Determination of phospholipase D-mediated hydrolysis of phosphatidylethanolamine

Overexpression of protein kinase C-epsilon and its regulatory domains in fibroblasts inhibits phorbol ester-induced phospholipase D activity

In fibroblasts, the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) stimulates phospholipase D (PLD)-mediated hydrolysis of both phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn) by PKC-alpha-mediated nonphosphorylating and phosphorylating mechanisms. Here we have used NIH 3T3 fibroblasts overexpressing holo PKC-epsilon and its regulatory, catalytic, and zinc finger domain fragments to determine if this isozyme also regulates PLD activity. Overexpression of holo PKC-epsilon inhibited the stimulatory effects of PMA (5-100 nM) on both PtdCho and PtdEtn hydrolysis. Overexpression of PKC-epsilon also was found to inhibit platelet-derived growth factor-induced PLD activity. Expression of the catalytic unit of PKC-epsilon had no effect on PMA-induced PLD activity. In contrast, expression of both the regulatory domain fragment and the zinc finger domain of PKC-epsilon resulted in significant inhibition of PMA-stimulated PtdCho and PtdEtn hydrolysis. Interestingly, although PKC-alpha also mediates the stimulatory effect of PMA on the synthesis of PtdCho by a phosphorylation mechanism, overexpression of holo PKC-epsilon or its regulatory domain fragments did not affect PMA-induced PtdCho synthesis. These results indicate that the PKC-epsilon system can act as a negative regulator of PLD activity and that this inhibition is mediated by its regulatory domain.

Alkyl lysophospholipids inhibit phorbol ester-stimulated phospholipase D activity and DNA synthesis in fibroblasts

The antineoplastic alkyl lysophospholipids (ALP) 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3) and 1-S-hexadecylthio-2-methoxymethyl-2-deoxy-rac-glycero-3-phosphocho line (BM41.440) were found to alter phospholipase D (PLD)-mediated phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn) hydrolysis in NIH 3T3 fibroblasts. After a shorter (50 min) treatment, 2.5-7.5 microg/ml concentrations of ALP stimulated PtdCho, but not PtdEtn, hydrolysis 2-4-fold. At the same time, 7.5-25 microg/ml concentrations of ALP significantly inhibited the larger (5.8-6.5-fold) stimulatory effects of phorbol 12-myristate 13-acetate (PMA) on both PtdCho and PtdEtn hydrolysis. When a brief (30 min) exposure of cells to 1-2.5 microg/ml concentrations of BM 41.440 was followed by incubation of washed cells for 3-16 h prior to the assay of PLD activity or DNA synthesis, the treated cells exhibited no increased PtdCho hydrolysis, while their responses to the stimulatory PMA effects on both PLD activity and DNA synthesis were strongly reduced. The results suggest that the PLD and protein kinase C systems may be important cellular targets of ALP actions.

Compound D609 inhibits phorbol ester-stimulated phospholipase D activity and phospholipase C-mediated phosphatidylethanolamine hydrolysis

Tricyclodecan-9-yl-xanthogenate (compound D609) has recently been used in various cellular systems to specifically inhibit the activity of phosphatidylcholine (PtdCho)-directed phospholipase C (PLC). Here we show that in intact NIH 3T3 fibroblasts, concentrations of D609 (35 to 50 micrograms/ml) which have been used to inhibit PLC activity also significantly inhibit phorbol ester-induced phospholipase D-mediated hydrolysis of both PtdCho and phosphatidylethanolamine (PtdEtn). In addition, in isolated membranes compound D609 also inhibited PLC-mediated PtdEtn hydrolysis. The results indicate that compound D609 cannot be considered as a specific inhibitor of PtdCho-directed PLC activity.

Differential roles of de novo sphingolipid biosynthesis and turnover in the "burst" of free sphingosine and sphinganine, and their 1-phosphates and N-acyl-derivatives, that occurs upon changing the medium of cells in culture

