Lipolysis of hepatic triacylglycerol stores

Silage for upcycling oil from saithe (Pollachius virens) viscera - Effect of raw material freshness on the oil quality

The main objective of this study was to investigate how the freshness of saithe (Pollachius virens) viscera affected the quality, composition and yield of oil obtained by silaging. Minced viscera with and without liver were stored separately for up to 3 days at 4 °C before silaging at pH 3.8 for 6 days at 10 °C. An antioxidant mixture was added to evaluate the effect on the lipid oxidation. Oil was extracted thermally from untreated raw material during storage (day 0-3) and after silaging. For oil obtained after silaging of viscera with liver, the oil yields increased significantly when the raw material was stored for more than one day prior to the treatment. Use of fresh raw material (collected at day 0) led to significantly lower oxidation compared to longer raw material storage. After one day of storage, the oxidation was less dependent on the freshness. Silaging with antioxidants resulted in significantly lower formation of oxidation products compared to acid without antioxidants and the most significant differences were observed after one day of storage. Contents of docosahexaenoic acid (DHA) and total omega-3 fatty acids decreased significantly when the raw material was stored for 1-3 days prior to silaging compared to fresh raw material. Results obtained by high resolution nuclear magnetic resonance (NMR) spectroscopy indicated that oxidation of esterified DHA might explain the DHA decrease. The free fatty acid content was highest when fresh raw material was used and was most likely affected by the formation of cholesteryl esters observed in NMR spectra after longer storage. The study shows that although the oil quality is reduced during silaging, processing shortly after catch and use of antioxidants can optimize the quality resulting in less oxidized oil richer in omega-3 fatty acids.

β-Adrenergic induction of lipolysis in hepatocytes is inhibited by ethanol exposure

In liver steatosis (i.e. fatty liver), hepatocytes accumulate many large neutral lipid storage organelles known as lipid droplets (LDs). LDs are important in the maintenance of energy homeostasis, but the signaling mechanisms that stimulate LD metabolism in hepatocytes are poorly defined. In adipocytes, catecholamines target the β-adrenergic (β-AR)/cAMP pathway to activate cytosolic lipases and induce their recruitment to the LD surface. Therefore, the goal of this study was to determine whether hepatocytes, like adipocytes, also undergo cAMP-mediated lipolysis in response to β-AR stimulation. Using primary rat hepatocytes and human hepatoma cells, we found that treatment with the β-AR agent isoproterenol caused substantial LD loss via activation of cytosolic lipases adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL). β-Adrenergic stimulation rapidly activated PKA, which led to the phosphorylation of ATGL and HSL and their recruitment to the LD surface. To test whether this β-AR-dependent lipolysis pathway was altered in a model of alcoholic fatty liver, primary hepatocytes from rats fed a 6-week EtOH-containing Lieber-DeCarli diet were treated with cAMP agonists. Compared with controls, EtOH-exposed hepatocytes showed a drastic inhibition in β-AR/cAMP-induced LD breakdown and the phosphorylation of PKA substrates, including HSL. This observation was supported in VA-13 cells, an EtOH-metabolizing human hepatoma cell line, which displayed marked defects in both PKA activation and isoproterenol-induced ATGL translocation to the LD periphery. In summary, these findings suggest that β-AR stimulation mobilizes cytosolic lipases for LD breakdown in hepatocytes, and perturbation of this pathway could be a major consequence of chronic EtOH insult leading to fatty liver.

The New Biology and Pharmacology of Glucagon

In the last two decades we have witnessed sizable progress in defining the role of gastrointestinal signals in the control of glucose and energy homeostasis. Specifically, the molecular basis of the huge metabolic benefits in bariatric surgery is emerging while novel incretin-based medicines based on endogenous hormones such as glucagon-like peptide 1 and pancreas-derived amylin are improving diabetes management. These and related developments have fostered the discovery of novel insights into endocrine control of systemic metabolism, and in particular a deeper understanding of the importance of communication across vital organs, and specifically the gut-brain-pancreas-liver network. Paradoxically, the pancreatic peptide glucagon has reemerged in this period among a plethora of newly identified metabolic macromolecules, and new data complement and challenge its historical position as a gut hormone involved in metabolic control. The synthesis of glucagon analogs that are biophysically stable and soluble in aqueous solutions has promoted biological study that has enriched our understanding of glucagon biology and ironically recruited glucagon agonism as a central element to lower body weight in the treatment of metabolic disease. This review summarizes the extensive historical record and the more recent provocative direction that integrates the prominent role of glucagon in glucose elevation with its under-acknowledged effects on lipids, body weight, and vascular health that have implications for the pathophysiology of metabolic diseases, and the emergence of precision medicines to treat metabolic diseases.

