Effects of vasopressin on the synthesis of phosphatidylethanolamines and phosphatidylcholines by isolated rat hepatocytes

Regulation of Phosphatidylethanolamine Homeostasis—The Critical Role of CTP:Phosphoethanolamine Cytidylyltransferase (Pcyt2)

Phosphatidylethanolamine (PE) is the most abundant lipid on the protoplasmatic leaflet of cellular membranes. It has a pivotal role in cellular processes such as membrane fusion, cell cycle regulation, autophagy, and apoptosis. CTP:phosphoethanolamine cytidylyltransferase (Pcyt2) is the main regulatory enzyme in de novo biosynthesis of PE from ethanolamine and diacylglycerol by the CDP-ethanolamine Kennedy pathway. The following is a summary of the current state of knowledge on Pcyt2 and how splicing and isoform specific differences could lead to variations in functional properties in this family of enzymes. Results from the most recent studies on Pcyt2 transcriptional regulation, promoter function, autophagy, and cell growth regulation are highlighted. Recent data obtained from Pcyt2 knockout mouse models is also presented, demonstrating the essentiality of this gene in embryonic development as well as the major physiological consequences of deletion of one Pcyt2 allele. Those include development of symptoms of the metabolic syndrome such as elevated lipogenesis and lipoprotein secretion, hypertriglyceridemia, liver steatosis, obesity, and insulin resistance. The objective of this review is to elucidate the nature of Pcyt2 regulation by linking its catalytic function with the regulation of lipid and energy homeostasis.

Thyroxine stimulates the acylation of lysophosphatidylethanolamine in rat heart

The acylation of cardiac lysophosphatidylethanolamine (LPE) was examined in rats treated with thyroid hormone. Rats were treated for five consecutive days with thyroxine (250 microg/kg) and controls were treated with saline. On the sixth day after an overnight fast, the hearts were removed and perfused in the Langendorff mode with 0.1 mM [1-14C]oleic acid. Radioactivity incorporated into phosphatidylethanolamine (PE) was increased 1.5-fold (P < 0.025) compared to controls. Radioactivity incorporated into phosphatidylcholine was not effected. The pool size of phosphatidylethanolamine and de novo biosynthesis of this phospholipid from [3H(G)]serine or [1,2-14C]ethanolamine were unaltered by thyroxine treatment. Treatment of rats with thyroxine resulted in a 1.5-fold (P < 0.025) increase in the relative percent of oleic acid in cardiac phosphatidylethanolamine. Thyroxine treatment resulted in a 1.8-fold (P < 0.025) increase in cardiac microsomal acyl-coenzyme A:1-acyl glycerophosphorylethanolamine acyltransferase activity compared to controls whereas, phospholipase A, acyl-coenzyme A hydrolase and fatty acyl-coenzyme A synthase activities were unaltered. The results demonstrate that the reacylation of cardiac LPE is regulated by thyroid hormone.

Cloning and expression of mouse liver phosphatidylserine synthase-1 cDNA. Overexpression in rat hepatoma cells inhibits the CDP-ethanolamine pathway for phosphatidylethanolamine biosynthesis

In eukaryotic cells, phosphatidylserine (PtdSer) is synthesized by two distinct synthases on the endoplasmic reticulum by a base-exchange reaction in which the polar head-group of an existing phospholipid is replaced with serine. We report the cloning and expression of a cDNA for mouse liver PtdSer synthase-1. The deduced protein sequence is >90% identical to that of PtdSer synthase-1 from Chinese hamster ovary cells and a sequence from a human myeloblast cell line. PtdSer synthase-1 cDNA was stably expressed in M.9.1.1 cells which are mutant Chinese hamster ovary cells defective in PtdSer synthase-1 activity, are ethanolamine auxotrophs, and have a reduced content of PtdSer and phosphatidylethanolamine (PtdEtn). The growth defect of M.9.1.1 cells was eliminated, and a normal phospholipid composition was restored in the absence of exogenous ethanolamine, implying that the cloned cDNA encoded PtdSer synthase. Mouse liver PtdSer synthase-1 was also expressed in McArdle 7777 rat hepatoma cells. In addition to a 3-fold higher in vitro serine-exchange activity, these cells also exhibited enhanced choline- and ethanolamine-exchange activities and incorporated more [3H]serine into PtdSer than did control cells. However, the levels of PtdSer and PtdEtn in cells overexpressing PtdSer synthase-1 activity were not increased. Excess PtdSer produced by the transfected cells was rapidly decarboxylated to PtdEtn and the degradation of PtdSer, and/or PtdEtn derived from PtdSer, was increased. Moreover, the CDP-ethanolamine pathway for PtdEtn biosynthesis was inhibited. These data suggest that (i) cellular levels of PtdSer and PtdEtn are tightly controlled, and (ii) the metabolism of PtdSer and PtdEtn is coordinately regulated to maintain phospholipid homeostasis.

