Effects of vasopressin and corticosterone on fatty acid metabolism and on the activities of glycerol phosphate acyltransferase and phosphatidate phosphohydrolase in rat hepatocytes

Seasonal variation of vasopressin and its relevance for the winter peak of cardiometabolic disease: A pooled analysis of five cohorts

BACKGROUND

Vasopressin concentration is typically higher at night, during stress, and in males, but readily lowered by water intake. Vasopressin is also a causal candidate for cardiometabolic disease, which shows seasonal variation.

OBJECTIVE

To study whether vasopressin concentration varies by season in a temperate climate.

METHODS

The vasopressin surrogate marker copeptin was analyzed in fasting plasma samples from five population-based cohorts in Malmö, Sweden (n = 25,907, 50.4% women, age 18-86 years). We investigated seasonal variation of copeptin concentration and adjusted for confounders in sinusoidal models.

RESULTS

The predicted median copeptin level was 5.81 pmol/L (7.18 pmol/L for men and 4.44 pmol/L for women). Copeptin exhibited a distinct seasonal pattern with a peak in winter (mid-February to mid-March) and nadir in late summer (mid-August to mid-September). The adjusted absolute seasonal variation in median copeptin was 0.62 pmol/L (95% confidence interval [CI] 0.50; 0.74, 0.98 pmol/L [95% CI 0.73; 1.23] for men and 0.46 pmol/L [95% CI 0.33; 0.59] for women). The adjusted relative seasonal variation in mean log copeptin z-score was 0.20 (95% CI 0.17; 0.24, 0.18 [95% CI 0.14; 0.23] in men and 0.24 [95% CI 0.19; 0.29] in women). The observed seasonal variation of copeptin corresponded to a risk increase of 4% for incident diabetes mellitus and 2% for incident coronary artery disease.

CONCLUSION

The seasonal variation of the vasopressin marker copeptin corresponds to increased disease risk and mirrors the known variation in cardiometabolic status across the year. Moderately increased water intake might mitigate the winter peak of cardiometabolic disease.

Copeptin relates to a fatty liver and measures of obesity in a South African population with mixed ethnicities

PURPOSE

Elevated copeptin, a vasopressin marker, is linked to metabolic disease, and obese rats with low-vasopressin concentration had a decreased risk of liver steatosis. We here investigated the association between copeptin and nonalcoholic fatty liver disease (NAFLD) and possible differences in copeptin concentration between ethnicities.

METHODS

In this cross-sectional study of 361 South Africans (n = 172 African black, 189 = Caucasian) with a mean age of 45 years and 45% men, plasma copeptin was measured and associated with NAFLD according to a validated fatty liver index accounting for measures of BMI, waist, triglycerides, and gamma-glutamyltransferase.

RESULTS

There was no significant difference in copeptin concentrations between ethnicities after age and gender adjustment (p = 0.24). Increasing copeptin tertile levels were significantly associated with obesity, overweight, and abdominal obesity, respectively, after multivariate adjustment for age, gender, ethnicity, and high HOMA-IR (p = 0.02 for all). Individuals in the second and third copeptin tertile had an increased odds (95% CI) of NAFLD of 1.77 (1.04-3.02) and 2.97 (1.74-5.06), respectively, compared to the bottom tertile (p < 0.001). The association between increasing copeptin tertile and NAFLD remained significant after adjustment for age, gender, ethnicity, high HOMA-IR, self-reported current alcohol intake, and statin treatment (p = 0.01).

CONCLUSIONS

Elevated plasma copeptin is independently associated with NAFLD in a population with mixed ethnicities, pointing at the pharmacologically modifiable vasopressin system as a new mechanism behind NAFLD.

Elevated plasma copeptin levels identify the presence and severity of non-alcoholic fatty liver disease in obesity

INTRODUCTION

Copeptin is the stable surrogate marker of vasopressin (VP), which is released in response to elevated plasma osmolality or low blood pressure. Elevated plasma copeptin levels are associated with higher risk of insulin resistance-related disorders, such as type 2 diabetes (T2DM), metabolic syndrome (MS), and cardiovascular disease, and experimental reduction of circulating VP levels is shown to significantly decrease hepatic fat content in obese rats, independently from body adiposity. However, the association between copeptin and non-alcoholic fatty liver disease and steatohepatitis (NAFLD/NASH) in humans has not been explored yet. The aim of this study was to explore the relationship between plasma copeptin and the presence/severity of NAFLD/NASH.

METHODS

For this study, we recruited 60 obese patients candidate to bariatric surgery for clinical purposes in which intraoperative liver biopsies were performed for diagnosing NAFLD/NASH. Circulating copeptin levels were also assessed in 60 age- and sex-comparable non-obese individuals without NAFLD at liver ultrasonography. Plasma copeptin was measured by sandwich immunoluminometric assay (Thermo Fisher Scientific).

RESULTS

Obese patients with biopsy-proven NAFLD (53%) had significantly higher copeptin levels than both obese individuals without NAFLD and non-obese subjects (ob/NAFLD+ 9.5 ± 4.9; ob/NAFLD- 6.4 ± 2.6; and non-ob/NAFLD- 7.4 ± 5.1 pmol/L; p = 0.004 and p = 0.01 respectively). Plasma copeptin concentration positively correlated with hepatic macro- and micro-vesicular steatosis (r = 0.36, p = 0.026; r = 0.31, p = 0.05), lobular inflammation (r = 0.37, p = 0.024) and significantly increased throughout degrees of NASH severity, as expressed as absence, borderline, and overt NASH at the liver biopsy (r = 0.35, p = 0.01). Greater circulating copeptin predicted the presence of NASH with OR = 1.73 (95% CI = 1.02-2.93) after multivariate adjustment for age, sex, renal function and presence of T2DM and MS components.