The glucagon-like peptide-1 analogue exendin-4 reverses impaired intracellular Ca(2+) signalling in steatotic hepatocytes

The release of Ca(2+) from the endoplasmic reticulum (ER) and subsequent replenishment of ER Ca(2+) by Ca(2+) entry through store-operated Ca(2+) channels (SOCE) play critical roles in the regulation of liver metabolism by adrenaline, glucagon and other hormones. Both ER Ca(2+) release and Ca(2+) entry are severely inhibited in steatotic hepatocytes. Exendin-4, a slowly-metabolised glucagon-like peptide-1 (GLP-1) analogue, is known to reduce liver glucose output and liver lipid, but the mechanisms involved are not well understood. The aim of this study was to determine whether exendin-4 alters intracellular Ca(2+) homeostasis in steatotic hepatocytes, and to evaluate the mechanisms involved. Exendin-4 completely reversed lipid-induced inhibition of SOCE in steatotic liver cells, but did not reverse lipid-induced inhibition of ER Ca(2+) release. The action of exendin-4 on Ca(2+) entry was rapid in onset and was mimicked by GLP-1 or dibutyryl cyclic AMP. In steatotic liver cells, exendin-4 caused a rapid decrease in lipid (half time 6.5min), inhibited the accumulation of lipid in liver cells incubated in the presence of palmitate plus the SOCE inhibitor BTP-2, and enhanced the formation of cyclic AMP. Hormone-stimulated accumulation of extracellular glucose in glycogen replete steatotic liver cells was inhibited compared to that in non-steatotic cells, and this effect of lipid was reversed by exendin-4. It is concluded that, in steatotic hepatocytes, exendin-4 reverses the lipid-induced inhibition of SOCE leading to restoration of hormone-regulated cytoplasmic Ca(2+) signalling. The mechanism may involve GLP-1 receptors, cyclic AMP, lipolysis, decreased diacylglycerol and decreased activity of protein kinase C.

Interactions between hepatic iron and lipid metabolism with possible relevance to steatohepatitis

The liver is an important site for iron and lipid metabolism and the main site for the interactions between these two metabolic pathways. Although conflicting results have been obtained, most studies support the hypothesis that iron plays a role in hepatic lipogenesis. Iron is an integral part of some enzymes and transporters involved in lipid metabolism and, as such, may exert a direct effect on hepatic lipid load, intrahepatic metabolic pathways and hepatic lipid secretion. On the other hand, iron in its ferrous form may indirectly affect lipid metabolism through its ability to induce oxidative stress and inflammation, a hypothesis which is currently the focus of much research in the field of non-alcoholic fatty liver disease/non-alcoholic steatohepatitis (NAFLD/NASH). The present review will first discuss how iron might directly interact with the metabolism of hepatic lipids and then consider a new perspective on the way in which iron may have a role in the two hit hypothesis for the progression of NAFLD via ferroportin and the iron regulatory molecule hepcidin. The review concludes that iron has important interactions with lipid metabolism in the liver that can impact on the development of NAFLD/NASH. More defined studies are required to improve our understanding of these effects.

Metabolomic and mass isotopomer analysis of liver gluconeogenesis and citric acid cycle: II. Heterogeneity of metabolite labeling pattern

In this second of two companion articles, we compare the mass isotopomer distribution of metabolites of liver gluconeogenesis and citric acid cycle labeled from NaH(13)CO(3) or dimethyl [1,4-(13)C(2)]succinate. The mass isotopomer distribution of intermediates reveals the reversibility of the isocitrate dehydrogenase + aconitase reactions, even in the absence of a source of alpha-ketoglutarate. In addition, in many cases, a number of labeling incompatibilities were found as follows: (i) glucose versus triose phosphates and phosphoenolpyruvate; (ii) differences in the labeling ratios C-4/C-3 of glucose versus (glyceraldehyde 3-phosphate)/(dihydroxyacetone phosphate); and (iii) labeling of citric acid cycle intermediates in tissue versus effluent perfusate. Overall, our data show that gluconeogenic and citric acid cycle intermediates cannot be considered as sets of homogeneously labeled pools. This probably results from the zonation of hepatic metabolism and, in some cases, from differences in the labeling pattern of mitochondrial versus extramitochondrial metabolites. Our data have implications for the use of labeling patterns for the calculation of metabolic rates or fractional syntheses in liver, as well as for modeling liver intermediary metabolism.

