De novo fatty acid synthesis and elongation of fatty acids by subcellular fractions of lung

PPARγ activation in late gestation does not promote surfactant maturation in the fetal sheep lung

Respiratory distress syndrome results from inadequate functional pulmonary surfactant and is a significant cause of mortality in preterm infants. Surfactant is essential for regulating alveolar interfacial surface tension, and its synthesis by Type II alveolar epithelial cells is stimulated by leptin produced by pulmonary lipofibroblasts upon activation by peroxisome proliferator-activated receptor γ (PPARγ). As it is unknown whether PPARγ stimulation or direct leptin administration can stimulate surfactant synthesis before birth, we examined the effect of continuous fetal administration of either the PPARγ agonist, rosiglitazone (RGZ; Study 1) or leptin (Study 2) on surfactant protein maturation in the late gestation fetal sheep lung. We measured mRNA expression of genes involved in surfactant maturation and showed that RGZ treatment reduced mRNA expression of LPCAT1 (surfactant phospholipid synthesis) and LAMP3 (marker for lamellar bodies), but did not alter mRNA expression of PPARγ, surfactant proteins (SFTP-A, -B, -C, and -D), PCYT1A (surfactant phospholipid synthesis), ABCA3 (phospholipid transportation), or the PPARγ target genes SPHK-1 and PAI-1. Leptin infusion significantly increased the expression of PPARγ and IGF2 and decreased the expression of SFTP-B. However, mRNA expression of the majority of genes involved in surfactant synthesis was not affected. These results suggest a potential decreased capacity for surfactant phospholipid and protein production in the fetal lung after RGZ and leptin administration, respectively. Therefore, targeting PPARγ may not be a feasible mechanistic approach to promote lung maturation.

Glial Hedgehog signalling and lipid metabolism regulate neural stem cell proliferation in Drosophila

The final size and function of the adult central nervous system (CNS) are determined by neuronal lineages generated by neural stem cells (NSCs) in the developing brain. In Drosophila, NSCs called neuroblasts (NBs) reside within a specialised microenvironment called the glial niche. Here, we explore non-autonomous glial regulation of NB proliferation. We show that lipid droplets (LDs) which reside within the glial niche are closely associated with the signalling molecule Hedgehog (Hh). Under physiological conditions, cortex glial Hh is autonomously required to sustain niche chamber formation. Upon FGF-mediated cortex glial overgrowth, glial Hh non-autonomously activates Hh signalling in the NBs, which in turn disrupts NB cell cycle progression and its ability to produce neurons. Glial Hh's ability to signal to NB is further modulated by lipid storage regulator lipid storage droplet-2 (Lsd-2) and de novo lipogenesis gene fatty acid synthase 1 (Fasn1). Together, our data suggest that glial-derived Hh modified by lipid metabolism mechanisms can affect the neighbouring NB's ability to proliferate and produce neurons.

Chlamydia trachomatis regulates growth and development in response to host cell fatty acid availability in the absence of lipid droplets

Chlamydia trachomatis (Ct) is a Gram-negative obligate intracellular pathogen of humans that causes significant morbidity from sexually transmitted and ocular diseases globally. Ct acquires host fatty acids (FA) to meet the metabolic and growth requirements of the organism. Lipid droplets (LDs) are storehouses of FAs in host cells and have been proposed to be a source of FAs for the parasitophorous vacuole, termed inclusion, in which Ct replicates. Previously, cells devoid of LDs were shown to produce reduced infectious progeny at 24 hr postinfection (hpi). Here, although we also found reduced progeny at 24 hpi, there were significantly more progeny at 48 hpi in the absence of LDs compared to the control wild-type (WT) cells. These findings were confirmed using transmission electron microscopy where cells without LDs were shown to have significantly more metabolically active reticulate bodies at 24 hpi and significantly more infectious but metabolically inert elementary bodies at 48 hpi than WT cells. Furthermore, by measuring basal oxygen consumption rates (OCR) using extracellular flux analysis, Ct infected cells without LDs had higher OCRs at 24 hpi than cells with LDs, confirming ongoing metabolic activity in the absence of LDs. Although the FA oleic acid is a major source of phospholipids for Ct and stimulates LD synthesis, treatment with oleic acid, but not other FAs, enhanced growth and led to an increase in basal OCR in both LD depleted and WT cells, indicating that FA transport to the inclusion is not affected by the loss of LDs. Our results show that Ct regulates inclusion metabolic activity and growth in response to host FA availability in the absence of LDs.

