Inhibition of calcium-dependent protein kinase C by hexadecylphosphocholine and 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine do not correlate with inhibition of proliferation of HL60 and K562 cell lines

Alkylphospholipids are Signal Transduction Modulators with Potential for Anticancer Therapy

BACKGROUND

Alkylphospholipids (APLs) are synthetically derived from cell membrane components, which they target and thus modify cellular signalling and cause diverse effects. This study reviews the mechanism of action of anticancer, antiprotozoal, antibacterial and antiviral activities of ALPs, as well as their clinical use.

METHODS

A literature search was used as the basis of this review.

RESULTS

ALPs target lipid rafts and alter phospholipase D and C signalling cascades, which in turn will modulate the PI3K/Akt/mTOR and RAS/RAF/MEK/ERK pathways. By feedback coupling, the SAPK/JNK signalling chain is also affected. These changes lead to a G2/M phase cell cycle arrest and subsequently induce programmed cell death. The available knowledge on inhibition of AKT phosphorylation, mTOR phosphorylation and Raf down-regulation renders ALPs as attractive candidates for modern medical treatment, which is based on individualized diagnosis and therapy. Corresponding to their unusual profile of activities, their side effects result from cholinomimetic activity mainly and focus on the gastrointestinal tract. These aspects together with their bone marrow sparing features render APCs well suited for modern combination therapy. Although the clinical success has been limited in cancer diseases so far, the use of miltefosine against leishmaniosis is leading the way to better understanding their optimized use.

CONCLUSION

Recent synthetic programs generate congeners with the increased therapeutic ratio, liposomal formulations, as well as diapeutic (or theranostic) derivatives with optimized properties. It is anticipated that these innovative modifications will pave the way for the further successful development of ALPs.

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.

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.

Modulation of protein kinase C in antitumor treatment

PKC isoenzymes were found to be involved in proliferation, antitumor drug resistance and apoptosis. Therefore, it has been tried to exploit PKC as a target for antitumor treatment. PKC alpha activity was found to be elevated, for example, in breast cancers and malignant gliomas, whereas it seems to be underexpressed in many colon cancers. So it can be expected that inhibition of PKC activity will not show similar antitumor activity in all tumors. In some tumors it seems to be essential to inhibit PKC to reduce growth. However, for inhibition of tumor proliferation it may be an advantage to induce apoptosis. In this case an activation of PKC delta should be achieved. The situation is complicated by the facts that bryostatin leads to the activation of PKC and later to a downmodulation and that the PKC inhibitors available to date are not specific for one PKC isoenzyme. For these reasons, PKC modulation led to many contradicting results. Despite these problems, PKC modulators such as miltefosine, bryostatin, safingol, CGP41251 and UCN-01 are used in the clinic or are in clinical evaluation. The question is whether PKC is the major or the only target of these compounds, because they also interfere with other targets. PKC may also be involved in apoptosis. Oncogenes and growth factors can induce cell proliferation and cell survival, however, they can also induce apoptosis, depending on the cell type or conditions in which the cells or grown. PKC participates in these signalling pathways and cross-talks. Induction of apoptosis is also dependent on many additional factors, such as p53, bcl-2, mdm2, etc. Therefore, there are also many contradicting results on PKC modulation of apoptosis. Similar controversial data have been reported about MDR1-mediated multidrug resistance. At present it seems that PKC inhibition alone without direct interaction with PGP will not lead to successful reversal of PGP-mediated drug efflux. One possibility to improve chemotherapy would be to combine established antitumor drugs with modulators of PKC. However, here also very contrasting results were obtained. Many indicate that inhibition, others, that activation of PKC enhances the antiproliferative activity of anticancer drugs. The problem is that the exact functions of the different PKC isoenzymes are not clear at present. So further investigations into the role of PKC isoenzymes in the complex and interacting signalling pathways are essential. It is a major challenge in the future to reveal whether modulation of PKC can be used for the improvement of cancer therapy.

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.

