Control of lipid metabolism in cultured cells

Amino Acids Rather than Glucose Account for the Majority of Cell Mass in Proliferating Mammalian Cells

Cells must duplicate their mass in order to proliferate. Glucose and glutamine are the major nutrients consumed by proliferating mammalian cells, but the extent to which these and other nutrients contribute to cell mass is unknown. We quantified the fraction of cell mass derived from different nutrients and found that the majority of carbon mass in cells is derived from other amino acids, which are consumed at much lower rates than glucose and glutamine. While glucose carbon has diverse fates, glutamine contributes most to protein, suggesting that glutamine's ability to replenish tricarboxylic acid cycle intermediates (anaplerosis) is primarily used for amino acid biosynthesis. These findings demonstrate that rates of nutrient consumption are indirectly associated with mass accumulation and suggest that high rates of glucose and glutamine consumption support rapid cell proliferation beyond providing carbon for biosynthesis.

The importance of acetyl coenzyme A synthetase for 11C-acetate uptake and cell survival in hepatocellular carcinoma

We analyzed the pattern of (11)C-acetate and (18)F-FDG uptake on PET/CT in patients with hepatocellular carcinoma (HCC). We also assessed the expression of important regulatory enzymes related to glycolysis and lipid synthesis in relation to (18)F-FDG and (11)C-acetate uptake in human HCC cell lines. The significance of (11)C-acetate uptake regulation was further evaluated with regard to cell viability.

METHODS

(18)F-FDG and (11)C-acetate uptake patterns in HCC in 11 patients and in 5 HCC cell lines were assessed. We evaluated the gene expression of metabolic enzymes related to glycolysis and lipid synthesis in a cell line with the highest (18)F-FDG uptake and another cell line with the highest (11)C-acetate uptake. They included hexokinase II, adenosine triphosphate citrate lyase, acetyl coenzyme A (CoA) synthetase 1 (ACSS1), acetyl CoA synthetase 2 (ACSS2), acetyl CoA carboxylase, and fatty acid synthase. In a cell line with high (11)C-acetate uptake, the enzymatic activities of ACSS1 and ACSS2 were blocked using respective small, interfering RNAs (siRNAs), and the impact on (11)C-acetate uptake and cell viability was assessed.

RESULTS

In all 11 patients and 4 of the 5 cell lines, the uptake patterns of the 2 radiotracers were complementary. ACSS1 and ACSS2 were highly expressed in a cell line with low (18)F-FDG uptake and high (11)C-acetate uptake, whereas only ACSS2 was expressed in a cell line with high (18)F-FDG uptake and low (11)C-acetate uptake. Fatty acid synthase expression was seen in cells with high (18)F-FDG or (11)C-acetate uptake. These findings indicate the possibility that both glucose and acetate can be a compensatory carbon source for lipid synthesis in cancer. Transient transfection with ACSS1 or ACSS2 siRNA in cells with high (11)C-acetate uptake decreased (11)C-acetate uptake and cell viability.

CONCLUSION

The patterns of (18)F-FDG and (11)C-acetate uptake seemed to complement each other in both human HCC and HCC cell lines. Fatty acid synthase expression was seen in cells with high (18)F-FDG or (11)C-acetate uptake, suggesting glucose- or acetate-dependent lipid synthesis. Acetyl CoA synthetase appears to be important in (11)C-acetate uptake and acetate-dependent lipid synthesis for the growth of cancer cells with a low-glycolysis phenotype. Inhibition of acetyl CoA synthetase in these cells may be promising for anticancer treatment.

A festschrift for J. Martyn Bailey, a biochemist extraordinaire

A festschrift for Dr. John Martyn Bailey, Professor of Biochemistry and Molecular Biology was organized by the Biochemistry department of the George Washington University School of Medicine and Health Sciences on December 4-5, 2006 to honor his 48 years of contributions. He made important contributions in the areas of essential fatty acids, prostaglandins, thromboxanes and lipoxygenase metabolites.

Modification of phospholipids fatty acid composition in reuber H35 hepatoma cells: Effect on HMG-CoA reductase activity

There is controversy about the effect of saturated and polyunsaturated fats on 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase, the main regulatory enzyme of cholesterogenic pathway. Results from dietary studies are difficult to interpret because diets normally contain a mixture of fatty acids. Therefore, we have used Reuber H35 hepatoma cells whose phospholipids were enriched in different individual fatty acids and have studied their effects on the cellular reductase activity. Lauric, myristic, eicosapentaenoic (EPA), and docosahexaenoic (DHA) acids were supplemented to the culture medium coupled to bovine serum albumin. The four fatty acids were incorporated into phospholipids from cells grown in media containing whole serum or lipoprotein-poor serum (LPPS). Reductase activity of cells cultivated in a medium with LPPS was three to four times higher than those cultivated in medium with whole serum. Saturated fatty acids increased reductase activity of cells grown in medium with whole serum, whereas n-3 polyunsaturated fatty acids (PUFA) decreased it. However, both saturated and polyunsaturated fatty acids increased reductase activity when serum lipoproteins were removed. In conclusion, this is one of the first reports demonstrating that saturated and n-3 PUFA only show differential effects on HMG-CoA reductase activity in the presence of lipoproteins.

