Phospholipid dependence of rat liver microsomal acyl: CoA synthetase and acyl-CoA : 1-Acyl-sn-glycero-3-phosphocholine O-acyltransferase

Summary Investigations were performed on the influence of the phospholipid composition and physicochemical properties of the rat liver microsomal membranes on acyl-CoA synthetase and acyl-CoA : 1-acyl-sn-glycero-3-phosphocholine O-acyltransferase activities. The phospholipid composition of the membranes was modified by incubation with different phospholipids in the presence of lipid transfer proteins or by partial delipidation with exogenous phospholipase C and subsequent enrichment with phospholipids. The results indicated that the incorporation of phosphatidylglycerol, phosphatidylserine and phosphatidylethanolamine induced a marked activation of acyl-CoA synthetase for both substrates used—palmitic and oleic acids. Sphingomyelin occurred as specific inhibitor for this activity especially for palmitic acid. Palmitoyl-CoA: and oleoyl-CoA : lacyl-sn-glycero-3-phosphocholine acyltransferase activities were found to depend on the physical state of the membrane lipids. The alterations in the membrane physical state were estimated using two different fluorescent probes—1,6-diphenyl-1,3,5-hexatriene and pyrene. In all cases of membrane fluidization this activity was elevated. On the contrary, in more rigid membranes obtained by incorporation of sphingomyelin and dipalmitoylphosphatidylcholine, acyltransferase activity was reduced for both palmitoyl-CoA and oleoyl-CoA. We suggest a certain similarity in the way of regulation of membrane-bound acyltransferase and phospholipase A₂ which both participate in the deacylation-reacylation cycle.     

Incorporation of acetate-1-14C into lipids by the perfused liver of normal, X-irradiated, or partially hepatectomized rats

In order to study lipid metabolism in the liver without interference due to transport from and to the liver the isolated livers of normal, X-irradiated, and partially hepatectomized rats were perfused with acetate-1-(14)C and the distribution of radioactivity in total lipids, total fatty acids, individual lipids, and fatty acids of individual lipids was determined. In X-irradiated animals, an increased incorporation of acetate into many lipids, particularly into cholesterol, was observed. Lipids in the liver of the partially hepatectomized rats exhibited a marked increase in triglyceride content together with a decreased rate of incorporation into all but the phospholipid fractions. It is concluded that the increase usually observed in lipid pontent of the regenerating liver is due to the changes in transcort rather than to changes in synthesis. The changes observed in irradiated liver could be the result of alterations in the metabolism of precursors common to most lipids.     

Effect of pregnenolone-16alpha-carbonitrile, a microsomal enzyme inducer, on the regenerating rat liver.

PCN, a microsomal enzyme inducer, given orally (10 mg in 1 ml water twice daily for 5 days), increased liver weight and mitotic activity in intact as well as in partially hepatectomized rats. Electron microscopy revealed SER proliferation in the hepatocytes of animals treated with PCN alone. Accumulation of SER membranes was also evident in the liver cell cytoplasm of untreated, partially hepatectomized rats; it was however, more pronounced in the hepatocytes of partially hepatectomized animals given PCN. These results indicate that the steriod has a marked effect on the regeneration rat liver.     

Alterations in the activities of hepatic plasma-membrane and microsomal enzymes during liver regeneration.

A marked increase in the activities of rat liver plasma-membrane (Na+ + K+)-stimulated ATPase and microsomal Ca2+-stimulated ATPase was observed 18h after partial hepatectomy. Lipid analyses for both membrane preparations reveal that in partially hepatectomized rats the cholesterol and sphingomyelin content are decreased with a subsequent decrease in the cholesterol/phospholipid molar ratio compared with those of sham-operated animals. Changes in the allosteric properties of plasma-membrane (Na+ + K+)-stimulated ATPase by F- (as reflected by changes in the Hill coefficient) indicated a fluidization of the lipid bilayer of both membrane preparations in 18 h-regenerating liver. The amphipathic dodecyl glucoside incorporated into the hepatic plasma membranes evoked a marked increase in the (Na+ + K+)-stimulated ATPase and 5'-nucleotidase activities. The lack of effect of the glucoside on the Lubrol-PX-solubilized 5'-nucleotidase indicates that changes in the activities of the membrane-bound enzymes caused by the glucoside are due to modulation of the membrane fluidity. Dodecyl glucoside appears to increase the membrane fluidity, evaluated through changes in the Hill coefficient for plasma-membrane (Na+ + K+)-stimulated ATPase. The biological significance of these data is discussed in terms of the differences and changes in the interaction of membrane-bound enzymes with membrane lipids during liver regeneration.     

