Lysosomal storage of sulfated glycosaminoglycans in cultured fibroblasts exposed to immunostimulatory acridine derivatives

Chemically induced accumulation of GAGs delays PrP(Sc) clearance but prolongs prion disease incubation time

Prion diseases are a group of fatal neurodegenerative diseases affecting humans and animals. The only identified component of the infectious prion is PrP(Sc), an aberrantly folded isoform of PrP(C). Glycosaminoglycans, which constitute the main receptor for prions on cells, play a complex role in the pathogenesis of prion diseases. For example, while agents inducing aberrant lysosomal accumulation of GAGs such as Tilorone and Quinacrine significantly reduced PrP(Sc) content in scrapie-infected cells, administration of Quinacrine to prion-infected subjects did not improve their clinical status. In this study, we investigated the association of PrP(Sc )with cells cultured with Tilorone. We found that while the initial incorporation of PrP(Sc) was similar in the treated and untreated cells, clearance of PrP(Sc) from the Tilorone-treated cells was significantly impaired. Interestingly, prolonged administration of Tilorone to mice prior to prion infection resulted in a significant delay in disease onset, concomitantly with in vivo accumulation of lysosomal GAGs. We hypothesize that GAGs may complex with newly incorporated PrP(Sc) in lysosomes and further stabilize the prion protein conformation. Over-stabilized PrP(Sc) molecules have been shown to comprise reduced converting activity.

Lysosomal glycosaminoglycan storage as induced by dicationic amphiphilic drugs: investigation into the mechanisms underlying the slow reversibility

Several dicationic amphiphilic compounds, such as the immunomodulator tilorone and analogues, impair the lysosomal catabolism of sulphated glycosaminoglycans (GAGs). Thereby they cause lysosomal GAG storage in rats and in cultured fibroblasts of several species including man. The GAG storage is rather slowly reversible in vivo; it persists for months after discontinuance of drug treatment. In the present study, we investigated the mechanisms underlying the slow reversibility. Cultured bovine corneal fibroblasts were pretreated for 4 days with tilorone (5 and 20 microM) or with compound CL-90.100 (3 and 10 microM) and further cultured in drug-free medium for periods up to 11 days. The intracellular GAG storage was analysed biochemically and demonstrated histochemically. The subcellular drug distribution (CL-90.100) was demonstrated by fluorescence microscopy. Dermatan sulphate (DS) provided the predominant contribution towards the GAG storage. After pretreatments with the low, as well as the high concentrations of either drug, the storage of DS was irreversible during the period of observation, whereas the minor storage of heparan sulphate was resolved. The enhanced secretion of the lysosomal enzyme beta-hexosaminidase (E.C. 3.2.1.52) caused by pretreatment with the high concentration of tilorone was also readily reversible. Thus, enzyme deprivation could not be the explanation for the sustained DS storage. The localization of the drug-related fluorescence within perinuclear cell organelles, presumably lysosomes, resembled that of the stored GAGs as visualized by histochemical staining. Both, the fluorescence and the positive GAG staining persisted with unchanged intracellular distribution throughout the recovery period. The present results suggest that the persistence of the DS storage is due to the formation of long-lived, non-degradable DS-drug complexes within the lysosomes.

