Synthesis and turnover of lipids in monolayer cultures of BHK-21 cells

Ceramide synthesis correlates with the posttranscriptional regulation of the sterol-regulatory element-binding protein

OBJECTIVE

Sterol-regulatory element-binding proteins (SREBPs) regulate transcription of genes of lipid metabolism. Ceramide decreases transcriptionally active SREBP levels independently of intracellular cholesterol levels. Mechanisms of the ceramide-mediated decrease of SREBP levels were investigated.

METHODS AND RESULTS

Experiments were performed in Chinese hamster ovary cells. Inhibition of ceramide synthesis with myriocin, cycloserine, or fumonisin decreases levels of transcriptionally active SREBP and reduces SRE-mediated gene transcription. When ceramide synthesis is increased through exogenous sphingosine or inhibition of sphingosine kinase, SRE-mediated gene transcription is increased. The important role of ceramide synthesis in SRE-mediated gene transcription is confirmed in LY-B cells that do not synthesize ceramide de novo. LY-B cells fail to increase SRE-mediated gene transcription in sterol depletion.

CONCLUSIONS

Ceramide synthesis correlates with the generation of transcriptionally active SREBP and SRE-mediated gene transcription. Inhibition of ceramide synthesis decreases levels of transcriptionally active SREBP and SRE-mediated gene transcription. It is hypothesized that the process of ongoing ceramide synthesis contributes to the physiological processing of SREBP, perhaps affecting ER-to-Golgi trafficking. Taken together, modification of ceramide synthesis could be a novel target for drug development in the pharmacologic modification of SRE-dependent pathways.

Unsaturated fatty acid-mediated decreases in sterol regulatory element-mediated gene transcription are linked to cellular sphingolipid metabolism

A major physiological feedback mechanism of cholesterol in transcription of a number of lipid metabolism-related genes is mediated by sterol regulatory elements (SREs) and their binding proteins (SREBPs). Polyunsaturated free fatty acids alone, as well as synergistically with sterols, decrease SRE-mediated gene expression up to 80% in a dose-dependent manner by decreasing levels of the active transcription factor SREBP. We investigated potential mechanisms for this effect. We hypothesized that free fatty acids reduce SREBP-mediated gene transcription by increasing intracellular cholesterol content through the hydrolysis of cellular sphingomyelin, which has a high affinity for free cholesterol. We also questioned whether the lipid second messenger ceramide, a product of sphingomyelin hydrolysis, can decrease SRE-mediated gene transcription. First we investigated the effect of fatty acids on sphingomyelin hydrolysis. Incubation of [(3)H]choline-labeled cells with unsaturated (but not saturated) fatty acids induced hydrolysis of [(3)H]choline-labeled sphingomyelin. Also, incubation of cell extracts from fatty acid-treated cells with [(3)H]sphingomyelin increased generation of [(3)H]ceramide compared with control cells in vitro. We found that addition of ceramide analogs alone and additively with fatty acids decreased SRE expression and that ceramide analogs reduced levels of the transcriptionally active forms of SREBP-1 and SREBP-2. Increasing intracellular ceramide levels by exogenous sphingomyelinase or inhibition of ceramidase decreased SRE-mediated gene expression. None of the above conditions induced apoptosis. Incubation with U18666A, a compound that inhibits intracellular cholesterol movement, increased SRE-mediated gene transcription. C(2)-ceramide abrogated the effect of U18666A on SRE-mediated gene transcription, suggesting cholesterol-independent regulation of SREBP. We provide evidence that sphingomyelin hydrolysis and intermediates of sphingomyelin metabolism (in addition to cholesterol and fatty acids) contribute to regulation of SRE-mediated gene transcription.

Interferon-gamma stimulates lipid metabolism in human monocytes

In nonactivated human monocytes, radiolabeled oleic, arachidonic, and palmitic acids are primarily incorporated into neutral lipids and phosphatidylcholine. Each of these fatty acids is also incorporated into phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, and sphingomyelin in characteristic proportions which do not differ between donors. The phospholipid head group precursors, choline and serine, are incorporated into phosphatidylcholine, and phosphatidylserine and serine is incorporated into phosphatidylethanolamine and sphingomyelin. The incorporation of these lipid precursors and the total lipid content of monocytes activated with interferon-gamma were compared to those of nonactivated monocytes. Fatty acid incorporation into interferon-gamma-activated monocytes was dramatically increased, particularly for palmitic acid. Palmitic acid incorporation into phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, and sphingomyelin was increased in activated cells by 167-387% at 2 hr and 215-274% at 4 hr compared to that of controls. The greatest increase in incorporation was for palmitic acid into sphingomyelin. Incorporation of arachidonic acid into phosphatidylinositol and serine into phosphatidylethanolamine was also increased in the interferon-gamma-activated monocytes. The total lipid content of activated and nonactivated monocytes did not differ. These results suggest that IFN-gamma activation induces a short-term stimulation of phospholipid metabolism which does not alter the gross lipid composition. Such modifications of phospholipid metabolism may be important in signal transduction as well as an indication of functional changes in the membranes of activated macrophages.

