Effect of acidic phospholipids on the activity of lysosomal phospholipases and on their inhibition by aminoglycoside antibiotics--I. Biochemical analysis

Ataluren suppresses a premature termination codon in an MPS I-H mouse

Abstarct

Suppressing translation termination at premature termination codons (PTCs), termed readthrough, is a potential therapy for genetic diseases caused by nonsense mutations. Ataluren is a compound that has shown promise for clinical use as a readthrough agent. However, some reports suggest that ataluren is ineffective at suppressing PTCs. To further evaluate the effectiveness of ataluren as a readthrough agent, we examined its ability to suppress PTCs in a variety of previously untested models. Using NanoLuc readthrough reporters expressed in two different cell types, we found that ataluren stimulated a significant level of readthrough. We also explored the ability of ataluren to suppress a nonsense mutation associated with Mucopolysaccharidosis I-Hurler (MPS I-H), a genetic disease that is caused by a deficiency of α-L-iduronidase that leads to lysosomal accumulation of glycosaminoglycans (GAGs). Using mouse embryonic fibroblasts (MEFs) derived from Idua-W402X mice, we found that ataluren partially rescued α-L-iduronidase function and significantly reduced GAG accumulation relative to controls. Two-week oral administration of ataluren to Idua-W402X mice led to significant GAG reductions in most tissues compared to controls. Together, these data reveal important details concerning the efficiency of ataluren as a readthrough agent and the mechanisms that govern its ability to suppress PTCs.

Key messages

Ataluren promotes readthrough of PTCs in a wide variety of contexts.

Ataluren reduces glycosaminoglyan storage in MPS I-H cell and mouse models.

Ataluren has a bell-shaped dose–response curve and a narrow effective range.

Preclinical Evaluation of Inhalational Spectinamide-1599 Therapy against Tuberculosis

The lengthy treatment time for tuberculosis (TB) is a primary cause for the emergence of multidrug resistant tuberculosis (MDR-TB). One approach to improve TB therapy is to develop an inhalational TB therapy that when administered in combination with oral TB drugs eases and shortens treatment. Spectinamides are new semisynthetic analogues of spectinomycin with excellent activity against Mycobacterium tuberculosis (Mtb), including MDR and XDR Mtb strains. Spectinamide-1599 was chosen as a promising candidate for development of inhalational therapy. Using the murine TB model and intrapulmonary aerosol delivery of spectinamide-1599, we characterized the pharmacokinetics and efficacy of this therapy in BALB/c and C3HeB/FeJ mice infected with the Mtb Erdman strain. As expected, spectinamide-1599 exhibited dose-dependent exposure in plasma, lungs, and ELF, but exposure ratios between lung and plasma were 12-40 times higher for intrapulmonary compared to intravenous or subcutaneous administration. In chronically infected BALB/c mice, low doses (10 mg/kg) of spectinamide-1599 when administered thrice weekly for two months provide efficacy similar to that of higher doses (50-100 mg/kg) after one month of therapy. In the C3HeB/FeJ TB model, intrapulmonary aerosol delivery of spectinamide-1599 (50 mg/kg) or oral pyrazinamide (150 mg/kg) had limited or no efficacy in monotherapy, but when both drugs were given in combination, a synergistic effect with superior bacterial reduction of >1.8 log₁₀ CFU was observed. Throughout the up to eight-week treatment period, intrapulmonary therapy was well-tolerated without any overt toxicity. Overall, these results strongly support the further development of intrapulmonary spectinamide-1599 as a combination partner for anti-TB therapy.

Quantification of Intracellular Accumulation and Retention of Lysosomotropic Macrocyclic Compounds by High-Throughput Imaging of Lysosomal Changes

Many marketed pharmaceuticals reach extremely high tissue concentrations due to accumulation in lysosomes (lysosomotropism). Quantitative prediction of intracellular concentrations of accumulating drugs is challenging, especially for macrocyclic compounds that mainly do not fit in current in silico models. We tested a unique library of 47 compounds (containing 39 macrocycles) specifically designed to cover the entire range of accumulation intensities observed with pharmaceuticals so far. For the first time, we show that intracellular concentration of compounds measured by liquid chromatography with tandem mass spectrometry correlates with the induction of phospholipidosis and inhibition of autophagy, but the highest correlation was observed with the increase of lysosomal volume (R = 0.95), all measured by high-throughput imaging assays. Based only on imaging data, we developed a 5-class in vitro model for the prediction of compound accumulation with the accuracy of 81%. The measured change of total lysosomal volume can thus be used in high-throughput screening for determination of the actual intensity of intracellular accumulation of new macrocyclic compounds. The models are largely based on macrocycles, greatly improving the screening and prediction of intracellular accumulation of this challenging class. However, all tested nonmacrocyclic compounds fitted well in the models, indicating potential use of the models in broader chemical space.

