Hexadecylphosphocholine inhibits translocation of CTP:choline-phosphate cytidylyltransferase in Madin-Darby canine kidney cells

Milteforan, a promising veterinary commercial product against feline sporotrichosis

Sporotrichosis, the cutaneous mycosis most commonly reported in Latin America, is caused by the Sporothrix clinical clade species, including Sporothrix brasiliensis and Sporothrix schenckii sensu stricto. Due to its zoonotic transmission in Brazil, S. brasiliensis represents a significant health threat to humans and domestic animals. Itraconazole, terbinafine, and amphotericin B are the most used antifungals for treating sporotrichosis. However, many strains of S. brasiliensis and S. schenckii have shown resistance to these agents, highlighting the importance of finding new therapeutic options. Here, we demonstrate that milteforan, a commercial veterinary product against dog leishmaniasis, whose active principle is miltefosine, is a possible therapeutic alternative for the treatment of sporotrichosis, as observed by its fungicidal activity in vitro against different strains of S. brasiliensis and S. schenckii. Fluorescent miltefosine localizes to the Sporothrix cell membrane and mitochondria and causes cell death through increased permeabilization. Milteforan decreases S. brasiliensis fungal burden in A549 pulmonary cells and bone marrow-derived macrophages and also has an immunomodulatory effect by decreasing TNF-α, IL-6, and IL-10 production. Our results suggest milteforan as a possible alternative to treat feline sporotrichosis.

IMPORTANCE

Sporotrichosis is an endemic disease in Latin America caused by different species of Sporothrix. This fungus can infect domestic animals, mainly cats and eventually dogs, as well as humans. Few drugs are available to treat this disease, such as itraconazole, terbinafine, and amphotericin B, but resistance to these agents has risen in the last few years. Alternative new therapeutic options to treat sporotrichosis are essential. Here, we propose milteforan, a commercial veterinary product against dog leishmaniasis, whose active principle is miltefosine, as a possible therapeutic alternative for treating sporotrichosis. Milteforan decreases S. brasiliensis fungal burden in human and mouse cells and has an immunomodulatory effect by decreasing several cytokine production.

Effect of Erufosine on Membrane Lipid Order in Breast Cancer Cell Models

Alkylphospholipids are a novel class of antineoplastic drugs showing remarkable therapeutic potential. Among them, erufosine (EPC3) is a promising drug for the treatment of several types of tumors. While EPC3 is supposed to exert its function by interacting with lipid membranes, the exact molecular mechanisms involved are not known yet. In this work, we applied a combination of several fluorescence microscopy and analytical chemistry approaches (i.e., scanning fluorescence correlation spectroscopy, line-scan fluorescence correlation spectroscopy, generalized polarization imaging, as well as thin layer and gas chromatography) to quantify the effect of EPC3 in biophysical models of the plasma membrane, as well as in cancer cell lines. Our results indicate that EPC3 affects lipid-lipid interactions in cellular membranes by decreasing lipid packing and increasing membrane disorder and fluidity. As a consequence of these alterations in the lateral organization of lipid bilayers, the diffusive dynamics of membrane proteins are also significantly increased. Taken together, these findings suggest that the mechanism of action of EPC3 could be linked to its effects on fundamental biophysical properties of lipid membranes, as well as on lipid metabolism in cancer cells.

Multi-target drugs active against leishmaniasis: A paradigm of drug repurposing

This mini-review focuses on leishmanicidal drugs that were sourced from small molecules previously approved for other diseases. The mechanisms of action of these molecules are herein explored, to probe the origins of their inter-species growth inhibitory activities. It is shown how the transversal action of the azoles - fluconazole, posaconazole and itraconazole - in both fungi and Leishmania is due to the occurrence of the same target, lanosterol 14-α-demethylase, in these two groups of species. In turn, the drugs miltefosine and amphotericin B are presented as truly multi-target agents, acting on small molecules, proteins, genes and even organelles. Steps towards future leishmanicidal drug candidates based on the multi-target strategy and on drug repurposing are also briefly presented.

Identification and Characterization of the Entamoeba Histolytica Rab8a Binding Protein: A Cdc50 Homolog

Membrane traffic plays a pivotal role in virulence in the enteric protozoan parasite Entamoeba histolytica. EhRab8A small GTPase is a key regulator of membrane traffic at the endoplasmic reticulum (ER) of this protist and is involved in the transport of plasma membrane proteins. Here we identified the binding proteins of EhRab8A. The Cdc50 homolog, a non-catalytic subunit of lipid flippase, was identified as an EhRab8A binding protein candidate by affinity coimmunoprecipitation. Binding of EhRab8A to EhCdc50 was also confirmed by reciprocal immunoprecipitation and blue-native polyacrylamide gel electrophoresis, the latter of which revealed an 87 kDa complex. Indirect immunofluorescence imaging with and without Triton X100 showed that endogenous EhCdc50 localized on the surface in the absence of permeabilizing agent but was observed on the intracellular structures and overlapped with the ER marker Bip when Triton X100 was used. Overexpression of N-terminal HA-tagged EhCdc50 impaired its translocation to the plasma membrane and caused its accumulation in the ER. As reported previously in other organisms, overexpression and accumulation of Cdc50 in the ER likely inhibited surface transport and function of the plasma membrane lipid flippase P4-ATPase. Interestingly, HA-EhCdc50-expressing trophozoites gained resistance to miltefosine, which is consistent with the prediction that HA-EhCdc50 overexpression caused its accumulation in the ER and mislocalization of the unidentified lipid flippase. Similarly, EhRab8A gene silenced trophozoites showed increased resistance to miltefosine, supporting EhRab8A-dependent transport of EhCdc50. This study demonstrated for the first time that EhRab8A mediates the transport of EhCdc50 and lipid flippase P4-ATPase from the ER to the plasma membrane.

Alkylphospholipids: An update on molecular mechanisms and clinical relevance

Alkylphospholipids (APLs) represent a new class of drugs which do not interact directly with DNA but act on the cell membrane where they accumulate and interfere with lipid metabolism and signalling pathways. This review summarizes the mode of action at the molecular level of these compounds. In this sense, a diversity of mechanisms has been suggested to explain the actions of clinically-relevant APLs, in particular, in cancer treatment. One consistently reported finding is that APLs reduce the biosynthesis of phosphatidylcholine (PC) by inhibiting the rate-limiting enzyme CTP:phosphocholine cytidylyltransferase (CT). APLs also alter intracellular cholesterol traffic and metabolism in human tumour-cell lines, leading to an accumulation of cholesterol inside the cell. An increase in cholesterol biosynthesis associated with a decrease in the synthesis of choline-containing phospholipids and cholesterol esterification leads to a change in the free-cholesterol:PC ratio in cells exposed to APLs. Akt phosphorylation status after APL exposure shows that this critical regulator for cell survival is modulated by changes in cholesterol levels induced in the plasma membrane by these lipid analogues. Furthermore, APLs produce cell ultrastructural alterations with an abundant autophagic vesicles and autolysosomes in treated cells, indicating an interference of autophagy process after APL exposure. Thus, antitumoural APLs interfere with the proliferation of tumour cells via a complex mechanism involving phospholipid and cholesterol metabolism, interfere with lipid-dependent survival-signalling pathways and autophagy. Although APLs also exert antiparasitic, antibacterial, and antifungal effects, in this review we provide a summary of the antileishmanial activity of these lipid analogues. This article is part of a Special Issue entitled: Membrane Lipid Therapy: Drugs Targeting Biomembranes edited by Pablo V. Escribá.

