Activation of phospholipase A2 by adriamycin in vitro. Role of drug-lipid interactions

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.

Oxidized phospholipids as potential novel drug targets

The interactions of three therapeutic agents, viz. the antipsychotics HPD and CPZ, and the antineoplastic anthracycline DOX, with oxidatively modified phospholipids were studied by monitoring the quenching of fluorescence of an incorporated pyrene-labeled lipid derivative. All three drugs bound avidly to the two oxidized PCs bearing either an aldehyde or carboxylic function at the end of the sn-2 nonanoyl chain, with the highest affinity measured between CPZ and the latter oxidized lipid. Subsequent dissociation of the above drugs from the oxidized lipids by DNA, acidic phospholipids, and NaCl revealed the binding of these drugs with the aldehyde lipid to be driven by hydrophobicity similarly to their binding to lysophosphatidylcholine, whereas a significant contribution of electrostatics was evident for the lipid with the carboxylic moiety. These results connect to previous experimental data, demonstrating the induction by these drugs of oxidative stress and binding to membrane phospholipids. These issues are elaborated with reference to their clinical use and side effects.

Modulation of the activity of secretory phospholipase A2 by antimicrobial peptides

The antimicrobial peptides magainin 2, indolicidin, and temporins B and L were found to modulate the hydrolytic activity of secretory phospholipase A(2) (sPLA(2)) from bee venom and in human lacrimal fluid. More specifically, hydrolysis of phosphatidylcholine (PC) liposomes by bee venom sPLA(2) at 10 micro M Ca(2+) was attenuated by these peptides while augmented product formation was observed in the presence of 5 mM Ca(2+). The activity of sPLA(2) towards anionic liposomes was significantly enhanced by the antimicrobial peptides at low [Ca(2+)] and was further enhanced in the presence of 5 mM Ca(2+). Similarly, with 5 mM Ca(2+) the hydrolysis of anionic liposomes was enhanced significantly by human lacrimal fluid sPLA(2), while that of PC liposomes was attenuated. These results indicate that concerted action of antimicrobial peptides and sPLA(2) could improve the efficiency of the innate response to infections. Interestingly, inclusion of a cationic gemini surfactant in the vesicles showed an essentially similar pattern on sPLA(2) activity, suggesting that the modulation of the enzyme activity by the antimicrobial peptides may involve also charge properties of the substrate surface.

Interactions of adriamycin, cytochrome c, and serum albumin with lipid monolayers containing poly(ethylene glycol)-ceramide

Poly(ethylene glycol)(2000)C(20)ceramide (PEG-Cer) containing monolayers at an air/water interface were characterized by measuring their surface pressure versus area/molecule (pi-A) and surface potential versus area/molecule (Delta V-A) isotherms. The behavior of pi-A as well as Delta V versus lipid density (Delta V-n) and Delta V-pi isotherms for PEG-Cer are in keeping with two transitions of the lipopolymer, starting at pi approximately equal to 9 and 21 mN/m. We also investigated the effects of PEG-Cer on the binding of adriamycin, cytochrome c and bovine serum albumin to monolayers containing varying mole fractions X of PEG-Cer. PEG-Cer impedes the penetration of these ligands into lipid monolayers with similar effects at both X = 0.04 and 0.08. This effect of PEG-Cer depends on the conformation of the lipopolymer and the interactions between the lipid surface and the surface-interacting molecule as well as the size of the latter.

Implication of radical oxygen species in ceramide generation, c-Jun N-terminal kinase activation and apoptosis induced by daunorubicin

Anthracyclines such as daunorubicin (DNR) generate radical oxygen species (ROS), which account, at least in part, for their cytotoxic effect. We observed that early ceramide generation (within 6-10 min) through neutral sphingomyelinase stimulation was inhibitable by the antioxidants N-acetylcysteine and pyrrolidine dithiocarbamate, which led to a decrease in apoptosis (>95% decrease in DNA fragmentation after 6 h). Furthermore, we observed that DNR triggers the c-Jun N-terminal kinase (JNK) and the transcription factor activated protein-1 through an antioxidant-inhibitable mechanism. Treatment of U937 cells with cell-permeant ceramides induced both an increase in ROS generation and JNK activation, and apoptosis, all of which were antioxidant-sensitive. In conclusion, DNR-triggered apoptosis implicates a ceramide-mediated, ROS-dependent JNK and activated protein-1 activation.

