. Although both agents neutralise the endosomal pH, only primaquine has a strong inhibitory effect on recycling of endocytosed proteins to the plasma membrane (Van Weert et al. (1995), J. Cell Biol. 130, 821-834). This suggests that primaquine interferes with a parameter, other than endosomal pH, that is essential for membrane recycling. In the presence of 0.3 mM primaquine, endocytosed transferrin-receptors accumulated intracellularly, but not in the additional presence of bafilomycin A1. Thus, at relative low concentrations proton pump-driven accumulation of primaquine in endosomes was required to inhibit membrane recycling, suggesting that the target of primaquine is associated with endosomes. The inhibitory effect of 1 mM primaquine on transferrin receptor recycling was not reversed by the additional presence of bafilomycin A1, indicating that osmotic swelling of endosomes due to accumulation of protonated primaquine could also not explain its effect. To study endosome swelling morphologically, we introduce a novel technique for fluorescent labelling of endosomes involving HRP-catalysed biotinylation. In the presence of 0.2 mM primaquine endosomal vacuoles with diameters up to 2 microm were observed. Endosome swelling was not observed when in addition to primaquine also bafilomycin A1 was present, supporting the notion that vacuolar proton pump activity lowers the dose response for primaquine. Factors that are crucial for membrane recycling and may be affected by primaquine are discussed.

Retrograde transport from the pre-Golgi intermediate compartment and the Golgi complex is affected by the vacuolar H+-ATPase inhibitor bafilomycin A1

The effect of the vacuolar H+-ATPase inhibitor bafilomycin A1 (Baf A1) on the localization of pre-Golgi intermediate compartment (IC) and Golgi marker proteins was used to study the role of acidification in the function of early secretory compartments. Baf A1 inhibited both brefeldin A- and nocodazole-induced retrograde transport of Golgi proteins to the endoplasmic reticulum (ER), whereas anterograde ER-to-Golgi transport remained largely unaffected. Furthermore, p58/ERGIC-53, which normally cycles between the ER, IC, and cis-Golgi, was arrested in pre-Golgi tubules and vacuoles, and the number of p58-positive approximately 80-nm Golgi (coatomer protein I) vesicles was reduced, suggesting that the drug inhibits the retrieval of the protein from post-ER compartments. In parallel, redistribution of beta-coatomer protein from the Golgi to peripheral pre-Golgi structures took place. The small GTPase rab1p was detected in short pre-Golgi tubules in control cells and was efficiently recruited to the tubules accumulating in the presence of Baf A1. In contrast, these tubules showed no enrichment of newly synthesized, anterogradely transported proteins, indicating that they participate in retrograde transport. These results suggest that the pre-Golgi structures contain an active H+-ATPase that regulates retrograde transport at the ER-Golgi boundary. Interestingly, although Baf A1 had distinct effects on peripheral pre-Golgi structures, only more central, p58-containing elements accumulated detectable amounts of 3-(2, 4-dinitroanilino)-3'-amino-N-methyldipropylamine (DAMP), a marker for acidic compartments, raising the possibility that the lumenal pH of the pre-Golgi structures gradually changes in parallel with their translocation to the Golgi region.

Secretion and pH-dependent self-processing of the pro-form of the Yarrowia lipolytica acid extracellular protease

The secretion and maturation of the acid extracellular protease (AXP) of the yeast Yarrowia lipolytica have been characterized using antiserum raised against this enzyme. A 42 kDa pro-enzyme form of AXP was identified from lysates of radiolabelled Y. lipolytica cells and found to contain no N-linked carbohydrate moieties. Using pulse-chase immune precipitation it was demonstrated that the AXP precursor was secreted into the extracellular medium where, under conditions of low pH, it underwent autocatalytic activation forming the mature enzyme. Conversion of the AXP pro-form in the presence of the protease inhibitor pepstatin indicated that an intramolecularly-catalysed reaction mechanism was involved in AXP maturation. Further evidence supporting the role of autocatalytic processing came from the side-chain specificity of mature AXP towards the oxidized B-chain of insulin.

