Actin dynamics in Amoeba proteus motility

We studied the distribution of the endogenous Arp2/3 complex in Amoeba proteus and visualised the ratio of filamentous (F-actin) to total actin in living cells. The presented results show that in the highly motile Amoeba proteus, Arp2/3 complex-dependent actin polymerisation is involved in the formation of the branching network of the contractile layer, adhesive structures, and perinuclear cytoskeleton. The aggregation of the Arp2/3 complex in the cortical network, with the exception of the uroid and advancing fronts, and the spatial orientation of microfilaments at the leading edge suggest that actin polymerisation in this area is not sufficient to provide the driving force for membrane displacement. The examined proteins were enriched in the pinocytotic pseudopodia and the perinuclear cytoskeleton in pinocytotic amoebae. In migrating amoebae, the course of changes in F-actin concentration corresponded with the distribution of tension in the cell cortex. The maximum level of F-actin in migrating amoebae was observed in the middle-posterior region and in the front of retracting pseudopodia. Arp2/3 complex-dependent actin polymerisation did not seem to influence F-actin concentration. The strongly condensed state of the microfilament system could be attributed to strong isometric contraction of the cortical layer accompanied by its retraction from distal cell regions. Isotonic contraction was limited to the uroid.

Lipids and signal transduction in the nucleus

During the last few years a growing amount of data has accumulated showing phospholipid participation in nuclear signal transduction. Very recent data strongly support the hypothesis that signal transduction in the nucleus is autonomic. Local production of inositol polyphosphates, beginning with the activation of phospholipase C is required for their specific function in the nucleus. Enzymes which modify polyphosphoinositols may control gene expression. Much less information is available about the role of other lipids in nuclear signal transduction. The aim of this minireview is to stress what is currently known about nuclear lipids with respect to nuclear signal transduction.

Lipids and signal transduction in the nucleus

During the last few years a growing amount of data has accumulated showing phospholipid participation in nuclear signal transduction. Very recent data strongly support the hypothesis that signal transduction in the nucleus is autonomic. Local production of inositol polyphosphates, beginning with the activation of phospholipase C is required for their specific function in the nucleus. Enzymes which modify polyphosphoinositols may control gene expression. Much less information is available about the role of other lipids in nuclear signal transduction. The aim of this minireview is to stress what is currently known about nuclear lipids with respect to nuclear signal transduction.

Serine base-exchange in rat liver nuclei

It has been shown that the incorporation of [(14)C]serine into phosphatidylserine (PS) in isolated rat liver nuclei is intrinsic to this organelle as attested by marker enzyme activity. Serine incorporation into PS was the highest in nuclei depleted of the outer membrane of the nuclear envelope (nucleoplasts) and negligible in the outer membrane. Trypsin treatment of nucleoplasts caused a strong inactivation of PS synthesis and only a moderate one of the NAD pyrophosphorylase activity, the marker enzyme of the inner nuclear membrane. We suggest that the serine base-exchange enzyme is located in the inner membrane of the nuclear envelope and accessible from the periplasmic surface of this membrane.

Different regulation of phospholipase D activity in glioma C6 cells by sphingosine, propranolol, imipramine and phorbol ester

In has been found that sphingosine, propranolol, imipramine and phorbol ester (12-O-tetradecanoylphorbol-13-acetate, TPA) have a stimulatory effect on phospholipase D activity in glioma C6 cells. The cells were prelabelled with [1-(14)C]palmitic acid and phospholipase D-mediated synthesis of [(14)C]phosphatidylethanol was measured. The enhancing effect of TPA was almost completely blocked by a specific protein kinase C inhibitor, GF 109203X. In contrast, GF 109203X failed to inhibit the sphingosine, imipramine and propranolol stimulatory effects, indicating that their stimulation was independent of protein kinase C. The effect of TPA on phospholipase D was also blocked by imipramine and propranolol, whereas sphingosine additively potentiated TPA-mediated phospholipase D activity, both at shorter and longer (2-60 min) times of incubation. These results suggest that in glioma C6 cells, sphingosine is not only involved in a different phospholipase D activation than the TPA regulatory system, but also that it operates in a different compartment of the cell.

