Stimulation Of Fatty Acid Synthesis Research Articles

Pyruvate carboxylase promotes thyroid cancer aggressiveness through fatty acid synthesis

BackgroundPyruvate carboxylase (PC) is an important anaplerotic enzyme in the tricarboxylic acid cycle (TCA) in cancer cells. Although PC overexpression has been observed in thyroid cancer (TC), the mechanisms involved in the carcinogenic effects of PC are still unclear.MethodsBioinformatics analysis and clinical specimens were used to analyze the relationship of PC expression with clinicopathological variables in TC. Fatty acid synthesis was monitored by LC/MS, Nile red staining, and triglyceride analysis. Mitochondrial oxygen consumption was evaluated by the Seahorse XF Mito Cell Stress Test. The correlation of PC with FASN and SREBP1c was assessed by qRT-PCR and IHC in 38 human TC tissues. Western blotting was used to evaluate the protein expression of PC, FASN, and SREBP1c and members of the AKT/mTOR and EMT pathways in TC cell lines. Wound-healing, CCK-8, and Transwell assays and a nude mouse xenograft model were used to verify the regulatory effects of PC and SREBP1c on thyroid tumor cell proliferation, migration and invasion.ResultsWe demonstrated that PC increased fatty acid synthesis, which then promoted TC progression and metastasis. Analysis of GEO data showed that the overexpression of PC in papillary thyroid cancer (PTC) was associated with PTC invasion and the fatty acid synthesis pathway. Analysis of clinical tissue specimens from PTC patients revealed that PC was more highly expressed in specimens from PTC patients with lymph node metastasis than in those from patients without metastasis. Multiple genes in the fatty acid synthesis signaling pathway, including FASN and SREBP1c, were downregulated in PC-knockdown TC cells compared to control cells. Lipid levels were also decreased in the PC-knockdown TC cells. Moreover, the ability of cells to grow, invade, and metastasize was also suppressed upon PC knockdown, suggesting that PC-mediated lipogenesis activation increases the aggressiveness of TC cells. In addition, PC was found to activate the AKT/mTOR pathway, thus improving FASN-mediated de novo lipogenesis in TC cells by upregulating SREBP1c expression. Studies in a nude mouse xenograft model showed that PC knockdown decreased tumor weight, but this effect was attenuated by forced expression of SREBP1c.ConclusionsOur results demonstrate that PC is strongly involved in the tumor aggressiveness of TC via its stimulation of fatty acid synthesis.

Abstract 1417: Stimulation of fatty acid synthesis by Spot14 enhances tumor cell proliferation but decreases metastasis in vivo

Abstract We have recently observed that Spot14 (S14), also known as Thyroid Hormone Responsive Protein, stimulates de novo fatty acid biosynthesis in mammary epithelial cells by increasing Fatty Acid Synthase (FASN) activity. In the presence of S14, FASN had increased Vmax, decreased Km, and shifted product formation toward medium chain fatty acids. Numerous studies have suggested that high expression of FASN is associated with higher tumor grade and poor clinical outcome, and likewise elevated expression of S14 has been linked to a poor clinical outcome for patients with invasive disease. In contrast, our analysis of S14 expression in databases of human breast tumors representing a variety of stages, grades, and subtypes suggests that high S14 is associated with ER-positive status, the luminal intrinsic subtype, and consequently, a favorable patient outcome. To determine the effect of S14 on mammary tumorigenesis in vivo, we crossed S14 null mice to MMTV-PyMT mice. Tumor latency was not altered in mice lacking S14; however, tumor cell proliferation was significantly decreased. Tumors arising in MMTV-PyMT mice lacking S14 were less dense and had a cystic morphology. There was a decrease in the phosphorylation of Src at Y416 and Akt at S473, indicating that activation of both signaling molecules was decreased in tumors lacking S14. Furthermore, tumors lacking S14 had reduced levels of de novo synthesized fatty acids. We also crossed MMTV-S14 transgenic mice expressing S14 in the mammary epithelium, to MMTV-Neu line 202 mice and observed that S14 overexpression significantly shortened tumor latency, increased tumor cell proliferation and increased concentration of fatty acids in tumor cells. Despite the increased proliferative index observed in the MMTV-Neu tumors expressing S14, there was a decrease in metastatic lesions detected in the lungs compared to MMTV-Neu mice. Gene expression profiling of mouse tumors revealed that tumors overexpressing S14 displayed features of lobular differentiation including increased expression of Elf5, a proposed master regulator of mammary differentiation, and expression of milk protein genes. Elevated fatty acid synthesis is a distinguishing feature of many solid tumors compared to adjacent normal tissue and this activity is thought to be acquired during tumor progression, as rapidly proliferating cancer cells have a heightened requirement for membrane phospholipid precursors. Our studies suggest that increased de novo fatty acid biosynthesis is sufficient to stimulate tumor cell proliferation; however, this is not correlated with increased metastasis, and in fact, tumors that expressed high levels of S14 appeared to have a more differentiated phenotype that may correlate with improved clinical outcome. Citation Format: Elizabeth A. Wellberg, Michael C. Rudolph, Andrew Lewis, Steven M. Anderson. Stimulation of fatty acid synthesis by Spot14 enhances tumor cell proliferation but decreases metastasis in vivo. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1417. doi:10.1158/1538-7445.AM2014-1417