Role of delta 9 desaturase activity in the maintenance of high levels of monoenoic fatty acids in hepatoma cultured cells

The incorporation and delta 9 desaturation of exogenous [14C]stearic acid were studied in HTC 7288c cells in suspension. We examined the uptake of the acid over a wide range of concentrations (0-160 microM) after incubating the cells for 6 h in a chemically-defined medium. Under this experimental condition, the uptake of the labeled acid was more extensive than that obtained from static cultures or from monolayer of isolated hepatocytes of rats. At an external concentration of 160 microM ca. 52 nmoles of acid per mg of cellular protein was taken up. The production of oleic acid from [14C]stearate (delta 9 desaturation) correlated well with the uptake curve between 0-80 microM concentration. For higher stearate concentrations, the biosynthesis of oleic acid declined substantially and a plateau of 22 nmoles/mg cellular protein was reached. The incorporation and desaturation of an initial exogeneous concentration of [14C]stearic acid (80 microM) was also studied from 0-6 h. The results obtained demonstrated that the uptake of the substrate into cellular lipids was fast and non saturable. Quantitative gas-liquid chromatography of total cellular lipids under the different experimental conditions demonstrated a negative correlation between the decrease in the palmitic and palmitoleic acids and the increase in the intracellular levels of stearic and oleic acids. These analytical modifications took place with no changes in the saturated/monoenoic fatty acid ratio. This work also demonstrated a significant contribution of the stearoyl-CoA desaturase system to the high levels of oleic acid present in this kind of hepatoma cells.

Incorporation and metabolic conversion of saturated and unsaturated fatty acids in SK-Hep1 human hepatoma cells in culture

We report here a study of the incorporation and metabolism of various long chain fatty acids in SK-Hep-1 cultured hepatoma cells. Medium supplementation with radiolabelled palmitic, stearic, linoleic, alpha-linolenic and eicosa-8,11,14-trienoic acids (1 microM, 24 H) resulted in an active uptake of each of these precursors by the cultures. Subsequent analysis of the cellular lipids indicated that they exhibit almost all the enzymic activities of polyunsaturated fatty acid metabolism that are characteristic of normal hepatic cells. With respect to the desaturation capacities of this cell line, although alpha-linolenic acid reacted more extensively than did linoleic acid and the conversion of 8,11,14-eicosatrienoic acid by the delta 5 specific enzyme was more avid than had been previously seen in normal rat or human liver: the saturated fatty acids constituted relatively poor substrates, being preferentially chain-elongated rather than (mono) desaturated at the delta 9 position. Analysis of the fatty acid profiles of total cellular lipids and of various lipid subclasses, however, revealed a relative paucity of essential fatty acids when compared with the abundance of endogenous monoenoic acids (particularly oleic). Of the total cellular fatty acids, 58% were present in the form of phospholipids; with 33% of the remaining 42% (i.e., the neutral lipids) being associated with triacylglycerol fraction. Within the total lipids, phosphatidyl-choline and phosphatidyl-ethanolamine were the major sites for the incorporation of all metabolic products derived from the incubated radiolabelled 16- and 18-carbon fatty acid precursors, whereas the phosphatidyl-inositol fraction was the predominant recipient of nascent arachidonic acid when the eicosatrienoate was the substrate. The express purpose of this investigation was to characterize the biochemical routes involved in the anabolism of various essential fatty acids in the human hepatocyte, through the use of cultured human hepatoma cells as an experimental model system. In view of the similarities between certain aspects of the polyunsaturated fatty acid metabolism of these cells and the corresponding properties of other mammalian hepatic or liver-derived tissues, the data presented here would thus constitute a significant beginning alone those lines. Moreover, considering the extreme difficulty in obtaining for such investigation relevant tissue samples from normal human sources, we regard these results- and the availability for use of this particular human hepatoma cell line-as important new developments in the effort to characterize a useful experimental model both for gaining immediate information and for designing future experiments.