Hsp47 binds to the KDEL receptor and cell surface expression is modulated by cytoplasmic and endosomal pH

Hsp47 is a novel glycoprotein that binds specifically to procollagen and is retained in the ER by its COOH-terminus RDEL peptide sequence (Satoh, M. et al. Jol. Cell Biol. 1996; 133: 469-83). In this paper, we report that erd2P, the KDEL receptor, is distributed, coprecipitates with, and binds to Hsp47. Also, under stress conditions and lowering of pHi, the cytoplasmic epitope of erd2P is not recognized by erd2P antibodies unless the cells are pretreated with NEM. Coincident with the masking of the cytoplasmic epitope of erd2P, following lowering of pHi, Hsp47 is not retained but eludes its retention receptor to be expressed on the cell surface. Alkalization of the endosomal compartments by treatment with NH4Cl or chloroquine also results in the loss of Hsp47 to the cell surface, presumably by inhibiting the retrieval of trans-Golgi network proteins from the cell surface. The expression of Hsp47 on the cell surface under conditions of stress and alteration of pHi and pHe posture Hsp47 as a serpin family protein that may modulate cell migration during development and invasion and metastasis in cancer.

Tissue distribution and metabolism of the [32P]-labeled oligodeoxynucleoside methylphosphonate-neoglycopeptide conjugate, [YEE(ah-GalNAc)3]-SMCC-AET-pUmpT7, in the mouse

Development of oligodeoxynucleotides (oligo-dNs) and their analogs as therapeutic agents is complicated by their low rate of transport across cellular membranes, which is required for interaction with the intracellular complementary nucleic acid sequences, and the lack of tissue-specific delivery. To overcome these obstacles, bioconjugates between cell surface receptor ligands and oligodeoxynucleoside methylphosphonates (oligo-MPs) have been constructed containing homogeneous, chemically defined covalent linkages. We have previously established that a model conjugate, [32P]-labeled [YEE(ah-GalNAc)3]-SMCC-AET-pUmpT7 (1), is delivered to Hep G2 cells in a ligand-specific manner, reaching a peak value of 26 pmol per 10(6) cells after 24 hours incubation at 37 degrees C (Hangeland et al., 1995). In this work, the in vivo behavior of this conjugate is explored. Administration of this conjugate to mice via tail vein injection demonstrates rapid uptake in liver to the extent of 69.9 +/- 9.9% of the injected dose after 15 minutes. Thereafter, the conjugate and its metabolites are rapidly cleared via the kidney and urine. Polyacrylamide gel electrophoresis analysis of extracts of Hep G2 cells and mouse liver reveal the conjugate 1 to be extensively metabolized. In contrast, the conjugate found in mouse urine is largely intact. These data show that this novel, biodegradable delivery vehicle represents a viable approach for the delivery of antisense oligo-MPs and other oligo-dN analogs to the liver for therapeutic and diagnostic applications.

Degradation of apolipoprotein B in cultured rat hepatocytes occurs in a post-endoplasmic reticulum compartment

The site of apolipoprotein B (apoB) degradation was investigated in cultured rat hepatocytes. Brefeldin A plus nocodazole completely blocked apoB degradation suggesting the involvement of a post-endoplasmic reticulum (ER) compartment. Monensin inhibited apoB degradation by 40% implying that a post-Golgi compartment could be involved in degradation of apoB. Ammonium chloride or chloroquine inhibited partially the degradation of apoB100 and apoB48, indicating some degradation in lysosomes, or in an acidic compartment such as trans-Golgi or endosomes. The degradations of apoB100 and apoB48 were blocked completely by (2S,3S)-trans-epoxysuccinyl-L-leucylamido-3-methylbutane ethyl ester (EST) during a chase of 90 min demonstrating that a cysteine protease was responsible for apoB degradation. Chymostatin, leupeptin, pepstatin, phenylmethylsulfonyl fluoride, and aprotinin had no significant effect on the degradation of apoB48. However, leupeptin and pepstatin decreased the degradation of apoB100 by 20-30%. Degradation of apoB100 and apoB48 occurred in isolated Golgi fractions with little degradation in heavy or light ER. Degradation of apoB in Golgi fractions was inhibited by EST and by preincubating hepatocytes with 10 nM dexamethasone. Immunofluorescent microscopy revealed that apoB accumulated in the Golgi region after EST treatment. It is concluded that a major part of apoB degradation in rat hepatocytes occurs in a post-ER compartment via the action of a cysteine protease that is regulated by glucocorticoids.

