Phosphatidylcholine-dependent phospholipase C in rat liver chromatin

Metabolic pathways for the degradation of phosphatidic acid in isolated nuclei from cerebellar cells

The aim of the present research was to analyse the pathways for phosphatidic acid metabolism in purified nuclei from cerebellar cells. Lipid phosphate phosphatase and diacylglyceride lipase activities were detected in nuclei from cerebellar cells. It was observed that DAGL activity makes up 50% of LPP activity and that PtdOH can also be metabolised to lysophosphatidic acid. With a nuclear protein content of approximately 40 μg, the production of diacylglycerol and monoacylglycerol was linear for 30 min and 5 min, respectively, whereas it increased with PtdOH concentrations of up to 250 μM. LysoPtdOH, sphingosine 1-phosphate and ceramide 1-phosphate, which are alternative substrates for LPP, significantly reduced DAG production from PA. DAG and MAG production increased in the presence of Triton X-100 (1 mM) whereas no modifications were observed in the presence of ionic detergent sodium deoxycholate. Ca²+ and Mg²+ stimulated MAG production without affecting DAG formation whereas fluoride and vanadate inhibited the generation of both products. Specific PtdOH-phospholipase A1 and PtdOH-phospholipase A2 were also detected in nuclei. Our findings constitute the first reported evidence of active PtdOH metabolism involving LPP, DAGL and PtdOH-selective PLA activities in purified nuclei prepared from cerebellar cells.

Phosphatidylcholine/sphingomyelin metabolism crosstalk inside the nucleus

It is known that phospholipids represent a minor component of chromatin. It has been highlighted recently that these lipids are metabolized directly inside the nucleus, thanks to the presence of enzymes related to their metabolism, such as neutral sphingomyelinase, sphingomyelin synthase, reverse sphingomyelin synthase and phosphatidylcholine-specific phospholipase C. The chromatin enzymatic activities change during cell proliferation, differentiation and/or apoptosis, independently from the enzyme activities present in nuclear membrane, microsomes or cell membranes. This present study aimed to investigate crosstalk in lipid metabolism in nuclear membrane and chromatin isolated from rat liver in vitro and in vivo. The effect of neutral sphingomyelinase activity on phosphatidylcholine-specific phospholipase C and sphingomyelin synthase, which enrich the intranuclear diacylglycerol pool, and the effect of phosphatidylcholine-specific phospholipase C activity on neutral sphingomyelinase and reverse sphingomyelin synthase, which enrich the intranuclear ceramide pool, was investigated. The results show that in chromatin, there exists a phosphatidylcholine/sphingomyelin metabolism crosstalk which regulates the intranuclear ceramide/diacylglycerol pool. The enzyme activities were inhibited by D609, which demonstrated the specificity of this crosstalk. Chromatin lipid metabolism is activated in vivo during cell proliferation, indicating that it could play a role in cell function. The possible mechanism of crosstalk is discussed here, with consideration to recent advances in the field.

Nuclear sphingomyelin-synthase and protein kinase C δ in melanoma cells

The aim of this research is to study the influence of protein kinase C delta on the nuclear phospholipids metabolism. Murine and human melanoma cells, in which overexpression of protein kinase delta was induced, were used. After purification of the nuclei, the phosphatidylcholine-dependent phospholipase C, sphingomyelin-synthase, and sphingomyelinase activities were measured. The results showed that the nuclear sphingomyelin-synthase activity increased and sphingomyelinase activity decreased in the protein kinase C delta overexpressive cells with respect to the controls. As a consequence, the ceramide pool decreased and diacylglycerol pool increased; this effect was not due to the phosphatidylcholine-dependent phospholipase C activity that did not change. The inhibition of sphingomyelinase could be due to protein kinase C delta as well as to existence of a sort of nuclear self-regulation between sphingomyelin-synthase and sphingomyelinase. The possible role of nuclear sphingomyelin-synthase in cell proliferation is discussed.