Two separate pools of sphingomyelin in BHK cells

In BHK cells labelled to equilibrium with [3H]choline and treated with sphingomyelinase the surface pool of sphingomyelin is degraded very rapidly (half-time 10 min) but the internal pool of sphingomyelin which accounts for about 30% of the total is only degraded slowly (half-time about 80 h) showing that the internal pool does not normally reach the surface. In [3H]choline incorporation experiments the internal pool begins to accumulate radioactivity at about the same time as phosphatidylcholine (30 min) but label does not enter the surface pool of sphingomyelin for a further 90 min. The internal and external pools reach the same specific activity only after about 20 h. Pulse-chase analysis with [3H]choline shows that radioactivity in each pool of sphingomyelin continues to increase when the specific radioactivity of phosphatidylcholine is decreasing, consistent with both pools being synthesised from a phosphatidylcholine precursor. The results suggest that sphingomyelin in BHK cells is present not only in the plasma membrane but also in a more rapidly labelling pool which does not mix with the surface pool.

An update of the enzymology and regulation of sphingomyelin metabolism

Sphingomyelin is found in plasma membranes and related organelles (such as endocytic vesicles and lysosomes) of all tissues, as well as in lipoproteins. Abnormalities in sphingomyelin metabolism have been associated with atherosclerosis, cancer and genetically transmitted diseases; however, except for Niemann-Pick disease, little is known about the mechanism for these disorders. Sphingomyelin biosynthesis de novo involves ceramide formation from serine and two mol of fatty acyl-CoA followed by addition of the phosphocholine headgroup. The headgroup appears to come from phosphatidylcholine, but other sources have not been ruled out. Factors that influence the rate of sphingomyelin synthesis include the availability of serine and palmitic acid, plus the relative activities of key enzymes of this pathway. Sphingomyelin turnover involves removal of the headgroup and amide-linked fatty acid by sphingomyelinases and ceramidases, respectively, which have been found in both lysosomes (with acidic pH optima) and plasma membranes (with neutral to alkaline pH optima). The enzymes of sphingomyelin turnover release ceramide and free sphingosine from endogenous substrates, which may have implications for the participation of a sphingomyelin/sphingosine cycle as another 'lipid second messenger' system.

Enzymatic deoxyglucosylation of ceramides by microsomes of BHK-21 cells. The effect of deoxyglucose treatment and herpesvirus infection

The microsomal fractions of cultured hamster fibroblasts (BHK-21 cells) catalyze the incorporation of glucose from UDPglucose or of deoxyglucose from UDPdeoxyglucose into a reaction mixture with liposomes consisting of ceramide and phosphatidylcholine. The microsomal fractions also catalyze the transfer of glucose from UDPglucose to endogenous acceptors. The specific activity of ceramide deoxyglucoside or ceramide glucoside formation was significantly higher when microsomal preparations obtained from deoxyglucose-treated or herpesvirus-infected BHK-21 cells were used as the glucosyltransferase source. Deoxyglucose was incorporated from UDPdeoxyglucose into hydroxy- and nonhydroxy-fatty acid-containing ceramides at approximately the same rate. Competitive inhibition of deoxyglucosylation of ceramides by UDPglucose suggests that both reactions were catalyzed by the same enzyme, viz. UDPglucose:ceramide glucosyltransferase. This inhibition of glycosphingolipid synthesis may account, in part, for the inhibitory effect of deoxyglucose on lipid-containing viruses.

Accumulation of N-3 polyunsaturated fatty acids cultured human Y79 retinoblastoma cells

The metabolism of the n-3 class of polyunsaturated fatty acids, which occur in relatively high quantities in neural tissues, was studied in human Y79 retinoblastoma cells. These cells contained low levels of n-3 polyunsaturates when grown in culture media supplemented with fetal bovine serum. The cells readily incorporated performed docosahexaenoic acid (22.6 n-3) into phospholipids, but human skin fibroblasts did this to a similar extent. When 10 to 30 mumol/ml linolenic acid (18:3 n-3) was added, the cell also accumulated 22:6 in phospholipids. The capacity to convert appreciable amounts of 18:3 to 22:6 appears to be a unique property of the retinoblastoma cells as compared with other continuously cultured cell lines. More 18:3 than linoleic acid (18:2 n-6) was incorporated into phospholipids by the retinoblastoma cultures, and 18:3 was channeled to a larger extent into the ethanolamine glycerophospholipid fraction. These findings indicate that retinoblastoma cells handle n-3 polyunsaturated fatty acids in a manner very similar to neural tissue in vivo. Based on the results obtained with this model system, it appears that three processes may contribute to the accumulation of 22:6 in retina and neural tissue: increased ability to incorporate 18:3, the capacity to convert 18:3 to 22:6, and channeling of 18:3 and its metabolites into ethanolamine glycerophospholipids.

