Regulation of phosphatidylcholine biosynthesis

Effect of hypoxia on phosphatidylcholine biosynthesis in the isolated hamster heart

In hamster heart, the majority of the phosphatidylcholine is synthesized via the CDP-choline pathway, and the rate-limiting step of this pathway is catalysed by CTP:phosphocholine cytidylyltransferase (EC 2.7.7.15). We have shown previously [Choy (1982) J. Biol. Chem. 257, 10928-10933] that, in the myopathic heart, the level of cardiac CTP was diminished during the development of the disease. In order to maintain the level of CDP-choline, and consequently the rate of phosphatidylcholine biosynthesis, cardiac cytidylyltransferase activity was increased. However, it was not clear if the same compensatory mechanism would occur when the cardiac CTP level was decreased rapidly. In this study, hypoxia of the hamster heart was produced by perfusion with buffer saturated with 95% N2. The heart was pulse-labelled with radioactive choline and then chased with non-radioactive choline for various periods under hypoxic conditions. There was a severe decrease in ATP and CTP levels within 60 min of hypoxic perfusion, with a corresponding fall in the rate of phosphatidylcholine biosynthesis. Analysis of the choline-containing metabolites revealed that the lowered ATP level did not affect the phosphorylation of choline to phosphocholine, but the lower CTP level resulted in the decreased conversion of phosphocholine to CDP-choline. Determination of enzyme activities revealed that hypoxic treatment resulted in the enhanced translocation of cytidylyltransferase from the cytosolic to the microsomal form. This enhanced translocation was probably caused by the accumulation of fatty acids in the heart during hypoxia. We postulate that the enhancement of translocation of the cytidylyltransferase to the microsomal form (a more active form) is a mechanism by which the heart can compensate for the decrease in CTP level during hypoxia in order to maintain phosphatidylcholine biosynthesis.

Glucocorticoid induction of fatty-acid synthase mediates the stimulatory effect of the hormone on choline-phosphate cytidylyltransferase activity in fetal rat lung

Fetal lung fatty-acid synthase and choline-phosphate cytidylyltransferase activities are increased by glucocorticoids. There is evidence that the hormone increases synthesis of fatty-acid synthase but only increases the catalytic activity of the cytidylyltransferase. Free fatty acids and a number of phospholipids have been reported to stimulate cytidylyltransferase activity in several organs, including the lung. We have addressed the question of whether glucocorticoid induction of fatty-acid synthase mediates the stimulatory effect of the hormone on choline-phosphate cytidylyltransferase activity. Explants of 18-day fetal rat lung were cultured for 48 h with dexamethasone and inhibitors of de novo fatty acid biosynthesis (agaric acid and hydroxycitric acid) being included in the medium for the final 20 h. Dexamethasone increased the activities of fatty acid synthase and choline-phosphate cytidylyltransferase by 84% and 60%, respectively. Agaric acid and hydroxycitric acid completely abolished the stimulatory effect of the hormone on cytidylyltransferase but not on fatty-acid synthase. The inhibitors had no effect on cytidylyltransferase activity in control cultures. Fetal lung choline-phosphate cytidylyltransferase can be maximally stimulated by inclusion of phosphatidylglycerol in the assay mixture and under this condition, cytidylyltransferase activity in control and dexamethasone-treated cultures in the presence and absence of the inhibitors were all increased to the same level. Therefore, the inhibitors did not diminish the capacity of cytidylyltransferase to be fully activated. We suggest that the glucocorticoid induction of fatty-acid synthase in fetal lung results in increased synthesis of fatty acids which in turn, either as free acids or after incorporation into phospholipids, activate choline-phosphate cytidylyltransferase.

Effect of dietary fat on phospholipid class distribution and fatty acid composition in rat fat cell plasma membrane

The effect of dietary fats on phospholipid class distribution and fatty acid composition was studied in rat fat cell plasma membrane. Three groups of male Wistar weanling rats were fed for 8 wk three diets differing in the amount and nature of the fats: 1.5% sunflower oil (low fat control; LFC), 10% sunflower oil (high fat, unsaturated; HFU), 1.5% sunflower oil + 8.5% cocoa butter (high fat, saturated; HFS). Plasma membranes were prepared from epididymal adipocytes. The amount and type of dietary fat significantly altered membrane phospholipid distribution. Phospholipid content was lowered with HFU as compared to LFC or HFS diets, but no changes were observed for cholesterol. Phosphatidylinositol (PI) and phosphatidylserine (PS) were less affected by dietary changes than were other phospholipid classes. Major changes were detected for phosphatidylcholine (PC), phosphatidylethanolamine (PE) and sphingomyelin (SM) contents. No large changes in PC and PE fatty acid compositions were observed between the LFC and HFS groups, but the HFU diet induced several changes. Correlations with plasma membrane 5'-nucleotidase activities are discussed.

1,2-Diacylglycerols overcome cyclic AMP-mediated inhibition of phosphatidylcholine synthesis in GH3 pituitary cells

Previous studies showed that phorbol esters and thyrotropin-releasing hormone (TRH) stimulated phosphatidylcholine synthesis via protein kinase C in GH3 pituitary cells [Kolesnick (1987) J. Biol. Chem. 262, 14525-14530]. In contrast, 1,2-diacylglycerol-stimulated phosphatidylcholine synthesis appeared independent of protein kinase C. The present studies compare phosphatidylcholine synthesis stimulated by these agents with inhibition via the cyclic AMP system. The potent phorbol ester phorbol 12-myristate 13-acetate (PMA, 10 nM) increased [32P]Pi incorporation into phosphatidylcholine at 30 min to 159 +/- 6% of control. The adenylate cyclase activator cholera toxin (CT; 10 nM) and the cyclic AMP analogue dibutyryl cyclic AMP (1 mM) abolished this effect. CT similarly abolished TRH-induced phosphatidylcholine, but not phosphatidylinositol, synthesis. This is the first report of inhibiton of receptor-mediated phosphatidylcholine synthesis by the cyclic AMP system. The 1,2-diacylglycerol 1,2-dioctanoylglycerol (diC8) also stimulated concentration-dependent phosphatidylcholine synthesis. DiC8 (3 micrograms/ml) induced an effect quantitatively similar to that of maximal concentrations of PMA and TRH, whereas a maximal diC8 concentration (30 micrograms/ml) stimulated an effect 3-4-fold greater than these other agents. CT decreased the effect of diC8 (3 micrograms/ml) by 80%. Higher diC8 concentrations overcame the CT inhibition. Similar results were obtained with dibutyryl cyclic AMP. Additional differences were found between low and high concentrations of diC8. Low concentrations of diC8 failed to induce additive phosphatidylcholine synthesis with maximal concentrations of PMA, whereas high concentrations were additive. Hence, low concentrations of 1,2-diacylglycerols appear to be regulated similarly to phorbol esters, and higher concentrations appear to act via a pathway unavailable to phorbol esters.

