Differential modulation of phospholipase D and phosphatidate phosphohydrolase during aging in rat cerebral cortex synaptosomes

Insulin modifies aging-related inhibition of 1-stearoyl, 2-arachidonoylglycerol phosphorylation in rat synaptic terminals

The purpose of the present study was to analyze diacylglycerol kinase (DAGK) activity in synaptic terminals from cerebral cortex (CC) and hippocampus (Hp) from adult (3-4 month-old) and aged (26-28 month-old) rats. The effect of insulin through DAGK activity on synaptosomes from adult and aged rats was also analyzed under conditions favoring saturated or unsaturated phosphatidic acid (PA) formation, using exogenous di-palmitoil glycerol (DPG) or 1-stearoyl-2-arachidonoylglycerol (SAG) as substrates. Results showed that the enzymatic activity preferentially uses SAG as substrate, thus indicating the presence of ɛ-type DAGK. A significant decrease in DAGK activity transforming SAG into PA was also observed in both tissues from aged rats. Western blot detection of DAGKɛ showed that enzyme content undergoes no changes with aging. [3H] inositol incorporation into phosphoinosites was also analyzed to evaluate the role of DAGKɛ in their synthesis. Data obtained from 3H-inositol incorporation into phosphoinositides revealed that in synaptosomes from aged rats phosphatidylinositol (PI) synthesis is lower than in adult animals. Interestingly, in the presence of SAG, PI synthesis was restored to adult values. DAGK activity over SAG was more highly stimulated by insulin in CC and Hp synaptosomes of aged rats with respect to adult rats. On the other hand, insulin exerted a stimulatory effect on PI and phosphatidylinositol 4 phosphate (PI(4)P) synthesis in synaptosomal CC from aged rats. Taken together, our findings indicate that in aged rats insulin triggers a stimulatory mechanism that reverts the diminished synaptosomal ability to synthesize arachidonoyl phosphatidic acid (20:4 PA). The recovery of this PA species indicates that insulin positively regulates phosphoinositide synthesis.

Regulation of phosphatidic Acid metabolism by sphingolipids in the central nervous system

This paper explores the way ceramide, sphingosine, ceramide 1-phosphate, and sphingosine 1-phosphate modulate the generation of second lipid messengers from phosphatidic acid in two experimental models of the central nervous system: in vertebrate rod outer segments prepared from dark-adapted retinas as well as in rod outer segments prepared from light-adapted retinas and in rat cerebral cortex synaptosomes under physiological aging conditions. Particular attention is paid to lipid phosphate phosphatase, diacylglycerol lipase, and monoacylglycerol lipase. Based on the findings reported in this paper, it can be concluded that proteins related to phototransduction phenomena are involved in the effects derived from sphingosine 1-phosphate/sphingosine or ceramide 1-phosphate/ceramide and that age-related changes occur in the metabolism of phosphatidic acid from cerebral cortex synaptosomes in the presence of either sphingosine 1-phosphate/sphingosine or ceramide 1-phosphate/ceramide. The present paper demonstrates, in two different models of central nervous system, how sphingolipids influence phosphatidic acid metabolism under different physiological conditions such as light and aging.

Selective localization of phosphatidylcholine-derived signaling in detergent-resistant membranes from synaptic endings

Detergent-resistant membranes (DRMs) are a class of specialized microdomains that compartmentalize several signal transduction processes. In this work, DRMs were isolated from cerebral cortex synaptic endings (Syn) on the basis of their relative insolubility in cold Triton X-100 (1%). The lipid composition and marker protein content were analyzed in DRMs obtained from adult and aged animals. Both DRM preparations were enriched in Caveolin, Flotillin-1 and c-Src and also presented significantly higher sphingomyelin (SM) and cholesterol content than purified Syn. Total phospholipid-fatty acid composition presented an increase in 16:0 (35%), and a decrease in 20:4n-6 (67%) and 22:6n-3 (68%) content in DRM from adults when compared to entire synaptic endings. A more dramatic decrease was observed in the 20:4n-6 and 22:6n-3 content in DRMs from aged animals (80%) with respect to the results found in adults. The coexistence of phosphatidylcholine-specific-phospholipase C (PC-PLC) and phospholipase D (PLD) in Syn was previously reported. The presence of these signaling pathways was also investigated in DRMs isolated from adult and aged rats. Both PC-PLC and PLD pathways generate the lipid messenger diacylglycerol (DAG) by catalyzing PC hydrolysis. PC-PLC and PLD1 localization were increased in the DRM fraction. The increase in DAG generation (60%) in the presence of ethanol, confirmed that PC-PLC was also activated when compartmentalized in DRMs. Conversely, PLD2 was excluded from the DRM fraction. Our results show an age-related differential fatty acid composition and a selective localization of PC-derived signaling in synaptic DRMs obtained from adult and aged rats.