Different contribution of phospholipid and triacylglycerol metabolism to esterification of free intracellular arachidonate: a study on SK-N-BE(2) human neuroblastoma cells

When SK-N-BE(2) human neuroblastoma cells were exposed for 1h to growth medium supplemented with [14C]arachidonic acid (AA) at final concentrations ranging from 1 microM to 100 microM, an amount of this fatty acid was uptaken ranging form a 2% to a 120% of that present in cells at steady state. As more [14C]AA was uptaken by cells, a larger fraction was progressively incorporated into triacylglycerols (TAG) in comparison to phospholipids (PL), with minor amounts remaining in a free form. By gas chromatographic analysis it was estimated that TAG from cells grown in ordinary medium contained about 2 nmoles AA per mg protein, but, after 1 h exposure to medium supplemented with 100 microM AA (label-free) this value rose to about 28 nmoles/mg protein; furthermore, as estimated on the basis of total fatty acid content, TAG mass was increased by a 16%. Cell exposure to medium enriched with 100 microM AA did not cause PL mass changes, whereas AA content was significantly increased only in phosphatidylcholine. Medium enrichment with 100 microM AA dramatically enhanced [3H]glycerol incorporation into TAG, as assessed after 1 h cell pulse, with minor but significant changes observed also for phosphatidylinositol and phosphatidylethanolamine, but not for phosphatidylcholine. In the light of these data, the contribution of PL and TAG to the removal of free intracellular AA is discussed.

Enhancement of phospholipid hydrolysis in vasopressin-stimulated BHK-21 and H9c2 cells

The hydrolysis of phospholipids in vasopressin-stimulated baby hamster kidney (BHK)-21 and H9c2 myoblastic cells was investigated. Phosphatidylcholine and phosphatidylethanolamine in these cells were pulse labelled with [3H]glycerol, [3H]myristate, [3H]choline or [3H]ethanolamine, and chased with the non-labelled precursor until linear turnover rates were obtained. When cells labelled with [3H]glycerol or [3H]myristate were stimulated by vasopressin, no significant decrease in the labelling of phosphatidylcholine was detected, but the labelling of phosphatidic acid was elevated. However, the labellings of phosphatidylethanolamine and its hydrolytic product were not affected by vasopressin stimulation. When the cells were pulse labelled with [3H]-choline, vasopressin stimulation caused a decrease in the labelled phosphatidylcholine with a corresponding increase in the labelled choline. The apparent discrepancy between the two types of labelling might be explained by the recycling of labelled phosphatidic acid back into phosphatidylcholine, thus masking the reduction in the labelled phospholipid during vasopressin stimulation. Alternatively, the labelled choline produced by vasopressin stimulation was released into the medium, thus reducing the recycling of label precursor back into the phospholipid and making the decrease in the labelling of phosphatidylcholine readily detectable. Further studies revealed that vasopressin treatment caused an enhancement of phospholipase D activity in these cells. The presence of substrate-specific phospholipase D isoforms in mammalian tissues led us to postulate that the differential stimulation of phospholipid hydrolysis by vasopressin was caused by the enhancement of a phosphatidylcholine-specific phospholipase D in both BHK-21 and the H9c2 cells.