Network analysis of intermediary metabolism using linear optimization. I. Development of mathematical formalism

Analysis of metabolic networks using linear optimization theory allows one to quantify and understand the limitations imposed on the cell by its metabolic stoichiometry, and to understand how the flux through each pathway influences the overall behavior of metabolism. A stoichiometric matrix accounting for the major pathways involved in energy and mass transformations in the cell was used in our analysis. The auxiliary parameters of linear optimization, the so-called shadow prices, identify the intermediates and cofactors that cause the growth to be limited on each nutrient. This formalism was used to examine how well the cell balances its needs for carbon, nitrogen, and energy during growth on different substrates. The relative values of glucose and glutamine as nutrients were compared by varying the ratio of rates of glucose to glutamine uptakes, and calculating the maximum growth rate. The optimum value of this ratio is between 2-7, similar to experimentally observed ratios. The theoretical maximum growth rate was calculated for growth on each amino acid, and the amino acids catabolized directly to glutamate were found to be the optimal nutrients. The importance of each reaction in the network can be examined both by selectively limiting the flux through the reaction, and by the value of the reduced cost for that reaction. Some reactions, such as malic enzyme and glutamate dehydrogenase, may be inhibited or deleted with little or no adverse effect on the calculated cell growth rate.

The fatty acid compositions of established fish cell lines after long-term culture in mammalian sera

The effect of long-term culture of fish cells in mammalian serum on the phospholipid fatty acid composition was investigated. All the cell lines studied had much lower levels of polyunsaturated fatty acids (PUFA) than those found in intact fish tissues. In particular (n-3) PUFA were considerably depleted in the cultured cell lines, leading to very low (n-3)/(n-6) ratios in all the phospholipid classes. In general the cells were rich in 18:1, 16:0, 18:0 and 16:1 with 20:4(n-6) and 22:6(n-3) as the major PUFA. The fatty acid composition reflected the composition of the fetal calf serum added to the media rather than their fish tissue origins. The results were discussed in relation to the roles of PUFA in general cell metabolism and more specifically the role of (n-3) PUFA in fish cells.

Accumulation of (n-9)-eicosatrienoic and docosatrienoic acids in human fibroblast phospholipids

An essential fatty acid (EFA) deficiency-like profile of fatty acids has been observed in HF-1 human skin fibroblasts cultured at clonal densities in MCDB 110 and 0.4% fetal bovine serum (FBS). The profile was characterized by an accumulation of 16:1n-7, 18:1n-9, 20:3n-9 and 22:3n-9, a reduction of n-6 fatty acids and a reduction in total polyunsaturated fatty acids (PUFA). The fatty acid composition of sphingomyelin (SPH), phosphatidylcholine (PC), phosphatidylserine (PS), phosphatidylinositol (PI) and phosphatidylethanolamine (PE) was determined and, except for SPH, each displayed an EFA deficiency-like profile. The triene to tetraene ratio (20:3n-9/20:4n-6) ranged from 5.3 in PI to 0.9 in PE. In addition, the highest percentage of 20:3n-9 was present in the PI and the highest percentage of 22:3n-9, in PE. Other human fibroblasts (normal, transformed and at different population doubling number levels [PDL]) were grown under the same conditions and were found to display triene to tetraene ratios (20:3n-9/20:4n-6) in total cellular lipids ranging from 0.7 to 4.5. The accumulation of 20:3n-9 and 22:3n-9 is due primarily to the existence of a basal nutrient medium (MCDB 110) that allows for the rapid clonal growth of human fibroblasts at reduced serum levels (0.4%). This culture procedure can be exploited to further elucidate various aspects of lipid metabolism in human fibroblasts.

Influence of environmental medium on fatty acid composition of human cells: leukocytes and fibroblasts

Fibroblasts in culture and leukocytes have been widely used to study fatty acid and lipoprotein cellular metabolism. The present investigations were designed to study the role of nutritional and environmental factors on lipid metabolism in these two types of cells. Leukocytes freshly isolated from human blood and fibroblasts cultured in media enriched in human serum (HS) have relatively similar fatty acid distributions. However, more important differences are observed in fibroblasts cultured in media enriched with HS or with fetal bovine serum (FBS). It is obvious that the quantity and quality of fatty acids are very different in FBS and HS, but intracellular regulation ensures relative homogeneity of saturated (SFA) and monounsaturated fatty acids (MUFA) in the cells, particularly in phospholipids. The first modifications induced by different media (FBS or HS) are detected on cellular growth; the differences seem to be due more to the fatty acid (FA) quantitative supply than to the FA quality of each culture medium. The major modifications in FA composition induced by different culture media concern the polyunsaturated fatty acids (PUFA) of phospholipids, especially the n-6 family. The intracellular linoleic acid level depends on the level in the medium, but intracellular n-6 metabolite levels depend both on the level in the medium and on the growth state of the cells. The n-3 family seems to be less affected by the quality of the medium in our experiment, and the cells maintain a stable docosahexaenoic acid (22:6n-3) level. A higher content of the n-3 family in the medium induces a higher level of eicosa- or docosapentaenoic acid, rather than docosahexaenoic acid itself.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of hormones on phospholipid metabolism in human cultured fibroblasts