Effect of ethanol on ATP-induced phospholipases C and D and serine base exchange in glioma C6 cells

The effect of extracellular ATP, a nucleotide receptor agonist in the central nervous system, was investigated in glioma C6 cells on the intracellular Ca2+ level and the formation of phosphatidylethanol and phosphatidic acid in the presence and absence of ethanol (150 mM). In the cells prelabeled with [14C]palmitic acid, 100 microM ATP induced both the hydrolysis and the transphosphatidylation reactions leading to the formation of [14C]phosphatidic acid; addition of ethanol generated [14C]phosphatidylethanol. However, ATP-mediated increase in the level of [14C]phosphatidic acid was not inhibited by ethanol. Furthermore, ethanol augmented ATP-induced transient and sustained increase in the intracellular Ca2+ concentration, whereas ethanol alone did not produce any change in the intracellular Ca2+ level. These results indicate that in glioma C6 cells, ATP induces activation of polyphosphoinositide-specific phospholipase C and phospholipase D and that ethanol enhances this effect. In the present investigation we have also shown that long-term (2 days) ethanol treatment, at concentration relevant to chronic alcoholism (100 mM), decreased the incorporation of [14C]serine into phosphatidylserine. Since the effect of ethanol on ATP-induced activities of phospholipase C and phospholipase D and on serine base-exchange in glioma C6 cells differs significantly from that in cultured neuronal cells, these results may contribute to a better understanding of the mechanisms of ethanol action in cells of glial origin.

Sphingosine, sphingosylphosphorylcholine and sphingosine 1-phosphate modulate phosphatidylserine homeostasis in glioma C6 cells

The effect of sphingosine, sphingosylphosphorylcholine and sphingosine 1-phosphate on L-[U-14C]serine incorporation into phosphatidylserine and phosphatidylserine-derived phosphatidylethanolamine was investigated in intact glioma C6 cells. Sphingosine, sphingosylphosphorylcholine and sphingosine 1-phosphate are potent signalling molecules which, due to their physicochemical features, may function as amphiphilic compounds. It has been found that sphingosine and sphingosylphosphorylcholine (amphiphilic cations) significantly increase [14C]phosphatidylserine synthesis and decrease the amount of 14C-labeled phosphatidylethanolamine. Sphingosine 1-phosphate (an amphiphilic anion) was without effect on phosphatidylserine synthesis but, similarly as sphingosine and sphingosylphosphorylcholine, reduced the conversion of phosphatidylserine to phosphatidylethanolamine. These results strongly suggest that sphingosine, sphingosylphosphorylcholine and sphingosine 1-phosphate can modulate cellular phospholipid homeostasis by stimulation of phosphatidylserine synthesis and an interference with phosphatidylserine decarboxylase.

Exogenous sphingosine 1-phosphate and sphingosylphosphorylcholine do not stimulate phospholipase D in C6 glioma cells

In the present study we investigate the effect of exogenous sphingosine, sphingosine 1-phosphate and sphingosylphosphorylcholine on phospholipase D (PLD) activity in glioma C6 cells. The cells were prelabeled with [1-14C]palmitic acid and PLD-mediated synthesis of [14C]phosphatidylethanol was measured. Sphingosine 1-phosphate and sphingosylphosphorylcholine did not stimulate [14C]phosphatidylethanol formation either at low (0.1-10 microM) or high (25-100 microM) concentrations. On the other hand, sphingosine at concentrations of 100-250 microM strongly stimulated PLD activity as compared to the effect of phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA), known as a PLD activator. The effect of TPA on PLD is linked to the activation of protein kinase C. The present study also shows that sphingosine additively enhances TPA-mediated PLD activity. This is in contrast to the postulated role of sphingosine as a protein kinase C inhibitor. These results demonstrate that in glioma C6 cells sphingosine not only affects PLD independently of its effect on protein kinase C, but also is unable to block TPA-mediated PLD activity.