Delayed secretory pathway contributions to VLDL-triglycerides from plasma NEFA, diet, and de novo lipogenesis in humans

Newly synthesized triglyceride (TG) may exit the liver immediately as VLDL-TG or be stored and secreted after a delay. We quantified the contributions from plasma NEFA, diet, and de novo lipogenesis (DNL) to VLDL-TG via immediate and delayed pathways in five lean, normolipidemic subjects; six obese, hypertriglyceridemic (HPTG) nondiabetics; and six obese, HPTG diabetics. Intravenous [(2)H(31)]palmitate and [1-(13)C(1)] acetate and oral [(2)H(35)]stearate were administered for 30 h preceding an overnight fast. [1,2,3,4-(13)C(4)]palmitate was infused during the subsequent 12 h fast. Contributions from plasma NEFA via the immediate pathway were 64 +/- 15, 33 +/- 6, and 58 +/- 2% in control, HPTG, and diabetic HPTG, respectively. Delayed pool fractional contributions were as follows: dietary FA, 2.0 +/- 0.9, 2.5 +/- 1, and 12 +/- 2%; DNL, 3 +/- 0.3, 14 +/- 3, and 13 +/- 4%; delayed NEFA, 15 +/- 4, 20 +/- 4, and 30 +/- 3%. VLDL-TG production rates and absolute input rates from the delayed pool were significantly higher in HPTG and diabetic HPTG than in controls. In conclusion, we provide direct kinetic evidence for a hepatic TG storage pool in humans and document its metabolic sources. The turnover time and sources of this pool differ in diabetic HPTG and nondiabetic HPTG, with potential therapeutic implications.

Mobilisation of triacylglycerol stores

Triacylglycerol (TAG) is an energy dense substance which is stored by several body tissues, principally adipose tissue and the liver. Utilisation of stored TAG as an energy source requires its mobilisation from these depots and transfer into the blood plasma. The means by which TAG is mobilised differs in adipose tissue and liver although the regulation of lipid metabolism in each of these organs is interdependent and synchronised in an integrated manner. This review deals principally with the mechanism of hepatic TAG mobilisation since this is a rapidly expanding area of research and may have important implications for the regulation of plasma very-low-density lipoprotein metabolism. TAG mobilisation plays an important role in fuel selection in non-hepatic tissues such as cardiac muscle and pancreatic islets and these aspects are also reviewed briefly. Finally, studies of certain rare inherited disorders of neutral lipid storage and mobilisation may provide useful information about the normal enzymology of TAG mobilisation in healthy tissues.

Limitations of the mass isotopomer distribution analysis of glucose to study gluconeogenesis. Substrate cycling between glycerol and triose phosphates in liver

Mass isotopomer distribution analysis allows studying the synthesis of polymeric biomolecules from 15N, 13C-, or 2H-labeled monomeric units in the presence of unlabeled polymer. The mass isotopomer distribution of the polymer allows calculation of (i) the enrichment of the monomer and (ii) the dilution of the newly synthesized polymer by unlabeled polymer. We tested the conditions of validity of mass isotopomer distribution analysis of glucose labeled from [U-13C3]lactate, [U-13C3]glycerol, and [2-13C]glycerol to calculate the fraction of glucose production derived from gluconeogenesis. Experiments were conducted in perfused rat livers, live rats, and live monkeys. In all cases, [13C]glycerol yielded labeling patterns of glucose that are incompatible with glucose being formed from a single pool of triose phosphates of constant enrichment. We show evidence that variations in the enrichment of triose phosphates result from (i) the large fractional decrease in physiological glycerol concentration in a single pass through the liver and (ii) the release of unlabeled glycerol by the liver, presumably via lipase activity. This zonation of glycerol metabolism in liver results in the calculation of artifactually low contributions of gluconeogenesis to glucose production when the latter is labeled from [13C]glycerol. In contrast, [U-13C3]lactate appears to be a suitable tracer for mass isotopomer distribution analysis of gluconeogenesis in vivo, but not in the perfused liver. In other perfusion experiments with [2H5]glycerol, we showed that the rat liver releases glycerol molecules containing one to four 2H atoms. This indicates the operation of a substrate cycle between extracellular glycerol and liver triose phosphates, where 2H is lost in the reversible reactions catalyzed by alpha-glycerophosphate dehydrogenase, triose-phosphate isomerase, and glycolytic enzymes. This substrate cycle presumably involves alpha-glycerophosphate hydrolysis.