Biphasic modulation of choline uptake and phosphatidylcholine biosynthesis by vasopressin in rat cardiac myocytes

The effect of vasopressin on choline uptake and phosphatidylcholine biosynthesis in isolated rat heart myocytes was investigated. Myocytes were incubated with labelled choline in the presence of 0.05-1.0 microM vasopressin. Uptake of choline was enhanced (25%) by a low concentration (0.2 microM) of vasopressin, but was attenuated (19%) by a higher vasopressin concentration (1.0 microM). The biosynthesis of phosphatidylcholine was also affected by vasopressin in a biphasic manner. At low concentrations of vasopressin, a general increase in cytosine triphosphate:phosphocholine cytidylyltransferase activity was observed that caused an enhanced conversion of phosphocholine to phosphatidylcholine via the cytidine diphosphocholine pathway. At high vasopressin concentrations, a decrease in the activity of cytidylyltransferase was detected, which was caused by the translocation of the enzyme from the microsomal fraction to the cytosolic fraction. The decrease in enzyme activity coincides with a reduction in the conversion of labelled phosphocholine to phosphatidylcholine. In view of the fact that phospholipid biosynthesis in rat hepatocytes is inhibited by vasopressin at all concentrations, the biphasic modulation of phosphatidylcholine biosynthesis in rat heart myocytes illustrates the diverse effects of this hormone in different mammalian tissues.

Phospholipid synthesis in the lymphomatous mouse liver studied by 31P nuclear magnetic resonance spectroscopy in vitro and by administration of 14C-radiolabelled compounds in vivo

High phosphomonoester to ATP ratios have been found in 31P magnetic resonance spectra from livers of patients with hepatic lymphoma (Dixon et al. (1990) Br. J. Cancer 63, 953-958). The present study of a murine lymphoma showed that the phosphomonoester in the lymphomatous liver was largely phosphoethanolamine, which is an intermediate of phospholipid metabolism. A significant positive correlation was found between the concentration of phosphoethanolamine, measured by high resolution 31P nuclear magnetic resonance spectroscopy of extracts, and the degree of infiltration, assessed by quantitative histology. The phosphoethanolamine concentration reached about 10 times its normal level, but the phosphocholine concentration remained the same as in the normal liver. Radiolabelling studies showed that while the rate of phosphoethanolamine synthesis from exogenous [14C]ethanolamine was higher in the lymphomatous mouse liver than in control livers, the rate of phosphatidylethanolamine synthesis was lower in the lymphomatous liver. The rate of phosphatidylcholine synthesis in lymphoma-bearing livers was not significantly different from that in control mouse livers.

Quantification of inositol phospholipid breakdown in isolated rat hepatocytes

The hydrolysis of inositol phospholipids induced by vasopressin in hepatocytes during 60 min was quantified chemically. There was a large release of myo-inositol which was abolished by Li+, indicating that it was derived from inositol phosphates and not from phospholipase D action on PtdIns. There was also a large release of inositol phosphates which was increased approx. 2-fold by Li+ at 30 min, but then remained constant, suggesting that inositol phospholipid breakdown declined substantially beyond this time. In cells prelabelled with myo-[3H]inositol and treated with Li+, [3H]PtdIns(4,5)P2 decreased maximally (50%) at 15 s and then recovered to a level at 5 min that was maintained at 25% below control for 40 min. [3H]PtdIns4P and [3H]PtdIns showed slower decreases to approx. 30% below control at 15 min, but with no further changes. Labelled Ins(1,4,5)P3 and Ins(1,3,4)P3 showed 2-4-fold increases within 30 s and then declined to values that were maintained at a constant level above the control, except for [3H]Ins(1,3,4)P3, which showed a second increase. [3H]Ins(1,4)P2 showed a very large increase over 10 min, whereas [3H]Ins4P and [3H]Ins1P showed little change before 6 and 15 min respectively. The total [3H]inositol phosphates showed little further increase after 20 min. These data are consistent with a rapid, but not sustained, hydrolysis of PtdIns-(4,5)P2, but not of PtdIns, by phospholipase C, but do not exclude PtdIns4P as a substrate. Phosphatidate was rapidly increased by vasopressin, whereas diacylglycerol was increased after a 1-2 min lag. Both were maintained at levels 2-3-fold above control for 60 min. The vasopressin-induced increase in inositol phosphates plus myo-inositol (approx. 120 nmol/100 mg) was greater than the increase in diacylglycerol plus phosphatidate (approx. 60 nmol/100 mg) between 10 and 40 min. This indicates that there was substantial further metabolism of these lipids. Addition of 75 mM ethanol resulted in rapid production of phosphatidylethanol in response to vasopressin and a 35% reduction in phosphatidate, but no decrease in diacylglycerol. In summary, the results indicate that inositol phospholipid hydrolysis by phospholipase C can account for most of the diacylglycerol and phosphatidate that accumulate during 60 min of vasopressin action, but that these phospholipids are probably not the major source of the phosphatidate that is formed during the first 2 min by phospholipase D, or of the diacylglycerol and phosphatidate that are formed beyond 30 min.