Membrane lipid compositional sensing by the inducible amphipathic helix of CCT

The amphipathic helical (AH) membrane binding motif is recognized as a major device for lipid compositional sensing. We explore the function and mechanism of sensing by the lipid biosynthetic enzyme, CTP:phosphocholine cytidylyltransferase (CCT). As the regulatory enzyme in phosphatidylcholine (PC) synthesis, CCT contributes to membrane PC homeostasis. CCT directly binds and inserts into the surface of bilayers that are deficient in PC and therefore enriched in lipids that enhance surface charge and/or create lipid packing voids. These two membrane physical properties induce the folding of the CCT M domain into a ≥60 residue AH. Membrane binding activates catalysis by a mechanism that has been partially deciphered. We review the evidence for CCT compositional sensing, and the membrane and protein determinants for lipid selective membrane-interactions. We consider the factors that promote the binding of CCT isoforms to the membranes of the ER, nuclear envelope, or lipid droplets, but exclude CCT from other organelles and the plasma membrane. The CCT sensing mechanism is compared with several other proteins that use an AH motif for membrane compositional sensing. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

Choline metabolism in malignant transformation

Abnormal choline metabolism is emerging as a metabolic hallmark that is associated with oncogenesis and tumour progression. Following transformation, the modulation of enzymes that control anabolic and catabolic pathways causes increased levels of choline-containing precursors and breakdown products of membrane phospholipids. These increased levels are associated with proliferation, and recent studies emphasize the complex reciprocal interactions between oncogenic signalling and choline metabolism. Because choline-containing compounds are detected by non-invasive magnetic resonance spectroscopy (MRS), increased levels of these compounds provide a non-invasive biomarker of transformation, staging and response to therapy. Furthermore, enzymes of choline metabolism, such as choline kinase, present novel targets for image-guided cancer therapy.

Hemolytic phospholipase C inhibition protects lung function during Pseudomonas aeruginosa infection

RATIONALE

The opportunistic pathogen Pseudomonas aeruginosa causes both acute and chronic lung infections and is particularly problematic in patients with cystic fibrosis and those undergoing mechanical ventilation. Decreased lung function contributes significantly to morbidity and mortality during P. aeruginosa infection, and damage inflicted by P. aeruginosa virulence factors contributes to lung function decline.

OBJECTIVES

We sought to describe direct contribution of a bacterial phospholipase C/sphingomyelinase, PlcHR, to alteration of host lung physiology and characterize a potential therapeutic for protection of lung function.

METHODS

We infected C57Bl/6 mice with P. aeruginosa wild-type or isogenic plcHR deletion strains and measured lung function using computer-controlled ventilators. For in vivo testing, miltefosine was delivered intraperitoneally 1 hour after infection. Infection and respiratory endpoints were at 24 hours after infection.

MEASUREMENTS AND MAIN RESULTS

P. aeruginosa wild-type infection caused significant lung function impairment, whereas the effects of a ΔplcHR strain infection were much less severe. Surfactometry analysis of bronchoalveolar lavage fluid indicated that PlcHR decreased pulmonary surfactant function. Miltefosine has structural similarity to the PC and sphingomyelin substrates of PlcHR, and we found that it inhibits the cleavage of these choline-containing lipids in vitro. Miltefosine administration after P. aeruginosa infection limited the negative effects of PlcHR activity on lung function.

CONCLUSIONS

We have directly linked production of a single virulence factor in P. aeruginosa with effects on lung function, and demonstrated that the inhibitor miltefosine protects lung function from PlcHR-dependent surfactant dysfunction.

Alterations in the homeostasis of phospholipids and cholesterol by antitumor alkylphospholipids

The alkylphospholipid analog miltefosine (hexadecylphosphocholine) is a membrane-directed antitumoral and antileishmanial drug belonging to the alkylphosphocholines, a group of synthetic antiproliferative agents that are promising candidates in anticancer therapy. A variety of mechanisms have been suggested to explain the actions of these compounds, which can induce apoptosis and/or cell growth arrest. In this review, we focus on recent advances in our understanding of the actions of miltefosine and other alkylphospholipids on the human hepatoma HepG2 cell line, with a special emphasis on lipid metabolism. Results obtained in our laboratory indicate that miltefosine displays cytostatic activity and causes apoptosis in HepG2 cells. Likewise, treatment with miltefosine produces an interference with the biosynthesis of phosphatidylcholine via both CDP-choline and phosphatidylethanolamine methylation. With regard to sphingolipid metabolism, miltefosine hinders the formation of sphingomyelin, which promotes intracellular accumulation of ceramide. We have demonstrated for the first time that treatment with miltefosine strongly impedes the esterification of cholesterol and that this effect is accompanied by a considerable increase in the synthesis of cholesterol, which leads to higher levels of cholesterol in the cells. Indeed, miltefosine early impairs cholesterol transport from the plasma membrane to the endoplasmic reticulum, causing a deregulation of cholesterol homeostasis. Similar to miltefosine, other clinically-relevant synthetic alkylphospholipids such as edelfosine, erucylphosphocholine and perifosine show growth inhibitory effects on HepG2 cells. All the tested alkylphospholipids also inhibit the arrival of plasma-membrane cholesterol to the endoplasmic reticulum, which induces a significant cholesterogenic response in these cells, involving an increased gene expression and higher levels of several proteins related to the pathway of biosynthesis as well as the receptor-mediated uptake of cholesterol. Thus, membrane-targeted alkylphospholipids exhibit a common mechanism of action through disruption of cholesterol homeostasis. The accumulation of cholesterol within the cell and the reduction in phosphatidylcholine and sphingomyelin biosyntheses certainly alter the ratio of choline-bearing phospholipids to cholesterol, which is critical for the integrity and functionality of specific membrane microdomains such as lipid rafts. Alkylphospholipid-induced alterations in lipid homeostasis with probable disturbance of the native membrane structure could well affect signaling processes vital to cell survival and growth.