Drug-induced lysosomal storage of sulphated glycosaminoglycans

1. Certain compounds (e.g., the immunomodulator tilorone and congeners) are able to induce lysosomal storage of sulphated glycosaminoglycans (GAG), thus, producing cytological and biochemical alterations reminiscent of the inherited mucopolysaccharidoses. The drug-induced GAG storage has been studied in cultured fibroblasts of several species and in rats, and it is likely to occur also in humans. 2. The cytological hallmarks of GAG storage are enlarged lysosomes congested with material that is intensely stained by cationic dyes. With respect to fixation techniques, one has to keep in mind that the GAGs are highly water-soluble and are leached during conventional fixation and tissue processing. Biochemically, the elevation of GAG contents in tissues and cultured fibroblasts is due to storage of dermatan sulphate, predominantly. 3. The molecular structure of the potent inducers of GAG storage is characterized by a planar tricyclic aromatic ring system that is symmetrically substituted with two side chains of 4-5 sigma bond length, each carrying a protonizable nitrogen atom. The lysosomal storage of GAG is accompanied by lysosomal accumulation of the inducing drug, with the molar ratio of drug to GAG-disaccharide unit amounting to > 1:1. The reversibility of GAG storage is rather slow. 4. The pathogenic mechanisms underlying the drug side effects are discussed and the following hypothesis is put forward: The compounds in question are lysosomotropic weak bases. They get trapped in the acidic lysosomes and accumulate highly there. Physicochemical data suggest that the drugs form complexes with the sulphated GAGs, particularly with dermatan sulphate: The positively charged nitrogen atoms of the drug side chains interact with the negative charges of sulphate and carboxy groups of the GAGs, thereby crosslinking at least two GAG helices. Moreover, the interlinking drug molecules form parallel stacks resulting from interaction of the aromatic pi-electrons of the planar ring systems. This further stabilizes the complexes. The GAGs within the complexes are thought to be resistant to the degrading lysosomal enzymes. 5. Drug-induced GAG storage has not been directly demonstrated in man. Yet, clinical reports on keratopathy and basophilic cytoplasmic inclusions in blood lymphocytes of tilorone-treated patients suggest that this drug side effect may also occur in man.

Drug-induced glycosaminoglycan storage: dose-dependent changes in the pattern of accumulated glycosaminoglycans in cultured bovine and human fibroblasts

The present study determines the amounts and patterns of glycosaminoglycans stored in cultured corneal fibroblasts after treatment with tilorone and three related compounds. The compounds have immunomodulatory properties and have been shown to impair the lysosomal degradation of glycosaminoglycans as a side effect. This side effect has been described as drug-induced mucopolysaccharidosis because the induced lysosomal storage of glycosaminoglycans leads to cellular lesions resembling those in patients with inherited mucopolysaccharidosis. In the present study, the dose-dependency of glycosaminoglycan storage was analyzed after treatment (96 hr) of bovine corneal fibroblasts. The investigated drug concentrations ranged from low concentrations inducing cytological lesions typical of drug-induced mucopolysaccharidosis to high concentrations at the borderline of cytotoxicity. The intracellular amounts of dermatan sulfate, heparan suflate, and chondroitin sulfate were quantified by densitometric scanning of Alcian Blue-stained bands after electrophoresis. All investigated compounds induced a predominant dermatan sulfate storage (3-4-fold accumulation) at low drug concentrations. With rising drug concentrations, a shift of the pattern of stored glycosaminoglycans was observed, characterized by the additional accumulation of heparan sulfate (up to 5-fold of control levels). In cultured human fibroblasts, tilorone also caused a marked dermatan sulfate storage, reaching maximum values at 5 microM and marked heparan sulfate storage at 20 microM. The present data provide evidence: (a) that selective dermatan sulfate accumulation is a characteristic feature of drug-induced glycosaminoglycan storage in cultured bovine and human fibroblasts, if these cells are treated with low concentrations (< or = 5 microM), that are likely to reflect the situation in vivo; and (b) that additional heparan sulfate storage is induced in vitro only by treatment with high concentrations that induce nonspecific cellular lesions.

The antimalarials quinacrine and chloroquine induce weak lysosomal storage of sulphated glycosaminoglycans in cell culture and in vivo

