The inhibition of cell signaling pathways by antitumor ether lipids.
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Animals↗
Biomedical subjects
R Bittman
Publications and source records attributed to R Bittman.
Evidence for involvement of mitogen-activated protein kinase, rather than stress-activated protein kinase, in potentiation of 1-beta-D-arabinofuranosylcytosine-induced apoptosis by interruption of protein kinase C signaling.
The stress-activated protein kinase (SAPK) and mitogen-activated protein kinase (MAPK) cascades mediate cytotoxic and cytoprotective functions, respectively, in the regulation of leukemic cell survival. Involvement of these signaling systems in the cytotoxicity of 1-beta-D-arabinofuranosylcytosine (ara-C) and modulation of ara-C lethality by protein kinase C PKC inhibition/down-regulation was examined in HL-60 promyelocytic leukemia cells. Exposure to ara-C (10 microM) for 6 hr promoted extensive apoptotic DNA damage and cell death, as well as activation of PKC. This response was accompanied by downstream activation of the SAPK and MAPK cascades. PKC-dependent MAPK activity seemed to limit ara-C action in that the toxicity of ara-C was enhanced by pharmacological reductions of PKC, MAPK, or both. Thus, ara-C action was (1) partially attenuated by diradylglycerols, which stimulated PKC and MAPK, but (2) dramatically amplified by sphingoid bases, which inhibited PKC and MAPK. The cytotoxicity of ara-C also was substantially increased by pharmacological reductions of PKC, including down-regulation of PKC by chronic preexposure to the macrocyclic lactone bryostatin 1 or inhibition of PKC by acute coexposure to the dihydrosphingosine analog safingol. Significantly, both of these manipulations prevented activation of MAPK by ara-C. Moreover, acute disruption of the MAPK module by AMF, a selective inhibitor of MEK1, suppressed both basal and drug-stimulated MAPK activity and sharply increased the cytotoxicity of ara-C, suggesting the direct involvement of MAPK as a downstream antiapoptotic effector for PKC. None of these chemopotentiating agents enhanced ara-CTP formation. Ceramide-driven SAPK activity did not seem to mediate drug-induced apoptosis, given that (1) neutralization of endogenous tumor necrosis factor-alpha with monoclonal antibodies or soluble tumor necrosis factor receptor substantially reduced ceramide generation and SAPK activation by ara-C, whereas the induction of apoptosis was unaffected; (2) pharmacological inhibition of sphingomyelinase by 3-O-methoxysphingomyelin reduced ceramide generation and SAPK activation without limiting the drug's cytotoxicity; and (3) potentiation of ara-C action by bryostatin 1 or safingol was not associated with further stimulation of SAPK. These observations collectively suggest a primary role for decreased MAPK, rather than increased SAPK, in the potentiation of ara-C cytotoxicity by interference with PKC-dependent signaling.
Antimetabolites, Antineoplastic↗
NADH dehydrogenase defects confer isoniazid resistance and conditional lethality in Mycobacterium smegmatis.
Isoniazid (INH) is a highly effective drug used in the treatment and prophylaxis of Mycobacterium tuberculosis infections. Resistance to INH in clinical isolates has been correlated with mutations in the inhA, katG, and ahpC genes. In this report, we describe a new mechanism for INH resistance in Mycobacterium smegmatis. Mutations that reduce NADH dehydrogenase activity (Ndh; type II) cause multiple phenotypes, including (i) coresistance to INH and a related drug, ethionamide; (ii) thermosensitive lethality; and (iii) auxotrophy. These phenotypes are corrected by expression of one of two enzymes: NADH dehydrogenase and the NADH-dependent malate dehydrogenase of the M. tuberculosis complex. The genetic data presented here indicate that defects in NADH oxidation cause all of the mutant traits and that an increase in the NADH/NAD+ ratio confers INH resistance.
Amino Acid Sequence↗
Sterol side chain length and structure affect the clearance of chylomicron-like lipid emulsions in rats and mice.