Age-related changes in the metabolization of phosphatidic acid in rat cerebral cortex synaptosomes

In this study, phosphatidic acid (PA) metabolization is found to generate diacylglycerol (DAG), monoacylglycerol (MAG) and glycerol by the sequential action of lipid phosphate phosphatase (LPP), diacylglycerol lipase (DAGL), and monoacylglycerol lipase (MAGL) in cerebral cortex (CC) synaptosomes. It is also demonstrated that PA is metabolized by phospholipases A (PLA)/lysophosphatidic acid phosphohydrolase (LPAPase) in synaptic endings. Age-related changes in the metabolization of PA have been observed in rat cerebral cortex synaptosomes in the presence of the alternative substrates for LPP, namely LPA, sphingosine 1-phosphate (S1P) and ceramide 1-phosphate (C1P). In addition, LPA and C1P up to concentrations of about 50 microM favor the metabolism in the direction of MAG and glycerol in aged and adult synaptosomes, respectively. At equimolecular concentrations with PA, LPA decreases DAG formation in adult and aged synaptosomes, whereas S1P decreases it and C1P increases it only in aged synaptosomes. Sphingosine (50 microM) or ceramide (100 microM) increase PA metabolism by the pathway that involves LPP/DAGL/MAGL action in aged membranes. Using RHC-80267, a DAGL inhibitor, we could observe that 50% and 33% of MAG are produced as a result of DAGL action in adult and aged synaptosomes, respectively. Taken together, our findings indicate that the ageing modifies the different enzymatic pathways involved in PA metabolization.

Insulin action on polyunsaturated phosphatidic acid formation in rat brain: an "in vitro" model with synaptic endings from cerebral cortex and hippocampus

The highly efficient formation of phosphatidic acid from exogenous 1-stearoyl-2-arachidonoyl-sn-glycerol (SAG) in rat brain synaptic nerve endings (synaptosomes) from cerebral cortex and hippocampus is reported. Phosphatidic acid synthesized from SAG or 1,2-dipalmitoyl-sn-glycerol (DPG) was 17.5 or 2.5 times higher, respectively, than from endogenous synaptosomal diacylglycerides. Insulin increased diacylglycerol kinase (DAGK) action on endogenous substrate in synaptic terminals from hippocampus and cerebral cortex by 199 and 97%, respectively. Insulin preferentially increased SAG phosphorylation from hippocampal membranes. In CC synaptosomes insulin increased phosphatidic acid (PA) synthesis from SAG by 100% with respect to controls. Genistein (a tyrosine kinase inhibitor) inhibited this stimulatory insulin effect. Okadaic acid or cyclosporine, used as Ser/Threo protein phosphatase inhibitors, failed to increase insulin effect on PA formation. GTP gamma S and particularly NaF were potent stimulators of PA formation from polyunsaturated diacylglycerol but failed to increase this phosphorylation when added after 5 min of insulin exposure. GTP gamma S and NaF increased phosphatidylinositol 4,5 bisphosphate (PIP2) labeling with respect to controls when SAG was present. On the contrary, they decreased polyphosphoinositide labeling with respect to controls in the presence of DPG. Our results indicate that a DAGK type 3 (DAGKepsilon) which preferentially, but not selectively, utilizes 1-acyl-2-arachidonoyl-sn-glycerol and which could be associated with polyphosphoinositide resynthesis, participates in synaptic insulin signaling. GTP gamma S and NaF appear to be G protein activators related to insulin and the insulin receptor, both affecting the signaling mechanism that augments phosphatidic acid formation.

PKCtheta promotes c-Rel-driven mammary tumorigenesis in mice and humans by repressing estrogen receptor alpha synthesis