The effect of hormones on phospholipid metabolism, pool size, 32P labeling and changes in fatty acid of human adult fibroblasts was determined. Simultaneously the change in membrane fluidity of single cells was recorded via fluorescence recovery after photobleaching under the influence of hormones. From all substances tested (isoproterenol, phenylephrine, adrenalin, histamine, angiotensin II, dansylcadaverine, propranolol) only isoproterenol and adrenalin slightly decreased total amount of phosphatidylcholine (PC). The amount of the other phospholipids analyzed remained unchanged. The 32P incorporation rate into phospholipids (PC, phosphatidylinositol (PI), phosphatidylethanolamine (PE)) was affected basicly different analyzing either PC, PI or PE. Histamine and propranolol provoked the highest incorporation of 32P (240% increase in PI labeling). Isoproterenol and adrenalin decreased PC labeling (45% and 18%) whereas isoproterenol decreased 32P incorporation into PI (18%), and adrenalin led to an increase (37%). PE labeling showed no or a slight increase in 32P incorporation applying the other agonists or antagonists. The fatty acid pattern of the respective phospholipids changed only to a minor extend. A decrease in hexadecanoic acid content of PI was found after administration of either isoproterenol, adrenalin or histamine. Parallel determination of membrane fluidity of single cells by fluorescence recovery after photobleaching showed an increase in the diffusion coefficient of a fluorescent lipid probe sticking in the membrane, following administration of isoproterenol and adrenalin, other substances tested exerted no effect. A relationship to changes in phospholipid metabolism became obvious. These results are discussed considering known mechanisms of receptor coupling and change in phospholipid metabolism and fluidity.

Diacylglycerol metabolism in phospholipase C-treated mammalian cells

Treatment of cultured cells with phospholipase C causes increased rates of hydrolysis of cellular phosphatidylcholine and increased rates of incorporation of choline into phosphatidylcholine. The fate of the diacylglycerol produced by the phospholipase C hydrolysis was examined in two cell lines, Chinese hamster ovary and HeLa. In the former cells, turnover of the glycerol moiety of phosphatidylcholine was not enhanced by phospholipase C treatment, indicating that the phospholipase C-generated diacylglycerol was recycled into new phosphatidylcholine. In HeLa cells, turnover of the glycerol backbone of phosphatidylcholine was enhanced by phospholipase C treatment, and the increased rate of turnover of the glycerol moiety was similar to that of the phosphate moiety. Thus, the fate of diacylglycerol generated at the plasma membrane was demonstrated to differ in these two cell lines. Incorporation of precursors of diacylglycerol into phosphatidylcholine was not enhanced by phospholipase C treatment in either cell line.

Methylmalonic and propionic acidemias: lipid profiles of normal and affected human skin fibroblasts incubated with [1-14C]propionate

Normal human skin fibroblasts and those from methylmalonic acidemia and propionic acidemia patients were grown in culture. Following incubation with [1-14C]propionate, the major lipid classes in the cells were separated by thin layer chromatography and isolated fractions analyzed by radio gas chromatography for the presence of odd-numbered long-chain fatty acids; the pattern of even-numbered long-chain fatty acids was obtained also. Normal fibroblasts incorporated a small percentage of propionate into odd-numbered fatty acids which were present in all lipids studied. The abnormal cells incorporated a larger amount while maintaining the characteristic ratios of odd-numbered fatty acids found in the normal line. Most of the radioactivity was associated with phospholipids which are the predominant constituents of cell membranes. A characteristic C15/C17 ratio was found for different phospholipids and the triglyceride fraction; pentadecanoic acid was the principal odd-numbered fatty acid utilized in the assembly of complex lipids. Compared to even-numbered long-chain fatty acids the absolute amount of odd-numbered fatty acids was low (1-2%), even in affected cells. An unusual polar lipid fraction was isolated in the course of the study. In the normal cell it contained several unlabeled eicosanoids which were missing from the same fraction of both affected cell lines.