Long-chain (sphingoid) bases are highly bioactive intermediates for sphingolipid metabolism, yet relatively little is known about how the amounts of these compounds are regulated. This study used J774A.1 cells to characterize the "burst" of sphinganine and sphingosine, or the transient increase of up to 10-fold in long-chain base mass, that occurs when cells in culture are changed to fresh medium. The increase in sphinganine was attributable to de novo sphingolipid biosynthesis because: 1) there is increased incorporation of [3H]serine and [3H]palmitate into sphinganine; 2) the incorporation of [3H]serine was equivalent to the increase in sphinganine mass; 3) beta-F-alanine, an inhibitor of serine palmitoyltransferase, blocked the sphinganine burst; 4) the magnitude of the burst depended on the concentration of serine in the medium, which is known to affect long-chain base biosynthesis; and 5) the appearance of sphinganine was relatively unaffected by lyso-osmotrophic agents (NH4Cl and chloroquine) that blocked sphingolipid hydrolysis in these cells. In contrast, the sphingosine burst arose mainly from turnover of complex sphingolipids because no incorporation of [3H]serine or [3H]palmitate into sphingosine was detected; sphingosine mass was not affected by beta-F-alanine or the serine concentration; and, the burst could be followed by the release of sphingosine and ceramide from complex sphingolipids (especially sphingomyelin) in a process that was inhibited by NH4Cl and chloroquine. Additionally, the fate of these long-chain bases differed: sphinganine was mostly (80-85%) acylated and incorporated into dihydroceramide and complex sphingolipids, whereas most of the sphingosine (70%) was phosphorylated and degraded, with incorporation of the resulting ethanolamine phosphate into phosphatidylethanolamine. Sphinganine, however, could be diverted toward degradation by adding an inhibitor of N-acylation (fumonisin B1). In accounting for the elevation in sphingosine and sphinganine after cells are changed to new medium, these studies have provided fundamental information about long-chain base metabolism. The existence of differential changes in sphinganine and sphingosine, as well as their 1-phosphates and N-acyl-derivatives, should be considered when evaluating the roles of sphingolipid metabolites in cell regulation.

Preferential inhibition of phorbol ester-induced hydrolysis of phosphatidylethanolamine by N-acetylsphingosine in NIH 3T3 fibroblasts

It has been reported that in rat fibroblasts cell-permeable ceramide analogs inhibit agonist-induced phospholipase D (PLD)-mediated hydrolysis of phosphatidylcholine (PtdCho). Here we demonstrate that relatively short (30 min) treatments of NIH 3T3 fibroblasts with 15-60 microM concentrations of N-acetylsphingosine result in preferential, although not exclusive, inhibition of phorbol 12-myristate 13-acetate-induced PLD-mediated hydrolysis of phosphatidylethanolamine (PtdEtn). The results suggest that in different cell types the PtdEtn- and PtdCho-hydrolyzing PLD activities are differentially sensitive to the inhibitory effect of ceramide.

The bisindolylmaleimide GF 109203X, a selective inhibitor of protein kinase C, does not inhibit the potentiating effect of phorbol ester on ethanol-induced phospholipase C-mediated hydrolysis of phosphatidylethanolamine

In fibroblasts, the protein kinase C (PKC) activator phorbol 12-myristate (PMA) either inhibits or stimulates phospholipase C-mediated hydrolysis of phosphatidylethanolamine in the absence or presence of ethanol, respectively. Here, we demonstrate that the specific PKC inhibitor bisindolylmaleimide GF 109203X prevents only the inhibitory, but not the stimulatory, PMA effect.

Tamoxifen stimulates phospholipase D activity by an estrogen receptor-independent mechanism

The effects of tamoxifen (TAM), a widely used agent in the treatment of breast cancer, were examined on phospholipase D (PLD)-mediated phospholipid hydrolysis. In drug-sensitive MCF-7 human breast carcinoma cells TAM, similar to several well-established activators of PLD, had no effect on phospholipid hydrolysis. In an estrogen receptor-deficient multidrug-resistant subline of MCF-7 cells, TAM preferentially stimulated the hydrolysis of phosphatidylethanolamine; two-fold stimulation required 2.5 or 5 microM TAM in the absence or presence of serum, respectively. In NIH 3T3 fibroblasts significant (4- to 4.8-fold) stimulation of phosphatidylethanolamine and phosphatidylcholine hydrolysis in the presence of serum required 10 microM TAM. These data establish that TAM can stimulate PLD activity by an estrogen receptor-independent mechanism.

