Perturbation of the structure and function of the Golgi complex by monovalent carboxylic ionophores

Mouse N2a Neuroblastoma Assay: Uncertainties and Comparison with Alternative Cell-Based Assays for Ciguatoxin Detection

The growing concern about ciguatera fish poisoning (CF) due to the expansion of the microorganisms producing ciguatoxins (CTXs) increased the need to develop a reliable and fast method for ciguatoxin detection to guarantee food safety. Cytotoxicity assay on the N2a cells sensitized with ouabain (O) and veratridine (V) is routinely used in ciguatoxin detection; however, this method has not been standardized yet. This study demonstrated the low availability of sodium channels in the N2a cells, the great O/V damage to the cells and the cell detachment when the cell viability is evaluated by the classical cytotoxicity assay and confirmed the absence of toxic effects caused by CTXs alone when using the methods that do not require medium removal such as lactate dehydrogenase (LDH) and Alamar blue assays. Different cell lines were evaluated as alternatives, such as human neuroblastoma, which was not suitable for the CTX detection due to the greater sensitivity to O/V and low availability of sodium channels. However, the HEK293 Nav cell line expressing the α1.6 subunit of sodium channels was sensitive to the ciguatoxin without the sensitization with O/V due to its expression of sodium channels. In the case of sensitizing the cells with O/V, it was possible to detect the presence of the ciguatoxin by the classical cytotoxicity MTT method at concentrations as low as 0.0001 nM CTX3C, providing an alternative cell line for the detection of compounds that act on the sodium channels.

Repurposing Polyether Ionophores as a New-Class of Anti-SARS-Cov-2 Agents as Adjunct Therapy

The emergence of SARS-CoV-2 and its variants have posed a significant threat to humankind in tackling the viral spread. Furthermore, currently repurposed drugs and frontline antiviral agents have failed to cure severe ongoing infections effectively. This insufficiency has fuelled research for potent and safe therapeutic agents to treat COVID-19. Nonetheless, various vaccine candidates have displayed a differential efficacy and need for repetitive dosing. The FDA-approved polyether ionophore veterinary antibiotic for treating coccidiosis has been repurposed for treating SARS-CoV-2 infection (as shown by both in vitro and in vivo studies) and other deadly human viruses. Based on selectivity index values, ionophores display therapeutic effects at sub-nanomolar concentrations and exhibit selective killing ability. They act on different viral targets (structural and non-structural proteins), host-cell components leading to SARS-CoV-2 inhibition, and their activity is further enhanced by Zn²⁺ supplementation. This review summarizes the anti-SARS-CoV-2 potential and molecular viral targets of selective ionophores like monensin, salinomycin, maduramicin, CP-80,219, nanchangmycin, narasin, X-206 and valinomycin. Ionophore combinations with Zn²⁺ are a new therapeutic strategy that warrants further investigation for possible human benefits.

Cascade Delivery to Golgi Apparatus and On-Site Formation of Subcellular Drug Reservoir for Cancer Metastasis Suppression

As the foremost cause of cancer-related death, metastasis consists of three steps: invasion, circulation, and colonization. Only targeting one single phase of the metastasis cascade may be insufficient since there are many alternative routes for tumor cells to disseminate. Here, to target the whole cascade of metastasis, hybrid erythrocyte and tumor cell membrane-coated nanoparticle (Hyb-NP) is designed with dual functions of increasing circulation time and recognizing primary, circulating, and colonized tumors. After loading with monensin, a recently reported metastasis inhibitor, the delivery system profoundly reduces spontaneous metastasis in an orthotopic breast cancer model. Underlying mechanism studies reveal that Hyb-NP can deliver monensin to its action site in the Golgi apparatus, and in return, monensin can block the exocytosis of Hyb-NP from the Golgi apparatus, forming a reservoir-like subcellular structure. Notably, the Golgi apparatus reservoir displays three vital functions for suppressing metastasis initialization, including enhanced subcellular drug retention, metastasis-related cytokine release inhibition, and directional migration inhibition. Collectively, based on metastasis cascade targeting at the tissue level, further formation of the Golgi apparatus drug reservoir at the subcellular level provides a potential therapeutic strategy for cancer metastasis suppression.

High content screening identifies monensin as an EMT-selective cytotoxic compound

Epithelial-to-mesenchymal transition (EMT) is implicated in cancer metastasis and drug resistance. Specifically targeting cancer cells in an EMT-like state may have therapeutic value. In this study, we developed a cell imaging-based high-content screening protocol to identify EMT-selective cytotoxic compounds. Among the 2,640 compounds tested, salinomycin and monensin, both monovalent cation ionophores, displayed a potent and selective cytotoxic effect against EMT-like cells. The mechanism of action of monensin was further evaluated. Monensin (10 nM) induced apoptosis, cell cycle arrest, and an increase in reactive oxygen species (ROS) production in TEM 4-18 cells. In addition, monensin rapidly induced swelling of Golgi apparatus and perturbed mitochondrial function. These are previously known effects of monensin, albeit occurring at much higher concentrations in the micromolar range. The cytotoxic effect of monensin was not blocked by inhibitors of ferroptosis. To explore the generality of our findings, we evaluated the toxicity of monensin in 24 human cancer cell lines and classified them as resistant or sensitive based on IC₅₀ cutoff of 100 nM. Gene Set Enrichment Analysis identified EMT as the top enriched gene set in the sensitive group. Importantly, increased monensin sensitivity in EMT-like cells is associated with elevated uptake of ³H-monensin compared to resistant cells.

The endoplasmic reticulum-resident chaperone heat shock protein 47 protects the Golgi apparatus from the effects of O-glycosylation inhibition

The Golgi apparatus is important for the transport of secretory cargo. Glycosylation is a major post-translational event. Recognition of O-glycans on proteins is necessary for glycoprotein trafficking. In this study, specific inhibition of O-glycosylation (Golgi stress) induced the expression of endoplasmic reticulum (ER)-resident heat shock protein (HSP) 47 in NIH3T3 cells, although cell death was not induced by Golgi stress alone. When HSP47 expression was downregulated by siRNA, inhibition of O-glycosylation caused cell death. Three days after the induction of Golgi stress, the Golgi apparatus was disassembled, many vacuoles appeared near the Golgi apparatus and extended into the cytoplasm, the nuclei had split, and cell death assay-positive cells appeared. Six hours after the induction of Golgi stress, HSP47-knockdown cells exhibited increased cleavage of Golgi-resident caspase-2. Furthermore, activation of mitochondrial caspase-9 and ER-resident unfolded protein response (UPR)-related molecules and efflux of cytochrome c from the mitochondria to the cytoplasm was observed in HSP47-knockdown cells 24 h after the induction of Golgi stress. These findings indicate that (i) the ER-resident chaperon HSP47 protected cells from Golgi stress, and (ii) Golgi stress-induced cell death caused by the inhibition of HSP47 expression resulted from caspase-2 activation in the Golgi apparatus, extending to the ER and mitochondria.