Changes in serum influence the fatty acid composition of established cell lines

The fatty acid composition of different kinds of commercially available serum used to supplement cell culture media differs widely. As compared with fetal bovine serum, horse and bovine calf serum have a very high content of linoleic acid (18:2) and are low in arachidonic acid (20:4). (Fatty acids are abbreviated as number of carbon atoms:number of double bonds). Swine serum contains substantial amounts of both 18:2 and 20:4. Only fetal bovine serum contains more than 1% docosahexaenoic acid (22:6). Considerable differences in fatty acid composition occur when cells are grown in media containing any of these different serum supplements. The 18:2 and 20:4 content of 3T3 mouse fibroblast phospholipids is highest when the medium contains horse serum, intermediate with bovine calf serum, and lowest with swine or fetal bovine serum. Likewise, the highest phospholipid 18:2 content in Madin-Darby canine kidney cells (MDCK) occurs when the medium contains horse serum. With MDCK cells, however, growth in swine serum produces the highest 20:4 content. The 3T3 cell phospholipids accumulate more than 1% 22:6 only when the medium contains fetal bovine serum, whereas in no case do the MDCK cell phospholipids accumulate appreciable amounts of 22:6. The fact that the cellular fatty acid composition is likely to change should be taken into account when changes are contemplated in the serum used to grow established cell lines.

Development and characterization of a tissue culture cell line with essential fatty acid deficiency

We have developed an essential fatty acid-deficient cell line from a parental cell line, HSDM1C1, which metabolizes arachidonic acid to prostaglandin E2 (PGE2). This cell line, designated EFD-1, is depleted of arachidonate, is unable to synthesize PGE2 in response to bradykinin, and has changes in fatty acid composition characteristic of tissues from animals with essential fatty acid deficiency. Within 15 min of repletion by arachidonate, the ability to synthesize PGE2 is restored. Linoleate also is able to restore PGE2 synthesis, indicating that deficient cells contain both the rate-limiting delta 6 desaturase enzyme and the delta 5 desaturase enzyme, which are required to form arachidonate. When parental cells are incubated in lipid-free medium, there is rapid induction of the ability to convert linoleate to arachidonate. Arachidonate prevents this induction, suggesting that icosanoid precursor availability controls the rate of arachidonate formation.

In vitro conversion of saturated to monounsaturated fatty acid by Ehrlich ascites cells

In this paper, evidence is presented on the capacity of Ehrlich ascites cells to synthesize in vitro monounsaturated fatty acids from radioactive palmitate. Localization of the double bond was determined by ozonolysis and subsequent reduction of the ozonides to aldesters followed by gas liquid chromatography. These results proved that Ehrlich ascites cells have a delta 9 desaturase that catalyzes the biosynthesis of palmitoleic acid from palmitic acid and oleic and vaccenic acid via elongation-desaturation and desaturation-elongation, respectively, using palmitic acid as substrate. Furthermore, it is shown that, as in the hepatic cells, delta 9 desaturase enzyme activity of the tumoral cells is associated with the endoplasmic reticulum. The electron transport components involved in the desaturase system, i.e., NADH-cytochrome b5 reductase and NADH-cytochrome c reductase, were also measured. The activities of these enzymes do not appear to be rate-limiting in the desaturase activity of these tumoral cells.

Phospholipid acyl group composition in normal and tumoral nerve cells in culture

We have studied the fatty acid composition of total phosphoglycerides from various types of nerve cells in culture. Primary cell cultures were compared with tumoral cell strains. Glial cells exhibited no characteristic pattern when compared to neurons. Tumoral cell phosphoglycerides contained much higher levels of octadecenoic acid and lower levels of C-20 to C-22 polyunsaturated fatty acids than normal cell phosphoglycerides. This observation seems to be a general feature in tumoral cell membranes. It could be of interest in respect to the membrane fluidity of cancer cells.

Fatty acid uptake by bloodstream forms of Trypanosoma brucei and other species of the kinetoplastida

1. Trypanosoma brucei and other species of the Kinetoplastida readily take up fatty acids from a simple incubation medium. This suggests that they at least partially satisfy their nutritional requirements for fatty acids from an exogenous source. 2. Uptake of fatty acids in T. brucei follows a biphasic time course, consisting of an initial rapid phase, thought to represent surface binding, followed by a slower accumulation phase. 3. Culture forms of T. brucei take up more than 10 times as much fatty acid during the rapid initial phase than do bloodstream forms, suggesting the presence of a greater number of fatty acid binding sites in the plasma membranes of the culture forms when compared to the bllodstream forms. 4. The uptake process is reversible. Measurements of the release of a preloaded fatty acid from bloodstream forms of T. brucei, in the presence and in the absence of albumin, provides some support for the hypothesis that fatty acid uptake and release is mediated by a direct interaction of free albumin and fatty acid-albumin complex with the cell surface.