CTP:cholinephosphate cytidylyltransferase in human and rat lung: association in vitro with cytoskeletal actin

CTP:cholinephosphate cytidylyltransferase activities were compared in saline homogenates of immature fetal (15-16 weeks gestation) and adult human lung. There were no differences in subcellular enzyme distribution, in Vmax activity, or in the phosphatidylglycerol-mediated stimulation of soluble enzyme activity. These results provide no support for a developmental translocation of cytidylyltransferase from a cytosolic to a microsomal location in human lung, such as that proposed to accompany the maturation of pulmonary surfactant phosphatidylcholine biosynthesis in rat. Soluble cytidylyltransferase activity from human but not rat lung was increased after manipulation in vitro. Resolution of human H form (greater than 10(3) kDa) and L form (200 kDa) enzyme by gel filtration led to an activity increase of 200%. Incubation at 37 degrees C for 2 h increased soluble enzyme recovery, although prior centrifugal removal of generated actin-rich aggregates was necessary in adult lung fractions. In contrast, 85% of soluble rat lung cytidylyltransferase was actin aggregate-associated after incubation. The apparent heteroassociation of rat and human lung enzyme with actin in the presence of poly(ethylene glycol) at 4 degrees C strongly suggested close in vitro and potential in vivo linkage. A partial co-purification of adult human lung cytidylyltransferase with actin was also consistent with this idea. We propose that some reported cytidylyltransferase translocation phenomena may be mediated by cytoskeletal interactions in vitro.

Phosphatidylcholine synthesis in the rat: the substrate for methylation and regulation by choline

Two lines of evidence led us to reexamine the possibility that methylation of phosphoethanolamine and its partially methylated derivatives, in addition to methylation of the corresponding phosphatidyl derivatives, plays a role in mammalian phosphatidylcholine biosynthesis: (a) Results obtained by Salerno and Beeler with rat [Salerno, D.M. and Beeler, D.A. (1973) Biochim. Biophys. Acta 326, 325-338] appear to strongly support such a role for methylation of phosphobases; (b) Such reactions have recently been shown to play major roles in phosphatidylcholine synthesis by higher plants [see Datko, A.H. and Mudd, S.H. (1988) Plant Physiol. 88, 854-861 and references therein]. We found that, following continuous labeling of rat liver with L-[methyl-3H]methionine for 10.4 min (intraperitoneal administration) or for 0.75 min (intraportal administration), virtually no 3H was detected in methylated derivatives of phosphoethanolamine, but readily detectable amounts of 3H were present in the base moiety of each methylated derivative of phosphatidylethanolamine. Thus, there was no indication that phospho-base methylation makes a significant contribution. Studies of cultured rat hepatoma cells showed definitively for the first time in a mammalian system that choline deprivation up-regulates the rate of flow of methyl groups originating in methionine into phosphatidylethanolamine and derivatives. Even under these conditions, methylation of phosphoethanolamine bases appeared to play a negligible role.

Effect of NaF and okadaic acid on the subcellular distribution of CTP: phosphocholine cytidylyltransferase activity in rat liver

The effect of preincubation of rat liver post-mitochondrial supernatant with NaF and okadaic acid on the subcellular distribution of CTP: phosphocholine cytidylyltransferase activity was investigated. NaF (20 mM) inhibited the time-dependent activation of cytidylyltransferase activity in post-mitochondrial supernatant. Subcellular fractionation of the post-mitochondrial supernatant revealed that cytidylyltransferase activity in the microsomal fraction was decreased and activity in the cytosolic fraction increased with time of preincubation with NaF compared to controls. Okadaic acid is a specific and potent inhibitor of type 1 and 2A phosphoprotein phosphatases. Preincubation of cytosol with 5 microM okadaic acid inhibited the time-dependent activation of cytosolic cytidylyltransferase activity. Preincubation of post-mitochondrial supernatants with 5 microM okadaic acid inhibited the time-dependent activation of cytidylyltransferase activity by 13% at 45 min and 16% at 60 min of preincubation compared to controls. Microsomal cytidylyltransferase activity was decreased 27% at 45 min and 31% at 60 min with a corresponding retention of cytosolic cytidylyltransferase activity of 21% at 45 min and 37% at 60 min of preincubation with okadaic acid compared to controls. We postulate that the activity of the type 1 and/or type 2A phosphoprotein phosphatases affect the subcellular distribution of CTP: phosphocholine cytidylyltransferase activity in rat liver.

Stimulation of CTP: phosphocholine cytidylyltransferase and phosphatidylcholine synthesis by incubation of rat hepatocytes with phospholipase A2

The effect of phospholipase A2 treatment of rat hepatocytes on CTP: phosphocholine cytidylyltransferase and phosphatidylcholine synthesis was investigated. Cytidylyltransferase is recovered from the cytosol and in a membrane-bound form with the microsomes. Digitonin treatment of cells causes rapid release into the medium of the cytosolic, but not the microsomal form of the cytidylyltransferase. Incubation of hepatocytes for 10 min with phospholipase A2 (0.9 units/dish) in the medium, resulted in a 33% decrease in the cytidylyltransferase activity released by digitonin treatment (2.5 +/- 0.15 nmol/min per mg compared to 3.9 +/- 0.10 nmol/min per mg in the control). In agreement with the digitonin experiments, incubation with 0.9 units/dish of phospholipase A2 resulted in a decrease in the cytidylyltransferase activity in the cytosol (from 4.3 +/- 0.10 nmol/min per mg to 2.6 +/- 0.14 nmol/min per mg) and a corresponding increase in the microsomal fraction (from 0.9 +/- 0.16 nmol/min per mg to 1.8 +/- 0.20 nmol/min per mg). The effect of phospholipase A2 on cytidylyltransferase translocation was concentration- and time-dependent. Incubation of hepatocytes in the presence of phospholipase A2 (0.9 units/dish) for 10 min prior to pulse-chase experiments resulted in an increase in radiolabel incorporation into phosphatidylcholine (from 2.4 +/- 0.02.10(-5) dpm/dish to 3.1 +/- 0.1.10(-5) dpm/dish) and a corresponding decrease in radiolabel associated with the choline (from 2.5 +/- 0.05.10(-5) to 1.4 +/- 0.03.10(-5) dpm) and phosphocholine fractions (from 8.5 +/- 0.07.10(-5) to 6.9 +/- 0.05.10(-5) dpm). We conclude that phospholipase A2 can cause a stimulation of CTP: phosphocholine cytidylyltransferase activity and phosphatidylcholine synthesis in cultured rat hepatocytes.