Intracellular binding is an important determinant of the avid hepatic uptake of the high clearance drug omeprazole

The contribution of intracellular storage to hepatic uptake of the high clearance drug, omeprazole, was examined in the recirculating isolated perfused rat liver preparation. Following injection of [3H]omeprazole (7.5 microCi, 5 mg) into the portal vein over 1 min, livers were perfused for 5 min (N = 3) or 30 min (N = 3) and then homogenized at 4 degrees and fractionated by differential centrifugation. Radiolabelled omeprazole and metabolites were determined by scintillation counting of fractions of eluant from HPLC. Seventy per cent of drug had been taken up by the liver at 5 min and 85% at 30 min, with unchanged drug representing 43 and 7.4%, respectively, of drug taken up. At both times, 70-75% of intracellular unchanged drug and the major metabolites were located in the cytosol, and the cytosol:perfusate concentration ratio was approximately 10:1. Mitochondrial, lysosomal and microsomal fractions contained relatively little drug. Extensive cytosolic binding of omeprazole therefore contributes substantially to the initial avid hepatic first-pass uptake of this drug.

Influenza virus M2 protein ion channel activity stabilizes the native form of fowl plague virus hemagglutinin during intracellular transport

The influenza A/fowl plague virus/Rostock/34 hemagglutinin (HA), which is cleaved intracellularly and has a high pH threshold (pH 5.9) for undergoing its conformational change to the low-pH form, was expressed from cDNA in CV-1 and HeLa T4 cells in the absence of other influenza virus proteins. It was found, by biochemical assays, that the majority of the HA molecules were in a form indistinguishable from the low-pH form of HA. The acidotropic agent, ammonium chloride, stabilized the accumulation of HA in its native form. Coexpression of HA and the homotypic influenza virus M2 protein, which has ion channel activity, stabilized the accumulation of HA in its pH neutral (native) form, and the M2 protein ion channel blocker, amantadine, prevented the rescue of HA in its native form. These data provide direct evidence that the influenza virus M2 protein ion channel activity can affect the status of the conformational form of cleaved HA during intracellular transport.

The modification and assembly of proteins in the endoplasmic reticulum

Proteins fold and assemble in the endoplasmic reticulum in an environment that is very different from the cytosol. The presence of relatively high concentrations of calcium, an oxidizing state, ATP and lumenal proteins are all important in mediating these events.

Mucin-type glycoproteins

Considerable advances have been made in recent years in our understanding of the biochemistry of mucin-type glycoproteins. This class of compounds is characterized mainly by a high level of O-linked oligosaccharides. Initially, the glycoproteins were solely known as the major constituents of mucus. Recent studies have shown that mucins from the gastrointestinal tract, lungs, salivary glands, sweat glands, breast, and tumor cells are structurally related to high-molecular-weight glycoproteins, which are produced by epithelial cells as membrane proteins. During mucin synthesis, an orchestrated sequence of events results in giant molecules of Mr 4 to 6 x 10(6), which are stored in mucous granules until secretion. Once secreted, mucin forms a barrier, not only to protect the delicate epithelial cells against the extracellular environment, but also to select substances for binding and uptake by these epithelia. This review is designed to critically examine relations between structure and function of the different compounds categorized as mucin glycoproteins.

