Long-chain fatty acids and their acyl-CoA esters cause the translocation of phosphatidate phosphohydrolase from the cytosolic to the microsomal fraction of rat liver

Lipin-1-derived diacylglycerol activates intracellular TRPC3 which is critical for inflammatory signaling

Exposure to Gram-negative bacterial LPS exacerbates host immune responses and may lead to sepsis, a life-threatening condition. Despite its high mortality and morbidity, no drugs specifically directed to treating sepsis are currently available. Using human cell genetic depletion, pharmacological inhibition, live-cell microscopy and organelle-targeted molecular sensors we present evidence that the channel TRPC3 is activated intracellularly during macrophage exposure to LPS and is essential for Ca²⁺ release from internal stores. In this manner, TRPC3 participates in cytosolic Ca²⁺ elevations, activation of the transcription factor NF-κB and cytokine upregulation. We also report that TRPC3 is activated by diacylglycerol generated by the phosphatidic acid phosphatase lipin-1. In accord with this, lipin-1-deficient cells exhibit reduced Ca²⁺ responses to LPS challenge. Finally, pharmacological inhibition of TRPC3 reduces systemic inflammation induced by LPS in mice. Collectively, our study unveils a central component of LPS-triggered Ca²⁺ signaling that involves intracellular sensing of lipin-1-derived DAG by TRPC3, and opens new opportunities for the development of strategies to treat LPS-driven inflammation.

Triglyceride Metabolism in the Liver

Triglyceride molecules represent the major form of storage and transport of fatty acids within cells and in the plasma. The liver is the central organ for fatty acid metabolism. Fatty acids accrue in liver by hepatocellular uptake from the plasma and by de novo biosynthesis. Fatty acids are eliminated by oxidation within the cell or by secretion into the plasma within triglyceride-rich very low-density lipoproteins. Notwithstanding high fluxes through these pathways, under normal circumstances the liver stores only small amounts of fatty acids as triglycerides. In the setting of overnutrition and obesity, hepatic fatty acid metabolism is altered, commonly leading to the accumulation of triglycerides within hepatocytes, and to a clinical condition known as nonalcoholic fatty liver disease (NAFLD). In this review, we describe the current understanding of fatty acid and triglyceride metabolism in the liver and its regulation in health and disease, identifying potential directions for future research. Advances in understanding the molecular mechanisms underlying the hepatic fat accumulation are critical to the development of targeted therapies for NAFLD. © 2018 American Physiological Society. Compr Physiol 8:1-22, 2018.

How lipid droplets "TAG" along: Glycerolipid synthetic enzymes and lipid storage

Triacylglycerols (TAG) serve as the predominant form of energy storage in mammalian cells, and TAG synthesis influences conditions such as obesity, fatty liver, and insulin resistance. In most tissues, the glycerol 3-phosphate pathway enzymes are responsible for TAG synthesis, and the regulation and function of these enzymes is therefore important for metabolic homeostasis. Here we review the sites and regulation of glycerol-3-phosphate acyltransferase (GPAT), acylglycerol-3-phosphate acyltransferase (AGPAT), lipin phosphatidic acid phosphatase (PAP), and diacylglycerol acyltransferase (DGAT) enzyme action. We highlight the critical roles that these enzymes play in human health by reviewing Mendelian disorders that result from mutation in the corresponding genes. We also summarize the valuable insights that genetically engineered mouse models have provided into the cellular and physiological roles of GPATs, AGPATs, lipins and DGATs. Finally, we comment on the status and feasibility of therapeutic approaches to metabolic disease that target enzymes of the glycerol 3-phosphate pathway. This article is part of a Special Issue entitled: Recent Advances in Lipid Droplet Biology edited by Rosalind Coleman and Matthijs Hesselink.

Lipin proteins and glycerolipid metabolism: Roles at the ER membrane and beyond

The regulation of glycerolipid biosynthesis is critical for homeostasis of cellular lipid stores and membranes. Here we review the role of lipin phosphatidic acid phosphatase enzymes in glycerolipid synthesis. Lipin proteins are unique among glycerolipid biosynthetic enzymes in their ability to transit among cellular membranes, rather than remain membrane tethered. We focus on the mechanisms that underlie lipin protein interactions with membranes and the versatile roles of lipins in several organelles, including the endoplasmic reticulum, mitochondria, endolysosomes, lipid droplets, and nucleus. We also review the corresponding physiological roles of lipins, which have been uncovered by the study of genetic lipin deficiencies. We propose that the growing body of knowledge concerning the biochemical and cellular activities of lipin proteins will be valuable for understanding the physiological functions of lipin proteins in health and disease. This article is part of a Special Issue entitled: Membrane Lipid Therapy: Drugs Targeting Biomembranes edited by Pablo V. Escribá.

Gender differences in the expression and cellular localization of lipin 1 in the hearts of fructose-fed rats

To give new insight to alterations of cardiac lipid metabolism accompanied by a fructose-rich diet (FRD), rats of both sexes were exposed to 10 % fructose in drinking water during 9 weeks. The protein level and subcellular localization of the main regulators of cardiac lipid metabolism, such as lipin 1, peroxisome proliferator-activated receptor α (PPARα), peroxisome proliferator-activated receptor γ coactivator-1 α (PGC-1α), carnitine palmitoyltransferase I (CPTI), and CD36 were studied. Caloric intake in fructose-fed rats (FFR) of both sexes was increased. Circulating triacylglyceroles (TAG) and non-esterified fatty acids were increased in male FFR, while females increased visceral adiposity and blood TAG. Total expression of lipin 1 in cardiac cell lysate and its cytosolic and microsomal level were increased in the hearts of male FFR. PPARα and PGC-1α content were decreased in the nuclear extract. In addition, cardiac deposition of TAG in male FFR was elevated, as well as inhibitory phosphorylation of insulin receptor substrate 1 (IRS-1). In contrast, in female FFR, lipin 1 level was increased in nuclear extract only, while overall CPTI expression and phosphorylation of IRS-1 at serine 307 were decreased. The results of our study suggest that fructose diet causes gender-dependent alterations in cardiac lipid metabolism. Potentially detrimental effects of FRD seem to be limited to male rats. Most of the observed changes might be a consequence of elevated expression and altered localization of lipin 1. Increased inhibitory phosphorylation of IRS-1 is possible link between cardiac lipid metabolism and insulin resistance in FFR.

Conserved residues in the N terminus of lipin-1 are required for binding to protein phosphatase-1c, nuclear translocation, and phosphatidate phosphatase activity

Lipin-1 is a phosphatidate phosphatase in glycerolipid biosynthesis and signal transduction. It also serves as a transcriptional co-regulator to control lipid metabolism and adipogenesis. These functions are controlled partly by its subcellular distribution. Hyperphosphorylated lipin-1 remains sequestered in the cytosol, whereas hypophosphorylated lipin-1 translocates to the endoplasmic reticulum and nucleus. The serine/threonine protein phosphatase-1 catalytic subunit (PP-1c) is a major protein dephosphorylation enzyme. Its activity is controlled by interactions with different regulatory proteins, many of which contain conserved RVXF binding motifs. We found that lipin-1 binds to PP-1cγ through a similar HVRF binding motif. This interaction depends on Mg(2+) or Mn(2+) and is competitively inhibited by (R/H)VXF-containing peptides. Mutating the HVRF motif in the highly conserved N terminus of lipin-1 greatly decreases PP-1cγ interaction. Moreover, mutations of other residues in the N terminus of lipin-1 also modulate PP-1cγ binding. PP-1cγ binds poorly to a phosphomimetic mutant of lipin-1 and binds well to the non-phosphorylatable lipin-1 mutant. This indicates that lipin-1 is dephosphorylated before PP-1cγ binds to its HVRF motif. Importantly, mutating the HVRF motif also abrogates the nuclear translocation and phosphatidate phosphatase activity of lipin-1. In conclusion, we provide novel evidence of the importance of the lipin-1 N-terminal domain for its catalytic activity, nuclear localization, and binding to PP-1cγ.

