Phospholipase D and phosphatidate phosphohydrolase activities in rat cerebellum during aging

Proliferative Glioblastoma Cancer Cells Exhibit Persisting Temporal Control of Metabolism and Display Differential Temporal Drug Susceptibility in Chemotherapy

Even in immortalized cell lines, circadian clocks regulate physiological processes in a time-dependent manner, driving transcriptional and metabolic rhythms, the latter being able to persist without transcription. Circadian rhythm disruptions in modern life (shiftwork, jetlag, etc.) may lead to higher cancer risk. Here, we investigated whether the human glioblastoma T98G cells maintained quiescent or under proliferation keep a functional clock and whether cells display differential time responses to bortezomib chemotherapy. In arrested cultures, mRNAs for clock (Per1, Rev-erbα) and glycerophospholipid (GPL)-synthesizing enzyme genes, ³²P-GPL labeling, and enzyme activities exhibited circadian rhythmicity; oscillations were also found in the redox state/peroxiredoxin oxidation. In proliferating cells, rhythms of gene expression were lost or their periodicity shortened whereas the redox and GPL metabolisms continued to fluctuate with a similar periodicity as under arrest. Cell viability significantly changed over time after bortezomib treatment; however, this rhythmicity and the redox cycles were altered after Bmal1 knock-down, indicating cross-talk between the transcriptional and the metabolic oscillators. An intrinsic metabolic clock continues to function in proliferating cells, controlling diverse metabolisms and highlighting differential states of tumor suitability for more efficient, time-dependent chemotherapy when the redox state is high and GPL metabolism low.

The Catalytic Efficiency of Lipin 1β Increases by Physically Interacting with the Proto-oncoprotein c-Fos

Phosphatidic acid (PA) is a central precursor for membrane phospholipid biosynthesis. The lipin family is a magnesium-dependent type I PA phosphatase involved in de novo synthesis of neutral lipids and phospholipids. The regulation of lipin activity may govern the pathways by which these lipids are synthesized and control the cellular levels of important signaling lipids. Moreover, the proto-oncoprotein c-Fos has an emerging role in glycerolipid synthesis regulation; by interacting with key synthesizing enzymes it is able to increase overall phospho- and glycolipid synthesis. We studied the lipin 1β enzyme activity in a cell-free system using PA/Triton X-100 mixed micelles as substrate, analyzing it in the presence/absence of c-Fos. We found that lipin 1β kcat value increases around 40% in the presence of c-Fos, with no change in the lipin 1β affinity for the PA/Triton X-100 mixed micelles. We also probed a physical interaction between both proteins. Although the c-Fos domain involved in lipin activation is its basic domain, the interaction domain is mapped to the N-terminal c-Fos. In conclusion, we provide evidence for a novel positive regulator of lipin 1β PA phosphatase activity that is not achieved via altering its subcellular localization or affinity for membranes but rather through directly increasing its catalytic efficiency.

The mouse liver displays daily rhythms in the metabolism of phospholipids and in the activity of lipid synthesizing enzymes

The circadian system involves central and peripheral oscillators regulating temporally biochemical processes including lipid metabolism; their disruption leads to severe metabolic diseases (obesity, diabetes, etc). Here, we investigated the temporal regulation of glycerophospholipid (GPL) synthesis in mouse liver, a well-known peripheral oscillator. Mice were synchronized to a 12:12 h light-dark (LD) cycle and then released to constant darkness with food ad libitum. Livers collected at different times exhibited a daily rhythmicity in some individual GPL content with highest levels during the subjective day. The activity of GPL-synthesizing/remodeling enzymes: phosphatidate phosphohydrolase 1 (PAP-1/lipin) and lysophospholipid acyltransferases (LPLATs) also displayed significant variations, with higher levels during the subjective day and at dusk. We evaluated the temporal regulation of expression and activity of phosphatidylcholine (PC) synthesizing enzymes. PC is mainly synthesized through the Kennedy pathway with Choline Kinase (ChoK) as a key regulatory enzyme or through the phosphatidylethanolamine (PE) N-methyltransferase (PEMT) pathway. The PC/PE content ratio exhibited a daily variation with lowest levels at night, while ChoKα and PEMT mRNA expression displayed maximal levels at nocturnal phases. Our results demonstrate that mouse liver GPL metabolism oscillates rhythmically with a precise temporal control in the expression and/or activity of specific enzymes.

