Enhancement of cytotoxicity of modeccin by nigericin in modeccin-resistant mutant cell lines

Enhanced killing of human epidermoid carcinoma (KB) cells by treatment with ricin encapsulated into sterically stabilized liposomes in combination with monensin

Ricin was encapsulated in various charged liposomes having 5 mol% PEG of different chain length on the surface. The cytotoxicity of ricin entrapped in these liposomal formulations was examined in human epidermoid carcinoma (KB) cells with a view to develop an optimum delivery system for ricin in vivo. It was observed that the cytotoxicity of ricin entrapped in various charged liposomes was significantly dependent on the surface charge as well as chain length of PEG. The maximum cytotoxicity of ricin was observed when it was delivered through negatively charged liposomes having 5 mol% PEG-2000 on the surface. Monensin enhances the cytotoxicity of ricin entrapped in various charged liposomes depending on the surface charge. Maximum potentiation of cytotoxicity of ricin was observed when it was delivered through negatively charged liposomes having 5 mol% PEG-2000 on the surface. Studies on the kinetics of inhibition of protein synthesis by ricin revealed that the lag period of inhibition of protein synthesis is significantly lengthened following its delivery through various charged liposomes. Monensin significantly reduced the lag period of action of ricin. It was also observed that the efficacies of monensin on the enhancement of cytotoxicity of ricin entrapped in various charged PEG-liposomes were highly related to their amount of cell association. The current study has demonstrated that by suitable adjustment of charge, density, and chain length of PEG on the surface of liposomes it would be possible to direct liposomal ricin to human tumor cells for their selective elimination in combination with monensin.

Effect of surface charge and density of distearylphosphatidylethanolamine-mPEG-2000 (DSPE-mPEG-2000) on the cytotoxicity of liposome-entrapped ricin: Effect of lysosomotropic agents

Ricin was encapsulated in various liposomes having neutral, negatively and positively charged and different density of DSPE-mPEG-2000 on the surface and cytotoxicity of ricin entrapped in these different charged liposomal formulations was studied in CHO pro(-) cells and compared with free ricin with a view to develop an optimum delivery system for ricin in vivo. It was observed that the cytotoxicity of ricin entrapped in various charged liposomes was significantly dependent on the charge on the surface of liposomes. The maximum cytotoxicity of ricin was observed when it was delivered through negatively charged liposomes. Monensin enhances the cytotoxicity of ricin entrapped in various charged liposomes and the extent of enhancement of the cytotoxicity is significantly dependent on the charge on the surface of liposomes. Maximum potentiation (213.14-fold) of cytotoxicity of ricin was observed when it was delivered through positively charged liposomes followed by negatively charged (83.36-fold) and neutral (71.30-fold) liposomes, respectively. Studies on the kinetics of inhibition of protein synthesis by ricin entrapped in various charged liposomes revealed that lag period of inhibition of protein synthesis is significantly lengthened following delivery through various charged liposomes. However, in the presence of monensin, the lag period was reduced. There is a marginal variation in the cytotoxicity of ricin entrapped in various charged liposomes after incorporation of 5mol% of DSPE-mPEG-2000 on the surface. However, there is a significant variation in the enhancing potency of monensin on the cytotoxicity of ricin entrapped in various charged liposomes in CHO pro(-) cells following incorporation of 5mol% DSPE-mPEG-2000 on the surface. Studies on the effect of variation of density of DSPE-mPEG-2000 on the surface of various charged liposomes on the enhancement of cytotoxicity of entrapped ricin by monensin in CHO pro(-) cells showed that the enhancing potency of monensin on the cytotoxicity of ricin entrapped in various charged liposomes is significantly dependent on the density of DSPE-mPEG-2000 on their surface. It was also observed that the efficacies of monensin on the enhancement of cytotoxicity of ricin entrapped in various charged PEG-liposomes in CHO pro(-) cells was highly related to their amount of cell-association. The present study has clearly shown that by suitable alteration of liposomal lipid composition, charge and density of hydrophilicity it would be possible to direct liposomal ricin to specific cells for their selective elimination in combination with monensin.