Selective down-regulation of protein kinase c-epsilon by carcinogens does not prevent stimulation of phospholipase D by phorbol ester and platelet-derived growth factor

It is well established that activators of protein kinase C (PKC) also enhance the activity of phospholipase D (PLD), and that this regulatory mechanism is altered in transformed cells. Here we used the C3H/10T1/2 mouse embryo fibroblast line, a cellular model for the study of carcinogenesis, to examine possible effects of carcinogens on the PKC isoenzyme pattern and on the regulation of PLD by the PKC activators phorbol 12-myristate 13-acetate (PMA) and platelet-derived growth factor (PDGF). Treatment of these fibroblasts with 0.5 microgram/ml 7,12-dimethyl-benz[a]anthracene or benzo[a]pyrene for 24 h greatly decreased (> 80%) the amount of immunoreactive PKC-epsilon. Of the remaining three isoenzymes identified, carcinogens alone had no effect on the cellular status of PKC-alpha and PKC-delta, although they appeared to promote slightly PMA-induced membrane translocation of the cytosolic forms of these isoenzymes in exponentially growing cells. Carcinogens and/or PMA had no effects on the cellular content or distribution of PKC-zeta. Chronic (24 h) treatments with carcinogens resulted in increased or decreased release of [14C]ethanolamine or [14C]choline from the appropriate prelabelled phospholipids, respectively. However, carcinogens failed to block the stimulatory effects of PMA and PDGF on the hydrolysis of phosphatidylethanolamine and phosphatidylcholine or on the synthesis of phosphatidylethanol mediated by PLD. These data indicate that in fibroblasts PKC-epsilon is not a major regulator of PLD activity.

Sphingosine-like stimulatory effects of propranolol on phospholipase D activity in NIH 3T3 fibroblasts

Propranolol and sphingosine exhibit several common biochemical effects, including inhibition of phosphatidic acid phosphohydrolase and protein kinase C (PKC) activities. In NIH 3T3 fibroblasts, sphingosine has also been shown to stimulate phospholipase D (PLD)-mediated hydrolysis of both phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn) (Kiss Z and Anderson WB, J Biol Chem 265: 7345-7350, 1990). The present study demonstrates that in [14C]palmitic acid-labeled NIH 3T3 fibroblasts, propranolol (50-100 microM) and sphingosine had similar stimulatory effects on PLD-mediated synthesis of phosphatidylethanol in the presence of ethanol. In [14C]choline- and [14C]-ethanolamine-labeled fibroblasts, both compounds also stimulated the hydrolysis of both [14C]PtdCho and [14C]PtdEtn. However, while sphingosine preferentially stimulated PtdEtn hydrolysis, propranolol had greater effects on PtdCho hydrolysis. At each time point examined (15-45 min), lower concentrations (25-50 microM) of propranolol and 100 nM phorbol 12-myristate 13-acetate (PMA) synergistically enhanced PtdEtn hydrolysis; a higher concentration (100 microM) of propranolol inhibited this PMA effect only when the incubation time was 45 min. On the other hand, propranolol (10-100 microM) had either no effect or it inhibited PMA-induced PtdCho hydrolysis after treatments for 15 or 45 min, respectively. These potentiating and inhibitory actions of propranolol on the hydrolysis of PtdCho and PtdEtn were similarly elicited by sphingosine. The present study identified the PLD system as another common target for the pharmacological actions of sphingosine and propranolol.

The zinc chelator 1,10-phenanthroline enhances the stimulatory effects of protein kinase C activators and staurosporine, but not sphingosine and H2O2, on phospholipase D activity in NIH 3T3 fibroblasts

Protein kinase C (PKC), an enzyme which is believed to mediate the stimulatory effects of the PKC activator phorbol 12-myristate 13-acetate (PMA) on phospholipase D (PLD) activity, has a zinc-dependent structure required for phorbol ester binding. Accordingly, zinc or zinc chelators would be expected to promote or inhibit, respectively, the stimulatory effects of PMA on PLD-mediated phospholipid hydrolysis. Instead, treatment of [14C]choline- and [14C]ethanolamine-labelled NIH 3T3 fibroblasts with the high-affinity zinc chelator 1,10-phenanthroline (0.2-1 mM) for 20-30 min was found to enhance the stimulatory effects of PMA on PLD-mediated hydrolysis of phosphatidylcholine and phosphatidylethanolamine. In [14C]palmitic acid-labelled fibroblasts, in the presence of ethanol, phenanthroline also enhanced the stimulatory effect of PMA on the synthesis of phosphatidylethanol, a marker of PLD activity. Addition of zinc (250 microM) to phenanthroline-treated fibroblasts reversed the stimulatory effects of the chelator. The potentiating effects of phenanthroline were also partially reversed by cadmium, whereas iron, lead, copper, magnesium and calcium were without effects. Of the other activators of PLD tested, phenanthroline also enhanced the stimulatory effects of platelet-derived growth factor and staurosporine, but not that of sphingosine and H2O2, on the hydrolysis of both phospholipids. These results suggest that regulation of PLD by PKC activators and staurosporine involves a common intermediate step, which is inhibited by a chelatable cellular pool of zinc.

