[CANCER RESEARCH 46, 6418-6422, December 1986] Ability of Inhibitors of and Synthesis to Sensitize Cells to , , Shigella , and Pseudomonas Toxin

Kirsten Sandvig, Tor Inge T0nnessen, and Sjur Olsnes Norsk Hydro's Institute for Cancer Research and The Norwegian Cancer Society, Montebello, Oslo, Norway

ABSTRACT MATERIALS AND METHODS

A number of compounds that interfere with glycoprotein synthesis and . Abrin, ricin, and Shigella toxin were purified as earlier transport have been tested for their ability to sensitize cells to cancero- described (10-12). Purified A was a static protein toxins. Tunicamycin, swainsonine, cycloheximide, and pur- generous gift from Dr. Stephen H. Leppla (SAMRIID, Frederick, MD). omycin sensitized Vero cells and HeLa cells to abrin and ricin, as we Ricin was coupled covalently to horseradish peroxidase with SPDP1 have found previously with monensin (K. Sandvig and S. Olsnes, J. Biol. essentially as described by Carlsson et al. (13). Ricin was treated with Chem., 257: 7504-7513, 1982). Cycloheximide, but not swainsonine, a 10-fold molar excess of SPDP, and horseradish peroxidase was treated sensitized Vero cells to A and Shigella toxin. The with a 20-fold molar excess. Unreacted SPDP was removed by gel ability of ricin to intoxicate cells was much lower at 19°Cthan at 37°C filtration on Sephadex G-25. Derivatized HRP was reduced with 50 and there was almost no sensitizing effect of cycloheximide and monensin mM dithiothreitol for 20 min at room temperature, run through a at 19°C.Studies by electron microscopy showed that ricin conjugated to Sephadex G-25 column, and then reacted with derivatized ricin over horseradish peroxidase appeared in trans Golgi elements in Vero cells. night at room temperature. Polyacrylamide gel electrophoresis revealed Possibly, transport of ricin into the cytosol requires passage through the that the conjugates formed contained ricin and horseradish peroxidase Golgi apparatus. The possibility that the sensitizing agents here described (1:1) as well as higher molecular weight conjugates. In order to obtain may be valuable in enhancing the action of is discussed. monovalent conjugates (i.e., ricin:HRP, 1:1), ricin-HRP was run through a Sephacryl S-200 column equilibrated with 0.14 M NaCl, 20 mM sodium phosphate (pH 7.4), and 0. l M lactose. Conjugates were INTRODUCTION well separated according to their molecular weight. Monovalent con jugates as shown by sodium dodecyl sulfate-polyacrylamide gel electro In recent years there has been an increasing interest in protein phoresis were used in the experiments. toxins with intracellular sites of action. A main reason for this Materials. Monensin and puromycin were obtained from Calbi- interest is the cancerostatic properties of the toxins abrin ochem-Behring Corp., La Jolla, CA; swainsonine and tunicamycin were and ricin (1-3) as well as the use of these toxins in the construc from Boehringer Mannheim GmbH, Mannheim, West Germany; cy tion of target-specific cytotoxic conjugates. Toxins that act on cloheximide and horseradish peroxidase type VI were from Sigma intracellular targets include the plant toxins abrin, ricin, mo- Chemical Co., St. Louis, MO; and SPDP was from Pharmacia, Upps deccin, and viscumin and the bacterial toxins toxin, ala, Sweden. Culture. HeLa cells, Vero cells (from African green monkey Shigella toxin, and Pseudomonas toxin. The mechanism of ), fetal hamster kidney cells, and mouse L-cells were propagated action of these toxins involves binding to cell surface receptors in monolayer cultures in minimum essential medium (Gibco, Europe, and subsequent of the bound toxin. An enzymatic- Ltd., Paisley, Scotland) with 10% fetal calf serum. ally active part of the toxins then crosses the limiting membrane Measurement of Protein Synthesis. After incubation with toxin in a of the vesicle and enters the cytosol where it inhibits protein Ar-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid containing me synthesis (for review, see Ref. 4). Due to the enzymatic effect dium with and without the indicated compounds as described in the of the toxins, few molecules are sufficient to kill cells. Whole legends to the figures, the cells were transferred to medium without toxins or enzymatically active fragments of toxins in this group inhibitors and the ability of the cells to incorporate [3H]leucine during 15 min was measured as earlier described (8). Incorporation of [3HJ- have therefore been conjugated to , e.g., antibodies, that bind to defined cell surface structures (immunotoxins). In those leucine into cells that had previously been exposed to monensin, swain cases where intact toxins are used the native binding sites are sonine, tunicamycin, cycloheximide, and puromycin but not to toxin was essentially the same as in cells that had not been treated with these commonly blocked by chemical modification or by addition of compounds. carbohydrates to which the toxins bind (5). Electron Microscopy. The cells were fixed with 1.5% glutaraldehyde The mechanism of toxin entry into the cytosol is only par in 0. l M cacodylate buffer at pH 7.4 for 20 min. Cytochemical dem tially understood. In spite of the structural similarity of these onstration of ricin-HRP was performed as described by Graham and toxins they have different requirements for entry and they seem Karnovsky (14). Cells were postfixed in 1% OsO4 in 0.1 M cacodylate to enter the cytosol from different intracellular compartments. buffer, pH 7.4. They were dehydrated in 50-96% ethanol and embedded Compounds that enhance the entry of the enzymatically active in Luft Epon through propylene oxide. Specimens were stained with part of the toxins into the cytosol may therefore give valuable lead citrate for 10 min. Ultrathin sections were examined with a Jeol clues to the nature of the entry mechanism. Such compounds 1200 electron microscope. may also improve the action of immunotoxins. Thus, NH4C1 and monensin, which sensitize cells to abrin and ricin (6-8), RESULTS AND DISCUSSION are now widely used to sensitize cells to immunotoxins. Since It has been shown earlier that low concentrations of monensin the low concentrations of monensin often used for this purpose sensitize cells to abrin and ricin as well as to a number of are known to affect the glycosylation of proteins (9) we decided immunotoxins (6-8, 15, 16). Although the low concentrations to investigate if other compounds that interfere with glycosyl of monensin commonly used for this purpose do not strongly ation or inhibit formation of glycoproteins have a similar sen affect the acidification of and lysosomes, they in sitizing effect. hibit the processing of glycoproteins by affecting transport Received 8/12/85; revised 2/28/86, 5/1/86, 7/22/86; accepted 8/18/86. within the Golgi apparatus (9, 17). We therefore tested the The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in 1The abbreviations used are: SPDP, Ar-succinimidyl-3-(2-pyridylthio)pro- accordance with 18 U.S.C. Section 1734 solely to indicate this fact. pionate; HRP, horseradish peroxidase. 