Differential effects of free and liposome-associated 1-O-octadecyl-2-O-methylglycerophosphocholine on protein kinase C

Incorporation of ET-18-OCH3 into well-characterized liposomes known as ELL-12 has eliminated its gastrointestinal and hemolytic toxicity without loss of growth inhibiting activity. ET-18-OCH3, but not ELL-12, blunted the increase in membrane protein kinase C (PKC) activity induced by 12-O-tetradecanoylphorbol 13-myristate (TPA) and markedly reduced levels of PKC alpha in NIH 3T3 fibroblasts. Furthermore, prolonged treatment with ELL-12 neither inhibited TPA-induced translocations of PKC alpha and PKC delta to the particulate fraction nor caused down-regulation, and did not affect the cellular distribution of TPA-insensitive PKC zeta. In Jurkat T cells, where ELL-12 markedly induced apoptosis that was blocked by an inhibitor of caspase-3-like activities, it had no effect on PKC activity or translocation induced by TPA. Thus, it seems unlikely that PKC is involved in the therapeutic effects of ELL-12.

The ether lipid ET-18-OCH3 increases cytosolic Ca2+ concentrations in Madin Darby canine kidney cells

The effect of the ether lipid 1-O-octadecyl-2-O-methyl-sn-glycero-3-phosphorylcholine (ET-18-OCH3) on the intracellular free Ca2+ concentration ([Ca2+]i) in Madin Darby canine kidney (MDCK) cells was studied using fura-2 as the Ca2+ probe. In Ca2+ medium, ET-18-OCH3 induced a significant rise in [Ca2+]i at concentrations between 10-100 microM with a concentration-dependent delay of 45-175 s. The [Ca2+]i signal was composed of a gradual rise and a sustained plateau. In Ca2+-free medium, ET-18-OCH3 (10-100 microM) induced a Ca2+ release from internal Ca2+ stores with a concentration-dependent delay of 45-175 s. This discharge of internal Ca2+ triggered capacitative Ca2+ entry in a concentration-dependent manner. This capacitative Ca2+ entry was not inhibited by econazole (25 microM), 1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole hydrochloride (SKF96365; 50 microM), nifedipine (10 microM), verapamil (10 microM), diltiazem (10 microM) and cadmium (0.5 microM). Methyl 2-(phenylthio)ethyl-1,4-dihydro-2,4,6-trimethylpyridine-3,5-dicarboxylat e (PCA-4248), a platelet-activating factor (PAF) receptor antagonist, inhibited 25 microM ET-18-OCH3-induced [Ca2+]i rise in a concentration-dependent manner between 1-20 microM, with 20 microM exerting a complete block. The [Ca2+]i rise induced by ET-18-OCH3 (25 microM) was not altered when the production of inositol 1,4,5-trisphosphate (IP3) was suppressed by the phospholipase C inhibitor U73122 (2 microM), but was partly inhibited by the phospholipase D inhibitor propranolol (0.1 mM) or the phospholipase A2 inhibitor aristolochic acid (20-40 microM). In Ca2+-free medium, pretreatment with 25 microM ET-18-OCH3 completely depleted the endoplasmic reticulum Ca2+ pump inhibitor thapsigargin-sensitive Ca2+ store. In contrast, pretreatment with thapsigargin abolished 0.1 mM ATP-induced [Ca2+]i rise without altering the ET-18-OCH3-induced [Ca2+]i rise. This suggests that ET-18-OCH3 depleted thapsigargin-sensitive Ca2+ stores and also released Ca2+ from thapsigargin-insensitive stores. The thapsigargin-insensitive stores involve mitochondria because the mitochondria uncoupler carbonylcyanide m-chlorophenylhydrazone (CCCP; 2 microM) induced a release of mitochondrial Ca2+ which was abolished by pretreatment with 25 microM ET-18-OCH3. ET-18-OCH3 (25 microM) induced a significant Mn2+ quench of fura-2 fluorescence at 360 nm excitation wavelength confirming that ET-18-OCH3 induced capacitative Ca2+ entry. La3+ (0.1 mM) or Gd3+ (50 microM) abolished the ET-18-OCH3-induced Mn2+ quench and [Ca2+]i rise. Our data imply that ET-18-OCH3 induced a [Ca2+]i rise in MDCK cells by activating PAF receptors leading to an internal Ca2+ release followed by capacitative Ca2+ entry. Phospholipase D and phospholipase A2, but not phospholipase C, might be involved in mediating the capacitative Ca2+ entry. La3+ abolished the ET-18-OCH3-induced [Ca2+]i rise presumably by inhibiting PAF receptors.