CONCLUSIONS

Increased plasma copeptin is independently associated with the presence and severity of NAFLD and NASH, pointing to a novel mechanism behind human fatty liver disease potentially modifiable by pharmacological treatment and lifestyle intervention.

Increased copeptin levels in metabolic syndrome from a Romanian population

Rationale: Arginine vasopressin (AVP) is secreted under conditions of water deprivation. Since AVP has a low half-life in the plasma, the C-terminal fragment of AVP-precursor (copeptin) was used to estimate the AVP levels. High copeptin levels increase the risk for the development of diabetes mellitus. Aim: This study was aimed to measure copeptin levels in the metabolic syndrome (MetS) in Romanians using a competitive enzyme immunoassay. Methods and results: Patients prone to present MetS (n = 63) were compared to controls (n = 42). In the MetS group, the syndrome was confirmed in 93.6%. Affected patients displayed 85.7% obesity and insulin resistance (HOMAIR of 4.9 ± 0.4 versus 1.1 ± 0.8 in controls). Low HDL-cholesterol was less represented (47.5%). Copeptin levels were 0.6 ± 0.0 in MetS versus 0.42 ± 0.0 ng/ mL in controls (P < 0.004). Higher copeptin (0.79 to 1.83 ng/ mL) was associated with MetS, P < 0.0018, OR 20, 95%CI [3.03 - 131.7]. In ANOVA, high copeptin was equally explained by MetS or obesity (P < 0.05,α = 3.8). The best correlation was found with high triglyceride levels (P < 0.013,α = 6.3) while the correlation with HOMAIR remained not significant. Discussion: These data indicated a concordant correlation between increased copeptin and MetS or its components. In the light of epidemiological data, indicating that more than 50% of the European population has a lower daily water intake and a fraction of 25% displaying high copeptin, our data further sustained that copeptin may be a good biomarker for MetS and/ or obesity, which should be further investigated with other members of the osmoregulation pathway at both pathogenesis and genetic levels.

Suppression of VLDL associated triacylglycerol secretion by both alpha- and beta-adrenoceptor agonists in isolated rat hepatocytes

The signal transduction pathways of intracellular calcium and adenosin 3',5'-cyclic monophosphate (cAMP) participate in the regulation of intrahepatic metabolism of very low density lipoproteins (VLDL). The adrenoceptors are linked to calcium and cAMP signal transduction pathways so it is proposed that they may be involved in the regulation of VLDL secretion. The current study is designed to test the effects of alpha- and beta-adrenoceptor agonists and antagonists on triacylglycerol secretion in freshly isolated rat hepatocytes. The inhibitory effect of epinephrine appeared at concentrations of more than 1 microM and reached a plateau at 100 microM. Epinephrine concentration for the half of the maximal bio-effect (EC(50)) was about 10 microM. Epinephrine at a concentration of 10 microM suppressed the secretion of triacylglycerol by 33% (P<or=0.01) and increased cellular content of triacylglycerol (18%, P<or=0.05) and total phospholipids (20%, P<or=0.05). Time course experiments for triacylglycerol secretion exhibited a linear relationship with a slope of 8.2+/-0.6 mug triacylglycerol/3 h mg cell protein. In the presence of epinephrine, cellular triacylglycerol and total phospholipids were slightly but significantly higher than the respective control at all points of time examined. The inhibitory effect elicited by epinephrine (10 microM) was abolished by the inclusion of the general alpha-adrenoceptor antagonist phentolamine (10 microM) and the specific alpha(1)-antagonist prazosin (1 microM) but not with the nonselective beta-antagonist propranolol (10 microM). Trifluoperazine an alpha-adrenoceptor antagonist and anticalmodulin agent, concealed the inhibitory effect of epinephrine in a concentration dependent manner, whereas theobromine a cAMP-phosphodiestrase inhibitor did not have any significant effect. The secretion of triacylglycerol was decreased not only by the alpha-adrenoceptor agonist phenylephrine (10 microM) but also by the beta-agonist isoproterenol (10 microM). Dibutyryl-cAMP (0.1 mM) also inhibited the secretion of triacylglycerol by 30% (P<or=0.01). The results suggest that epinephrine inhibits the secretion of triacylglycerol from rat hepatocytes via the alpha(1)-adrenoceptor while stimulation of beta- as well as alpha-adrenoceptors can also exert a similar effect.

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.

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.

Inhibition of phosphatidylcholine and phosphatidylethanolamine biosynthesis by cytochalasin B in cultured glioma cells: potential regulation of biosynthesis by Ca(2+)-dependent mechanisms

The major route of phosphatidylcholine (PtdCho) biosynthesis in mammalian cells is the sequence: choline (Cho)----phosphocholine (PCho)----cytidinediphosphate choline (CDP-Cho)----PtdCho. Recently, we have found that intermediates of this pathway are not freely diffusible in cultured rat glioma (C6) cells but are channeled towards PtdCho biosynthesis (George et al. (1989). Biochim. Biophys. Acta. 1004, 283-291). Channeling of intermediates in other mammalian systems is thought to be mediated through adsorption of enzymes to membranes and cytoskeletal elements to form multienzyme complexes. In this study, agents which perturb the structure and function of cytoskeletal elements were tested for effects on phospholipid metabolism in glioma cells. The filament-disrupting agent cytochalasin B (CB), but not other cytochalasins or the microtubule depolymerizer colchicine inhibited PtdCho and phosphatidylethanolamine (PtdEtn) biosynthesis as judged by dose-dependent reduction of labeling from [3H]Cho and [14C]ethanolamine (Etn). 32Pi pulse-labeling indicated that CB selectively decreased PtdCho and PtdEtn biosynthesis without affecting synthesis of other phospholipids. Synthesis of water-soluble intermediates of PtdCho metabolism was unaffected but the conversion of phosphoethanolamine to CDP-ethanolamine was reduced by CB. Effects of CB on phospholipid biosynthesis were not due to inhibition of glucose uptake as shown by experiments with 2-deoxyglucose, glucose-starved cells and other cytochalasins. Experiments with Ca(2+)-EGTA buffers and digitonin-permeabilized cells, and the Ca(2+)-channel blocker verapamil suggest that effects of CB on PtdCho and PtdEtn biosynthesis are due to alteration of intracellular Ca2+. Taken together, these results suggest that CB acts at sites distinct from glucose transport and cellular microfilaments to specifically inhibit PtdCho and PtdEtn biosynthesis by mechanisms dependent on intracellular Ca2+.