Differential effects of late gestation maternal overnutrition on the regulation of surfactant maturation in fetal and postnatal life

KEY POINTS

Offspring of overweight and obese women are at greater risk for respiratory complications at birth. We determined the effect of late gestation maternal overnutrition (LGON) in sheep on surfactant maturation, glucose transport and fatty acid metabolism in the lung in fetal and postnatal life. There were significant decreases in surfactant components and numerical density of surfactant producing cells in the alveolar epithelium due to LGON in the fetal lung. However, there were no differences in the levels of these surfactant components between control and LGON lambs at 30 days of age. The reduced capacity for surfactant production in fetuses as a result of LGON may affect the transition to air breathing at birth. There was altered glucose transport and fatty acid metabolism in the lung as a result of LGON in postnatal life. However, there is a normalisation of surfactant components that suggests accelerated maturation in the lungs after birth.

ABSTRACT

With the increasing incidence of obesity worldwide, the proportion of women entering pregnancy overweight or obese has increased dramatically. The fetus of an overnourished mother experiences numerous metabolic changes that may modulate lung development and hence successful transition to air breathing at birth. We used a sheep model of maternal late gestation overnutrition (LGON; from 115 days' gestation, term 147 ± 3 days) to determine the effect of exposure to an increased plane of nutrition in late gestation on lung development in the fetus (at 141 days' gestation) and the lamb (30 days after birth). We found a decrease in the numerical density of surfactant protein positive cells, as well as a reduction in mRNA expression of surfactant proteins (SFTP-A, -B and -C), a rate limiting enzyme in surfactant phospholipid synthesis (phosphate cytidylyltransferase 1, choline, α; PCYT1A), and glucose transporters (SLC2A1 and SLC2A4) in the fetal lung. In lambs at 30 days after birth, there were no differences between Control and LGON groups in the surfactant components that were downregulated in the LGON fetuses. However, mRNA expression of SFTP-A, PCYT1A, peroxisome proliferator activated receptor-γ, fatty acid synthase and fatty acid transport protein were increased in LGON lambs compared to controls. These results indicate a reduced capacity for surfactant production in late gestation. While these deficits are normalised by 30 days after birth, the lungs of LGON lambs exhibited altered glucose transport and fatty acid metabolism, which is consistent with an enhanced capacity for surfactant synthesis and restoration of surfactant maturity in these animals.

Fatty Acid Oxidation Mediated by Acyl-CoA Synthetase Long Chain 3 Is Required for Mutant KRAS Lung Tumorigenesis

KRAS is one of the most commonly mutated oncogenes in human cancer. Mutant KRAS aberrantly regulates metabolic networks. However, the contribution of cellular metabolism to mutant KRAS tumorigenesis is not completely understood. We report that mutant KRAS regulates intracellular fatty acid metabolism through Acyl-coenzyme A (CoA) synthetase long-chain family member 3 (ACSL3), which converts fatty acids into fatty Acyl-CoA esters, the substrates for lipid synthesis and β-oxidation. ACSL3 suppression is associated with depletion of cellular ATP and causes the death of lung cancer cells. Furthermore, mutant KRAS promotes the cellular uptake, retention, accumulation, and β-oxidation of fatty acids in lung cancer cells in an ACSL3-dependent manner. Finally, ACSL3 is essential for mutant KRAS lung cancer tumorigenesis in vivo and is highly expressed in human lung cancer. Our data demonstrate that mutant KRAS reprograms lipid homeostasis, establishing a metabolic requirement that could be exploited for therapeutic gain.