Glycosidated phospholipids: uncoupling of signalling pathways at the plasma membrane

Cell expansion and metastasis are considered hallmarks of tumour progression. Therefore, efforts have been made to develop novel anti-cancer drugs that inhibit both the proliferation and the motility of tumour cells. Synthetic alkylphospholipids, compounds with aliphatic side chains that are ether linked to a glycerol backbone, are structurally derived from platelet-activating factor and represent a new class of drugs with anti-proliferative properties in tumour cells. These compounds do not interfere with the DNA or mitotic spindle apparatus of the cell. Instead, they are incorporated into cell membranes, where they accumulate and interfere with lipid metabolism and lipid-dependent signalling pathways. Recently, it has been shown that the most commonly studied alkylphospholipids inhibit proliferation by inducing apoptosis in malignant cells while leaving normal cells unaffected. This review focuses on a novel group of synthetic alkylphospholipids, the glycosidated phospholipids, which contain carbohydrates or carbohydrate-related molecules at the sn-2 position of the glycerol backbone. Members of this subfamily also exhibit anti-proliferative capacity and modulate the cell adhesion, differentiation, and migration of tumour cells. Among this group, Ino-C2-PAF shows the highest efficacy and low cytotoxicity. Apart from its anti-proliferative effect, Ino-C2-PAF strongly reduces cell motility via its inhibitory effect on the phosphorylation of the cytosolic tyrosine kinases FAK and Src. Signalling pathways under the control of the FAK/Src complex are normally required for both migration and proliferation and play a prominent role in tumour progression. We intend to highlight the potential of glycosidated phospholipids, especially Ino-C2-PAF, as a promising new group of drugs for the treatment of hyperproliferative and migration-based skin diseases.

Alkylphospholipids inhibit capillary-like endothelial tube formation in vitro: antiangiogenic properties of a new class of antitumor agents

Synthetic alkylphospholipids (APLs), such as edelfosine, miltefosine and perifosine, constitute a new class of antineoplastic compounds with various clinical applications. Here we have evaluated the antiangiogenic properties of APLs. The sensitivity of three types of vascular endothelial cells (ECs) (bovine aortic ECs, human umbilical vein ECs and human microvascular ECs) to APL-induced apoptosis was dependent on the proliferative status of these cells and correlated with the cellular drug incorporation. Although confluent, nondividing ECs failed to undergo apoptosis, proliferating ECs showed a 3-4-fold higher uptake and significant levels of apoptosis after APL treatment. These findings raised the question of whether APLs interfere with new blood vessel formation. To test the antiangiogenic properties in vitro, we studied the effect of APLs using two different experimental models. The first one tested the ability of human microvascular ECs to invade a three-dimensional human fibrin matrix and form capillary-like tubular networks. In the second model, bovine aortic ECs were grown in a collagen gel sandwich to allow tube formation. We found that all three APLs interfered with endothelial tube formation in a dose-dependent manner, with a more than 50% reduction at 25 micromol/l. Interference with the angiogenic process represents a novel mode of action of APLs and might significantly contribute to the antitumor effect of these compounds.

Lysosomal and mitochondrial pathways in miltefosine-induced apoptosis in U937 cells

Hexadecylphosphocholine (HePC) is an anticancer agent whose effect has been shown to involve apoptosis induction but the signaling pathways leading to apoptosis remain to be elucidated. We show here that HePC induces activation of caspase-9, -3, and -8 via the intrinsic pathway, release of cytochrome c, activation and relocation of Bax to the mitochondria as well as the cleavage of Bid. Moreover, a lysosomal pathway characterized by partial lysosomal rupture, cathepsin B activation and relocation from lysosomes to the cytosol, is involved in HePC-induced apoptosis. A cathepsin B/L inhibitor partially suppresses caspase activation and apoptosis induction, indicating signaling between lysosomes and mitochondria. Conversely, the pancaspase inhibitor Q-VD-OPH inhibits lysosomal rupture, but only at early time points, suggesting that immediate lysosomal rupture involves caspases. Overexpression of Bcl-2, an anti-apoptotic protein known to prevent mitochondrial dysfunction, totally abrogates lysosomal destabilization and cell death.

Miltefosine affects lipid metabolism in Leishmania donovani promastigotes

Miltefosine (hexadecylphosphocholine [HePC]) is the first orally active antileishmanial drug. Transient HePC treatment of Leishmania donovani promastigotes at 10 microM significantly reduced the phosphatidylcholine content and enhanced the phosphatidylethanolamine (PE) content in parasite membranes, suggesting a partial inactivation of PE-N-methyltransferase. Phospholipase D activity did not seem to be affected by HePC. In addition, the enhancement of the lysophosphatidylcholine content could be ascribed to phospholipase A2 activation. Moreover, transient HePC treatment had no effect on the fatty acid alkyl chain length or the fatty acid unsaturation rate. Concerning sterols, we found a strong reduction of the C24 alkylated sterol content, and the enhancement of the cholesterol content could be the result of the HePC condensation effect with sterols. Because some of the effects observed after transient HePC treatment were different from those previously observed in HePC-resistant parasites, it could be hypothesized that continuous in vitro drug pressure induces the mechanisms of regulation in Leishmania lipid metabolism.

Effect of anti-tumor ether lipids on ordered domains in model membranes

1-O-octadecyl-2-O-methyl-sn-glycero-3-phosphocholine (OMPC, edelfosine) and 1-hexadecylphosphocholine (HePC, miltefosine) represent two groups of synthetic ether lipid analogues with anti-tumor activity. Because of their hydrophobic nature, they may become incorporated into plasma membranes of cells, and it has been argued that they may act via association with lipid rafts. With the quenching of steady-state fluorescence of probes preferentially partitioning into sterol-rich ordered domains (cholestatrienol and trans-parinaric acid), we showed that OMPC and HePC by themselves did not form sterol-rich domains in fluid model membranes, in contrast to the two chain ether lipid 1,2-O-dihexadecyl-sn-glycero-3-phosphocholine. Nevertheless, all three ether lipids significantly stabilized palmitoyl-sphingomyelin/cholesterol-rich domains against temperature induced melting. In conclusion, this study shows that anti-tumor ether lipids are likely to affect the properties of cholesterol-sphingomyelin domains (i.e., lipid rafts) when incorporated into cell membranes.