Rapid activation of phosphatidate phosphohydrolase in mesangial cells by lipid A

Knowledge of rapid events in cell signaling initiated by lipid A, the core moiety of bacterial lipopolysaccharide, is limited. In the present study we have demonstrated that cis-parinaric acid (cis-PnA) rapidly labels 1,2-sn-diacylglycerol (DAG) subsequent to labeling of phosphatidic acid (PA). Stimulation of microsomal membranes with lipid A decreased the level of PA labeled with cis-PnA within 5 s and increased the proportion of fluorescent label in DAG. Lipid A stimulation of DAG synthesis at 5-15 s was inhibited by incubation of mesangial cells with pertussis toxin prior to isolation of microsomal membranes. Inhibition of DAG formation was accompanied by an accumulation of the mass and fluorescent label in the cis-PnA-labeled phosphatidic acid pool. GTP gamma S caused a decrease in labeled PA and an increase in labeled 1,2-DAG. We conclude that the PA pool was enlarged via the lipid A sensitive lyso-PA acyl transferase (lyso-PA-AT) and was decreased by a phosphatidate phosphohydrolase to form DAG. The phosphatidate phosphohydrolase was at least partly regulated by a pertussis-sensitive G-protein. Lipid A or 1,2-dilinoleyl-PA, a product of lyso-PA-AT, induced cell activation as monitored by actin reorganization and cellular shape changes. Pretreatment of cells with pertussis toxin prevented the morphological changes normally induced by lipid A or 1,2-dilinoleyl-PA. In contrast, 1-oleoyl-2-acetylglycerol induced rapid actin reorganization and shape change, presumably bypassing the pertussis blockade. We propose that specific pools of PA and PA-derived DAG are key elements in rapid signaling in mesangial cells and are independent of the PI cycle and phospholipase C.

Unsaturated fatty acids activate glycogen phosphorylase in cultured rat hepatocytes

Oleate, linoleate, linolenate, arachidonate and eicosapentaenoate, but not myristate, palmitate and stearate, stimulated glycogen phosphorylase activity by 2-8-fold when added to cultured rat hepatocytes. Addition of BSA or Ca2- to the incubation medium decreased the stimulating effects of the unsaturated fatty acids. The combination of oleate or linolenate, with corticosterone, testosterone or estradiol produced synergistic stimulations of phosphorylase activity. The stimulation of glycogen phosphorylase activity by linolenate was inhibited by staurosporine or sphingosine. Staurosporine (80 nM) alone also decreased basal phosphorylase activities by about 60%. The results show that unsaturated fatty acids can be used as model agonists to stimulate phosphorylase activity by a mechanism that probably involves protein kinase C. On the basis of the fatty acid: BSA ratios used, this stimulation should only occur in vivo at high fatty acid concentrations when accompanied by hypoalbuminaemia.

Regulation of the metabolism of 1,2-diacylglycerols and inositol phosphates that respond to receptor activation

This review assimilates information on the regulation of the metabolism of those inositol phosphates and diacylglycerols that respond to receptor activation. Particular emphasis is placed on the regulation of specific enzymes, the occurrence of isoenzymes, and metabolic compartmentalization; the overall aim is to demonstrate the significance of these activities in relation to the physiological impact of the various cell signalling processes.

Interactions of thiophosphatidic acid with enzymes which metabolize phosphatidic acid. Inhibition of phosphatidic acid phosphatase and utilization by CDP-diacylglycerol synthase

Thiophosphatidic acid (1,2-diacyl-sn-glycero-3-phosphorothioate; thioPA) was chemically synthesized from egg phosphatidylcholine-derived 1,2-diacylglycerol and PSCl3 and tested for its effects on enzymes which utilize phosphatidic acid (PA) in phospholipid biosynthesis. The compound was not a substrate for rat liver cytosolic PA phosphatase and strongly inhibited this enzyme activity. ThioPA was also a potent inhibitor of purified membrane-associated PA phosphatase from Saccharomyces cerevisiae in a competitive manner and exhibited an apparent Ki = 60 microM. In contrast, purified CDPdiacylglycerol synthase (PA:CTP cytidylyltransferase) from this organism was able to convert thioPA to CDP-diacylglycerol. The apparent Vmax for thioPA was 7-fold lower than that for PA, whereas the apparent Km for thioPA (70 microM) was 4-fold lower than that for PA. Calculation of the specificity constant (Vmax/Km) demonstrated that PA was the preferred substrate. These properties of thioPA indicate that this substance may prove useful in studies of phospholipid metabolism and function.

Rapid activation of glycogen phosphorylase by steroid hormones in cultured rat hepatocytes

Testosterone (40-300 microM), oestradiol (20-500 microM), progesterone (20-500 microM), dexamethasone (10 nM-1 microM) and corticosterone (1-10 microM) activate glycogen phosphorylase rapidly when added directly to hepatocytes. The activation of phosphorylase was concentration-dependent and occurred after 10 min for dexamethasone, 30 min for testosterone and 60 min for oestradiol and progesterone. This rapid effect does not appear to be dependent on a stimulation of protein synthesis, it is independent of an increase in cyclic AMP, and it is not diminished by the presence of ornithine decarboxylase inhibitors. The stimulation of phosphorylase activity is diminished by depleting the incubation medium of Ca2+ in the presence of 0.5 mM-EGTA, and therefore it may involve changes in the distribution of Ca2+ in the hepatocytes. These results may explain some of the pharmacological effects of sex steroids, and also might contribute to the physiological actions of glucocorticoids.