Carcinogens stimulate phosphorylation of ethanolamine derived from increased hydrolysis of phosphatidylethanolamine in C3H/101/2 fibroblasts

Many human tumors contain high concentrations of ethanolamine phosphate (EtnP). An important question is whether increased formation of EtnP is merely the consequence of cell transformation, or is it associated with the process of carcinogenesis. Here we show that in C3H/10T1/2 embryonic fibroblasts, an established cellular model for the study of carcinogenesis, the environmental carcinogens, 7,12-dimethylbenz[a]anthracene (DMBA) and benzo[a]pyrene (B[a]P) (0.1-1 microgram/ml concentration; 24 h treatment), stimulate phosphorylation of ethanolamine derived from increased hydrolysis of phosphatidylethanolamine. The results suggest that increased formation of EtnP is associated with the early stages of carcinogenesis. This observation may have prognostic value.

Protein kinase C-dependent stimulation of phospholipase D in phospholipase C-treated fibroblasts

Treatment of [14C]choline- or [14C]ethanolamine-labeled NIH 3T3 fibroblasts with Bacillus cereus phosphatidyl-choline-specific phospholipase C (PLC) enhanced phospholipase D (PLD)-mediated hydrolysis of the respective 14C-labeled phospholipids. PLD activity was stimulated by 1.5 U/mL of PLC and by 100 nM of the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) to similar extents. Treatment of [14C]palmitic acid-labeled fibroblasts with PLC in the presence of ethanol also enhanced PLD-mediated formation of phosphatidylethanol; the effects of PLC and PMA were nonadditive. PLC had no effect on PLD activity in fibroblasts in which PKC was down-regulated by prolonged (24 h) treatment with 300 nM PMA. These data indicate that treatment of fibroblasts with exogenous PLC results in PKC-dependent activation of PLD.

Phospholipase D: regulation and functional significance

PLD is a major route for hydrolysis of PC in most tissues, consistent with it playing an important role in signal transduction. The enzyme appears to be activated by a variety of different mechanisms in different tissues, suggesting there might be several different isoforms. Little, however, is known at present about its enzymology and molecular biology. There is little direct evidence to indicate the functional significance of PLD activation but an accumulation of indirect evidence links PLD with prolonged changes in cell function. In particular, two areas where there is strong evidence for a role for PLD are mitogenesis and leukocyte hyperresponsiveness. An important area for future work will be the investigation of how products from the PLD pathway exert these effects. Current evidence suggests an important role for Ca(2+)-independent PKC isoforms and probably also for novel cellular targets for the putative second messenger PA.

Regulation of phospholipase D by sphingosine involves both protein kinase C-dependent and -independent mechanisms in NIH 3T3 fibroblasts

Previously, the protein kinase C (PKC) inhibitor sphingosine was found to stimulate phospholipase D (PLD)-mediated hydrolysis of both phosphatidylethanolamine (PtdEtn) and phosphatidylcholine (PtdCho) in NIH 3T3 fibroblasts [Kiss & Anderson (1990) J. Biol. Chem. 265, 7345-7350]. Here we examined the possible relationship between the opposite effects of sphingosine on PKC-mediated protein phosphorylation and PLD activation. After treatments for 3-5 min, sphingosine (25 microM) and the PKC activators phorbol 12-myristate 13-acetate (PMA) (100 nM), bryostatin (100 nM) or platelet-derived growth factor (50 ng/ml) synergistically stimulated the hydrolysis of both PtdEtn and PtdCho in NIH 3T3 fibroblasts prelabelled with [14C]ethanolamine or [14C]choline. Inhibition of PMA-induced phospholipid hydrolysis could also be elicited by sphingosine, but this process required prolonged (60 min) treatments of fibroblasts with 40-60 microM-sphingosine. Similarly to sphingosine, the protein phosphatase inhibitor okadaic acid also had either potentiating or inhibitory effects on PMA-stimulated PLD activity, depending on the length of incubation time and the concentration of PMA. Consistent with the presence of an inhibitory component in the overall action of PKC, the PKC inhibitor staurosporine and down-regulation of PKC activity by prolonged (24 h) treatment with PMA similarly enhanced PLD activity. Data suggest that (a) sphingosine may enhance PMA-mediated phospholipid hydrolysis by neutralizing the action of an inhibitory PKC isoform, and that (b) the stimulatory PKC isoform is less sensitive to the inhibitory action of sphingosine.