Synergistic activation of CTP:phosphocholine cytidylyltransferase by phosphatidylethanolamine and oleic acid

CTP:phosphocholine cytidylyltransferase present in rat liver cytosol was activated almost 30-fold when assayed in the presence of liposomes containing 60 mole % dioleoyl phosphatidylethanolamine (DOPE). During the assay, some of the DOPE was degraded to lysoPE and oleic acid. Whereas cytidylyltransferase activity was not affected when assayed in the presence of liposomes containing lysoPE, liposomes containing oleic acid activated the enzyme. Activation by oleic acid could be eliminated by the addition of fatty acid-free bovine serum albumin (BSA) to the assay. When cytidylyltransferase activity was measured in the presence of both BSA and liposomes containing DOPE, enzyme activity was increased almost 20-fold, as compared with assays performed in the absence of added lipid. The 1.5-fold difference in cytidylyltransferase activity observed when cytosol was assayed with DOPE containing liposomes in the absence or presence of BSA (30-fold stimulation vs 20-fold stimulation) cannot be explained by the loss of activation attributable to oleic acid alone. Activation of the enzyme in the presence of liposomes containing DOPE and oleic acid is several-fold greater than the sum of the activations caused by the individual compounds. These data suggest that PE and oleic acid act synergistically in activating the cytidylyltransferase.

Unique molecular species of phosphatidylcholine containing very-long-chain (C24-C38) polyenoic fatty acids in rat brain

Rat brain has been shown to contain polyenoic very-long-chain fatty acids (VLCFA) belonging to the n-3 and n-6 series with four, five and six double bonds and even-carbon chain lengths from 24 to 38. These fatty acids are almost exclusively located in unusual molecular species of phosphatidylcholine at the sn-1 position of the glycerol backbone, whereas saturated, monoenoic and polyenoic fatty acids with less than 24 carbon atoms are present at the sn-2 position. Polyenoic VLCFA phosphatidylcholine in neonatal rat brain is enriched with n-6 pentaenoic and n-3 hexaenoic VLCFA with up to 36 carbon atoms, whereas the corresponding phospholipid in adult rat brain mainly contains n-6 tetraenoic and n-3 pentaenoic VLCFA with up to 38 carbon atoms. The total amount of polyenoic VLCFA associated with phosphatidylcholine is highest in the brain of immature animals. Polyenoic VLCFA phosphatidylcholine appears to be predominantly confined to nervous tissue in rats, and it is envisaged that this phospholipid is of physiological significance.

Diacylglycerol production in Xenopus laevis oocytes after microinjection of p21ras proteins is a consequence of activation of phosphatidylcholine metabolism

Microinjection of p21Ha-ras proteins into Xenopus laevis oocytes induces a rapid increase of 1,2-diacylglycerol (DAG) levels. The observed alterations in DAG levels were consistent with the ability of the protein to induce maturation, measured by germinal vesicle breakdown (GVBD). Both the increase in DAG levels and GVBD activity were dependent on the ability of the proteins to undergo membrane translocation. Alterations of DAG levels or GVBD activity did not correlate with changes in the levels of inositol phosphates. However, at minimal doses sufficient to achieve maximal biological response, a biphasic increase in the amounts of phosphocholine and CDP-choline was observed. The first burst of phosphocholine and CDP-choline preceded the increase in DAG levels. The second peak paralleled the appearance of DAG. Choline kinase activity was also increased in oocyte extracts after p21ras microinjection. These results suggest that both the synthesis and degradation of phosphatidylcholine are activated after microinjection of ras proteins into Xenopus oocytes, resulting in a net production of DAG.

Inhibition of the translocation of cytidylyltransferase can be a delayed mechanism to control phosphatidylcholine biosynthesis "in vivo"

To determine whether CTP: Phosphocholine cytidylyltransferase is responsible for the previously observed decrease in phosphatidylcholine biosynthesis during thioacetamide treatment, rats were injected daily i.p. with either this agent (50 mg/kg/day) or diluent for 3 and 60 days. Microsomal and cytosolic enzymatic activities were estimated. Results demonstrated that cytosolic enzyme activity appears significantly decreased; however microsomal enzyme only appears significantly decreased after 60 days of treatment. At this time period microsomal percentage of enzyme activity is also significantly decreased. From these results it can be deduced that inhibition of enzyme translocation can be a delayed adaptative response "in vivo".

Diacylglycerol production in Xenopus laevis oocytes after microinjection of p21ras proteins is a consequence of activation of phosphatidylcholine metabolism

Microinjection of p21Ha-ras proteins into Xenopus laevis oocytes induces a rapid increase of 1,2-diacylglycerol (DAG) levels. The observed alterations in DAG levels were consistent with the ability of the protein to induce maturation, measured by germinal vesicle breakdown (GVBD). Both the increase in DAG levels and GVBD activity were dependent on the ability of the proteins to undergo membrane translocation. Alterations of DAG levels or GVBD activity did not correlate with changes in the levels of inositol phosphates. However, at minimal doses sufficient to achieve maximal biological response, a biphasic increase in the amounts of phosphocholine and CDP-choline was observed. The first burst of phosphocholine and CDP-choline preceded the increase in DAG levels. The second peak paralleled the appearance of DAG. Choline kinase activity was also increased in oocyte extracts after p21ras microinjection. These results suggest that both the synthesis and degradation of phosphatidylcholine are activated after microinjection of ras proteins into Xenopus oocytes, resulting in a net production of DAG.