The effect of alpha-melanophor-stimulating hormone on liver regeneration and incorporation of amino acid in rats' liver protein.

Following 25 mug/day synthetic alpha-MSH administration, the liver regeneration of partially hepatectomized rats proved to be increased. The hormone treatment resulted in an enhanced alanine incorporation of the liver proteins, but this effect was uncertain on partially hepatectomized rats. Due to the hormone treatment the low liver protein content of the operated rats became normal. The pseudocholinesterase activity of the liver homogenate of alpha-MSH treated rats was also elevated. On the basis of these experiments authors are supposing some protein synthesis increasing effect of synthetic alpha-MSH.     

Sequential changes in alanine metabolism following partial hepatectomy in the rat

After partial hepatectomy, the liver undergoes an array of metabolic changes until regeneration is complete. Since carbons derived from alanine can be incorporated into most metabolic pools, we studied the metabolism of (14)C-labeled alanine during the early phase of regeneration. Sham operated (controls) and partially hepatectomized rats weighing about 200 g each were injected intraperitoneally with 1-[U-(14)C]alanine at 9, 18, and 36 hours after surgery. The animals were killed 2 hours after injection. Compared to the controls, alanine oxidation was markedly depressed (P < 0.05) in the 9- and 18-hour groups, but was restored in the 36-hour group. The specific activity of plasma glucose and hepatic glycogen was elevated 9 and 18 hours after partial hepatectomy. There was a corresponding increase in the activities of fructose-1,6-diphosphatase and phosphoenolpyruvate carboxykinase. Hepatic protein specific activity increased by 30, 74, and 120%, respectively 9, 18, and 36 hours after partial hepatectomy. Hepatic fatty acids followed a similar pattern. In a separate set of experiments, the distribution of radioactivity in glutamic acid was measured. The results showed that alanine carbons enter the citric acid cycle primarily via the acetyl CoA pathway in the controls, but via the oxaloacetate pathway in partially hepatectomized rats. The results demonstrate significant changes in the activities of metabolic pathways of alanine in the early phase of hepatic regeneration.     

Erythrocyte defects precede the onset of CCl4-induced liver cirrhosis. Protection by silymarin.

The time-course of some alterations produced in erythrocytes during the onset of CCl4-induced liver cirrhosis was studied in rats. Erythrocyte membranes were isolated to measure Na+, K+ and Ca+2-ATPase activities. Membrane lipid composition was determined to calculate the cholesterol/phospholipid ratio and serum samples were used to measure lipoperoxidation. The results demonstrated that as CCl4 treatment progressed, serum lipoperoxidation and membrane cholesterol/phospholipid ratio increased while ATPase activities decreased. ATPase activities in red blood cells of cirrhotic rats were 50% below normal values but those determined in cells of animals treated simultaneously with CCl4 + silymarin were significantly improved. Silymarin co-treatment also preserved the normal cholesterol/phospholipid ratio in the membranes. Our results suggest that the measure of ATPase activities in erythrocytes membranes could be a simple, safe and useful early marker of liver damage and also valuable to test the effectiveness of a given drug therapy.     