6418 Downloaded from cancerres.aacrjournals.org on September 25, 2021. © 1986 American Association for Cancer Research. SENSITIZATION TO PROTEIN TOXINS effect on toxin entry of two other compounds known to interfere but an even larger fraction of the ricin-HRP was now found in with the assembly of glycoproteins. Tunicamycin inhibits the secondary lysosomes. In cells treated with 10~7 M monensin there was a marked addition of carbohydrate to asparagine by preventing the syn thesis of the lipid intermediate involved in this process (18). difference in morphological appearance. Cells were preincu Swainsonine, which inhibits the Golgi enzyme, mannosidase II, bated for 30 min with monensin and then ricin-HRP was added to the cells. After l h incubation at 4°Cthe cells were washed prevents the removal of carbohydrate from one branch of the high mannose intermediate formed during glycosylation (19), and then incubated at 37"C. After 15 min at 37°Cricin-HRP and it thus inhibits the subsequent additions of carbohydrates. was found in the same elements as in cells not treated with As shown in Fig. 1, swainsonine sensitized Vero cells to ricin monensin, but the Golgi stacks and trans-Go\gi elements had to the same extent as monensin. Also tunicamycin had a sen already started to swell. After 45 min numerous large vesicular sitizing effect, although smaller than that found with the other profiles with electron-dense vesicles around them were seen in two compounds. Tunicamycin, swainsonine, and monensin sen the Golgi region. These probably represent swollen Golgi ele sitized Vero cells to abrin to the same extent as to ricin (data ments, and they contained large amounts of ricin-HRP. After 2 h incubation at 37°Cthere were fewer typical second not shown). The sensitizing effect was observed also at a toxin concentration high enough to saturate all the cell surface- ary lysosomes than in cells not treated with monensin indicating binding sites (10 Mg/ml toxin). Furthermore, the effect was not that ricin-HRP had been trapped in the Golgi region (Fig. 2b). limited to Vero cells. Thus, tunicamycin, swainsonine, and In another experiment cells were treated with swainsonine (1 monensin also sensitized HeLa cells and mouse L-cells to ricin ¿ig/ml)in the same way as described for monensin. After 15 and abrin. min ricin-HRP was found in the same elements as in the control To study the effect of the inhibitors on the uptake and experiment, but there was less labeling of endosomes and intracellular routing of ricin, cells were treated with ricin con somewhat more labeling of tubular elements. After 45 min at 37°Cclusters of small tubular and vesicular structures appeared jugated to HRP, and we determined by electron microscopy the localization of the conjugate after different periods of time. To in the trans-Golgi region. Even 2-3 stacks which could belong syncronize the uptake, cells were first incubated with ricin-HRP to the Golgi apparatus were labeled. After 2 h incubation more at 4°Cfor 1 h, to allow binding, but not entry to occur. The lysosomes were labeled than in the control experiment (Fig. cells were then washed and incubated at 37°C.After 15 min 2e). incubation at 37°Cricin-HRP was found mainly on the cell Possibly, ricin must be modified by enzymes present in the surface and in small vesicular and tubular profiles just inside Golgi apparatus or transported through Golgi elements before the . Also larger vacuolar profiles, presumably it can enter the cytosol. The fact that ricin is observed only in endosomes, were labeled. Similarly, Nicolson found ricin in fra/w-Golgi elements, while tunicamycin and swainsonine affect endocytic compartments (20). At this stage some staining was processing of glycoproteins before they reach this location (22), found in vesicles in the irans-Go\gi network in accordance with would imply that, in the absence of inhibitors, ricin competes the results published by Gonatas et al. (21). with newly formed glycoproteins for transport or for processing. After 45 minutes of incubation at 37°Ca larger fraction of Experiments were also carried out where the formation of the ricin-HRP conjugate had moved deeper into the cell interior glycoproteins was blocked with the inhibitors cyclo- and was found in numerous small vesicular and tubular profiles heximide and puromycin. As shown in Fig. 3, both these throughout the . Many of these structures appeared compounds sensitized Vero and HeLa cells to ricin. None of to belong to the frans-Golgi network and ricin-HRP was also these drugs affected the endocytic uptake of ricin. Similar occasionally found in structures that appeared to be trans-Go\gi results were obtained with abrin (not shown). Since puromycin stacks. With increasing time of incubation less material was induces depolymerization of the polysomes while cyclohexi- found in structures presumed to be endosomes, and more was mide has the opposite effect (23, 24), it is unlikely that the found in vesicles in the Golgi region and in lysosomes (Fig. la). sensitization to the toxins is due to increased exposure of the After 2 h incubation at 37°Cthe same elements were labeled, toxin-sensitive site of the ribosomes. It should be noted that in cells incubated with cycloheximide and then transferred to medium without this compound the ability to synthesize protein was immediately restored. Also the sensitivity to ricin was restored to the normal level upon removal of the drug. Clearly toxin only 80- therefore, the sensitizing effect of cycloheximide is not due to cell damage or to a synergistic effect with ricin on protein synthesis. The data suggest that the sensitizing effect is due to lì swainsonine an increased transport of the enzymatically active fragment into * 8 40 the cytosol, and the results are consistent with the hypothesis I ° that ricin competes or interacts with newly synthesized glyco ¡•£.20 proteins. Electron microscopic examination of cycloheximide-treated