Uronema marinum: identification and biochemical characterization of phosphatidylcholine-hydrolyzing phospholipase C

Phosphatidylcholine (PC)-specific phospholipase D (PC-PLD) and phosphatidylcholine (PC)-specific phospholipase C (PC-PLC) activities have been detected in Uronema marinum. Partial purification of PC-PLC revealed that two distinct forms of PC-PLC (named as mPC-PLC and cPC-PLC) were existed in membrane and cytosol fractions. The two PC-PLC enzymes showed the preferential hydrolyzing activity for PC with specific activity of 50.4 for mPC-PLC and 28.3 pmol/min/mg for cPC-PLC, but did not hydrolyze phosphatidylinositol or phosphatidylethanolamine. However, the biochemical characteristics and physiological roles of both enzymes were somewhat different. mPC-PLC had a pH optimum in the acidic region at around, pH 6.0, and required approximately 0.4 mM Ca2+ and 2.5 mM Mg2+ for maximal activity. cPC-PLC had a pH optimum in the neutral region at around, pH 7.0, and required 1.6 mM Ca2+ and 2.5 mM Mg2+ for maximal activity. cPC-PLC, but not mPC-PLC, showed a dose-dependent inhibitory effect on the luminal-enhanced chemiluminescence (CL) responses and the viability of zymosan-stimulated phagocytes of olive flounder, indicating that cPC-PLC may contribute to the parasite evasion against the host immune response. Our results suggest that U. marinum contains PC-PLD as well as two enzymatically distinct PC-PLC enzymes, and that mPC-PLC may play a role in the intercellular multiplication of U. marinum and cPC-PLC acts as a virulence factor, serving to actively disrupt the host defense mechanisms.

Presence of different phospholipase C isoforms in the nucleus and their activation during compensatory liver growth

Phospholipase C (PLC) was purified from the membrane-depleted rat liver nuclei. About 60% of the total PLC-activity corresponded to beta1b isoform, 30% to PLC-gamma1 and less than 10% to PLC-delta1. PLC-beta1b and -gamma1 were found in the nuclear matrix, while PLC-delta1 was detected in the chromatin. Two peaks of an increase in the total PLC-activity were detected occurring at 6 and 20 h after partial hepatectomy. An early increase in PLC-beta1b activity in the nuclear matrix was associated with serine phosphorylation of the enzyme, while the later increase paralleled the increase in the amount of protein. The increase in the PLC-gamma1 activity measured at 6 and 20 h after partial hepatectomy was associated with tyrosine phosphorylation of the enzyme. The activity of PLC-delta1 and the amount of the protein found in the chromatin was increased only at 20 h after partial hepatectomy.

Increase of Nuclear Ceramide through Caspase-3-Dependent Regulation of the “Sphingomyelin Cycle” in Fas-Induced Apoptosis

Regardless of the existence of ceramide-related molecules, such as sphingomyelin (SM), neutral sphingomyelinase (nSMase), and SM synthase, in the nucleus, the regulation of ceramide in the nucleus is poorly understood in stress-induced apoptosis. In Fas-induced Jurkat T-cell apoptosis, we found a time- and dose-dependent increase of ceramide content in the nuclear and microsomal fractions. Fas-induced increase of ceramide content in the nucleus also was detected by confocal microscopy using anticeramide antibody. Activation of nSMase and inhibition of SM synthase were evident in the nuclear fraction after Fas cross-linking, whereas nSMase was activated, but SM synthase was not affected, in the microsomal fraction. Pretreatment with D-609, a putative SM synthase inhibitor, enhanced Fas-induced increase of ceramide in the nucleus and induction of apoptosis along with increase of Fas-induced inhibition of nuclear SM synthase. Fas-induced activation of caspase-3 was detected in the nuclear fraction and in whole cell lysate. A caspase-3 inhibitor, acetyl-Asp-Glu-Val-Asp-chloromethyl ketone, blocked not only Fas-induced increases of apoptosis and ceramide content but also Fas-induced activation of nSMase and inhibition of SM synthase in the nuclear fraction. Taken together, it is suggested that the nucleus is a site for ceramide increase and caspase-3 activation in Fas-induced Jurkat T-cell apoptosis and that caspase-3-dependent regulation of the "SM cycle" consisting of nSMase and SM synthase plays a role in Fas-induced ceramide increase in the nucleus.