Enhancement of the cytotoxicity of liposomal ricin by the carboxylic ionophore monensin and the lysosomotropic amine NH4Cl in Chinese hamster ovary cells

Ricin was encapsulated in negatively charged liposomes and its effect on the cytotoxicity was compared with native ricin in Chinese hamster ovarian (CHO) cells. The cytotoxicity of ricin, as measured by a marker protein synthesis (incorporation of 3H-leucine), was reduced markedly (300-fold) following encapsulation in liposomes. Lactose, a potent inhibitor of ricin cytotoxicity, had no effect on the binding, internalization, and cytotoxicity of liposomal ricin, indicating that liposomal ricin enter into mammalian cells by an alternative route, bypassing galactose-mediated endocytic pathway. Both monensin (a carboxylic ionophore) and NH4Cl (a lysosomotropic amine) markedly enhances the cytotoxicity of liposomal ricin, indicating endocytotic uptake of liposomal ricin. The degree of potentiation of the cytotoxicity of liposomal ricin by both monensin and NH4Cl was significantly higher (441- and 51-fold) as compared to native ricin (62.5- and 12.5-fold). The extent of exocytosis of free ricin was found to be much higher as compared to liposomal ricin; on the other hand, the extent of degradation of free and liposomal ricin was identical. Consequently, the intracellular level of liposomal ricin was increased to 3.5-fold. This higher level of intracellular liposomal ricin may allow more efficient ricin A-chain release into the cytosol under the influence of NH4Cl and monensin. Monensin-induced potentiation of liposomal ricin was prevented by brefeldin A, indicating that in the presence of monensin, the liposomal ricin was efficiently routed through the Golgi apparatus en route to the cytosol. Thus, liposomal ricin in combination with monensin may have potential application for selective elimination of malignant cells.

In vitro fusion of endocytic vesicles: effects of reagents that alter endosomal pH

Ricin, a plant toxin that binds to galactose-terminated glycoproteins and glycolipids on the cell surface, is internalized into endosomes before reaching the cytosol where it exerts its toxic activity. Fusion of early endosomes containing ricin or transferrin was demonstrated by using postnuclear supernatant fractions from K-562 cells. For both ligands, fusion depended on time, temperature, and ATP and was blocked by preincubation with N-ethylmaleimide. Some reagents that increase endosomal pH, the ionophores monensin and nigericin and the weak base chloroquine, stimulated the rate of fusion. However, bafilomycin A1, a specific inhibitor of vacuolar H(+)-ATPases, did not alter the rate of fusion. Moreover, it reduced or eliminated stimulation caused by monensin, nigericin, or chloroquine. Thus, the increased rate of fusion did not correlate with the higher lumenal pH of the endosome. The results suggest instead that fusion was stimulated by reagents that promoted accumulation of cations within the vesicles.

Characterization of 3'-azido-3'-deoxythymidine inhibition of ricin and Pseudomonas exotoxin A toxicity in CHO and Vero cells

Ricin (RIC), modeccin (MOD), Pseudomonas exotoxin A (PE), and diphtheria toxin (DT) are protein toxins that enter cells by receptor-mediated endocytosis. After intracellular transport and membrane translocation to the cytosol, these toxins inhibit protein synthesis by enzymatically removing a specific adenine residue from ribosomal RNA (RIC, MOD), or by ADP-ribosylation of elongation factor-2 (PE, DT). Recently, Thompson and Pace (1992) reported that AZT (3'-azido-3'-deoxythymidine) inhibited RIC toxicity in Vero cells, and this inhibition was not due to a block of RIC enzymatic activity. This paper extends these findings and examines the effects of AZT treatment on the toxicities of other protein toxins in Chinese hamster ovary (CHO) and Vero cell lines. AZT treatment did not significantly alter the toxicity of DT or MOD in either cell line, but it markedly reduced RIC and PE toxicity in both cell lines. The ID50 values (concentration of toxin required to inhibit protein synthesis by 50%) for RIC and PE in CHO cells increased approximately 6.5- and 12.5-fold, respectively; while in Vero cells the ID50 values increased ca. 8.5- and 4.5-fold, respectively. Results of further studies revealed differences in the mechanisms by which AZT inhibited RIC and PE toxicity. Results of cell-free translation indicated that, unlike its effects on RIC, AZT blocked the ability of PE to perform its enzymatic activity. As AZT did not block RIC enzymatic activity, we examined the effects of AZT on earlier steps in the RIC intoxication process. AZT treatment did not inhibit cell-surface binding or internalization of [125I]-RIC. Results of kinetic studies showed that when AZT was incubated with cells at the time of RIC exposure, it caused no major change in the lag phase, during which RIC reaches the site of translocation. However, it clearly reduced the subsequent first-order reduction in the rate of protein synthesis, suggesting an effect on translocation. Monensin (an ionophore that perturbs intracellular trafficking and increases the toxicities of RIC and PE) reduced AZT protection against both toxins. Nocodazole and colchicine (agents that disrupt microtubules and some routes of intracellular trafficking) reduced the ability of AZT to inhibit RIC, but not PE, toxicity. In summary, our results suggest that (1) AZT acts within the cytosol to inhibit (directly or indirectly) the enzymatic action of PE, and (2) the AZT inhibition of RIC cytotoxicity does not involve perturbations of RIC cell-surface binding, internalization, or enzymatic activity but might result from an alteration in RIC translocation.