Phosphatidylethanolamine metabolism in rat liver after partial hepatectomy. Control of biosynthesis of phosphatidylethanolamine by the availability of ethanolamine

The effect of partial (70%) hepatectomy on phosphatidylethanolamine (PE) synthesis was studied in rat liver during the first 4 post-operative days. Between 4 and 96 h after partial hepatectomy, the mass of PE increased from 30% to 80% of sham-operation values. In line with the increase in PE mass, the rate of PE synthesis in vivo from [14C]ethanolamine was stimulated 1.6- and 1.3-fold at 22 and 48 h after partial hepatectomy respectively. Surprisingly, the activity of CTP:phosphoethanolamine cytidylyltransferase (EC 2.7.7.14) was virtually unchanged after partial hepatectomy. In addition, neither ethanolamine kinase (EC 2.7.1.82) nor ethanolaminephosphotransferase (EC 2.7.8.1) showed any changes in activity over the time period studied. Hepatic levels of ethanolamine and phosphoethanolamine were drastically increased after partial hepatectomy, as compared with sham operation, whereas levels of CDP-ethanolamine and microsomal diacylglycerol were not affected. Interestingly, partial hepatectomy caused the concentration of free ethanolamine in serum to increase from 29 microM to approx. 50 microM during the first day after surgery. In hepatocytes isolated from non-operated animals, incorporation of [3H]ethanolamine into PE was stimulated by increasing the ethanolamine concentration from 10 up to 50 microM, whereas the radioactivity associated with phosphoethanolamine only increased at ethanolamine concentrations higher than 30 microM. Taken together, our results indicate that the observed increase in serum ethanolamine concentration after partial hepatectomy is probably responsible for both the increase in PE biosynthesis and the accumulation of ethanolamine and phosphoethanolamine in regenerating liver.

Okadaic acid inhibits phosphatidylethanolamine biosynthesis in rat hepatocytes

Okadaic acid, a specific inhibitor of protein phosphatase 1 and 2A, inhibited the synthesis of phosphatidylethanolamine via the CDPethanolamine pathway in isolated hepatocytes. Pulse-chase experiments and measurement of the enzyme activity demonstrated that the inhibition of phosphatidylethanolamine synthesis was not caused by an inhibition of CTP:phosphoethanolamine cytidylyltransferase, the putative regulatory enzyme. However, okadaic acid decreased the cellular diacylglycerol level to 30% of that in control cells. The data suggest that the availability of diacylglycerol limits phosphatidylethanolamine synthesis in okadaic acid-treated hepatocytes.

Phosphatidylcholine metabolism in rat liver after partial hepatectomy. Evidence for increased activity and amount of CTP:phosphocholine cytidylyltransferase

The effect of partial (70%) hepatectomy on phosphatidylcholine (PC) synthesis in rat liver was investigated during the first 4 post-operative days. Between 4 and 96 h after partial hepatectomy, the mass of PC increased from 30% to 80% of sham-operation values, being comparable with the restoration of total liver mass after partial hepatectomy. Relative to control (sham-operation), the incorporation in vivo of [3H]choline into PC was stimulated 2.6-fold at 22 h after partial hepatectomy. Moreover, CTP:phosphocholine cytidylyltransferase (EC 2.7.7.15) activity was significantly enhanced, and the pool size of phosphocholine decreased at 22 and 48 h after partial hepatectomy, whereas the activity of choline kinase (EC 2.7.1.32) was augmented at a later stage of liver regeneration (48 and 96 h). Stimulation of CTP:phosphocholine cytidylyltransferase activity by partial hepatectomy occurred in both the microsomal and cytosolic fractions. The stimulatory effect in the cytosolic fraction was mainly due to an increase in the number of enzyme molecules, as demonstrated by immunotitration of the amount of cytosolic cytidylyltransferase protein.