Thymidine metabolism in regenerating rat liver one to two hours after partial hepatectomy

1. When [(3)H]thymidine was injected intravenously into rats in amounts up to 40mg/kg body wt. and the (3)H radioactivity in the livers measured at 30min, saturation kinetics for thymidine uptake were not found. If the animals were examined 3 min after intravenous injection, saturation could be attained in normal rats with 12mg of thymidine/kg and in partially hepatectomized rats with 4mg/kg. At concentrations of thymidine close to saturation, no differences were found in rate or amount of uptake/g of liver between normal and partially hepatectomized rats 1-2h after operation. 2. Perfusion techniques were used to compare thymidine uptakes in the two sets of rats at concentrations up to 40mum-thymidine. Uptakes with tracer amounts of thymidine after 30min were identical in vivo and in the perfusion studies and were twice as great in livers from partially hepatectomized rats with concentrations up to 40mum-thymidine. 3. At 1.5h after operation there was nearly twice as much beta-aminoisobutyrate present per g of liver from partially hepatectomized as compared with normal rats.     

Effect of propionyl-L-carnitine treatment on membrane phospholipid fatty acid turnover in diabetic rat erythrocytes

In this work we have examined the effect of the oral administration of propionyl-L-carnitine (PLC) on the membrane phospholipid fatty acid turnover of erythrocytes from streptozotocin-induced diabetic rats. A statistically significant reduction in radioactive palmitate, oleate, and linoleate, but not arachidonate, incorporation into membrane phosphatidylcholine (PC) of diabetic rat erythrocytes with respect to control animals was found. Changes in radioactive fatty acid incorporation were also found in diabetic red cell phosphatidylethanolamine (PE), though they were not statistically significant. Oral propionyl-L-carnitine (PLC) treatment of diabetic rats partially restored the ability of intact red cells to reacylate membrane PC with palmitate and oleate, and reacylation with linoleate was fully restored. The analysis of the membrane phospholipid fatty acid composition revealed a consistent increase of linoleate levels in diabetic rat red cells, and a modest decrease of palmitate, oleate and arachidonate. The phospholipid fatty acid composition of diabetic red blood cells was not affected by the PLC treatment. Lysophosphatidylcholine acyl-CoA transferase (LAT) specific activity measured with either palmitoyl-CoA or oleyl-CoA was significantly reduced in diabetic erythrocyte membranes in comparison to controls. In addition LAT kinetic parameters of diabetic erythrocytes were altered. The reduced LAT activity could be partially corrected by PLC treatment of diabetic rats. Our data suggest that the impaired erythrocyte membrane physiological expression induced by the diabetic disease may be attenuated by the beneficial activity of PLC on the red cell membrane phospholipid fatty acid turnover.Abbreviations LAT lysophosphatidylcholine acyl-CoA transferase - PC phosphatidylcholine - PE phosphatidylethanolamine - PLC propionyl-L-carnitine - STZ streptozotocin     

Glucose homeostasis during the early stages of liver regeneration in fasted rats

Kinetic studies with [2-3H]glucose in vivo and gluconeogenic activity measurements in vivo and in vitro were performed in 70% hepatectomized rats submitted to fasting, which represents an extra burden for glucose synthesis but does not impair liver regeneration. Rates of glucose replacement, under steady-state conditions, 14 and 24 h postoperatively, did not differ in partially hepatectomized fasted rats and sham-operated controls. Phosphoenolpyruvate carboxykinase activities increased more rapidly during fasting in remnant livers than in intact livers from controls. Rates of incorporation of 14C from alanine into circulating glucose in hepatectomized rats were already maximal 14 h after surgery, whereas in controls they continued to augment. The maximal rates after partial hepatectomy could not be surpassed by performing the operation in diabetic animals. It is concluded that the relatively high blood sugar levels during fasting in hepatectomized rats do not depend on a reduced peripheral utilization of glucose, but only on a rapid increase in the gluconeogenic activity. The data suggest that hepatocytes in remnant liver can proliferate under conditions of maximal gluconeogenic and low glycolytic activities.     