In previous work we found that sterols such as cholesterol were essential for physiological plasma clearance of lipid emulsions mimicking the structure of mammalian triglyceride-rich lipoproteins. In the present study we compared the clearances of emulsions prepared with sterols of varying alkyl chain length (straight chains, n-C3 to n-C7, or branched chains, i-C5 to i-C10) at the C-17 position. Our studies show that the length of the alkyl chain at the C-17 position of sterols markedly affects the removal of remnant particles from the plasma of rats traced by emulsion cholesteryl oleate label. An alkyl chain of 7 carbons or more was needed for normal remnant clearance. Straight and branched chains of similar length were cleared similarly, showing that the presence of a branch at the end of the alkyl chain had no effect on remnant clearance. For side chains of 7 carbons or less, substitution of sterols with an unsaturation in the alkyl chain close to the terminal carbon markedly decreased the clearance of remnants. Triolein label was used to estimate lipolysis of the injected emulsions. Lipolysis was little affected by the structure of the sterol side chain, except that lipolysis was markedly higher with emulsions containing sterols with an alkyl chain having 4 carbon atoms (n-C4) or with an unsaturation in the 4 carbon alkyl chain. We conclude that the length of the alkyl side chain is an important element in the essentiality of cholesterol as a regulator of metabolism of lipid emulsion models of triglyceride-rich lipoproteins.
Animals↗
Enantioselective synthesis and antiproliferative properties of an ilmofosine analog, 2'-(trimethylammonio)ethyl 3-(hexadecyloxy)-2-(methoxymethyl)propyl phosphate, on epithelial cancer cell growth.
An asymmetric synthesis of the 1-alkyloxy analog of the thioether phosphocholine ilmofosine (BM 41.440, rac-1), 2'-(trimethylammonio)ethyl 3-(hexadecyloxy)-2-(methoxymethyl)propyl phosphate (2), is described. Stereoselectivity was obtained in an asymmetric hydroboration-oxidation sequence carried out on a 2,2-disubstituted 1-alkene, 3-(hexadecyloxy)-2-(methoxymethyl)-1-propene (9), which was prepared by starting with either ethyl acrylate or ethyl alpha-(hydroxymethyl)acrylate (3). (R)- and (S)-2 and rac-1 were highly effective in inhibiting the proliferation of the breast adenocarcinoma cell line MCF-7 (IC50, 2 microM), moderately effective against A549 (non-small-cell lung adenocarcinoma) (IC50, 8-10 icroM), and less effective against A427 (large cell lung carcinoma) (IC50, approximately 20 microM). The in vitro cytotoxicity against the three epithelial cancer cell lines was independent of the configuration about C-2 of the glycerol backbone of 2 and was also not altered by substitution of oxygen for sulfur in the sn-1 ether linkage of ilmofosine.
Antineoplastic Agents↗
Has nature designed the cholesterol side chain for optimal interaction with phospholipids?
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Calorimetry, Differential Scanning↗
A convenient synthesis of D-myo-inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) and L-myo-inositol 1,4,5-trisphosphate (Ins(3,5,6)P3).
An efficient synthesis of an optically active inositol derivative that is a precursor to D-myo-inositol 1,4,5-trisphosphate (Ins(1,4,5)P3, (-)) is described. Crystallization of the diastereomers of (+/-)-1-O-[(+)-menthoxycarbonyl]-6-O-benzyl-2,3:4,5-di-O-isopropyl idene-myo- inositol diastereomers from methanol gives only one diastereomer. Alkaline hydrolysis gives the useful inositol derivative (-)-6-O-benzyl-2,3:4,5-di-O-isopropylidene-myo-inositol. Likewise, crystallization of the diastereomers of (+/-)-3-O-[(-)-menthoxycarbonyl]-4-O-benzyl-1,2:5,6-di-O-isopropyl idene-myo- inositol from methanol gave a pure compound which could be hydrolyzed to give (+)-4-O-benzyl-1,2:5,6-di-O-isopropylidene-myo-inositol, a precursor to D-myo-inositol 3,5,6-trisphosphate (Ins(3,5,6)P3,(+)). The ease with which these enantiomerically pure inositol derivatives were isolated may facilitate the synthesis of more complex inositol phosphate derivatives such as D-myo-inositol 1,3,4,5-tetrakisphosphate.
Inositol 1,4,5-Trisphosphate↗
Inhibition of phospholipase C-delta 1 catalytic activity by sphingomyelin.