Characterization of cytosolic forms of CTP: choline-phosphate cytidylyltransferase in lung, isolated alveolar type II cells, A549 cell and Hep G2 cells

The subcellular forms of cytidylyltransferase (EC 2.7.7.15) in rat lung, rat liver, Hep G2 cells, A549 cells and alveolar Type II cells from adult rats were separated by glycerol density centrifugation. Cytosol prepared from lung, Hep G2 cells, A549 cells and alveolar Type II cells contained two forms of the enzyme. These species were identical to the L-Form and H-Form isolated previously from lung cytosol by gel filtration. Liver cytosol contained only the L-Form. Rapid treatment of Hep G2 cells with digitonin released all of the cytoplasmic cytidylyltransferase activity. The released activity was present in both H-Form and L-Form. The molecular weight of L-Form was determined from sedimentation coefficients and Stokes radius values to be 97,690 +/- 10,175. Thus, the L-Form appears to be a dimer of the Mr 45,000 catalytic subunit. The f/f degrees value of 1.5 indicated that the protein molecule has an axial ratio of 10, assuming a prolate ellipsoid shape. The estimated molecular weight of the H-Form was 284,000 +/- 25,000. The H-Form was dissociated into L-Form by incubation of cytosol at 37 degrees C. Triton X-100 (0.1%) and chlorpromazine (1.0 mM) also dissociated the H-Form into L-Form. Western blot analysis indicated that both forms contained the catalytic subunit. An increase in Mr 45,000 subunit coincided with the increase in cytidylyltransferase activity in L-Form, which resulted from the dissociated of H-Form. The L-Form was dependent on phospholipid for activity. The H-Form was active without lipid. Phosphatidylinositol was present in the H-Form isolated from Hep G2 cells. The phosphatidylinositol dispersed when the H-Form was dissociated into L-Form. Phosphatidylinositol and phosphatidylglycerol cause L-Form to aggregate into a form similar to H-Form. Phosphatidylcholine/oleic acid (1:1 molar ratio) and oleic acid also aggregated the L-Form. Phosphatidylcholine did not produce aggregation. We conclude that the H-Form is the active form of cytidylyltransferase in cytoplasm. The H-Form appears to be a lipoprotein consisting of an apoprotein (L-Form dimer of the Mr 45,000 subunit) complexed with lipids. A change in the relative distribution of H-Form and L-Form in cytosol would alter the cellular activity and thus may be important in the regulation of phosphatidylcholine synthesis.

Comparison of albumin-mediated release of lysophosphatidylcholine and lysophosphatidylethanolamine from cultured rat hepatocytes

We have investigated the albumin-stimulated release from cultured rat hepatocytes of lysophosphatidylcholine derived from methylation of phosphatidylethanolamine and of lysophosphatidylethanolamine. In the absence [corrected] of albumin, neither lysophosphatidylethanolamine nor lysophosphatidylcholine was released into the culture medium. Albumin stimulated the accumulation of both phospholipids in the medium. After 2 h, 14.1 nmol of lysophosphatidylcholine and 2.0 nmol of lysophosphatidylethanolamine per 3 x 10(6) cells had accumulated in the medium. The rate of release of [3H]ethanolamine-labelled lysophosphatidylethanolamine was rapid in the first 2 h and then was decreased, whereas there was a 1 h lag in the release of [3H]ethanolamine-labelled lysophosphatidylcholine. This apparent lag probably reflected the time necessary for the synthesis of phosphatidylcholine from phosphatidylethanolamine in the cells. Albumin caused a decrease in labelled cellular lysophosphatidylethanolamine and lysophosphatidylcholine which only partially accounted for the accumulation of the labelled phospholipids in the medium. Albumin also stimulated the release of labelled phosphatidylethanolamine (almost 3-fold) and phosphatidylcholine (2-fold) into the medium. There was no detectable change in the labelling of the cellular pools of these phospholipids, most likely owing to the large amounts in the cells compared with the medium. The labelled lysophospholipids did not arise from catabolism of the parent phospholipid in the medium. Analysis of the fatty acids of the secreted lysophospholipids showed a preferential release of unsaturated fatty acyl species of lysophosphatidylcholine, whereas lysophosphatidylethanolamine contained similar amounts of saturated and unsaturated fatty acids.

GPC phosphodiesterase and phosphomonoesterase activities of renal cortex and medulla of control, antidiuresis and diuresis rats

Glycerylphosphorylcholine (GPC) concentration was reported to be elevated in renal medulla of experimental animals deprived of water. The activities of GPC phosphodiesterases were similar in homogenates and membrane subfractions of renal cortex prepared from control, diuresis and antidiuresis rats. There were no differences in these preparations' ability to hydrolyze phosphorylcholine. In contrast, there was a nearly 50% reduction of non-specific phosphomonoesterase activity, using p-nitrophenylphosphate as substrate and membrane subfractions prepared from the antidiuresis animals. It is suggested that as a consequence, a pathway for the formation from L-alpha-glycerylphosphate is activated.

Partial purification and characterization of CTP:cholinephosphate cytidylyltransferase from castor bean endosperm

CTP: cholinephosphate cytidylyltransferase (EC 2.7.7.15) has been purified approximately 600-fold from postgermination endosperm of castor bean. The enzyme was solubilized with n-octyl beta-D-glucopyranoside and then subjected to ion exchange and gel filtration chromatography. The Km's of the purified enzymatic activity were 0.37 and 1.1 mM for CTP and choline phosphate, respectively. Magnesium was required for activity. The purified cytidylyltransferase activity was inhibited by both phosphate and ATP. The extent of ATP inhibition was dependent on preincubation time, temperature, and Mg2+ and Ca2+ concentrations. The possible regulation of cytidylyltransferase in castor bean endosperm by protein phosphorylation is discussed.

The acquisition of lipids AfrAfrican trypanosomes

Bloodstream forms of African trypanosomes are dependent on their host for fatty acids, choline and other components of membrane lipids. The bulk of their choline requirement is met by their ability to take up lysophospholipids from the host tissue fluids. Trypanosoma brucei has ocyltransferose and phospholipase A(1) activities for the metabolism of exogenous lysophospholipids. The rate of uptake of lysophospholipids can be controlled by changes in the extrocellulor concentration of fatty ocyl-coenzyme A, and this control has potential for chemotherapy.