Hexadecylphosphocholine disrupts cholesterol homeostasis and induces the accumulation of free cholesterol in HepG2 tumour cells

Hexadecylphosphocholine (HePC) is a synthetic lipid belonging to the alkylphosphocholines (APC), a new group of antiproliferative agents that are proving to be promising candidates in anticancer therapy. We reported in a previous study that HePC interferes with phosphatidylcholine (PC) synthesis in HepG2 cells via both CDP-choline and phosphatidylethanolamine (PE) methylation. We have subsequently extended our studies to show that HePC interferes with sphingolipid metabolism by hindering the formation of sphingomyelin (SM), an effect accompanied by a substantial increase in the incorporation of the exogenous lipogenic precursors into ceramides. Interestingly, we demonstrate for the first time that HePC strongly inhibits the esterification of free cholesterol (FC) by acting at the level of acyl CoA:cholesterol acyltransferase (ACAT) (EC 2.3.1.26) activity. This effect is accompanied by a considerable increase in the synthesis of cholesterol, which leads to a rise in the levels of FC in cells. We are left in no doubt that the imbalance in the metabolism of membrane-lipid components vital to cell survival may well be responsible for the observed DNA fragmentation and activation of caspase-3, an enzyme involved in the cell apoptosis found in this study.

In vitro activity of hexadecylphosphocholine (miltefosine) against metronidazole-resistant and -susceptible strains of Trichomonas vaginalis

OBJECTIVES

Trichomonas vaginalis is the causative agent of trichomoniasis, a sexually transmitted disease with worldwide significance. Trichomoniasis can be treated with metronidazole; however, resistant strains of T. vaginalis have been isolated and there is a lack of useful alternative drugs. The aim of the present study was to examine the activity of hexadecylphosphocholine (HePC; miltefosine), a membrane-active alkylphospholipid, that is licensed as an antileishmanial agent against T. vaginalis.

METHODS

The efficacy of HePC after 30 min, 1 h, 16 h and 24 h against four different T. vaginalis strains (with varying resistance to metronidazole) was evaluated.

RESULTS

It was shown that all isolates, including the metronidazole-resistant strains, were susceptible to HePC, with EC50s of between 8 and 40 microM and EC90s of between 8 and 80 microM depending on time and on the medium used for the experiments. Treatment of trichomonads with HePC resulted in rounding up and, at concentrations of >or=40 microM, in subsequent total lysis of the organisms.

CONCLUSIONS

HePC may be a promising new candidate for the treatment of trichomoniasis.

Miltefosine induces apoptosis-like death in Leishmania donovani promastigotes

Miltefosine (hexadecylphosphocholine [HePC]) has proved to be a potent oral treatment for human visceral leishmaniasis due to Leishmania donovani. The molecular mechanisms that contribute to the antileishmanial activity of HePC are still unknown. We report that in wild-type promastigotes of Leishmania donovani HePC is able to induce a cell death process with numerous cytoplasmic, nuclear, and membrane features of metazoan apoptosis, including cell shrinkage, DNA fragmentation into oligonucleosome-sized fragments, and phosphatidylserine exposure. None of these changes were detected in an HePC-resistant clone treated with the same drug concentration. Therefore, HePC does not appear to kill L. donovani promastigotes by a direct toxic mechanism but, rather, kills the promastigotes by an indirect one. Pretreatment of wild-type promastigotes with two broad caspase inhibitors, z-Val-Ala-DL-Asp(methoxy)-fluoromethylketone and Boc-Asp(methoxy)-fluoromethylketone, as well as a broad protease inhibitor, calpain inhibitor I, prior to drug exposure interfered with DNA fragmentation but did not prevent cell shrinkage or phosphatidylserine externalization. These data suggest that at least part of the apoptotic machinery operating in wild-type promastigotes involves proteases. Identification of the death-signaling pathways activated in HePC-sensitive parasites appears to be essential for a better understanding of the molecular mechanisms of action and resistance in these parasites.

Effects of etherlipid analogs on cell membrane functions

Hexadecylphosphocholine and other etherlipid-derived substances show a pronounced antiproliferative activity on neoplastic cells and a broad spectrum of other biological effects on many cell types in vitro and in vivo. Though the precise molecular mechanism by which these etherlipid analogs act still remains unresolved, it seems clear that it most probably involves some essential function of the cell membrane. We investigated the effect of different etherlipids with and without cytotoxic activity in etherlipid-susceptible and -resistant tumor cell lines on three important membrane functions. We observed various inhibitory activities on endocytosis and the uptake of small precursor molecules as sugars, amino acids, and alcohols by toxic and nontoxic substances in resistant as well as susceptible cells. There was no correlation between the antiproliferative characteristics of the compounds and the effects on these membrane transport functions. Furthermore, the substances reduced the number of membrane tumor necrosis factor-alpha receptors regardless of their antiproliferative properties. The results of these investigations suggest that etherlipid analogs may interfere with many membrane functions in an unspecific manner. Therefore, many of the previously reported biological effects of etherlipids have to be viewed under a different light. Future investigation on these compounds should always contain appropriate control substances and cell models to really prove the specificity of the observed effects.

Hexadecylphosphocholine inhibits phosphatidylcholine biosynthesis and the proliferation of HepG2 cells

Hexadecylphosphocholine (HePC) is a synthetic lipid representative of a new group of antiproliferative agents, alkylphosphocholines (APC), which are promising candidates in anticancer therapy. Thus we have studied the action of HePC on the human hepatoblastoma cell line HepG2, which is frequently used as a model for studies into hepatic lipid metabolism. Non-toxic, micromolar concentrations of HePC exerted an antiproliferative effect on this hepatoma cell line. The incorporation into phosphatidylcholine (PC) of the exogenous precursor [methyl-14C]choline was substantially reduced by HePC. This effect was not due to any alteration in choline uptake by the cells, the degradation rate of PC or the release of PC into the culture medium. As anaccumulation of soluble choline derivatives points to CTP:phosphocholine cytidylyltransferase (CT) as the target of HePC activity we examined its effects on the different enzymes involved in the biosynthesis of PC via CDP-choline. Treatment with HePC altered neither the activity of choline kinase (CK) nor that of diacylglycerol cholinephosphotransferase (CPT), but it did inhibit CT activity in HepG2 cells. In vitro HePC also inhibited the activity of cytosolic but not membrane-bound CT. Taken together our results suggest that HePC interferes specifically with the biosynthesis of PC in HepG2 cells by depressing CT translocation to the membrane, which may well impair their proliferation.

Hexadecylphosphocholine does not influence phospholipase D and sphingomyelinase activity in human leukemia cells

Hexadecylphosphocholine (HePC) is the first representative of the alkylphosphocholines (APC), a new group of biologically active compounds. HePC has pronounced antiproliferative effects on neoplastic cells in vitro and in vivo. The molecular mechanism by which HePC exerts its biological effects is still under investigation. Recently there has been growing evidence that HePC probably interferes with cellular signalling via phospholipases. It has been shown to inhibit both forms of phospholipase C (PLC), the phosphatidylinositol- and the phosphatidylcholine-specific PLC, and phospholipase A2. Here we present data showing that HePC inhibits the activity of phospholipase D in vitro, whereas the action of this enzyme in leukemic cell lines is not affected. Furthermore HePC does not seem to disturbed the activity of sphingomyelinase, another enzyme of phospholipid metabolism which has been shown to play an important role in cellular signalling as well.