Binding of adriamycin to liposomes as a probe for membrane lateral organization

A stopped-flow spectrofluorometer equipped with a rapid scanning emission monochromator was utilized to monitor the binding of adriamycin to phospholipid liposomes. The latter process is evident as a decrease in fluorescence emission from a trace amount of a pyrene-labeled phospholipid analog (PPDPG, 1-palmitoyl-2-[(6-pyren-1-yl)]decanoyl-sn-glycero-3-phospho-rac-++ +glyce rol) used as a donor for resonance energy transfer to adriamycin. For zwitterionic 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) liposomes, fluorescence decay was slow, with a half-time t1/2 of approximately 2 s. When the mole fraction of the acidic phospholipid, 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-rac-glycerol (POPG), was increased to XPG >/= 0.04, the decay of fluorescence became double exponential, and an additional, significantly faster process with t1/2 in the range between 2 and 4 ms was observed. Subsequently, as XPG was increased further, the amplitude of the fast process increased, whereas the slower process was attenuated, its t1/2 increasing to 20 s. Increasing [NaCl] above 50 mM or [CaCl2] above 150 microM abolished the fast component, thus confirming this interaction to be electrostatic. The critical dependence of the fast component on XPG allows the use of this process to probe the organization of acidic phospholipids in liposomes. This was demonstrated with 1, 2-palmitoyl-sn-glycero-3-phosphocholine (DPPC) liposomes incorporating PPDPG (XPPDPG = 0.03), i.e., conditions where XPG in fluid bilayers is below the required threshold yielding the fast component. In keeping with the presence of clusters of PPDPG, the fast component was observed for gel-state liposomes. At approximately 34 degreesC (i.e., 6 degrees below Tm), the slower fluorescence decay also appeared, and it was seen throughout the main phase transition region as well as in the liquid-crystalline state. The fluorescence decay behavior at temperatures below, above, and at the main phase transition temperature is interpreted in terms of thermal density fluctuations and an intermediate state between gel and liquid-crystalline states being involved in the phospholipid main phase transition. This is the first observation of a cluster constituted by acidic phospholipids controlling the membrane association of a drug.

Binding of quinacrine to acidic phospholipids and pancreatic phospholipase A2. Effects on the catalytic activity of the enzyme

Binding of quinacrine to phospholipids and porcine pancreatic phospholipase A2 (PLA2) was investigated using fluorescence resonance energy transfer, Langmuir films, assay for the enzymatic activity, and molecular modeling. No significant binding of this drug to the zwitterionic phosphatidylcholine was observed whereas a high affinity for acidic phospholipids was revealed by quenching of pyrene-labeled phospholipid analogues. Partial reversal of this binding was observed due to the addition of 4 mM CaCl2. Quinacrine efficiently and independently of the lipid surface pressure penetrated into monolayers of phosphatidylglycerol while only a weak penetration into phosphatidylcholine films was evident. Quinacrine also bound to eosin-labeled PLA2, and the addition of 4 mM CaCl2 reversed this interaction almost completely. In the presence of acidic phospholipids both the drug and the enzyme were attached to the lipid surface. Studies on the influence of quinacrine on the activity of PLA2 toward pyrene-labeled phospholipid analogues revealed that the hydrolysis of phosphatidylcholine was progressively reduced as a function of increasing [quinacrine]. At low [CaCl2] and low quinacrine:lipid molar ratios (<1:5) quinacrine enhanced slightly the rate of hydrolysis of acidic phospholipids whereas at higher drug:lipid molar ratios (>1:2) an inhibition was observed. In the presence of 1 mM CaCl2 quinacrine inhibited PLA2-catalyzed hydrolysis of phosphatidylglycerol only when the drug:lipid molar ratio exceeded 1:1. The presence of 4 mM CaCl2 abolished nearly completely the inhibition with all the substrate analogues used. Our data suggest that the inhibition of PLA2 by quinacrine is due to its binding to the enzyme. This is supported also by molecular modeling which suggested a binding site for quinacrine close to the active site and Ca2+ binding site of the enzyme. Importantly, our data indicate that quinacrine binds avidly to acidic phospholipids and their presence may influence the drug-enzyme interaction and the inhibition of the enzyme action. Accordingly, presence of quinacrine may interfere also with other processes that require the presence of acidic lipids and/or Ca2+, such as the function of the nicotinic acetylcholine receptor.