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 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.

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.

Interleukin-1-induced ether-linked diglycerides inhibit calcium-insensitive protein kinase C isotypes. Implications for growth senescence

It is hypothesized that inflammatory cytokines and vasoactive peptides stimulate distinct species of diglycerides that differentially regulate protein kinase C isotypes. In published data, we demonstrated that interleukin-1, in contrast to endothelin, selectively generates ether-linked diglyceride species (alkyl, acyl- and alkenyl, acylglycerols) in rat mesangial cells, a smooth muscle-like pericyte in the glomerulus. We now demonstrate both in intact cell and in cell-free preparations that these interleukin-1 receptor-generated ether-linked diglycerides inhibit immunoprecipitated protein kinase C delta and epsilon but not zeta activity. Neither interleukin-1 nor endothelin affect de novo protein expression of these protein kinase C isotypes. As down-regulation of calcium-insensitive protein kinase C isotypes has been linked to antimitogenic activity, we investigated growth arrest as a functional correlate for IL-1-generated ether-linked diglycerides. Cell-permeable ether-linked diglycerides mimic the effects of interleukin-1 to induce a growth-arrested state in both G-protein-linked receptor- and tyrosine kinase receptor-stimulated mesangial cells. This signaling mechanism implicates cytokine receptor-induced ether-linked diglycerides as second messengers that inhibit the bioactivity of calcium-insensitive protein kinase C isotypes resulting in growth arrest.

1-O-Octadecyl-2-O-methylglycerophosphocholine inhibits protein kinase C-dependent phosphorylation of endogenous proteins in MCF-7 cells

Studies with leukaemic cells, based primarily on in vitro assays, have suggested that antitumour ether lipids have only a moderate effect on protein kinase C (PKC) activity, and, furthermore, inhibition of PKC is unlikely to be involved in the mechanism of inhibition of cell proliferation by these compounds. To determine if this is also the case for epithelial cancer cells, we examined the effect of 1-O-octadecyl-2-O-methylglycerophosphocholine (ET18-OCH3) on PKC-induced phosphorylation of endogenous proteins in MCF-7 cells under incubation conditions where the drug inhibited cell proliferation. As expected, stimulation of quiescent 32P-labelled MCF-7 cells with 1 microM PMA resulted in the phosphorylation of a number of proteins. The PMA-induced phosphorylation of the proteins was abolished by preincubation of the cells with Ro 31-8220 (5 microM) for 20 min, or 10 microg/ml ET18-OCH3 for 3 h before stimulation with PMA. Thus under incubation conditions where ET18-OCH3 inhibited the proliferation of MCF-7 cells, the ether lipid potently inhibited the activity of PKC in intact cells. This inhibition was unlikely to be due to the effect of the compound on PKC translocation since there was little effect of ET18-OCH3 on the translocation of the alpha, gamma and epsilon species of PKC. These results suggest that a role for the inhibition of PKC activity by ET18-OCH3 in the mechanism of inhibition of cell proliferation by ET18-OCH3 cannot yet be discounted in epithelial cancer cells. In addition, we also observed that ET18-OCH3 enhanced the phosphorylation of selected proteins under basal unstimulated conditions. Although some of these proteins were also observed to be phosphorylated in response to PMA stimulation, the phosphorylation induced by ET18-OCH3 was not inhibited by Ro 31-8220, indicating that this was not mediated by PKC.