Fatty acid intake and the risk of community-acquired pneumonia in U.S. women

OBJECTIVE

Despite substantial progress in the treatment of community-acquired pneumonia, there are limited data on dietary risk factors. Fatty acid intake may influence community-acquired pneumonia risk by modulating the immune system. Our study prospectively examined the association between fatty acid intake and community-acquired pneumonia risk.

METHODS

The study population included 83165 women from the Nurses' Health Study II cohort who were 27 to 44 y old in 1991. The women reported lifestyle habits on biennial questionnaires and dietary intake every 4 y by validated semiquantitative food frequency questionnaires. There were 925 pneumonia cases over 10 y of follow-up. We examined independent associations for six fatty acids using Cox's proportional hazards regression.

RESULTS

Women in the highest quintile of palmitic acid intake had a 54% greater risk of pneumonia compared with those in the lowest quintile (multivariate relative risk 1.54, 95% confidence interval 1.12-2.12, P for trend = 0.002). Oleic acid intake was inversely associated with pneumonia risk (highest quintile multivariate relative risk 0.75, 95% confidence interval 0.55-1.04, P for trend = 0.02). Women in the highest quintile of docosahexanoic acid and eicosapentaenoic acid intake had a 24% greater risk of community-acquired pneumonia than did those in the lowest quintile (multivariate relative risk 1.24, 95% confidence interval 1.00-1.55, P for trend = 0.08). No significant associations were found for linoleic acid, alpha-linolenic acid, or docosahexanoic acid alone.

CONCLUSION

Fatty acid intake may affect the risk of community-acquired pneumonia in young and middle-aged women. Higher dietary intake of palmitic acid and possibly docosahexanoic and eicosapentaenoic acids may increase the risk of community-acquired pneumonia in women, whereas higher oleic acid intake may decrease the risk.

The fatty acid chain elongation system of mammalian endoplasmic reticulum

Much has been learned about FACES of the endoplasmic reticulum since its discovery in the early 1960s. FACES consists of four component reactions, requires the fatty acid to be activated in the form of a CoA derivative, utilizes reducing equivalents in the form of NADH or NADPH, is induced by a fat-free diet, resides on the cytoplasmic surface of the endoplasmic reticulum, appears to function in concert with the desaturase system and appears to exist in multiple forms (either multiple condensing enzymes connected to a single pathway or multiple pathways). FACES has been found in all tissues investigated, namely, liver, brain, kidney, lung, adrenals, retina, testis, small intestine, blood cells (lymphocytes and neutrophils) and fibroblasts, with one exception--the heart has no measurable activity. Yet, much more needs to be learned. The critical, inducible and rate-limiting condensing enzyme has resisted solubilization and purification; the purification of the other components has met with limited success. We know nothing about the site of synthesis of each component of FACES. How is each component enzyme integrated into the endoplasmic reticulum membrane? Is there a single mRNA directing synthesis of all four components or are there four separate mRNAs? How are elongation and desaturation coordinated? What is (are) the physiological regulator(s) of FACES--ADP, AMP, IP3, G-proteins, phosphorylation, CoA, Ca2+, cAMP, none of these? The molecular biology of FACES is only in the fetal stage of development. We are only scratching the surface--it is an undiscovered country.

Bioreactivity of intratracheally administered slate dust in rats: incorporation of 14C-acetate into lung lipids

The effect of intratracheally instilled slate dust on the phospholipid profile, and 14C-acetate incorporation into the lipids of lung lavage, whole lung tissue and its subcellular fractions, has been studied in rats. The acellular fraction of lung lavage showed a decrease in the phospholipid content at 4 days and then an increase at 40 days of dust exposure, whereas the cellular fraction showed the reverse. The order of 14C-acetate incorporation into total lipids and individual phospholipids showed a parallel trend. The rate of incorporation with total lipids of lung tissue was higher at the two stages of dust exposure and a similar pattern prevailed in the subcellular fractions, i.e. mitochondrial, microsomal and cytosolic fraction. Acetate incorporation was highest in mitochondria, followed by the microsomes. An increase in the microsomal and mitochondrial cholesterol levels was also observed. There was no significant change in the solvent-extracted 14C-counts of whole plasma, trichloroacetic acid (TCA) precipitate and TCA supernatant of plasma. The results indicate that slate dust causes an enhanced synthesis of pulmonary surfactant and other lung lipids and, therefore, has an effect on the metabolism of type II alveolar epithelial cells.