Binding of daidzein to liposomes

Turbidity and differential scanning calorimetry measurements revealed the plant derived antineoplastic isoflavone, daidzein, to bind to large unilamellar liposomes. Comparing different unsaturated phospholipids most pronounced aggregation due to daidzein was observed for phosphatidylinositol (PI) while the inclusion of cholesterol strongly attenuated the aggregation. Interestingly, aggregation was not observed for the structurally very closely related isoflavone, genistein. The extent of aggregation was nonlinearly dependent on the content of PI in egg phosphatidylcholine (eggPC) vesicles. The saturated dimyristoyl phospholipids, phosphatidylserine, phosphatidylcholine, phosphatidic acid, as well as phophatidylglycerol were also extensively aggregated by daidzein at 10 degrees C, i.e., below their main phase transition temperature whereas their aggregation at 35 degrees C in the fluid phase was strongly reduced. Vesicle aggregation could be accompanied by membrane fusion, however, neither contents mixing nor lipid mixing of the LUVs (large unilamellar vesicles) was observed in the presence of daidzein. Strong perturbation of the thermal phase behaviour of both dimyristoyl phosphatidylcholine (DMPC) and dimyristoyl phosphatidylserine (DMPS) multilamellar vesicles by daidzein was revealed by differential scanning calorimetry. More specifically, for DMPC increasing quantities of daidzein progressively decreased both the main transition temperature Tm and its enthalpy whereas for DMPS a decrease in delta H was not observed, thus indicating the modes of interaction of daidzein with these phospholipids to differ. Our results indicate daidzein to reside in the polar headgroup/interfacial region of PI and PS membranes. The interactions of daidzein with phospholipids could represent an additional contributor to the growing list of effects of this isoflavone on cellular functions.

Activation of phospholipase A2 by amyloid beta-peptides in vitro

Amyloid beta-peptides (A beta) are centrally involved in the pathogenesis of Alzheimer's disease. Using secretory phospholipase A2 (PLA2) from porcine pancreas as a model and in the presence of a limiting Ca2+ concentration of approximately 50 nM, the synthetic peptide A beta 1-42 activates the hydrolysis of the pyrene-labeled acidic phospholipid analog 1-palmitoyl-2-[(pyren-1-yl)]hexanoyl-sn-glycero-3-phosphoglycerol (PPHPG) maximally 2.3-fold, whereas an inhibition of PLA2 action by 50% on the corresponding phosphatidylcholine derivative (PPHPC) was observed. The above effects were evident at 0.24 nM A beta 1-42 corresponding to A beta 1-42:phospholipid and A beta 1-42:PLA2 molar ratios of 1:10 650 and 1:7.6, respectively. The presence of 10 mol % 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG) in PPHPC reversed the inhibitory effect of A beta 1-42 peptide and for these vesicles the hydrolytic activity of PLA2 toward the fluorescent phosphatidylcholine was enhanced approximately 1.8-fold by A beta 1-42. In contrast, inclusion of 10 mol % POPG into PPHPG did not influence either the hydrolytic rate toward the latter lipid or the activating effect of A beta 1-42. Ca2+ concentrations exceeding 15 microM abolished the enhancing effect of A beta 1-42 on the hydrolysis of PPHPG whereas a slight activation of PPHPC hydrolysis now became evident. With limiting [Ca2+] preaggregated A beta 1-42 enhanced the hydrolysis of both PPHPG as well as PPHPC but the peptide concentrations required were higher by 3-4 orders of magnitude. The synthetic peptide A beta 25-35 corresponding to the hydrophobic membrane-spanning segment of the beta amyloid precursor protein activated PLA2 when using PPHPG as a substrate; however, compared to A beta 1-42 the extent of activation was less (approximately 2-fold) and required higher (1 nM) peptide. A beta 25-35 did not affect the hydrolysis of the phosphatidylcholine derivative. The hydrophilic peptide A beta 1-28 had no effect on PLA2-catalyzed hydrolysis of either PPHPG or PPHPC under the conditions used in the present study. Interestingly, the above activating effects of A beta 1-42 and A beta 25-35 on PLA2-catalyzed hydrolysis of the acidic phospholipid substrate parallel their toxicity on cultured neurons whereas A beta 1-28 had no influence either on cultured cells or on PLA2 activity.