Nonsense Suppression as an Approach to Treat Lysosomal Storage Diseases

In-frame premature termination codons (PTCs) (also referred to as nonsense mutations) comprise ~10% of all disease-associated gene lesions. PTCs reduce gene expression in two ways. First, PTCs prematurely terminate translation of an mRNA, leading to the production of a truncated polypeptide that often lacks normal function and/or is unstable. Second, PTCs trigger degradation of an mRNA by activating nonsense-mediated mRNA decay (NMD), a cellular pathway that recognizes and degrades mRNAs containing a PTC. Thus, translation termination and NMD are putative therapeutic targets for the development of treatments for genetic diseases caused by PTCs. Over the past decade, significant progress has been made in the identification of compounds with the ability to suppress translation termination of PTCs (also referred to as readthrough). More recently, NMD inhibitors have also been explored as a way to enhance the efficiency of PTC suppression. Due to their relatively low threshold for correction, lysosomal storage diseases are a particularly relevant group of diseases to investigate the feasibility of nonsense suppression as a therapeutic approach. In this review, the current status of PTC suppression and NMD inhibition as potential treatments for lysosomal storage diseases will be discussed.

A metabolomics cell-based approach for anticipating and investigating drug-induced liver injury

In preclinical stages of drug development, anticipating potential adverse drug effects such as toxicity is an important issue for both saving resources and preventing public health risks. Current in vitro cytotoxicity tests are restricted by their predictive potential and their ability to provide mechanistic information. This study aimed to develop a metabolomic mass spectrometry-based approach for the detection and classification of drug-induced hepatotoxicity. To this end, the metabolite profiles of human derived hepatic cells (i.e., HepG2) exposed to different well-known hepatotoxic compounds acting through different mechanisms (i.e., oxidative stress, steatosis, phospholipidosis, and controls) were compared by multivariate data analysis, thus allowing us to decipher both common and mechanism-specific altered biochemical pathways. Briefly, oxidative stress damage markers were found in the three mechanisms, mainly showing altered levels of metabolites associated with glutathione and γ-glutamyl cycle. Phospholipidosis was characterized by a decreased lysophospholipids to phospholipids ratio, suggestive of phospholipid degradation inhibition. Whereas, steatosis led to impaired fatty acids β-oxidation and a subsequent increase in triacylglycerides synthesis. The characteristic metabolomic profiles were used to develop a predictive model aimed not only to discriminate between non-toxic and hepatotoxic drugs, but also to propose potential drug toxicity mechanism(s).

Gallium Potentiates the Antibacterial Effect of Gentamicin against Francisella tularensis

The reasons why aminoglycosides are bactericidal have not been not fully elucidated, and evidence indicates that the cidal effects are at least partly dependent on iron. We demonstrate that availability of iron markedly affects the susceptibility of the facultative intracellular bacterium Francisella tularensis strain SCHU S4 to the aminoglycoside gentamicin. Specifically, the intracellular depots of iron were inversely correlated to gentamicin susceptibility, whereas the extracellular iron concentrations were directly correlated to the susceptibility. Further proof of the intimate link between iron availability and antibiotic susceptibility were the findings that a ΔfslA mutant, which is defective for siderophore-dependent uptake of ferric iron, showed enhanced gentamicin susceptibility and that a ΔfeoB mutant, which is defective for uptake of ferrous iron, displayed complete growth arrest in the presence of gentamicin. Based on the aforementioned findings, it was hypothesized that gallium could potentiate the effect of gentamicin, since gallium is sequestered by iron uptake systems. The ferrozine assay demonstrated that the presence of gallium inhibited >70% of the iron uptake. Addition of gentamicin and/or gallium to infected bone marrow-derived macrophages showed that both 100 μM gallium and 10 μg/ml of gentamicin inhibited intracellular growth of SCHU S4 and that the combined treatment acted synergistically. Moreover, treatment of F. tularensis-infected mice with gentamicin and gallium showed an additive effect. Collectively, the data demonstrate that SCHU S4 is dependent on iron to minimize the effects of gentamicin and that gallium, by inhibiting the iron uptake, potentiates the bactericidal effect of gentamicin in vitro and in vivo.