Peroxisomal beta-oxidation of fatty acids in bovine and rat liver

Hepatic peroxisomal beta-oxidation rates were compared in liver homogenates from cows and rats during different nutritional and physiological states. Peroxisomal oxidation in liver homogenates from cows represented 50% and 77% of the total capacity for the initial cycle of beta-oxidation of palmitate and octanoate, respectively, but only 26% and 65% for rats. Lactation or food deprivation did not alter rates of hepatic peroxisomal beta-oxidation of palmitate or octanoate in cows. Fasting and clofibrate treatment increased rates of total and peroxisomal beta-oxidation of palmitate and octanoate in rat liver.

Cytoplasmic triacylglycerols and cholesteryl esters are degraded in two separate catabolic pools in cultured human fibroblasts

The sources and the catabolic pathways of cytoplasmic pools of triacylglycerols and cholesteryl esters have been comparatively investigated in cultured fibroblasts from normal subjects and from patients affected with neutral lipid storage disease (NLSD) and Wolman disease (WD). (i) Endogenously biosynthesized triacylglycerols and cholesteryl esters were degraded extra-lysosomally since they were catabolized at similar rates in normal and in WD fibroblasts. In NLSD fibroblasts, the degradation of endogenous triacylglycerols was severely deficient, whereas that of endogenous cholesteryl esters was in the normal range. (ii) Reconstituted high density lipoproteins (HDL) containing radiolabelled [3H]triolein and cholesteryl [14C]oleate were taken up by cultured fibroblasts and rapidly degraded in a non-lysosomal compartment. In NLSD fibroblasts the degradation of HDL-[3H]triolein was blocked whereas that of HDL-[14C]cholesteryl oleate was in the normal range. These data suggest that: (i) the cytoplasmic pools of triacylglycerols and cholesteryl esters originate from HDL uptake and from endogenous biosynthesis as well; (ii) cytoplasmic (non-lysosomal) triacylglycerols and cholesteryl esters are degraded by two separate catabolic pathways.

The lipolysis/esterification cycle of hepatic triacylglycerol. Its role in the secretion of very-low-density lipoprotein and its response to hormones and sulphonylureas

In hepatocyte cultures maintained in the absence of extracellular fatty acids, at least 70% of the secreted very-low-density lipoprotein (VLDL) triacylglycerol was derived via lipolysis of intracellular triacylglycerol. This proportion was unchanged when the cells were exposed for 24 h to insulin or glucagon, hormones which decreased the overall secretion of intracellular triacylglycerol, or to chloroquine or tolbutamide, agents which inhibit lysosomal lipolysis. The rate of intracellular lipolysis was 2-3-fold greater than that required to maintain the observed rate of triacylglycerol secretion. Most of the fatty acids released were returned to the intracellular pool. Neither insulin nor glucagon had any significant effect on the overall lipolysis and re-esterification of intracellular triacylglycerol. In these cases a greater proportion of the released fatty acids re-entered the cellular pool, rather than being recruited for VLDL assembly. Tolbutamide inhibited intracellular lipolysis, but suppressed VLDL secretion to a greater extent. 3,5-Dimethylpyrazole did not affect lipolysis or VLDL secretion. The increased secretion of VLDL triacylglycerol observed after exposure of cells to insulin for 3 days was not accompanied by an increased rate of intracellular lipolysis. However, a larger proportion of the triacylglycerol secreted under these conditions may not have undergone prior lipolysis.

Purification and characterization of hepatic triacylglycerol lipase isolated from rainbow trout,Oncorhynchus mykiss

Trialcylglycerol (TG) lipase was isolated and partially purified from rainbow trout liver. Triacylglycerol lipase activity was assayed by measuring(14)C-oleic acid release from(14)C-triolein.(14)C-oleic acid release was linear for up to two hours. Optimal activity occurred at pH 7.0 and 15°C. Most of the lipase activity was recovered in the cytosolic fraction. A 27,000-fold purification was achieved after Sepharose (Bio-gel A 0.5 M, 200-400 mesh) chromatography of a resuspended 20% ammonium sulfate fraction. The molecular weight of the trout hepatic lipase as determined by size-exclusion chromatography and by SDS-polyacrylamide gel electrophoresis was 40-43 kD. Lipase-mediated hydrolysis of TG resulted in the production of diacylglycerols, monoacylglycerols, and fatty acids. Kinetic analysis indicated that Vmax=0.016 nmol/h/mg protein and that Km=0.28 mM triolein. Lipolytic activity was enhanced in the presence of cAMP/ATP-Mg(2+). These results suggest that the liver of trout possesses a neutral TG lipase that is responsible for mobilizing stored TG and is catalytically activated by phosphorylation.