The vast majority of primary human breast cancer tissues display aberrant nuclear NF-kappaB c-Rel expression. A causal role for c-Rel in mammary tumorigenesis has been demonstrated using a c-Rel transgenic mouse model; however, tumors developed with a long latency, suggesting a second event is needed to trigger tumorigenesis. Here we show that c-Rel activity in the mammary gland is repressed by estrogen receptor alpha (ERalpha) signaling, and we identify an epigenetic mechanism in breast cancer mediated by activation of what we believe is a novel PKCtheta-Akt pathway that leads to downregulation of ERalpha synthesis and derepression of c-Rel. ERalpha levels were lower in c-Rel-induced mammary tumors compared with normal mammary gland tissue. PKCtheta induced c-Rel activity and target gene expression and promoted growth of c-Rel- and c-RelxCK2alpha-driven mouse mammary tumor-derived cell lines. RNA expression levels of PKCtheta and c-Rel target genes were inversely correlated with ERalpha levels in human breast cancer specimens. PKCtheta activated Akt, thereby inactivating forkhead box O protein 3a (FOXO3a) and leading to decreased synthesis of its target genes, ERalpha and p27(Kip1). Thus we have shown that activation of PKCtheta inhibits the FOXO3a/ERalpha/p27(Kip1) axis that normally maintains an epithelial cell phenotype and induces c-Rel target genes, thereby promoting proliferation, survival, and more invasive breast cancer.

Activation of phosphoinositide-3 kinase/Akt pathway by FeSO4 in rat cerebral cortex synaptic endings

The aim of this work was to study the involvement of the phosphoinositide-3-kinase (PI3K)/Akt pathway in synaptic endings incubated under oxidative stress conditions. Synaptosomes purified from rat cerebral cortex were exposed to FeSO4 (50 microM) for different periods of time. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) reduction and lactate dehydrogenate (LDH) leakage were significantly affected after 5 min of incubation in the presence of FeSO4, with respect to control conditions. In whole synaptosomes incubated in the presence of [gamma-(32)P]ATP, phosphoinositide (PPI) labeling was increased after 5 min of Fe2+ exposure. This effect was prevented by the specific PI3K inhibitor LY294002. Anti-p85 immunoprecipitates (IPs) obtained from synaptosomes preincubated with Fe2+ (5 min) showed a PI3K activity two-fold higher than the activity recovered under control conditions. Additionally, Akt activation was temporally coincident with PI3K activation. LY294002 was not able to prevent the LDH leakage and diminution of MTT reduction induced by Fe2+. Our results demonstrate that free iron provokes the early activation of PI3K/Akt pathway, but this activation is not sufficient for protecting synaptic endings from oxidative damage.

Insulin promotes diacylglycerol kinase activation by different mechanisms in rat cerebral cortex synaptosomes

The mechanism by which insulin increases diacylglycerol kinase (DAGK) activity has been studied in cerebral cortex (CC) synaptosomes from adult (3-4 months of age) rats. The purpose of this study was to identify the role of phospholipases C and D (PLC and PLD) in DAGK activation by insulin. Neomycin, an inhibitor of PLC phosphatidylinositol-bisphosphate (PIP2) specific; ethanol, an inhibitor of phosphatidic acid (PA) formation by the promotion of a transphosphatidyl reaction of phosphatidylcholine phospholipase D (PC-PLD); and DL propranolol, an inhibitor of phosphatidate phosphohydrolase (PAP), were used in this study. Insulin (0.1 microM) shielded an increase in PA synthesis by [32P] incorporation using [gamma-32P]ATP as substrate and endogenous diacylglycerol (DAG) as co-substrate. This activated synthesis was strongly inhibited either by ethanol or DL propranolol. Pulse chase experiments also showed a PIP2-PLC activation within 1 min exposure to insulin. When exogenous unsaturated 18:0-20:4 DAG was present, insulin increased PA synthesis significantly. However, this stimulatory effect was not observed in the presence of exogenous saturated (di-16:0). In the presence of R59022, a selective DAGK inhibitor, insulin exerted no stimulatory effect on [32P]PA formation, suggesting a strong relationship between increased PA formation by insulin and DAGK activity. These data indicate that the increased synthesis of PA by insulin could be mediated by the activation of both a PC-PLD pathway to provide DAG and a direct DAGK activation that is associated to the use of 18:0-20:4 DAG species. PIP2-PLC activation may contribute at least partly to the insulin effect on DAGK activity.

Age-associated changes of insulin action on the hydrolysis of diacylglycerol generated from phosphatidic acid

Age-related changes in insulin action on diacylglycerol (DAG) degradation was studied in rat cerebral cortex synaptosomes. The generation of monoacylglycerol (MAG) and water soluble products (WSP, glycerol plus glycerol-3-phosphate) from DAG was studied in cerebral cortex (CC) synaptosomes from adult (4-month-old) and aged (28-month-old) rats. Additionally, the effect of porcine insulin and tyrosine phosphorylation was evaluated in the same group of animals. In this study we demonstrate that the age-related increase in WSP generation was accompanied by unmodified MAG levels. In the presence of diacylglycerol lipase (DAG lipase) inhibitor, RHC-80267, a lower inhibitory effect on MAG production was observed in CC synaptosomes from aged rats with respect to that in adult membranes. Under these experimental conditions, WSP formation was only diminished in aged membranes. Insulin stimulated MAG and WSP formation at long incubation times (30 min) in adult animals, while it had an inhibitory effect in aged animals. Insulin plus vanadate (as tyrosine-phosphatase inhibitor) inhibited MAG production at short incubation times whereas the same effect was observed in aged animals at long times of incubation. WSP formation was stimulated by insulin plus vanadate both in adult and aged animals at 30 min of incubation. Our results show that insulin differentially modulates MAG and WSP production from exogenous PA in CC synaptosomes from aged rats compared with adult rats.