Screening for the drug-phospholipid interaction: correlation to phospholipidosis

Phospholipid bilayers represent a complex, anisotropic environment fundamentally different from bulk oil or octanol, for instance. Even "simple" drug association to phospholipid bilayers can only be fully understood if the slab-of-hydrocarbon approach is abandoned and the complex, anisotropic properties of lipid bilayers reflecting the chemical structures and organization of the constituent phospholipids are considered. The interactions of drugs with phospholipids are important in various processes, such as drug absorption, tissue distribution, and subcellular distribution. In addition, drug-lipid interactions may lead to changes in lipid-dependent protein activities, and further, to functional and morphological changes in cells, a prominent example being the phospholipidosis (PLD) induced by cationic amphiphilic drugs. Herein we briefly review drug-lipid interactions in general and the significance of these interactions in PLD in particular. We also focus on a potential causal connection between drug-induced PLD and steatohepatitis, which is induced by some cationic amphiphilic drugs.

Neamine inhibits xenografic human tumor growth and angiogenesis in athymic mice

PURPOSE

We have previously shown that the aminoglycoside antibiotic neomycin blocks the nuclear translocation of angiogenin and inhibits its angiogenic activity. However, neomycin has not been considered as a favorable drug candidate for clinical development because of its known nephrotoxicity and ototoxicity. The aim of this study is to determine whether neamine, a nontoxic derivative of neomycin, possesses antitumor activity.

EXPERIMENTAL DESIGN

The effect of neamine on the nuclear translocation of angiogenin was examined by means of immunofluorescence and Western blotting. The antitumor activity of neamine was determined with three different animal models.

RESULTS

Neamine effectively blocked the nuclear translocation of angiogenin in endothelial cells and inhibited angiogenin-induced cell proliferation. It inhibited the establishment of human tumor xenografts in athymic mice in both ectopic and orthotopic tumor models. It also inhibited the progression of established human tumor transplants, whereas the structurally related antibiotic paromomycin had no effect. Immunohistochemical staining showed that both angiogenesis and cancer cell proliferation are inhibited by neamine.

CONCLUSION

These results suggest that the nontoxic aminoglycoside antibiotic neamine is an effective inhibitor of nuclear translocation of angiogenin and may serve as an inhibitor for angiogenin-induced angiogenesis and cancer progression.

Modulation of the in vitro activity of lysosomal phospholipase A1 by membrane lipids

Lysosomal phospholipases play a critical role for degradation of cellular membranes after their lysosomal segregation. We investigated the regulation of lysosomal phospholipase A1 by cholesterol, phosphatidylethanolamine, and negatively-charged lipids in correlation with changes of biophysical properties of the membranes induced by these lipids. Lysosomal phospholipase A1 activity was determined towards phosphatidylcholine included in liposomes of variable composition using a whole-soluble lysosomal fraction of rat liver as enzymatic source. Phospholipase A1 activity was then related to membrane fluidity, lipid phase organization and membrane potential as determined by fluorescence depolarization of DPH, 31P NMR and capillary electrophoresis. Phospholipase A1 activity was markedly enhanced when the amount of negatively-charged lipids included in the vesicles was increased from 10 to around 30% of total phospholipids and the intensity of this effect depended on the nature of the acidic lipids used (ganglioside GM1<phosphatidylinositol approximately phosphatidylserine approximately phosphatidylglycerol approximately phosphatidylpropanol<phosphatidic acid). For liposomes containing phosphatidylinositol, this increase of activity was not modified by the presence of phosphatidylethanolamine and enhanced by cholesterol only when the phosphatidylinositol content was lower than 18%. Our results, therefore show that both the surface-negative charge and the nature of the acidic lipid included in bilayers modulate the activity of phospholipase A1 towards phosphatidylcholine, while the change in lipid hydration or in fluidity of membrane are less critical. These observations may have physiological implications with respect to the rate of degradation of cellular membranes after their lysosomal segregation.

Interaction of the Macrolide Antibiotic Azithromycin with Lipid Bilayers: Effect on Membrane Organization, Fluidity, and Permeability

PURPOSE

To investigate the effect of a macrolide antibiotic, azithromycin, on the molecular organization of DPPC:DOPC, DPPE:DOPC, SM:DOPC, and SM:Chol:DOPC lipid vesicles as well as the effect of azithromycin on membrane fluidity and permeability.

METHODS

The molecular organization of model membranes was characterized by atomic force microscopy (AFM), and the amount of azithromycin bound to lipid membranes was determined by equilibrium dialysis. The membrane fluidity and permeability were analyzed using fluorescence polarization studies and release of calcein-entrapped liposomes, respectively.