Sensitivity of liver cells formed after partial hepatectomy to glucagon, vasopressin and angiotensin II

During the active proliferation which follows partial hepatectomy, the sensitivity of liver cells to glucagon is markedly diminished. In hepatocytes obtained from rats partially hepatectomized 3 days before experiments were performed, the dose-response curves to glucagon were shifted to the right by about two orders of magnitude as compared to those of the control cells. Later on (7 days after surgery) the dose-response to glucagon was still shifted to the right but by only one order of magnitude. These data are consistent with the diminution in the number of glucagon receptors in liver plasma membrane during liver regeneration reported by other authors. No stimulation of glycogenolysis, gluconeogenesis or ureogenesis was produced by vasopressin or angiotensin II in hepatocytes from rats partially hepatectomized 3 days before experimentation. However, phosphatidylinositol labeling was stimulated in these cells to a similar extent as in the controls. The ionophore A23187 was also ineffective in stimulating glycogenolysis in these cells. Later, 7 days after surgery, the hepatic responsiveness to vasopressin and angiotensin II was restored. The data suggest that, during the initial stages of liver regeneration, the enzymatic machinery of the hepatocyte is not sensitive to calcium-signalling.     

Dehydroepiandrosterone (DHEA) facilitates liver regeneration after partial hepatectomy in rats

In this study we investigated whether or not liver regeneration is facilitated by dehydroepiandrosterone (DHEA) after partial (70%) hepatectomy in rats. Treatment with DHEA (300 mg/kg body weight) did not cause any significant increase in the expression ratio of proliferating cell nuclear antigen (PCNA) in sham-operated controls; however, in partially hepatectomized rats it caused a significant increase in the ratio in hepatocytes 24 and 36 hr after hepatectomy. In partially hepatectomized rats, DHEA treatment significantly accelerated the restoration of liver 48, 60, and 72 hr after partial hepatectomy. The restoration rate in DHEA-treated hepatectomized rats at 72 hr was 1.3-fold greater than in partially hepatectomized controls. Treatment with androstenedione (300 mg/kg body weight), the first metabolite of DHEA, did not cause any significant increase in the expression of PCNA in either sham-operated controls or partially hepatectomized rats. These results indicate that DHEA itself promotes the liver regenerative process after partial hepatectomy in rats.     

Urethane interaction with nucleic acids and proteins from non-malignant fast growing tissues

The interaction of urethane metabolite(s) with macromolecules in tissues of pregnant and partially hepatectomized mice was studied. Pregnant mice were treated with tritiated urethane on the 17th day of pregnancy. Partially hepatectomized mice were studied at 72 h and 198 h after the operation. Operated mice were injected with tritiated urethane, 6 h before killing. It was observed that the specific radioactivity of lung DNA was highest in pregnant mice whereas in partially hepatectomized mice the specific radioactivities of lung DNA and regenerating liver DNA were comparable. It was also observed that binding to macromolecules was greatest at 72 h when the highest mitotic activity in regenerating liver occurs.     

The effect of bile salts and calcium on isolated rat liver mitochondria

Intact mitochondria were incubated with and without calcium in solutions of chenodeoxycholate, ursodeoxycholate, or their conjugates. Glutamate dehydrogenase, protein and phospholipid release were measured. Alterations in membrane and organelle structure were investigated by electron paramagnetic resonance spectroscopy. Chenodeoxycholate enhanced enzyme liberation, solubilized protein and phospholipid, and increased protein spin label mobility and the polarity of the hydrophobic membrane interior, whereas ursodeoxycholate and its conjugates did not damage mitochondria. Preincubation with ursodeoxycholate or its conjugate tauroursodeoxycholate for 20 min partially prevented damage by chenodeoxycholate. Extended preincubation even with 1 mM ursodeoxycholate could no longer prevent structural damage. Calcium (from 0.01 mM upward) augmented the damaging effect of chenodeoxycholate (0.15-0.5 mM). The combined action of 0.01 mM calcium and 0.15 mM chenodeoxycholate was reversed by ursodeoxycholate only, not by its conjugates tauroursodeoxycholate and glycoursodeoxycholate. In conclusion, ursodeoxycholate partially prevents chenodeoxycholate-induced glutamate dehydrogenase release from liver cell mitochondria by membrane stabilization. This holds for shorter times and at concentrations below 0.5 mM only, indicating that the different constitution of protein-rich mitochondrial membranes does not allow optimal stabilization such as has been seen in phospholipid- and cholesterol-rich hepatocyte cell membranes, investigated previously.     