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

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

Palmitate dependence of insulin secretion, "de novo" phospholipid synthesis and 45Ca2+-turnover in glucose stimulated rat islets

Palmitate ability to modify D-[U-14C]glucose incorporation into different lipids ("de novo" synthesis), as well as sugar-stimulation of insulin release and 45Ca2+-fluxes, was investigated in islets of fed and 48-h starved rats. The fatty-acid induced dose-dependent, correlative increments of insulin secretion, 45Ca2+-influx and the "de novo" synthesis of each phospholipid fraction analysed at 20 mmol/l (but not 3 mmol/l) glucose. Omission of calcium reduced drastically (p less than 0.001) insulin release and the "de novo" synthesis of neutral glycerolipids, leaving unaltered that of acidic phospholipids (phosphatidate and phosphoinositides). The increased synthesis of the latter is therefore not the consequence of stimulated secretion. It could initiate or contribute to maintain an increased turnover of islet phosphoinositides, thus generating some mediators of the calcium signalling system (inositol phosphates). Starvation led to a drastic reduction (p less than 0.001) of both insulin secretion, "de novo" synthesis of each lipid fraction, and 45Ca2+-influx in response to glucose and palmitate. The presence of a fatty-acid oxidation inhibitor (2-bromostearate or 2-tetradecylglycidate) prevented the effect of starvation on 45Ca2+-influx, as it has been shown to do on insulin secretion and palmitate incorporation into islet lipids. It is finally suggested that palmitate might amplify the insulin secretory response of islets to glucose, through the stimulation of the "de novo" synthesis of phosphoinositides and the subsequent generation of inositol phosphates, which would contribute to accelerated calcium turnover.

Factors regulating the secretion of lysophosphatidylcholine by rat hepatocytes compared with the synthesis and secretion of phosphatidylcholine and triacylglycerol. Effects of albumin, cycloheximide, verapamil, EGTA and chlorpromazine

1. The synthesis and secretion of glycerolipid by monolayer cultures of rat hepatocytes was measured by determining the incorporations of [3H]glycerol, [3H]oleate and [14C]choline and by the absolute concentration of triacylglycerol. 2. The presence of albumin in the medium stimulated the accumulation of lysophosphatidylcholine in the medium by 11-13-fold. 3. Cycloheximide did not significantly alter the accumulation of lysophosphatidylcholine. 4. This process was particularly sensitive to inhibition by chlorpromazine and verapamil, compared with the secretion of triacylglycerol and phosphatidylcholine. By contrast, it was relatively less sensitive to EGTA. 5. It is suggested that intracellular Ca2+ may be important in the production of lysophosphatidylcholine, which then accumulates in the medium by binding to albumin. In vivo this lysophosphatidycholine may be a means of delivering choline and polyunsaturated fatty acids to other organs.

Fatty acid specificity for the synthesis of triacylglycerol and phosphatidylcholine and for the secretion of very-low-density lipoproteins and lysophosphatidylcholine by cultures of rat hepatocytes

1. The synthesis and secretion of glycerolipids by monolayer cultures of rat hepatocytes was measured by using radioactive choline, glycerol and fatty acids and by measuring the concentration of triacylglycerols in the cells. 2. The incorporation of glycerol into triacylglycerol and the accumulation of this lipid in hepatocytes showed little specificity for fatty acids, except for eicosapentaenoate, which stimulated least. Oleate was more effective at stimulating triacylglycerol secretion than were palmitate, stearate, arachidonate and eicosapentaenoate. 3. Linoleate, linolenate, arachidonate and eicosapentaenoate stimulated the incorporation of glycerol and choline into phosphatidylcholine that was secreted into the medium. By contrast, palmitate and stearate produced relatively high incorporations into the phosphatidylcholine that remained in the cells. 4. The incorporation of glycerol and choline into lysophosphatidylcholine in the medium was stimulated 2-3-fold by all of the unsaturated fatty acids tested, whereas palmitate and stearate failed to stimulate if the acids were added separately. When 1 mM-stearate was added with 1 mM-linoleate, the incorporation of linoleate into lysophosphatidylcholine was about 4 times higher than that of stearate. 5. It is proposed that the secretion of lysophosphatidylcholine by the liver could provide a transport system for choline and essential unsaturated fatty acids to other organs.

Triton X-100 promotes the accumulation of phosphatidic acid and inhibits the synthesis of phosphatidylcholine in human decidua and chorion frondosum tissues in vitro

Triton X-100 is known to affect phospholipid metabolism and the generation of various signal molecules from cellular phospholipids. In the present work the effect of Triton X-100 on phospholipid metabolism of human decidua and of the primordial placenta (chorion frondosum) was studied. Triton X-100 (0.05%, v/v) added to tissue mince 30 min before the end of a 60 min incubation stimulated 2-4-fold (decidua) and 4-6-fold (placenta) the incorporation of [32P]phosphate ([32P]Pi) into phosphatidic acid, while markedly decreasing the labeling of phosphatidylcholine. Triton X-100 had no effect on the labeling of phosphatidylinositol in the decidua, and only a slight increase was observed in the placenta. When labeled glucose was used to assess phospholipid synthesis, the addition of Triton had no effect on phosphatidic acid, while decreasing the synthesis of phosphatidylcholine. Incorporation of [32P]Pi into phosphatidic acid was not accelerated by a submicellar concentration (0.01%) of Triton, whereas the synthesis of phosphatidylcholine was decreased irrespective of detergent concentration. Anionic or cationic detergents could not mimic the action of Triton on phosphatidic acid synthesis. Although Triton inhibited the synthesis of ATP in a dose-dependent manner, this could not account for the above results. Instead, it is suggested that diacylglycerol kinase and phosphocholine:CTP cytidylyltransferase are possible targets of the action of Triton X-100.