Cooperative effects of ethanol and protein kinase C activators on phospholipase-D-mediated hydrolysis of phosphatidylethanolamine in NIH 3T3 fibroblasts

In a previous study, ethanol was shown to enhance the stimulatory effect of phorbol 12-myristate 13-acetate (PMA), a prominent activator of protein kinase C (PKC), on phospholipase-D (PLD)-mediated hydrolysis of phosphatidylethanolamine (PtdEtn) in NIH 3T3 fibroblasts (Kiss et al. (1991) Eur. J. Biochem. 197, 785-790). Here, the mechanism and possible significance of ethanol-stimulated PtdEtn hydrolysis was further studied. In [14C]ethanolamine-labeled NIH 3T3 fibroblasts, 10 mM ethanol enhanced PMA-induced hydrolysis of PtdEtn 1.5-2.0-fold during a 2.5-15-min incubation period. Other alcohols, including glycerol, methanol, and 1-propanol, also enhanced PMA-induced PtdEtn hydrolysis. Of the other PLD activators tested, ethanol potentiated the PKC-dependent stimulatory effect of bombesin but failed to alter the apparently PKC-independent stimulatory effect of serum. Pretreatment of [14C]ethanolamine-labeled fibroblasts with 200 mM ethanol for 20 min resulted in increased (approx. 2-fold) hydrolysis of [14C]PtdEtn in isolated membranes. In membranes from ethanol-treated, but not from untreated, cells, PMA further enhanced (approx. 1.5-fold) the production of [14C]ethanolamine. Ethanol exerted none of the above stimulatory effects on phosphatidylcholine hydrolysis. These results suggest that the specific stimulatory action of ethanol on PLD-mediated PtdEtn hydrolysis can occur in vivo and may involve increased binding of a regulatory PKC-isoform to membranes.

The long-term combined stimulatory effects of ethanol and phorbol ester on phosphatidylethanolamine hydrolysis are mediated by a phospholipase C and prevented by overexpressed alpha-protein kinase C in fibroblasts

The protein kinase C (PKC) activator 12-O-tetradecanoylphorbol 13-acetate (TPA) has been shown to potentiate the stimulatory effect of ethanol on the hydrolysis of phosphatidylethanolamine (PtdEtn) in NIH 3T3 fibroblasts. Following an initial 20-min period, the main product of PtdEtn degradation in cells treated with TPA plus ethanol was ethanolamine phosphate. Here, we have examined the regulatory role of PKC and the possible catalytic role of phospholipase C in the formation of ethanolamine phosphate. TPA, bryostatin, and bombesin, direct or indirect activators of PKC, had similar potentiating effects on ethanol-induced formation of [14C]ethanolamine phosphate from [14C]PtdEtn in [14C]ethanolamine-prelabelled NIH 3T3 fibroblasts. At lower concentrations of ethanol (40-80 mM), significant stimulation of ethanolamine phosphate formation required longer treatments (2 h or longer). The combined effects of TPA (100 nM) and ethanol (50-200 mM) on ethanolamine phosphate formation were not inhibited by the PKC inhibitors staurosporine or 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7). In contrast, these inhibitors significantly inhibited TPA-induced formation of ethanolamine, catalyzed by a phospholipase-D-type enzyme. In membranes isolated from TPA+ethanol-treated cells, enhanced formation of ethanolamine phosphate was maintained for at least 20 min. Down-regulation of PKC by prolonged (24-h) treatment of NIH 3T3 fibroblasts by 300 nM TPA enhanced, while overexpression of alpha-PKC in Balb/c fibroblasts diminished, the stimulatory effect of ethanol on the formation of ethanolamine phosphate. Finally, addition of the protein phosphatase inhibitor okadaic acid (2 microM) to fibroblasts inhibited TPA+ethanol-induced formation of ethanolamine phosphate. These results suggest that alpha-PKC-mediated protein phosphorylation may negatively regulate PtdEtn hydrolysis and that the potentiating effect of TPA may result, at least partly, from increased degradation of this PKC isoform.