Comparative studies on human skin fibroblasts: life span and lipid metabolism in medium containing fetal bovine or human serum

Three strains of human skin fibroblasts were cultivated in nutrient medium supplemented either with human serum or fetal bovine serum, and growth and lipid synthesis were compared. Rates of cellular growth were similar in both kinds of medium, but the replicative life spans of all three strains were curtailed significantly in human-serum medium. Incorporation of label into the major classes of neutral lipids from [14C]acetate and 3H2O was increased also in human-serum medium. Since human serum contained higher concentrations of cholesterol known to reduce endogenous cholesterol synthesis, these results were unexpected. Nonlipid factors in human serum may account for the shortened cellular life spans and increased lipogenesis and perhaps for the potential to develop atherosclerosis.

Acetate as a carbon source for lipid synthesis in cultured cells

Lipid synthesis from acetate and glucose has been studied in fibroblast cultures grown in lipid free medium containing various concentrations of acetate and glucose. Their role in lipid synthesis was evaluated by comparing the specific activities of total cell lipid, and sterol and fatty acid subfractions, to that of the carbon source in the medium. When glucose was present at 1000 microgram/ml (5.6 mM) and acetate at 100 microgram/ml (1.2 mM), the majority of the total cell lipid and subfractions were derived from glucose. As acetate concentrations increased, lipid synthesis from acetate increased, and glucose declined as a carbon source; in cells grown in the presence of 1000 microgram/ml (12 mM) acetate more of the lipid was derived from acetate than from glucose. When acetate was present at 1000 microgram/ml (12 mM) and glucose was varied from 1000 to 200 microgram/ml (5.6 to 1.1 mM), the relative amount of lipid synthesis from acetate was further increased. The data indicate that acetate can serve as a significant carbon source for lipid synthesis in cultured fibroblasts when it is presented to the cells at high extracellular concentrations relative to glucose.

Essential fatty acid metabolism in cultured astroblasts

The fetal calf serum on which two astroblast cell lines were grown was shown to be deficient in essential fatty acids. The fatty acid profiles of lipids of these two cell lines showed very low amounts of polyunsaturated fatty acids. In order to know if this low level of unsaturation was due to the lack of essential fatty acids in serum, or to a lack of desaturase activities in the cells, we have investigated the modifications of the cell lipid fatty acid patterns when serum was enriched in essential fatty acids. Linoleic acid was incorporated in rather high amounts in the cell lipids, while linolenic acid was very poorly incorporated. These two essential fatty acids were converted into polyunsaturated fatty acids only when they were added alone to the serum. Both cell clones exhibited a lack in the delta 4 desaturase activity. No morphological changes of the cells occured after nine days of culture with modified serum.

Lipids of cultured hepatoma cells: VIII. Utilization of D-[1-14C] glucose for lipid biosynthesis

Minimal deviation hepatoma 7288C cells (HTC) were incubated in serum-supplemented and serum-free Swim's 77 medium in the presence of D-[1-14C] glucose for 1, 2, 4, 8, 12 and 24 hr. Glucose oxidation to CO2, incorporation into total cell mass, and incorporation into cell and medium lipids were determined. The percentage distribution of total cell lipid radioactivity in individual neutral and polar lipid classes was followed as a function of time. Degradation studies of individual lipid classes were performed to ascertain the percentage of radioactivity in acyl and glycerol moieties. The percentage of D-[1-14C] glucose oxidized to 14CO2, incorporated into cell matter and cell lipids was elevated in cells incubated in serum-free medium as opposed to serum-supplemented medium. The percentage distribution of total cell lipid radioactivity into individual neutral lipid classes from both serum-free and serum-supplemented cultures was as follows: sterols greater than triglycerides greater than free fatty acids greater than sterol esters. The percentage distribution of total cell lipid radioactivity into individual polar lipid classes of serum-supplemented cultures was as follows: phosphatidylcholine greater than phosphatidylinositol greater than sphingomyelin greater than phosphatidylethanolamine greater than phosphatidylserine. The distribution of glucose radiolabel into individual polar lipid classes of serum-free HTC cells was different from their serum-supplemented counterparts: sphingomyelin greater than phosphatidylcholine greater than phosphatidylinositol greater than phosphatidylethanolamine greater than phosphatidylserine. Glycerol from glyceride classes contained a higher percentage of radioactivity than the acyl moieties, with this percentage significantly elevated in serum-free cultures. The data indicate that, although glucose is a substrate for HTC cell lipids, other precursors present in the culture system also contribute to the lipid constituency of this hepatoma cell line.