How early studies on secreted and membrane protein quality control gave rise to the ER associated degradation (ERAD) pathway: the early history of ERAD

All newly synthesized proteins are subject to quality control check-points, which prevent aberrant polypeptides from harming the cell. For proteins that ultimately reside in the cytoplasm, components that also reside in the cytoplasm were known for many years to mediate quality control. Early biochemical and genetic data indicated that misfolded proteins were selected by molecular chaperones and then targeted to the proteasome (in eukaryotes) or to proteasome-like particles (in bacteria) for degradation. What was less clear was how secreted and integral membrane proteins, which in eukaryotes enter the endoplasmic reticulum (ER), were subject to quality control decisions. In this review, we highlight early studies that ultimately led to the discovery that secreted and integral membrane proteins also utilize several components that constitute the cytoplasmic quality control machinery. This component of the cellular quality control pathway is known as ER associated degradation, or ERAD. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

Role of an acidic cluster/dileucine motif in cation-independent mannose 6-phosphate receptor traffic

The endocytic trafficking of the cation-independent mannose 6-phosphate receptor (CI-MPR) involves multiple sorting steps. A cluster of acidic amino acids followed by a dileucine motif in the cytoplasmic tail has been proposed to mediate receptor sorting from the trans Golgi network (TGN) to late endosomes. Mutations in this motif impair lysosomal enzyme sorting by preventing association of CI-MPR with coat proteins. The role of the acidic cluster/dileucine motif in the post-endocytic transport of the receptor was examined using the CI-MPR mutants, AC01 and D160E (Chen HJ, Yuan J, Lobel P. J Biol Chem 1997;272:7003-7012). Following internalization, wild type (WT) CI-MPR is transported through sorting endosomes into the endocytic recycling compartment (ERC), after which it traffics to the TGN and other organelles. However, the mutants localize mostly to the ERC and only a small portion reaches the TGN, suggesting that the sorting of the CI-MPR mutants from the ERC into the TGN is severely impaired. We observed no defect in receptor internalization or in the rate of tail mutant recycling to the cell surface compared to the WT. These results demonstrate that the acidic cluster/dileucine motif of CI-MPR is critical for receptor sorting at early stages of intracellular transport following endocytosis.

Modulation of Nav1.7 and Nav1.8 peripheral nerve sodium channels by protein kinase A and protein kinase C

Voltage-gated Na+ channels (VGSC) are transmembrane proteins that are essential for the initiation and propagation of action potentials in neuronal excitability. Because neurons express a mixture of Na+ channel isoforms and protein kinase C (PKC) isozymes, the nature of which channel is being regulated by which PKC isozyme is not known. We showed that DRG VGSC Nav1.7 (TTX-sensitive) and Nav1.8 (TTX-resistant), expressed in Xenopus oocytes were differentially regulated by protein kinase A (PKA) and PKC isozymes using the two-electrode voltage-clamp method. PKA activation resulted in a dose-dependent potentiation of Nav1.8 currents and an attenuation of Nav1.7 currents. PKA-induced increases (Nav1.8) and decreases (Nav1.7) in peak currents were not associated with shifts in voltage-dependent activation or inactivation. The PKA-mediated increase in Nav1.8 current amplitude was prevented by chloroquine, suggesting that cell trafficking may contribute to the changes in Nav1.8 current amplitudes. A dose-dependent decrease in Nav1.7 and Nav1.8 currents was observed with the PKC activators phorbol 12-myristate, 13-acetate (PMA) and phorbol 12,13-dibutyrate. PMA induced shifts in the steady-state activation of Nav1.7 and Nav1.8 channels by 6.5 and 14 mV, respectively, in the depolarizing direction. The role of individual PKC isozymes in the regulation of Nav1.7 and Nav1.8 was determined using PKC-isozyme-specific peptide activators and inhibitors. The decrease in the Nav1.8 peak current induced by PMA was prevented by a specific epsilonPKC isozyme peptide antagonist, whereas the PMA effect on Nav1.7 was prevented by epsilonPKC and betaIIPKC peptide inhibitors. The data showed that Nav1.7 and Nav1.8 were differentially modulated by PKA and PKC. This is the first report demonstrating a functional role for epsilonPKC and betaIIPKC in the regulation of Nav1.7 and Nav1.8 Na+ channels. Identification of the particular PKC isozymes(s) that mediate the regulation of Na+ channels is essential for understanding the molecular mechanism involved in neuronal ion channel regulation in normal and pathological conditions.

Differential expansion of T-cell receptor variable beta subsets after antigenic stimulation in patients with different outcomes of hepatitis C infection

Persistent antigenic stimulation during chronic hepatitis C may alter the T-cell receptor variable chain beta (TCR BV) repertoire as well as the cytokine responses of hepatitis C virus (HCV)-specific T lymphocytes. We analysed the distribution of the TCR BV subsets 2.1, 3.1, 5.1, 6.1, 8, 13.1, 13.6, 14.1, 17.1, 21.3 in relation to intracytoplasmic expression of interleukin-2, interferon-gamma, interleukin-4 and interleukin-10. Using flow cytometry, CD45RO+ memory T cells of 27 patients with chronic hepatitis C, eight patients with resolved HCV infection and 16 non-HCV-related controls were studied with and without stimulation by the HCV core, NS3, NS4, NS5a and NS5b proteins. Patients with chronic and resolved hepatitis C differed by larger basal TCR BV2.1+, BV6.1+, BV17.1+ and BV21.3+ subsets in chronic hepatitis C, which were correlated to the numbers of T cells with spontaneous interleukin-2 and interferon-gamma production (r=0.51-0.73, P<0.05). Upon HCV-specific stimulation these subsets did not expand, whereas a marked in vitro expansion of TCR BV8+ T cells in response to all HCV proteins was selectively noted in chronic hepatitis C (P<0.05). This expansion of TCR BV8+ memory T cells was significantly correlated to HCV-induced interleukin-10 expression (r=0.58-0.98, P<0.01). Thus, differential involvement of selected TCR BV subsets may be related to the outcome of HCV infection.

HCV-specific cytokine induction in monocytes of patients with different outcomes of hepatitis C

AIM

Cytokine release by macrophages critically determines the type of immune response to an antigen. Therefore, we studied hepatitis C virus (HCV)-specific induction of interleukins-1 beta, -10, -12 (IL-1 beta, IL-10, IL-12), and tumor necrosis factor-alpha (TNF-alpha) in monocytes.

METHODS

Intracellular cytokine expression was studied by flow cytometry in 23 patients with chronic hepatitis C, 14 anti-HCV seropositives without viremia and 11 controls after stimulation of peripheral blood mononuclear cells with recombinant core, NS3, NS4, NS5a and NS5b proteins.

RESULTS

Patients with HCV viremia revealed greater spontaneous expression of IL-1beta, TNF-alpha, and IL-10. Furthermore, greater than twofold higher IL-10 expression was induced by the HCV antigens in chronic hepatitis C than in the other two groups (P<0.05). In contrast, neither IL-12 nor TNF-alpha was induced preferentially.