Coexistence of phosphatidylcholine-specific phospholipase C and phospholipase D activities in rat cerebral cortex synaptosomes

DAG derived from phosphatidylcholine (PtdCho) acts as a lipid second messenger. It can be generated by the activation of phospholipase D (PLD) and the phosphatidic acid phosphohydrolase type 2 (PAP2) pathway or by a PtdCho-specific phospholipase C (PtdCho-PLC). Our purpose was to study PtdCho-PLC activity in rat cerebral cortex synaptosomes (CC Syn). DAG production was highly stimulated by detergents such as Triton X-100 and sodium deoxycholate. Ethanol and tricyclodecan-9-yl-xanthate potassium salt decreased DAG generation by 42 and 61%, respectively, at 20 min of incubation. These data demonstrate that both the PLD/PAP2 pathway and PtdCho-PLC contribute to DAG generation in CC Syn. PtdCho-PLC activity remained located mainly in the synaptosomal plasma membrane fraction. Kinetic studies showed Km and Vmax values of 350 microM and 3.7 nmol DAG x (mg protein x h)(-1), respectively. Western blot analysis with anti-PtdCho-PLC antibody showed a band of 66 KDa in CC Syn. Our results indicate the presence of a novel DAG-generating pathway in CC Syn in addition to the known PLD/PAP2 pathway.

Phosphatidic acid and diacylglycerol generation is regulated by insulin in cerebral cortex synaptosomes from adult and aged rats

Insulin receptor associated with the cerebral cortex (CC) has been shown to be involved in brain cognitive functions. Furthermore, deterioration of insulin signaling has been associated with age-related brain degeneration. We have reported previously that aging stimulates phospholipase D/phosphatidate phosphohydrolase 2 (PLD/PAP2) pathway in CC synaptosomes from aged rats, generating a differential availability of their reaction products: diacylglycerol (DAG) and phosphatidic acid (PA). The aim of this work was to determine the effect of aging on DAG kinase (DAGK), as an alternative pathway for PA generation, and to evaluate the effect of insulin on PLD/PAP2 pathway and DAGK. PLD, PAP2, and DAGK activities were measured using specific radiolabeled substrates in CC synaptosomes from adult (4 months old) and aged rats (28 months old). In adult animals, in the presence of the tyrosine phosphatase inhibitor (sodium o-vanadate), insulin stimulated PLD activity at 5 min incubation. DAGK activity was also increased at the same time of incubation and PAP2 was inhibited. In aged animals, PLD activity was not modified by the presence of insulin plus vanadate, PAP2 was inhibited, and DAGK was stimulated by the hormone. Insulin, vanadate, and the combination of both induced protein tyrosine phosphorylation in adult CC synaptosomes. Aged rats showed a lower level of protein phosphorylation with respect to adult rats. Our results show that insulin modulates PA and DAG availability through the regulation of PLD/PAP2 and DAGK pathways in adult rat CC synaptosomes. Additionally, we demonstrated that PA and DAG generation is regulated differentially by insulin during aging.

Phospholipase D and phosphatidate phosphohydrolase activities in rat cerebellum during aging

Aging is a process that affects different organs, of which the brain is particularly susceptible. PA and DAG are central intermediates in the phosphoglyceride as well as in the neutral lipid biosynthetic pathway, and they have also been implicated in signal transduction. Phospholipase D (PLD) and phosphatidate phosphohydrolase (PAP) are the enzymes that generate PA and DAG. The latter can be transformed into MAG by diacylglycerol lipase (DGL). In the present study, we examine how aging modulates the PLD, PAP, and DGL isoforms in cerebellar subcellular fractions from 4- (adult), 28-, and 33-mon-old (aged) rats. PI-4,5-bisphosphonate (PIP2)-dependent PLD, PAP1, and DGL1 were distributed in different percentages in all cerebellum subcellular fractions. On the other hand, PAP2 and DGL2 activities were observed in all subcellular fractions except in the cytosolic fraction. Aging modified the enzyme distribution pattern. In addition, aging decreased nuclear (45%), mitochondrial-synaptosomal (55%), and cytosolic (71%) PAP1 activity and increased (28%) microsomal PAP1 activity. DGL1 activity was decreased in nuclear (85%) and mitochondrial-synaptosomal (63%) fractions by aging. On the other hand, PIP2-dependent PLD activities were increased in the mitochondrial-synaptosomal fraction. PAP2 and DGL2 were increased in the microsomal fraction by 87 and 114%, respectively, and they were decreased in the nuclear fraction. The changes observed in cerebellum PAP1 and DGL1 activities from aged rats with respect to adult rats could be related to modifications in lipid metabolism. Differential PA metabolization during aging through PIP2-dependent PLD/PAP2/DGL2 activities could be related to alterations in the neural signal transduction mechanisms.