RESULTS

In situ AFM images revealed that azithromycin leads to the erosion and disappearance of DPPC and DPPE gel domains, whereas no effect was noted on SM and SM:cholesterol domains. Although azithromycin did not alter the permeability of DPPC:DOPC, DPPE:DOPC, SM:DOPC, and SM:Chol:DOPC lipid vesicles, it increased the fluidity at the hydrophilic/hydrophobic interface in DPPC:DOPC and DPPE:DOPC models. This effect may be responsible for the ability of azithromycin to erode the DPPC and DPPE gel domains, as observed by AFM.

CONCLUSIONS

This study shows the interest of both AFM and biophysical methods to characterize the drug-membrane interactions.

Piracetam inhibits the lipid-destabilising effect of the amyloid peptide Abeta C-terminal fragment

Amyloid peptide (Abeta) is a 40/42-residue proteolytic fragment of a precursor protein (APP), implicated in the pathogenesis of Alzheimer's disease. The hypothesis that interactions between Abeta aggregates and neuronal membranes play an important role in toxicity has gained some acceptance. Previously, we showed that the C-terminal domain (e.g. amino acids 29-42) of Abeta induces membrane permeabilisation and fusion, an effect which is related to the appearance of non-bilayer structures. Conformational studies showed that this peptide has properties similar to those of the fusion peptide of viral proteins i.e. a tilted penetration into membranes. Since piracetam interacts with lipids and has beneficial effects on several symptoms of Alzheimer's disease, we investigated in model membranes the ability of piracetam to hinder the destabilising effect of the Abeta 29-42 peptide. Using fluorescence studies and 31P and 2H NMR spectroscopy, we have shown that piracetam was able to significantly decrease the fusogenic and destabilising effect of Abeta 29-42, in a concentration-dependent manner. While the peptide induced lipid disorganisation and subsequent negative curvature at the membrane-water interface, the conformational analysis showed that piracetam, when preincubated with lipids, coats the phospholipid headgroups. Calculations suggest that this prevents appearance of the peptide-induced curvature. In addition, insertion of molecules with an inverted cone shape, like piracetam, into the outer membrane leaflet should make the formation of such structures energetically less favourable and therefore decrease the likelihood of membrane fusion.

Membrane destabilization induced by beta-amyloid peptide 29-42: importance of the amino-terminus

Increasing evidence implicates interactions between Abeta-peptides and membrane lipids in Alzheimer's disease. To gain insight into the potential role of the free amino group of the N-terminus of Abeta29-42 fragment in these processes, we have investigated the ability of Abeta29-42 unprotected and Abeta29-42 N-protected to interact with negatively-charged liposomes and have calculated the interaction with membrane lipids by conformational analysis. Using vesicles mimicking the composition of neuronal membranes, we show that both peptides have a similar capacity to induce membrane fusion and permeabilization. The fusogenic effect is related to the appearance of non-bilayer structures where isotropic motions occur as shown by 31P and 2H NMR studies. The molecular modeling calculations confirm the experimental observations and suggest that lipid destabilization could be due to the ability of both peptides to adopt metastable positions in the presence of lipids. In conclusion, the presence of a free or protected (acetylated) amino group in the N-terminus of Abeta29-42 is therefore probably not crucial for destabilizing properties of the C-terminal fragment of Abeta peptides.

Gentamicin causes endocytosis of Na/Pi cotransporter protein (NaPi-2)

BACKGROUND

Renal toxicity is a major side-effect of aminoglycoside antibiotics and is characterized by an early impairment in proximal tubular function. In a previous study, we have shown that gentamicin administration to the rat causes an early impairment in sodium gradient-dependent phosphate (Na/Pi) cotransport activity. The purpose of our current study was to determine the molecular mechanisms of the impairment in Na/Pi cotransport activity, specifically the role of the proximal tubular type II Na/Pi cotransporter.

METHODS

Rats were treated for one, two, and three days with two daily injections of 30 mg/kg body weight gentamicin or the vehicle.