W^ IO"11 IT* «T cells revealed that there was no clear difference between cells treated with cycloheximide and untreated cells after 15 min Ricin concentration (M) incubation at 37°C.However, after 45 min more endosomes Fig. I. Ability of tunicamycin. swainsonine, and monensin to sensitize Vero and endocytic tubular elements were labeled than in cells not cells to ricin. Vero cells, growing in 24-well disposable trays, were preincubated treated with cycloheximide. After 2 h incubation at 37°Cthe for 30 min in the absence and presence of tunicamycin and for 15 min in the absence and presence of swainsonine and monensin. Increasing concentrations of trans stack in many Golgi apparatuses were swollen. This stack ricin were then added, and the incubation was continued for 3 h at 37°C.Finally, protein synthesis was measured during a 15-min interval as described in "Mate and not seldom structures that appeared to be one or two rials and Methods." The additions were: O, 10~7M tunicamycin; A, swainsonine additional Golgi stacks were labeled with ricin-HRP (Fig. 2c). (1 /Jg/ml): •,IO"7M monensin; X, none. More secondary dense lysosomes were labeled as well as many 6419 Downloaded from cancerres.aacrjournals.org on September 25, 2021. © 1986 American Association for Cancer Research. % E, «¿ C >*•té «MV c Go cGo