Daily rhythms of glycerophospholipid synthesis in fibroblast cultures involve differential enzyme contributions

Circadian clocks regulate the temporal organization of several biochemical processes, including lipid metabolism, and their disruption leads to severe metabolic disorders. Immortalized cell lines acting as circadian clocks display daily variations in [(32)P]phospholipid labeling; however, the regulation of glycerophospholipid (GPL) synthesis by internal clocks remains unknown. Here we found that arrested NIH 3T3 cells synchronized with a 2 h-serum shock exhibited temporal oscillations in a) the labeling of total [(3)H] GPLs, with lowest levels around 28 and 56 h, and b) the activity of GPL-synthesizing and GPL-remodeling enzymes, such as phosphatidate phosphohydrolase 1 (PAP-1) and lysophospholipid acyltransferases (LPLAT), respectively, with antiphase profiles. In addition, we investigated the temporal regulation of phosphatidylcholine (PC) biosynthesis. PC is mainly synthesized through the Kennedy pathway with choline kinase (ChoK) and CTP:phosphocholine cytidylyltranferase (CCT) as key regulatory enzymes. We observed that the PC labeling exhibited daily changes, with the lowest levels every ~28 h, that were accompanied by brief increases in CCT activity and the oscillation in ChoK mRNA expression and activity. Results demonstrate that the metabolisms of GPLs and particularly of PC in synchronized fibroblasts are subject to a complex temporal control involving concerted changes in the expression and/or activities of specific synthesizing enzymes.

Mouse lipin-1 and lipin-2 cooperate to maintain glycerolipid homeostasis in liver and aging cerebellum

The three lipin phosphatidate phosphatase (PAP) enzymes catalyze a step in glycerolipid biosynthesis, the conversion of phosphatidate to diacylglycerol. Lipin-1 is critical for lipid synthesis and homeostasis in adipose tissue, liver, muscle, and peripheral nerves. Little is known about the physiological role of lipin-2, the predominant lipin protein present in liver and the deficient gene product in the rare disorder Majeed syndrome. By using lipin-2-deficient mice, we uncovered a functional relationship between lipin-1 and lipin-2 that operates in a tissue-specific and age-dependent manner. In liver, lipin-2 deficiency led to a compensatory increase in hepatic lipin-1 protein and elevated PAP activity, which maintained lipid homeostasis under basal conditions, but led to diet-induced hepatic triglyceride accumulation. As lipin-2-deficient mice aged, they developed ataxia and impaired balance. This was associated with the combination of lipin-2 deficiency and an age-dependent reduction in cerebellar lipin-1 levels, resulting in altered cerebellar phospholipid composition. Similar to patients with Majeed syndrome, lipin-2-deficient mice developed anemia, but did not show evidence of osteomyelitis, suggesting that additional environmental or genetic components contribute to the bone abnormalities observed in patients. Combined lipin-1 and lipin-2 deficiency caused embryonic lethality. Our results reveal functional interactions between members of the lipin family in vivo, and a unique role for lipin-2 in central nervous system biology that may be particularly important with advancing age. Additionally, as has been observed in mice and humans with lipin-1 deficiency, the pathophysiology in lipin-2 deficiency is associated with dysregulation of lipid intermediates.

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.

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.

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.

The cardiac pacemaker current

In mammals cardiac rate is determined by the duration of the diastolic depolarization of sinoatrial node (SAN) cells which is mainly determined by the pacemaker I(f) current. f-channels are encoded by four members of the hyperpolarization-activated cyclic nucleotide-gated gene (HCN1-4) family. HCN4 is the most abundant isoform in the SAN, and its relevance to pacemaking has been further supported by the discovery of four loss-of-function mutations in patients with mild or severe forms of cardiac rate disturbances. Due to its selective contribution to pacemaking, the I(f) current is also the pharmacological target of a selective heart rate-reducing agent (ivabradine) currently used in the clinical practice. Albeit to a minor extent, the I(f) current is also present in other spontaneously active myocytes of the cardiac conduction system (atrioventricular node and Purkinje fibres). In working atrial and ventricular myocytes f-channels are expressed at a very low level and do not play any physiological role; however in certain pathological conditions over-expression of HCN proteins may represent an arrhythmogenic mechanism. In this review some of the most recent findings on f/HCN channels contribution to pacemaking are described.