RESULTS

Gentamicin caused a progressive decrease in superficial cortical apical brush-border membrane (SC-BBM) Na/Pi cotransporter activity (856 +/- 93 in control vs. 545 +/- 87 pmol/mg BBM protein in 3-day gentamicin, P < 0.01). Western blot analysis showed a parallel and progressive decrease in SC-BBM Na/Pi cotransporter protein abundance, a 50% decrease after one day of treatment, a 63% decrease after two days of treatment, and an 83% decrease after three days treatment with gentamicin. In contrast, gentamicin treatment had no effect on Na/Pi cotransport activity or Na/Pi cotransporter protein abundance in BBM isolated from the juxtamedullary cortex (JMC-BBM). Immunofluorescence microscopy showed a major decrease in the expression of Na/Pi cotransporter protein in the apical membrane of the proximal convoluted tubule, with progressive intracellular accumulation of Na/Pi protein. Colocalization studies showed that in gentamicin-treated rats, Na/Pi protein was colocalized in the early endosomes and especially in the lysosomes. Northern blot analysis of cortical RNA interestingly showed no reduction in Na/Pi cotransporter mRNA abundance even after three days of gentamicin treatment.

CONCLUSION

We conclude that gentamicin inhibits Na/Pi cotransport activity by causing a decrease in the expression of the type II Na/Pi cotransport protein at the level of the proximal tubular apical BBM and that inhibition of Na/Pi cotransport activity is most likely mediated by post-transcriptional mechanisms.

Inhibition of in vitro endosomal vesicle fusion activity by aminoglycoside antibiotics

The effects of two aminoglycoside antibiotics, neomycin and Geneticin, on the endocytic pathway were studied using a cell-free assay that reconstitutes endosome-endosome fusion. Both drugs inhibit the rate and extent of endosome fusion in a dose-dependent manner with IC50 values of approximately 45 microM and approximately 1 mM, respectively. Because the IC50 for neomycin falls within the range of affinities reported for its binding to acidic phospholipids, notably phosphatidylinositol 4,5-bisphosphate (PIP2), these data suggest that negatively charged lipids are required for endosome fusion. A role for negatively charged lipids in membrane traffic has been postulated to involve the activity of a PIP2-dependent phospholipase D (PLD) stimulated by the GTP-binding protein ADP-ribosylation factor (ARF). Although neomycin blocks endosome fusion at a stage of the in vitro reaction that is temporally related to steps inhibited by cytosolic ARFs when they bind guanosine-5'-gamma-thiophosphate (GTPgammaS), these inhibitors appear to act in a synergistic manner. This idea is confirmed by the fact that addition of a PIP2-independent PLD does not suppress neomycin inhibition of endosome fusion; moreover, in vitro fusion activity is not affected by the pleckstrin homology domain of phosphoinositide-specific phospholipase C delta1, which binds to acidic phospholipids, particularly PIP2, with high affinity. Thus, although aminoglycoside-sensitive elements of endosome fusion are required at mechanistic stages that are also blocked by GTPgammaS-bound ARF, these effects are unrelated to inhibition of the PIP2-dependent PLD activity stimulated by this GTP-binding protein. These results argue that there are additional mechanistic roles for acidic phospholipids in the endosomal pathway.

Interaction of the macrolide azithromycin with phospholipids. I. Inhibition of lysosomal phospholipase A1 activity

Azithromycin, the first clinically developed dicationic macrolide antibiotic, displays an exceptional accumulation in lysosomes of cultured cells. In fibroblasts incubated with 50 mg/l (66.6 microM), it induces a distinct phospholipidosis as evidenced by biochemical and ultrastructural criteria, which strikingly resembles alterations described previously with gentamicin, a pentacationic aminoglycoside antibiotic which inhibits the lysosomal catabolism of phospholipids. We show that both drugs inhibit, in an equimolar manner, the activity of phospholipase A1 (assayed for phosphatidylcholine, included in negatively charged liposomes), in a way consistent with the model of 'charge neutralization' proposed already for gentamicin (Mingeot-Leclercq et al., 1988, Biochem. Pharmacol. 37, 591). Both drugs bind to negatively charged liposomes. Yet, in spite of this binding, azithromycin does not induce aggregation or fusion of negatively charged vesicles, under conditions in which gentamicin (or spermine, a fully hydrophilic polycation) causes a massive aggregation, and bis(beta-diethylaminoethylether)hexestrol (a dicationic amphiphile) causes fusion. The molecular interactions of azithromycin with acidic phospholipids are further examined in a companion paper.