Effects of hexadecylphosphocholine on phosphatidylcholine and phosphatidylserine metabolism in human lymphoma cells

Hexadecylphosphocholine (HePC) belongs to a new group of antineoplastic agents, the alkylphosphocholines (APC). HePC shows a broad spectrum of biological effects in various cells in vitro and in vivo. It has pronounced antiproliferative effects on neoplastic cells. The molecular mechanism by which HePC exerts its biological effects is still under investigation. By generating a HePC-resistant cell variant of the lymphoma cell line Raji, we established a model to investigate which molecular mechanism may be responsible for the antiproliferative action of HePC. Here we present data showing that HePC substantially interferes with the metabolism of cellular phosphatidylcholine (PC) and phosphatidylserine (PS) in the human lymphoma cell line Raji. HePC leads to an inhibition of PC synthesis via CDP-choline and as a compensatory mechanism enhances the generation of PC via PS indicating that the reduced PC synthesis seems to significantly disturb cellular homeostasis. In HePC-resistant Raji cells, PC synthesis via CDP-choline is constitutively less active and is not further reduced by HePC. Resistant Raji cells do not seem to use the alternative pathway of PC synthesis via PS nor can this be induced by HePC. The resistance mechanism may therefore be independent of cell membrane metabolism.

Effects of miltefosine and other alkylphosphocholines on human intestinal parasite Entamoeba histolytica

The protozoan parasite Entamoeba histolytica is the cause of amoebic dysentery and liver abscess. It is therefore responsible for significant morbidity and mortality in a number of countries. Infections with E. histolytica are treated with nitroimidazoles, primarily with metronidazole. At this time, there is a lack of useful alternative classes of substances for the treatment of invasive amoebiasis. Alkylphosphocholines (alkyl-PCs) such as hexadecyl-PC (miltefosine) were originally developed as antitumor agents, but recently they have been successfully used for the treatment of visceral leishmaniasis in humans. We examined hexadecyl-PC and several other alkyl-PCs with longer alkyl chains, with and without double bond(s), for their activity against two strains of E. histolytica. The compounds with the highest activity were oleyl-PC, octadecyl-PC, and nonadecenyl-PC, with 50% effective concentrations for 48 h of treatment between 15 and 21 microM for strain SFL-3 and between 73 and 98 microM for strain HM-1:IMSS. We also tested liposomal formulations of these alkyl-PCs and miltefosine. The alkyl-PC liposomes showed slightly lower activity, but are expected to be well tolerated. Liposomal formulations of oleyl-PC or closely related alkyl-PCs could be promising candidates for testing as broad-spectrum antiprotozoal and antitumor agents in humans.

Growth inhibition of human mammary carcinoma by liposomal hexadecylphosphocholine: Participation of activated macrophages in the antitumor mechanism

This study was undertaken to investigate the antitumor effect of liposomal hexadecylphosphocholine (L-HPC), a synthetic phospholipid encapsulated into multilamellar vesicles (MLV). The effect of these liposomes was tested in an orthotopic nude mouse model using the human mammary carcinomas MDA-MB 435 and 231. The main interest of the investigation was to study whether activated macrophages are substantially involved in the tumor growth inhibition mechanism. The growth of both MDA-MB 435 and 231 tumors in the mammary fat pad was significantly inhibited by a 14-day intraperitoneal therapy with L-HPC. The remaining tumors were shown to be heavily infiltrated with macrophages. In vitro studies of mPEM demonstrated a significant induction of macrophage-mediated tumor cytotoxicity (MMCTX) against the 2 cell lines by L-HPC. The L-HPC-mediated activation mechanism was characterized to be IL-6 and TNFalpha dependent but rather independent of IL-1alpha and nitric oxide (NO). NMA, a specific inhibitor of NO production, did not inhibit L-HPC-induced MMCTX. Furthermore, L-HPC was shown to upregulate the matrixmetalloproteinases MMP-9 and MMP-2 secretion into the supernatant. Considering cytokine release and production of collagenases, the L-HPC-induced macrophage activation cascade is assumed to be comparable with that of classical activators such as lipopolysaccharide (LPS) and interferon (IFN) gamma. As far as NO production is considered, the L-HPC activation mechanism differs from that caused by LPS and IFN gamma.

Surface remodeling associated with vasopressin-induced membrane traffic in L6 myogenic cells

The plasma membrane is dynamically remodeled as a function of the cell cycle, motility and membrane traffic. We have previously shown that arg8-vasopressin (AVP) stimulation of L6 myoblasts induces the activation of phosholipase D during the first minutes of stimulation, and the differentiation of 1,6 myoblasts as a long term effect. We now report that AVP also induces two types of morphological responses in L6 cells within a few minutes of stimulation: exocytosis, apparent as uncoated pits, and the generation of membrane projections and reffles. Thus, such an experimental model is suitable for the study of hormone-induced morphological surface modifications and their regulatory mechanisms. In L6 cells, AVP-induced projection generation depends on the integrity of microfilaments, intermediate filaments, and microtubules. Moreover, projection generation and exocytosis appear to be independently regulated phenomena: in fact, inhibition of the de novo synthesis of phosphatidylcholine inhibits membrane traffic but fails to block projection appearance. Conversely, the latter phenomenon, unlike exocytosis, is mediated by PI3-kinase signaling. Thus, AVP induces two early, independently regulated morphological modifications in L6 cells: exocytosis, involved in plasma membrane phospholipid turnover, and membrane projections, likely involved in cell migration.

Biosynthesis of phosphatidylcholine from a phosphocholine precursor pool derived from the late endosomal/lysosomal degradation of sphingomyelin

Previous studies suggest that the steps of the CDP- choline pathway of phosphatidylcholine synthesis are tightly linked in a so-called metabolon. Evidence has been presented that only choline that enters cells through the choline transporter, and not phosphocholine administered to cells by membrane permeabilization, is incorporated into phosphatidylcholine. Here, we show that [(14)C]phosphocholine derived from the lysosomal degradation of [(14)C]choline-labeled sphingomyelin is incorporated as such into phosphatidylcholine in human and mouse fibroblasts. Low density lipoprotein receptor-mediated endocytosis was used to specifically direct [(14)C]sphingomyelin to the lysosomal degradation pathway. Free labeled choline was not found either intracellularly or in the medium, not even when the cells were energy-depleted. Deficiency of lysosomal acid phosphatases in mouse or alkaline phosphatase in human fibroblasts did not affect the incorporation of lysosomal [(14)C]sphingomyelin-derived [(14)C]phosphocholine into phosphatidylcholine, supporting our finding that phosphocholine is not degraded to choline prior to its incorporation into phosphatidylcholine. Inhibition studies and analysis of molecular species showed that exogenous [(3)H]choline and sphingomyelin-derived [(14)C]phosphocholine are incorporated into phosphatidylcholine via a common pathway of synthesis. Our findings provide evidence that, in fibroblasts, phosphocholine derived from sphingomyelin is transported out of the lysosome and subsequently incorporated into phosphatidylcholine without prior hydrolysis of phosphocholine to choline. The findings do not support the existence of a phosphatidylcholine synthesis metabolon in fibroblasts.