Biotin stores in rodent lungs: localization to Clara and type II alveolar cells

Biotin is a cofactor for carboxylases used in fatty acid synthesis, gluconeogenesis, and energy production by the citric acid cycle. Although lung has low levels of this vitamin overall, high concentrations were demonstrated histochemically in Clara cells of mouse, rat, hamster, and guinea pig using avidin conjugated to peroxidase. Lesser concentrations were found in type II cells of mouse, rat, and hamster but not guinea pig. By electron microscopy, biotin stores in mouse Clara cells were localized to mitochondria, while those in type II cells were present in both mitochondria and the cytoplasmic matrix. Biotin stores in type II cells are probably used mainly in fatty acid synthesis but also in gluconeogenesis and energy production. The reason for particularly high concentrations in the mitochondria of Clara cells is unknown.

The activity and properties of an acidic triacylglycerol lipase from adult and fetal rat lung

Triacylglycerol lipase with maximal activity at pH 5 was present in adult and fetal lung. The activity was inhibited by serum concentrations used to measure lipoprotein lipase and by 0.5 M NaCl. The activity in homogenates from fetal lung was about 40% of the activity in adult lung homogenates. The activity increased to 80% of the adult levels during the first 24-48 h following birth. Acidic triacylglycerol lipase was present in all subcellular fractions from adult lung. However, the major amount of activity appeared to be associated with lysosomes. Fetal lung contained significantly more activity in the cytosolic fraction compared to the adult. The reaction produced free fatty acids (65%), 1,2(2,3)-diacylglycerol (22%) and 2-monoacylglycerol (12%). Minimal amounts of 1,3-diacylglycerol and 1(3)-monoacylglycerol were formed. Diacylglycerol lipase and monoacylglycerol hydrolase activities at pH 5 were independently determined and both were higher than the triacylglycerol lipase activity. The subcellular distribution of diacylglycerol lipase and monoacylglycerol hydrolase differed from that of triacylglycerol lipase. Overall, the results indicated that the lung has considerable intracellular lipase activity and therefore could readily hydrolyze intracellular triacylglycerol to free fatty acids. The reaction also produced significant amounts of 1,2-diacylglycerol which suggests that triacylglycerol could be a direct source of diacylglycerol for phospholipid synthesis.

Effect of starvation on enzymes related to lipid metabolism in guinea-pig lungs

The changes in activities of acetyl CoA carboxylase, microsomal fatty acid elongation enzyme, choline phosphotransferase, triglyceride lipase, phospholipase A1 and phospholipase A2 were followed in guinea-pig lungs at 24, 48 and 72 h after food deprivation. Triglyceride lipase was elevated and phospholipase A1 and phospholipase A2 were unaffected, while the other activities decreased. The significance of these findings in relation to food deprivation is discussed.

Studies on the fate of labelled palmitic acid in rat lung

1. Pulmonary palmitic acid metabolism was studied in rats in basal conditions and after experimental talcosis. 2. Palmitic acid was rapidly incorporated into phospholipids and stored in the form of esterified cholesterol. 3. During the experiment a de novo synthesis of triglycerides was not detected.

Unsaturated fatty acids in the postnatally developing rat lung

Fatty acid desaturase activity specific for the C-9 position is present in lung microsomes prepared from rats of all ages. This activity is significantly lower in neonatal rat lung compared with adult lung. A rapid increase in C-9 fatty acid desaturase activity seen at the approximate time of weaning may be related to a decrease in the polyunsaturated fatty acid (PUFA) content of the diet as the rat begins to consume laboratory chow instead of mother's milk. The 900 X g supernatant fraction of rat lung parenchymal cell homogenates is capable of incorporating linoleate, linolenate, and arachidonate into both triacylglycerols and phospholipids. Lung tissue from rats less than 20 days old incorporates these PUFA into phospholipids at a greater rate than lung tissue from adult rats. The incorporation of these PUFA into phospholipids in neonatal lung tissue occurred at a greater rate rate than their incorporation into triacylglycerols. In contrast, lung tissue from adult rats incorporated PUFA into triacylglycerols at a greater rate than into phospholipids. These data show that PUFA, known to be elevated in neonatal rat lungs, are used primarily for phospholipid biosynthesis in neonatal lung tissue whereas in adult lung tissue they are preferentially esterified to glycerol.