Age-related changes in central nervous system phosphatidylserine decarboxylase activity

A significant portion of brain phosphatidylethanolamine (PE) is synthesized by a pathway involving the mitochondrial enzyme phosphatidylserine decarboxylase (PSDC), in a process by which phosphatidylserine (PS) is transferred from the endoplasmic reticulum to mitochondria. Aging changes the fatty acid composition of brain phospholipids, PS and PE being the most affected. The present study was carried out to determine PSDC activity in cerebral cortex (CC) and cerebellum (CRBL) mitochondrial fraction from adult (4-month-old) and aged (30-month-old) rats and to compare these activities with that found in liver. To study the effect of 22:6n-3 content on the PSDC activity, PSs from different sources were prepared: rPS (from bovine retina, containing 36 mol % of 22:6n-3); adPS (from adult rat CC microsomal membranes, with 25 mole % 22:6n-3 content) and agPS (from aged rat CC microsomal membranes, with 21 mole % 22:6n-3 content). For aged CC PSDC, the preferred substrate was agPS (the physiological substrate for aged animals), whereas in adult CC PSDC the substrate preference was inverse (rPS > adPS > agPS). Furthermore, CRBL PSDC does not show any substrate preference based on 22:6n-3 content. CRBL PSDC activity in aged membranes using agPS as substrate is lower than PSDC activity in adult membranes in the presence of adPS. These results indicate that under physiological conditions, cerebellar PSDC is inhibited during aging. Liver PSDC activity showed the same substrate preference in adult and aged rats as adult CC PSDC. These findings lead us to conclude that PSDC activity has a differential tissue-dependent substrate preference characteristic of the aging process.

Age-associated changes in central nervous system glycerolipid composition and metabolism

In this review, changes in brain lipid composition and metabolism due to aging are outlined. The most striking changes in cerebral cortex and cerebellum lipid composition involve an increase in acidic phospholipid synthesis. The most important changes with respect to fatty acyl composition involve a decreased content in polyunsaturated fatty acids (20:4n-6, 22:4n-6, 22:6n-3) and an increased content in monounsaturated fatty acids (18:1n-9 and 20:1n-9), mainly in ethanolamine and serineglycerophospholipids. Changes in the activity of the enzymes modifying the phospholipid headgroup occur during aging. Serine incorporation into phosphatidylserine through base-exchange reactions and phosphatidylcholine synthesis through phosphatidylethanolamine methylation increases in the aged brain. Phosphatidate phosphohydrolase and phospholipase D activities are also altered in the aged brain thus producing changes in the lipid second messengers diacylglycerol and phosphatidic acid.

Age-dependent agonist-specific dysregulation of membrane-mediated signal transduction: emergence of the gate theory of aging

Although a general mechanism for the limited responsiveness of senescent cells has yet to be established, reduced responsiveness may in part be ascribed to deficits in the apparatus required for cell surface receptor-mediated signal transduction. Age-related changes of receptor-mediated signal transduction occur at many levels, and are known to include quantitative and qualitative changes in growth factor receptors, G-protein coupled receptors, and many other downstream signaling molecules. Here, we emphasize the prime role of the cellular surface in the perception and transmission of external stimuli in response to the aging process. As major means of cellular signal transduction, the receptor tyrosine kinase (RTK) system and the G protein-coupled receptor (GPCR) system of senescent cells were investigated. We observed that the RTK system was severely damaged, while the GPCR system was only partially inactivated by aging. These results suggest that the agonist-dependent dysregulation of and imbalance of signal transduction pathways might be responsible for the functional deterioration of senescent cells, and indicate a possibility of the functional recovery of senescent cells through agonist-specific signal system activation. Moreover, those data evoke the emerging concept that the senescent phenotype may be modulated by the membrance-associated signal system, implying the gate theory of aging.