Binding of DNA to liposomes containing different derivatives of sphingosine

Binding of DNA to dimyristoylphosphatidylcholine (DMPC) liposomes containing different sphingosine derivatives was investigated. DNA labelled with adriamycin was used as a fluorescence quencher and its membrane association was observed by resonance energy transfer from liposomes incorporating a pyrene-derivatized lipid bisPDPC as a donor and containing 19 mol% of sphingosine, dihydro-, phyto- or dimethylsphingosine. As revealed by differential scanning calorimetry, the thermal phase behaviour of multilamellar liposomes containing these sphingolipids was also significantly altered by DNA. Attachment of DNA to liposomes containing sphingosylphosphorylcholine was much weaker, and no binding of DNA to membranes containing N-acetylsphingosine, N-stearoylsphingosine or sphingomyelin was observed. The membrane binding of DNA was dependent on pH and could be reversed by the inclusion of phosphatidic acid (eggPA) into the liposomes. Analogously, the association of cytochrome c with eggPA could be reversed by the DNA-binding sphingosines. These findings lend support to our previous proposal that the DNA-sphingosine interaction is electrostatic and requires the presence of a positive charge in the latter. Accordingly, sphingosines carrying a protonated amino group attach DNA to membranes, while blocking of the amino group by N-acylation abolishes this interaction.

Differential scanning calorimetry study on the binding of nucleic acids to dimyristoylphosphatidylcholine-sphingosine liposomes

Binding of DNA and RNA to sphingosine-containing dimyristoylphosphatidylcholine (DMPC) liposomes was characterized by differential scanning calorimetry. The thermal phase behaviour of neat DMPC liposomes was unaffected by the presence of the nucleic acids. However, significant alterations in the melting profiles of the DMPC/sphingosine composite membranes were produced by DNA and RNA, thus revealing their binding to the liposomes. For example, for 79:21 (molar ratio) DMPC/sphingosine liposomes a single endotherm at 29.1 degrees C with an enthalpy of 6.3 kcal/mol lipid was observed. In the presence of DNA at the nucleotide/sphingosine ratio of 0.6 this endotherm separated into three distinct peaks at 28.0, 31.4 and 35.1 degrees C, together with an approximately 22% reduction in the total enthalpy. Further increase in DNA concentration up to 1.5 nucleotides per sphingosine led to complete loss of the original heat absorption peak of the DMPC/sphingosine liposomes, while an endotherm at 34.3 degrees C with delta H of 2.7 kcal/mol developed. By visual inspection, rapid and extensive aggregation of the liposomes due to DNA was evident. Evidence for DNA-induced phase separation was also provided by compression isotherms of sphingosine containing DMPC monolayers recorded over an aqueous buffer both in the presence and absence of DNA. The effects of RNA on the thermal phase behaviour of the composite liposomes were qualitatively similar to those described above for DNA. Notably, the presence of eggPA abolished the nucleic acid induced heat capacity changes for DMPC/sphingosine liposomes probably because of neutralization of the positive charge of sphingosine. The binding of DNA to DMPC/sphingosine liposomes occurred both below and above the lipid phase transition temperature, as shown by fluorescence resonance energy transfer utilizing adriamycin-labelled DNA as a quencher and membrane incorporated pyrene-labelled phospholipid as a donor. However, the apparent binding to liquid crystalline liposomes was slightly more effective.

Nanomolar arachidonic acid influences the respiratory burst in eosinophils and neutrophils induced by GTP-binding protein. A comparative study of the respiratory burst in bovine eosinophils and neutrophils