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.

1-O-octadecyl-2-O-methyl-glycerophosphocholine inhibits the transduction of growth signals via the MAPK cascade in cultured MCF-7 cells

1-O-Octadecyl-2-O-methyl-glycerophosphocholine (ET18-OCH3) is an ether lipid with selective antiproliferative properties whose mechanism of action is still unresolved. We hypothesized that since ET18-OCH3 affects a wide variety of cells, its mechanism of action was likely to involve the inhibition of a common widely used pathway for transducing growth signals such as the mitogen-activated protein kinase (MAPK) cascade. To test this, we established conditions whereby quiescent MCF-7 cells took up ET18-OCH3 in sufficient quantities that inhibited cell proliferation subsequent to the addition of growth medium and examined the activation of components of the MAPK cascade under these conditions. ET18-OCH3 inhibited the sustained phosphorylation of MAPK resulting in a decrease in the magnitude and duration of activation of MAPK in cells stimulated with serum or EGF. ET18-OCH3 had no effect on the binding of EGF to its receptors, their activation, or p21ras activation. However, an interference in the association of Raf-1 with membranes and a resultant decrease in Raf-1 kinase activity in membranes of ET18-OCH3-treated cells was observed. ET18-OCH3 had no direct effect on MAPK or Raf-1 kinase activity. A direct correlation between ET18-OCH3 accumulation, inhibition of cell proliferation, Raf association with the membrane, and MAPK activation was also established. These results suggest that inhibition of the MAPK cascade by ET18-OCH3 as a result of its effect on Raf-1 activation may be an important mechanism by which ET18-OCH3 inhibits cell proliferation.

Alkyl and alkoxyethyl antineoplastic phospholipids

Two series of phosphodiester ether lipid analogs with (N-methylmorpholino)ethyl or (N-methylpiperidino)ethyl polar head groups and long aliphatic or alkoxyethyl chains in the nonpolar portion of the molecule were synthesized as potential antineoplastic agents. The cytotoxic activity of these compounds (9-19) was evaluated in vitro against a panel of six human tumor xenografts and in two biochemical, mechanism-based screens (cdc2 kinase and cdc25 phosphatase). Analogs 13, 14, 17, and 19 showed activity in the in vitro tests. Specifically, 14 and 17 were more active than the reference compound hexadecylphosphocholine (Miltefosine, He-PC) while 13 and 19 possessed activity similar to that of the control. Of the analogs tested the one with the highest potency and least toxicity (17) has an N-methylpiperidino head group and a C16 alkyl chain. In the mechanism-based tests 11 showed weak inhibitory activity in the cdc25 phosphatase screen.

Ether lipid-induced cell damage of neuroblastoma cells is only weakly correlated with increased intracellular Ca2+ levels

The cell-damaging action of the ether lipid ET-18-OCH3 was studied in single NH15-CA2 neuroblastoma x glioma hybrid cells using light microscopy, and correlated with changes of the free intracellular calcium concentration, [Ca2+]i, as measured with the fluorescent Ca2+ indicator Fura-2. Addition of 3-100 microM ET-18-OCH3 to the cultures caused disintegration of neurites, cell rounding and detachment of the cells from the bottom of the culture dish. The effects occurred within 30-240 min, the faster, the higher the ET-18-OCH3 concentration. Also [Ca2+]i increased in a concentration-dependent manner, however, within seconds, and stayed high during the recording time of 20 min. The presence of 50 microM lanthanum or gadolinium ions prevented the ET-18-OCH3-induced increases of [Ca2+]i, but had no effect on neurite destruction and cell rounding. Preincubation with 1 mM diisopropylfluorphosphate or 100 microM leupeptin, both membrane-permeant inhibitors of intracellular proteases, did not prevent the effects. Nor was neurite destruction prohibited in the presence of 10 microM of the actin-stabilizing agent phalloidin or 2 microM taxol, a microtubule-stabilizer. We conclude that [Ca2+]i, although being increased during ET-18-OCH3-induced cell damage, is not the key factor of cell destruction.