Modulation of phospholipase D by hexadecylphosphorylcholine: a putative novel mechanism for its antitumoral activity

Hexadecylphosphorylcholine (HePC, D-18506, INN: Mitelfosine) belongs to the family of alkylphosphocholines with anticancer activity. Previous reports have related its antitumoral activity to their ability to interfere with phospholipid metabolism. However a clear mechanism of action has not been established yet. We have investigated the effect of HePC on two enzymes recently reported to play a role in cell growth proliferation, phospholipase D (PLD) and choline kinase (ChoK). Our results demonstrate that treatment with HePC induces a rapid stimulation of PLD, that may be achieved by PKC dependent or independent mechanisms, depending on the cell line investigated. Both PLD1 and PLD2 isoenzymes are sensitive to HePC activation. By contrast, no effect was observed by HePC on ChoK, a new target for anticancer drug development. Furthermore, in all cell lines tested, a chronic exposure of the cells to HePC abrogates PLD activation by either phorbol esters or HePC itself with no effect on total cellular PLD levels. This is reflected in a strong inhibition of PLD activity. We suggest that the inhibitory effects on PLD by HePC may be related to its antitumoral action.

Alkyl-lysophospholipids as anticancer agents and enhancers of radiation-induced apoptosis

Synthetic alkyl-lysophospholipids (ALPs, also referred to as ether-phospholipids) have been studied as antitumor agents for more than a decade. Classical examples of these ALPs include 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OCH(3); Edelfosine) and hexadecylphosphocholine (HePC; Miltefosine). Unlike most currently available chemotherapeutic drugs that target the nuclear DNA, ALPs exert their action at the plasma membrane level, where they interfere with mitogenic signal transduction pathways. Whereas malignant cells are highly sensitive to the lethal action of ALPs, normal cells remain relatively unaffected, illustrating the potential selective antitumor properties of this class of drugs. Recently, ALPs have regained interest because of their capacity to induce apoptosis in various tumor cell lines. Moreover, in combination with other (conventional) anticancer regimens, ALPs seem to cause an additive and sometimes synergistic cytotoxic effect. These biologic properties make ALPs attractive drugs for further clinical evaluation. The present review discusses recent insights into the mode(s) of action of ALPs, their interaction with ionizing radiation, and clinical application.

Molecular events that regulate cell proliferation: an approach for the development of new anticancer drugs

Cancer chemotherapy is the object of many fundamental and clinical researches. The development in molecular techniques and structural studies at the molecular level have led to the discovery of key proteins involved in the regulation of cell proliferation. This opened perspectives to characterize new anticancer drugs in order to reduce the limitations found with conventional drugs such as the lack of selectivity for cancer cells and resistance phenomena. This review presents the anticancer drugs in clinical investigations that target molecules involved in the signal transduction impairment, the cell cycle deregulation and the differentiation with comments on their mechanisms of action.

Activity of the phosphatidylcholine biosynthetic pathway modulates the distribution of fatty acids into glycerolipids in proliferating cells

PtdCho accumulation is a periodic, S phase-specific event that is modulated in part by cell cycle-dependent fluctuations in CTP:phosphocholine cytidylyltransferase (CCT) activity. A supply of fatty acids is essential to generate the diacylglycerol (DG) precursors for phosphatidylcholine (PtdCho) biosynthesis but it is not known whether the DG supply is also coupled to the cell cycle. Although the rate of fatty acid synthesis in a macrophage cell line was dramatically stimulated in response to the growth factor, CSF-1, it was not regulated by the cell cycle. Increased fatty acid synthesis correlated with elevated acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS) steady-state mRNA levels. Cellular fatty acid synthesis was essential for membrane PL synthesis. Cerulenin inhibition of endogenous fatty acid synthesis also inhibited PtdCho synthesis, which was not relieved by exogenous fatty acids. Inhibition of CCT activity by the addition of lysophosphatidylcholine (lysoPtdCho) or temperature-shift of a conditionally defective CCT diverted newly synthesized DG to the TG pool where it accumulated. Enforced expression of CCT stimulated PtdCho biosynthesis and reduced TG synthesis. Thus, the cellular DG supply did not regulate PtdCho biosynthesis and CCT activity governs the partitioning of DG into either the PL or TG pools, thereby controlling both PtdCho and TG biosynthesis.

Modulation of polymorphic properties of dielaidoylphosphatidylethanolamine by the antineoplastic ether lipid 1-O-octadecyl-2-O-methyl-glycero-3-phosphocholine

The capacity of the antineoplastic ether lipid 1-O-octadecyl-2-O-methyl-glycero-3-phosphocholine (ET-18-OCH3) to modulate the polymorphic properties of dielaidoylphosphatidylethanolamine has been studied using biophysical techniques. Differential scanning calorimetry showed that ET-18-OCH3 depresses the onset of the Lbeta to Lalpha phase transition, decreasing also DeltaH of the transition. At the same time, the onset of the transition from Lalpha to inverted hexagonal HII phase was gradually increased as the ether lipid concentration was increased, totally disappearing at concentrations higher than 5 mol%. Small-angle X-ray diffraction and 31P-NMR confirmed that ET-18-OCH3 induced that the appearance of the inverted hexagonal HII phase was shifted towards higher temperatures completely disappearing at concentrations higher than 5 mol%. These results were used to elaborate a partial phase diagram and they were discussed as a function of the molecular action of ET-18-OCH3.

Cytotoxic etherphospholipid analogues

1. Alkyllyso-derivatives of physiologic cell membrane phospholipids show remarkable cytostatic and cytotoxic activity on many malignant tumor cell lines and tumors in vitro and in vivo. Three of these etherphospholipid analogues have already been tested in clinical phase II studies and one of these compounds, hexadecylphosphocholine (HePC), is now commercially available as a drug for the treatment of mammary carcinoma in Germany. 2. Etherphospholipid analogues possess a variety of interesting biological characteristics like induction of cellular maturation, inhibition of tumor cell invasion or modulation of the immune response with high potential value for tumor therapy. 3. Though there have been extensive investigations on the biochemical mode of action of these substances, the precise mechanism responsible for the majority of biological effects has not yet been identified. 4. In recent years growing evidence has been accumulated that etherphospholipid analogues substantially interfere with intracellular signal transduction.