To investigate a possible role of phospholipase A2 (PLA2) in the respiratory burst in bovine eosinophilic and neutrophilic leukocytes dependent on GTP-binding protein (G-protein), we permeabilized these cells with Staphylococcus aureus alpha-toxin and induced NADPH oxidase activity with the non-hydrolysable GTP analogue GTP[S] or the aluminium tetrafluoro complex AlF4-. Under same experimental conditions, cells responded with different onset times. The onset time for eosinophils was 50-200 s, for neutrophils it was only a few seconds. GTP[S] stimulated in neutrophils only 5% of the respiratory burst compared to eosinophils, whereas AlF4(-)-induced comparable responses (neutrophils 120% of eosinophils). GDP inhibited these responses with an IC50 value of 2.4 mM. Arachidonic acid showed, with the exception of AlF4- stimulated neutrophils, on both stimuli and cell types an enhancing effect (150%) that reached its maximum at 0.1-1 microM. The PLA2 inhibitor 4-bromophenacylbromide reduced the GTP[S]- and AlF4(-)-induced response almost completely (10 microM) and the inhibition was not significantly different for eosinophils and neutrophils (IC50 1-3 microM). If the respiratory burst was reduced with 4-bromophenacylbromide to 1-4% of the original value, 10% of the basal NADPH oxidase activity could be restored by addition of only 20-100 nM arachidonic acid. In addition, the PLA2 activator adriamycin enhanced the response in a dose-dependent manner and in the same order as arachidonic acid did. The results presented above suggest that the respiratory burst may be regulated by different low-molecular-mass and/or heterotrimeric G-proteins and an active role for arachidonic acid or its metabolites in the activation and the maintenance of the direct G-protein-stimulated respiratory burst in bovine eosinophils and neutrophils.

Sphingosine-mediated membrane association of DNA and its reversal by phosphatidic acid

Resonance energy transfer was measured between egg phosphatidylcholine liposomes containing the intramolecular excimer forming pyrene-labelled phospholipid analogue 1,2-bis[pyren-1-(-yl)]decanoyl-sn-glycero-3-phosphocholine (bisPDPC) as a donor and DNA-bound adriamycin as an acceptor. Membrane association of DNA turned out to be critically dependent on the presence of sphingosine in the liposomes. Identical result was obtained by measuring the extent of quenching of the fluorescent DNA-bound dye Hoechst 33258 due to energy transfer to the lipophilic stain Nile Red incorporated in egg phosphatidylcholine liposomes containing varying amounts of sphingosine. The attachment of DNA to sphingosine-containing membranes could be reversed by the further inclusion of the negatively charged phosphatidic acid up to approximately 1:2 PA/sphingosine molar ratio in the liposomes, thus suggesting the involvement of electrostatic interactions. Differential scanning calorimetry measurements confirmed a lack of association between DNA and dimyristoylphosphatidylcholine liposomes. Instead drastic changes were produced by DNA in the heat capacity scans measured for liposomes also incorporating sphingosine. Fluorescence microscopy revealed an extensive aggregation of sphingosine containing pyrene-phosphatidylcholine-labelled egg phosphatidylcholine liposomes in the presence of DNA. Together with other available data on the effects of sphingosine, the present findings suggest that sphingosine could directly alter the chromatin structure. Accordingly, such alterations may contribute to the control of replication and gene expression.

Effect of anthracyclines on phospholipase A2 activity and prostaglandin E2 production in rat gastric mucosa

The purpose of this study was to investigate in rats the effects of three anthracyclines, pirarubicin, doxorubicin and epirubicin on gastric prostaglandin E2 (PGE2) metabolism and phospholipase A2 (PLA2, EC 3.1.1.4) activity. The level of the membrane precursor, arachidonic acid, and the stability of the membrane were investigated by analysis of the composition of fatty acids. Enzymatic activities involved in the turnover of membrane phospholipids such as lysophospholipase (LPase, EC 3.1.1.5) and acyl-CoA lysophosphatidylcholine: acyltransferase (ACLAT, EC 2.3.1.23), and in the detoxification of lipid hydroperoxides, selenium-dependent glutathione-peroxidase (GSH-PX, EC 1.11.1.9) were measured after injection of the drugs for 4 consecutive days. Pirarubicin does not give rise to any changes in these activities but doxorubicin and epirubicin decreased PGE2 production and the activities of PLA2, LPase and ACLAT. GSH-PX activity was not changed by any of the drugs. The decrease in PLA2 activity does not seem to be related to variations in membrane lipid composition because the total phospholipids content was unchanged. The P/S (polyunsaturated/saturated) ratio increased in the doxorubicin group and decreased in the epirubicin group, and the unsaturation index was moderately modified. Arachidonic acid was increased only in the doxorubicin group. In vitro, PLA2 activity was not inhibited by the three drugs in the micromolar range. A marked inhibition was observed at 2.5 mM for pirarubicin and at 1.0 mM for doxorubicin and epirubicin. The Lineweaver-Burk representation showed that these inhibitions were of an uncompetitive type. Pirarubicin may therefore be considered to be an anthracycline without marked side-effects on gastric mucosa. However, the in vitro inhibition of PLA2 activity by anthracyclines does not fully explain the in vitro decrease in PLA2 specific activity observed after doxorubicin and epirubicin treatment, and in this context membrane structure modifications unconnected with the lipid composition can not be excluded. In vivo these phenomena may affect PGE2 synthesis, whose level was lower in the doxorubicin and epirubicin groups than in control group.