Aminoglycoside antibiotics prevent the formation of non-bilayer structures in negatively-charged membranes. Comparative studies using fusogenic (bis(beta-diethylaminoethylether)hexestrol) and aggregating (spermine) agents

Aminoglycoside antibiotics cause aggregation but not fusion of negatively-charged liposomes at an extent proportional to their capacity to interact with acidic phospholipids (Van Bambeke et al., 1995, Eur. J. Pharmacol., 289, 321-333). To understand why aggregation is not followed by fusion, we have examined here the influence of two aminoglycosides with markedly different toxic potential (gentamicin > isepamicin) on lipid phase transition in negatively-charged liposomes using 31P-NMR spectroscopy, in comparison with spermine (an aggregating agent) and bis(beta-diethylaminoethylether)hexestrol or DEH (a fusogenic cationic amphiphile). Gentamicin, spermine, and, to a lesser extent, isepamicin inhibit the appearance of the isotropic signal seen upon warming of control liposomes and denoting the presence of mobile structures. This non-bilayer signal appeared most prominently when liposomes were incubated with DEH, a strong fusogenic agent. We conclude that aminoglycosides, like spermine, have the potential to prevent membrane fusion, by inhibiting the development of a critical change in membrane organization, which is associated with fusion. We suggest that this capacity could be a determinant in aminoglycoside toxicity.

Aminoglycoside antibiotics induce aggregation but not fusion of negatively-charged liposomes

The binding of aminoglycoside antibiotics to acidic phospholipids of membranes is an essential step in the development of both their renal and auditory toxicities, which could be associated with critical modifications of the membrane properties. This work examines the capacity of aminoglycosides to induce membrane aggregation and fusion. Three techniques were used in parallel: (i) measurement of the dequenching rate of a lipid-soluble fluorescent probe (octadecylrhodamine B) incorporated at self-quenched concentration in membranes; (ii) measurement of the increase in the energy transfer between two fluorescent derivatives of phospholipids; and (iii) electron microscopy of negatively-stained replicas. The results were compared with those obtained with spermine (an aggregating polycation) and melittin (a fusogenic peptide). The three approaches indicate that aminoglycosides induce liposomes aggregation, but not fusion. Aggregation is related to the capacity of each drug studied to bind phosphatidylinositol, as evaluated by its energy of interaction with this acidic phospholipid, and to its toxic potential. Membrane aggregation occurring in vivo could therefore contribute to, or be a determinant of this toxicity, which could rationally be screened for new derivatives by the methods applied here.

Molecular parameters involved in aminoglycoside nephrotoxicity

Aminoglycoside antibiotics are hydrophilic molecules consisting of an animated cyclitol associated with amino sugar. They bind in vivo as well as in vitro to negatively charged membranes. Their use as chemotherapeutic agents is unfortunately accompanied by oto- and nephrotoxic reactions, and the purpose of this review is to examine the role of the molecular interactions between aminoglycosides and membranes in the development of nephrotoxicity. 31P Nuclear magnetic resonance (NMR) and fluorescence depolarization have been used to characterize the effect of aminoglycosides on phosphate heads and fatty acyl chains of phospholipids. 15N NMR has been used to obtain interesting information on regioselective interactions of amino groups of antibiotics with phospholipids. The binding of aminoglycosides with negatively charged membranes is associated with impairment of phospholipid catabolism, change in membrane permeability, and membrane aggregation. Biochemical analysis and 1H NMR spectroscopy have brought information on the molecular mechanism involved in the impairment of phospholipid catabolism. Nephrotoxic aminoglycosides could induce sequestration of phosphatidylinositol and therefore reduce the amount of negative charge available for optimal lysosomal phospholipase activity toward phosphatidylcholine included in liposomes that also contain cholesterol and sphingomyelin. Conformational analysis shows that aminoglycosides, which have a high potency to inhibit lysosomal phospholipase activity, adopt an orientation parallel to the lipid/water interface. This orientation of the aminoglycoside molecule at the interface is also critical to explain the marked increase of membrane permeability induced by less nephrotoxic aminoglycosides such as isepamicin and amikacin. This effect is indeed only observed with aminoglycosides oriented perpendicular to this interface, probably related to the creation of a local condition of disorder. The impairment of phospholipid catabolism, which is considered to be an early and significant step in the development of aminoglycoside toxicity, is therefore not related to the change in membrane permeability. However, the role of this latter phenomenon and of membrane aggregation for aminoglycoside nephrotoxicity could be further investigated.