Phosphorylation of lipin 1 and charge on the phosphatidic acid head group control its phosphatidic acid phosphatase activity and membrane association

The lipin gene family encodes a class of Mg(2+)-dependent phosphatidic acid phosphatases involved in the de novo synthesis of phospholipids and triglycerides. Unlike other enzymes in the Kennedy pathway, lipins are not integral membrane proteins, and they need to translocate from the cytosol to intracellular membranes to participate in glycerolipid synthesis. The movement of lipin 1 within the cell is closely associated with its phosphorylation status. Although cellular analyses have demonstrated that highly phosphorylated lipin 1 is enriched in the cytosol and dephosphorylated lipin 1 is found on membranes, the effects of phosphorylation on lipin 1 activity and binding to membranes has not been recapitulated in vitro. Herein we describe a new biochemical assay for lipin 1 using mixtures of phosphatidic acid (PA) and phosphatidylethanolamine that reflects its physiological activity and membrane interaction. This depends on our observation that lipin 1 binding to PA in membranes is highly responsive to the electrostatic charge of PA. The studies presented here demonstrate that phosphorylation regulates the ability of the polybasic domain of lipin 1 to recognize di-anionic PA and identify mTOR as a crucial upstream signaling component regulating lipin 1 phosphorylation. These results demonstrate how phosphorylation of lipin 1 together with pH and membrane phospholipid composition play important roles in the membrane association of lipin 1 and thus the regulation of its enzymatic activity.

The human lipodystrophy protein seipin is an ER membrane adaptor for the adipogenic PA phosphatase lipin 1

Disruption of the gene BSCL2 causes a severe, generalised lipodystrophy, demonstrating the critical role of its protein product, seipin, in human adipose tissue development. Seipin is essential for adipocyte differentiation, whilst the study of seipin in non-adipose cells has suggested a role in lipid droplet formation. However, its precise molecular function remains poorly understood. Here we demonstrate that seipin can inducibly bind lipin 1, a phosphatidic acid (PA) phosphatase important for lipid synthesis and adipogenesis. Knockdown of seipin during early adipogenesis decreases the association of lipin 1 with membranes and increases the accumulation of its substrate PA. Conversely, PA levels are reduced in differentiating cells by overexpression of wild-type seipin but not by expression of a mutated seipin that is unable to bind lipin 1. Together our data identify lipin as the first example of a seipin-interacting protein and reveals a novel molecular function for seipin in developing adipocytes.

Phosphatidate phosphatase, a key regulator of lipid homeostasis

Yeast Pah1p phosphatidate phosphatase (PAP) catalyzes the penultimate step in the synthesis of triacylglycerol. PAP plays a crucial role in lipid homeostasis by controlling the relative proportions of its substrate phosphatidate and its product diacylglycerol. The cellular amounts of these lipid intermediates influence the synthesis of triacylglycerol and the pathways by which membrane phospholipids are synthesized. Physiological functions affected by PAP activity include phospholipid synthesis gene expression, nuclear/endoplasmic reticulum membrane growth, lipid droplet formation, and vacuole homeostasis and fusion. Yeast lacking Pah1p PAP activity are acutely sensitive to fatty acid-induced toxicity and exhibit respiratory deficiency. PAP is distinguished in its cellular location, catalytic mechanism, and physiological functions from Dpp1p and Lpp1p lipid phosphate phosphatases that utilize a variety of substrates that include phosphatidate. Phosphorylation/dephosphorylation is a major mechanism by which Pah1p PAP activity is regulated. Pah1p is phosphorylated by cytosolic-associated Pho85p-Pho80p, Cdc28p-cyclin B, and protein kinase A and is dephosphorylated by the endoplasmic reticulum-associated Nem1p-Spo7p phosphatase. The dephosphorylation of Pah1p stimulates PAP activity and facilitates the association with the membrane/phosphatidate allowing for its reaction and triacylglycerol synthesis. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Myocardial fatty acid metabolism and lipotoxicity in the setting of insulin resistance

Management of diabetes and insulin resistance in the setting of cardiovascular disease has become an important issue in an increasingly obese society. Besides the development of hypertension and buildup of atherosclerotic plaques, the derangement of fatty acid and lipid metabolism in the heart plays an important role in promoting cardiac dysfunction and oxidative stress. This review discusses the mechanisms by which metabolic inflexibility in the use of fatty acids as the preferred cardiac substrate in diabetes produces detrimental effects on mechanical efficiency, mitochondrial function, and recovery from ischemia. Lipid accumulation and the consequences of toxic lipid metabolites are also discussed.

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.

Insulin-stimulated interaction with 14-3-3 promotes cytoplasmic localization of lipin-1 in adipocytes

Lipin-1 is a bifunctional protein involved in lipid metabolism and adipogenesis. Lipin-1 plays a role in the biosynthesis of triacylglycerol through its phosphatidate phosphatase activity and also acts as a transcriptional co-activator of genes involved in oxidative metabolism. Lipin-1 resides in the cytoplasm and translocates to the endoplasmic reticulum membrane to catalyze the phosphatidate phosphatase reaction. It also possesses a nuclear localization signal, which is required for its translocation to the nucleus and may therefore be important for lipin-1 co-activator function. Thus, subcellular localization may be an important factor in the regulation of this protein. Here, we show that the nuclear localization signal alone is not sufficient for lipin-1 nuclear localization, and identify lipin-1 interaction with 14-3-3 as a determinant of its subcellular localization. We demonstrate that lipin-1 interacts with 14-3-3 proteins and that overexpression of 14-3-3 promotes the cytoplasmic localization of lipin-1 in 3T3-L1 adipocytes. The effect of 14-3-3 is mediated through a serine-rich domain in lipin-1. Functional mapping of the 14-3-3-interacting region within the serine-rich domain indicates redundancy and cooperativity among several sites, including five phosphorylated serine and threonine residues. Insulin stimulation of 3T3-L1 adipocytes results in increased lipin-1 phosphorylation, enhanced interaction with 14-3-3, and predominantly cytoplasmic localization. In summary, our studies suggest that insulin may modulate the cellular function of lipin-1 by regulating its subcellular localization through interactions with 14-3-3 proteins.

Phosphatidate degradation: phosphatidate phosphatases (lipins) and lipid phosphate phosphatases

Three lipid phosphate phosphatases (LPPs) regulate cell signaling by modifying the concentrations of a variety of lipid phosphates versus their dephosphorylated products. In particular, the LPPs are normally considered to regulate signaling by the phospholipase D (PLD) pathway by converting phosphatidate (PA) to diacylglycerol (DAG). LPP activities do modulate the accumulations of PA and DAG following PLD activation, but this could also involve an effect upstream of PLD activation. The active sites of the LPPs are on the exterior surface of plasma membranes, or on the luminal surface of internal membranes. Consequently, the actions of the LPPs in metabolizing PA formed by PLD1 or PLD2 should depend on the access of this substrate to the active site of the LPPs. Alternatively, PA generated on the cytosolic surface of membranes should be readily accessible to the family of specific phosphatidate phosphatases, namely the lipins. Presently, there is only indirect evidence for the lipins participating in cell signaling following PLD activation. So far, we know relatively little about how individual LPPs and specific phosphatidate phosphatases (lipins) modulate cell signaling through controlling the turnover of bioactive lipids that are formed after PLD activation.

Early stages of Golgi vesicle and tubule formation require diacylglycerol

We have investigated the role for diacylglycerol (DAG) in membrane bud formation in the Golgi apparatus. Addition of propranolol to specifically inhibit phosphatidate phosphohydrolase (PAP), an enzyme responsible for converting phosphatidic acid into DAG, effectively prevents formation of membrane buds. The effect of PAP inhibition on Golgi membranes is rapid and occurs within 3 min. Removal of the PAP inhibitor then results in a rapid burst of buds, vesicles, and tubules that peaks within 2 min. The inability to form buds in the presence of propranolol does not appear to be correlated with a loss of ARFGAP1 from Golgi membranes, as knockdown of ARFGAP1 by RNA interference has little or no effect on actual bud formation. Rather, knockdown of ARFGAP1 results in an increase in membrane buds and a decrease of vesicles and tubules suggesting it functions in the late stages of scission. How DAG promotes bud formation is discussed.