Influence of Ca2+ and ethanol on the aggregation and thermal phase behaviour of L-dihexadecylphosphatidylcholine liposomes

The influence of Ca2+ and ethanol on vesicle aggregation and thermal phase behaviour of the diether lipid 1,2-dihexadecylphosphatidylcholine (DHPC) was studied by light absorbance and DSC. At temperatures below the pretransition the ethanol-injected vesicles of L-DHPC were rapidly aggregated by Ca2+. Upon raising the cation concentration a biphasic increase in aggregation saturating at an approximate [Ca2+]/[lipid] ratio of 1.5:1 was observed. Further increase in [Ca2+] up to [Ca2+]/[lipid] stoichiometries exceeding 2.5:1 led to the loss of aggregation. Removal of ethanol by dialysis abolished Ca(2+)-induced aggregation. Ethanol-injected vesicles of the ester-linked L-dipalmitoylphosphatidylcholine (L-DPPC) or the racemic DL-DHPC were not aggregated by Ca2+ thus indicating the importance of the absence of ester carbonyls as well as the stereochemical configuration of the lipid in determining the mode of interaction of DHPC with Ca2+. Differential scanning calorimetry of multilamellar liposomes of L-DHPC showed an increase by 8 degrees in the pretransition temperature Tp in the presence of 250 mM ethanol. Both with and without ethanol, increasing concentrations of Ca2+ corresponding to [Ca2+]/[lipid] ratios of 1:1 to 20:1 caused a gradual decrease in Tp and finally the disappearance of the pretransition. Concomitantly a slight elevation in Tm occurred. No principal differences were observed in the thermal phase behaviour of the L-isomer and racemic DL-DHPC.

Effect of doxorubicin on the order of the acyl chains of anionic and zwitterionic phospholipids in liquid-crystalline mixed model membranes: absence of drug-induced segregation of lipids into extended domains

We investigated the effect of the antineoplastic drug doxorubicin on the order of the acyl chains in liquid-crystalline mixed bilayers consisting of dioleoylphosphatidylserine (DOPS) or -phosphatidic acid (DOPA), and dioleoylphosphatidylcholine (DOPC) or -phosphatidylethanolamine (DOPE). Previous 2H-NMR studies on bilayers consisting of a single species of di[11,11-2H2]oleoyl-labeled phospholipid showed that doxorubicin does not affect the acyl chain order of pure zwitterionic phospholipid but dramatically decreases the order of anionic phospholipid [de Wolf, F. A., et al. (1991) Biochim. Biophys. Acta 1096, 67-80]. In the present work, we studied mixed bilayers in which alternatively the anionic or the zwitterionic phospholipid component was 2H-labeled so as to monitor its individual acyl chain order. Doxorubicin decreased the order parameter of the mixed anionic and zwitterionic lipids by approximately the same amount and did not induce a clear segregation of the lipid components into extended, separate domains. The drug had a comparable disordering effect on mixed bilayers of unlabeled cardiolipin and 2H-labeled zwitterionic phospholipid, indicating the absence of extensive segregation also in that case. Upon addition of doxorubicin to bilayers consisting of 67 mol% DOPE and 33 mol% anionic phospholipid, a significant part of the lipid adopted the inverted hexagonal (HII) phase at 25 degrees C. This bilayer destabilization, which occurred only in mixtures of anionic phospholipid and sufficient amounts of DOPE, might be of physiological importance. Even upon formation of extended HII-phase domains, lipid segregation was not clearly detectable, since the relative distribution of 2H-labeled anionic phospholipid and [2H]DOPE between the bilayer phase and HII phase was very similar. Our findings argue against a role of extensive anionic/zwitterionic lipid segregation in the mechanism of action and toxicity of doxorubicin.