Involvement of the Golgi region in the intracellular trafficking of cholera toxin

The intracellular pathway following receptor-mediated endocytosis of cholera toxin was studied using brefeldin A (BFA), which inhibited protein secretion and induced dramatic morphological changes in the Golgi region. In both mouse Y1 adrenal cells and CHO cells, BFA at 1 micrograms/ml caused a 80-90% inhibition of the cholera toxin (CT)-induced elevation of intracellular cAMP. The inhibition of the cytotoxicity of CT by BFA was also observed in a rounding assay of Y1 adrenal cells. The inhibition of CT cytotoxicity by BFA was dose dependent, with the ID50 value similar to the LD50 of BFA in Y1 adrenal cells. Binding and internalization of [125I]-labeled cholera toxin in Y1 adrenal cells was not affected by BFA. Unlike the BFA-sensitive cell lines such as Y1 adrenal and CHO cells, BFA at 1 micrograms/ml did not inhibit the cytotoxicity of CT in PtK1 cells, of which the Golgi structure was BFA-resistant. These results strongly suggest that a BFA-sensitive Golgi is required for the protection of CT cytotoxicity by BFA. In contrast, elevation of the intracellular cAMP by forskolin, which acts directly on the plasma membrane adenylate cyclase, was not affected by BFA. These observations indicate that the intoxication of target cells by CT requires an intact Golgi region for its intracellular trafficking and/or processing. In this respect, CT shares a common intracellular pathway with ricin, Pseudomonas toxin, and modeccin, even though their structures and modes of action are very different.

Monensin intercalation in liposomes: effect on cytotoxicities of ricin, Pseudomonas exotoxin A and diphtheria toxin in CHO cells

Monensin, a carboxylic ionophore was intercalated in liposomes (liposomal monensin) and its effect on cytotoxicities of ricin, Pseudomonas exotoxin A and diphtheria toxin in CHO cells was studied. Intercalation of monensin in liposomal bilayer is found to have no effect on its stability and interaction with cells. Liposomal monensin (1 nM) substantially enhance the cytotoxicities of ricin (62-fold) and Pseudomonas exotoxin A (11.5-fold) while it has no effect on diphtheria toxin. This observed effect is highly dependent on the liposomal lipid composition. The potentiating ability of monensin (1 nM) in neutral vesicles is significantly higher (2.2-fold) as compared to negatively charges vesicles. This ability is drastically reduced by incorporation of stearylamine in liposomes and is found to be dependent on the density of stearylamine as well as on the concentration of serum in the medium. Monensin in liposomes containing 24 mol% stearylamine has a very marginal effect on the cytotoxicity of ricin (7.5-fold) which is further reduced (1.5-fold) in the presence of 20% serum. The uptake of 125I-gelonin from neutral vesicles is significantly higher (approximately 2.0-fold) than that from the negative vesicles. The uptake from positive vesicles is highly dependent on the concentration of stearylamine. The reduction in the lag period (30 min) of ricin action by monensin in neutral and negative vesicle is comparable with free monensin. However, monensin in positive vesicle has no effect on it. These studies have suggested that liposomes could be used as a delivery vehicle for monensin for selective elimination of tumor cells in combination with hybrid toxins.