We measured the ability of sphingomyelin (SPM) to inhibit phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] hydrolysis catalyzed by human phospholipase C-delta 1 (PLC-delta 1) in model membranes and detergent phospholipid mixed micelles. SPM strongly inhibited PLC-delta 1 catalytic activity measured in large unilamellar vesicles (LUVs) composed of egg phosphatidylcholine (PC), PI(4,5)P2, and SPM from brain or egg. At 37 or 45 degrees C, the rate of PI(4,5)P2 hydrolysis in PC/SPM/PI(4,5)P2 vesicles (15:80:5 mol:mol) was less than 25% of that observed in PC/PI(4,5)P2 vesicles (95:5). By contrast, catalysis was only weakly inhibited by equivalent concentrations of the SPM analog, 3-deoxy-2-O-stearoyl-SPM, which lacks hydrogen bond-donating groups at the C-3 and C-2 positions of the sphingolipid backbone. Inhibition by SPM was not observed in detergent/phospholipid mixed micelles. The binding affinity of PLC-delta 1 for vesicles containing PC and PI(4,5)P2 was slightly diminished by inclusion of SPM in the lipid mixture, but not enough to account for the decreased rate of catalysis. We could find no evidence of specific binding of the enzyme to SPM, which argues against a simple negative allosteric mechanism. To understand the cause of inhibition, the effects of SPM and 3-deoxy-2-O-stearoyl-SPM on the bulk properties of the substrate bilayers were examined. Increasing the mole fraction of SPM altered the fluorescence emission spectra of two sets of head group probes, 6-lauronyl(N,N-dimethylamino)naphthalene and N-[5-(dimethylamino)naphthalene-1-sulfonyl]-1,2-dihexadecanoyl-sn- glycero-3-phosphoethanolamine, that are sensitive to water content at the membrane/solution interface. Results obtained with both probes suggested a reduction in hydration with increasing SPM content. Vesicles containing 3-deoxy-2-O-stearoyl-SPM produced intermediate changes. Our results are most consistent with a model in which SPM inhibits PLC by increasing interlipid hydrogen bonding and by decreasing membrane hydration; both factors raise the energy barrier for activation of PLC-delta 1 at the membrane/protein microinterface.
Animals↗
Synthesis and growth inhibitory properties of glycosides of 1-O-hexadecyl-2-O-methyl-sn-glycerol, analogs of the antitumor ether lipid ET-18-OCH3 (edelfosine).
Glycosylated antitumor ether lipids (GAELs), analogs of 1-O-octadecyl-2-O-methyl-sn-glycero-3-phosphocholine (1, ET-18-OCH3, edelfosine), were synthesized in good overall yields by glycosylation of 1-O-alkyl-2-O-methyl-sn-glycerol and tested for in vitro antineoplastic activity against a variety of murine and human tumor cell lines. Stereospecific glycosylation was achieved by the use of 2-O-acetyl-3,4,6-tri-O-benzylglucopyranosyl and -mannopyranosyl trichloroacetimidates as donors, with trimethylsilyl trifluoromethanesulfonate as catalyst in the presence of molecular sieves at -78 degrees C. The GAELs differ from 1 in having the sn-3-phosphocholine residue replaced by one of the following monosaccharide residues: beta- and alpha-2-deoxy-D-arabino-hexopyranosyl, alpha-D-mannopyranosyl, 2-O-methyl-beta-D-glucopyranosyl, and 2-O-methyl-alpha-D-mannopyranosyl. 1-O-Hexadecyl-2-O-methyl-3-O-(2'-deoxy-beta-D-arabino-hexopyranosyl)- sn-glycerol (2) was more effective than 1 in inhibiting the growth of MCF-7 (human breast cancer) and its adriamycin-resistant form MCF-7/adriamycin, and murine Lewis lung cancer cells. 2-Deoxy-beta-D-arabino-hexopyranoside 2 was also an effective growth inhibitor of two drug-resistant leukemic cell lines, P388/Adr and L1210/vmdr.
Animals↗
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.
Calcium-Calmodulin-Dependent Protein Kinases↗
Induction of apoptosis and potentiation of ceramide-mediated cytotoxicity by sphingoid bases in human myeloid leukemia cells.