Regulation of fatty acid biosynthesis in Ehrlich cells by ascites tumor plasma lipoproteins

Fatty acid biosynthesis in Ehrlich cells in vitro was reduced when very low density lipoproteins (VLDL) isolated from the ascites tumor plasma were added to the incubation medium. The degree of inhibition was dependent on the VLDL concentration. At the VLDL concentrations usually present in the ascites plasma, there was a 30% decrease in biosynthesis as measured by (3)H(2)O incorporation into fatty acids. Analysis of the labeled fatty acids by gas liquid chromatography indicated that this decrease was due to a reduction in fatty acid de novo biosynthesis and that chain elongation actually was increased when VLDL were present. Although ascites plasma low- and high density lipoproteins also produced a concentration-dependent inhibition of fatty acid biosynthesis, their effects were much smaller than those of the VLDL. Studies employing VLDL and radioactive free fatty acids indicated that the cells took up utlilzed fatty acids derived from these lipoproteins. When VLDL were present, labeled free fatty acid incorporation into cell phospholipids, cholesteryl esters, and CO(2) decreased, whereas its incorporation into the cell free fatty acid pool increased. By contrast, the cells incorporated only very small amounts of fatty acid from either low- or high density lipoproteins. This suggests that the VLDL exert their inhibitory effect on fatty acid synthesis by supplying exogenous fatty acids to the cells.

Lipid accumulation cells derived from porcine aorta and grown under anaerobic conditions

Fibroblast-like cells, derived from porcine aorta, were cultured under aerobic and anaerobic conditions. Light and electron microscopic examinations, lipid composition measurements, and incorporation of radioactive precursors into lipids of these cells were performed. Anaerobically grown cells accumulated oil red O stainable droplets and within 6 hr the triacylglycerol content increased to 4 times the level determined in cells grown under aerobic conditions. This ratio remained constant throughout an additional 12 hr of growth. The fatty acid composition of the triacylglycerols which accumulated under anaerobic conditions differed from the composition of fatty acids in the triacylglycerols present in the growth medium. The cellular unesterified fatty acids of the anaerobically grown cells differed only slightly in composition from the fatty acids in the growth medium, while the unesterfied fatty acids of aerobically grown cells differed to a greater extent from those of the growth medium.

Lipid metabolism in cultured aortic smooth muscle cells and comparison with other cell types. Part I. Composition of cells grown in hyperlipemic serum

The lipid compositions of cultured rabbit aortic smooth muscle cells, adventitial fibroblasts and skin fibroblasts were determined for cells grown in media containing either normolipemic or hyperlipemic serum. No significant changes were found in cell phospholipid composition. Each of the threee cell types responded similarly to hyperlipemic serum, accumulating esterified cholesterol and triglyerides.

Incorporation of I-14C-acetate into the lipids of isolated epidermal cells

Suspensions of epidermal cells were prepared by trypsinization of rat epidermis and incubated with I-14C-acetate in Eagle's minimum essential medium. The incorporation of radioactivity into the total lipids of the cells was increased by the addition of serum to the medium. The pattern of incorporation into the main lipid classes was affected by the addition of serum to the medium and by alterations in the density of the cell suspensions. The incorporation of radioactivity into the total lipids increased approximately linearly over a 24-h period of incubation and the pattern of incorporation into the phospholipids and glycolipids altered with time. However, at each time interval studied, reporducible patterns of incorporation were obtained under controlled conditions. The incorporation of radioactive acetate therefore provides the basis for a very sensitive method of lipid analysis.

Cholesteryl ester metabolism in tissue culture cells: II. Source of accumulated esterified cholesterol in Fu5AH rat hepatoma cells

Previous investigations had demonstrated that Fu5AH rat hepatoma cells accumulated large quantities of esterified cholesterol when grown in hyperlipemic rabbit serum. The present investigation has determined the sources of the cellular esterified cholesterol when the cells were grown in hyperlipemic serum. Cellular esterification of endogenous and exogenous free cholesterol contributed 10% and 30%, respectively. The remaining 60% of the accumulated cellular esterfied cholesterol was derived from exogenous (serum) cholesteryl esters. Evidence for the hydrolysis of a portion of the incorporated esterified cholesterol is presented. A stimulation of free cholesterol incorporation and cellular esterification is elicited by hyperlipemic serum and serum lipoproteins when compared to normolipemic serum present at equivalent exogenous cholesterol concentrations. The effect of hyperlipemic serum is reduced by Tween-80 and Triton WR-1339. Comparative data on esterified cholesterol accumulation, free cholesterol incorporation, and cellular cholesterol esterification in Fu5-5 rat hepatoma cells, L-cells, and rabbit aortic medial cells are presented.