Differential effects of platelet-derived growth factor, serum and bombesin on phospholipase D-mediated hydrolysis of phosphatidylethanolamine in NIH 3T3 fibroblasts

In previous studies, activators of protein kinase C, sphingosine, ATP and various oncogenes were each found to enhance phospholipase D-mediated hydrolysis of phosphatidylethanolamine (PtdEtn) in NIH 3T3 fibroblasts. Here I examined possible stimulation of PtdEtn hydrolysis by various growth-stimulatory agents, including serum, bombesin, platelet-derived growth factor (PDGF), fibroblast growth factor (FGF) and insulin. Treatment of NIH 3T3 fibroblasts, prelabelled with [14C]Etn or [32P]PtdEtn, with PDGF-BB resulted in enhanced formation of [14C]Etn or [32P]phosphatidic acid from the respective labelled cellular pools of PtdEtn. A maximal effect (approximately 3-fold stimulation) on PtdEtn hydrolysis was obtained with 50 ng of PDGF/ml after 5 min of treatment. Phosphatidylcholine (PtdCho) was also hydrolysed, although less extensively than PtdEtn, in PDGF-stimulated cells. PDGF-stimulate hydrolysis of both PtdEtn and PtdCho was prevented by prolonged (30 h) treatment of cells with 400 nM-phorbol 12-myristate 13-acetate (PMA). Similar to PDGF, fetal calf serum (1-10%) also stimulated PtdEtn hydrolysis. However, in contrast to PDGF, the effect of serum on PtdEtn hydrolysis (i) was not diminished by pretreatment with PMA, and (ii) was synergistic with that of PMA after a 1 h incubation. Compared with PDGF and serum, bombesin had less effect on PtdEtn hydrolysis, while FGF and insulin had no effects at all. In contrast to PDGF or serum, bombesin inhibited the effect of PMA on PtdEtn hydrolysis.

Ethanolamine is released from glial cells in primary culture on stimulation with foetal calf serum and phorbol ester

Ethanolamine is released to the external medium at increased rates over basal levels when [3H]ethanolamine-labelled glial cells in primary culture, subcultures of astrocytes and O-2A lineage glia or C6 glioma cells are treated with phorbol ester or foetal calf serum. On equilibrium labelling of mixed glial cultures 90-95% of incorporated [3H]ethanolamine was in ethanolamine-containing phospholipids and 2% in the phosphatidylcholine component. On stimulation of glia with phorbol ester or foetal calf serum ethanolamine, and not phosphoethanolamine or choline, was released to the medium suggesting that a phospholipase D-mediated turnover of lipid, perhaps partially linked to protein kinase C, is involved. The results show that cells in primary culture behave similarly to cell lines in that signal transduction pathways involve a stimulated turnover of ethanolamine-phospholipids with the external release of ethanolamine.

Selective inhibition by 4-hydroxynonenal of sphingosine-stimulated phospholipase D in NIH 3T3 cells

In fibroblasts, the mitogenic effects of sphingosine involves a rapid rise in the cellular content of phosphatidic acid (PtdOH) which may be due to the stimulation of phospholipase D, or inhibition of PtdOH phosphohydrolase, or both. Here, we demonstrate that in fibroblasts, 4-hydroxynonenal is a selective inhibitor of sphingosine-stimulated phospholipid hydrolysis, and it also inhibits sphingosine-induced formation of PtdOH.

Adenosine triphosphate (ATP) and other nucleotides stimulate the hydrolysis of phosphatidylethanolamine in intact fibroblasts

Addition of adenosine triphosphate (ATP) to [14C]ethanolamine-prelabeled NIH 3T3 fibroblasts resulted in rapid formation of [14C]ethanolamine from the prelabeled cellular phosphatidylethanolamine (PtdEtn) pool. After 2-min exposure, 10 microM ATP had near maximal effects on PtdEtn hydrolysis. Several other nucleotides, including UTP, ITP, and the stable ATP analog adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S), also had stimulatory effects on PtdEtn hydrolysis with a potency comparable to that observed with ATP. The same nucleotides which acted on PtdEtn hydrolysis also had similar stimulatory effects on the hydrolysis of phosphatidylcholine (PtdCho) in [14C]choline-labeled cells. In isolated membranes, Mg2+ greatly enhanced the stimulatory effects of ATP and ATP gamma S, but not of other nucleotides, on the hydrolysis of PtdEtn and PtdCho. Results indicate that in isolated membranes, both ATP and ATP gamma S stimulate phospholipid hydrolysis by two different mechanisms, but in intact cells only one of these mechanisms appears to be responsive to externally added nucleotides.