Uptake of exogenous phosphatidylserine by human neuroblastoma cells stimulates the incorporation of [methyl-14C]choline into phosphatidylcholine

The phosphatidylserine (PtdSer) content of human cholinergic neuroblastoma (LA-N-2) cells was manipulated by exposing the cells to exogenous PtdSer, and the effects on phospholipid content, membrane composition, and incorporation of choline into phosphatidylcholine (PtdCho) were investigated. The presence of liposomes containing PtdSer (10-130 microM) in the medium caused time- and concentration-dependent increases in the PtdSer content of the cells, and smaller and slower increases in the contents of other membrane phospholipids. The PtdSer levels in plasma membrane and mitochondrial fractions prepared by discontinuous sucrose density gradient centrifugation increased by 50 and 100%, respectively, above those in control cells after 24 h of exposure to PtdSer (130 microM). PtdSer caused a concomitant, concentration-dependent increase of up to twofold in the incorporation of [methyl-14C]choline chloride into PtdCho at a choline concentration (8.5 microM) compatible with activation of the CDP-choline pathway, suggesting that the levels of PtdSer in membranes may serve as a stimulus to regulate overall membrane composition. PtdSer caused a mean increase of 41% in PtdCho labeling, but the phorbol ester, phorbol 12-myristate 13-acetate (PMA), which stimulates PtdCho synthesis in a number of cell lines, increased [14C]PtdCho levels by only 14% in LA-N-2 cells, at a concentration (100 nM) which caused complete translocation of the calcium- and phospholipid-dependent enzyme protein kinase C to the membrane. The translocation was inhibited by prior exposure of the cells to PtdSer. Treatment with PMA for 24 h diminished protein kinase C activity by 80%, but increased the labeling of PtdCho in both untreated and PtdSer-treated cells. These data suggest that uptake of PtdSer by LA-N-2 cells alters both the phospholipid composition of the membrane and synthesis of the major membrane phospholipid PtdCho; the latter effect does not involve activation of protein kinase C.

Regulation of phosphatidylcholine biosynthesis in activated alveolar type II cells

The biosynthesis of phosphatidylcholine was studied in a population of activated Type II cells isolated from the lungs of rats treated with silica. Type II cells were separated by centrifugal elutriation into two populations, designated Type IIA and Type IIB. The Type IIB or activated population consisted of Type II cells that were larger than normal cells; Type IIA cells were morphologically similar to normal Type II cells. Type IIB cells incorporated more [Me-14C]choline into both total phosphatidylcholine and disaturated phosphatidylcholine than did Type IIA or control Type II cells. Measurement of the pool sizes of the choline-containing precursors to phosphatidylcholine indicated that the biosynthesis of phosphatidylcholine was increased 4- to 5-fold in Type IIB cells. Increased conversion of cholinephosphate to CDP-choline was associated with increased phosphatidylcholine biosynthesis in Type IIB cells. Cholinephosphate cytidylyltransferase activity was increased approximately threefold in Type IIB cells. Subcellular fractionation indicated that essentially all of the increase in cytidylyltransferase activity was associated with the particulate fraction (100,000 x g pellet). In Type IIB cells, the particulate fraction contained 83% of the total cellular cytidylyltransferase activity; in control cells, this fraction contained 67% of the total activity. The specific activity of the cytidylyltransferase associated with the particulate fraction was increased twofold in Type IIB cells. The specific activity of the cytosolic enzyme was similar to that in control cells. Cholinephosphotransferase specific activity was increased approximately twofold in the activated Type II cells. The specific activity of choline kinase was the same as that in control Type II cells. These results demonstrate that the increased biosynthesis of phosphatidylcholine in Type IIB cells is a result of stimulation of the CDP-choline pathway. This study indicates that both cholinephosphate cytidylyltransferase and cholinephosphotransferase may be involved in regulating the de novo biosynthesis of phosphatidylcholine in alveolar Type II cells.

Effects of chlorpromazine and trifluoperazine on choline metabolism and phosphatidylcholine biosynthesis in cultured chick heart cells under normoxic and anoxic conditions

The effects of chlorpromazine and trifluoperazine on phosphatidylcholine biosynthesis in the heart were investigated in isolated cardiac cells under normoxic and anoxic conditions. The cells were obtained from 7-day-old chick embryos and were maintained in culture. After 96 hr, cells were maintained either in an incubator with oxygen at room air concentration (normoxia) or in an incubator containing 95% nitrogen and 5% CO2 (anoxia). Pulse chase experiments with [methyl-3H]choline were conducted using a 2-hr incubation with choline. Chlorpromazine and trifluoperazine at 10(-5) M produced a significant (P less than 0.05) increase in the incorporation of choline into both phosphocholine and phospholipid. High concentrations of chlorpromazine or trifluoperazine i.e. 10(-4) M, damaged myocardial cells as reflected in a significant (P less than 0.05) reduction in cellular protein and a further reduction in labelled choline in phosphocholine or phospholipid after adjusting for the lower protein concentrations. Anoxia altered choline metabolism but 6 hr of anoxia was the minimum time needed for the effect to be observable. Anoxia, for 24 hr, produced a significant (P less than 0.05) reduction in labelled choline in phosphocholine without a significant change in incorporation of label in phospholipid or cellular protein. Both chlorpromazine and trifluoperazine at 10(-5) M prevented anoxic-induced changes in phosphocholine metabolism. Thus, chlorpromazine and trifluoperazine affect phospholipid biosynthesis in cardiac cells and prevent anoxia-induced changes in phosphatidylcholine biosynthesis.

Dideoxycytidine, an anti-HIV drug, selectively inhibits growth but not phosphatidylcholine metabolism in neuroblastoma and glioma cells

Dideoxycytidine (ddCyd), an inhibitor of AIDS-related HIV, has been examined for effects on cell proliferation and phosphatidylcholine synthesis in tumor lines of nervous system origin. Uptake and metabolism of [3H]ddCyd, observed in all cells, was greatest in one human neuroblastoma line, HTB-10. Growth of the HTB-10 line was markedly inhibited by 40 microM ddCyd, whereas growth of C6 glioma and N1E-115 or HTB-11 neuroblastoma cells was unaltered. Phosphatidylcholine synthesis in the presence or absence of stimulation by phorbol ester was not specifically altered by ddCyd. Thus, ddCyd was incorporated and inhibited growth in a cell-specific manner but had little effect on cytidine-dependent phospholipid synthesis. This suggests that some cells derived from the nervous system may be more susceptible than others with respect to the positive and negative effects of ddCyd as a potential antiviral drug.

Co-ordinate control of CDP-choline and phosphatidylethanolamine methyltransferase pathways for phosphatidylcholine biosynthesis occurs in response to change in diet fat

Brain microsomal and synaptic plasma membrane phosphatidylcholine composition and biosynthetic activity were examined in relation to the composition of diet fat fed. Phosphocholinetransferase and methyltransferase activities are shown to be modulated by the diet, and by changes in the membrane phospholipid content of long-chain polyunsaturated fatty acids. This physiological modulation is co-ordinated such that the rate of phosphatidylcholine synthesis via one route is inversely regulated with activity of the alternate pathway.