Prior studies demonstrated that ceramide promotes apoptotic cell death in the human myeloid leukemia cell lines HL-60 and U937 (Jarvis, W. D., Kolesnick, R. N., Fornari, F. A., Jr., Traylor, R. S., Gewirtz, D. A., and Grant, S. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 73-77), and that this lethal process is potently suppressed by diglyceride (Jarvis, W. D., Fornari, F. A., Jr., Browning, J. L., Gewirtz, D. A., Kolesnick, R. N., and Grant, S. (1994) J. Biol. Chem. 269, 31685-31692). The present findings document the intrinsic ability of sphingoid bases to induce apoptosis in HL-60 and U937 cells. Exposure to either sphingosine or sphinganine (0. 001 10 microM) for 6 h promoted apoptotic degradation of genomic DNA as indicated by (a) electrophoretic resolution of 50-kilobase pair DNA loop fragments and 0.2-1.2-kilobase pair DNA fragment ladders on agarose gels, and (b) spectrofluorophotometric determination of the formation and release of double-stranded fragments and corresponding loss of integrity of bulk DNA. DNA damage correlated directly with reduced cloning efficiency and was associated with the appearance of apoptotic cytoarchitectural traits. At sublethal concentrations (</=750 nM), however, sphingoid bases synergistically augmented the apoptotic capacity of ceramide (10 microM), producing both a leftward shift in the ceramide concentration-response profile and a pronounced increase in the response to maximally effective levels of ceramide. Thus, sphingosine and sphinganine increased both the potency and efficacy of ceramide. The apoptotic capacity of bacterial sphingomyelinase (50 milliunits/ml) was similarly enhanced by either (a) acute co-exposure to highly selective pharmacological inhibitors of protein kinase C such as calphostin C and chelerythrine or (b) chronic pre-exposure to the non-tumor-promoting protein kinase C activator bryostatin 1, which completely down-modulated total assayable protein kinase C activity. These findings demonstrate that inhibition of protein kinase C by physiological or pharmacological agents potentiates the lethal actions of ceramide in human leukemia cells, providing further support for the emerging concept of a cytoprotective function of the protein kinase C isoenzyme family in the regulation of leukemic cell survival.
Apoptosis↗
Synthesis and evaluation of the antiproliferative effects of 1-O-hexadecyl-2-O-methyl-3-O-(2'-acetamido-2'-deoxy-beta-D- glucopyranosyl)-sn-glycerol and 1-O-hexadecyl-2-O-methyl-3-0- (2'-amino-2'-deoxy-beta-D-glucopyranosyl)-sn-glycerol on epithelial cancer cell growth.
Two ether glucosyl diglyceride analogs were synthesized, and their antiproliferative activity against four epithelial cancer cell lines was evaluated. 1-O-Hexadecyl-2-O-methyl-3-O-(2'-acetamido-2'-deoxy-beta-D- glucopyranosyl)-sn-glycerol (4) was synthesized by reaction of 2-acetamido-2-deoxy-3,4,6-tri-O-acetyl-alpha-D-glucopyranosyl chloride with 1-O-hexadecyl-2-O-methyl-sn-glycerol followed by deacetylation by methanolic hydrolysis. The N-acetyl group of 4 was removed by hydrolysis with ethanolic potassium hydroxide to form 1-O-hexadecyl-2-O-methyl-3-O-(2'-amino-2'-deoxy-beta-D-glucopyranosyl)- sn-glycerol (5). Compounds 4 and 5 inhibited the proliferation of MCF-7, A549, A427, and T84 cancer cell lines. The IC(50) values for 5 ranged from 6.5 to 12.2 microM, whereas 4 was more effective against A549 cells (IC(50) 9 microM) than against MCF-7 (IC(50) 17 microM) and A427 (IC(50) 25 microM) cells and was inactive against T84 cells. Under identical incubation conditions, compounds 4 and 5 were potent inhibitors of the proliferation of OVCAR-3 cells with IC(50) values of 12 and 4 microM, respectively, whereas ET-18-OCH(3), hexadecylphosphocholine, and erucylphosphocholine had IC(50) values of 24, >30, and >30 microM, respectively. The cell-inhibitory profile of these ether-linked glucosyl diglycerides strengthens the hypothesis that such glycolipids represent a distinct group of antitumor ether lipids, having antineoplastic activities that differ from the well-known alkylphosphocholines and alkyllysophospholipids.
Antineoplastic Agents↗
The effect of side-chain analogues of cholesterol on the thermotropic phase behavior of 1-stearoyl-2-oleoylphosphatidylcholine bilayers: a differential scanning calorimetric study.