The antimalarial agents quinacrine and chloroquine are well known as potent inducers of lysosomal storage of polar lipids (lipidosis) in cell culture and in vivo. In previous experiments on cultured fibroblasts, chloroquine was shown to additionally cause weak lysosomal storage of sulphated glycosaminoglycans (GAGs) thus inducing mucopolysaccharidosis (MPS). In the present study, quinacrine was investigated for this ability, because we wished to know whether or not the acridine ring system in quinacrine would enhance the MPS-inducing potency as compared to chloroquine carrying an isoquinoline ring system. Tilorone (2,7-bis[2-(diethylamino)ethoxy]fluoren-9-one) known as a potent inducer of MPS served as reference compound. The compounds were compared at a concentration (3 microM) which did not enhance the secretion of the lysosomal enzyme beta-hexosaminidase (E.C. 3.2.1.52), since this would be an indication of unspecific drug effects upon the endosomal/lysosomal compartments of the cell. Additionally the liver of quinacrine- and chloroquine-treated rats was examined with the question whether the lysosomal GAG storage induced by either drug in cell culture had an equivalent in intact organisms. Both, in cell culture and in vivo, quinacrine was found to be a more potent inducer of lysosomal GAG storage than was chloroquine. The results suggest that the acridine ring system favours this drug side effect as compared with the bicyclic isoquinoline ring system. On the other hand, quinacrine was significantly less potent than tilorone and the Symmetrically substituted acridine derivative 3,6-bis[2-(diethylamino)ethoxy]acridine investigated previously. This suggests that the asymmetric structure of the quinacrine molecule reduces the potency as compared to the symmetrically substituted bisbasic compounds with planary tricyclic ring systems such as tilorone and congeners.

Tilorone-induced lysosomal storage of sulphated glycosaminoglycans can be separated from tilorone-induced enhancement of lysosomal enzyme secretion

This investigation deals with a drug side-effect. The immunomodulatory drug tilorone (2,7-bis[2-(diethylamino)ethoxy]fluoren-9-one) and congeners induce lysosomal storage of sulphated glycosaminoglycans (GAGs) in animals and in cultured cells. At high tilorone concentrations, GAG storage in cultured fibroblasts was previously reported to be accompanied, and presumably caused by, disturbance of intracellular targeting of lysosomal enzyme precursors, which leads to enhanced secretion and thus loss of lysosomal enzymes. The purpose of the present study was to examine whether the GAG storage induced in cultured bovine fibroblasts by low tilorone concentrations is also accompanied by enhanced lysosomal enzyme release. Enhanced secretion of beta-hexosaminidase (EC 3.2.1.52) was taken as indicating the intracellular mistargeting of lysosomal enzyme precursors. Dose-response curves were established for (a) the intracellular accumulation of 35S-GAGs and (b) the release of beta-hexosaminidase after exposure (72 hr) to tilorone (1-35 microM). For positive controls, the classical lysosomotropic agents NH4Cl (1-30 mM) and chloroquine (1-60 microM) were used. With NH4Cl, 35S-GAG storage was accompanied by enhanced enzyme release throughout the concentration range (EC50 at 3.3 mM for either effect). With chloroquine, low concentrations (< or = 5 microM) caused a small increase in 35S-GAG accumulation without abnormal enzyme secretion; at higher concentrations both drug effects were produced (EC50 around 15 microM for either effect). With tilorone, low concentrations (< or = 5 microM) caused marked 35S-GAG accumulation without enhancement of enzyme release. The EC50 for tilorone-induced 35S-GAG storage was 3 microM, as opposed to 15 microM for enzyme release. The results indicate that GAG storage induced by low concentrations of tilorone is due to mechanisms other than mistargeting and loss of lysosomal enzymes. On the basis of previous results it may be hypothesized that tilorone and other symmetrically substituted dicationic compounds form complexes with the polyanionic GAG chains and thereby impair their enzymic degradation.

Drug-induced intralysosomal storage of sulfated glycosaminoglycans (GAGs): a methodical pitfall occurring with acridine derivatives

The present communication deals with an adverse drug action which is exerted by a series of dicationic amphiphilic compounds such as the immunomodulatory drug tilorone and congeners. The drugs induce lysosomal storage of sulfated glycosaminoglycans (GAGs) in intact organisms and in cultured cells by impairing the lysosomal GAG degradation. This impairment was proposed to be due to the formation of non-degradable GAG-drug complexes. GAGs are highly water-soluble and not preservable by aldehyde fixatives. Therefore, usually the lysosomes appear optically empty in histological preparations, unless the fixative is supplemented with a GAG-precipitating agent. When acridine derivatives were used for the induction of GAG-storage, the lysosomal storage material displayed unexpected and unsystematic variability with regard to its preservability and ultrastructure. In the present study, evidence is presented that the acridine derivatives (a) remain bound to the stored GAGs for some time after glutaraldehyde fixation; and (b) they precipitate GAGs in vitro. Thus, apart from their unwanted action in the living cell, i.e., disturbing lysosomal GAG-degradation, the drugs function as precipitants and "fixatives" for the intralysosomal GAGs. The uncontrolled persistence of the drugs after tissue fixation leads to variable degree of GAG-preservation and thus to unpredictable variability of the ultrastructure of the storage lysosomes. If this pitfall is not realized, the resulting inconsistencies may rise confusion among toxicologic pathologists who deal with drug-induced lysosomal storage disorders.