Bafilomycin A1 inhibits the targeting of lysosomal acid hydrolases in cultured hepatocytes

Effects of bafilomycin A1, an inhibitor of vacuolar H(+)-ATPase, on the synthesis and processing of cathepsin D and cathepsin H were investigated in primary cultured rat hepatocytes. Pulse-chase experiments showed that after being synthesized as procathepsin D and procathepsin H the precursors were converted into mature forms in the control cells as the chase time elapsed. However, in the presence of 5 x 10(-7) M of bafilomycin A1, both precursors were largely secreted into the medium and no mature forms were found within the cells. Thus bafilomycin A1 mimics lysosomotropic amines with regard to perturbation of the targeting of lysosomal acid hydrolases. In contrast, bafilomycin A1 was found not to inhibit processings of proalbumin and procomplement component 3, which are thought to occur at the acidic trans-Golgi, implying that the proteolytic event of the proproteins is not sensitive to an increase of intra-Golgi pH. The results suggest that bafilomycin A1 is useful as a pH-perturbant to study the role of acidity in living cells.

Increased cytotoxicity of ricin in a putative Golgi-defective mutant of Chinese hamster ovary cell

We have studied the cytotoxicity of ricin in a monensin-resistant mutant (MonR-31) of Chinese hamster ovary (CHO) cell line which is presumably altered in Golgi functions/structures. The cytotoxicity of ricin was increased in MonR-31 mutant cells compared with that in its parental CHO cells. In wild-type CHO cells, the cytotoxicity of ricin was enhanced by HN4Cl, bafilomycin A1, or nigericin. The enhancement of ricin cytotoxicity by these compounds was greatly reduced in MonR-31 mutant cells. Brefeldin A (BFA), which disrupts the structure of the Golgi apparatus, inhibits the cytotoxicity of ricin in both CHO and MonR-31 cells. We have also examined the effects of glycosylation inhibitors and the removal of high mannose oligosaccharide chains in ricin on the ricin hypersensitivity in MonR-31 cells. The hypersensitivity of MonR-31 cells to ricin is apparently not due to any difference in glycosylation between CHO and MonR-31 cells or in the processing of oligosaccharides on ricin by the target cells. Nigericin at low concentration (10 nM), which has no effect on the cytotoxicity of diphtheria toxin, enhances the ricin cytotoxicity, but inhibits the modeccin cytotoxicity. Our results suggest that important step(s) in the intoxication process of CHO cells by ricin and modeccin take place in the Golgi region.

Structure and biosynthesis of the xylose-containing carbohydrate moiety of rice alpha-amylase

Suspension-cultured cells of rice secrete alpha-amylase into the culture medium. It has been shown that the mature form of the alpha-amylase contains xylose-bearing N-linked oligosaccharide: (formula; see text) We demonstrate that suspension-cultured cells of rice secrete alpha-amylase containing oligomannose-type oligosaccharides in the presence of 1-deoxymannojirimycin or tris(hydroxymethyl)aminomethane. On the other hand, alpha-amylase purified from germinated rice seedlings contains several kinds of oligomannose-type and N-acetyllactosamine-type oligosaccharides. The processing pathway of oligosaccharide moieties in rice cells is discussed on the basis of a comparison of these oligosaccharides structures.

Alteration of intracellular traffic by monensin; mechanism, specificity and relationship to toxicity