Coordinated regulation of esterification and lipolysis by palmitate, H2O2 and the anti-diabetic sulfonylurea drug, glimepiride, in rat adipocytes

Inhibition of lipolysis by palmitate, H2O2 and the anti-diabetic sulfonylurea drug, glimepiride, in isolated rat adipocytes has previously been shown to rely on the degradation of cyclic adenosine monophosphate by the phosphodiesterase, Gce1, and the 5'-nucleotidase, CD73. These glycosylphosphatidylinositol (GPI)-anchored proteins are translocated from plasma membrane lipid rafts to intracellular lipid droplets upon H2O2-induced activation of a GPI-specific phospholipase C (GPI-PLC) in response to palmitate and glimepiride in intact adipocytes and, as demonstrated here, in cell-free systems as well. The same agents are also known to stimulate the incorporation of fatty acids into triacylglycerol. Here the involvement of H2O2 production, GPI-PLC activation and translocation of Gce1 and CD73 in the agent-induced esterification and accompanying lipid droplet formation was tested in rat adipocytes using relevant inhibitors. The results demonstrate that upregulation of the esterification and accumulation of triacylglycerol by glimepiride depends on the sequential H2O2-induced GPI-PLC activation and GPI-protein translocation as does inhibition of lipolysis. In contrast, stimulation of the esterification and triacylglycerol accumulation by palmitate relies on insulin-independent tyrosine phosphorylation and thus differs from its anti-lipolytic mechanism. As expected, insulin regulates lipid metabolism via typical insulin signalling independent of H2O2 production, GPI-PLC activation and GPI-protein translocation, albeit these processes are moderately stimulated by insulin. In conclusion, triacylglycerol and lipid droplet formation in response to glimepiride and H2O2 may involve the hydrolysis of cyclic adenosine monophosphate by lipid droplet-associated Gce1 and CD73 which may regulate lipid droplet-associated triacylglycerol-synthesizing and hydrolyzing enzymes in coordinated and inverse fashion.

Three mammalian lipins act as phosphatidate phosphatases with distinct tissue expression patterns

We previously identified mutations in the Lpin1 gene, encoding lipin-1, as the underlying cause of lipodystrophy in the fatty liver dystrophy (fld) mutant mouse. Lipin-1 is normally expressed at high levels in adipose tissue and skeletal muscle, and deficiency in the fld mouse causes impaired adipose tissue development, insulin resistance, and altered energy expenditure. We also identified two additional lipin protein family members of unknown function, lipin-2 and lipin-3. Han et al. (Han, G. S., Wu, W. I., and Carman, G. M. (2006) J. Biol. Chem. 281, 9210-9218) recently demonstrated that the single lipin homolog in yeast, Smp2, exhibits phosphatidate phosphatase type-1 (PAP1) activity, which has a key role in glycerolipid synthesis. Here we demonstrate that lipin-1 accounts for all of the PAP1 activity in white and brown adipose tissue and skeletal muscle. However, livers of lipin-1-deficient mice exhibited normal PAP1 activity, indicating that other members of the lipin protein family could have PAP1 activity. Consistent with this possibility, recombinant lipin-2 and lipin-3 possess PAP1 activity. Each of the three lipin family members showed Mg2+-dependent activity that was specific for phosphatidate under the conditions employed. The different lipins showed distinct tissue expression patterns. Our results establish the three mammalian lipin proteins as PAP1 enzymes and explain the biochemical basis for lipodystrophy in the lipin-1-deficient fld mouse.

Purification of Mg2+-dependent phosphatidate phosphohydrolase from rat liver: new steps and aspects

A new procedure for the partial purification of Mg2+-dependent, N-ethylmaleimide-sensitive phosphatidate phosphohydrolase (Mg2+-PAP; EC 3.1.3.4) from rat liver cytosol is described, using protein precipitation with MgCl2, gel filtration on Sephacryl S-400, chromatography on DEAE-cellulose and affinity chromatography on calmodulin-agarose. From the parallel change in staining intensity and in the level of the specific activity of enzyme fractions, a relationship between a 90-kDa SDS gel band, identified as the beta-isoform of the 90-kDa heat shock protein, and Mg2+-PAP could be detected.

Relationship between the inhibition of phosphatidic acid phosphohydrolase-1 by oleate and oleoyl-CoA ester and its apparent translocation

Phosphatidic acid phosphohydrolase-1 (PAP-1) activity is reversibly inhibited by fatty acids and their acyl-CoA esters and it appears paradoxical that these effectors have been reported to increase the liver's esterification capacity by translocating the rate-limiting enzyme PAP-1 from cytosol to the endoplasmic reticulum. Therefore, we have examined the effect of oleate, oleoyl-CoA, and spermine on the activation and translocation of PAP-1 of rat liver. PAP-1 activity is directly inhibited by oleic acid and oleoyl-CoA ester in an allosteric manner, resulting in the formation of inactive PAP-1-fatty acid (or -acyl-CoA) complex, even in the absence of any subcellular structures. Such association/aggregation of PAP-1 can be easily collected by centrifugation and may explain the apparent translocation phenomenon of this enzyme to a particular structure in the presence of fatty acids or acyl-CoA esters as reported in many works. Indeed, incubation of cytosol fraction alone with oleate or oleoyl-CoA at 37 degrees C, followed by centrifugation, induces a significant increase (sevenfold) in PAP-1 activity in the pellet fraction. This displacement is accompanied by an increase in the specific activity of PAP-1 in the pellet fraction. Spermine is less effective than oleate in inducing the displacement of PAP-1 activity from cytosol to the pellet fraction in the absence of any membrane structures. This apparent translocation of PAP-1 is also promoted when homogenate fraction was incubated with oleate prior to the preparation of cytosol and microsomal fraction. Thus, many of the announced factors, including fatty acids, would promote the in vitro association/aggregation of PAP-1 enzyme rather than its translocation, and therefore, re-evaluation of the reported effects on PAP-1 translocation phenomenon is required. It is proposed that fatty acids and their esters would favour beta-oxidation over esterification by promoting the forming of inactive associated PAP-1 in situations such as starvation and metabolic stress in which there is an increased supply of fatty acids to the liver.

Regulation of phosphatidic acid phosphohydrolase 1 by fatty acids

In the starved state and during metabolic stress, free fatty acids (FFA) are the principal hepatic energy supply, undergoing beta-oxidation. Accordingly, it appears paradoxical that FFA have been reported to increase the liver's esterification capacity by translocating the rate-limiting enzyme phosphatidic acid phosphohydrolase (PAP-1) from the cytosol to the endoplasmic reticulum. We have therefore investigated the regulation of rat liver PAP-1. Oleic acid inhibited PAP activity in all subcellular fractions, with PAP-1 activity in cytosol being the most sensitive. Inhibition was also observed with oleoyl-CoA, linoleate, and palmitate. Fatty acids and their derivatives show detergent effects at high concentrations, and such effects can lead to enzyme inhibition. Inhibition by oleate, however, was reversed by phosphatidic acid and albumin and exhibited sigmoidal kinetics. These results demonstrate that PAP-1 is reversibly inhibited by FFA and their CoA esters, which may play a role in directing hepatic FFA to beta-oxidation during times of increased energy demand.