Induction of apoptosis in two mammalian cell lines results in increased levels of fructose-1,6-bisphosphate and CDP-choline as determined by 31P MRS

Programmed cell death or apoptosis was induced in human promyelocytic leukemia (HL-60) and Chinese hamster ovary (CHO-K1) cells using several cytotoxic drugs that have different modes of action, including camptothecin, ceramide, chelerythrine, etoposide, farnesol, geranyl geraniol, and hexadecylphosphocholine. The consequent changes in cellular metabolism were monitored using 31P MRS measurements on intact cells and cell extracts. Cells undergoing programmed cell death exhibited characteristic changes in the levels of glycolytic and phospholipid metabolites. The most significant changes were increases in the concentration of the glycolytic intermediate, fructose-1,6-bisphosphate and in the concentration of CDP-choline, which is an intermediate in phosphatidylcholine biosynthesis. In HL-60 cells, the increase in fructose-1,6-bisphosphate levels could be explained by depletion of cellular NAD(H) levels. All of the agents used to induce apoptosis caused the accumulation of CDP-choline. Since the resonances of this compound occur in a relatively well resolved region of tissue spectra, it could provide a marker for apoptosis that would allow the noninvasive detection of the process in vivo using 31P MRS measurements.

Induction of ceramide-mediated apoptosis by the anticancer phospholipid analog, hexadecylphosphocholine

The prototype of a new class of antiproliferative phospholipid analogs, hexadecylphosphocholine (HePC), has been shown to inhibit tumor growth and is currently used for the treatment of cutaneous metastases of mammary carcinomas. Although several cellular targets of HePC, e.g. protein kinase C and CTP:phosphocholine cytidylyltransferase, have been proposed, the mechanisms of HePC-induced anticancer activity are still unclear. Considering that the antiproliferative effect of HePC correlates with inhibition of phosphatidylcholine biosynthesis, which is tightly coupled to sphingomyelin biosynthesis, we tested the hypothesis that treatment of cells with the anticancer drug leads to increased cellular ceramide and subsequently to apoptotic cell death. In the present study, we showed that 25 micromol/liter HePC induced apoptosis. In further experiments, we demonstrated that HePC inhibited the incorporation of radiolabeled choline into phosphatidylcholine and at a later time point into sphingomyelin. This was confirmed by metabolic labeling of the lipid backbone using radiolabeled serine, and it was shown that HePC decreased the incorporation of serine into sphingomyelin by 35% and simultaneously increased the incorporation of serine into ceramide by 70%. Determination of the amount of ceramide revealed an increase of 53% in HePC-treated cells compared with controls. In accordance with the hypothesis that elevated ceramide levels may be the missing link between the metabolic effects of HePC and its proapoptotic properties, HePC-induced apoptosis was blocked by fumonisin B1, an inhibitor of ceramide synthesis. Furthermore, we found that membrane-permeable ceramides additively increased the apoptotic effect of HePC.

Apoptosis triggered by 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine is prevented by increased expression of CTP:phosphocholine cytidylyltransferase

A HeLa cell line was constructed for the regulation of CTP:phosphocholine cytidylyltransferase (CCT) expression via a tetracycline-responsive promoter to test the role of CCT in apoptosis triggered by exposure of cells to the antineoplastic phospholipid 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3). Basal CCT expression in the engineered HeLa cell line was the same as in control HeLa cells lines, and CCT activity and protein were elevated 25-fold following 48 h of induction with doxycycline. Increased CCT expression prevented ET-18-OCH3-induced apoptosis. Acylation of exogenous lysophosphatidylcholine circumvented the requirement for CCT activity by providing an alternate route to phosphatidylcholine, and heightened CCT expression and lysophosphatidylcholine supplementation were equally effective in reversing the cytotoxic effect of ET-18-OCH3. Neither CCT overexpression nor lysophosphatidylcholine supplementation allowed the HeLa cells to proliferate in the presence of ET-18-OCH3, indicating that the cytostatic property of ET-18-OCH3 was independent of its effect on membrane phospholipid synthesis. These data provide compelling genetic evidence to support the conclusion that the interruption of phosphatidylcholine synthesis at the CCT step by ET-18-OCH3 is the primary physiological imbalance that accounts for the cytotoxic action of the drug.

The antiproliferative effect of hexadecylphosphocholine toward HL60 cells is prevented by exogenous lysophosphatidylcholine

The mechanisms that account for the anti-proliferative properties of the biologically active lysophospholipid analog hexadecylphosphocholine (HexPC) were investigated in HL60 cells. HexPC inhibited the incorporation of choline into phosphatidylcholine and the pattern of accumulation of soluble choline-derived metabolites pinpointed CTP:phosphocholine cytidylyltransferase (CT) as the inhibited step in vivo. HexPC also inhibited recombinant CT in vitro. HexPC treatment led to accumulation of cells in G2/M phase, triggered DNA fragmentation and caused morphological changes associated with apoptosis. The supplementation of HexPC-treated cells with exogenous lysophosphatidylcholine (LPC) completely reversed the cytotoxic effects of HexPC and restored HL60 cell proliferation in the presence of the drug. LPC provided an alternate pathway for phosphatidylcholine synthesis via the acylation of exogenous LPC. This result contrasted with the response of HL60 cells to 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3) where LPC overcame the cytotoxic effects but did not support continued cell proliferation. Morphological integrity, DNA stability and cell viability were maintained in cells treated with LPC plus either antineoplastic agent. Thus the inhibition of phosphatidylcholine biosynthesis at the CT step accounts for the cytotoxicity of both HexPC and ET-18-OCH3 which is overridden by providing an alternate pathway for phosphatidylcholine synthesis via the acylation of exogenous LPC.

Expression of two 50 kDa proteins is associated with sensitivity towards etherlipid analogues in the human leukaemia cell line HL 60

Hexadecylphosphocholine (HePC) is a new etherphospholipid derived substance with pronounced antineoplastic activity. So far the mode of action of this compound has not been resolved. Therefore, we decided to approach this problem by generating HePC resistant sublines of susceptible cells. The human leukaemia cell line HL 60 was successfully adapted to high concentrations of HePC over a period of 14 months. The resistant cell line HL 60 R shows similar functional characteristics as the original HL 60. Both lines can be induced to terminal differentiation into a granulocytic phenotype by DMSO. In this process, normal HL 60 cells also become resistant towards HePC. Determinations of cellular membrane lipid composition did not show significant changes, which would explain the resistance mechanism. Analysis of cellular proteins by 2D-gelelectrophoresis revealed two 50 kDa proteins expressed in HL 60 and differentiated HL 60 cells, which were not expressed in HL 60 R. Reversion of resistance of HL 60 R after prolonged cultivation without HePC led to re-expression of the two proteins, indicating at a possible involvement of these proteins in HePC sensitivity.