Inhibition of phosphatidylethanolamine synthesis by glucagon in isolated rat hepatocytes

Exposure of isolated rat hepatocytes to glucagon or chlorophenylthio cyclic AMP led to an inhibition of the incorporation of [1,2-14C]ethanolamine into phosphatidylethanolamine. Pulse-chase experiments and measurement of the activities of the enzymes involved in the CDP-ethanolamine pathway provided evidence that the inhibitory effect of glucagon on the synthesis de novo of phosphatidylethanolamine was not caused by a diminished conversion of ethanolamine phosphate into CDP-ethanolamine. The observations suggested that the glucagon-induced inhibition of the biosynthesis of phosphatidylethanolamine is probably due to a decreased supply of diacylglycerols, resulting in a decreased formation of phosphatidylethanolamine from CDP-ethanolamine and diacylglycerols.

Regulation of phosphatidylcholine biosynthesis in Plasmodium-infected erythrocytes

Plasmodium knowlesi-infected erythrocytes efficiently incorporated choline and metabolize it into phosphatidylcholine via the de novo Kennedy pathway. No formation of either betaine or acetylcholine was detected. At physiological concentrations of external choline, isotopic equilibrium between intracellular choline and phosphocholine was reached in less than 1 h, whereas labeled phosphatidylcholine accumulated constantly, until at least 210 min. During this time, intracellular CDP-choline remained quite low compared to phosphocholine, which suggests that choline-phosphate cytidylyltransferase (EC 2.7.7.15) is the rate-limiting step of the Kennedy pathway. However, this activity was probably not saturated in situ by phosphocholine, since the external choline concentration, up to 100 microM, can regulate phosphatidylcholine biosynthesis via the level of intracellular phosphocholine. This was corroborated by the respective velocities and affinity characteristics of the three enzymatic steps involved in the Kennedy pathway. These results, together with the localization of both choline metabolites and enzyme activities, provide a precise scheme of the dynamics of de novo phosphatidylcholine biosynthesis. Concerning the alternative pathway for phosphatidylcholine biosynthesis via the methylation of phosphatidylethanolamine, we show that an increase in de novo phosphatidylcholine biosynthesis could instigate a concomitant decrease in the steps of phosphatidylethanolamine methylation, indicating that the parasite is able to modulate its phosphatidylcholine biosyntheses.

Induction of phosphatidylcholine biosynthesis via CDPcholine pathway in lung and liver of rats following intratracheal administration of DDT and endosulfan

The induction of phosphatidylcholine (PC) biosynthesis via the CDPcholine pathway in lung and liver of rats has been shown following the intratracheal administration of 1,1,1-trichloro-2m2-bis(p-chlorophenyl) ethane (DDT) (5 mg/100 g body weight) and endosulfan (1 mg/100 g body weight) for 3 days. Controls received only the vehicle solution (groundnut oil, 0.1 m1/100 g body weight). The treatment of DDT and endosulfan significantly increased the PC contents and the incorporation of radioactive [methyl-3H]choline into PC of lung and liver microsomes. The incorporation of radioactive [methyl-14C]methionine into microsomal PC of lung and liver was not affected significantly by treatment with either of the insecticides. 1,4,5,6,7-hexachloro-5-norbornene-2,3-dimethano cyclic sulfite (endosulfan) administration significantly increased the activity of choline kinase and phosphocholine cytidylyltransferase (both cytosolic and microsomal) of lung, whereas DDT increased the activity of only latter. In liver, both DDT and endosulfan administration significantly increased the activity of choline kinase and phosphocholine cytidylyltransferase (both cytosolic and microsomal). However, the activity of phosphocholinetransferase was not affected in both lung and liver microsomes of rats treated with these insecticides. The PC precursor pool sizes, choline and phosphorylcholine, of lung and liver tissues were not altered by DDT and endosulfan treatments. The present results suggest that the increased level of PC and incorporation of radioactive [methyl-3H]choline into microsomal PC could be the result of increased activity of choline kinase and phosphocholine cytidylyltransferase of lung and liver of rats following intratracheal administration of DDT and endosulfan.

Effects of ethanol feeding on hepatic lipid synthesis

Rats were fed a high-fat, liquid diet containing either 36% of total calories as ethanol or an isocaloric amount of sucrose, for a period up to 35 days. At different time intervals we measured the effects of ethanol administration on the activities of a number of key enzymes involved in hepatic lipid synthesis. At the start of the experimental period the activities of acetyl-CoA carboxylase and fatty acid synthase, measured in liver homogenates, increased in the control as well as in the ethanol-fed group. After 35 days these enzyme activities were still elevated but there were no significant differences between the two groups. In hepatocytes isolated from controls as well as from ethanol-fed rats, short-term incubations with ethanol induced an increase in the rate of fatty acid synthesis and in the activities of acetyl-CoA carboxylase and fatty acid synthase. However, no alterations in the regulation of these enzymes by short-term modulators of lipogenesis were apparent in hepatocytes isolated from alcohol-treated animals. The results do not indicate a major role for the enzymes of de novo fatty acid synthesis in the development of the alcoholic fatty liver. The amount of liver triacylglycerols increased in ethanol-fed rats during the entire treatment period, whereas the hepatic levels of phosphatidylcholine and phosphatidylethanolamine were not affected by ethanol ingestion. Ethanol administration for less than 2 weeks increased the activities of phosphatidate phosphohydrolase, diacylglycerol acyltransferase, and microsomal phosphocholine cytidylyltransferase, whereas the cytosolic activity of phosphocholine cytidylyltransferase was slightly decreased. Upon prolonged ethanol administration the activities of these enzymes were slowly restored to control values after 35 days, suggesting development of some kind of adaptation. It is interesting that, although the activities of phosphatidate phosphohydrolase and diacylglycerol acyltransferase were restored to the levels found in the control rats, this effect was not accompanied by a stabilization or decrease of the concentration of hepatic triacylglycerols.