Alterations in membrane permeability induced by aminoglycoside antibiotics: studies on liposomes and cultured cells

Aminoglycoside antibiotics bind to negatively-charged membranes in vitro as well as in vivo. We have examined if this binding could be associated with a change in the properties of membrane permeability. We have used a series of aminoglycoside derivatives and two independent test systems, namely (i) the release of calcein and of Mn2+ from phosphatidylinositol-containing large unilamellar vesicles, and (ii) the influx of Ca2+ into cultured macrophages. We found that certain aminoglycosides (e.g., streptomycin, isepamicin) markedly increase the membrane permeability whereas others (e.g., gentamicin) barely or do not influence it. This increase, when it occurs, is slower or less extensive than observed with pore-forming agents (mellitin, nystatin) or a Ca(2+)-ionophore (ionomycin). It is not observed with an agent [bis(beta-diethylaminoethylether)hexestrol] known to cause membrane fusion, and is not associated with any detectable change in membrane fluidity. In computer-aided conformational analysis of mixed monolayers between phosphatidylinositol and the aminoglycosides studied, it was found that those derivatives inducing an increase in membrane permeability in our experiments adopted an orientation rather perpendicular to the interface, whereas those with no or only a moderate effect were placed in a parallel orientation to this interface. The perpendicular orientation might cause a local condition of disorder which could explain the effects observed.

Accessibility of aminoglycosides, isolated and in interaction with phosphatidylinositol, to water. A conformational analysis using the concept of molecular hydrophobicity potential

The mode of interaction between aminoglycosides and negatively charged phospholipids plays a critical role in the inhibition of lysosomal phospholipases induced by these antibiotics and therefore in their nephrotoxicity. Previous works suggested that accessibility of the drug interacting with phospholipids to water could be crucial in this respect. We have used the concept of molecular hydrophobicity potential described by Brasseur [J Med Chem 266: 16120-16127, 1991] to visualize the hydrophobic and hydrophilic envelopes around aminoglycosides assembled with phosphatidylinositol molecules, and to obtain a three-dimensional representation of the complex formed. Using a series of different aminoglycosides, we showed that molecules with a lower inhibitory potential (gentamicin B, amikacin and isepamicin) are surrounded by both hydrophobic and hydrophilic envelopes whereas aminoglycosides which are more inhibitory are enveloped primarily by either hydrophilic (kanamycin A or B) or hydrophobic (gentamicin C1a) envelopes. This approach, which is here for the first time applied to the study of drug-lipid complexes, could help in the better understanding of the molecular mechanism of lysosomal phospholipase inhibition induced by aminoglycosides.

Interactions of aminoglycoside antibiotics with phospholipids. A deuterium nuclear magnetic resonance study

The effect of several aminoglycoside (AG) antibiotics on aqueous multilamellar dispersions of mixtures of phosphatidylinositol (PI) and deuterated phosphatidylcholine (PC) has been studied by deuterium (2H) NMR. Isepamicin and amikacin gave rise to no significant changes in 2H-NMR lineshape relative to that of the lipid mixture without antibiotic. Both kanamycin A and B, which have a greater affinity for PI than the other two antibiotics examined in this study, induced temperature-dependent changes in 2H-NMR lineshapes and associated spectral moments. The results are consistent with an antibiotic-induced lateral phase separation giving rise to PC-enriched domains free of drug and PI-AG domains. These effects are correlated with the inhibitory potency of aminoglycosides towards PC degradation.

Interaction of phospholipids with proteins and peptides. New advances 1990

1. The review deals with the recent achievements in the study of the various interactions of phospholipids with proteins and peptides. 2. The interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. 3. The effect of the interaction on the structure and biological activity of the interacting molecules is also discussed.

Effect of substrate organization on the activity and on the mechanism of gentamicin-induced inhibition of rat liver lysosomal phospholipase A1

Aminoglycoside antibiotics, such as gentamicin, induce a lysosomal phospholipidosis in the kidney cortex of experimental animals and humans. In vitro, gentamicin binds to negatively charged phospholipids, such as phosphatidylinositol, and decreases the activity of lysosomal phospholipases towards a neutral phospholipid (phosphatidylcholine) included in lipid vesicles. The mechanism of such an inhibition was not unequivocally established. On one hand Mingeot-Leclercq et al. (Biochem Pharmacol 37: 591-599, 1988) observed that the activity of phospholipase A1 is modulated by the negative charges of the bilayer and that the inhibitory potency of gentamicin is inversely related to the phosphatidylinositol content of the vesicles, and therefore proposed that inhibition is due to charge neutralization. On the other hand, Hostetler and Jellison (J Pharmacol Exp Ther 254: 188-191, 1990) observed that the activity of phospholipase A1 is not modulated by the negative charges of the vesicles and that the inhibitory potency of gentamicin is directly related to the phosphatidylinositol content of the bilayer, and therefore proposed that inhibition is due to substrate depletion. However, the experimental designs of these two models differed in several respects such as the source (liver versus kidney) and nature of the enzyme (native lysosomal extract versus purified delipidated phospholipase A1), and the composition of lipid vesicles (those containing constant amounts of phosphatidylcholine and cholesterol, and inversely varying amounts of phosphatidylinositol and sphingomyelin versus those containing inversely related amounts of phosphatidylcholine and phosphatidylinositol only). In order to assess the nature of the differences between these models, we compared the activity of phospholipase A1 and its inhibition by gentamicin using only one source of enzyme, the rat liver lysosomal extract, and the two types of lipid vesicles as used in the above models. Our results showed that both models are true within the frame work of their respective experimental designs. However, since the composition of the lipid vesicles as well as the nature of the enzyme preparation (whole lysosomal extract) in the "charge neutralization" model is closer to in vivo conditions, we suggest that this model may be more relevant to the in vivo situation.