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

The effect of vasopressin on the biosynthesis of phosphatidylcholines and phosphatidylethanolamines was investigated in freshly isolated rat hepatocytes in suspension. Treatment of hepatocytes with vasopressin inhibits the incorporation of [Me-14C]choline into phosphatidylcholines in a dose-dependent manner. The hormone does not affect the uptake, phosphorylation or oxidation of choline. Pulse-chase studies indicate that CTP:cholinephosphate cytidylyltransferase might be subject to hormonal regulation by vasopressin. In contrast with the inhibitory effect of vasopressin on the synthesis of phosphatidylcholines, this hormone stimulates the incorporation of [1,2-14C]ethanolamine into phosphatidylethanolamines in a dose-dependent manner. Pulse and pulse-chase studies with labelled ethanolamine show that the conversion of ethanolaminephosphate to CDPethanolamine as well as the formation of phosphatidylethanolamines from CDPethanolamine and diacylglycerol are enhanced. Determination of the effect of vasopressin on the activity of the enzymes of the synthesis de novo of phosphatidylethanolamines demonstrates an increase of the activity of ethanolaminephosphotransferase, probably as a result of the increased amount of diacylglycerol in vasopressin-treated cells.

Modulation of rat brain cytosolic phosphatidate phosphohydrolase: effect of cationic amphiphilic drugs and divalent cations

The effects of three cationic amphiphilic drugs on rat brain cytosolic phosphatidate phosphohydrolase and their mechanisms of action were studied utilizing membrane-bound, emulsified, and emulsified sonicated phosphatidate as substrates. With the membrane-bound substrate, chlorpromazine, desmethylimipramine, and propranolol inhibited the activity in a dose-dependent fashion with an IC50 of 30-50 microM. In the presence of the emulsified substrate, chlorpromazine was a more potent inhibitor than desmethylimipramine or propranolol but 200 microM was needed for 50% inhibition of activity. Addition of heat-inactivated microsomes to the emulsified substrate, to simulate the conditions with the membrane-bound substrate, did not alter this value. Both Mg2+ and Ca2+ stimulated the enzyme activity but only Ca2+ counteracted the effect of chlorpromazine. Kinetic studies indicate that chlorpromazine acts as a noncompetitive inhibitor of the enzyme. Emulsified sonicated phosphatidate was a good substrate at low (less than 10 microM) concentrations. It was a poor substrate at 1 mM, but at this concentration chlorpromazine stimulated the activity instead of inhibiting. This drug altered the integrity of phosphatidate vesicle membranes as visualized by electron microscopy. The different results obtained with the three types of substrate indicate the importance of the configuration of phosphatidate for the expression of enzyme activity and for its susceptibility to the action of cationic amphiphilic drugs.

Effects of insulin, glucagon, dexamethasone, cyclic GMP and spermine on the stability of phosphatidate phosphohydrolase activity in cultured rat hepatocytes

Hepatocytes were preincubated with 10mM-glucagon and 100 microM-corticosterone to increase phosphatidate phosphohydrolase activity. Addition of 10 nM-glucagon or 100 microM-8-bromo cyclic GMP to a second incubation mixture that contained cycloheximide increased the half-life of the phosphohydrolase activity. Dexamethasone (100 nM) had no significant effect, but insulin (500 pM) or spermine (1 mM) decreased the half-life. None of these compounds altered the general rate of degradation of proteins labelled with [3H]leucine. There appears to be a specific control of the half-life of phosphatidate phosphohydrolase activity, which could contribute to its long-term regulation in the liver.

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.

Effect of catecholamines on the metabolic fate of nonesterified fatty acids in isolated hepatocytes from newborn rats

Isolated hepatocytes from newborn rats are able to produce ketone bodies from added medium-chain and long-chain fatty acids. Carnitine enhances the rate of ketone body synthesis from palmitate as well as from caprinoate. The 3-OHB/AcAc ratios indicate a highly reduced state of the mitochondrial redox carriers in the presence of both fatty acids and carnitine. Ketogenesis from palmitate accounts for about 90% of the total beta-oxidation. At recovery of 95% of the radioactivity two thirds of totally fatty acid uptake are channeled into esterification, whereas the remainder is oxidized. alpha- and beta-agonists stimulate glycogen degradation and glucose release and reduce net lactate production in hepatocytes from newborn rats. The (1-14C)-palmitate uptake is not altered by alpha- and beta-agonists. Phenylephrine significantly enhances 14CO2 production from (1-14C)-palmitate. Neither of the agonists affects the rate of esterification or of ketone body production with palmitate as substrate. Isoproterenol, however, stimulates ketogenesis from caprinoate even in the presence of optimal carnitine concentrations.

Insulin antagonises the growth hormone-mediated increase in the activity of phosphatidate phosphohydrolase in isolated rat hepatocytes

Rat hepatocytes were incubated in monolayer culture, under serum-free conditions for 8 h. Rat growth hormone (up to 100 nM) increased the activity of phosphatidate phosphohydrolase by up to 47%. Insulin (500 pM or 35 nM), cycloheximide or actinomycin D reversed this effect. The ability of growth hormone to modify the effects of insulin is discussed in relation to the control of the phosphohydrolase activity and glycerolipid synthesis.