CONCLUSION

In chronic hepatitis C antigen-specific cytokine induction in monocytes is apparently shifted towards predominant IL-10 induction - not counterbalanced by antiviral type 1 cytokines. This may contribute to persistent viral replication.

Binding of human neutrophils to cell-surface anchored Tamm-Horsfall glycoprotein in tubulointerstitial nephritis

BACKGROUND

Human Tamm-Horsfall glycoprotein (T-H) is a glycosylphosphatidylinositol-anchored protein exposed at the surface of distal nephron cells, and urinary T-H is the released soluble counterpart. The latter has been implicated in tubulointerstitial nephritis, and the proinflammatory potential has been related to its ability to bind in vitro human neutrophils (PMNs). We have examined the conditions required for the binding of neutrophils to cell-surface anchored T-H and the consequent effects.

METHODS

A HeLa cell-line derivative permanently transformed with human T-H cDNA and expressing T-H at the cell surface was used throughout the study. The adhesion of PMNs to cells expressing T-H was analyzed by immunofluorescence microscopy before and after the opsonization of cells with anti-T-H antibodies. The oxidative burst induced by adhesion of PMNs to the cells was determined by the activation of myeloperoxidase. Quantitative and qualitative changes in the release of T-H under the adhesion of activated PMNs were determined by dot-blot and Western blot analysis.

RESULTS

No binding of neutrophils to cell-surface-anchored T-H was observed. On the contrary, the opsonization of cells with anti-T-H antibodies resulted in a dramatic adhesion of neutrophils. Such an adhesion induced the oxidative burst of PMNs and a large increment in the release of T-H, as well as the release of the slightly faster migrating T-H form, which is normally retained intracellularly.

CONCLUSIONS

These results support the notion that, after the autoimmune response, the adhesion of neutrophils to cell-surface T-H contributes to the pathogenesis of tubulointerstitial nephritis, favoring a further accumulation of T-H in the interstitium and inducing the loss of cell integrity via reactive oxygen metabolites generated by activated neutrophils.

Phosphorylation of rat insulin-like growth factor binding protein-1 does not affect its biological properties

Insulin-like growth factors (IGFs) I and II stimulate growth and expression of specific genes through binding to cell membrane receptors. IGF binding proteins also bind IGF-I with higher affinity than the receptor. They are found in the circulation and tissues and can modulate IGF actions. Human IGFBP-1 is phosphorylated on serine residues, which increases its affinity for IGF-I. An acidic, presumably phosphorylated, form of human IGFBP-1 inhibits IGF-I-stimulated DNA synthesis in cultured cells, while a less acidic, unphosphorylated form potentiates this function. Phosphorylation of human IGFBP-3, however, does not affect its affinity for IGF-I. Previously we found that multiple forms of rat IGFBP-1 are obtained by anion-exchange chromatography, raising the possibility that it also is phosphorylated, which led us to examine its properties. Phosphopeptide analysis of 32P-labeled, immunoprecipitated rat IGFBP-1 synthesized by H-4-II-EC3 rat hepatoma cells indicated that it is phosphorylated on two sites that were deduced to be ser107 and ser132 in the central nonconserved domain. Dephosphorylation of purified phosphorylated rat IGFBP-1 did not affect its affinity for IGF-I or its specific binding activity, and the dephosphorylated form inhibited DNA synthesis in 3T3 cells. Incubation of cells labeled with radioactive proline in the presence of monensin and brefeldin A, which inhibit secretion at different sites, led to intracellular accumulation of the least phosphorylated form of rat IGFBP-1, but prevented further phosphorylation. The results suggested that phosphorylation occurs at two sites in cells, the cis-Golgi and the trans-Golgi network. In summary, these studies have shown that rat IGFBP-1 is phosphorylated on two sites by reactions that occur in different secretory organelles and that similar to human IGFBP-3, but unlike human IGFBP-1, phosphorylation does not affect its affinity for IGF-I.

Intracellular transport, cell-surface exposure and release of recombinant Tamm-Horsfall glycoprotein

Human Tamm-Horsfall glycoprotein (T-H), first described as the major urinary glycoprotein, is a glycosylphosphatidyl-inositol (GPI)-anchored membrane protein which mainly resides at the luminal face of cells of the thick ascending limb of Henle's loop (TAL) and early distal convoluted tubules of nephron. Since no human renal cell-line producing T-H is available, T-H cDNA was transfected in HeLa cells and a cell line was selected in which 95% of the cells stably expressed T-H, in order to elucidate the biosynthesis, mechanisms regulating the transport of T-H along the exocytic pathway, exposure at the cell surface and release in soluble form. Treatment of cells with an exogenous reducing agent results in a drastic delay in the conversion from precursor to mature T-H. Since the accumulating T-H-precursor carries glycans not yet processed by Golgi-mannosidases, we propose that the formation of a correct set of intrachain disulphide bonds is required for T-H exit out the endoplasmic reticulum. Even the treatment of cells with an inhibitor of GPI-anchor biosynthesis results in an intracellular accumulation of T-H precursor, loss of T-H localization into Golgi apparatus and reduced surface exposure. These results indicate that the GPI-anchor addition is necessary for T-H delivery to the cell-surface. The release rate of new synthesized T-H shows an initial lag time very likely depending on the time required for T-H surface exposure. A portion of released T-H appears to contain ethanolamine, a component of GPI anchor, indicating that, at least in HeLa cells, a GPI-specific phospholipase contributes to the T-H release. Exposure of cells to monensin and brefeldin A results in a loss of accumulation of T-H in the Golgi perinuclear region and a reduced delivery to the cell surface. Under monensin treatment an intermediate T-H form non-exposed at the cell surface is released in the medium, indicating that a soluble T-H may be produced inside the cell under conditions that alter the Golgi apparatus. If such an event occurs in polarized kidney cells, a T-H release from the basolateral face may be postulated, inasmuch as the GPI-anchor is an apical sorting signal. Since T-H is a powerful autoantigen, the accumulation of soluble T-H in the interstitium of TAL may cause the formation of immunocomplexes.