Drug-phospholipid interactions: role in aminoglycoside nephrotoxicity

Aminoglycoside antibiotics are known to be transported and accumulated within lysosomes of renal proximal tubular cells and to cause proximal tubular cell injury and necrosis. The pathogenesis of aminoglycoside nephrotoxicity is postulated to be related to the capacity of these organic polycations to interact electrostatically with membrane anionic phospholipids and to disrupt membrane structure and function. Aminoglycoside antibiotics have been shown to bind to anionic phospholipids of model membranes and to alter membrane permeability and promote membrane aggregation. In vivo these drugs induce phospholipiduria and a renal cortical phospholipidosis. The latter reflects the accumulation of phospholipid-containing myeloid bodies within the lysosomal compartment consequent to aminoglycoside-induced inhibition of lysosomal phospholipases. The mechanism of drug-induced inhibition of phospholipases has been shown to be secondary to the binding of these cationic drugs to anionic phospholipids. As the lysosomes became progressively distended with myeloid bodies, they become unstable and eventually rupture, which results in the release of acid hydrolases as well as high concentrations of aminoglycosides into the cytoplasm where they interact with and disrupt the function of other membranes and organelles including mitochondria and microsomes. It is postulated that the redistribution of drug from the lysosomal compartment to organellar membranes is the critical event which triggers the irreversible injury cascade. Polyaspartic acid is a polyanionic peptide which when administered in vitro or in vivo forms electrostatic complexes with aminoglycoside antibiotics and prevents these drugs from interacting with anionic phospholipids, from perturbing phospholipid metabolism and from causing cell injury and necrosis.

Effect of acidic phospholipids on the activity of lysosomal phospholipases and on their inhibition induced by aminoglycoside antibiotics--II. Conformational analysis

In a companion paper (Mingeot-Leclercq et al. Biochem Pharmacol 40: 489-497, 1990), we showed that the inhibitory potency of gentamicin on the activity of lysosomal phospholipases, measured towards phosphatidylcholine included in negatively-charged liposomes, is markedly influenced by the nature of the acidic phospholipid used (phosphatidylinositol, phosphatidylserine, phosphatidic acid), whereas the binding of the drug to the three types of liposomes is similar. This result challenged previous conclusions pointing to a key role exerted by drug binding to phospholipid membranes and presumably charge neutralization, for phospholipases inhibition (Carlier et al. Antimicrob Agents Chemother, 23: 440-449, 1983; Mingeot-Leclercq et al., Biochem Pharmacol 37:591-599, 1988). Conformational analysis of mixed monolayers of gentamicin and each of the three acid phospholipids shows that gentamicin systematically adopts an orientation largely parallel to the hydrophobic-hydrophilic interface, but that (i) the energies of interaction are largely different (phosphatidylinositol greater than phosphatidylserine greater than phosphatidic acid), and (ii) the apparent accessibility of the bound drug to water varies in an inverse relation with the energies of interaction. Amikacin, a semisynthetic derivative of kanamycin A with a lower inhibitory potential towards phospholipases than gentamicin in the three types of liposomes used, also showed similar differences in energies of interaction and accessibility to water, but constantly exhibited an orientation perpendicular to the hydrophobic-hydrophilic interface. We conclude that impairment of lysosomal phospholipase activities towards phosphatidylcholine included in negatively-charged membranes by aminoglycoside antibiotics is indeed dependent upon drug binding to the bilayer, but is also modulated by (i) the nature of the acidic phospholipid, which influences the energy of interaction and the accessibility of the drug with respect to the hydrophilic phase, and (ii) the orientation of the drug, which it itself related to its chemical structure. Inasmuch as phospholipases inhibition is related to aminoglycoside nephrotoxicity, these findings may help in better defining the molecular determinants and mechanisms responsible for this adverse effect.