Ultrastructural evidence of pulmonary capillary endothelial damage from paraquat

Because of lack of agreement concerning the toxicity of paraquat to the pulmonary microvasculature, we have undertaken an electron microscopic study of lungs of paraquat-treated rats. Rats were injected with paraquat or sterile water (controls) intraperitoneally; the animals were then killed at 24-h intervals for 10 days post-injection. In the control animals, lung ultrastructure remained normal throughout the study. In treated animals, the initial evidence of alveolar epithelial injury occurred 24 h post-paraquat. By 48 h, severe fragmentation and desquamation of membranous pneumocytes occurred, and both alveolar and interstitial edema were present. Epithelial damage was maximal 72-96 h post-paraquat. Pulmonary capillary endothelial abnormalities were less extensive than the alveolar epithelial lesions. Endothelial damage was first observed 48 h post-paraquat. In endothelial cells on the septal (thick) side of the capillaries, the number of pinocytotic vesicles was significantly increased (P less than 0.05) from 48 to 96 h post-paraquat. In endothelium adjacent to damaged epithelium, abnormalities included hydration, fragmentation, discontinuity, and widened intercellular junctions; these were maximal 72-96 h post-paraquat. Although other mechanisms are probably important, damaged pulmonary capillary endothelium seems to be a factor favoring paraquat-induced pulmonary edema.

De novo fatty acid synthesis in the perfused rat lung. Incorporation of palmitate into phospholipids

1. The incorporation of exogenously derived [14C]palmitate and endogenously synthesized [3H]palmitate (from 3H2O) was measured in the isolated perfused lung. 2. Over 40% of the fatty acid esterified into lung disaturated phosphatidylcholine was derived from de novo synthesis. 3. A major portion of the palmitate synthesized de novo was incorporated in the 2 position of disaturated phosphatidylcholine. 4. Streptozotocin-induced diabetes and the compound 5-(tetradecyloxy)-2-furoic acid markedly inhibited de novo fatty acid synthesis while the incorporation of exogenously supplied palmitate increased into disaturated phosphatidylcholine, primarily in the 2 position. 5. Treatment with insulin resulted in an increase in [14C]glucose incorporation into lung phospholipid, with the largest increase appearing in the glyceride-glycerol fraction of the phosphatidylcholine species. 6. Insulin neither stimulated de novo fatty acid synthesis nor increased exogenous palmitate incorporation. 7. These data show: (1) that de novo fatty acid synthesis in the perfused rat lung is involved in the remodeling reactions in the synthesis of phosphatidylcholine, and (2) that diabetes affects the relative contribution of de novo synthesized and exogenously supplied palmitate available for the esterification of lung phospholipid.

Phospholipids of the lung in normal, toxic, and diseased states

The highly pulmonary concentration of 1,2-dipalmitoyl-sn-glycerol-3-phosphorylcholine (dipalmitoyllecithin) and its implication as an important component of lung surfactant have promoted investigation of phospholipid metabolism in the lung. This review will set the contents including recent informations for better understanding of phospholipid metabolism of the lung in normal state (physiological significances of lung phospholipids, characteristics of phospholipids in lung tissue and alveolar washing, biosynthetic pathways of dipalmitoyllecithin, etc.) as well as in toxic states (pulmonary oxygen toxicity, etc.) and in diseased states (idiopathic respiratory distress syndrome, pulmonary alveolar proteinosis, etc.) Since our main concern has been to clarify the most important route for supplying dipalmitoyllecithin, this review will be focused upon the various biosynthetic pathways leading to the formation of different molecular species of lecithin and their potential significance in the normal, toxic, and diseased lungs.