Monensin, a monovalent ion-selective ionophore, facilitates the transmembrane exchange of principally sodium ions for protons. The outer surface of the ionophore-ion complex is composed largely of nonpolar hydrocarbon, which imparts a high solubility to the complexes in nonpolar solvents. In biological systems, these complexes are freely soluble in the lipid components of membranes and, presumably, diffuse or shuttle through the membranes from one aqueous membrane interface to the other. The net effect for monensin is a trans-membrane exchange of sodium ions for protons. However, the interaction of an ionophore with biological membranes, and its ionophoric expression, is highly dependent on the biochemical configuration of the membrane itself. One apparent consequence of this exchange is the neutralization of acidic intracellular compartments such as the trans Golgi apparatus cisternae and associated elements, lysosomes, and certain endosomes. This is accompanied by a disruption of trans Golgi apparatus cisternae and of lysosome and acidic endosome function. At the same time, Golgi apparatus cisternae appear to swell, presumably due to osmotic uptake of water resulting from the inward movement of ions. Monensin effects on Golgi apparatus are observed in cells from a wide range of plant and animal species. The action of monensin is most often exerted on the trans half of the stacked cisternae, often near the point of exit of secretory vesicles at the trans face of the stacked cisternae, or, especially at low monensin concentrations or short exposure times, near the middle of the stacked cisternae. The effects of monensin are quite rapid in both animal and plant cells; i.e., changes in Golgi apparatus may be observed after only 2-5 min of exposure. It is implicit in these observations that the uptake of osmotically active cations is accompanied by a concomitant efflux of H+ and that a net influx of protons would be required to sustain the ionic exchange long enough to account for the swelling of cisternae observed in electron micrographs. In the Golgi apparatus, late processing events such as terminal glycosylation and proteolytic cleavages are most susceptible to inhibition by monensin. Yet, many incompletely processed molecules may still be secreted via yet poorly understood mechanisms that appear to bypass the Golgi apparatus. In endocytosis, monensin does not prevent internalization. However, intracellular degradation of internalized ligands may be prevented.(ABSTRACT TRUNCATED AT 400 WORDS)

UDP-GlcNAc transport across the Golgi membrane: electroneutral exchange for dianionic UMP

We have examined the coupling and charge stoichiometry for UDP-GlcNAc transport into Golgi-enriched vesicles from rat liver. In the absence of added energy sources, these Golgi vesicles concentrate UDP-GlcNAc at least 20-fold, presumably by exchange with endogenous nucleotides. Under the conditions used, extravesicular degradation of UDP-GlcNAc has been eliminated, and less than 15% of the internalized radioactivity becomes associated with endogenous macromolecules. Of the remaining intravesicular label, 85% remains unmetabolized UDP-[3H]GlcNAc, and approximately 15% is hydrolyzed to [3H]GlcNAc-1-phosphate. Efflux of accumulated UDP-[3H]GlcNAc is induced by addition of UMP, UDP, or UDP-galactose to the external medium. Permeabilization of Golgi vesicles causes a rapid and nearly complete loss of internal UDP-[3H]GlcNAc, indicating that the results reflect transport and not binding. Moreover, transport of UDP-[3H]GlcNAc into these Golgi vesicles was stimulated up to 5-fold by mechanically preloading vesicles with either UDP-GlcNAc or UMP. The response of UMP/UMP exchange and UMP/UDP-GlcNAc exchange to alterations in intravesicular and extravesicular pH suggests that UDP-GlcNAc enters the Golgi apparatus in electroneutral exchange with the dianionic form of UMP.

Proalbumin is processed to serum albumin in COS-1 cells transfected with cDNA for rat albumin

Synthesis and processing of rat albumin were investigated in COS-1 cells transiently expressing rat albumin. Analysis using isoelectric focusing revealed that serum-type albumin, which is indistinguishable from the counterpart isolated from rat hepatocyte cuture medium, was secreted from the transfected COS-1 cells, indicating that proalbumin is effectively converted into serum albumin in the COS-1 cells, if not completely. Furthermore methylamine was found to cause the diminution of serum albumin released from the cells, substantiating that the proteolytical conversion of proalbumin occurs in the Golgi complex before discharge from the COS-1 cells.