Aberrant O-glycosylation in the collagenous domain of pro alpha2(I) procollagen subunits synthesized by chemically transformed hamster fibroblasts

Chemically transformed Syrian hamster embryo fibroblasts (NQT-SHE) do not synthesize the pro alpha1(I) subunit of type I collagen, but they secrete two forms of the pro alpha2(I) subunit (N33 and N50) with abnormal post-translational modifications localized in the alpha2CB3,5 cyanogen bromide peptide of the collagenous domain (B. Peterkofsky and W. Prather (1992) J. Biol. Chem. 267 5388-5395). Isoelectric focusing and treatment of the modified chains with glycosidases and biotinylated Jacalin lectin identified the modifications as Gal beta1,3-GalNAc-O-Ser/Thr with or without a terminal sialic acid in an alpha2,6 linkage. Unhydroxylated N33 alpha-chains also reacted with Jacalin, confirming that the abnormal modification was O-glycosylation and not hyperhydroxylation of proline or lysine. Cells were treated with benzyl GalNAc, a competitive inhibitor of galactosyl transferase that prevents addition of Gal to GalNAc-O-Ser/Thr and thus blocks elongation of O-glycosyl chains. Treated cells secreted pro alpha2(I) chains containing GalNAc-O-Ser/Thr but no galactose or sialic acid, which suggested that Gal addition takes place before sialylation. Treatment of NQT-SHE cells with monensin and brefeldin A inhibited secretion and led to intracellular accumulation of pro alpha2(I) chains that contained only GalNAc. Therefore, it appears that GalNAc addition to pro alpha2(I) chains in NQT-SHE cells occurs in the cis-Golgi, while sialic acid and galactose are added in the trans-Golgi network. The pro alpha2(I) chains produced by NQT-SHE cells most likely are modified because they are in the denatured state, and thus potential O-glycosylation sites become available that would not be exposed in normal triple helical procollagen.

Expression of mycobacterial antigens on murine dissociated Schwann cells infected with Mycobacterium leprae

Mycobacterial antigens were detected in a cell surface ELISA on murine dissociated Schwann cells infected with Mycobacterium leprae. The time kinetics of expression and its refractoriness to modulation with monensin suggests that the antigens are likely to be integrated into the membrane during bacterial entry. This may be partially responsible for the defective Schwann cell functions leading to subsequent peripheral nerve damage.

Dynamic measurement of the pH of the Golgi complex in living cells using retrograde transport of the verotoxin receptor

The B subunit of verotoxin (VT1B) from enterohemorrhagic Escherichia coli is responsible for the attachment of the holotoxin to the cell surface, by binding to the glycolipid, globotriaosyl ceramide. After receptor-mediated endocytosis, the toxin is targeted to the Golgi complex by a process of retrograde transport. We took advantage of this unique property of VT1B to measure the pH of the Golgi complex in intact live cells. Purified recombinant VT1B was labeled with either rhodamine or fluorescein for subcellular localization by confocal microscopy. After 1 h at 37 degrees C, VT1B accumulated in a juxtanuclear structure that colocalized with several Golgi markers, including alpha-mannosidase II, beta-COP, and NBD-ceramide. Moreover, colchicine and brefeldin A induced dispersal of the juxtanuclear staining, consistent with accumulation of VT1B in the Golgi complex. Imaging of the emission of fluorescein-labeled VT1B was used to measure intra-Golgi pH (pHG), which was calibrated in situ with ionophores. In intact Vero cells, pHG averaged 6.45 +/- 0.03 (standard error). The acidity of the Golgi lumen dissipated rapidly upon addition of bafilomycin A1, a blocker of vacuolar-type ATPases, pHG remained constant despite acidification of the cytosol by reversal of the plasmalemmal Na+/H+ antiport. Similarly, pHG was unaffected by acute changes in cytosolic calcium. Furthermore, pHG recovered quickly toward the basal level after departures imposed with weak bases. These findings suggest that pHG is actively regulated, despite the presence of a sizable H+ "leak" pathway. The ability of VT1B to target the Golgi complex should facilitate not only studies of acid-base regulation, but also analysis of other ionic species.

Heat shock protein synthesis is affected by intracellular pH: inhibition by monensin-induced alkalosis in C6 rat glioma cells

The effect of intracellular pH (pHi) on heat shock protein (HSP) synthesis was investigated in C6 rat glioma cells. pHi changes were analysed by means of fluorescence spectroscopy in a perfused monitoring system allowing continuous measurements before, during and after treatments. HSP induction was determined by means of Western blots and autoradiographs. A 20 min heat shock (HS) of 44 degrees C decreased the pHi from 7.36 to 7.05 during exposure [17] and elicited the synthesis of heat shock proteins 2-8 h later. A pHi decrease, brought about by low extracellular pH (pHe) of 4.5 and 5.0 or 5.5, induced HSP synthesis after 1 h or 3 h, respectively. During these treatments, pHi decreased to values significantly lower than that caused by HS. Three h exposure to pHe 6.2, however, was not inductive. These results indicate that the heat shock-induced pHi decrease alone is not sufficient to stimulate HSP synthesis. In order to investigate the effect of alkaline pHi on the induction of HSP by heat, pHi was increased prior to HS treatments. Preincubation of cells at pHe ranging from 6.8 to 8.0 had little effect on pHi and on HSP synthesis. A shift of pHi to more alkaline values was achieved by adding the H+/Na+ exchanger monensin at alkaline pHe. Twenty microM monensin raised the pHi and inhibited the HSP induction depending on the pHe values: as pHe was increased from pH 7.2 to 8.0 HSP synthesis was increasingly inhibited. Monensin also diminished the HS-induced drop of pHi particularly at higher pHe. The result showed that neither a lower pHi nor a drop of pHi during HS is a necessary prerequisite for the induction, whereas alkalosis inhibits the synthesis of HSP.

Defective pH regulation of acidic compartments in human breast cancer cells (MCF-7) is normalized in adriamycin-resistant cells (MCF-7adr)

Alkalinization of normally acidic intracellular compartments or acidification of a mildly alkaline cytoplasm by biochemical or genetic manipulation has been demonstrated to inhibit both endocytosis and secretion (Tartakoff, 1983a; Cosson et al., 1989; Mellman et al., 1986; Davoust et al., 1987; Cosson et al., 1989; van Deurs et al., 1989; Maxfield & Yamashiro, 1991; Hansen et al., 1993). These results provide the basis for the conclusion that the maintenance of pH gradients between acidic vesicular compartments and a mildly alkaline cytoplasm is an essential biochemical requirement for the correct functioning of the endocytotic and secretory machinery. Tumor cells have been shown to have an abnormally acidic cytoplasmic pH (Warburg, 1956; Simon & Schindler, 1994). Here we report that the intracellular vesicular compartments in tumor cells (MCF-7) derived from a human breast cancer fail to acidify. This failure results in a significant decrease in the pH gradient (0.9 pH unit) between the vesicular luminal compartments and the cytoplasm. These defects are correlated with a disruption in the organization and function of the trans-Golgi network (TGN) and the pericentriolar recycling compartment (PRC). In marked distinction, drug-resistant tumor cells (MCF-7adr) derived from the MCF-7 line that are resistant to the most widely employed chemotherapeutic drug, adriamycin, appear normal in both acidification and organization of the PRC and TGN. Treatment of drug-resistant MCF-7adr cells with nigericin and monensin, ionophores demonstrated to disrupt vesicular acidification (Tartakoff, 1983b), leads to a resensitization of these cells to adriamycin. Drug sensitivity is proposed to result from an acidification defect within vesicles of the recycling and secretory pathways. A functional consequence of this defect is the diminished capacity of cells to remove cytotoxic drugs from the cytoplasm by sequestration of protonated drugs within the vesicles, followed by drug secretion through the activity of the secretory and recycling pathways.