Biosynthesis and secretion of triacylglycerol in rat liver after partial hepatectomy

Partially hepatectomized rats were used to investigate the mechanism of fatty-liver development in the regenerating rat liver. After partial hepatectomy the amount of hepatic triacylglycerol increased by almost 4-fold compared with sham-operated rats. The activities of both cytosolic and microsomal phosphatidate phosphohydrolase were enhanced at 12 h after surgery. The activity of diacylglycerol acyltransferase was increased at a later stage of regeneration. Analysis of plasma lipoproteins showed a significant decrease of lipids associated with very-low-density lipoproteins (VLDL). Relative to control, the rate of hepatic triacylglycerol synthesis from [3H]glycerol in vivo was stimulated at 22 h after partial liver resection. However, secretion of glycerol-labelled triacylglycerol in VLDL was the same in control and hepatectomized rats. In cultures of hepatocytes from hepatectomized donor rats, the concentration of triacylglycerol and the biosynthesis of this lipid from [3H]glycerol or from [3H]oleate were enhanced. The secretion of total triacylglycerol into the medium was not affected, resulting in a net accumulation of intracellular triacylglycerol. The rate of secretion of leucine-labelled apolipoproteins B and E associated with VLDL was similar in cell cultures from hepatectomized and sham-operated rats. The results of this study show that the enhancement of the biosynthesis of triacylglycerol in hepatectomized livers is not accompanied by an increase of the secretion of VLDL.

Ca2(+)-dependence of arachidonic acid redistribution among phospholipids of cultured mouse keratinocytes

Mouse keratinocytes cultured in a medium containing less than 0.1 mM Ca2+ (low Ca2+) incorporated [1-14C]arachidonic acid (AA) into phospholipids by kinetics including; (i) a rapid labelling of phosphatidylinositol (PtdIns), phosphatidylserine (PtdSer) and both acid-stable and alkenylacyl forms of phosphatidylcholine (PtdCho); and (ii) a slow but long-lasting radiolabel incorporation into both acid-stable and alkenylacyl forms of phosphatidylethanolamine (PtdEtn), partly associated with a net radioactivity loss from acid stable-PtdCho. Under low Ca2+ conditions no radioactivity transfer apparently occurred between PtdIns and other phospholipid classes. When cells were prelabelled for 24 h with [1-14C]AA and reincubated in label-free medium containing 1.2 mM Ca2+ (normal Ca2+), an early and extensive loss of radioactivity from PtdIns was observed, reasonably in connection with Ca2+ stimulation of phosphoinositide turnover. Cell shift to normal Ca2+ did not result in an increased synthesis of labelled eicosanoids, but was consistent with an increase of radioactivity incorporation into diacylglycerol (DAG) and with a complex pattern of [1-14C]AA redistribution, eventually leading to a marked radioactivity incorporation into acid stable-PtdEtn (but not into alkenylacyl-PtdEtn) and to a labelling decrease of acid stable-PtdCho. The possible mechanisms driving AA recycling after cell shift to normal Ca2+ are discussed.

Stimulation of CTP: phosphocholine cytidylyltransferase and phosphatidylcholine synthesis by calcium in rat hepatocytes