Aging promotes a different phosphatidic acid utilization in cytosolic and microsomal fractions from brain and liver

Among the morphological and biochemical changes taking place in the membranes of aged tissues, we reported in previous studies on alterations in phospholipid synthesis and phospholipid-specific fatty acid composition. Phosphatidic acid (PA) and diacylglycerol (DAG) are central intermediates in phosphoglyceride and neutral lipid biosynthetic pathways and have also recently been implicated in signal transduction. The present paper shows the effect of aging on phosphatidate phosphohydrolase (PAPase) activiy, which operates on phosphatidic acid to synthesize diacylglycerol. Two forms of mammalian PAPase can be indentified on the basis of subcellular localization and enzyme properties, one involved predominantly in lipid synthesis (PAP 1) and the other in signal transduction (PAP 2). Microsomal and cytosolic fractions of brain and liver from 3.5-month-old (adult) and 28.5-month-old (aged) rats were used. PAPase isoform activities were differentiated on the basis of N-ethylmaleimide (NEM) sensitivity and Mg(2+)-dependency. Our results demonstrate that aging caused PAP 2 to increase in brain microsomal fractions but did not affect PAP 1, whereas in brain cytosolic fractions, it caused a strong decrease in PAP 1 (57%). The distribution of enzymes between microsomes and cytosol changed in aged rats with respect to adult rats, showing a translocation of PAP 1 from cytosol to microsomes. In addition, an increase in the production of monoacylglycerol (MAG) was observed in microsomes from aged brain. PAP 2 activity in liver microsomal fractions from aged rats showed no changes with respect to adult rats whereas PAP 1 activity increased 228% in microsomal fractions and 76% in cytosolic fractions in this tissue. The distribution of PAP 1 activity between microsomal and cytosolic fractions in liver tissue was also affected in aged rats, indicating a translocation of this form of the enzyme from cytosolic to microsomal fractions. The production of monoacylglycerol in liver microsomes also increased, whereas there was a decrease in MAG formation from cytosolic fraction. The changes observed in the two PAPase forms in brain and liver of aged rats with respect to adult rats suggest that PA is differently utilized by the PAPase isoforms, probably generating aging-related DAGs different to those present in adults and required for specific cellular functions. The changes observed in liver PAP 1 from aged with respect to adult rats suggest that such changes could be related with modifications in lipid homeostasis induced by age-altered hormonal balance. However, PA-modified utilization during aging through PAP 2 activity could be related to alterations in neural signal transduction mechanisms.

Phosphatidate phosphatases of mammals, yeast, and higher plants

Phosphatidate phosphatase (EC 3.1.3.4) catalyzes the hydrolysis of phosphatidate to yield sn-1,2-diacylglycerol and inorganic phosphate. In mammalian systems, forms of phosphatidate phosphatase involved in glycerolipid synthesis and signal transduction have been identified. Forms of the enzyme involved in signal transduction have been purified and partially characterized. In yeast, phosphatidate phosphatases associated with the endoplasmic reticulum and mitochondria have also been purified and partially characterized. Information on phosphatidate phosphatases from mammals and yeast is useful in characterizing the enzyme from plant systems. This review examines progress on the characterization of phosphatidate phosphatases from mammals, yeast, and higher plants. The purification and characterization of the phosphatidate phosphatase involved in glycerolipid synthesis in developing oilseeds may lead to the identification of the encoding gene. Increasing our understanding of the enzymes of lipid synthesis in developing seeds will aid in the development of biotechnological strategies for seed oil modification.

Purification and characterization of N-ethylmaleimide-insensitive phosphatidic acid phosphohydrolase (PAP2) from rat liver

N-Ethylmaleimide-insensitive phosphatidic acid phosphohydrolase (PAP; EC 3.1.3.4) was purified 5900-fold from rat liver. The enzyme was solubilized from membranes with octylglucoside, fractionated with (NH4)2SO4, and purified in the presence of Triton X-100 by chromatography on Sephacryl S300, hydroxyapatite, heparin-Sepharose and Affi-Gel Blue. Silver-stained SDS/PAGE indicated that the enzyme was an 83 kDa polypeptide. Sephacryl S-300 gel filtration also produced a second peak of enzyme activity, which was eluted from all of the chromatography columns at a different position from the purified enzyme. SDS/PAGE indicated that it contained three polypeptides (83 kDa, 54 kDa and 34 kDa), and gel filtration suggested that it was not an aggregate of the purified enzyme. Both forms were sensitive to inhibition by amphiphilic amines, Mn2+ and Zn2+, but not by N-ethylmaleimide. Purified PAP required detergent for activity, but was not activated by Mg2+, fatty acids or phospholipids. The enzyme was able to dephosphorylate lysophosphatidic acid or phosphatidic acid, and was inhibited by diacylglycerol and monoacylglycerol. No evidence was obtained for regulation of PAP by reversible phosphorylation.

Subcellular distribution of N-ethylmaleimide-sensitive and -insensitive phosphatidic acid phosphohydrolase in rat brain

The dephosphorylation of phosphatidic acid by phosphatidic acid phosphohydrolase (PAP) is important in both cell-signalling and in glycerolipid metabolism. However, these roles are apparently performed by two different enzymes, which can be distinguished by their sensitivity in vitro to N-ethylmaleimide (NEM). Both of these enzymes are present in rat brain as well as a wide range of other rat tissues. However, the quantity and specific activity of each enzyme varies considerably between different tissues, as does the ratio of the two enzymes in each tissue. Tissues rich in glycerolipids are abundant in NEM-sensitive PAP, whereas there is no obvious pattern to the distribution of the NEM-insensitive enzyme in the different tissues tested. Studies on brain cortex, which is relatively rich in both forms of PAP, indicate that the NEM-insensitive PAP is located in the synaptosomes, and the NEM-sensitive enzyme present in the cytosol and microsomes. The NEM-sensitive PAP can also be translocated from the cytosol to the microsomes by oleate. When assayed against a range of phosphatidic acids, NEM-sensitive PAP showed a preference for phosphatidic acids with short acyl chains and for those containing arachidonate, whereas NEM-insensitive PAP had a preference for short and unsaturated acyl chains. The two isozymes also had different activity profiles against these substrates suggesting that they are in fact different enzymes. The implications for these results on the putative roles of the two forms of PAP are discussed.

A study on lipid metabolism in heart and liver of cholesterol- and pectin-fed rats

Pectin is known as a cholesterol-reducing dietary fibre, and in the present study we addressed the question whether pectin affected the quantity of lipid in droplets in the myocardial cells and of lipid in the liver cells. Male Wistar rats received either a diet containing cholesterol or a standard diet without cholesterol with 0, 50 or 100 g pectin/kg incorporated for 10 d. The fractional volume of lipid droplets in the myocardial cells decreased as a function of pectin dose in both the standard-fed and the cholesterol-fed rats. Serum cholesterol was significantly reduced in both groups after addition of 100 g pectin/kg diet. The cholesterol diet increased the liver cholesterol level, and 100 g pectin/kg diet resulted in a lower concentration of liver cholesterol in the cholesterol-fed animals, but the influence on standard-fed rats was modest. Hydroxymethylglutaryl-CoA reductase (EC 1.1.1.88; HMG-CoA reductase) activity increased when pectin was given in the standard diet. Liver triacylglycerol level increased after cholesterol and pectin feeding. Mitochondrial fatty acid oxidation and phosphatidate phosphohydrolase (EC 3.1.3.4) activity tended to decrease, whereas the peroxisomal fatty acid oxidation and acyl-CoA oxidase activity were unchanged. Increased hepatic triacylglycerol content by cholesterol and pectin treatment may be due to inhibited mitochondrial fatty acid oxidation along with increased availability of fatty acid for esterification and triacylglycerol synthesis. The presence of pectin in the diets of cholesterol-fed rats resulted in increased hepatic concentration of triacylglycerols and increased mitochondrial fatty acid oxidation. In this case the hepatic accumulation of triacylglycerol may be mediated by a reduced efflux of triacylglycerols from the liver.