Drug-induced lysosomal storage of sulfated glycosaminoglycans. Studies on the underlying structure-activity relationships

Some immunomodulatory drugs have previously been shown to induce lysosomal storage of sulfated glycosaminoglycans (sGAG) in intact organisms and cultured cells. These compounds consist of a planary aromatic ring system and two symmetric side chains each carrying a protonizable nitrogen. The purpose of this study was to test a larger collection of such compounds for their potencies to induce lysosomal storage of sGAG in cultured fibroblasts of rat cornea. The cells were exposed (72 h) to various compounds differing with respect to the aromatic ring system or the side chains. Lysosomal sGAG-storage was demonstrated by selective cytochemical staining with cuprolinic blue. The threshold concentration, i.e., the concentration necessary to induce cuprolinic blue-positive cytoplasmic inclusions in at least 1% of the cells, was determined for each compound. The threshold concentrations were distributed over a range of 0.3-30 microM. It should be emphasized that the threshold concentration of a given compound is not a constant, but depends on the volume of cell culture medium per surface area of cell monolayer, since the lysosomal accumulation lowers the initial drug concentration in the medium. If the ratio of medium volume:cell monolayer surface is increased as compared with standard cell culture conditions, the threshold concentration will be lowered. The compounds were ranked according to their threshold concentrations as determined under standard conditions. The following conclusions can be drawn from the ranking: the type of the central aromatic ring system and the distance between the ring system and the protonizable nitrogen atoms of the side chains influence the potency to induce lysosomal sGAG-storage. Regarding the ring system, the potency decreases as follows: acridine approximately anthrachinone > fenfluorenone approximately fenfluorene > xanthenone; xanthene > dibenzofuran approximately dibenzothiophene. In intact organisms, these structure-activity relationships may be superimposed by drug metabolism and pharmacokinetic factors.

Induction of mucopolysaccharidosis in rats by treatment with immunostimulatory acridine derivatives

In the accompanying paper, four dibasic acridine derivatives were reported to induce lysosomal storage of sulfated glycosaminoglycans (sGAG), i.e., mucopolysaccharidosis, in cultured fibroblast (Handrock et al. Toxicol. Appl. Pharmacol. 114, 1992). The purpose of the present morphological and biochemical investigation was to examine whether two representatives of the acridine derivatives, namely 3,6-bis[2-(diethylamino)ethoxy]-acridine and the piperidino analogue, induce mucopolysaccharidosis in intact organisms. Rats were orally treated with 60-80 mg/kg up to 22 weeks. Morphological examination of liver, spleen, and blood lymphocytes yielded cytochemical evidence of mucopolysaccharidosis. Biochemically, up to a 48-fold increase of the urinary excretion of sGAG was found. In the liver and spleen of chronically treated rats, the sGAG contents were elevated by factors up to 56 and 23, respectively. Heparan sulfate and dermatan sulfate contributed most to the total increase of sGAG; chondroitin sulfate was stored to a minor degree. For one compound, the tissue concentrations were determined. It was found that the drug was accumulated in the tissues. Due to their fluorescent properties, the drugs could be detected by fluorescence microscopy to be present in high concentrations within the sGAG-storing lysosomes. On the basis of these observations and of the biochemical data it appears justified to assume a ratio of at least one drug molecule per disaccharide unit of the sGAG to be present in the lysosomes. It is proposed that this leads to the formation of sGAG-drug complexes in the lysosomes. Such complexes may be indigestible substrates for the lysosomal enzymes, thus leading to mucopolysaccharidosis. For toxicologic practice, the cytochemical examination of lymphocytes is recommended as a simple measure for early detection and monitoring of this adverse drug effect.