Cholesterol ester hydrolysis and hormone-sensitive lipase in lactating rat mammary tissue

Neutral cholesterol esterase activity is expressed in extracts of mammary epithelial cells. The identity of the enzyme catalyzing this hydrolysis was investigated. Anti-hormone-sensitive lipase immunoglobulin elicited the total inhibition of this activity and also immunoprecipitated a single phosphoprotein of Mr 84 kDa from mammary cell extracts previously phosphorylated in vitro with [gamma-32P]ATP and cyclic AMP-dependent protein kinase. It is concluded that mammary cell cholesterol esterase activity results from the presence of hormone-sensitive lipase.

Cholesterol ester hydrolase activity in mammary tissue of the lactating rat

Cholesterol ester hydrolase activity has been studied in mammary glands of rats. Subcellular fractionation of the glands obtained in mid-lactation indicated that around 80% of the recovered activity was associated with particulate fractions. Two distinct cholesterol ester hydrolase activities were identified, one with an optimum pH of 7.5-9.0 and the second (approximately 5% of the total activity) with a more acidic pH optimum. Although the neutral cholesterol ester hydrolase had some properties in common with the lipoprotein lipase in mammary tissue, it was shown to be a separate entity by several criteria. Its activity could be increased following treatment with Mg-ATP and cAMP-dependent protein kinase, suggesting identity with the hormone sensitive lipase of adipose tissue. The cholesterol ester hydrolase activity in mammary glands just after parturition was greater than in glands obtained either from late-pregnant or midlactating animals. The subcellular distribution of the neutral cholesterol ester hydrolase suggested that it may have a different function to the neutral cholesterol ester hydrolase of adrenals and other tissues. Nevertheless the fact that the activity of the enzyme can be modulated by cAMP-dependent protein kinase suggests the possibility that hormonal control of this enzyme may be involved in the regulation of cholesterol metabolism in the mammary gland.

Storage, mobilization and secretion of cytosolic triacylglycerol in hepatocyte cultures. The role of insulin

Cytosolic triacylglycerol labelled from [3H]oleate accounted for almost 50% (57 +/- 22 nmol/mg of protein) of the total cellular triacylglycerol which was newly synthesized by cultured hepatocytes during a 24 h incubation. Insulin decreased the export of triacylglycerol as very-low-density lipoprotein (VLDL) during this period. This resulted in a sequestration of newly synthesized triacylglycerol in the cytosol, rather than in the particulate fraction of the cell. Longer periods of incubation with [3H]oleate resulted in increased concentrations of newly synthesized triacylglycerol within the cell, most of which (78 +/- 3% after 48 h; 80 +/- 3% after 72 h) was located within the cytosolic fraction. The quantity of newly synthesized triacylglycerol in the cell cytosol was further increased by insulin. During these periods there were decreases in the amounts of triacylglycerol associated with the particulate fraction of the cell, irrespective of the presence or absence of insulin. In no case was a decrease in VLDL triacylglycerol secretion in response to insulin accompanied by an increased triacylglycerol content in the particulate fraction of the cell. In some experiments, the fate of the cytosolic triacylglycerol was studied by pulse labelling with [3H]oleate. In these cases, when insulin was removed from the medium of cells to which they had previously been exposed, more newly synthesized triacylglycerol was secreted compared with cells which had not been exposed to insulin. This extra triacylglycerol was mobilized from the cytosolic rather than from the particulate fraction of the cell. Subsequent addition of insulin to the medium prevented the mobilization of cytosolic triacylglycerol. These results suggest that insulin enhances the storage of hepatocellular triacylglycerol in a cytosolic pool. Deficiency of insulin in the medium stimulates the mobilization of this pool which is channelled into the secretory pathway, entering the extracellular medium as VLDL.