Cytotoxic activity of lutropin-gelonin conjugate in mouse Leydig tumor cells: potentiation of the hormonotoxin activity by different drugs

A hormonotoxin preparation composed of gelonin, a basic protein of 30,000 Da isolated from the plant Gelonium multiflorum and the luteinizing hormone (LH, lutropin) isolated from the sheep pituitary has been studied for its cytotoxic action on mouse testicular Leydig tumor cells (MA-10 cells). Gelonin modified with 2-iminothiolane and conjugated with hormone modified by N-succinimidyl-3-2-pyridyl dithiopropionate was able to inhibit protein synthesis in Leydig tumor cells. An enhancement of the cytotoxicity of the hormonotoxin was obtained in the presence of drugs like quinacrine, chloroquine, verapamil and monensin. We report that the cytotoxicity of hormonotoxin was enhanced 10-15 times with quinacrine (7.6 microM), chloroquine (29 microM), verapamil (40 microM) and monensin (0.29 microM). While quinacrine, chloroquine and verapamil were not cytotoxic to MA-10 cells for up to 48 h, monensin alone reduced protein synthesis significantly in 48 h. All the drugs studied here inhibited steroidogenic action of the native hormone even at concentrations which were not detrimental to protein synthesis. On the basis of the above studies, we suggest that it may be feasible to develop combination strategies to destroy gonadal cells bearing gonadotropin (LH) receptors. In cells not bearing LH receptors (COS-7 cell line) there was no cytotoxicity either with hormonotoxin alone or in combination with the drugs, suggesting specificity of action.

Developmental effects of monensin on Drosophila melanogaster

Extracellular matrix and membrane proteins and their correct secretion probably are key elements in morphogenesis and differentiation in Drosophila. In this study, we have analysed the effects of monensin, a Na⁺-H⁺-ionophore which blocks normal secretion, applied during cellular blastoderm formation on further development. Normal cell morphology and intercellular contacts are lost and the extracellular matrix becomes disorganized. Gastrulation is blocked and abnormal foldings can be observed. Cuticle phenotypes showed different degrees of ventral, dorsal, head and posterior defects. The results are discussed in the context of what is known about membrane and extracellular matrix proteins in Drosophila.

Monensin action on the Golgi complex in perfused rat liver: evidence against bile salt vesicular transport

Several studies suggest that bile salts are transported from the basolateral to the canalicular membrane of hepatocytes by a vesicular pathway, possibly in part via the Golgi complex. To test this hypothesis, the present study examined, in the perfused rat liver, the influence of the Na+ ionophore monensin on the biliary secretion of taurocholate and biliary lipids. The effects of the drug have been checked by the study of the ultrastructural modifications of the Golgi complex, secretion of horseradish peroxidase, and bile salt uptake. An infusion of monensin (1, 3, or 5 mumol/L) into the liver induced considerable swelling of the Golgi complex within 5 minutes. After a bolus injection of horseradish peroxidase during monensin infusion, the biliary secretion of the protein was delayed (1 mumol/L monensin) and markedly reduced (5 mumol/L monensin). Bile salt uptake was virtually unchanged except with 5 mumol/L monensin. This suggests that monensin has the same effects on the subcellular traffic in the perfused liver as in cultured cells. After a bolus injection of taurocholate (0.25, 5.0, or 8.5 mumol/100 g body wt) during monensin infusion, the pattern of biliary secretion of the bile salt was identical to that of controls. During continuous infusion of taurocholate, a 10-minute monensin infusion (1 or 3 mumol/L) had no effect on the biliary secretion of taurocholate and on the secretion of lecithin and cholesterol induced by taurocholate. High concentrations (5 mumol/L) or prolonged infusions (20 minutes) of monensin decreased the biliary secretion of bile salts but corresponded to a marked decrease of taurocholate uptake. In summary, the Na+ ionophore monensin altered the Golgi complex and the vesicular transport of horseradish peroxidase, whereas taurocholate biliary secretion was not influenced unless taurocholate biliary secretion was not influenced unless taurocholate uptake by the liver was markedly decreased. It may be concluded that taurocholate and biliary lipid secretion, under these conditions, does not depend essentially on pathways involving acidic transporting vesicles and particularly the trans-Golgi complex.

Entry of newly synthesized gangliosides into myelin

Ganglioside synthesis and transport to myelin was studied in brainstem slices prepared from 19-21-day-old rats. The slices were incubated for up to 2 h in the presence of [3H]glucosamine to label primarily the hexosamine portion of complex gangliosides. The amount of radioactivity incorporated into gangliosides during slice incubations was only 10-15% of the amount of the label incorporated during in vivo labeling of brainstem gangliosides using equivalent amounts of [3H]glucosamine. Among individual gangliosides this inhibition was greater for the more complex gangliosides. When labeled gangliosides were isolated from homogenate and myelin fractions prepared from brain slices, the complex total gangliosides of both fractions showed a lag in labeling kinetics but with a lower specific radioactivity for the myelin fraction, reflecting the larger pool size and slower turnover rate exhibited by myelin components. Chase experiments showed that more complex gangliosides in homogenate exhibited almost no effect of chase after 30 min. Addition of the Golgi-disrupting agent monensin to slice incubations inhibited the labeling of all gangliosides except GM3, GM2, and GD3, and transport to myelin of all complex gangliosides except GM2. These results show that a monensin-sensitive mode of transport is responsible for the translocation of most newly synthesized gangliosides into myelin.

Carboxy terminally truncated forms of ribophorin I are degraded in pre-Golgi compartments by a calcium-dependent process

Two COOH terminally truncated variants of ribophorin I (RI), a type I transmembrane glycoprotein of 583 amino acids that is segregated to the rough portions of the ER and is associated with the protein-translocating apparatus of this organelle, were expressed in permanent HeLa cell transformants. Both variants, one membrane anchored but lacking part of the cytoplasmic domain (RL467) and the other consisting of the luminal 332 NH2-terminal amino acids (RI332), were retained intracellularly but, in contrast to the endogenous long lived, full length ribophorin I (t 1/2 = 25 h), were rapidly degraded (t 1/2 less than 50 min) by a nonlysosomal mechanism. The absence of a measurable lag phase in the degradation of both truncated ribophorins indicates that their turnover begins in the ER itself. The degradation of RI467 was monophasic (t 1/2 = 50 min) but the rate of degradation of RI332 molecules increased about threefold approximately 50 min after their synthesis. Several pieces of evidence suggest that the increase in degradative rate is the consequence of the transport of RI332 molecules that are not degraded during the first phase to a second degradative compartment. Thus, when added immediately after labeling, ionophores that inhibit vesicular flow out of the ER, such as carbonyl cyanide m-chlorophenylhydrazone (CCCP) and monensin, suppressed the second phase of degradation of RI332. On the other hand, when CCCP was added after the second phase of degradation of RI332 was initiated, the degradation was unaffected. Moreover, in cells treated with brefeldin A the degradation of RI332 became monophasic, and took place with a half-life intermediate between those of the two normal phases. These results point to the existence of two subcellular compartments where abnormal ER proteins can be degraded. One is the ER itself and the second is a non-lysosomal pre-Golgi compartment to which ER proteins are transported by vesicular flow. A survey of the effects of a variety of other ionophores and protease inhibitors on the turnover of RI332 revealed that metalloproteases are involved in both phases of the turnover and that the maintenance of a high Ca2+ concentration is necessary for the degradation of the luminally truncated ribophorin.