Retention of human skin fibroblast fatty acid modifications during maintenance culture

The fatty acid composition of cultured human skin fibroblasts was modified by adding either oleic or linoleic acid to the growth medium. After the cultures became confluent, they were washed and transferred to different maintenance media in order to determine the stability of the various fatty acyl modifications. Some changes in fatty acid composition occurred under all conditions. When the maintenance medium was supplemented with fatty acid, the cellular neutral lipid and phospholipid fatty acyl composition were altered markedly within 16 to 24 hr. If no supplemental fatty acid was available during the maintenance period, however, the modified fatty acyl compositions were sufficiently retained so that appreciable differences between the cells enriched with oleate and linoleate persisted for at least 48 to 72 hr. This considerable degree of stability occurred when either 10% delipidized fetal bovine serum or 10% fetal bovine serum containing its inherent lipids were present in the maintenance medium. Although the triglyceride content of the fatty acid-modified cells was quite labile, neither the cholesterol nor phospholipid content changed appreciably during culture in any of the maintenance media. Since the fatty acid compositional differences persisted during several days of maintenance under certain conditions, these modified cultures appear to be a useful experimental system for assessing the effect of lipid structure on fairly long-term cellular functions.

Phospholipid composition and ultrastructure of A549 cells and other cultured pulmonary epithelial cells of presumed type II cell origin

In order to assess the usefulness of A549, L-2, and AK-D cell lines as model systems for alveolar type II cells, we compared their phospholipid composition to that of fibroblasts grown under similar conditions. The percentage of disaturated phosphatidylcholine and phosphatidylglycerol, key phospholipids of purified surface-active material, was the same in epithelial cells and fibroblasts. When A549 cells were maintained in serum-free media for two days, ultrastructural examination showed an increase in cytoplasmic lamellar inclusions but there was no change in the percentage of disaturated phosphatidylcholine or phosphatidylglycerol. Because the lipid content of these cultured cells was very different from that of freshly isolated rat type II cells, we conclude that their suitability as model cell systems for type II cells is questionable.

Lipids of rabies virus and BHK-21 cell membranes

The lipid composition of highly purified Flury strain of rabies virus (HEP) propagated in BHK-21 cells in a chemically defined medium was observed to be 6.7% neutral lipids, 15.8% phospholipids, and 1.5% glycolipids. In the virion, phosphatidylethanolamine, phosphatidylcholine, and sphingomyelin were the most abundant phospholipids, accounting for 90% of the total, and the molar ratio of cholesterol to phospholipid was 0.48. Uninfected BHK-21 cell membranes were obtained by nitrogen cavitation techniques and separated by density gradient centrifugation, and the membranes were assayed for purity using 5'-nucleotidase, cytochrome oxidase, and reduced nicotinamide adenine dinucleotide phosphate diaphorase activities. Lipids of the plasma membrane were enriched in cholesterol, phosphatidylcholine, and phosphatidylethanolamine. In contrast, membranes of the endoplasmic reticulum were enriched in phosphatidylcholine, but contained smaller amounts of phosphatidylethanolamine and sphingomyelin. Comparison of the fatty acyl chains of virus and membranes from uninfected cells revealed the virion to have the lowest ratio of C18:1 to C18:0 (1.771), compared with values of about 3.0 for the plasma membrane and endoplasmic reticulum. Total polyenoic fatty acids were enriched in the plasma membrane, whereas the virus contained higher amounts of total saturates than either of the two membrane preparations. Analysis of the polar and neutral lipid fractions as well as the acyl chain analysis suggests the virion has a lipid composition that is intermiediate to that of the plasma membrane and endoplasmic reticulum and is consistent with the view that numerous viral particles are synthesized de novo by not utilizing a preexisting membrane template. From the ratio of cholesterol to phospholipid of 0.48, we calculated that 1.92 X 10(5) molecules of lipid would cover 4.14 X 10(4) nm2 in the form of a bilayer. Considerations of the molecular dimensions of the rabies envelope (total surface area, 5 X 10(4) nm2) as a bilayer suggest that some penetration of lipids by envelope proteins (M and G) is necessary.