Regulation of gating and rundown of HCN hyperpolarization-activated channels by exogenous and endogenous PIP2

The voltage dependence of activation of the HCN hyperpolarization-activated cation channels is shifted in inside-out patches by −40 to −60 mV relative to activation in intact cells, a phenomenon referred to as rundown. Less than 20 mV of this hyperpolarizing shift can be due to the influence of the canonical modulator of HCN channels, cAMP. Here we study the role of phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) in HCN channel rundown, as hydrolysis of PI(4,5)P₂ by lipid phosphatases is thought to underlie rundown of several other channels. We find that bath application of exogenous PI(4,5)P₂ reverses the effect of rundown, producing a large depolarizing shift in HCN2 activation. A synthetic short chain analogue of PI(4,5)P₂, dioctanoyl phosphatidylinositol 4,5-bisphosphate, shifts the HCN2 activation curve to more positive potentials in a dose-dependent manner. Other dioctanoyl phosphatidylinositides with one or more phosphates on the lipid headgroup also shift activation, although phosphatidylinositol (PI) is ineffective. Several lines of evidence suggest that HCN2 is also regulated by endogenous PI(4,5)P₂: (a) blockade of phosphatases slows the hyperpolarizing shift upon patch excision; (b) application of an antibody that binds and depletes membrane PIP₂ causes a further hyperpolarizing shift in activation; (c) the shift in activation upon patch excision can be partially reversed by MgATP; and (d) the effect of MgATP is blocked by wortmannin, an inhibitor of PI kinases. Finally, recordings from rabbit sinoatrial cells demonstrate that diC₈ PI(4,5)P₂ delays the rundown of native HCN currents. Thus, both native and recombinant HCN channels are regulated by PI(4,5)P₂.

Phospholipase D1 is up-regulated in the mitochondrial fraction from the brains of Alzheimer's disease patients

Mitochondrial dysfunction may play an important role in sporadic Alzheimer's disease (AD) progression. Recently, we have reported that amyloid precursor protein (APP) stimulates phospholipase D (PLD) activity and beta-amyloid (Abeta) region of APP is involved in the interaction with PLD1. To elucidate the involvement of PLD in the pathophysiology of AD, we examined the expression of PLD1 and alteration of membrane phospholipid in mitochondrial membranes of control and AD brains using Western blot and phospholipid analysis by thin layer chromatography (TLC). We have found that protein expression of PLD1 was significantly increased in mitochondrial fraction of brains of AD patients compared with that in control brains. Furthermore, the concentration of mitochondrial phospholipids such as phosphatidylcholine (PC) and phosphatidylethanolamine (PE) was increased and the content of phosphatidic acid (PA), a product of PLD activity, was up-regulated in the mitochondrial membrane fractions of AD brain compared with that of control brain. These results suggest that up-regulation of PLD1 in the mitochondrial fraction of AD brain might affect the composition of membrane phospholipids and provide a clue to the mechanism underlying the mitochondrial dysfunction associated with AD.

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.

Phospholipase D1 is associated with amyloid precursor protein in Alzheimer's disease

Amyloid precursor protein (APP) is a widely expressed transmembrane protein of unknown function that is involved in the pathogenesis of Alzheimer's disease (AD). We investigated the involvement of phospholipase D (PLD) in the pathophysiology of AD. We showed dramatic upregulation of PLD1 immunoreactivity in reactive astroglial cells in brain tissue sections from authentic AD patients. Expression and activity of PLD1 were up-regulated in brain tissues from AD patients, especially caveolae membrane fraction, compared with those of control brains. Interestingly, PLD1 physically interacts and colocalizes with APP and caveolin-3. We found that APP was associated with the pleckstrin homology domain of PLD1, and the amyloid region of APP interacted with PLD. Elevated expression of APP stimulated PLD activity in human astroglioma cells. These results suggest that up-regulation of PLD might have a role in the neuronal pathology associated with AD.

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.