Assembly and secretion of hepatic very-low-density lipoprotein

In contrast to water-soluble fuels such as glucose or ketone bodies, the use of lipids as an energy source for tissues has required the development of complex structures for their transport through the aqueous plasma. In the case of endogenously synthesized triacylglycerol this is achieved by the assembly and secretion of hepatic VLDL which provides the necessary stability in an aqueous medium. An essential component of this assembly process is apo B. Dietary changes which require an increase in hepatic VLDL secretion appear to be accompanied by increases in the availability of functional apo B. Interesting questions relate to: (a) the intracellular site(s) of triacylglycerol association with apo B, and (b) the mechanism(s) by which the availability of functional apo B at this site responds to metabolic and hormonal signals which reflect dietary status and, thus, the need to secrete triacylglycerol. As regards the latter, although in some cases changes in apo B synthesis occur in response to VLDL secretion hepatic apo B mRNA levels appear to be quite stable in vitro. Intracellular switching of apo B between the secretory and degradative pathways may be important in controlling VLDL assembly and post-translational modifications of the apoprotein may also play a role by influencing its ability to bind to triacylglycerol. Transport is not the only problem associated with the utilization of a concentrated energy source such as triacylglycerol and the complex problems of waste product disposal and recycling have to be dealt with. In the case of triacylglycerol, potentially toxic waste products include atherogenic remnants and LDL. The overall problem, then, in the long-term, involves the development of a 'safe' means of utilizing triacylglycerol and this requirement accounts for much of the complexity of plasma lipoprotein metabolism. In this area, the rat could teach the human a few tricks. One of these appears to be the utilization of hepatic apo B48 rather than apo B100 for VLDL assembly in response to increases in the extrahepatic utilization of hepatically synthesized triacylglycerol. Under these conditions, the remnants of hepatic triacylglycerol utilization by peripheral tissues are cleared from the plasma much more readily via a process which seems to involve the cycling of more triacylglycerol back to the liver than that which occurs in humans. The means by which this is achieved, though, are obscure and may involve a chylomicron remnant receptor, the nature of which, itself, remains controversial.(ABSTRACT TRUNCATED AT 400 WORDS)

Triacylglycerol synthesis in cultured rat hepatocytes. The rate-limiting role of diacylglycerol acyltransferase

The limiting role of diacylglycerol acyltransferase with respect to triacylglycerol synthesis in cultured rat hepatocytes was evaluated by following the inhibition of the overall synthetic flux by 2-bromooctanoate acting as an inhibitor of the diacylglycerol acyltransferase step. The flux-control coefficient of diacylglycerol acyltransferase in intact cultured hepatocytes amounted to 0.76 in the presence of saturating glycerol and either palmitate or oleate as the fatty acyl substrates. The flux-control coefficient of diacylglycerol acyltransferase in lysolecithin-permeabilized cultured hepatocytes amounted to 0.80 and 0.99 in the presence of saturating glycerol 3-phosphate and either palmitate or oleate as the fatty acyl substrate, respectively. Hence, triacylglycerol synthesis in liver cells under the experimental conditions employed is rate-limited by the diacylglycerol acyltransferase.

Effects of oestrogen on adenylate cyclase system and glucose output in rat liver

Effects of chronic oestrogen treatment on catecholamine- and glucagon-sensitive adenylate cyclase activity and glucose output in hepatocytes of castrated male rats were studied. In hepatocytes from male intact or castrated rats, the beta-adrenergic agonist isoprenaline did not stimulate adenylate cyclase activity and glycogenolysis, but glucagon markedly stimulated all these activities. Treatment of castrated animals with 17 beta-oestradiol for 7 days led to the appearance of beta-adrenergic-stimulated increases in both cyclic AMP generation and glucose output. The basal, glucagon- or fluoride-stimulated activities of adenylate cyclase of hepatic membranes prepared from oestrogen-treated rats were similar to those of control animals. Treatment with oestrogen did not influence the number or affinity of beta-adrenergic receptors. In hepatic plasma membranes from control rats, GTP failed to decrease the affinity of beta-adrenergic receptors for agonists, whereas the GTP-induced shift was apparently observed in those from oestrogen-treated animals. These results suggest that oestrogen is able to facilitate the coupling of hepatic beta-adrenergic receptors to the enzyme by increasing the effectiveness of receptor-guanine nucleotide regulation.