Sialagogue-stimulated protein phosphorylation related to ornithine decarboxylase induction in cultured rat parotid explants

Both beta-adrenergic (isoproterenol) and cholinergic (carbachol) sialagogues increase amylase secretion, ornithine decarboxylase activity and DNA synthesis in murine parotid gland in vivo and in vitro. These agonists enhanced the incorporation of labelled inorganic orthophosphate into parotid proteins in rat parotid explants cultured on siliconized lens paper floating on serum-free 199 medium. Analysis of the labelled proteins by SDS-PAGE and autoradiography revealed that isoproterenol enhanced the phosphorylation of four proteins with apparent molecular weights of 17, 20, 31 and 32 kDa and carbachol stimulated the phosphorylation of 31 and 32 K proteins. Isoproterenol-dependent ornithine decarboxylase induction and phosphorylation of the proteins were selectively suppressed by monensin but not by polymyxin B, whereas carbachol-dependent ornithine decarboxylase induction and protein phosphorylation were inhibited by polymyxin B but not by monensin. Neither monensin nor polymyxin B suppressed isoproterenol- or carbachol-stimulated amylase secretion. Time course experiments showed that sialagogue-stimulated protein phosphorylation preceded the increase of ornithine decarboxylase activity and had almost disappeared when it was maximal. Propranolol and atropine, antagonists of isoproterenol and carbachol, respectively, completely inhibited not only amylase secretion and ornithine decarboxylase induction but also protein phosphorylation stimulated by the corresponding agonists. These findings suggest that increased phosphorylation of specific proteins is associated with sialagogue-stimulated ornithine decarboxylase induction but not amylase secretion.

Pharmacological inhibition of the intracellular transport of low-density lipoprotein-derived cholesterol in Chinese hamster ovary cells

Mammalian cells, cultured in the presence of serum lipoproteins, acquire cholesterol necessary for growth from the uptake and lysosomal hydrolysis of low-density lipoproteins (LDL). The mechanism(s) of intracellular transport of LDL-derived cholesterol from lysosomes to other cellular sites is unknown. In this study, various pharmacological agents were assessed for their ability to inhibit the movement of LDL-cholesterol from lysosomes to the plasma membrane. The only pharmacological agent tested in these experiments that specifically inhibited LDL-cholesterol movement was U18666A. Ketoconazole impaired the intracellular transport of LDL-cholesterol; however, ketoconazole also had a general effect on cholesterol movement, since it impeded the desorption of endogenously synthesized cholesterol into the medium. Other drugs that affected cholesterol movement appeared to be nonspecific. Cholesterol transport from lysosomes to plasma membranes was not significantly altered by agents that affect lysosomal function or cytoskeletal organization, as well as energy poisons and cycloheximide.

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.

The effects of monensin on the transport of horseradish peroxidase into intracellular lumina in cultured rat hepatocytes

A kinetic study of horseradish peroxidase transport was carried out on short-term cultured adult rat hepatocytes. After 4 hr of plating, cells were preincubated with monensin (1 microM) for 1 hr before their incubation with horseradish peroxidase for different times. Monensin treatment resulted in the dilatation of the Golgi apparatus and caused the appearance of numerous intracellular lumina lined with microvilli in associated as well as isolated hepatocytes but did not modify newly formed bile canaliculi. The frequency of their appearance increased to 28% in cells pretreated with monensin compared to 2% in controls. Intracellular lumina membranes had the morphological features of apical membranes and were stained with horseradish peroxidase more frequently than those of newly formed bile canaliculi. This work therefore provides a model for studying bile secretion in cultured hepatocytes. Our results also suggest that the biliary transport of horseradish peroxidase does not involve the Golgi apparatus, since horseradish peroxidase was never observed in the Golgi stacks even after monensin treatment.

Fibroblasts that proliferate near denervated synaptic sites in skeletal muscle synthesize the adhesive molecules tenascin(J1), N-CAM, fibronectin, and a heparan sulfate proteoglycan

Four adhesive molecules, tenascin(J1), N-CAM, fibronectin, and a heparan sulfate proteoglycan, accumulate in interstitial spaces near synaptic sites after denervation of rat skeletal muscle (Sanes, J. R., M. Schachner, and J. Covault. 1986. J. Cell Biol. 102:420-431). We have now asked which cells synthesize these molecules, and how this synthesis is regulated. Electron microscopy revealed that mononucleated cells selectively accumulate in perisynaptic interstitial spaces beginning 2 d after denervation. These cells were identified as fibroblasts by ultrastructural and immunohistochemical criteria; [3H]thymidine autoradiography revealed that their accumulation results from local proliferation. Electron microscopic immunohistochemistry demonstrated that N-CAM is associated with the surface of the fibroblasts, while tenascin(J1) is associated with collagen fibers that abut fibroblasts. Using immunofluorescence and immunoprecipitation methods, we found that fibroblasts isolated from perisynaptic regions of denervated muscle synthesize N-CAM, tenascin(J1), fibronectin, and a heparan sulfate proteoglycan in vitro. Thus, fibroblasts that selectively proliferate in interstitial spaces near synaptic sites are likely to be the cellular source of the interstitial deposits of adhesive molecules in denervated muscle. To elucidate factors that might regulate the accumulation of these molecules in vivo, we analyzed the expression of tenascin(J1) and fibronectin by cultured fibroblasts. Fibroblasts from synapse-free regions of denervated muscle, as well as skin, lung, and 3T3 fibroblasts accumulate high levels of tenascin(J1) and fibronectin in culture, showing that perisynaptic fibroblasts are not unique in this regard. However, when they are first placed in culture, fibroblasts from denervated muscle bear more tenascin(J1) than fibroblasts from innervated muscle, indicating that expression of this molecule by fibroblasts is regulated by the muscle's state of innervation; this difference is no longer apparent after a few days in culture. In 3T3 cells, accumulation of tenascin(J1) is high in proliferating cultures, depressed in confluent cultures, and reactivated in cells stimulated to proliferate by replating at low density or by wounding a confluent monolayer. Thus, synthesis of tenascin(J1) is regulated in parallel with mitotic activity. In contrast, levels of fibronectin, which increase less dramatically after denervation in vivo, are similar in fibroblasts from innervated and denervated muscle and in proliferating and quiescent 3T3 cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of brefeldin A on the synthesis and secretion of egg white proteins in primary cultured oviduct cells of laying Japanese quail (Coturnix coturnix japonica)

We investigated the effect of brefeldin A (BFA) and monensin on the secretion of egg white proteins in primary cultured oviduct cells. Monensin inhibited the secretion of egg white proteins, but this drug also caused morphological changes of the cells and inhibited their protein synthesis. BFA inhibited protein secretion without any remarkable morphological changes of the cells and without significant inhibition of protein synthesis. In the presence of BFA at 1 microgram/ml, only 10% of synthesized ovalbumin was secreted into the medium even after 6 h. A similar effect of BFA was shown in the case of conalbumin. When the cells were cultured in the presence of BFA, precursors of mature ovalbumin accumulated which were not secreted. Their affinity for lectin binding affinity columns (Con-A Sepharose and WGA-agarose) suggested that they contained immature high-mannose-type chains. The above results suggest that the conversion of high-mannose-type oligosaccharides to hybrid-type ones, which is not usually observed in mammalian species but only in avian oviduct tissues, proceeds between the Golgi complex and the endoplasmic reticulum.