Regulation of bile acid, cholesterol, and fatty acid synthesis in chicken primary hepatocytes by different concentrations of T0901317, an agonist of liver X receptors

Liver X receptors (LXRs) are members of the nuclear receptor family of transcription factors. They play a crucial role in lipid metabolism processes such as bile acid and fatty acid synthesis, as well as minor or limited roles in the regulation of cholesterol synthesis and uptake in mammals. In avian species, however, little is known about the role of LXRs except for the fact that they are involved in the stimulation of fatty acid synthesis. In this study, we characterize the expression profile of genes related to bile acid, cholesterol, and fatty acid synthesis and VLDL secretion in chicken primary hepatocytes treated with T0901317, a synthetic agonist of LXR. The activity of chicken cholesterol 7α hydroxylase (CYP7A1), a key enzyme in bile acid synthesis, mRNA expression, and bile acid excretion, was stimulated by supplementation of the culture medium with a low concentration (0.01 μM) of T0901317. In contrast, the levels of sterol regulatory element binding protein (SREBP)-1, fatty acid synthase mRNA, and VLDL-triacylglycerol in cells cultured in the presence of a high concentration (10 μM) of T0901317 were higher than those cultured in zero or low concentrations of T0901317. These results suggest that cellular responses to this LXR agonist were similar to those present in mammals. A novel finding of this study concerned changes to the regulation of cholesterol synthesis and uptake in chicken hepatocytes treated with T0901317. Levels of SREBP-2,3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) and low-density lipoprotein receptor (LDLr) mRNA expression increased as a function of increasing T0901317 (up to 1.0 μM), but remained similar to those in cells cultured under control conditions when the concentration of T0901317 was increased to 10 μM. These results suggest that LXRs play an important role in cholesterol synthesis and uptake in chicken hepatocytes and, as such, differ to findings in mammals where the effect of LXR agonists on cholesterol synthesis plays only a minor role in the regulation of cellular sterol homeostasis.

A feed-forward loop amplifies nutritional regulation of PNPLA3

The upsurge in prevalence of obesity has spawned an epidemic of nonalcoholic fatty liver disease (NAFLD). Previously, we identified a sequence variant (I148M) in patatin-like phospholipase domain-containing protein 3 (PNPLA3) that confers susceptibility to both hepatic triglyceride (TG) deposition and liver injury. To glean insights into the biological role of PNPLA3, we examined the molecular mechanisms by which nutrient status controls hepatic expression of PNPLA3. PNPLA3 mRNA levels, which were low in fasting animals, increased approximately 90-fold with carbohydrate feeding. The increase was mimicked by treatment with a liver X receptor (LXR) agonist and required the transcription factor SREBP-1c. The site of SREBP-1c binding was mapped to intron 1 of Pnpla3 using chromatin immunoprecipitation and electrophoretic mobility shift assays. SREBP-1c also promotes fatty acid synthesis by activating several genes encoding enzymes in the biosynthetic pathway. Addition of fatty acids (C16:0, C18:1, and C18:2) to the medium of cultured hepatocytes (HuH-7) increased PNPLA3 protein mass without altering mRNA levels. The posttranslational increase in PNPLA3 levels persisted after blocking TG synthesis with triascin C. Oleate (400 muM) treatment prolonged the half-life of PNPLA3 from 2.4 to 6.7 h. These findings are consistent with nutritional control of PNPLA3 being effected by a feed-forward loop; SREBP-1c promotes accumulation of PNPLA3 directly by activating Pnpla3 transcription and indirectly by inhibiting PNPLA3 degradation through the stimulation of fatty acid synthesis.