Sphingosine and phorbol ester modulate protein kinase C activity and modify ATP-evoked calcium mobilization in glioma C6 cells

The effect of sphingosine and 12-O-tetradecanoyl-phorbol-13-acetate (TPA) on ATP-evoked Ca(2+) mobilization in glioma C6 cells was studied with the Fura-2 video-imaging technique. Treatment of the cells with TPA, an activator of protein kinase C, reduced the ATP-evoked release of Ca(2+) from the intracellular stores, whereas sphingosine, known from in vitro studies as a protein kinase C inhibitor, potentiated Ca(2+) release synergistically with ATP. ATP-induced Ca(2+) mobilization was also enhanced by a specific protein kinase C inhibitor, GF 109203X. Pretreatment of the cells with GF 109203X prevented TPA action, whereas TPA diminished the stimulatory effect of sphingosine. However, this sphingosine effect was only observed after a short (1 min) treatment, whereas a longer treatment (5 min) reduced ATP-evoked Ca(2+) release. It is therefore concluded that sphingosine has two apparent actions: it inhibits protein kinase C providing a positive feedback regulation of receptor signals and it releases Ca(2+) from intracellular stores by an unknown mechanism, possibly independent of protein kinase C.

[Cross-talk between signal transduction pathways in the cell-- the role of G proteins in these processes]

Family of heterotrimeric G proteins plays an essential role in transducing signals across the plasma membrane in animal cells. Members of G protein family have been grouped into four subtypes: Gs, Gi, Gq and G12. A cross-talk between Gi- and Gq-; Gs- and Gi-; and Gs- and Gi-coupled receptors are presented. Interactions between G protein coupled receptors and receptor tyrosine kinases and between G12 protein and the small molecular weight guanosine trisphosphate protein Rho are also described.

Sphingosine, sphingosylphosphorylcholine and sphingosine 1-phosphate modulate phosphatidylserine homeostasis in glioma C6 cells

The effect of sphingosine, sphingosylphosphorylcholine and sphingosine 1-phosphate on L-[U-14C]serine incorporation into phosphatidylserine and phosphatidylserine-derived phosphatidylethanolamine was investigated in intact glioma C6 cells. Sphingosine, sphingosylphosphorylcholine and sphingosine 1-phosphate are potent signalling molecules which, due to their physicochemical features, may function as amphiphilic compounds. It has been found that sphingosine and sphingosylphosphorylcholine (amphiphilic cations) significantly increase [14C]phosphatidylserine synthesis and decrease the amount of 14C-labeled phosphatidylethanolamine. Sphingosine 1-phosphate (an amphiphilic anion) was without effect on phosphatidylserine synthesis but, similarly as sphingosine and sphingosylphosphorylcholine, reduced the conversion of phosphatidylserine to phosphatidylethanolamine. These results strongly suggest that sphingosine, sphingosylphosphorylcholine and sphingosine 1-phosphate can modulate cellular phospholipid homeostasis by stimulation of phosphatidylserine synthesis and an interference with phosphatidylserine decarboxylase.

Exogenous sphingosine 1-phosphate and sphingosylphosphorylcholine do not stimulate phospholipase D in C6 glioma cells

In the present study we investigate the effect of exogenous sphingosine, sphingosine 1-phosphate and sphingosylphosphorylcholine on phospholipase D (PLD) activity in glioma C6 cells. The cells were prelabeled with [1-14C]palmitic acid and PLD-mediated synthesis of [14C]phosphatidylethanol was measured. Sphingosine 1-phosphate and sphingosylphosphorylcholine did not stimulate [14C]phosphatidylethanol formation either at low (0.1-10 microM) or high (25-100 microM) concentrations. On the other hand, sphingosine at concentrations of 100-250 microM strongly stimulated PLD activity as compared to the effect of phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA), known as a PLD activator. The effect of TPA on PLD is linked to the activation of protein kinase C. The present study also shows that sphingosine additively enhances TPA-mediated PLD activity. This is in contrast to the postulated role of sphingosine as a protein kinase C inhibitor. These results demonstrate that in glioma C6 cells sphingosine not only affects PLD independently of its effect on protein kinase C, but also is unable to block TPA-mediated PLD activity.