Contribution of cytoplasmic storage triacylglycerol to VLDL-triacylglycerol in isolated rat hepatocytes

The cytoplasmic triacylglycerol (TG) storage pool of isolated hepatocytes was labelled in order to evaluate its incorporation into very low density lipoproteins (VLDL). Rats were injected with [1-14C]oleate 2 min prior to surgery and cell incubations began 90-100 min thereafter. In keeping with the equilibration of the two TG pools (in smooth endoplasmic reticulum, SER, and cytoplasm) in 120 min (Stein, Y. and Shapiro, B. (1959) Am. J. Physiol. 196, 1238-1241) the bulk of radioactive TG at time zero was in the cytoplasm and TG specific activities were similar in cytoplasm and SER. Radioactive and total VLDL-TG secretions were greatly inhibited after 80 min by chloroquine which is assumed to block lysosomal hydrolysis of cytoplasmic TG. When the SER-TG pool was labelled by addition of [1-14C]oleate in vitro, chloroquine affected neither [1-14C]oleate uptake and esterification nor its incorporation into VLDL-TG from 15-20 min until 80 min. After 100 min, when [1-14C]oleate-TG was transferred back from cytoplasm to SER, chloroquine began to decrease radioactive VLDL-TG output and by 210 min caused the same inhibition as under the in vivo labelling condition. These results are consistent with an inhibition by chloroquine of the lysosomal hydrolysis of cytoplasmic TG resulting in a blockage of their back transfer to SER membranes whereas other steps of VLDL production were not affected, at least up to 100 min. This study also showed that stored TG is a quantitatively important VLDL precursor, sustaining VLDL production for several hours in the absence of exogenous fatty acids.

Characterization of MgATP-driven H+ uptake into a microsomal vesicle fraction from rat pancreatic acinar cells

In microsomal vesicles, as isolated from exocrine pancreas cells, MgATP-driven H+ transport was evaluated by measuring H+-dependent accumulation of acridine orange (AO). Active H+ uptake showed an absolute requirement for ATP with simple Michaelis-Menten kinetics (Km for ATP 0.43 mmol/liter) with a Hill coefficient of 0.99. H+ transport was maximal at an external pH of 6.7, generating an intravesicular pH of 4.8. MgATP-dependent H+ accumulation was abolished by protonophores, such as nigericin (10(-6) mol/liter) or CCCP (10(-5) mol/liter), and by inhibitors of nonmitochondrial H+ ATPases, such as NEM or NBD-Cl, at a concentration of 10(-5) mol/liter. Inhibitors of both mitochondrial and nonmitochondrial H+ pumps, such as DCCD (10(-5) mol/liter) or Dio 9 (0.25 mg/ml), reduced microsomal H+ transport by about 90%. Vanadate (2 x 10(-3) mol/liter), a blocker of those ATPases, which form a phosphorylated intermediate, did not inhibit H+ transport. The stilbene derivative DIDS (10(-4) mol/liter), which inhibits anion transport systems, abolished H+ transport completely. MgATP-dependent H+ transport was found to be anion dependent in the sequence Cl- greater than Br- greater than gluconate-; in the presence of SO2-4, CH3COO- or No-3, no H+ transport was observed. MgATP-dependent H+ accumulation was also cation dependent in the sequence K+ greater than Li+ greater than Na+ = choline+. As shown by dissipation experiments in the presence of different ion gradients and ionophores, both a Cl- and a K+ conductance, as well as a small H+ conductance, were found in the microsomal membranes. When membranes containing the H+ pump were further purified by Percoll gradient centrifugation (ninefold enrichment compared to homogenate), no correlation with markers for endoplasmic reticulum, mitochondria, plasma membranes, zymogen granules or Golgi membranes was found. The present data indicate that the H+ pump located in microsomes from rat exocrine pancreas is a vacuolar- or "V" -type H+ ATPase and has most similarities to that described in endoplasmic reticulum, Golgi apparatus or endosomes.