Acid lipase activity in neonatal rat liver cell types. Effect of starvation

The acid lipase activity in the liver of neonatal (1-day-old) rats was studied. It was found that (i) in whole liver, the activity was 50% lower than in adult rats; (ii) in neonatal livers, the activity was 7.7-fold higher in hepatocytes than in hemopoietic cells; (iii) neonatal hepatocytes contained about 25% of the activity detected in adult hepatocytes; (iv) all the differences disappeared when expressed per mg of protein; and (v) starvation did not affect the activity either in adult or in neonatal rat liver.

Triglyceride accumulation and very low density lipoprotein secretion by rat and goat hepatocytes in vitro

Utilization of [1-14C] oleate by freshly isolated rat and goat hepatocytes was compared. Intracellular [14C] triglyceride accumulation by hepatocytes did not differ between species. At 2 h of incubation, rat hepatocytes secreted approximately 25 times more [14C] triglyceride than goat hepatocytes. Very low density lipoprotein secretion was greatest by hepatocytes incubated in media containing 4:1 oleate:bovine serum albumin. Rat hepatocytes converted three to four times more [1-14C] oleate to 14CO2 and acid-soluble products than goat hepatocytes. Rate of 14CO2 formation by both rat and goat hepatocytes increased as incubation time increased and as rate of cellular triglyceride accumulation decreased. The ratio of 14CO2:[14C] acid-soluble products formed was greater for rat than goat hepatocytes, which indicated rat hepatocytes may oxidize fatty acid more completely. Differences in metabolic rate, based on oxygen consumption, between isolated goat and rat hepatocytes were minor and could not account for marked differences in very low density lipoprotein secretion. Goat hepatocytes did not incorporate detectable quantities of labeled fatty acid into low or high density lipoproteins. Ruminants may be susceptible to fatty liver when the liver takes up large amount of nonesterified fatty acid due to an inability to efficiently export fatty acid as very low density lipoprotein triglyceride.

Lipid dynamics in fish: aspects of absorption, transportation, deposition and mobilization

1. Aspects of lipid metabolism, including absorption and depositional processes, appear quite different in fish as compared to homeothermic vertebrates. 2. Dietary lipids in fish are absorbed as fatty acids and as triacylglycerols aggregated into chylomicra particles. 3. Interorgan transport of lipids, like that of mammals, consists of an exogenous (dietary) loop and an endogenous loop. 4. Fish store lipids among several depot organs, including mesenteric membranes, liver and muscle. 5. Several fast-acting and slow-acting agents modulate depot lipid mobilization. 6. Mobilized lipids may be transported in the serum as free fatty acids bound to specific carrier proteins.

Comparative effects of dietary fish oil and carbohydrate on plasma lipids and hepatic activities of phosphatidate phosphohydrolase, diacylglycerol acyltransferase and neutral lipase activities in the rat

In rats fed a fish oil-enriched diet, plasma triacylglycerols were lowered 51%. At the same time there was a mean 45% reduction in Mg2+-dependent phosphatidate phosphohydrolase activity in liver microsomes and a mean 20% decrease in microsomal triacylglycerol (neutral) and diacylglycerol hydrolase activities, but not of diacylglycerol acyltransferase. These observations support the hypothesis that decreases in the activities of phosphatidate phosphohydrolase and of both lipases are involved in the expression of the inhibitory effects of fish oil feeding on hepatic lipoprotein triacylglycerol secretion. Conversely, the feeding of a sucrose-enriched diet resulted in a mean 39% rise in plasma triacylglycerols, a 19% increase in triacylglycerol hydrolase and a mean 45% increase in Mg2+-dependent microsomal phosphohydrolase activity. The effects of the two nutritional interventions on phosphatidate phosphohydrolase activity confirm a key function for this enzyme in triacylglycerol formation.

Cyclic AMP increases incorporation of exogenous fatty acids into triacylglycerols in hamster fibroblasts

Incorporation of various exogenous saturated or unsaturated [14C]labeled fatty acids (palmitic, stearic, oleic, linoleic and arachidonic) into triacylglycerols by hamster fibroblasts was markedly enhanced (two- to fourfold) in the presence of theophylline or dibutyryl cyclic adenosine monophosphate (dbcAMP). This effect was observed for short-term (1-6 hr) as well as long-term (15-24 hr) preincubation with dbcAMP. In the presence of sodium fluoride, a phosphoprotein phosphatase inhibitor, measurement of diacylglycerol acyltransferase (DGAT) activity in cells pretreated with dbcAMP pointed out a marked increase (3 X) in specific activity. The results suggest that DGAT activity in fibroblasts could be activated by a cAMP-dependent phosphorylation process.