Phospholipid fatty acid modification of rat liver microsomes affects acylcoenzyme A:cholesterol acyltransferase activity

The effect of phospholipid fatty acyl composition on the activity of acylcoenzyme A:cholesterol acyltransferase was investigated in rat liver microsomes. Specific phosphatidylcholine replacements were produced by incubating the microsomes with liposomes and bovine liver phospholipid-exchange protein. Although the fatty acid composition of the microsomes was modified appreciably, there was no change in the microsomal phospholipid or cholesterol content. As compared to microsomes enriched for 2 h with dioleoylphosphatidylcholine, those enriched with dipalmitoylphosphatidylcholine exhibited 30-45% less acyl-CoA:cholesterol acyltransferase activity. Enrichment with 1-palmitoyl-2-linoleoylphosphatidylcholine increased acyl-CoA:cholesterol acyltransferase activity by 20%. By contrast, dilinoleoylphosphatidylcholine abolished microsomal acyl-CoA:cholesterol acyltransferase activity almost completely. Addition of cofactors that stimulated microsomal lipid peroxidation inhibited acyl-CoA:cholesterol acyltransferase activity by only 10%, however, and did not increase the inhibition produced by submaximal amounts of dilinoleoylphosphatidylcholine. Certain of the phosphatidylcholine replacements produced changes in palmitoyl-CoA hydrolase, NADPH-dependent lipid peroxidase, glucose-6-phosphatase and UDPglucuronyl transferase activities, but they did not closely correlate with the alterations in acyl-CoA:cholesterol acyltransferase activity. Electron spin resonance measurements with the 5-nitroxystearate probe indicated that microsomal lipid ordering was reduced to a roughly similar extent by dioleoyl- or by dilinoleoylphosphatidylcholine enrichment. Since these enrichments produce widely different effects on acyl-CoA:cholesterol acyltransferase activity, changes in bulk membrane lipid fluidity cannot be the only factor responsible for phospholipid fatty acid compositional effect on acyl-CoA:cholesterol acyltransferase. The present results are more consistent with a modulation resulting from either changes in the lipid microenvironment of acyl-CoA:cholesterol acyltransferase or a direct interaction between specific phosphatidylcholine fatty acyl groups and acyl-CoA:cholesterol acyltransferase.     

Relation between membrane potential changes and DNA, RNA and protein synthesis in regenerating liver cells after partial hepatectomy

Variations in the membrane potential and in DNA, RNA and protein synthesis were studied in experiments on rats at varying times after hepatectomy. Hyperpolarization of the hepatocyte plasmatic membrane was shown to develop during liver regeneration along with activation of DNA, RNA and protein synthesis. Actinomycin D prevented the development of hyperpolarization and activation of RNA and protein synthesis. Administration of liver filtrates from hepatectomized rats to intact recipients induced hyperpolarization of liver cells. Activation of protein biosynthesis following hepatectomy resulted in hyperpolarization of the cell membrane, this action being mediated via formation of a specific membrane-active factor.     

Biosynthetic labelling of membrane lipids of eukaryotic cells in tissue culture by a novel type of fluorescent fatty acids.

W-Anthryl labelled fatty acids with hydrocarbon chains of different lengths (C8, C11, C15) and different degrees of unsaturation have been incorporated into the membrane lipids of three different cell lines in tissue culture by addition of these 3H-labelled precursor fatty acids to the growth medium. The cell lines were baby hamster kidney cells (BHK 21), Chang liver cells and the RN6 cell line derived from a chemically induced Schwannoma tumor cell clone. Cell growth was normal. The quantitative analysis on the basis of radioactivity determinations demonstrated that the fluorescent-labelled fatty acids were introduced into the neutral lipid fraction (triglycerides, diglycerides, and cholesterol esters, all present in small amounts), but mainly into the phospholipid classes phosphatidylcholine, -ethanolamine and -serine, and to a lesser extent, as N-acyl component of sphingolipids (sphingomyelins, ceramides, mono- and diglycosylceramides). Cell fractionation studies indicated that the membranes of all subcellular particles were labelled with the fluorescent probes in their lipid moieties. These w-anthryl fatty acids are the first type of fluorescent lipid precursors which can be incorporated biosynthetically in vivo into membrane lipids of eukaryotic cells. The effective incorporation of the bulky fluorescent anthryl group in the terminal position of fatty acids of different chain lengths into the complex membrane lipids of the cell gives proff of 1) their uninhibited membrane transport, 2) their activation by the acyl-CoA synthetase and 3) their substrate properties for the O- acyl and N-acyl transferases in phospho- and sphingolipid biosynthesis.     