Induction of resistance to hexadecylphosphocholine in the highly sensitive human epidermoid tumour cell line KB

Hexadecylphosphocholine (HePC, Miltefosine) is a representative of the group of alkyl-lysophosphocholines showing remarkable antitumoral activity in in vitro experiments and in experimental animal tumour models. The epidermoid tumour cell line KB, which is highly sensitive to HePC (half-maximal growth inhibiting concentration, IC50: 1.2 microM; half lethal concentration, LC50: 2.8 microM), was slowly adapted to increasing concentrations of HePC. After 14 months, the adaptation process was stopped at a concentration of 10 micrograms/ml (23.5 microM). At this point, the KB cells tolerated high doses of HePC (IC50: 41.2 microM; LC50: 87.1 microM). The resistant cells (KBr) also showed crossresistance to the other well studied ether-lysophospholipids, Edelfosine (1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine, OMG-3PC; ET18OCH3) and Ilmofosine (1 S-hexadecyl-2-methoxymethyl-rac-(1-thio-3-hydroxy)propyl-3-phosphocho lin e, BM 41.440). Comparison of the KB and KBr cells showed that total lipid phosphate, ether-lipid content, vinyl-ether-lipid content, protein content as well as cholesterol content were unchanged. Furthermore, no changes were observed in the lipid composition between KB and KBr cells. Uptake of choline was also unchanged in both cells, but the uptake of D-myo-inositol was lower by a factor of two in the KBr cells. However, in KB cells, the addition of HePC induced a 50% reduction of D-myo-inositol-uptake, whereas in KBr cells inositol uptake was unchanged. Differences in HePC uptake and HePC metabolism were apparent between the KB and KBr cell lines. KBr cells showed a 3-fold lower uptake for HePC and a 3- to 4-fold faster metabolism of HePC than KB cells. However, the amount of non-metabolised HePC after 2 days of incubation with 1 microgram/ml HePC (LC50: 1.2 microgram/ml) in KB cells was 3- to 4-fold lower than the amount of HePC in KBR cells at 10 micrograms/ml (LC50: 37 micrograms/ml), indicating that KBr cells can incorporate higher amounts of HePC than KB cells without adverse effects for cell growth and viability. This seems to indicate that mechanisms other than slower uptake and faster metabolism are involved in the induction of resistance to HePC in KBr cells.

Analogs of alkyllysophospholipids: chemistry, effects on the molecular level and their consequences for normal and malignant cells

In the search for new approaches to cancer therapy, the first alkyllysophospholipid (ALP) analogs were designed and studied about two decades ago, either as potential immunomodulators or as antimetabolites of phospholipid metabolism. In the meantime, it has been demonstrated that they really act in this way. However, their special importance is based on the fact that, in addition, they interfere with key events of signal transduction, such as hormone (or cytokine)-receptor binding or processing, protein kinase C or phospholipase C function and phosphatidylinositol and calcium metabolism. There are no strict structural requirements for their activity. Differences in the cellular uptake or the state of cellular differentiation seem to be mainly responsible for higher or lower sensitivities of cells towards ALP analogs. Consequences of the molecular effects mentioned on the cellular level are cytostasis, induction of differentiation (while in contrast the effects of known inducers of differentiation such as 12-O-tetradecanoylphorbol-13-acetate are inhibited, probably as a consequence of protein kinase C inhibition) and loss of invasive properties. Already in sublytic concentrations, alterations in the membrane structure were observed, and lysis may begin at concentrations not much higher than those causing the other effects described. Few ALP analogs have already entered clinical studies or are in clinical use. ALP analogs are the only antineoplastic agents that do not act directly on the formation and function of the cellular replication machinery. Therefore, their effects are independent of the proliferative state of the target cells. Because of their interference with cellular regulatory events, including those failing in cancer cells, ALP analogs, beyond their clinical importance, are interesting model compounds for the development of new, more selective drugs for cancer therapy.