Changes in rat lung microsomal lipids after p-xylene: relationship to inhibition of benzo[a]pyrene metabolism

The relationship between p-xylene's effects on microsomal membranes, cytochrome P-450, and benzo[a]pyrene (BaP) metabolism was studied. p-Xylene (1 g/kg, ip, 1 h) inhibited 3-hydroxy BaP (3-OH) formation and decreased arylhydrocarbon hydroxylase (AHH) activity approximately 40% in rat lung microsomes. BaP dihydrodiol and quinone formation were unchanged by p-xylene administration. Cytochrome P-450 was below the limit of detection in lung microsomes from p-xylene-treated rats. Total phospholipid (PL) and phosphatidylcholine (PC) in microsomal membranes were decreased 28% and 17%, respectively. Cholesterol (CL), phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylinositol (PI), and sphingomyelin (SM) were unchanged. The net activity of enzymes involved in the synthesis of PC, phosphatidylethanolamine-N-methyltransferase I and II (PMT I and PMT II), was slightly elevated by p-xylene. PL/CL and PC/PE ratios, indicators of membrane fluidity, were decreased 34% and 13%, respectively, in microsomes from p-xylene-treated rats. Analysis of fluidity by fluorescence polarization showed that the actual fluidity of treated microsomes was slightly decreased (5%) as compared to controls. The decrease in P-450, PL, and PC is considered to contribute to the inhibition of BaP metabolism.

Lipid composition of platelets from patients with atherosclerosis: effect of purified eicosapentaenoic acid ethyl ester administration

Although eicosapentaenoic acid (EPA) has been shown to have beneficial effects in the prevention of atherosclerosis, the mechanism by which these effects occur is not entirely clear. We investigated the lipid composition of platelets in paired subjects with and without atherosclerotic disease, either hypercholesterolemic (low density lipoprotein [LDL] cholesterol [Chol] greater than or equal to 170 mg/dl) or normocholesterolemic (LDL-Chol less than 170 mg/dl). Platelets from patients with atherosclerotic disease had a lower phosphatidylcholine (PC)/Chol ratio, when compared with those from patients without atherosclerotic disease, irrespective of LDL-Chol levels. Eleven patients with atherosclerotic disease were treated with purified EPA ethyl ester (1.8 g/day), and changes in lipid composition of platelets were investigated. Plasma levels of total Chol and LDL-Chol decreased significantly after EPA administration. The phospholipid (PL)/Chol ratio and the PC/Chol ratio in platelets from patients with atherosclerotic disease increased significantly after 4-10 wk EPA treatment. The EPA content in platelets increased, while the arachidonic acid (AA) content decreased. EPA-induced changes in the PL/Chol and the PC/Chol ratios of platelets, as well as fatty acyl chain shifts, may be related to the beneficial effects in preventing atherosclerosis, possibly by increase in the membrane fluidity.

Alterations in lipid metabolism and cellular morphology in the liver of female rats treated with ethmozine

Alterations in lipid metabolism and cellular morphology in the liver were examined in female rats treated with 100 mg Ethmozine/kg/day for 7 days. The incorporation of either [3H]acetate with [methyl-14C]choline, or [methyl-14C]methionine was used to monitor the effect of the drug on neutral and phospholipid syntheses. Ethmozine (ETH) reduced the incorporation of choline into phosphatidylcholine (PC) by 50%, but the transmethylation of phosphatidylethanolamine to form PC was unaffected. The formation of lyso-PC was reduced by one-half irrespective of the donor radiolabel. An accumulation of both micro- and macrovesicles (fat) was found in the centri- and midlobular zones of the liver, which is likely the result of increased synthesis and decreased secretion of triacylglycerol (TAG). Incorporation of acetate into TAG was increased fivefold by ETH treatment, and to a lesser degree into cholesterol and cholesterylester/squalene.

Fatty acids stimulate phosphatidylcholine synthesis and CTP:choline-phosphate cytidylyltransferase in type II pneumocytes isolated from adult rat lung

The regulation by cyclic AMP and fatty acids of phosphatidylcholine (PC) synthesis in rat alveolar type II cells was studied. In contrast with results with hepatocytes, cyclic AMP and its potent chlorophenylthio analogue had no inhibitory effect on [Me-14C]choline incorporation into PC in pulse-chase studies with alveolar type II cells. The inclusion of the fatty acids palmitate, oleate or linoleate in the chase incubation medium stimulated the incorporation of [Me-14C]choline into PC by type II cells. The effect of palmitate, which was more pronounced than that of the other fatty acids, appeared to be concentration-dependent. Increased [Me-14C]choline incorporation into PC was paralleled by an accelerated disappearance of the radiolabel from choline phosphate, which is consistent with an activation of CTP:choline-phosphate cytidylyltransferase. This enzyme is considered to be rate-limiting in the synthesis of PC de novo by type II cells. As fatty acids are also substrate for PC synthesis, their effect could also be due to compensation for a fatty acid deficiency. To test this possibility, fatty acid synthesis in the type II cells was stimulated by addition of lactate. Even then, an additional stimulation of PC synthesis by palmitate was observed, which supports the regulatory influence of exogenous fatty acids. Incubation of type II cells in the presence of 0.2 mM-palmitate resulted in a 45% increase in the membrane-bound CTP:choline-phosphate cytidylyltransferase activity, whereas the soluble activity remained unchanged. Choline kinase activity in the soluble fraction increased by 48%. However, the increase in choline kinase is unlikely to be responsible for the increased metabolic flux through the choline phosphate pathway, because there is a relatively large pool of choline phosphate in type II cells. Therefore it is suggested that the microsomal CTP:choline-phosphate cytidylyltransferase is the form of this enzyme which is active in surfactant PC synthesis, and possibly has a regulatory role in this pathway.

Albumin stimulates the release of lysophosphatidylcholine from cultured rat hepatocytes

The effect of albumin on the release of [3H]lysophosphatidylcholine from cultured rat hepatocytes prelabelled with [Me-3H]choline was studied. In the absence of serum and albumin from the medium, the cells released essentially no [3H]lysophosphatidylcholine. Albumin stimulated this process dramatically, and it reached a plateau at 2 mg/ml. After an initial lag of 30 min, the release of [3H]lysophosphatidylcholine was linear for at least 4 h. At low concentrations, albumin slightly stimulated [3H]phosphatidylcholine release. The albumin had no measurable effect on the metabolism of cellular [3H]phosphatidylcholine, [3H]lysophosphatidylcholine or [3H]glycerophosphocholine. In addition, albumin did not alter the release of 3H-labelled water-soluble compounds, including [3H]glycerophosphocholine, into the medium. The possibility that the [3H]lysophosphatidylcholine was arising from catabolism of [3H]phosphatidylcholine in the medium by secreted enzymes was excluded. The effect on [3H]lysophosphatidylcholine secretion was also observed when the cells were incubated with alpha-cyclodextrin, a cyclic polysaccharide that has the ability to bind lysophosphatidylcholine. The albumin-released lysophosphatidylcholine was enriched in unsaturated fatty acids. Alteration of the fatty acid composition of cellular phosphatidylcholine gave rise to parallel changes in phosphatidylcholine and lysophosphatidylcholine in the medium. It is concluded that phosphatidylcholine is constantly being degraded in the rat hepatocyte to lysophosphatidylcholine which is released into the medium only when a suitable acceptor is present.