Fasciola hepatica: the effect of the sodium ionophore monensin on the adult tegument

The effect on the tegument of adult Fasciola hepatica of incubation in the sodium ionophore monensin, the Na+/K+-ATPase inhibitor ouabain and ouabain pretreatment followed by monensin has been determined in vitro by scanning and transmission electron microscopy (SEM, TEM). With monensin incubation alone (1 x 10(-6) M), a flattening of the tegument with some loss of spines on the ventral surface is evident from 0.5 h onwards. Internally, the subtegumental musculature becomes grossly swollen, although there is no swelling of the infoldings of the basal plasma membrane of the tegument, even after 24 h incubation. Ouabain incubation (1 x 10(-3) M) induces folding of the apical surface of the tegument from 0.5 h onwards, and this is accompanied by the formation of blebs and microvilli. Brief (0.5 h) exposure to ouabain (1 x 10(-3) M) followed by monensin treatment (1 x 10(-4) M, 3 h) leads to gross "vacuolation" of the tegument, but this is not due to swelling of the basal infoldings. The other main feature of ouabain-pretreated flukes is the projection of basal lamina-like material into the tegumental syncytium. Monensin treatment alone (1 x 10(-6) M) results in the Golgi complexes of the tegumental cells becoming very diffuse from 1.5 h onwards, and relatively few secretory bodies are present in the cytoplasm. After 0.5 h incubation in ouabain (1 x 10(-3) M), the Golgi complexes of the tegumental cells are indistinct, although numerous secretory bodies are still present. The classical monensin-induced swelling of the Golgi cisternae is observed in the tegumental cells only when monensin treatment (1 x 10(-4) M, 3 h) was preceded by brief (0.5 h) exposure to ouabain (1 x 10(-3) M). The results are discussed in relation to the postulated osmoregulatory role of the tegument and the role of sodium pumps in membrane function in the fluke.

Diamphenethide--a reassessment of its pharmacological action

At a concentration of 1 x 10(-4) M (28.84 micrograms/ml), with a solvent concentration of 1.0% (v/v) ethanol, the deacetylated (amine) metabolite of diamphenethide (DAMD) causes an initial stimulation of activity, followed by suppression, leading to a paralysis within 3 h. These changes are accompanied by an increase in muscle tone of more than 200 mg. However, ethanol alone at a concentration of 1.0% (v/v) causes an initial stimulation of activity and increase in muscle tone (approximately 550 mg). If the concentration of DAMD is kept at 1 x 10(-4) M (28.84 micrograms/ml) but the solvent concentration reduced [e.g., 0.05% (v/v) dimethyl sulphoxide], then only a suppression of motility and flaccid paralysis are observed. This response is also seen at the lower concentration of 10 micrograms/ml, which corresponds to the maximum blood levels of DAMD in vivo. The sodium ionophore monensin induces a suppression of motility, leading to a rapid flaccid paralysis (in approximately 1.5 h at 1 x 10(-7) M, and within a few minutes at higher concentrations). Ouabain, an inhibitor of Na+/K+-ATPase activity, also causes a suppression of motility, but this is accompanied by an increase in muscle tone, leading to a spastic paralysis (in approximately 2.5 h at 1 x 10(-3) M, and 3.5 h at 1 x 10(-4) M). Pretreatment with ouabain (1 x 10(-3) M for 15 min) followed by monensin (1 x 10(-5) M) reverses the original effect of monensin by inducing a rapid spastic paralysis (in approximately 50 min).(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of the incorporation of 3H-galactose and 3H-sialic acid into thyroglobulin in relation to the block of intracellular transport induced by monensin. Studies with isolated porcine thyroid follicles

The Na+/K+ ionophore monensin is known to arrest the intracellular transport of newly synthesized proteins in the Golgi complex. In the present investigation the effect of monensin on the secretion of 3H-galactose-labeled and 3H-sialic acid-labeled thyroglobulin was studied in open thyroid follicles isolated from porcine thyroid tissue. Follicles were incubated with 3H-galactose at 20 degrees C for 1 h; at this temperature the labeled thyroglobulin remains in the labeling compartment (Ring et al. 1987a). The follicles were then chased at 37 degrees C for 1 h in the absence or presence of 1 microM monensin. Without monensin substantial amounts of labeled thyroglobulin were secreted into the medium, whereas in the presence of the ionophore secretion was inhibited by 80%. Since we have previously shown (Ring et al. 1987b) that monensin does not inhibit secretion of thyroglobulin present on the distal side of the monensin block we conclude that galactose is incorporated into thyroglobulin on the proximal side of this block. Secretion was also measured in follicles continuously incubated with 3H-galactose for 1 h at 37 degrees C in the absence or presence of monensin. In these experiments secretion of labeled thyroglobulin was inhibited by about 85% in the presence of monensin. Identically designed experiments with 3H-N-acetylmannosamine, a precursor of sialic acid, gave similar results, i.e., almost complete inhibition of secretion of labeled thyroglobulin in the presence of monensin. The agreement between the results of the galactose and sialic acid experiments indicates that sialic acid, like galactose, is incorporated into thyroglobulin on the proximal side of the monensin block.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of monensin on thyroglobulin secretion. Studies with isolated follicles from pig thyroids

The effect of monensin on the secretion of thyroglobulin was studied in open follicles isolated from pig thyroid tissue; in this system, thyroglobulin is secreted into the incubation medium. When monensin was present during a 4-h chase incubation after pulse-labelling with 3H-leucine, the secretion of labelled thyroglobulin was reduced by about 85%; in electron-microscopic autoradiographs of rat thyroid lobes labelled and chase-incubated under similar conditions the relative number of grains over follicle lumina was strongly reduced when monensin was present during the chase. These observations are in agreement with the consensus that monensin arrests transport of secretory proteins in the Golgi complex. In other experiments, pulse-labelled follicles were chase-incubated for 1.5 h whereby labelled thyroglobulin was transported from the RER to exocytic vesicles. Monensin present during a subsequent chase of 0.5 h caused only a moderate decrease of labelled thyroglobulin secretion. TSH present during the second chase-stimulated secretion in both control and monensin-exposed follicles. TSH also caused a drastic reduction of exocytic vesicles in rat thyroid lobes, and the number of vesicles remaining in the cells was the same in controls and lobes exposed to the ionophore. The observations are interpreted to show that monensin does not inhibit the basal or TSH-stimulated transport of thyroglobulin from the site of monensin-induced arrest in the Golgi complex to the apical cell surface or the exocytosis of thyroglobulin.