Pathways of palmitate metabolism in the isolated rat lung

Plasma fatty acids represent major precursors of lung lipids. In this study, the pathways of palmitate metabolism were measured in an isolated perfused rat lung. Lungs were ventilated with 5% CO2 in air and perfused with Krebs-Ringer bicarbonate containing 3% serum albumin and 0.25 mM [U-14C] and [9, 10(-3)H] palmitate. Fatty acid utilization was estimated by recovery of radiolabel in products of metabolism. Fourteen percent of a total 14C-fatty-acid utilization of 4.5 mumol fatty acid/100 min/g dry wt. was recovered as 14CO2. Degradation of fatty acid to acetyl CoA was indicated by a 3H2O production that was twice fatty acid oxidation to CO2. The majority of palmitate was recovered in lung phosphatidylcholines with a 14C to 3H ratio of 1.4 accounting for differences between 14C and 3H2O productions. Addition of glucose to the perfusate decreased fatty acid oxidation to CO2 by 32% but had no effect on 14C recovery in phospholipids. Perfusion with the uncoupler of oxidative phosphorylation 2,4-dinitrophenol stimulated fatty acid oxidation twofold but decreased 14C incorporation into lipids. These data together with estimates of fatty acid synthesis based on 3H2O incorporation into lipids, suggested that exogenous fatty acids and glucose both represent sources of carbon for de novo fatty acid synthesis and energy production.

The perfused isolated lung as a possible model for the study of lipid synthesis by type II cells in their natural environment

The incorporation of radioactively labeled palmitate and acetate into total and disaturated phosphatidylcholines was studied in the perfused whole lung, in surfactant secreted during perfusion, and in isolated alveolar type II cells. Exogenously added palmitate was found to be incorporated preferentially into the 2-position of total and disaturated phosphatidylcholines in all cases. Acetate, when supplied at a high concentration, was incorporated preferentially into the 2-position in all cases. However, acetate supplied at a low concentration was incorporated preferentially into the 2-position in type II cells and in surfactant, but preferentially into the 1-position in the whole lung. The dissimilarity in incorporation of acetate between isolated type II cells and perfused whole lung and the similarity in this respect between isolated type II cells and surfactant indicate that the perfused isolated lung may only be a good model for studying the synthesis of surfactant components by the type II cells in their natural environment if the products of processes in type II cells are separated from products of other cells after the perfusion. Both in surfactant and in lavaged lung tissue, labeled palmitate and acetate incorporated mainly into the 2-position of phosphatidylglycerol. This indicates that remodeling reactions are involved in the synthesis of dipalmitoylphosphatidylglycerol.

The in vivo labeling with acetate and palmitate of lung phospholipids from developing and adult rabbits

The labeling with radiolabeled acetate and palmitate of lung, microsomes isolated from lung, and surfactant phospholipids from adult, 3-day-old, and newborn rabbits was studied. The half-life of phosphatidylcholine from lung and microsomal fractions was shorter when labeled with acetate than when labeled with palmitate. Half-time values similarly measured for phosphatidylglycerol, phosphatidylinositol or phosphatidylethanolamine were not different for the two labels. Acetate and palmitate-labeled phospholipids appeared in the surfactant fraction with similar accumulation curves. The relative specific activities of acetate-labeled phosphatidylcholine from adult, 3-day-old, and newborn rabbits, respectively, were 1.30, 1.86 and 1.77 times those measured for those measured for the palmitate label. Surfactant phosphatidylinositol and phosphatidylethanolamine from 3-day-old animals similarly were labeled preferentially with acetate. However, phosphatidylglycerol purified from the surfactant fraction contained equivalent relative amounts of the acetate and palmitate labels in 3-day-old and adult rabbits. These results suggest that the type II pneumocyte may use acetate preferentially for the synthesis of palmitic acid which then is incorporated into surfactant phospholipids.