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Fig. 2. Localization of ricin-HRP in Vero cells. In a ricin-HRP was incubated with cells at 4"C for 1 h. The cells were then washed and incubated in N-2- hydroxyethylpiperazine-N'-cthanesulfonic acid medium at 37'C for 2 h. Ricin-HRP is seen in endosomes (E) and in tubular and vesicular structures in the trans- Golgi network (¡Go)as well as in rrani-Golgi stacks. The m-Golgi (cGo) is unlabeled. In b cells were incubated with 10"' M monensin for 15 min at 37*C before transfer to 4"C and incubation with ricin-HRP as in a. Ricin-HRP is localized in large vesicular profiles frequently found in the frans-Golgi region. Bar, 0.5 um. In t1 cells were treated with cycloheximide (10 .u ml) in the same way as with monensin. Ricin-HRP is found in small electron-dense vesicles in the irani-Golgi network as well as in Golgi stacks. Frequently frans-Golgi stacks were swollen (arrowhead) and often 2-3 stacks were labeled (arrow). In d cells were incubated at !9°Cfor 2 h after incubation of ricin-HRP with the cells at 4°Cfor I h. There was almost no ricin-HRP in the iranj-Golgi network in these cells. In e cells were treated with swainsonine (1 Mg/ml) after the same protocol as in b and c. Golgi complexes were often found to be elongated with extensive labeling in 2-3 stacks as well as in vesicular profiles in /rans-Golgi region. Labeling is also seen in endosomes and lysosomes (L). Bars, 0.5 ¿im. 6420 Downloaded from cancerres.aacrjournals.org on September 25, 2021. © 1986 American Association for Cancer Research. SENSITIZATION TO PROTEIN TOXINS

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cycloheximide, 37'C

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Fig. 4. Effect of temperature on the ability of monensin and cycloheximide to sensitize cells to ricin. Vero cells growing in 24-well disposable trays were incubated in the absence and presence of cycloheximide and monensin for 15 min at 37°Cand at 19°C.Then increasing concentrations of ricin were added and the incubation was continued for 18 h. Finally, protein synthesis was measured during a 15-min interval at 37°Cas described in "Materials and Methods." O, A, D, 18 h at 37°C;•,A, •,18 h at 19°C.The additions were: O, •,cycloheximide (10 Mg/ml); A, A, 10~7M monensin; D, •none.