Phosphatidylcholine metabolism in cultured cells: catabolism via glycerophosphocholine

The catabolism of phosphatidylcholine (PtdCho) has been studied in cultured murine neuroblastoma (N1E-115), C6 glioma, rat brain primary glia, and human fibroblast cells. Cells were pulse labelled for 96 h with [methyl-3H]choline followed by a chase for up to 24 h in medium containing 4 mM choline. Measurement of the radioactivity and mass of choline-containing compounds in these cells indicated that the major degradative pathway is PtdCho----lysophosphatidylcholine (lysoPtdCho)----glycerophosphocholine (GroPCho)----choline. At all times during the chase, PtdCho, sphingomyelin and lysoPtdCho comprised 72-92% of the cell-associated radioactivity; the remaining 10-30% was water-soluble and was chiefly GroPCho (30-80%) in all cell lines. In fibroblasts, however, phosphocholine (PCho) was also a major labelled water-soluble component (33-54%). The specific activity of GroPCho closely parallelled that of PtdCho in fibroblasts, but decreased faster than PtdCho in C6 and N1E-115 cells. We postulate that this may be due to distinct pools of PtdCho in the cell with differing rates of turnover. The changes in specific activity of PCho suggest that the major portion is formed by synthesis rather than as a degradative product. However, the inability to reduce the specific activity of this fraction to that of the intracellular choline suggests that a portion may be derived from either PtdCho or GroPCho.

Density-dependent variations in the lipid content and metabolism of cultured human keratinocytes

Cultured human foreskin keratinocytes have been utilized extensively to study modulations in protein content during epidermal differentiation. In this study we examined their usefulness as a model system for differentiation-linked changes in lipid content and metabolism. First-to-third passage keratinocytes were grown in 10% fetal calf serum on a mitomycin-treated 3T3 feeder layer and harvested at intervals before, during, and after reaching confluence for determination of lipid, protein, and DNA content. Lipid synthesis, determined as acetate incorporation into lipid, was most active in pre-confluent cultures and at all times closely paralleled the growth activity of the cultures. Post-confluent cultures were characterized by an increase in total lipid content and by increased triglyceride content and synthesis. Pulse-chase studies demonstrated that labeling of the triglyceride pool was labile and suggested that even in post-confluent cultures, triglycerides provide a fatty acid reservoir for phospholipid biosynthesis. A novel band, which co-migrated with monoalkyldiacylglycerol in two solvents systems was present in confluent and post-confluent cultures, but absent in pre-confluent cultures. Sphingolipids constituted less than 10% of total lipid at all stages of growth, and cholesterol sulfate was present only in small quantities. These studies illustrate the relationship of lipid synthesis to growth and demonstrate that human foreskin keratinocytes, cultured under standard conditions, reproduce incompletely the lipid composition of epidermis in vivo.

Phosphatidylcholine synthesis by peritoneal mesothelium: its implications for peritoneal dialysis

This study investigated the possibility that the peritoneum is capable of synthesizing phosphatidylcholine (PC), a lubricant surfactant, in an amount similar to that produced by pulmonary alveoli. The synthesis of PC by rat lung (positive control), liver (negative control), and transparent mesentery (test tissue) was determined by in vitro incubation of these tissues in the presence of (methyl-14C) choline chloride for three hours at 37 degrees C in Warburg flasks. All lipid material present in tissue and incubation media was extracted by the Folch technique. Carrier egg PC was added to each sample and total PC was isolated using high performance thin-layer chromatography. The PC fractions were counted for total radioactivity. No statistically significant difference was observed between the mean radioactive value for mesenteric tissue compared with lung tissue. The mean radioactive value for liver when compared with mesenteric and lung tissue was significantly lower (P less than 0.001). Thus, under the conditions of the experiment, we have demonstrated for the first time that peritoneal tissue is capable of synthesizing PC in amounts similar to that produced by the lung. Electron microscopy of transparent mesentery (test tissue) showed that mesothelium constituted the bulk of the cell population. Therefore, it is most likely that the PC that has been detected in peritoneal fluid is produced by the mesothelial cell. These findings are of significant relevance to developing concepts on the role of surfactant phospholipids in the physiology of peritoneal dialysis.

Effect of diethyl ether on phosphatidylcholine biosynthesis in hamster organs

The effect of diethyl ether anesthesia on phosphatidylcholine biosynthesis in hamster organs was investigated. Ether administration did not affect the incorporation of radioactive choline into phosphatidylcholine in the liver, heart, lung, brain and spleen. A significant (29%) decrease in the labeling of phosphatidylcholine was detected in the kidney of ether-treated hamsters. Reduction in phosphatidylcholine labeling was not due to a diminished radioactive choline uptake but a decrease in the conversion of phosphocholine to CDP-choline. The accumulation of labeled phosphocholine was caused by the translocation of CTP:phosphocholine cytidylyltransferase from microsomal (more-active) form to cytosolic (less-active) form. Ether administration appears to modulate the cytidylyltransferase in hamster kidney differently than that in other hamster organs.

Stimulation of protein phosphorylation in mixed glial cell primary cultures and subcultures by the phorbol ester 12-O-tetradecanoylphorbol-13-acetate

Glial cell primary cultures consisting of protoplasmic and fibrous astrocytes, oligodendrocytes and progenitor glial cells incubated in medium containing 0.5% foetal calf serum and treated with 25 nM 12-o-tetradecanoylphorbol-13-acetate (TPA) for periods between 15 and 60 min showed a stimulation of protein phosphorylation which was most prominent in a polypeptide with a molecular weight of about 80,000 Da. Glial subcultures consisting mainly of Type 2 astrocytes, oligodendrocytes and progenitor glia showed a similar TPA stimulation of 80,000 Da protein phosphorylation detectable within 1 min of phorbol ester addition. TPA treatment of primary glial cultures led to an enhancement of phospholipid turnover but exposure of primary glial cultures to concentrations of TPA up to 250 nM caused no morphological change in protoplasmic astrocytes. 4-Phorbol (4-PH) or dimethylsulfoxide (DMSO) was without effect on protein phosphorylation or lipid turnover in glial cultures.