Comparison of hexadecylphosphocholine with fish oil as an antitumor agent

Hexadecylphosphocholine (HePC) reduced the growth of the human mammary tumor, MX-1, in the athymic nude mouse similar to the fish oil, MaxEPA. When used together, HePC and MaxEPA were additive towards reducing tumor growth. An unsaturated alkylphosphocholine mixture, ShisoPC, was not as effective as HePC in reducing tumor growth. MaxEPA reduced tumor PGE2 levels greater than 90%, while HePC and the ShisoPC only reduced tumor PGE2 40-60% with HePC being slightly better than ShisoPC. MaxEPA markedly increased the cellular omega 3 fatty acids and decreased 20:4 omega 6, the substrate for PGE2. HePC did not alter the tumor fatty acid composition, but it significantly lowered the total fatty acid concentration of the tumor by about 47%. In addition, phosphatidylcholine and sphingomyelin decreased in tumors from animals treated with HePC, and alterations in other phospholipids also were noted. These data suggest that different mechanisms exist for HePC and fish oil in reducing tumor growth.

Bcl-2 antagonizes apoptotic cell death induced by two new ceramide analogues

Ceramides which arise in part from the breakdown of sphingomyelin comprise a class of antiproliferative lipids and have been implicated in the regulation of programmed cell death better known as apoptosis. In the present study, two new synthetic ceramide analogues, N-thioacetylsphingosine and FS-5, were used in Molt 4 cells to induce cell death. Besides their cytotoxic effects at concentrations > or = 14 microM the data obtained clearly show that both analogues induced apoptosis at concentrations below this critical concentration as assessed by trypan blue exclusion and cleavage of the death substrate poly-(ADP-ribose) polymerase (PARP). Additional experiments in bcl-2-transfected Molt 4 cells revealed that the apoptotic but not the lytic effects of the analogues were antagonized by the apoptosis inhibitor Bcl-2. Furthermore, neither N-thio-acetylsphingosine nor FS-5 induced PARP cleavage in bcl-2-transfected Molt 4 cells indicating that the induction of apoptotic cell death by cell permeable ceramides is not due to unspecific disturbance of the cell membrane.

Synergistic cytotoxic effects of ether phospholipid analogues and ionizing radiation in human carcinoma cells

BACKGROUND AND PURPOSE

There is growing evidence in recent years that the antiproliferative effects of ionizing radiation may not be exclusively mediated via DNA damage but also by interactions and alterations of cell membrane associated processes. Here, we tested the hypothesis that membrane active cytotoxic ether lipids and analogues may interact with ionizing radiation, enhancing its antiproliferative effects.

MATERIALS AND METHODS

The two epithelial tumor cell lines HTB 43 and KB, and the ether lipid resistant subline KBr were treated by a combination of radiation and ether lipids. Cytotoxic effects were measured by colony forming assays and the effects on membrane phospholipids were determined by quantitative thin-layer chromatography of cell lipid extracts.

RESULTS

We present evidence that some ether lipids show supra-additive cytotoxic effects with ionizing radiation. These effects seem to depend on the same structural properties of ether lipids that determine their intrinsic cytostatic and cytotoxic activity. Identical growth inhibitory results were achieved when cells were treated before, or 30 min after irradiation. Analysis of major membrane phospholipids revealed no statistically significant differences of phospholipid distribution pattern in cells exposed to both treatment modalities.

CONCLUSION

Our data indicate that changes of overall membrane phospholipid composition do not seem to be the mechanism of synergistic antiproliferative activity of ether lipids and ionizing radiation.

Differential regulation of phospholipase A2 in human leukemia cells by the etherphospholipid analogue hexadecylphosphocholine

Hexadecylphosphocholine (HePC) is the main representative of a new group of antineoplastic agents, the alkylphosphocholines, which were originally derived from cytotoxic etherlysophospholipids. HePC shows antiproliferative action against a whole variety of tumor cells and tumors in vitro and in vivo. Furthermore, it also induces differentiation in some hematologic cell lines and prevents invasive growth of neoplastic cells in vitro. To date, the precise molecular mechanisms mediating the biological effects of HePC have not been identified yet. As etherlysophospholipids seem to inhibit some pathways of lipid-dependent intracellular signalling, similar effects may be relevant for HePC. We therefore investigated the influence of HePC on phospholipase A2 (PLA2-EC 3.1.1) in the human leukemia cell line U 937. HePC seems to inhibit enzyme activity independently of protein kinase C (PKC) in differentiated U 937 cells stimulated by tumor necrosis factor alpha (TNFalpha). Inhibition of purified secretory PLA2 from snake venom (EC 3.1.1.4) in vitro shows characteristics of a non-competitive mode. In contrast, HePC leads to an enhancement of PLA2 activity in immature cells which cannot be explained by changes in membrane composition. Our data suggest that PLA, inhibition is most probably not the mechanism by which HePC mediates its antiproliferative effects.

[Spectrographic studies on the radioresistance of Miltex and miltefosine]

AIM

With simultaneous application of Miltex and radiation therapy in the combined treatment of topical relapses and skin metastases in breast carcinoma patients the question arises, how radioresistant is the new cytotoxic agent. Because of the long penetration times of the active agent miltefosine the answer is important with particular regard to the time of the external application of Miltex.

MATERIAL AND METHOD

After the application of a single dose of 10 Gy we studied the stability of the commercial preparation and its active agent miltefosine by means of absorption spectroscopy.

RESULTS

Immediately following the irradiation no alterations in absorption spectra of Miltex and miltefosine were found. However, 2 and 8 h post radiation the absorption curves of Miltex and miltefosine solutions were distinctly changed. The radiation induced changes of Miltex dilutions were smaller than those of the miltefosine solutions. For the commercial preparation the amount of the radiation-induced destruction is 0.10.

CONCLUSIONS

Consequently Miltex has shown a sufficient radioresistance or its decrease in the effectiveness is small. With daily single doses of 2 Gy in the radiotherapy of the topical relapses and skin metastases the destruction degree should be reduced to 0.02 assuming linear changes. Because of the distinct changes in the spectra and relative slow penetration of miltefosine in various cell lines [10, 11, 14] we will propose an application of the commercial cytotoxic agent 5 h before the radiation fractions. The smaller effect on Miltex is discussed in relation to the solution mediators of the active agent.