An in vivo comparison of acetate and palmitate as precursors of surfactant phosphatidylcholine

3-Day-old rabbits were injected simultaneously with [(3)H]acetate and [(14)C]-palmitic acid. The specific activities of lung, lamellar body and surfactant phosphatidylcholine and disaturated phosphatidylcholine were measured at time intervals from 10 min to 23 h following isotope administration. Palmitic acid contained 87% of the acetate radioactivity recovered from lung and surfactant phosphatidylcholine. The relative specific activities of surfactant phosphatidylcholine and disturated phosphatidylcholine labeled with acetate were 2.02 and 1.86 times those measured using the palmitic acid label. Apparently the palmitic acid synthesized from acetate is preferentially incorporated into lung phosphatidylcholines and disaturated phosphatidylcholines which are destined to become surfactant.

Pulmonary fatty acid synthesis. II. Amino acids as fatty acid precursors in rat lung

The incorporation of various 14C-labeled amino acids into CO2 and lipids by rat lung slices was examined. Alanine, valine, leucine, isoleucine, aspartate, and glutamate were oxidized by lung tissue, whereas glycine and phenylalanine were not oxidized. Carbon originating from alanine, leucine, and glutamate was incorporated into pulmonary fatty acids by a mechanism indicative of de novo synthesis. Experiments with specifically labeled [14C]aspartate and [14C]glutamate revealed that the complete citrate-malate cycle described by Patel et al. (25) is of minor importance in pulmonary lipogenesis due to the extremely low activity of NADP-malate dehydrogenase. Glucose and pyruvate were also actively incorporated into fatty acids, and it is suggested that citrate in pulmonary tissue, as in other tissues, plays an important role in the transport of acetyl units from the mitochondria to the cell cytosol during lipogenesis from various carbohydrate and amino acid substrates.

Pulmonary fatty acid synthesis. I. Mitochondrial acetyl transfer by rat lung in vitro

Incorporation of tritiated water into fatty acids by rat adipose tissue and lung tissue slices incubated with 5 mM glucose indicated a level of fatty acid synthesis in rat lung approximately 15% that observed in adipose tissue in vitro. (-)-Hydroxycitrate, and inhibitor of ATP citrate lyase, markedly reduced tritiated water incorporation into fatty acids by lung tissue slices. The effects of (-)-hydroxycitrate and n-butymalonate on the incorporation of 14C-labeled glucose, pyruvate, acetate, and citrate suggested that citrate is a major acetyl carrier for de novo fatty acid synthesis in lung tissue. Alternative mechanisms to citrate as an acetyl carrier were also considered. Lung mitochondrial preparations formed significant levels of acetylcarnitine in the presence of pyruvate and carnitine. However, the effect of carnitine on the incorporation of 14C-labeled glucose, pyruvate, acetate, and citrate into fatty acids by lung tissue slices indicated that acetylcarnitine may not be a significant acetyl carrier for fatty acid synthesis but may serve as an acetyl "buffer" in the control of mitochondrial acetyl-CoA levels. Additionally, it appears unlikely that either acetylaspartate or acetoacetate are of major importance in acetyl transfer in lung tissue.

Citrate formation by rat lung mitochondrial preparations

Rat lung mitochondrial preparations were incubated in the preasence of pyruvate and malate. The principal metabolic products measured were citrate and CO2. Citrate formation from pyruvate was found to be dependent on the presence of malate. Significant citrate was formed in the presence of isocitrate and the rate of citrate formation was increased by the addition of pyruvate. Small amounts of citrate were formed by lung mitochondrial preparations in the presence of 2-oxoglutarate and succinate only after the addition of pyruvate. The level of acetyl-CoA was significantly greater in the presence of pyruvate than in the presence of pyruvate plus malate. The addition of malate to lung mitoochondrial preparations increased 14CO2 production from [2-14C] pyruvate into malate and citrate. A low level of pyruvate-dependent H14CO3-incorporation into acid-stable products was observed, principally citrate and malate, but this rate did not exceed 5% of the rate of net citrate formation in the presence of malate and pyruvate. The capacity of rate lung mitochondria to form oxaloacetate from pyruvate alone in vitro is very limited, and would appear to cast doubt on a major role of pyruvate carboxylase in citrate formation. It is concluded that the rate of citrate formation from pyruvate is limited by the availability of intramitochondrial oxaloacetate and the rate of citrate efflux across the mitochondrial membrane.