The metabolism of the phosphonium analogue of choline in cultured cells. A useful nuclear-magnetic-resonance probe for membrane phosphatidylcholine

1. Replacement of choline by the phosphonium analogue does not affect the growth rate of P815Y, NIL, 3T3, and SV40/3T3 cells in culture. 2. The fatty acid composition of the resulting phosphonium phosphatidylcholine is similar to that of phosphatidylcholine. 3. The rate of synthesis and degradation of phosphatidylcholine and of the phosphonium analogue are similar. 4. Phospholipid-exchange protein does not distinguish between phosphatidylcholine and the phosphonium analogue. 5. By contrast, incorporation of phosphonium choline into sphingomyelin occurs to only a minor extent. 6. It is concluded that, since the enzymes involved in the turnover of phosphatidylcholine do not discriminate between quaternary N and quaternary P in the polar head-group region, phosphonium choline should prove to be a useful probe for 31P nuclear-magnetic-resonance (n.m.r.) studies of natural membranes.

[Gas-liquid chromatography of non-esterified fatty acids of the rat myocardium]

Non-esterified fatty acids in cardiac cell cultures of postnatal rat, in postnatal rat heart and in ventricle muscle of adult rat were analyzed by gas-liquid chromatography. Among cultured heart cells, three different types were found. Type A0 appeared when arachidonic acid was lacking in tissue culture medium; it had the highest percentage in saturated fatty acids, palmitoleic and linolenic acids. Type A1 and type A2 appeared when arachidonic acid was proved to be present in culture medium. Non esterified fatty acid composition of type A1 was closely similar to that of postnatal rat heart, while type A2 ranged between that of postnatal heart and that of adult ventricular muscle.

Differential labeling of glycerol moieties of phospholipids and triacylglycerols of cultured mammalian cells by [U-14 C] glucose

Rabbit liver cells, in which fatty acid synthesis was suppressed by the rabbit serum component of the medium, were grown through 8- to 120-fold increases in cell numbers and mass of cell lipid in the presence of [U-14 C]-glucose. Triacylglycerols, phosphatidylcholine, and phosphatidylethanolamine were isolated from the total cell lipid and deacylated. Carbons 1 and 3 of the glycerol from the triacylglycerols and the no. 1 glycerol carbons of the two deacylated phospholids were oxidized by periodate and isolated as the dimedon derivative of formaldehyde. The specific activities of the glycerol carbons indicated that 58, 44, and 37 percent of the glycerol of the triacylglycerols. phosphatidylcholine, and phosphatidylethanolamine, respectively, were derived from the glucose of the medium. An additional 8 percent and 1-2 percent of the glycerol of each lipid was derived, respectively, from [U-14 C] glycerol and U14C-labeled amino acids added to the medium. In agreement with an experiment with albumin-bound [9,10- minus 3H]-oleic acid, and with smilar earlier experiments, it appears likely that appriacylglycerols originated from serum lipoproteins, or their partial hydrolysis products. An appreciable part of the ethanolamine of the cells' phosphatidylethanolamine originated from exogenous U- minus 14 C-labeled amino acids. Phosphatidyl-ethanolamine, however, was not a primary source of phosphatidylcholine. Labeling of the fatty acids of triacylglycerols and phospholipids by radioactive glucose, glycerol and amino acids was negligible.

Turnover of phospholipids in rat aortic smooth muscle cells in culture

Smooth muscle cells derived from rat aortic media were explanted and grown in culture for 14 to 60 days. During that time they formed a confluent multilayer and depostied extracellular material resembling newly formed elastin. The lipid composition of the cells in culture differed slightly from the parent cells in the intact aorta with respect to a higher phospholipid/DNA ratio and a higher lecithin content. The cholesterol content resembled that of parent cells. After incubation with labeled precursor the cultured cells show an active lipid synthesis; choline is incorporated mainly into lecithin, whereas glycerol and palmitate appear in phospholipids and to a lesser extent in neutral lipids. After a 2 hour pulse and up to 96 hour chase there is a linear fall in the specific acitivty of lecithin with a half-time of 28 to 30 hours. The rate of fall in specific activity of glycerol- or choline-labeled lecithin was found to be similar, indicating that choline does not turn over by an exchange reaction and is a suitable marker for studying phospholipid turnover in cultured cells. The results provide a basis for investigation of the effect of increasing cellular cholesterol content on phospholipid turnover.

Effect of exogenous lipid on lipid synthesis by bone and bone cell cultures

Newborn rat calvaria and isolated calvaria cells are capable of de novo lipid synthesis when grown in the presence or absence of exogenous lipid sources. Synthesis decreases when exogenous lipids are supplied. Several cholesterol precursors were found in these tissues and the presence of dihydrocholesterol was established for the first time.