Retinoic acid alters the metabolic 3H-labelling of glycosphingolipids

Retinoic acid increases the incorporation of radioactivity from a mixture of [3H]-galactose and [3H]-glucosamine into glycosphingolipids of serum-starved quiescent human foreskin fibroblasts with a preferential labelling of ceramide mono- and dihexoside as compared to ceramide tri- and tetrahexoside. Under the conditions used, no similar change in the specific labelling of glycoprotein is observed. Alteration in [3H]-precursor uptake into glycolipids comparable to that seen under the influence of retinoic acid does not occur in the presence of phorbolester, colchicine, butyrate or after infection with cytomegalovirus.

Effect of cooling on intracellular transport and secretion of thyroglobulin

The effect of cooling to 20 degrees C on the intracellular transport and secretion of thyroglobulin was studied by incubating open thyroid follicles isolated from porcine thyroid tissue. Follicles were labeled with 3H-leucine or 3H-galactose and the secretion of labeled thyroglobulin into the incubation medium was followed by chase incubations under various experimental conditions. The observations indicate that the transport of thyroglobulin is inhibited at three sites of the intracellular pathway by cooling to 20 degrees C, i.e., between the RER cisternae and the Golgi cisternae, between the latter and the exocytic vesicles, and between these vesicles and the extracellular space (corresponding to the follicle lumen). The secretion of 3H-leucine-labeled thyroglobulin decreased linearly between 37 degrees and 20 degrees C; within this temperature range the activation energy for secretion, calculated from Arrhenius plots, was found to be 37 kcal/mol. Below 20 degrees C the secretion was scarcely measurable. It is suggested that the three transport blocks at 20 degrees C result mainly from inhibition of membrane fission and fusion due to phase transition in membrane lipids.

Effect of monensin and nocodazole on the intestinal lipid esterification in mouse jejunal organ culture

The ability of mouse jejunal explants to esterify a lipid emulsion containing oleic acid, palmitic acid and monopalmitin has been studied in different in vitro experimental conditions. The incubating lipid solution must have a minimum volume for obtaining optimal triglyceride esterification by the cultured intestinal mucosa. In our incubating conditions the exchange of oleic for palmitic acid does not significantly modify the amount of lipids esterified by the explants in 15 min. Monensin or nocodazole, added to the culture medium of intestinal explants for 3 hr, significantly change the amount of lipids esterified and secreted. The inhibition observed after nocodazole treatment disappears, however, when the explants are rinsed and the culture is allowed to continue for an additional 3 hr in a drug-free medium. These results suggest that the regulation of lipid metabolism can be studied in organ culture.

Influence of monensin on biosynthesis, processing and secretion of proteodermatan sulfate by skin fibroblasts

The influence of monensin on biosynthesis, processing and secretion of proteodermatan sulfate from human skin fibroblasts was studied with the aid of a specific immunological procedure. Double-labeling experiments with [3H]leucine and [35S]sulfate indicated that monensin caused a dose-dependent parallel decrease of sulfate incorporation into total and of secretion of 3H-labeled proteodermatan sulfate. Compared with the untreated control, a greater proportion of incorporated [35S]sulfate than of incorporated [3H]leucine became secreted. Other monensin effects were a moderate intracellular accumulation of glycosaminoglycan-free core protein, a reduced chain length and a greatly reduced epimerization of D-glucuronic to L-iduronic acid residues. In contrast to the formation of N-acetylgalactosamine 4-sulfate residues 6-sulfation was not affected. Conversion of high-mannose-type oligosaccharides to complex-type N-glycans which normally occurred concomitantly with glycosaminoglycan biosynthesis was inhibited. Withdrawal of monensin made possible an additional sulfation of intracellularly accumulated proteodermatan sulfate. The newly formed sulfate esters did not cluster at the non-reducing ends of the glycosaminoglycan chains. Cells preexposed to monensin and labeled with [3H]glucosamine either in the absence or continuous presence of the drug incorporated similar amounts of 3H radioactivity into proteodermatan sulfate. The results suggest that epimerization of D-glucuronic acid residues and 4-sulfation occur predominantly in the trans cisternae of the Golgi apparatus whereas chain polymerisation and 6-sulfation take place predominantly in the cis Golgi complex.

The confined function model of the Golgi complex: center for ordered processing of biosynthetic products of the rough endoplasmic reticulum

The organized and characteristic elements of the Golgi complex (GC) are the stacked smooth-surfaced cisternae, which are found in the centrosphere of all eukaryotic cells. These cisternae, in conjunction with other associated smooth-surfaced membranes, are responsible for executing net unidirectional intracellular transport (ICT) from the rough endoplasmic reticulum (RER) toward more distally located structures. This chapter focuses on the broad range of accessory activities that occur during transport, the family of “posttranslational modifications.” These events are, in all likelihood, not essential for the “primary” function of the GC yet they are crucial in allowing the cell to tailor its biosynthetic products for its own needs and the needs of the organism as a whole. In addition to modifying products of the rough endoplasmic reticulum, the GC may be involved in processing events because of its participation in other routes of vesicular traffic—for example, centripetal traffic from the cell surface. Various nonequivalent criteria have been used to ascribe processing events to the GC-autoradiography, preparative or analytic subcellular fractionation, interruption by ICT inhibitors, and delay in the impact of cycloheximide.

The role of subcompartments of the Golgi complex in protein intracellular transport

The functioning of the Golgi complex in protein intracellular transport is most simply understood in terms of its being composed of a sequence of functionally distinct subcompartments. For example, the influence of perturbation of cellular Na+-K+ balance on the transport of secretory and membrane glycoproteins is to greatly slow their passage from relatively proximal to relatively distal subcompartments. To further the understanding of the nature of these subcompartments a rat IgM myeloma has been subjected to analytical subcellular fractionation. Fractions selectively enriched in distinct Golgi-associated activities have been prepared and their membrane proteins compared with those of rough microsomal fractions. The subfractionation is extensive and suggests the possibility of obtaining well resolved Golgi subfractions. Myeloma cells stained intracellularly with Concanavalin A- and wheatgerm agglutinin--peroxidase conjugates show distinct labelling patterns. Concanavalin A stains the entirety of the rough endoplasmic reticulum as well as the proximal face of the Golgi stack. Wheatgerm agglutinin stains the distal face of the stack of Golgi cisternae. The staining patterns are not due to immunoglobulin as they are also observed in myeloma variants that fail to synthesize immunoglobulin.