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HT" KT1i IO' KT Toxin concentration (M) puromycin Fig. 3. Ability of cycloheximide and puromycin to sensitize cells to ricin. Vero cells and HeLa cells growing in 24-well disposable trays were incubated for 15 min in the absence and presence of cycloheximide and puromycin. Then increasing concentrations of ricin were added, and the incubation was continued for 3 h at 37°C.Finally, protein synthesis was measured as described in "Materials and 10 10 Methods." The additions were: O, cycloheximide (10 fig/ml); A, 1 m\i puromycin; x. none. Toxin concentration (M) Fig. 5. Effect of swainsonine. cycloheximide, and puromycin on the sensitivity of Vero cells to Pseudomonas toxin. Vero cells growing in 24-well disposable large multivesicular bodies. These structures were very seldom trays were incubated for 15 min in the absence and presence of swainsonine, cycloheximide, and puromycin. Then increasing concentrations of Pseudomonas found in cells not treated with cycloheximide. toxin were added and the incubation was continued for 6 h at 37°C.Finally, Although the electron microscopic examinations showed that protein synthesis was measured as described in "Materials and Methods." The all the sensitizing agents lead to an increased labeling with additions were: •,swainsonine (1 jig/ml); A, cycloheximide (10 fig/ml); O, I mivi ricin-HRP in the Golgi region, we cannot exclude the possibility puromycin; x, none. that entry of ricin into the cytosol occurs somewhere else. Inhibition of glycoprotein synthesis could increase the amount Pseudomonas toxin (Fig. 5), while swainsonine did not have of toxin transported to the cytosol by specifically inhibiting this effect. Also the sensitivity of Vero cells to Shigella toxin formation of toxin-inactivating enzymes. However, since ricin was increased 10-fold by cycloheximide, but not by swainsonine and abrin are very resistant to proteolytic enzymes, we consider (not shown). It should be noted that, in contrast to abrin and this less likely. ricin, Pseudomonas toxin and Shigella toxin are not glyco- In order to further characterize the sensitizing effect of cyclo sylated. It is therefore possible that the bacterial toxins do not heximide and monensin on ricin entry, we studied the effect of compete with newly synthesized glycoproteins for transport or temperature on this process. As shown in Fig. 4 there was a processing in the same way as abrin and ricin may do. Further strong reduction in the toxic effect of ricin when the incubation more, there is no evidence that Pseudomonas toxin and Shigella was performed at 19°Crather than at 37°C,and at 19°Cthere toxin bind to glycoprotein as ricin and abrin do. Therefore, was no sensitizing effect of monensin and only a slight sensitiz binding of Pseudomonas toxin and Shigella toxin to newly ing effect of cycloheximide. It has been shown that fusion of formed glycoproteins would probably not occur to the same intracellular vesicles and transport within the Golgi apparatus extent as in the case of ricin and abrin. are inhibited at low temperature (25-27). In electron micro It is not clear if inhibition of protein synthesis sensitizes cells scopic examination of cells incubated at 19°Cwith ricin-HRP, to all four toxins by the same mechanism since the protein hardly any labeling was found in the Golgi region and in toxins here described have different requirements for entry (4). secondary lysosomes indicating that transport from early en- Thus, the entry mechanism of abrin and ricin seems to differ docytic compartments (endosomes and tubular elements) to from that of Shigella toxin and Pseudomonas toxin, suggesting Golgi and lysosomes was inhibited (Fig. 2d). This further that also the mechanism behind the sensitization here described, strengthens the hypothesis that the Golgi apparatus is impor differs. This is now being investigated. Since monensin and tant for the action of ricin. Possibly, the residual toxic effect ammonium chloride sensitize cells to ricin and abrin and have observed with ricin at 19°Cisdue to transport mechanism other any even larger sensitizing effect on some immunotoxins, it is than the efficient one operating at 37°C. possible that the new sensitizing agents here described may Cycloheximide and puromycin also sensitized Vero cells to prove to be of practical use in work with immunotoxins. 6421 Downloaded from cancerres.aacrjournals.org on September 25, 2021. © 1986 American Association for Cancer Research. SENSITIZATION TO PROTEIN TOXINS