Inhibition of neuroglandular antigen (NGA) glycosylation by phorbol ester in human melanoma cells

NGA is a human melanoma-associated antigen recognized by a panel of murine monoclonal antibodies developed in this laboratory. NGA consists of a 23.5 kDa core protein which is glycosylated in vivo to give a family of glycoproteins (30-60 kDa). Treatment of human melanoma G361 cells with the phorbol ester PMA resulted in apparent partial inhibition of NGA glycosylation. After PMA treatment, NGA appeared as 3 different bands of 24, 29 and 34 kDa on SDS-PAGE. The 29 kDa band is similar to the one obtained by treatment with the ionophore monensin, which inhibits NGA O-glycosylation. PMA can modulate plasma membrane ion exchange, most likely by activating protein kinase C. In G361 cells PMA may produce the same net effect as monensin, by impairing transport in the Golgi complex and consequently inhibiting protein O-glycosylation through an ionophore-like effect. Treatment of G361 cells with both PMA and protein kinase C inhibitors re-established the usual NGA glycosylation pattern. Thus the observed effect of PMA on NGA glycosylation is reversible and appears to be mediated by protein kinase C activation.

Immunocytochemical Localization of the Vacuolar H-ATPase in Maize Root Tip Cells

The vacuolar H(+)-ATPase of maize (Zea mays L.) root tip cells has been localized at the EM level using rabbit polyclonal antibodies to the 69 kilodalton subunit and protein A-colloidal gold. Intracellular gold particles were detected mainly on the tonoplast and Golgi membranes. Only about 27% of the vacuoles were labeled above background. The absence of gold particles on the majority of vacuoles suggests either that the tonoplast H(+)-ATPase is degraded during tissue preparation or that the small vacuoles of root tip cells are specialized with respect to H(+)-ATP ase activity. The pattern of gold particles on the labeled vacuoles ranged from uniform to patchy. Virtually all of the Golgi bodies were labeled by the antibody, but the particle densities were too low to determine whether the H(+)-ATPase was associated with specific regions, such as the trans-face. Cell wall-labeling was also observed which could be partially prevented by the inclusion of gelatin as a blocking agent. The immunocytochemical results confirm previous biochemical studies with isolated membrane fractions (A Chanson, L Taiz 1985 Plant Physiol 78: 232-240).

Monensin inhibits ligand dissociation only transiently and partially and distinguishes two galactosyl receptor pathways in isolated rat hepatocytes

Monensin has been shown to inhibit the dissociation of internalized asialoorosomucoid (ASOR) from galactosyl (Gal) receptors in hepatocytes (Harford et al., J. Cell. Biol., 96:1824, 1983). Examination of the long-term kinetics of dissociation of a single round of surface-bound 125I-ASOR in the presence of monensin revealed, however, that dissociation resumed after a lag of 30-40 min. Dissociation proceeded slowly with apparent first order kinetics (k = 0.006-0.022 min-1) and reached a plateau after 4 h, both in freshly isolated cells in suspension and in cells cultured for 24 h. Only a portion of the ligand bound to surface Gal receptors was capable of dissociating. The degree of dissociation was correlated with the expression of a subpopulation of receptors we have recently designated as state 1 Gal receptors (Weigel et al., Biochem. Biophys. Res. Commun. 140:43, 1986). The recovery and dissociation of a portion of 125I-ASOR-receptor complexes after the lag period is not due to a depletion of monensin, since a second addition of the drug has no affect once dissociation resumes. Furthermore, as assessed by the accumulation of the fluorescent dye acridine orange, cells have not recovered the ability to acidify intracellular compartments during the time that dissociation occurs. The results support a model for the hepatic Gal receptor system, in which there are two functionally different receptor populations, recycling pathways, and ligand processing pathways. Monensin blocks dissociation of 125I-ASOR from receptors in the major pathway completely. In the minor pathway dissociation proceeds to completion only after a lag. In this minor pathway monensin appears to temporarily delay a maturation or translocation process that must occur prior to dissociation. We conclude that the observed dissociation in the presence of monensin cannot be mediated by low pH, or by pH or pNa gradients.