In this study we have examined the effects of analogues of cholesterol differing with respect to alkyl side-chain length and structure on the thermotropic phase behavior of bilayers formed from 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC), an important subclass of naturally occurring phosphatidylcholines (PCs). The synthetic sterols we studied contained either a terminally unbranched (n-series) or a single methyl-branched (iso-series) side chain of 3 to 10 carbon atoms. The phase transition behavior was examined by high-sensitivity differential scanning calorimetry (DSC). The main phase transition endotherm of SOPC/sterol bilayers consists of superimposed sharp and broad components, which represent the hydrocarbon chain melting of sterol-poor and sterol-rich phospholipid domains, respectively. The transition temperature and the cooperativity of the sharp component are moderately reduced upon sterol incorporation and the enthalpy decreases to zero when sterol levels of 20-30 mol% are reached. The enthalpy of the broad component transition initially increases to a maximum around 25 or 25-30 mol% sterol and thereafter decreases with further increases in sterol concentration. However, the broad transition of SOPC bilayers containing both short (C-22, i-C5 and n-C3) and long (i-C9 and i-C10) side-chain sterols still persists at levels of 50 mol% sterol. Thus the effective stoichiometry of SOPC-sterol interactions varies with changes in sterol alkyl side-chain length. The incorporation of short linear or branched side-chain sterols (C-22, n-C3, n-C4, i-C5) causes the broad component transition temperature and cooperativity to decrease dramatically, whereas the incorporation of medium- and long-chain sterols in both the n- and iso-series has less effect on the transition temperature and cooperativity of the broad component. Overall, no significant differences were found between the n- and iso-series sterols for a given side-chain length. A comparison of the phase behavior of dipalmitoylphosphatidylcholine (DPPC)/sterol (McMullen et al. (1995) Biophys. J. 69, 169-176) and SOPC/sterol mixtures indicates that the primary factor responsible for changes in the thermotropic phase behavior of these systems is the extent of the hydrophobic mismatch between the sterol and the host lipid bilayer. However, sterol miscibility in PC bilayers, and thus the stoichiometry of lipid-sterol interactions, also appears to depend on the degree of unsaturation of the host lipid bilayer.
Calorimetry, Differential Scanning↗
Stereospecific induction of apoptosis in U937 cells by N-octanoyl-sphingosine stereoisomers and N-octyl-sphingosine. The ceramide amide group is not required for apoptosis.
We investigated the ability of N-octanoyl-sphingosine (C8-Cer) stereoisomers, N-octanoyl-DL-erythro-dihydrosphingosine (DL-e-DHC8-Cer), and a new ceramide derivative, N-octyl-D-erythro-sphingosine (D-e-C8-Ceramine), to induce apoptosis in U937 cells. We found the C8-Cer stereoisomers to be stereospecific with the D- and L-threo stereoisomers being severalfold more potent than the erythro in inducing nucleosomal fragmentation. The order of potency was: D-t-C8-Cer = L-t-C8-Cer > L-e-C8-Cer > D-e-C8-Cer > DL-e-DHC8-Cer. The importance of the carbonyl group in apoptosis was investigated by using a new ceramide derivative, D-e-C8-Ceramine, in which the carbonyl group was replaced by a methylene group. The carbonyl group was not necessary for triggering apoptosis. In fact, replacement of the carbonyl group decreased substantially the time required for cells to die, with maximum DNA fragmentation occurring at 6 h as opposed to the 18 h required by D-e-C8-Cer. To explore possible mechanisms by which these compounds trigger the apoptotic pathway, we tested their ability to increase the endogenous levels of cellular ceramide and to differentially activate a ceramide-activated protein kinase (CAPK). While the potent DNA fragmentation-inducing compounds D-e-C8-Ceramine and L-t-C8-Cer failed to increase the cellular ceramide levels, D-e-C8-Cer, D-t-C8-Cer and D-e-C8-Ceramine activated the CAPK equally. These studies suggest that the DNA fragmentation-inducing ability of the threo stereoisomers and D-e-C8-Ceramine cannot be attributed either to an increase in the activity of CAPK, or, as illustrated by D-e-C8-Ceramine and L-t-C8-Cer, to the differential elevation of endogenous ceramide. The phosphatase inhibitor okadaic acid failed to protect U937 cells from apoptosis induced by D-e-C8-Cer.
Apoptosis↗
BCL-2 suppresses ceramide-induced cell killing.
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Letter↗
Structural requirements of sphingosylphosphocholine and sphingosine-1-phosphate for stimulation of activator protein-1 activity.