Capacitative calcium entry. Glioma C6 as a model of nonexcitable cells

Although the mechanism of the capacitative Ca2+ entry is still a mysterious process, it has been presently accepted that it occurs through plasma membrane channel pores rather than through a carrier mechanism. As it has been proposed by Putney (Cell Calcium, 1986, 7, 1-12), Ca2+ entry is directly dependent on the state of filling of the endoplasmic reticulum Ca2+ stores, i.e. it is activated by the depletion of the endoplasmic reticulum Ca2+ pool. However, the nature of the signal for activation of Ca2+ entry is still unknown. The biphasic capacitative Ca2+ entry involves inositol phosphate system and is ubiquitous in all nonexcitable cells. We have shown that glioma C6 cells belong to such type of cells and are characterized by a typical capacitative Ca2+ entry pathway. The characteristics of this Ca2+ influx is summarized and the hypotheses about its mechanism of activation are discussed.

Serine base exchange enzyme activity is modulated by sphingosine and other amphiphilic compounds: possible role of positive charge in increasing the synthesis of phosphatidylserine

It has been found that sphingosine and sphingosylphosphorylcholine (amphiphilic cations) have a stimulatory, and cholesterol 3-sulfate (an amphiphilic anion), an inhibitory, effect on [14C]serine incorporation into phosphatidylserine in glioma C6 and rat liver microsomes. In glioma intact cells sphingosine stimulates phosphatidylserine synthesis in a process independent of protein kinase C, but suppressed by thapsigargin. We suggest that the stimulation of the enzyme occurs by the interaction of amphiphilic cations with the membrane cosubstrate phospholipids, leading to a charge redistribution on their phosphate groups, and hence facilitating the enzyme action. A new hypothesis concerning the mechanism of the serine base exchange reaction is discussed.

Intracellular Ca2+ signals induced by ATP and thapsigargin in glioma C6 cells. Calcium pools sensitive to inositol 1,4,5-trisphosphate and thapsigargin

In glioma C6 cells, extracellular ATP generates inositol 1,4,5-trisphosphate (InsP3), indicating the presence of purinergic receptors coupled to phosphoinositide turnover. To identify the effect of ATP (acting via InsP3) and thapsigargin (acting without InsP3 production as a specific inhibitor of the endoplasmic reticulum Ca(2+)-ATPase) on intracellular Ca2+ pools we used video imaging of Fura-2 loaded into single, intact glioma C6 cells. It has been shown that ATP and thapsigargin initiate Ca2+ response consistent with the capacitative model of Ca2+ influx. When the cells were stimulated by increasing concentrations of ATP (1, 10, 50 and 100 microM) the graded, quantal Ca2+ response was observed. In the absence of extracellular Ca2+ thapsigargin and ionomycin-releasable Ca2+ pools are overlapping, demonstrating that Ca2+ stores are located mainly in the endoplasmic reticulum. After maximal Ca2+ mobilization by ATP, thapsigargin causes further increase in cytosolic Ca2+ concentration, whereas emptying of thapsigargin-sensitive intracellular stores prevents any further Ca2+ release by ATP. Thus, the thapsigargin-sensitive intracellular pool of Ca2+ in glioma C6 cells seems to be larger than that sensitive to InsP3. Two hypothesis to explain this result are proposed. One postulates a presence of two different Ca2+ pools, sensitive and insensitive to InsP3 and both discharged by thapsigargin, and the other, the same intracellular pool of Ca2+ completely emptying by thapsigargin and only partially by InsP3. These results may contribute to understanding the mechanism of Ca2+ signalling mediated by ATP, the most potent intracellular Ca2+ mobilizing agonist in all types of glial cells.