Protein production and secretion in exocrine cells

Acinar cells of exocrine glands are highly specialized for producing, storing, and discharging secretory proteins for use on surfaces that represent interfaces between the organism and the surrounding environment. These functions are achieved through the secretory pathway that includes a series of functionally distinct intracellular compartments--the endoplasmic reticulum, subcompartments of the Golgi complex, and the secretion granule in which exportable macromolecules are stored at high concentrations. Most secretion occurs by granule exocytosis in response to external hormonal or neural stimuli. Although these processes have been traced in a variety of morphological and biochemical studies, very little is known about the mechanisms involved in facilitating and maintaining secretory storage, orchestrating discharge at the apical cell surface, and in ensuring conservation and re-internalization of the granule membrane. Recent studies initiated on cell fractions obtained from the rat parotid gland have provided significant insight into the protein storage conditions that prevail in the granule interior and the components of the granule membrane that are likely to be involved in general secretory function such as exocytosis.

Reduced temperature alters Pseudomonas exotoxin A entry into the mouse LM cell

The movement of Pseudomonas exotoxin A (PE) into the cytoplasm of mouse LM fibroblasts was followed by using inhibition of protein synthesis as a biochemical index of toxin activity; biotinyl-PE and avidin-gold colloids were used for electron microscopy. At 37 degrees C both specific antitoxin and pronase-trypsin protected cells against PE toxicity when added within seconds of warming cells, whereas methylamine was protective when added during the first 7 min of endocytosis. Lowering the temperature to 19 degrees C afforded protection when the temperature transition was accomplished within 15 min of the original endocytic event. These data suggest that PE enters an acidic compartment before reaching a step blocked by shifting cells from 37 to 19 degrees C. PE expressed toxicity for LM cells at 19 degrees C, but at a concentration 1 order of magnitude higher than that required at 37 degrees C. At 19 degrees C, antitoxin or trypsin-pronase protection was rapidly ablated. In contrast cells were fully protected by methylamine for 90 min. Using electron microscopy we demonstrated that toxin moved normally (30 s) to coated areas at 19 degrees C, but remained at this site for up to 20 min before being internalized. The majority of the toxin internalized at 19 degrees C remained in endosomes or in Golgi-associated vesicles and was not delivered to lysosomes. The results suggest that, under physiological conditions (37 degrees C), PE rapidly enters cells through coated areas, moves to an acidic compartment (i.e., the endosome), and then probably to the Golgi region en route to lysosomes. The evidence suggests that movement of toxin from endosomes or Golgi vesicles to lysosomes is blocked at 19 degrees C. We hypothesize that the active form of PE enters the cytosol, where it expresses its toxicity during fusion of Golgi-derived, toxin-laden vesicles with lysosomes.

Golgi and secreted galactosyltransferase

Galactosyltransferase (GT) belongs to the glycosyltransferases. In several tissues and cell lines, the enzyme is localized by immunocytochemistry to the two to three trans cisternae of the Golgi complex and may thus be considered a specific membrane component of this type of endomembrane. As a consequence, it is the most common Golgi "marker" enzyme in cell fractionation studies. Study of its biosynthesis, membrane orientation, and turnover in several tissues and cultured cell lines has broadened our knowledge about Golgi function itself. The enzyme is oriented towards the lumen of the cisternal space. In this orientation, it catalyzes the transfer of galactose to glycoprotein-bound acetylglucosamine and, in the presence of alpha-lactalbumin, to glucose, as shown in the Golgi complex of mammary gland epithelial cells. The enzymatic properties of GT are well known. The metabolism of GT has been extensively studied in HeLa and human hepatoma cells. The enzyme is synthesized in the rough endoplasmic reticulum (RER) and provided with one N-linked oligosaccharide and palmitate residues. In the Golgi complex, terminal sugars are attached to the N-linked oligosaccharide and extensive O-glycosylation takes place. The half-life of the enzyme is about 20 hr, after which a soluble form appears in the culture medium. Release of GT into the medium is observed in all cell lines studied. This phenomenon is in accordance with the presence of soluble GT in body fluids such as serum, ascites, milk, and saliva. In patients suffering from ovarian and breast cancer, increased levels of GT enzyme activity have been reported. Whether extracellular GT is of biological significance is still a point of discussion.