Effects of dietary fatty acid composition on glycerophosphate acyltransferase activity in rat adipose tissue and liver

1. How source of dietary fat influences glycerophosphate acyltransferase (GPAT) activity in homogenates of adipose tissue and liver was determined using 5 rats per diet that contained 12% fat from either corn- (CO), high-oleate-safflower (HOSO), palm- or high-stearate-oils. 2. Activity was greatest in adipose tissue homogenates of rats fed HOSO and CO whereas no treatment effects were observed in liver homogenates. 3. No differences in fat pad weights were observed; thus HOSO- and CO-diets likely depressed other lipid synthetic rates. 4. Data indicate that mono- and diunsaturated fatty acid diets supported greater adipose tissue GPAT activity than did saturated fat diets when fed to rats.

The activity of the metabolic form of hepatic phosphatidate phosphohydrolase correlates with the severity of alcoholic fatty liver in human beings

Increased esterification of fatty acids to triglyceride is common to most of the mechanisms proposed to explain the causation of alcoholic fatty liver. However, it is unclear whether this is caused by increased substrate supply or whether direct stimulation of the enzymes of the esterification pathway occurs after excessive alcohol intake. The rate-limiting step in triglyceride synthesis is catalyzed by the enzyme phosphatidate phosphohydrolase, which is present in the cytosol and microsomes and is sensitive to inhibition by N-ethylmaleimide. This enzyme is physically distinct from a second form of phosphatidate phosphohydrolase that is located predominantly in the plasma membrane, is insensitive to N-ethylmaleimide inhibition and has a putative role in cell-signaling. We have investigated whether the activity of the N-ethylmaleimide-sensitive ("metabolic") form of phosphatidate phosphohydrolase is increased in patients with alcoholic liver disease and whether any increased activity correlates with the severity of steatosis. N-ethylmaleimide-sensitive and -insensitive phosphatidate phosphohydrolase activities were measured in needle liver biopsy specimens from 42 alcoholic patients and 6 patients with primary biliary cirrhosis and in wedge biopsy specimens from 6 normal patients undergoing routine cholecystectomy. Steatosis was "scored" on coded slides from 0 to 3. N-ethylmaleimide-sensitive activity was higher in alcoholic biopsy specimens scoring 3 (3.25 +/- 0.4 units/mg protein, n = 10) than in those scoring either 0 (1.21 +/- 0.2, n = 14) or 1 to 2 (1.58 +/- 0.2, n = 18), and it was also higher than in biopsy specimens from normal and primary biliary cirrhosis patients (1.65 +/- 0.3, n = 12; p < 0.0001, analysis of variance).(ABSTRACT TRUNCATED AT 250 WORDS)

Oleate stimulation of diacylglycerol formation from phosphatidylcholine through effects on phospholipase D and phosphatidate phosphohydrolase

Hydrolysis of exogenous phosphatidylcholine (PtdCho) to 1,2-diacylglycerol by rat liver plasma membranes was stimulated by oleate concentrations as low as 0.1 mM. In the presence of 75 mM ethanol, the fatty acid also enhanced phosphatidylethanol (PtdEtOH) formation from PtdCho. These effects were also observed with linoleate and arachidonate, but not with saturated fatty acids or detergents, and were minimal in microsomes or mitochondria. Release of [3H]choline from exogenous Ptd[3H]Cho was stimulated by oleate, whereas phosphoryl[3H]choline formation was inhibited. Oleate and other unsaturated, but not saturated, fatty acids also stimulated the conversion of exogenous [14C]phosphatidic acid to [14C]diacylglycerol. These data are consistent with stimulatory effects of these fatty acids on both phospholipase D and phosphatidate phosphohydrolase in liver plasma membranes. The stimulatory effect of guanosine 5'-O-[3-thio]triphosphate) (20 microM) on PtdEtOH and diacylglycerol formation from PtdCho was enhanced by low concentrations of oleate. Phospholipase A2 also stimulated PtdEtOH and diacylglycerol formation from exogenous PtdCho. It is proposed that unsaturated fatty acids may play a physiological role in the regulation of diacylglycerol production through activation of phospholipase D and phosphatidate phosphohydrolase.

Effects of sphingosine, albumin and unsaturated fatty acids on the activation and translocation of phosphatidate phosphohydrolases in rat hepatocytes

The activities of two phosphatidate phosphohydrolases were measured in cultured rat hepatocytes incubated with 0.1 mM albumin. The activity, which is inhibited by N-ethylmaleimide (PAP-1) is located in the cytosolic and membrane fractions. PAP-1 activity is stimulated by Mg2+ and it can be translocated from the cytosol to the membranes by relatively low (0.5-1 mM) concentrations of fatty acids. In addition, higher concentrations (1-3 mM) of fatty acids cause an increase in the total PAP-1 activity. Translocation of PAP-1 activity in the hepatocytes is preferentially promoted by unsaturated fatty acids (C18:1, C18:2, C18:3, C20:4 and C20:5), rather than by saturated acids (C14:0, C16:0, C18:0). Increasing the extracellular concentration of albumin from 30 microM to 1 mM displaces PAP-1 activity from the membrane fraction. Sphingosine, but not staurosporine, can inhibit the redistribution of PAP-1 activity induced by oleate. The amphiphilic amines, sphingosine, chlorpromazine and propranolol, also decrease membrane-bound PAP-1 activity in the absence of fatty acids, but they do not alter, significantly, the activity of the cytosolic PAP-1. In the presence of 1 mM oleate, sphingosine, chlorpromazine and propranolol decrease the translocation of PAP-1 from the cytosol to the membranes. The phosphohydrolase activity, which is insensitive to N-ethylmaleimide (PAP-2), is specifically located in the plasma membrane (Jamal, Z., Martin, A., Gomez-Muñoz, A. and Brindley, D.N. (1991) J. Biol. Chem. 266, 2988-2996) and it is not stimulated by Mg2+. Saturated fatty acids, albumin, sphingosine and propranolol have no significant effects on PAP-2 activity. However, chlorpromazine decreases PAP-2 activity by about 14%. Linolenate, arachidonate and eicosapentaenoate at 1 mM also produced small (7-10%) decreases in PAP-2 activity. It is proposed that both PAP-1 and PAP-2 activities may be involved in signal transduction, although the main function of PAP-1 seems to be involved in the synthesis of glycerolipids.

Physical evidence for the presence of two forms of phosphatidate phosphohydrolase in rat liver

Phosphatidate phosphohydrolase (PAP)-catalysed dephosphorylation of phosphatidic acid to diacylglycerol is an important step in glycerolipid metabolism and cell-signalling. Gel filtration chromatography on Superose 6 and anion-exchange chromatography on Mono Q of rat liver subcellular fractions has provided physical evidence for the presence of two distinct forms of PAP activity. One form was sensitive to inhibition by N-ethylmaleimide (NEM), had an apparent M(r) of 540,000 and was eluted from the anion-exchange column by 0.35 M NaCl, while the other was insensitive to inhibition by NEM, had an apparent M(r) of 240,000 and was eluted from the anion-exchange column by 0.15 M NaCl. Studies on the subcellular distribution of these two enzymes, using 5'-nucleotidase as a plasma membrane marker, demonstrated that the NEM-sensitive form was predominantly cytosolic but translocated to the microsomal membranes in response to oleate. The NEM-insensitive form was predominantly located in the plasma membrane but a small proportion (approx. 10%) of total cell activity was present on the endoplasmic reticulum. The implications of these results for the likely roles of the two different forms of PAP in fatty acid esterification and cell-signalling are discussed.