Relationship between the displacement of phosphatidate phosphohydrolase from the membrane-associated compartment by chlorpromazine and the inhibition of the synthesis of triacylglycerol and phosphatidylcholine in rat hepatocytes

Glycerolipid synthesis was studied in isolated hepatocytes by using 177 microM [14C]oleate and 1 mM [3H]glycerol. Chlorpromazine (25-400 microM) inhibited the synthesis of phosphatidylcholine and triacylglycerol. This was accompanied by an average increase of 12-fold in the accumulation of the labelled precursors in phosphatidate at 200 microM chlorpromazine and a decrease in the conversion of phosphatidate to diacylglycerol of 76%. These results indicate that part of the inhibition of the synthesis of phosphatidylcholine and triacylglycerol occurs at the level of phosphatidate phosphohydrolase. The relative rate of triacylglycerol synthesis at different concentrations of chlorpromazine was approximately proportional to the rate of conversion of phosphatidate to diacylglycerol. Phosphatidylcholine synthesis increased at higher rates of conversion of phosphatidate to diacylglycerol, but it was relatively independent of the latter rate when this was inhibited by more than about 30% with chlorpromazine. The addition of oleate to the hepatocytes caused a translocation of phosphatidate phosphohydrolase from the cytosol to the membrane-associated compartment. Chlorpromazine had the opposite effect and displaced the phosphohydrolase from the membranes in the presence or absence of oleate. There was a highly significant correlation between the activity of phosphatidate phosphohydrolase that was associated with the membranes of the hepatocytes and the calculated conversion of [3H]phosphatidate to diacylglycerol. Chlorpromazine also antagonized the association of the phosphohydrolase with microsomal membranes when cell-free preparations were incubated with combinations of oleate and spermine. Furthermore, it inhibited the transfer of the soluble phosphohydrolase to microsomal membranes that were labelled with [14C]phosphatidate and thereby decreased diacylglycerol production. It is concluded that part of the action of chlorpromazine in inhibiting the synthesis of triacylglycerol and phosphatidylcholine occurs because it prevents the interaction of the soluble phosphatidate phosphohydrolase with the membranes on which glycerolipid synthesis occurs. This in turn prevents the conversion of phosphatidate to diacylglycerol.

Effects of dexamethasone and insulin on the synthesis of triacylglycerols and phosphatidylcholine and the secretion of very-low-density lipoproteins and lysophosphatidylcholine by monolayer cultures of rat hepatocytes

Rat hepatocytes in monolayer culture were preincubated for 19 h with 1 microM-dexamethasone, and the incubation was continued for a further 23 h with [14C]oleate, [3H]glycerol and 1 microM-dexamethasone. Dexamethasone increased the secretion of triacylglycerol into the medium in particles that had the properties of very-low-density lipoproteins. The increased secretion was matched by a decrease in the triacylglycerol and phosphatidylcholine that remained in the hepatocytes. Preincubating the hepatocytes for the total 42 h period with 36 nM-insulin decreased the amount of triacylglycerol in the medium and in the cells after the final incubation for 23 h with radioactive substrates. However, insulin had no significant effect on the triacylglycerol content of the cell and medium when it was present only in the final 23 h incubation. Insulin antagonized the effects of dexamethasone in stimulating the secretion of triacylglycerol from the hepatocytes, especially when it was present throughout the total 42 h period. The labelling of lysophosphatidylcholine in the medium when hepatocytes were incubated with [14C]oleate and [3H]glycerol was greater than that of phosphatidylcholine. The appearance of this lipid in the medium, unlike that of triacylglycerol and phosphatidylcholine, was not stimulated by dexamethasone, or inhibited by colchicine. However, the presence of lysophosphatidylcholine in the medium was decreased when the hepatocytes were incubated with both dexamethasone and insulin. These findings are discussed in relation to the control of the synthesis of glycerolipids and the secretion of very-low-density lipoproteins and lysophosphatidylcholine by the liver, particularly in relation to the interactions of glucocorticoids and insulin.

Interactions of insulin, glucagon and dexamethasone in controlling the activity of glycerol phosphate acyltransferase and the activity and subcellular distribution of phosphatidate phosphohydrolase in cultured rat hepatocytes

Rat hepatocytes were incubated in monolayer culture for 8 h. Glucagon (10nM) increased the total phosphatidate phosphohydrolase activity by 1.7-fold. This effect was abolished by adding cycloheximide, actinomycin D or 500 pM-insulin to the incubations. The glucagon-induced increase was synergistic with that produced by an optimum concentration of 100 nM-dexamethasone. Theophylline (1mM) potentiated the effect of glucagon, but it did not affect the dexamethasone-induced increase in the phosphohydrolase activity. The relative proportion of the phosphohydrolase activity associated with membranes was decreased by glucagon when 0.15 mM-oleate was added 15 min before the end of the incubations to translocate the phosphohydrolase from the cytosol. This glucagon effect was not seen at 0.5 mM-oleate. Since glucagon also increased the total phosphohydrolase activity, the membrane-associated activity was maintained at 0.15 mM-oleate and was increased at 0.5 mM-oleate. This activity at both oleate concentrations was also increased in incubations that contained dexamethasone, particularly in the presence of glucagon. Insulin increased the relative proportion of phosphatidate phosphohydrolase that was associated with membranes at 0.15 mM-oleate, but not at 0.5 mM-oleate. It also decreased the absolute phosphohydrolase activity on the membranes at both oleate concentrations in incubations that also contained glucagon and dexamethasone. None of the hormonal combinations significantly altered the total glycerol phosphate acyltransferase activity. However, glucagon significantly increased the microsomal activities, and insulin had the opposite effect. Glucagon also decreased the mitochondrial acyltransferase activity. There was a highly significant correlation between the total phosphatidate phosphohydrolase activity and the synthesis of neutral lipids from glycerol phosphate and 0.5 mM-oleate in homogenates of cells from all of the hormonal combinations. Phosphatidate phosphohydrolase activity is increased in the long term by glucocorticoids and also by glucagon through cyclic AMP. In the short term, glucagon increases the concentration of fatty acid required to translocate the cytosolic reservoir of activity to the membranes on which phosphatidate is synthesized. Insulin opposes the combined actions of glucagon and glucocorticoids. The long-term events explain the large increases in the phosphohydrolase activity that occur in vivo in a variety of stress conditions. The expression of this activity depends on increases in the net availability of fatty acids and their CoA esters in the liver.