Phosphatidylserine synthesis in glioma C6 cells is inhibited by Ca2+ depletion from the endoplasmic reticulum: effects of 2,5-di-tert-butylhydroquinone and thimerosal

The effects of 2,5-di-tert-butylhydroquinone (DBHQ) and thimerosal on phosphatidylserine synthesis by the base exchange reaction and on calcium mobilization in intact glioma C6 cells were compared with that of thapsigargin, a selective inhibitor of the endoplasmic reticulum Ca(2+)-ATPase. It has been found that all these agents inhibit phosphatidylserine synthesis by 70%, but their effectiveness are different. The data show that this inhibition is caused by Ca2+ depletion of the endoplasmic reticulum, indicating that phosphatidylserine synthesis requires high concentration of Ca2+ within this structure. On this basis and on literature data, a new model for the localization of the serine base exchange enzyme in the endoplasmic reticulum membrane is proposed.

Sphingosine stimulates calcium mobilization and modulates calcium signals evoked by thapsigargin in glioma C6 cells

The effect of sphingosine on intracellular calcium signalling in glioma C6 cells was studied with Fura-2 video imaging technique. Sphingosine had a direct effect on changes in cytosolic Ca2+ concentration only when applied at high concentration of 100 microM, causing the cytosolic Ca2+ level to rise. However, at a much lower concentration of 15 microM sphingosine diminished calcium responses triggered by thapsigargin (a specific inhibitor of calcium pump in the endoplasmic reticulum) and ionomycin (calcium ionophore). Since responses to thapsigargin and ionomycin were blocked in Ca(2+)-free medium, we postulate that sphingosine is acting on the intracellular calcium stores. Additionally, sphingosine (at 15 microM and 100 microM) markedly decreases thapsigargin-induced sustained elevation in cytosolic Ca2+ concentration, indicating its inhibitory effect on thapsigargin-evoked Ca2+ influx. Sphingosine is a known inhibitor of protein kinase C and the involvement of this enzyme is postulated in the modulatory effects of sphingosine on intracellular calcium dynamics.

Phosphatidylserine synthesis in phorbol ester treated glioma C6 cells

Phosphatidylserine (PS) synthesis was studied in glioma C6 cells with [14C]serine and in the presence or absence of the phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA). It was found that incubation of the cells with 10 nM or 100 nM TPA for 1 h inhibited PS formation by 30% and 60%, respectively. Long-term (18 h) treatment of the cells with 100 nM TPA diminished PS formation and further addition of TPA to down-regulated cells did not affect PS synthesis. The data show that the changes in PS synthesis can be associated with alterations in morphology of cell and the actin cytoskeleton organization. The role of protein kinase C in this process is discussed.

Changes in Ca2+ concentration in phorbol ester and thapsigargin treated glioma C6 cells. The role of protein kinase C in regulation of Ca2+ entry

Glioma C6 cells treated with 12-0-tetradecanoyl-phorbol-13-acetate, TPA (10 nM and 100 nM) manifested slow increase in intracellular calcium concentration ([Ca2+]i), dependent upon both Ca2+ release from intracellular stores and Ca2+ entry, and ranging from 50 to 500 nM in different cells. The effect of TPA was abolished by the down-regulation procedure and by protein kinase C inhibitors, such as staurosporine (100 nM), suramin (100 microM), and sphingosine (100 microM), pointing to a role of protein kinase C (PKC) in this process. On the other hand, thapsigargin (100 nM), a selective inhibitor of the endoplasmic reticulum Ca(2+)-ATPase, produced a rapid increase in [Ca2+]i (up to 800 nM). This increase consisted of a transient initial phase followed by sustained elevation in [Ca2+]i, typical of Ca2+ release from intracellular stores and of Ca2+ entry, respectively. However, when the cells were exposed to TPA (100 nM) prior to thapsigargin (100 nM), then thapsigargin produced only a transient rise in [Ca2+]i. We suggest that TPA, a PKC activator, affects thapsigargin-induced Ca2+ entry, probably by PKC-mediated changes in cytoskeleton structures.