Relationship between translocation of long-chain acyl-CoA hydrolase, phosphatidate phosphohydrolase and CTP:Phosphocholine cytidylyltransferase and the synthesis of triglycerides and phosphatidylcholine in rat liver

Translocation of long-chain acyl-coenzyme A hydrolase from the microsomal fraction to the cytosolic fraction was promoted in cell-free extracts of rat liver by palmitic acid, oleic acid, tetradecylthioacetic acid, and tetradecylthiopropionic acid, and by their CoA esters. The CoA esters were more effective than the non-esterified acids in the translocation of the enzyme. Treatment of normolipidemic rats with sulfur-substituted non-beta-oxidizable fatty acid analogues resulted in a transitory increase in hepatic concentration of long-chain acyl-CoA. Longer feeding times almost normalized the hepatic long-chain acyl-CoA content. Microsomal long-chain acyl-CoA hydrolase activity was inhibited, whereas the activity of the cytosolic form was stimulated. The rise in enzyme activity coincided with a reduction in liver content of triglyceride and an increase in hepatic phospholipid content. The results suggest that the activity of long-chain acyl-CoA hydrolase in the cytosol may control the amount of acyl-CoA thioesters in the liver. Esterified and non-esterified fatty acids caused in vitro translocation of phosphatidate phosphohydrolase and cytidine 5'-triphosphate (CTP):phosphocholine cytidylyltransferase from the cytosolic fraction to the microsomal fraction. However, the translocation of these two enzyme systems was not obtained in vivo. The activity of phosphatidate phosphohydrolase decreased in microsomal and cytosolic fractions while the activity of cytidylyltransferase in these fractions increased. The activities of soluble phosphatidate phosphohydrolase and long-chain acyl-CoA hydrolase appeared to be inversely correlated.(ABSTRACT TRUNCATED AT 250 WORDS)

Mitochondrial phosphatidate is converted to triacylglycerol in rat hepatocytes

Phosphatidate is formed in both the endoplasmic reticulum and the outer mitochondrial membrane in rat liver. To investigate whether the phosphatidate synthesized in mitochondria can be converted to triacylglycerol in vivo, two experimental approaches were employed. (i) [3H]Phosphatidate-labeled mitochondria were enclosed in plasma membrane vesicles and these fused, in the presence of inactivated Sendai virus and calcium ions, to hepatocytes in monolayer culture. The recovery of radioactivity in various cell-associated lipids was measured. (ii) Mitochondrial phosphatidate was labeled with [14C]palmitate in hepatocytes which had been permeabilized with lysophosphatidylcholine and in which the microsomal glycerolphosphate acyltransferase had been inhibited with N-ethylmaleimide. The recovery of radioactivity in various lipids after incubation with particle free supernatant was measured. Evidence was obtained from both these experimental approaches that mitochondrial phosphatidate can be converted to triacylglycerol in rat hepatocytes. The results are discussed in relation to the role of mitochondrial phosphatidate in liver lipid metabolism.

Effects of hypothyroidism and high-fat feeding on mRNA concentrations for the low-density-lipoprotein receptor and on acyl-CoA:cholesterol acyltransferase activities in rat liver

1. Induction of hypothyroidism in rats by feeding propylthiouracil (PTU) significantly increased serum cholesterol concentrations, and the effect was more pronounced for cholesterol in low-density lipoproteins (LDL) rather than high-density lipoproteins (HDL). The concentrations of serum triacylglycerol were decreased in hypothyroidism. These effects on serum lipids were also seen when the normal rats were pair-fed with the PTU-treated group. 2. Feeding a diet rich in saturated fat and cholesterol further increased cholesterol concentrations in LDL and also elevated that in very-low-density lipoprotein (VLDL) of hypothyroid rats. In euthyroid rats such a diet resulted in a relatively small increase in VLDL cholesterol, whereas LDL cholesterol was decreased. 3. Steady-state concentrations of mRNA for the hepatic LDL receptor were significantly decreased in the livers of hypothyroid rats, but were not significantly changed by high-fat feeding in euthyroid or hypothyroid rats. 4. The expression of the LDL receptor in hepatocytes cultured from hypothyroid rats was decreased relative to the euthyroid controls. 5. Whereas the esterification of cholesterol with oleate in hepatocytes cultured from hypothyroid rats was decreased, the activity of acyl-CoA:cholesterol acyltransferase (ACAT) in the livers of these animals was not changed. 6. High-fat feeding increased the hepatic ACAT activity in normal and hypothyroid rats. 7. Incubation of rat hepatocytes with 10 nM-tri-iodothyronine for 4 h increased the relative concentration of the mRNA for the LDL receptor by 25%. 8. It is therefore concluded that thyroid hormones stimulate the synthesis and expression of the hepatic LDL receptor. Elevated cholesterol concentrations in LDL in hypothyroidism probably result from a primary defect in the expression of the hepatic receptor, rather than indirectly via changes in ACAT activity.

Effect of n-3 fatty acids on the key enzymes involved in cholesterol and triglyceride turnover in rat liver

The effect of long-chain n-3 fatty acids on hepatic key enzymes of cholesterol metabolism and triglyceride biosynthesis was investigated in two rat models. In the first model, rats were intravenously infused for two weeks with a fat emulsion containing 20% of triglycerides in which either n-6 or n-3 fatty acids predominated. The treatment with n-3 fatty acids led to a reduction primarily of serum cholesterol (45%), but also of serum triglycerides (18%). HMG-CoA reductase activity and cholesterol 7 alpha-hydroxylase activity were reduced by 45% and 36%, respectively. There were no significant effects on diacylglycerol acyltransferase (DGAT) or phosphatidate phosphohydrolase (PAP) activities. In the second model, rats were fed a diet enriched with sucrose, coconut oil and either sunflower oil (n-6 fatty acids) or fish oil (long-chain n-3 fatty acid ethyl esters). The treatment with n-3 fatty acids decreased serum triglycerides (41%) and, to a lesser extent, serum cholesterol (17%). Neither glycerol 3-phosphate acyltransferase (GPAT) or DGAT were affected by n-3 fatty acids. In contrast, PAP activity was reduced by 26%. HMG-CoA reductase was not significantly affected, whereas cholesterol 7 alpha-hydroxylase activity was reduced by 36%. The results indicate that part of the TG-lowering effect of long-chain n-3 fatty acids may be mediated by inhibition of the soluble phosphatidate phosphohydrolase. The effect on serum cholesterol may be partly due to inhibition of HMG-CoA reductase.

Properties of phosphatidate phosphohydrolase and diacylglycerol acyltransferase activities in the isolated rat heart. Effect of glucagon, ischaemia and diabetes

Myocardial triacylglycerol hydrolysis is subject to product inhibition. After hydrolysis of endogenous triacylglycerols, the main proportion of the liberated fatty acids is re-esterified to triacylglycerol, indicating the importance of fatty acid re-esterification in the regulation of myocardial triacylglycerol homoeostasis. Therefore, we characterized phosphatidate phosphohydrolase (PAP) and diacylglycerol acyltransferase (DGAT) activities, enzymes catalysing the final steps in the re-esterification of fatty acids to triacylglycerols in the isolated rat heart. The PAP activity was mainly recovered in the microsomal and soluble cell fractions, with an apparent Km of 0.14 mM for both the microsomal and the soluble enzyme. PAP was stimulated by Mg2+ and oleic acid. Oleic acid, like a high concentration of KCl, stimulated the translocation of PAP activity from the soluble to the particulate (microsomal) fraction. Myocardial DGAT had an apparent Km of 3.8 microM and was predominantly recovered in the particulate (microsomal) fraction. Both enzyme activities were significantly increased after acute streptozotocin-induced diabetes, PAP from 15.6 +/- 1.1 to 28.1 +/- 3.6 m-units/g wet wt. (P less than 0.01) and DGAT from 2.23 +/- 0.11 to 3.01 +/- 0.11 m-units/g wet wt. (P less than 0.01). In contrast with diabetes, low-flow ischaemia during 30 min did not affect PAP and DGAT activity in rat hearts. Perfusion with glucagon (0.1 microM) during 30 min did not affect total PAP activity, but changed the subcellular distribution. More PAP activity was recovered in the particulate fraction. DGAT activity was lowered by glucagon treatment from 0.37 +/- 0.03 to 0.23 +/- 0.02 m-unit/mg of microsomal protein (P less than 0.05). The role of PAP and DGAT activity and PAP distribution in the myocardial glucose/fatty acid cycle is discussed.