Triacylglycerol secretion and fatty acid synthesis by the liver in acute uremic rats

In bilaterally nephrectomized rats fatty acid synthesis (incorporation of 3H from 3H2O into fatty acids by the liver) and triacylglycerol (TG) secretion by the liver (Triton method) were investigated 24 h after surgery. Starved and fed rats were employed. In uremic rats fatty acid synthesis occurs at a normal rate; prevention of acidosis during uremia by bicarbonate treatment has no influence on lipid synthesis. Following intravenous injections of Triton WR 1339 the TG plasma levels are considerably lower in uremic rats than in corresponding controls. As the plasma volume is significantly higher in uremic animals, there is only an insignificant decrease in the absolute TG secretion rate in these animals. The removal of TG from the blood to the tissues in severely reduced in uremia with acidosis. The normal rates of fatty acid synthesis and nearly normal rates of TG secretion may, therefore, be considered as permissive factors for the development of uremic hypertriglyceridemia. The lesser degree of hypertriglyceridemia in uremia without acidosis is only caused by the better removal of TG from the blood (as could be demonstrated earlier). Fatty acid synthesis and TG secretion do not play any role in this case.

Fatty acids reverse the cyclic AMP inhibition of triacylglycerol and phosphatidylcholine synthesis in rat hepatocytes

The influence of cyclic AMP analogues and fatty acids on glycerolipid biosynthesis in monolayer cultures of rat hepatocytes was investigated. Chlorophenylthio-cyclic AMP and adenosine 3':5'-cyclic phosphorothioate inhibited the rate of triacylglycerol synthesis from [1(3)-3H]glycerol, and phosphatidylcholine synthesis from [Me-3H]-choline. Supplementation of the hepatocytes with palmitate (1 mM) reversed chlorophenylthio-cyclic AMP inhibition of triacylglycerol synthesis. Similarly, cyclic AMP analogue-inhibition of phosphatidylcholine synthesis was abolished when the cells were simultaneously incubated with oleate (3 mM). Reactivation of phosphatidylcholine synthesis in chlorophenylthio-cyclic AMP-supplemented cells with oleate was accompanied by conversion of CTP: phosphocholine cytidylyltransferase into the membrane-bound form, since these cells released the enzyme more slowly after treatment with digitonin. The opposing actions of cyclic AMP and fatty acids are discussed in relation to the regulation of glycerolipid biosynthesis during starvation, diabetes and stress.

Salt-resistant (hepatic) lipase. Evidence for its presence in bovine liver and adrenal cortex

Alkaline lipolytic activities in bovine liver and adrenal cortex have been investigated; each tissue has a salt-resistant, hepatic-type lipase activity of which we describe a partial purification. Properties of the partially purified enzymes have been compared directly with those of authentic hepatic lipase prepared from rat liver. Furthermore, a similar activity has been detected in bovine post-heparin plasma. These findings contrast with a previous report that bovine post-heparin plasma and liver extracts lack hepatic salt-resistant lipase.

Effect of ethanol, noradrenaline and 3',5'-cyclic AMP on oxidation of fatty acids and lipolysis in isolated rat hepatocytes

Improved methods for the determination of the rate of fatty acid oxidation and lipolysis are presented and evaluated. Hepatocytes from fed rats were incubated with 1.3 mM palmitate. The rate of oxidation of exogenous fatty acids was determined with [1-14C]palmitate. The rate of oxidation of endogenous fatty acids was determined either as the difference between total fatty acid oxidation and oxidation of exogenous fatty acids, or as the oxidation of intracellular lipids, prelabelled with 14C. The two methods agreed completely, indicating that the only endogenous source of acetylCoA was fatty acids. The rate of oxidation of exogenous and endogenous fatty acids was estimated to be 858 and 284 nmol C2-units/min 10(8) cells, respectively, and the rate of lipolysis to be 1640 nmol C2-units/min 10(8) cells. Ethanol caused a 17% and 70% inhibition of the oxidation of exogenous and endogenous fatty acids, respectively. Lipolysis was inhibited approximately 10% by ethanol. Noradrenaline was without effect on the oxidation of exogenous and endogenous fatty acids and on lipolysis. 3',5'-Cyclic AMP enhanced the oxidation of exogenous and endogenous fatty acids by 25% and 31%, respectively, and the rate of lipolysis by 38%, suggesting the presence in hepatocytes of a cAMP sensitive lipase.