Plasmalogens in rat liver chromatin: New molecules involved in cell proliferation

A minor component of chromatin, the phospholipid fraction, changes during cell cycle as result of the activation of intranuclear lipid metabolism enzymes including phosphatidylcholine-dependent phospholipase C activity. It is known that this enzyme may be activated by phosphatidylcholine plasmalogen (Plg). Until now, there has been little evidences for the presence of Plgs inside the nucleus. The aim of our study is to ascertain if they are present in the nucleus and are responsible of the activation of phosphatidylcholine-dependent phospholipase C during cell proliferation and apoptosis. Therefore, we have analysed the Plg composition of the whole homogenate, cytosol, nuclei and chromatin of hepatocytes. The phosphatidylcholine-dependent phospholipase C activity was assayed using both phosphatidylcholine and plasmalogenyl-phosphatidylcholine as substrates. Our results show, for the first time, that Plgs are present in chromatin and the plasmalogenyl-phosphatidylcholine stimulates the phosphatidylcholine-dependent phospholipase C activity more than phosphatidylcholine. Finally, in order to verify the possible role of these molecules during cell proliferation and apoptosis, we used liver of rats fed with ciprofibrate which stimulates hepatocytes proliferation during the treatment and, after withdrawal, apoptosis. After 3 days of ciprofibrate treatment, the chromatin plasmalogenyl-phosphatidylcholine increases as well as the phosphatidylcholine-dependent phospholipase C activity. After drug withdrawal, when the hepatocytes undergo to apoptosis, the plasmalogenyl-phosphatidylcholine content together with phosphatidylcholine-dependent phospholipase C activity decreases. Therefore, it can be concluded that plamalogens are present in the chromatin, and probably may have a function both in regulating phosphatidylcholine dependent phospholipase C and cell cycle.

Reverse sphingomyelin-synthase in rat liver chromatin

The chromatin phospholipid fraction is enriched in sphingomyelin content which changes during cell maturation and proliferation. Recently, we have demonstrated that the sphingomyelin variations can be due to chromatin neutral sphingomyelinase and sphingomyelin-synthase activities which differ in pH and K(m) optima from those present in nuclear membranes. The sphingomyelin can be used also as a source of phosphorylcholine for phosphatidylcholine synthesis by reverse sphingomyelin-synthase. In the present work we have studied the possible existence of reverse sphingomyelin-synthase activity in nuclear membrane and chromatin. A very low activity was detected in the homogenate, cytosol and nuclear membrane (0.93+/-0.14, 2.61+/-0.33 and 0.87+/-0.13 pmol/mg protein/min, respectively), whereas the activity present in chromatin was 37.09+/-2.05 pmol/mg protein/min. The reverse sphingomyelin-synthase decreases the intranuclear diacylglycerol pool and increases the intranuclear ceramide pool, whereas sphingomyelin-synthase has an opposite effect. The possible correlation between these enzymes is discussed.

A possible role of cholesterol-sphingomyelin/phosphatidylcholine in nuclear matrix during rat liver regeneration

BACKGROUND/AIMS

Phospholipids and cholesterol in chromatin have been previously demonstrated. The lipid fraction changes during cell proliferation in relation to activation of enzymes of phospholipid metabolism. The aim of the present work is to clarify if chromatin lipids may derive or not from nuclear matrix and if they have different roles.

METHODS

The subnuclear fractions were isolated from rat hepatocyte nuclei and the lipid fraction was extracted and analysed by chromatography in normal and regenerating liver. The phosphatidylcholine-sphingomyelin metabolism enzymes activity was assayed, by using radioactive substrates.

RESULTS

In nuclear matrix, cholesterol and sphingomyelin are respectively five and three times higher than those present in chromatin; the amount of phosphatidylcholine, which it is enriched in saturated fatty acids, is lower, thus indicating a less fluid structure. The lower content in phosphatidylcholine may be justified by the phosphatidylcholine-dependent phospholipase C activity, which increases during liver regeneration, reaching a peak at the beginning of S-phase, when also cholesterol and sphingomyelin increase.