Effects of cyclic AMP, glucocorticoids and insulin on the activities of phosphatidate phosphohydrolase, tyrosine aminotransferase and glycerol kinase in isolated rat hepatocytes in relation to the control of triacylglycerol synthesis and gluconeogenesis

Rat hepatocytes were incubated in monolayer culture in modified Leibovitz L-15 medium containing either 10% (v/v) newborn-calf serum or 0.2% (w/v) fatty-acid-poor bovine serum albumin. The addition of 100 nM-dexamethasone increased the activities of both phosphatidate phosphohydrolase and tyrosine aminotransferase by about 3.5-fold after 8h, and these activities continued to rise until at least 24h. Incubating the hepatocytes in the albumin-containing medium with 10 microM- or 100 microM-8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate increased the activities of the phosphohydrolase and aminotransferase by 2.6- and 3.4-fold respectively after 8h. These increases were blocked by actinomycin D. The increases in the activities that were produced by the cyclic AMP analogue and dexamethasone were independent and approximately additive. Insulin when added alone did not alter the phosphohydrolase activity, but it increased the aminotransferase activity by 34%. The dexamethasone-induced increase in the phosphohydrolase activity was completely blocked by 7-144 microM-insulin, whereas that of the aminotransferase was only partly suppressed. Insulin had no significant Effects on the increases in the activities of phosphatidate phosphohydrolase and tyrosine aminotransferase that were produced by the cyclic AMP analogue, but this may be because the analogue is fairly resistant to degradation by the phosphodiesterase. The activity of glycerol kinase was not significantly changed by incubating the hepatocytes with insulin, dexamethasone and the cyclic AMP analogue alone or in combinations. It is proposed that high concentrations of cyclic AMP and glucocorticoids increase the total activity of phosphatidate phosphohydrolase in the liver and provide it with an increased capacity for synthesizing triacylglycerols and very-low-density lipoproteins, which is expressed when the availability of fatty acids is high. There appears to be a co-ordinated hormonal control of triacyglycerol synthesis and gluconeogenesis in diabetes and in metabolic stress to enable the liver to supply other organs with energy.

Spermine promotes the translocation of phosphatidate phosphohydrolase from the cytosol to the microsomal fraction of rat liver and it enhances the effects of oleate in this respect

Spermine (0.5-2 mM) promoted the translocation of phosphatidate phosphohydrolase from the soluble to the microsomal fraction in a cell-free system derived from rat liver. By contrast, spermidine (1 mM) and putrescine (1 mM) had no significant effect on the translocation when added alone. Spermine, and to a lesser extent, spermidine, enhanced the translocating action of oleate and increased its effectiveness in transferring the phosphohydrolase from the soluble to the microsomal fraction. It is proposed that the phosphohydrolase becomes metabolically active when it combines with membranes and that polyamines might help to regulate this interaction. This could facilitate the action of fatty acids and enable cells to increase their capacity for triacylglycerol synthesis to match an increased availability of fatty acids.

Persistent activity modification of phosphatidate phosphohydrolase and fatty acyl-CoA synthetase on incubation of adipocytes with the tumour promoter 12-O-tetradecanoylphorbol 13-acetate

Incubation of rat adipocytes with the phorbol ester TPA caused a dose-dependent, persistent increase in phosphatidate phosphohydrolase activity and decrease in fatty acyl-CoA synthetase activity. Half-maximal effects were obtained with 1-2 X 10(-10) M TPA. TPA did not alter basal or noradrenaline-stimulated lipolysis. It is suggested that these two enzymes might be modified by protein kinase C.

The role of Mg2+-dependent phosphatidate phosphohydrolase in pulmonary glycerolipid biosynthesis

Rat lung microsomes washed with increasing concentrations of NaCl show a displacement of protein from microsomes to the wash supernatant. Among the proteins removed from the microsomal surface was the Mg2+-dependent phosphatidate phosphohydrolase, while the Mg2+-independent activity remained associated with the microsomes. The Mg2+-dependent activity could be quantitatively assayed in the wash supernatant. Microsomes washed with increasing concentrations of NaCl showed a progressive impairment in the synthesis of labelled neutral lipid and phosphatidylcholine from [14C]glycerol 3-phosphate with a concomitant increase in the labelling of phosphatidic acid. The impairment was sigmoidal and correlated highly with the decrease in Mg2+-dependent phosphatidate phosphohydrolase activity. When Mg2+-dependent phosphatidate phosphohydrolase from wash supernatant was incubated with microsomes previously washed with high salt concentrations, the labelling of neutral lipid and phosphatidylcholine was returned to control levels. Labelling of neutral lipids and phosphatidylcholine could be restored upon addition of a cytosolic Mg2+-dependent phosphatidate phosphohydrolase isolated by gel filtration. Mg2+-independent phosphatidate phosphohydrolase isolated from cytosol was incapable of restoring the labelling of neutral lipids and phosphatidylcholine. These findings confirm that the Mg2+-dependent phosphatidate phosphohydrolase of rat lung is involved in pulmonary glycerolipid biosynthesis. The role of the Mg2+-independent phosphatidate phosphohydrolase activity remains unknown.