Temporal dynamics and regional distribution of [14C]serine uptake into mouse brain

In order to examine the uptake of L-serine into brain structures and brain metabolic compartments, L-[U-14C]serine was injected into tail vein of mice. The uptake was examined 30 min, 90 min, 3 h and 5 h after injection by both quantitative autoradiography of coronal brain sections and by biochemical analysis. Brain radioactivity was extracted and partitioned into protein associated pellets, metabolites soluble in aqueous phase and lipids soluble in the organic phase. Most of the radioactivity was found in the aqueous phase, about 10% was incorporated into lipids. Among phospholipids the highest label was found in phosphatidylserine, then in phosphatidylethanolamine and in phosphatidylcholine, it amounted to 52%, 30% and 18% of label by 90 min after injection, respectively. The brain distribution of L-serine uptake resembled that described for strychnine-insensitive [3H]glycine binding, with cortical structures being preferentially labelled.

Effect of ionophore A23187 and thapsigargin on serine incorporation into phosphatidylserine in intact and permeabilized glioma C6 cells at high and low Ca2+ concentrations

Intact and permeabilized glioma C6 cells were incubated with [14C]serine in media containing low (100 nM) or high (2 mM) [Ca2+] and serine incorporation into phosphatidylserine was examined. In all cases thapsigargin, a blocker of the endoplasmic reticulum Ca(2+)-ATPase, diminished this process, whereas the action of the ionophore A23187 was dependent on the external calcium concentration and time of incubation. In permeabilized cells incubated at 100 nM Ca2+, serine incorporation into phosphatidylserine was diminished when the endoplasmic reticulum Ca2+ levels were lowered by the ionophore. In intact cells incubated at 2 mM CaCl2, addition of A23187 had no effect for the first 30 min and later decreased [14C]serine incorporation. This result seems to be not connected with the degradation of already formed phosphatidylserine, or with an enhanced metabolic conversion of this phospholipid, but with the decrease of its synthesis. The mechanism of this last process appears to be involved in the ionophore-induced perturbation of cellular Ca2+ homeostasis. Our results indicate that phosphatidylserine synthesis is a Ca(2+)-regulated process.

Metabolic conversion of phosphatidylserine via phosphatidylethanolamine into phosphatidylcholine in rat brain

Exogenous, liposomal [14C]phosphatidylserine, and that synthesized from [14C]serine, were very slowly metabolized in cortex and hippocampus slices of rat brain; phosphatidylethanolamine (PE) formed from phosphatidylserine (PS) was not methylated to phosphatidylcholine (PC) for up to 6 hours of incubation. Among homogenates prepared from 7 separate brain regions, the cerebellum showed the highest, and the striatum and pons the lowest rate of PS synthesis and its further decarboxylation to PE; in all of these regions the stepwise methylation of PE to PC was very low. Isolated microsomal and mitochondrial fractions of whole rat brain, mixed together and incubated with [14C]serine and S-adenosylmethionine, displayed a high level of newly synthesized mitochondrial PE, and a low level of methylated PC in the microsomes. Moreover, PE formed in brain microsomes by the base exchange reaction was converted into PC in an insignificant range. These data show a limited activity for sequential methylation of PE into PC in rat brain, and suggest that it is probably not caused by the slow movement of mitochondrial PE.

Effect of the ionophore A23187, thapsigargin, caffeine and heparin on phosphatidylserine synthesis in rat liver microsomal fraction

It has been shown that the incorporation of [14C]serine into phosphatidylserine in rat liver microsomal fractions is distinctly stimulated by ATP and Mg2+ at low (10 microM) Ca2+ concentration. This stimulation occurred in untreated microsomes, but did not occur in fully disrupted microsomal vesicles. The stimulatory effect of ATP and Mg2+ on phosphatidylserine synthesis was reduced by calcium ionophore A23187 and by thapsigargin, which is a specific blocker of Ca(2+)-ATPase. It was also diminished by caffeine and enhanced by heparin, agents known to modulate Ca2+ release by receptor channels. It is therefore postulated that the synthesis of phosphatidylserine occurs on the luminal side of the endoplasmic reticulum and can be regulated by events responsible for Ca2+ release and entry into these structures.