In vivo potentiation of ricin toxicity by monensin delivered through liposomes

Monensin, a carboxylic ionophore, which is known to raise intravesicular pH, was intercalated in liposomes and its effect on the toxicity of ricin in mice was studied. The toxicity of ricin in vivo was found to be significantly enhanced by the administration of monensin intercalated in liposomes (liposomal monensin). The observed enhancement of the toxicity of ricin by monensin was highly dose-dependent and was maximal when ricin was injected within 60 min of monensin injection. The survival time was found to be reduced in the range of 8-20 h, depending on the dose of ricin used, by liposomal monensin. Stability of liposomes containing monensin as inferred from the release of entrapped calcein or FITC-dextran under both in vivo and in vitro conditions was comparable to that observed for liposomes without monensin. Liposomal monensin remains in circulation for 2 h and was cleared from the blood stream after 4 h. In contrast, 15 min was required for the clearance of monensin when administered in free form. Studies on the distribution of liposomal monensin and 125I-ricin in various tissues have revealed that monensin is mainly localized in the liver and spleen which are also the major sites for ricin accumulation. Our observation on the substantial enhancement of ricin toxicity in vivo by liposomal monensin strongly supports the potential usefulness of the latter as a potentiating agent in the enhancement of the toxicity of immunotoxin or hormonotoxin for selective elimination of cancer cells.

Interaction of gelonin with macrophages: effect of lysosomotropic amines

The cytotoxic effects of gelonin on various phagocytic and nonphagocytic cells were studied. Peritoneal exudate cells (PEC) are found to be more sensitive to gelonin compared to P388D1 and J774A.1 cells, nonphagocytic cells being the least sensitive. While chloroquine markedly enhances the cytotoxicity of gelonin in macrophages (greater than 100-fold) ammonium chloride confers protection. A higher rate of uptake of 125I-gelonin in PEC (7 times the rate observed in other cells) is probably mediated by an interaction of terminal mannose residues of gelonin with mannose receptors on PEC plasma membrane as inferred from a pronounced inhibitory effect of mannan. In contrast to a pronounced inhibitory effect of mannan on the uptake of gelonin in PEC (7-fold), the cytotoxicity is reduced only by 2-fold. On the other hand, mannan has little or no effect on enhancement of the toxicity of gelonin by chloroquine. The studies have suggested that the internalization of gelonin in PEC may involve two pathways: (a) mannose receptor-mediated endocytosis, which plays a minor role in the intoxication process, and (b) nonspecific fluid phase pinocytosis, which is susceptible to enhancement by chloroquine and has a major role to play in the manifestation of the toxic effect of gelonin. In macrophage-like cells only the latter pathway operates.

Disruption of the Golgi apparatus by brefeldin A inhibits the cytotoxicity of ricin, modeccin, and Pseudomonas toxin

We have studied the cytotoxicity of ricin in cells treated with brefeldin A (BFA), which dramatically disrupts the structure of the Golgi apparatus causing Golgi content and membrane to redistribute to the ER. BFA inhibits the cytotoxicity of ricin in Chinese hamster ovary, normal rat kidney, and Vero cells and abolishes the enhancement of ricin cytotoxicity by NH4Cl, nigericin, swainsonine, and tunicamycin or by a mutation in endosomal acidification. BFA protects cells from the cytotoxicities of modeccin and Pseudomonas toxin, but has no effect on the intoxication by diphtheria toxin. Pretreatment of BFA does not protect cells from ricin treatment in the absence of BFA. Our results suggest that ricin, modeccin, and Pseudomonas toxin share a common pathway of intracellular transport from endosomes to the Golgi region where they are released into the cytosol. In contrast, the lack of protection of Vero cells from diphtheria toxin by BFA indicates that diphtheria toxin is released from acidified endosomes without involving the Golgi region.

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.

Lectin-resistant CHO cells: selection of seven new mutants resistant to ricin

In attempts to isolate new CHO glycosylation mutants, selection protocols using plant lectins that bind galactose residues of cell surface carbohydrates were applied to mutagenized CHO populations. The lectins were used alone or in combination to obtain seven ricin-resistant phenotypes. Each mutant had distinctive properties compared with previously described ricin-resistant CHO cells. One of the new phenotypes was dominant in somatic cell hybrids, and the others were recessive. Complementation analyses between related lectin-resistant (LecR) phenotypes indicated that each new isolate represented a novel genotype. Five of the mutants had properties typical of new CHO glycosylation mutants. The remaining two mutants were not readily categorized. Although they did not appear to be ricin-internalization or protein-synthesis mutants, they also did not display the marked alterations in sensitivity to several lectins of different sugar specificity expected for glycosylation mutants. The seven new LecR mutants described in these studies brings the total number of different LecR CHO mutants isolated by this and other laboratories to about 40. Criteria for identifying new LecR mutations in CHO cells are discussed.