CONCLUSIONS

The nuclear matrix lipids are independent from chromatin lipids; the ratio cholesterol-sphingomyelin/phosphatidylcholine is higher and, as a consequence, nuclear matrix is less fluid in relation to DNA synthesis, suggesting a specific role of nuclear matrix as a structure involved in DNA duplication.

Functional role of phospholipids in the nuclear events

This review presents the structural and functional role of phospholipids in chromatin and nuclear matrix as well as the difference in composition and turnover compared to those present in the nuclear membrane. Nuclei have a very active lipid metabolism which seems to play an important role in the transduction of the signals to the genome in response to agonists acting at the plasma membrane level. The evidence on the presence of phospholipid-calcium-dependent protein kinase C (PKC) in nuclei and enzymes of phospholipids turnover is given. Protein kinase C interacts with nuclear phosphoinositol and sphingomyelin cycles products. This fact evidences about possibility that signal transduction events could also occur at the nuclear level during induction of cell proliferation, differentiation and apoptosis.

Nuclear and chromatin lipids: metabolism in normal and gamma-irradiated rats

The data on nuclear and chromatin lipid metabolism are reviewed. The amount of neutral lipids and phospholipids in nuclei of rat thymus, liver and neocortex neuron as well as the amount of lipids in rat thymus and liver chromatin are described. The metabolic responses of nuclear and chromatin lipids from thymus to different doses and dose rates of gamma-irradiation of rats are discussed. In most cases, the nuclear and chromatin lipid responses are distinct. Changes in nuclear and chromatin lipid metabolism in response to gamma-irradiation are suggested to connect with the signal transduction pathway and the regulation of the transcriptional and replicative chromatin activity. The influence of beta-carotene and picrotoxin on rat liver nuclear lipids and neocortex neuronal nuclear lipids, respectively, was analyzed. The possible involvement of the lipid traffic in the chromatin lipid responses to gamma-irradiation and other agents is suggested.

Phosphatidylcholine metabolism in nuclei of phorbol ester-activated LA-N-1 neuroblastoma cells

The agonist stimulation of a variety of cells results in the induction of specific lipid metabolism in nuclear membranes, supporting the hypothesis of an important role of the lipids in nuclear signal transduction. While the existence of a phosphatidylinositol cycle has been reported in cellular nuclei, little attention has been given to the metabolism of phosphatidylcholine in nuclear signaling. In the present study the metabolism of phosphatidylcholine in the nuclei of neuroblastoma cells LA-N-1 was investigated. The incubation of LA-N-1 nuclei with radioactive choline, phosphocholine or CDP-choline led to the production of labelled phosphatidylcholine. The incorporation of choline and phosphocholine but not CDP-choline was enhanced in nuclei of TPA treated cells. Moreover the presence of choline kinase, phosphocholine cytidylyltransferase and phosphocholine transferase activities were detected in the nuclei and the TPA treatment of the cells stimulated the activity of the phosphocholine cytidylyltransferase. When cells prelabelled with [3H]palmitic acid were stimulated with TPA in the presence of ethanol, an increase of labelled diacylglycerol and phosphatidylethanol in the nuclei was observed. Similarly, an increase of labelled diacylglycerol and phosphatidic acid but not of phosphatidylethanol occurred in [3H]palmitic acid prelabelled nuclei stimulated with TPA in the presence of ethanol. However the production of phosphatidylethanol was observed when the nuclei were treated with TPA in the presence of ATP and GTPgammaS. The stimulation of [3H]choline prelabelled nuclei with TPA also generated the release of free choline and phosphocholine. The results indicate the presence of PLD and probably PLC activities in LA-N-1 nuclei and the involvement of phosphatidylcholine in the production of nuclear lipid second messengers upon TPA stimulation of LA-N-1 cells. The correlation of the disappearance of phosphatidylcholine, the production of diacylglycerol and phosphatidic acid with the stimulation of phosphatidylcholine synthesis in nuclei of TPA treated LA-N-1 suggests the existence of a phosphatidylcholine cycle in these nuclei.