Intracellular translocation of phosphatidate phosphatase in maturing safflower seeds: a possible mechanism of feedforward control of triacylglycerol synthesis by fatty acids

Phosphatidate phosphatase activity was found both in the cytosol and in the microsomal membrane of maturing safflower seeds. The combined and relative activities of these two forms varied with seed maturation. During the period of rapid triacylglycerol accumulation in the cell, most of the phosphatidate phosphatase activity was membrane-bound; at the initial and last stages of seed development when triacylglycerol synthesis was at an insignificant level, the majority of the activity was soluble. The potassium salts of palmitic, stearic and oleic acids, which are the fatty acid products of proplastids, caused the translocation of the cytosolic phosphatidate phosphatase to the microsomal membrane, while laurate and linoleate, which are not products of proplastids, showed no effect. Oleoyl-CoA did not convert the soluble form of the enzyme into the membrane-bound form. The translocation induced by oleate was reversible. The cytosolic phosphatidate phosphatase of safflower seeds was not transferred to the microsomal membranes prepared from soybean, a plant species of Leguminosae, and from rapeseed, a species of Cruciferae, but was transferred to that from sunflower, which belongs to the same family as safflower, Compositae. These observations suggest that in maturing oil seeds the rate of fatty acid synthesis in proplastids may regulate the species-specific translocation of phosphatidate phosphatase between the cytosol and the endoplasmic reticulum membrane where triacylglycerol synthesis occurs and that in turn the translocation of this ambiquitous enzyme could control the rate of triacylglycerol synthesis in the cell.

Translocation to rat liver mitochondria of phosphatidate phosphohydrolase

When a particle-free supernatant fraction from rat liver was incubated at 37 degrees C with mitochondria and oleate, some of the enzyme phosphatidate phosphohydrolase (PAP), initially present in the particle-free supernatant, was recovered, after the incubation, bound to mitochondria. This translocation of PAP from cytosol to mitochondria was stimulated by oleate or palmitate in a similar fashion to the stimulation of translocation of PAP to endoplasmic reticulum [Martin-Sanz, Hopewell & Brindley (1984) FEBS Lett. 175, 284-288]. Translocation of PAP from particle-free supernatant to a partially purified mitochondrial-outer-membrane preparation was also stimulated by oleate. More PAP was bound to a mitochondrial-outer-membrane fraction washed in 0.5 M-NaCl before resuspension in sucrose than to a sucrose-washed mitochondrial-outer-membrane preparation. In contrast, washing of microsomal membranes in 0.5 M-NaCl did not enhance the binding of PAP to these membranes. PAP also binds to phosphatidate-loaded mitochondria or microsomes (microsomal fractions). In the experimental system employed, more PAP bound to mitochondria loaded with phosphatidate than to microsomes loaded with phosphatidate. The results are discussed in relation to the role of mitochondrial phosphatidate in liver lipid metabolism.

Effects of free fatty acids and acyl-coenzyme A on diacylglycerol kinase in rat brain

Our earlier studies have indicated the presence of diacylglycerol kinase activity in rat brain cytosol as well as subcellular membrane fractions (Strosznajder et al.: Neurochemistry International 8(2):213-221, 1986). There is much evidence indicating the release of diacylglycerols due to stimulation of polyphosphoinositide hydrolysis by hormones and receptor agonists. In turn, diacylglycerols have been linked to a second messenger role for activation of protein kinase C. The present study tests the ability of free fatty acids and acyl-coenzyme A (acyl-CoA) to regulate diacylglycerol kinase activity. In a system containing brain cytosol and microsomes, addition of oleic acid (0.5 mM) resulted in large stimulation of diacylglycerol kinase activity as well as some translocation of the enzyme from cytosol to microsomes. On the other hand, oleoyl-CoA (0.1 mM), but neither palmitoyl-CoA nor arachidonoyl-CoA, was effective in translocation of the diacylglycerol kinase. Unlike oleic acid, which preferred to associate with membranes, most of the oleoyl-CoA remained in the cytosolic fraction. Since free fatty acids in brain are stringently controlled and are released during ischemic insult, a condition which also elicits the breakdown of polyphosphoinositide to diacylglycerols, results here suggest a plausible mechanism for regulation of diacylglycerol metabolism by free fatty acids and acyl-CoA.

Activation of rat liver cytosolic phosphatidic acid phosphatase by nucleoside diphosphates

Phosphatidic acid phosphatase (EC 3.1.3.4) was purified 30-fold by ammonium sulfate fractionation and hydroxyapatite chromatography from the soluble fraction of rat liver. ADP was found to stimulate the enzyme activity with half-maximal stimulation at 0.2 mM. Similar effects were seen when ADP was replaced by GDP or CDP. In contrast, ATP inhibited the enzyme; half-maximal inhibition observed at 0.2 mM. Again, the degree of inhibition did not differ when GTP or CTP replaced ATP. Thus, the structure of the base part of the nucleotide was not critical for mediating these effects. The positions of the phosphate groups in the nucleotide structure were however found to be of importance for the enzyme activity. Variations in the structure of the phosphate ester bound at the 5'-position had a pronounced effect on phosphatidic acid phosphatase activity. The effect of nucleotides depended on pH, and the inhibition by ATP was more pronounced at pH levels lower than 7.0, whereas the stimulatory effect of ADP was virtually the same from pH 6.0 to pH 8.0. The enzyme showed substrate saturation kinetics with respect to phosphatidic acid, with an apparent Km of 0.7 mM. Km increased in the presence of ATP, whereas both apparent Vmax and Km increased in the presence of ADP, suggesting different mechanisms for the action of the two types of nucleotides. The results indicated that physiological levels of nucleotides with a diphosphate or a triphosphate ester bound at the 5'-position of the ribose moiety influenced the activity of phosphatidic acid phosphatase. The possibility is discussed that these effects might be of importance for the regulation of triacylglycerol biosynthesis.

Age-related changes in the translocation of phosphatidate phosphohydrolase from the cytosol to microsomal membranes in rat liver

The effects of oleate, spermine and chlorpromazine were assayed in the presence or absence of 0.15 M KCl on the translocation of phosphatidate phosphohydrolase activity from cytosol to endoplasmic reticulum membranes in liver homogenates obtained from rats aged 1, 30, 60, 180 and 360 days. Marked age-associated decreases in phosphatidate phosphohydrolase distribution onto the membranes were demonstrated under nearly all conditions. In liver homogenates taken from 1-day-old rats and incubated with 0.15 M KCl, most of the enzyme was active (associated with the membranes). Physiological salt concentration (0.15 M KCl) produced a 2-fold increase of oleate-induced translocation of phosphatidate phosphohydrolase activity in liver homogenates from 1-day-old rats; it had no effect on those from 60-day-old rats, and produced a notable decline in liver homogenates obtained from 180- and 360-day-old rats. The promoting effect of spermine on oleate-induced translocation of this enzyme activity was higher in younger rats when incubated in the absence of 0.15 M KCl. Chlorpromazine did not show its usual antagonizing effect on oleate-induced translocation of phosphatidate phosphohydrolase when added to homogenates taken from 1-day-old rats. The antagonizing effect was slightly apparent in liver homogenates from 30-day-old rats and was more pronounced in those from 60-day-old rats in which the values diminished to one-half and to one-third either in the presence or absence of 0.15 M KCl.