Inhibition of phosphatidylserine synthesis by glutamate, acetylcholine, thapsigargin and ionophore A23187 in glioma C6 cells

Phosphatidylserine synthesis was studied in glioma C6 cells with [14C]serine and in the presence or absence of agents which increase the level of [Ca2+]i. It was found that glutamate and acetylcholine inhibited this synthesis by up to 40%, whereas thapsigargin and the ionophore A23187 inhibited by up to 70%. The inhibitory effect of thapsigargin and the A23187 was observed in Ca(2+)-free medium. The data show that the inhibition of this synthesis is caused by the Ca(2+)-depletion from endoplasmic reticulum, suggesting that the synthesis of phosphatidylserine occurs on the luminal side of these structures and can be regulated by transmembrane signaling systems.

Transport of phosphatidic acid within the mitochondrion

Transfer of phosphatidic acid from the outer to the inner membrane within intact rat liver mitochondria was assessed by measuring the ratio of lipid 32P to the marker enzyme of the outer membrane, rotenone-insensitive NADH-cytochrome c reductase, in the outer and inner membrane fractions obtained after incubation of mitochondria under conditions for net synthesis of [32P]phosphatidic acid. This transfer was found to proceed with time, to occur only under high ionic strength of the external medium and to be insensitive to N-ethylmaleimide and factors reducing the number of contact sites between the two mitochondrial membranes. These results are interpreted as supporting the idea that phosphatidic acid transport within the mitochondrion occurs as free diffusion through the aqueous phase and not being mediated by phospholipid transfer protein(s).

Mechanism of the ATP-dependent phosphatidylserine synthesis in liver subcellular fractions

It has been shown that the ATP-dependent incorporation of [14C]serine into phosphatidylserine in rat liver mitochondrial and microsomal fractions is prevented by EGTA. On the other hand, at low (microM) Ca2+ concentrations, serine incorporation is strongly stimulated by ATP and Mg2+. This stimulatory effect is reduced by calcium ionophore A23187. It is therefore suggested that the ATP-dependent process is that of serine base-exchange reaction, stimulated by endogenous Ca2+ accumulated inside the microsomal vesicles by Ca2+,Mg2+-ATPase. The mitochondrial activity can be accounted for by contamination by the endoplasmic reticulum.

Non-protein-mediated transfer of phosphatidic acid between microsomal and mitochondrial membranes

The transfer of phosphatidic acid between rat liver microsomes loaded with [32P]-phosphatidic acid and rat liver mitochondria was studied in the absence of added lipid transfer proteins. It was found that during 1 h at 37 degrees C in the medium containing 100 mM KCl, 20-30% of phosphatidic acid but only 2.5% of phosphatidylcholine were transferred. This spontaneous transfer of phosphatidic acid remained the same after pretreatment of microsomes and mitochondria with 125 mM KCl or microsomes alone with 1 mM Tris, pH 8.6, procedures reported to remove adsorbed lipid transfer proteins. This transfer was insensitive to thiol-blocking reagents. The initial rate of this non-protein-mediated transfer of phosphatidic acid was virtually independent of the concentration of the acceptor membranes (mitochondria), thus indicating that it occurs by diffusion of the phospholipid through the aqueous phase rather than by membrane collision. About 80% of phosphatidic acid synthesized in the outer mitochondrial membrane was recovered in the inner membrane after a 1-h incubation, pointing to a high rate of the intermembrane transfer of this phospholipid within intact mitochondrion.

Transfer of phosphatidic acid between microsomal and mitochondrial outer and inner membranes

A protein fraction from rat liver cytoplasm, precipitable at 50-95% saturation of ammonium sulphate, binds phosphatidic acid from mitochondrial and microsomal membranes. Protein-bound phosphatidic acid was eluted from Sephadex G-75 in fractions corresponding to a molecular weight of about 10 000. No such binding was observed with mitochondrial soluble proteins, either total or precipitated with ammonium sulphate between 50 and 95% saturation. The transfer of phosphatidic acid from microsomes to mitochondria was increased by liver cytoplasmic proteins precipitable at 50-95% saturation of ammonium sulphate but not with mitochondrial soluble proteins. This increase by cytoplasmic proteins was pronounced in 200 mM sucrose but was negligible in 100 mM KCI where the spontaneous transfer was quite high. Cytoplasmic proteins stimulated the synthesis of cardiolipin and phosphatidylglycerol in mitochondria deprived of the outer membrane but not in intact mitochondria when phosphatidic acid was supplied either by microsomes or liposomes. It is suggested that the transfer of phosphatidic acid from the outer to the inner mitochondrial membrane is not mediated by transfer proteins but occurs either by direct contact of the membranes or as free diffusion through the aqueous phase.