The interaction of lithium with thyrotropin-releasing hormone-stimulated lipid metabolism in GH3 pituitary tumour cells. Enhancement of stimulated 1,2-diacylglycerol formation

Treatment of GH3 cells with thyrotropin-releasing hormone (TRH) for periods up to 60 min resulted in a prolonged reduction in the cellular content of phosphatidylinositol (PtdIns) with no lasting change in the levels of the other inositol-containing phospholipids. Accompanying this was a maintained increase in the GH3 cell 1,2-diacylglycerol content and a slower decline in the level of cellular triacylglycerol. When the cells were suspended in lithium-containing balanced salt solution for 30 min (in the absence of exogenous myo-inositol), there was a 15% decrease in GH3 cell inositol levels. This was associated with a small, but significant, increase in the cellular content of phosphatidylinositol 4,5-bisphosphate (PtdIns4,5P2) and 1,2-diacylglycerol. Addition of TRH to cells suspended in lithium-containing medium depleted cellular inositol levels by around 65% within 30 min. By this time, there was also a 50% reduction in the cellular content of PtdIns and a 20% reduction in phosphatidylinositol 4-phosphate (PtdIns4P). Control levels of PtdIns4,5P2 were maintained in the combined presence of TRH and lithium. Under those conditions, TRH no longer depleted cellular triacylglycerol and there was a marked increase in the ability of TRH to elevate the GH3 cell content of 1,2-diacylglycerol. The effect of TRH on the cellular content of phosphatidic acid was not altered by the presence of lithium. The results show, firstly, that when PtdIns resynthesis is inhibited by lithium-induced inositol depletion, its glycerol backbone accumulates, at least in part, in 1,2-diacylglycerol and, secondly, that GH3 cells preserve their cellular levels of PtdIns4,5P2 in the face of a considerable reduction in the cellular content of PtdIns.

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

A translocation of phosphatidate phosphohydrolase from the cytosolic to the microsomal fraction was promoted in cell-free extracts of rat liver by oleate and palmitate and their CoA esters. Oleate was more potent in this respect than palmitate and the CoA esters were more effective than the unesterified acids. Octanoate, octanoyl-CoA and CoA did not cause the translocation. It is proposed that the interaction of phosphatidate phosphohydrolase with the membranes that synthesize glycerolipids causes it to become metabolically active. This enables the liver to increase its capacity for triacylglycerol synthesis in response to an increased supply of fatty acids.

Can phosphorylation of phosphatidate phosphohydrolase by a cyclic AMP-dependent mechanism regulate its activity and subcellular distribution and control hepatic glycerolipid synthesis?

Incubating the particle-free supernatant of rat liver with alkaline phosphatase decreased the activity of phosphatidate phosphohydrolase by 21-29%. When the particle-free supernatant was incubated with various combinations of Mg2+, ATP, cyclic AMP and cyclic AMP-dependent protein kinase this failed to alter significantly phosphatidate phosphohydrolase activity under the conditions employed. The incubation of hepatocytes in monolayer culture with 0.5 mM-8-(4-chlorophenylthio)adenosine 3',5'-monophosphate increased the total activity of phosphatidate phosphohydrolase as measured in vitro. This also decreased the proportion of the phosphohydrolase that was associated with the membrane fraction of the cells and increased that in the cytosolic fraction. Adding 1 mM-oleate to the hepatocytes promoted the translocation of phosphatidate phosphohydrolase from the cytosol to the membrane-associated compartment. Oleate overcame the effect of the cyclic AMP analogue in favouring the cytosolic distribution of the phosphohydrolase. These results are discussed in relation to the interaction of hormonal balance and substrate supply in controlling the synthesis of phosphatidylcholine and triacylglycerol in the liver in stress and in diabetes. It is proposed that the cytosolic phosphatidate phosphohydrolase activity represents a reservoir of potential activity that becomes expressed when the enzyme translocates to the membranes on which the synthesis of glycerolipids occurs.

A transient increase in diacylglycerols is associated with the action of vasopressin on hepatocytes

Vasopressin induced a transient increase of 50% in the total concentration of diacylglycerols (determined by g.l.c.) in isolated hepatocytes. The increase was maximal at 0.25 min, and the concentration of diacylglycerols in cells treated with vasopressin had returned to the basal value by 4 min. No change in the concentration of diacylglycerols was observed after the treatment of cells with glucagon. The dependency of this effect on the concentration of vasopressin was similar to that of the effect of the hormone on 45Ca2+ efflux measured at 0.1 mM extracellular Ca2+. Vasopressin increased the proportion of arachidonic acid and stearic acid and decreased the proportion of oleic acid present in the diacylglycerols. In hepatocytes prelabelled with [14C]arachidonic acid, vasopressin increased the amount of [14C]diacylglycerol. The effects of vasopressin on the total concentration of diacylglycerols and [14C]diacylglycerol were mimicked by an exogenous phospholipid phosphodiesterase (phospholipase C) from Clostridium perfringens. The results are consistent with the conclusion that the transient increase in diacylglycerols induced by vasopressin is caused by the rapid hydrolysis of both the phosphoinositides and one or more other phospholipids.

Partial purification and characterization of the soluble phosphatidate phosphohydrolase of rat liver

A method is described by which the Mg2+-stimulated phosphatidate phosphohydrolase can be purified from the soluble fraction of liver from ethanol-treated rats. The increase in specific activity was about 416-fold. This involved purification by adsorption on calcium phosphate, chromatography on DE-52 DEAE-cellulose, separation on Ultrogel AcA-34 and chromatography on CM-Sepharose 6B. The effects of phosphatidylcholine, phosphatidate and Mg2+, Mn2+ and Zn2+ on the activity are described. Inhibitor studies indicate that the phosphohydrolase contains functional thiol groups and arginine residues.

Oleic acid promotes the activation and translocation of phosphatidate phosphohydrolase from the cytosol to particulate fractions of isolated rat hepatocytes

The incubation of hepatocytes with 1-4mM-oleate increased the total activity of phosphatidate phosphohydrolase that was measured in the presence of Mg2+ to about 2-fold. This was accompanied by an increase in the proportion of the enzyme that was isolated with the particulate fractions. Conversely, the addition of up to 4mM-oleate decreased the recovery of phosphatidate phosphohydrolase in the cytosolic fraction from about 70% to 3% when hepatocytes were lysed with digitonin. Most of the increase in the membrane-associated phosphohydrolase activity was isolated after cell fractionation in the microsomal fraction that was enriched with the endoplasmic-reticulum marker arylesterase. It is proposed that the translocation of phosphatidate phosphohydrolase facilitates the increased synthesis of triacylglycerols in the liver when it is presented with an increased supply of fatty acids.