Phosphatidylinositol kinase, phosphatidylinositol-4-phosphate kinase and diacylglycerol kinase activities in rat brain subcellular fractions

Subcellular fractions isolated and purified from rat brain cerebral cortices were assayed for phosphatidylinositol (PI-), phosphatidylinositol-4-phosphate (PIP-), and diacylglycerol (DG-) kinase activities in the presence of endogenous or exogenously added lipid substrates and [gamma-32P]ATP. Measurable amounts of all three kinase activities were observed in each subcellular fraction, including the cytosol. However, their subcellular profiles were uniquely distinct. In the absence of exogenous lipid substrates, PI-kinase specific activity was greatest in the microsomal and non-synaptic plasma membrane fractions (150-200 pmol/min per mg protein), whereas PIP-kinase was predominantly active in the synaptosomal fraction (136 pmol/min per mg protein). Based on percentage of total protein, total recovered PI-kinase activity was most abundant in the cytosolic, synaptosomal, microsomal and mitochondrial fractions (4-11 nmol/min). With the exception of the microsomal fraction, a similar profile was observed for PIP-kinase activity when assayed in the presence of exogenous PIP (4 nmol/20 mg protein in a final assay volume of 0.1 ml). Exogenous PIP (4 nmol/20 mg protein) inhibited PI-kinase activity in most fractions by 40-70%, while enhancing PIP-kinase activity. PI- and PIP-kinase activities were observed in the cytosolic fraction when assayed in the presence of exogenously added PI or PIP, respectively, but not in heat-inactivated membranes containing these substrates. When subcellular fractions were assayed for DG-kinase activity using heat-inactivated DG-enriched membranes as substrate, DG-kinase specific activity was predominantly present in in the cytosol. However, incubation of subcellular fractions in the presence of deoxycholate resulted in a striking enhancement of DG-kinase activities in all membrane fractions. These findings demonstrate a bimodal distribution between particulate and soluble fractions of all three lipid kinases, with each exhibiting its own unique subcellular topography. The preferential expression of PIP-kinase specific activity in the synaptic membranes is suggestive of the involvement of PIP2 in synaptic function, while the expression of PI-kinase specific activity in the microsomal fraction suggests additional, yet unknown, functions for PIP in these membranes.

A distinction in vitro between rat liver phosphatidate phosphatase and phospholipase C

Hepatocellular membranes (1000 X g) containing membrane-associated, labeled phosphatidic acid were incubated (1-30 min) with 2 mM oleate or 5 mM bromobenzene in the presence or absence of various metals and NaF. Under the appropriate incubation conditions, membranes displayed rapid and significant oleate- and bromobenzene-dependent increases in the dephosphorylation of labeled phosphatidic acid. However, oleate and bromobenzene activated the dephosphorylation of phosphatidate by phosphatidate phosphatase and phospholipase C, respectively. This conclusion is supported by the observation that the phosphatase stimulated by oleate is: (1) Mg2+ -dependent; (2) inhibited by other metals, such as Ca2+; (3) inhibited by NaF; (4) specific for phosphatidic acid; and (5) associated with a rise in liver cell triacylglycerol production. Bromobenzene, however, activated a phospholipase C that is: (1) stimulated by various metals, such as Mg2+, Ca2+ and Ba2+; (2) insensitive to NaF; (3) associated with the degradation of various membrane phospholipids; (4), associated with liver cell injury; and (5) not associated with a rise in liver cell triacylglycerol formation. These results suggest that under appropriate conditions in vitro the dephosphorylation of phosphatidic acid can be used to assess changes in phosphatidate phosphatase and/or phospholipase C activity. The distinction between these enzymes is important, since phosphatidate phosphatase and phospholipase C regulate key steps in phospholipid biosynthesis and degradation, respectively.

Role of acetaldehyde in ethanol-induced increase in the activity of phosphatidate phosphatase in rat liver

The effect of ethanol on the activity of phosphatidate phosphatase was studied in rat liver using membrane-bound phosphatidate and phosphatidate emulsion as substrate. A single large dose of ethanol (5 g/kg body wt) caused an increase in the enzyme activity measured with membrane-bound phosphatidate after an approximate 2-hr lag period in both cytosolic and microsomal fraction and the increase was approximately 2.2- and 1.8-fold that in control rats at 5 hr in cytosol and microsomes, respectively. A similar time-course of the increase was obtained with phosphatidate emulsion as substrate. These ethanol-induced increases in the activity of cytosolic and microsomal phosphatidate phosphatase were blocked by the pretreatment of rats with actinomycin D. The ethanol-induced rise in the activity of cytosolic and microsomal phosphatidate phosphatase measured with membrane-bound phosphatidate was abolished when rats were injected with pyrazole prior to ethanol administration. On the other hand, pretreatment with cyanamide enhanced the increase in cytosolic activity produced by a suboptimal dose of ethanol (1 g/kg), while microsomal activity was not affected by the same treatment, suggesting that acetaldehyde may be selectively involved in the ethanol-induced increase in the activity of cytosolic phosphatidate phosphatase. This hypothesis was supported by a finding that administration of paraldehyde, a cyclic trimer of acetaldehyde, produced an increase (35%) in cytosolic activity, but not in microsomal activity.

Stimulatory effect of mevinolin on rat liver phosphatidic acid phosphatase

The activity of the soluble form of phosphatidic acid phosphatase in rat liver was stimulated about 2.5-fold by inclusion of mevinolin, a competitive hydroxymethylglutaryl-CoA reductase inhibitor, in the diet (0.1%). The stimulatory effect of mevinolin was present also after dietary addition of cholestyramine (5%) or intraperitoneal administration of ethanol. Addition of cholesterol (2%) to the diet totally abolished the stimulation by mevinolin on phosphatidic acid phosphatase. The results support a correlation between the synthesis of the rate-limiting enzyme in cholesterol biosynthesis and the activity of the apparent rate-limiting enzyme in triacylglycerol biosynthesis.

Translocation of CTP: phosphocholine cytidylyltransferase from cytosol to membranes in HeLa cells: stimulation by fatty acid, fatty alcohol, mono- and diacylglycerol

Addition of oleate, oleyl alcohol, or palmitate to HeLa cell medium resulted in a rapid stimulation of PC synthesis and activation of CTP: phosphocholine cytidylyltransferase. Stimulation was optimal with 0.35 mM oleate, 0.3 mM oleyl alcohol and 5 mM palmitate, or 1 mM palmitate if EGTA were added to the medium. The cytidylyltransferase was activated by translocation of the inactive cytosolic form to membranes. In untreated cells approx. 30% of the total cytidylyltransferase was membrane bound, while in treated cells, 80-90% was membrane associated. Addition of bovine serum albumin (10 mg/ml) to cells previously treated with oleate (0.35 mM) rapidly removed cellular fatty acid, and the membrane-bound cytidylyltransferase activity returned to approx. 30%. Similar results were obtained by extraction of membranes with albumin in vitro. Although 95% of the free fatty acid was extracted, 30-40% of the membrane cytidylyltransferase remained bound. Translocation of cytidylyltransferase between isolated cytosol and microsomal fractions was promoted by addition of oleate, palmitate, oleyl alcohol, and monoolein. Addition of diacylglycerol, lysophosphatidylcholine, lysophosphatidylethanolamine, calcium palmitate, and detergents such as Triton X-100, cholate or Zwittergent did not stimulate translocation of the enzyme. Addition of oleoyl-CoA promoited translocation, however, 40% of it was hydrolyzed releasing free oleic acid. Cytosolic cytidylyltransferase bound to microsomes pre-treated with phospholipase C, which had 7-fold elevated diacylglycerol content. Fatty acid-promoted translocation was blocked by Triton X-100, but not by 1 M KCl. These results suggest that a variety of compounds with differing head group size and charge, and number of hydrocarbon chains can function as translocators, and that hydrophobic rather than ionic interactions mediate the binding of cytidylyltransferase to membranes.

Spermine antagonises the effects of dexamethasone, glucagon and cyclic AMP in increasing the activity of phosphatidate phosphohydrolase in isolated rat hepatocytes

Rat hepatocytes were incubated in monolayer culture, under serum free conditions, for 8 h. Glucagon (10 nM), 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (100 microM) and dexamethasone (100 nM) increased the activity of phosphatidate phosphohydrolase by approx. 2-, 3.6- and 3.3-fold, respectively. Spermine alone had no significant effect. Spermine (2.5 mM) almost completely inhibited the glucagon induced increase in phosphohydrolase activity. It only partially inhibited the dexamethasone and cyclic AMP mediated inductions. Spermidine had no significant effect in this respect. The results are discussed in relation to the known effects of polyamines on glycerolipid synthesis, in particular, and on intermediary metabolism.