Thymidine kinase and deoxyribonucleic acid metabolism in growing and regenerating livers from rats on controlled feeding schedules

Rats accustomed to eating during the first 8h of a daily 12h dark period re-established about 80% of intact liver weight, protein and DNA within 4 days following partial hepatectomy; further increases were not observed. Liver thymidine kinase activity and thymidine incorporation into liver DNA exhibited marked daily oscillations during liver regeneration. Maximum values were observed near the end of the dark period both in intact growing rats and in rats partially hepatectomized 2h before the end of the dark period. The time of day of surgery affected thymidine kinase activity and thymidine incorporation into DNA at specific times following partial hepatectomy. This seriously affects the interpretation of reports of experiments where the time of day of killing has been held constant and time of surgery varied. Highly significant correlation coefficients were observed for thymidine incorporation before killing versus thymidine kinase activity at time of killing and for thymidine versus orotic acid incorporation into DNA of livers from rats partially hepatectomized 2h before the end of the dark period and killed at 12h intervals. Thymidylate phosphatase activity returned to the normal amount at a rate similar to that for liver protein. Thymidylate phosphatase did not affect the validity of the thymidine kinase assay. The relationship of [(14)C]orotic acid to [(3)H]thymidine incorporation into liver DNA varied with the time of day, with the age of the rat and during the regeneration of the liver.     

The effect of choline deficiency on the outer membranes of rat liver mitochondria

The outer membranes of mitochondria prepared from the liver of rats kept 12 days on a choline-deficient diet were analyzed for changes in phospholipid and protein content. The total amount of phospholipid in the outer membranes was not affected by the deficiency. There was, however, a significant decrease in the amount of phosphatidylcholine and an increase in phosphatidylethanolamine. The alterations in the membrane phospholipids were reflected in a reduction in the fluorescence of the membrane probe, 8-anilino-1-naphthalene sulfonate. Choline deficiency also affected the protein composition of the outer membranes as judged by electrophoretic analysis; however, the activity of several enzymes which serve as markers for the outer membrane was not affected by the deficiency.     

The phospholipid-dependence of uridine diphosphate glucuronyltransferase. Effect of protein deficiency on the phospholipid composition and enzyme activity of rat liver microsomal fraction

After force-feeding a protein-free diet to male rats for 5-7 days a substantial (2.4-fold) increase in the specific activity of the liver microsomal enzyme UDP-glucuronyltransferase (EC 2.4.1.17) was observed. A similar activation of the enzyme occurred when rats were fed on a low-protein (5%, w/w, casein) diet for 60 days. Although both the short- and long-term protein-deficient diets decreased the contents of microsomal protein and phospholipid in liver tissue they did not significantly alter the ratio of these major membrane components. Protein deficiency profoundly altered the phospholipid composition of microsomal membranes. The most striking difference in microsomal phospholipid composition between control and protein-deficient rats was their content of lysophosphatides. Whereas microsomal membranes from protein-deficient rats contained significant proportions of lysophosphatidylcholine and lysophosphatidylethanolamine very little or no lysophosphatides were detected in control preparations. Pretreatment of microsomal fractions from normal rats with phospholipase A markedly increased their UDP-glucuronyltransferase activity as did their pretreatment with lysophosphatidylcholine. It is concluded that the quantities of lysophosphatides present in microsomal membranes from protein-deficient rats were sufficient to have caused the increased UDP-glucuronyltransferase activities of these preparations. Evidence is presented suggesting that these changes in microsomal phospholipid composition and UDP-glucuronyltransferase activity caused by protein deficiency reflect changes that occur in vivo. The possible physiological significance of these findings is discussed.