Heart and liver membrane phospholipid homeostasis during acute administration of various antitumoral drugs to the rat

The purpose of this study was to investigate in the rat heart and liver the effects of an acute administration of three anthracyclines, doxorubicin, epirubicin and pirarubicin, and an anthracenedione, mitoxantrone, on the membrane peroxidative status, which was estimated by the composition of polyunsaturated fatty acids (PUFA), and on the activities of the enzymes involved in membrane repair processes and lipid hydroperoxide detoxification. Rats were injected for four consecutive days with the drugs or saline (control) and killed 24 hr after the last injection. All the drugs induced an increase in plasma thiobarbituric reactive substances and alpha-tocopherol concentrations, both expressed per milligram of plasma lipids. Plasma vitamin A was decreased by about a factor of two by all the drugs. The fatty acid profile in the heart lipids showed that the polyunsaturated species (20:4 n-6, 22:6 n-3) remained at the same or even higher levels after anthracycline treatment. This can be explained by the fact that the activities of the enzymes involved in either the recycling of membrane phospholipids, such as phospholipases A1 and A2 (EC 3.1.1.4 and EC 3.1.1.32), lysophospholipases (EC 3.1.1.5) and acylCoA:lysophosphatidylcholine acyltransferases (EC 2.3.1.23), or hydroperoxide detoxification, such as selenium-dependent glutathione peroxidase (GSH-PX, EC 1.11.1.9) and glutathione S-transferases (GSH-T, EC 2.1.5.18), were maintained at the same level of activity after the antitumoral treatment. In liver, membrane phospholipid levels of PUFA were maintained as well as the activities of phospholipid-metabolizing enzymes. GSH-PX activity was not affected whereas that of GSH-T was slightly lowered by the drugs. These results suggest that during acute antitumoral-induced lipid peroxidation of membranes, the multi-enzymatic complex of the immediate processes of repair and detoxification is fully operational, allowing the membrane to rapidly recover its functional status. The results are discussed in the context of the equivocal relationships between antitumoral-induced lipid peroxidation and cardiac disturbances.

Substrate level modulation of the activity of phospholipase A2 in vitro by 12-O-tetradecanoylphorbol-13-acetate

The action of porcine pancreatic phospholipase A2 towards fluorescent phospholipid analogs is either enhanced or suppressed by 4 beta-12-O-tetradecanoylphorbol-13- acetate (TPA), depending on the chemical structure of the substrate and the concentration of Ca2+. In the presence of nmolar Ca2+ concentrations increasing [TPA] enhanced by approx. 5-fold the rate of hydrolysis of the pyrene-labelled acidic alkyl-acyl phospholipid, 1-octacosanyl-2-[6- (pyrene-1-yl)] hexanoyl-sn-glycero-3-phosphatidylmethanol. Maximal effect was obtained at high TPA/substrate molar ratios approaching 1:2. In the presence of 4 mM CaCl2 maximal activation was reduced to approximately 1.5-fold. With the corresponding phosphatidylcholine derivative as a substrate increasing [TPA] reduced fatty acid release maximally by 90% both at low [Ca2+] as well as in the presence of 4 mM CaCl2. Essentially identical results were obtained using 4 alpha-TPA, a stereoisomer which does not activate protein kinase C.

Membrane structure, toxins and phospholipase A2 activity

The phospholipid-hydrolyzing enzyme phospholipase A2 (PLA2) (EC 3.1.1.4) exists in several forms which can be located in the cytosol or on cellular membranes. We review briefly cellular regulatory mechanisms involving covalent modification by protein kinase C and the action of Ca2+, cytokines, G proteins and other cellular proteins. The major focus is the role of phospholipid structure on PLA2 activity, including (1) the mechanism of PLA2 action on synthetic phospholipid bilayers, (2) perturbation of synthetic and cellular membranes with lipophilic agents and membrane-interactive peptides and (3) the ability of these agents to activate endogenous PLA2 activity, with emphasis on the venom and plant toxins melittin, cardiotoxin and Pyrularia thionein.