The sphingolipids sphingosine-1-phosphate (SPP) and sphingosylphosphocholine (SPC) stimulate mitogenesis in Swiss 3T3 fibroblasts and stimulate DNA binding activity of activator protein-1 (AP-1). We show that SPP and SPC were more potent agents than nonphosphorylated sphingosines and N-acyl-sphingolipids (ceramides) with respect to DNA synthesis, AP-1 DNA binding activity, and AP-1 trans-activation, illustrating the importance of the terminal phosphate group. The free 2-amino group and the 4E double bond of SPC and SPP were found to be important for these activities. Although the combination of decreasing the sphingoid backbone chain length of SPC by two carbons and hydrogenating the 4E bond only slightly reduced its effects, in contrast, the same modifications in SPP significantly decreased its mitogenic and AP-1 trans-activation effects. Furthermore, substitution of the 3-hydroxyl group in SPP with hydrogen decreased its ability to stimulate DNA synthesis and to stimulate AP-1 transcriptional activity. Thus, critical sphingolipid structural components for AP-1 activation and mitogenic stimulation include the free 2-amino group, the free 3-hydroxyl group, the 4,5-trans double bond, and terminal phosphorylation. These observations may be relevant for clinical uses of these compounds in applications such as wound healing and inhibition of metastasis.
3T3 Cells↗
N-palmitoyl-serine and N-palmitoyl-tyrosine phosphoric acids are selective competitive antagonists of the lysophosphatidic acid receptors.
Lysophosphatidic acid is the best characterized member of a lipid mediator family with growth factor-like activities that act through a class of G protein-coupled plasma membrane receptors. In Xenopus laevis oocytes, lysophosphatidate activates at least two pharmacologically distinct receptor subtypes distinguished by 1-acyl-sn-glycero-2,3-cyclic phosphate. Both of these naturally occurring ligands elicit oscillatory Cl- currents in the oocyte through G protein-coupled activation of the phosphoinositide/Ca2+ second messenger system, which in turn leads to the opening of Ca(2+)-activated Cl- channels. We developed an improved chemical synthesis and purification procedure for two N-acylated amino acid phosphates. N-Palmitoyl-serine and N-palmitoyl-tyrosine phosphoric acids inhibited the lysophosphatidate-activated Cl- currents with IC50 values of 5.4 +/- 0.7 and 6.5 +/- 1.5 nM at the high affinity site and 805 +/- 97 and 172 +/- 36 nM at the low affinity receptor site, respectively. In selective activation of the cyclic lysophosphatidate receptor, IC50 values of 330 +/- 30 and 490 +/- 40 nM were obtained, respectively. The D- and L-stereoisomers were equally effective when applied extracellularly. In contrast, they were ineffective when microinjected into the oocyte, indicating an extracellular site of inhibition. The inhibitors did not alter currents elicited by the different acetylcholine, serotonin, and glutamate receptors expressed heterologously in the oocyte. Pharmacological analysis of the results indicates that N-palmitoyl-serine and N-palmitoyl-tyrosine phosphoric acids are potent and specific competitive inhibitors of the lysophosphatidate receptors in the X. laevis oocyte.
Acetylcholine↗
Differential scanning calorimetry study of the influence of phospholipid analogs with a carbonyl-terminated sn-2 chain on the interdigitated phases formed by 1-stearoyl-2-capryl-sn-glycero-3-phosphatidylcholine (C18:C10-PC).
Synthetic glycerophosphocholines with highly asymmetric chain lengths form interdigitated bilayers in the gel phase. In nature, phospholipids with one hydrocarbon chain approximately twice as long as the other can arise from the autooxidation of unsaturated linkages in the acyl chain, and thus the oxidation products would contain a carbonyl group at the chain terminus. In this study, we have investigated the thermotropic behavior of bilayers prepared from mixtures of the well-studied, mixed-chain phospholipid, 1-stearoyl-2-capryl-sn-glycero-3-phosphocholine (C18:C10-PC, 1), with synthetic 1-stearoyl-2-acyl-sn-glycero-3-phosphocholines in which the sn-2 chain is approximately one-half the length of the sn-1 chain and contains a C==O group near the omega terminus. Phase diagrams of binary mixtures of 1 with a chain-terminal ketone-PC analog (2) or with a chain-terminal ester-PC analog (3) in excess water exhibited gel-phase immiscibility over a wide compositional range, but miscibility in the liquid-crystalline phase. However, 1 was completely miscible with C18:C10:1 delta 10-PC (compound 4), which bears a chain-terminal carbon-carbon double bond, in both the gel and liquid-crystalline phases. The calorimetric data suggest that phosphatidylcholines (PC) with carbonyl-terminated chains, which can be produced by autooxidation of naturally abundant 1-saturated-2-unsaturated phospholipids such as 1-stearoyl-2-oleoyl-PC, may not form the normal triple-chain mixed interdigitated structure characteristic of hydrocarbon-terminated PCs in gel-phase bilayers.
Calorimetry, Differential Scanning↗