13. Carlsson, J., Drevin, H., and Axén,R. Protein thiolation and reversible ACKNOWLEDGMENTS protein-protein conjugation. A'-Succinimidyl-3-(2-pyridyldithio)propionate, a new heterobifunctional reagent. Biochem. J., 173: 723-737, 1978. Jorunn Jacobsen and Trine Ryghseter provided skillful technical 14. Graham, R. C., and Karnovsky, M. J. The early stages of absorption of assistance. injected horseradish peroxidase in the proximal tubules of mouse kidney: ultrastructural cytochemistry by a new technique. J. Histochem. Cytochem., 14: 291 -302, 1966. REFERENCES 15. Casellas, P., Bourrie, B. J. P., Gros, P., and Jansen, F. K. Kinetics of cytotoxicity induced by immunotoxins. J. Biol. Chem., 259: 9359-9364, 1. Fodstad, 0., Kvalheim, G., Godal, A.. Lotsberg, J., Aamdal, S., Hast, H., 1984. and Pihl, A. Phase I study of the plant protein ricin. Cancer Res., 44: 862- 16. Raso, V., and Lawrence, J. Carboxylic ionophores enhance the cytotoxic 865, 1984. potency of ligand- and antibody-delivered ricin a chain. J. Exp. Med., 160: 2. Fodstad, 0., Olsnes, S., and Pihl, A. Inhibitory effect of abrin and ricin of 1234-1240, 1984. the growth of transplantable murine tumors and of abrin on human cancers 17. Marsh, M., Wellsteed, J., Kern, H., Harms, E., and Helenius, A. Monensin in nude mice. Cancer Res., 37: 4559-4567, 1977. inhibits Semliki Forest virus penetration into culture cells. Proc. Nati. Acad. 3. Lin, J-Y., Tserng, K-Y., Chen, C-C, Lin, L-T.. and Tung, T-C. Abrin and Sci. USA, 79:5297-5301, 1982. ricin: new antitumor substances. Nature (Lond.), 227: 292-293, 1970. 18. Elbein, A. D. The tunicamycins—useful tools for studies on glycoproteins. 4. Olsnes, S., and Sandvig, K. Entry of toxic proteins into cells. In: P. Cuatre- Trends Biochem. Sci., 6: 219-221, 1981. casas and T. F. Roth (eds.). -mediated Endocytosis, Ser. B, Vol. 15, 19. Tulsiani, D. R. P., Harris, T. M.. and Touster, O. Swainsonine inhibits the pp. 189-236, London: Chapman and Hall. 1983. biosynthesis of complex glycoproteins by inhibition of Golgi mannosidase II. 5. Vitetta, E. S., Krolick, K. A., Miyama-Inaba. M., Cushley, W., and Uhr, J. J. Biol. Chem., 257: 7936-7939, 1982. W. Immunotoxins: a new approach to cancer therapy. Science (Wash. DC) 20. Nicolson, G. L. Ultrastructural analyses of toxin binding and entry into 2/9:644-650, 1983. mammalian cells. Nature (Lond.), 25/: 628-630, 1974. 6. Ray, B.. and Wu, H. C. Enhanced internalization of ricin in nigericin- 21. Gonatas, N. K., Kim, S. U., Stieber, A., and Avrameas, S. Internalization of pretreated Chinese hamster ovary cells. Mol. Cell. Biol., /: 552-559, 1981. in neuronal GERL. J. Cell Biol., 73: 1-13, 1977. 7. Sandvig, K.. Olsnes, S., and Pihl, A. Inhibitory effect of ammonium chloride 22. Rothman. J. E. The compartmental organization of the Golgi apparatus. Sci. and chloroquine on the entry of the toxic modeccin into I Ici .a cells. Am. 253: 84-95. 1985. Biochem. Biophys. Res. Commun.. 90:648-655, 1979. 23. Haselkorn, R., and Rothman-Denes, R. Protein synthesis. Annu. Rev. Mi- 8. Sandvig, K., and Olsnes, S. Entry of the toxic proteins abrin, modeccin, ricin, crobiol., 43: 397-438, 1973. and into cells. II. Effect of pH. metabolic inhibitors, and 24. Pestka, S. Inhibitors of ribosome functions. Annu. Rev. Microbiol., 25:487- ionophores and evidence for toxin penetration from endocytotic vesicles. J. 562, 1971. Biol. Chem., 257: 7504-7513, 1982. 25. Dunn, W. A., Hubbard, A. L., and Aronson, N. N., Jr. Low temperature 9. Tartakoff, A. M. Perturbation of vesicular traffic with the carboxylic iono- selectively inhibits fusion between pinocytic vesicles and lysosomes during phore moncnsin. Cell. 32: 1026-1028, 1983. heterophagy of 125I-asialofetuin by the perfused rat . J. Biol. Chem., 255: 10. Olsnes, S.. and Pihl, A. Isolation and properties of abrin: a toxic protein 5971-5978, 1980. inhibiting protein synthesis. Eur. J. Biochem., 35: 179-185, 1973. 26. Matlin, K. S., and Simons, K. Reduced temperature prevents transfer of a 11. Olsnes. S., and Pihl, A. Different biological properties of the two constituent membrane glycoprotein to the cell surface but does not prevent terminal chains of ricin, a toxic protein inhibiting protein synthesis. Biochem glycosylation. Cell, 34: 233-243. 1983. istry, 12: 3121-3126, 1973. 27. Saraste, J., and Kuismanen, E. Pre- and post-Golgi vacuoles operate in the 12. Olsnes. S., and Eiklid, K. Isolation and characterization of Shigella shigae transport of Semliki Forest virus membrane glycoproteins to the cell surface. cytotoxin. J. Biol. Chem., 255: 284-289, 1980. Cell, 38: 535-549, 1984.

6422 Downloaded from cancerres.aacrjournals.org on September 25, 2021. © 1986 American Association for Cancer Research. Ability of Inhibitors of Glycosylation and Protein Synthesis to Sensitize Cells to Abrin, Ricin, Shigella Toxin, and Pseudomonas Toxin

Kirsten Sandvig, Tor Inge Tønnessen and Sjur Olsnes

Cancer Res 1986;46:6418-6422.

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