The effects of Ca2+, ionophore A23187, and vasopressin on CTP:phosphocholine cytidylyltransferase were investigated. Cytidylyltransferase is present in the cytosol and in a membrane-bound form on the microsomes. Digitonin treatment caused release of the cytosolic form rapidly. Addition of 7 mM Ca2+ to hepatocyte medium resulted in a 3-fold decrease in cytidylyltransferase released by digitonin treatment (1.7 +/- 0.1 nmol/min per mg compared to 5.1 +/- 0.2 nmol/min per mg in the control). Verapamil, a calcium channel blocker, partially overcame this effect of Ca2+. Ionophore A23187 and vasopressin both mimicked the effect of Ca2+ and resulted in a decrease in cytidylyltransferase release (2.4 +/- 0.1 nmol/min per mg and 2.5 +/- 0.2 nmol/min per mg, respectively) compared to control (3.4 +/- 0.1 nmol/min per mg). In agreement with the digitonin experiments, incubation with 7 mM Ca2+ resulted in a decrease in cytidylyltransferase in the cytosol (from 4.0 to 1.2 mol/min per mg) and a corresponding increase in the microsomes (from 0.6 to 2.4 nmol/min per mg). Verapamil partially blocked this translocation caused by Ca2+. Ionophore A23187 and vasopressin also caused translocation of the cytidylyltransferase from the cytosol to the microsomes. The addition of Ca2+ also resulted in an increase in PC synthesis. With 7 mM Ca2+ in the medium, the label associated with PC increased to 3.8 +/- 0.1.10(6) dpm/dish from 2.7 +/- 0.1.10(6) dpm/dish after 10 min. PC degradation was also affected, since 7 mM Ca2+ in the medium resulted in an increase in LPC formation both in the cell and the medium. We conclude that high concentrations of calcium in the hepatocyte medium can cause a stimulation of CTP:phosphocholine cytidylyltransferase and PC synthesis in cultured hepatocytes.

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.

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.

Effects of dietary conditions on the pool sizes of precursors of phosphatidylcholine and phosphatidylethanolamine synthesis in rat liver

We developed a new method for the determination of choline- and ethanolamine-containing precursors of phosphatidylcholine and phosphatidylethanolamine after their separation by HPLC and we have studied the effects of different dietary conditions on the pool sizes of these metabolites in rat liver. Fasting for 48 h induced only a small decrease in the amounts of phosphatidylethanolamine and its water-soluble precursors. Upon refeeding with a high-sucrose, fat-free diet for 72 h, the levels of ethanolamine-containing compounds were only slowly restored. The effects of various dietary conditions on the amounts of phosphatidylcholine and its water-soluble precursors were much more pronounced. Fasting induced a sharp decrease, especially of the amount of cholinephosphate. However, the levels of phosphatidylcholine and the choline-containing precursors were rapidly restored upon refeeding for 24 h. Continued refeeding for an additional 48 h enhanced the cholinephosphate pool size to a level more than double that found in normally fed rats. The latter effect was accompanied by an inhibition of the enzyme CTP:choline-phosphate cytidylyltransferase. The results are discussed in view of a possible regulatory mechanism that may balance the amounts of phosphatidylcholine and phosphatidylethanolamine.

Stimulation of phosphatidylethanolamine synthesis in isolated rat hepatocytes by phorbol 12-myristate 13-acetate

Incubation of freshly isolated rat hepatocytes in the presence of phorbol 12-myristate 13-acetate stimulates the incorporation of [1,2-14C]ethanolamine into phosphatidylethanolamines. This stimulation is strongly dependent on the ethanolamine concentration in the medium and becomes apparent at ethanolamine concentrations above 25 microM. Treatment of hepatocytes with phorbol 12-myristate 13-acetate results in a decreased labelling of intracellular ethanolamine, ethanolaminephosphate and CDPethanolamine. Exposure of cells to phorbol 12-myristate 13-acetate induces an increase of the activity of the enzymes CTP: ethanolaminephosphate cytidylyltransferase and ethanolaminephosphotransferase. These effects are accompanied by a decrease of the pool size of ethanolaminephosphate and CDPethanolamine and an increase of the level of diacylglycerols after 30 min of incubation in the presence of phorbol 12-myristate 13-acetate. Upon prolonged incubation, the CDPethanolamine and diacylglycerol pools are restored to the level found in untreated cells. These results indicate that stimulation of phosphatidylethanolamine synthesis by phorbol 12-myristate 13-acetate is probably exerted at the level of CTP : ethanolaminephosphate cytidylytransferase, although there may be an additional effect on the subsequent step of phosphatidylethanolamine synthesis, the formation of phosphatidylethanolamines from CDPethanolamine and diacylglycerols.