Phosphatidylserine biosynthesis in mitochondria from Ehrlich ascites tumor cells

1. Isolated mitochondria of Ehrlich ascites tumor cells incorporated [14C]serine into phosphatidylserine and partly into its decarboxylation product, phosphatidylethanolamine, while phosphatidylserine was the sole product with microsomes. 2. The incorporation of [14C]serine into mitochondrial phospholipids was stimulated by Ca2+ which indicated the operation of the Ca2+-dependent base-exchange mechanism, virtually absent in mammalian tissue mitochondria. The finding cannot be attributed to microsomal contamination. 3. The incorporation of [14C]serine into mitochondrial phospholipids was also stimulated by ATP, both in the presence and in the absence of calcium-complexing agents. The stimulation by ATP was insensitive to penicillin and streptomycin, thus pointing that this process was not of bacterial origin. 4. The latter process was further stimulated by phosphatidic acid and phosphatidic acid precursors, but not by CDP diacylglycerol. 32P from neither [gamma-32P]ATP nor [32P]phosphatidic acid was incorporated into phosphatidylserine in the presence of CTP and L-serine. The release of [14C]CMP from [14C]CDP diacylglycerol was not stimulated by l-serine. 5. It is concluded that [14C]serine incorporation into mitochondrial phospholipids by ATP-dependent process does not fit to any of the pathways of phopholipid biosynthesis described so far.

Synthesis of phospholipids in mitochondria and other membrane fractions of rabbit reticulocytes

1. Reticulocytosis of 40-50% was obtained in rabbits by daily bleeding. Reticulocytes (plus erythrocytes) were subfractionated into plasma membrane fraction, mitochondria and the post-mitochondrial fraction. 2. In all fractions, fatty acids were incorporated into phospholipids. This process was ATP dependent and represented acylation of lysophospholipids. 3. Incorporation of fatty acids into lysophosphatidic and phosphatidic acids occurred only in the presence of sn-glycerol 3-phosphate and was observed in mitochondria and the post-mitochondrial fraction. It represents a two-step acylation of sn-glycerol 3-phosphate. 4. Incorporation of phosphorylcholine from CDPcholine into phosphatidylcholine was observed in the mitochondrial and the post-mitochondrial fractions. This activity was correlated with NADPH-cytochrome c reductase and was probably connected with the remnants of the endoplasmic reticulum.

Role of lamellar bodies in the biosynthesis of phosphatidylcholine in mouse lung

1. A lamellar body-enriched fraction was isolated from whole lung homogenates of mouse lung and its contamination with microsomes, mitochondria, and cytosol protein assessed by marker enzyme analyses. 2. By measuring the activity of cholinephosphotransferase (EC 2.7.8.2) in varying amounts of microsomes in the presence and absence of a fixed quantity of lamellar bodies, it could be demonstrated unequivocally that lamellar bodies of mouse lung lack the capacity to synthesize phosphatidylcholine de novo. 3. A similar approach allowed the conclusion that lamellar bodies of mouse lung do not contain lysophosphatidylcholine acyltransferase (EC 2.3.1.23) and lysophosphatidylcholine:lysophosphatidylcholine acyltransferase (EC 2.3.1.--), enzymes which play a putative role in the formation of pulmonary 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine. The activities of these enzymes observed in lamellar body fractions could be attributed completely to contaminating microsomes and cytosol respectively. 4. Lamellar bodies contributed to the activity of microsomal lysophosphatidylcholine acyltransferase by a cooperative effect. The possible role of this cooperation in the biosynthesis of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine is discussed.