Redox Regulation of Protein Tyrosine Phosphatase 1B by Manipulation of Dietary Selenium Affects the Triglyceride Concentration in Rat Liver

Protein tyrosine phosphatase 1B (PTP1B) is a key enzyme in the counter-regulation of insulin signaling and in the stimulation of fatty acid synthesis. Selenium (Se), via the activities of glutathione peroxidase (GPx) and thioredoxin reductase (TrxR), is involved in the removal of H2O2 and organic peroxides, which are critical compounds in the modulation of PTP1B activity via glutathionylation. Our study with growing rats investigated how the manipulation of dietary Se concentration influences the regulation of PTP1B and lipogenic effects mediated by PTP1B. Weanling albino rats were divided into 3 groups of 10. The negative control group (NC) was fed a Se-deficient diet for 8 wk. Rats in groups Se75 and Se150 received diets supplemented with 75 or 150 μg Se/kg. Se supplementation of the rats strongly influenced expression and activity of the selenoenzymes cytosolic GPx, plasma GPx, phospholipidhydroperoxide GPx, and cytosolic TrxR, and liver PTP1B. Liver PTP1B activity was significantly higher in groups Se75 and Se150 than in the NC group and this was attributed to a lowered inhibition of the enzyme by glutathionylation. The increased liver PTP1B activity in groups Se75 and Se150 resulted in 1.1- and 1.4-fold higher liver triglyceride concentrations than in the NC rats. The upregulation of the sterol regulatory element binding protein-1c and of fatty acid synthase, 2 PTP1B targets, provided a possible explanation for the lipogenic effect of PTP1B due to the manipulation of dietary Se. We therefore conclude that redox-regulated proteins, such as PTP1B, represent important interfaces between dietary antioxidants such as Se and the regulation of metabolic processes.

Soy protein reduces hepatic lipotoxicity in hyperinsulinemic obese Zucker fa/fa rats

Hepatic steatosis is commonly present during the development of insulin resistance, and it is a clear sign of lipotoxicity attributable in part to an accelerated lipogenesis. There is evidence that a soy protein diet prevents the overexpression of hepatic sterol-regulatory element binding protein-1 (SREBP-1), decreasing lipid accumulation. Therefore, the aim of the present work was to study whether a soy protein diet may prevent the development of fatty liver through the regulation of transcription factors involved in lipid metabolism in hyperinsulinemic and hyperleptinemic Zucker obese fa/fa rats. Serum and hepatic cholesterol and triglyceride levels, as well as VLDL-triglyceride and LDL-cholesterol, were significantly lower in rats fed soy protein than in rats fed a casein diet for 160 days. The reduction in hepatic cholesterol was associated with a low expression of liver X receptor-alpha and its target genes, 7-alpha hydroxylase and ABCA1. Soy protein also decreased the expression of SREBP-1 and several of its target genes, FAS, stearoyl-CoA desaturase-1, and delta5 and delta6 desaturases, decreasing lipogenesis even in the presence of hyperinsulinemia. Reduction in SREBP-1 was not associated with the presence of soy isoflavones. Finally, soy protein reduced SREBP-1 expression in adipocytes, preventing hypertrophy, which also helps prevent the development of hepatic lipotoxicity.

Central role for liver X receptor in insulin-mediated activation of Srebp-1c transcription and stimulation of fatty acid synthesis in liver.

Transcription of the gene encoding sterol regulatory element-binding protein 1c (SREBP-1c) is known to be activated by insulin in the liver. The resultant SREBP-1c protein activates transcription of the genes required for fatty acid synthesis. Here, we use SREBP-1c promoter reporter constructs to dissect the mechanism of insulin activation in freshly isolated rat hepatocytes. The data show that a complete insulin response (increase of 6- to 11-fold) requires two binding sites for liver X receptors (LXRs), which are nuclear receptors that are activated by oxygenated sterols. Disruption of these binding sites did not lower basal transcription but severely reduced the response to insulin. In contrast, disruption of the closely linked binding sites for SREBPs and nuclear factor Y lowered basal transcription drastically but still permitted a 4- to 7-fold increase in response to insulin. Arachidonic acid, an inhibitor of LXR activation, blocked the response to insulin. We conclude that insulin activates the SREBP-1c promoter primarily by increasing the activity of LXRs, possibly through production of a ligand that activates LXRs or their heterodimerizing partner, the retinoid X receptor. Nuclear SREBPs and nuclear factor Y play permissive roles.

Alpha-Tocopherol and NADPH in the Erythrocytes and Plasma of Multiple Sclerosis Patients

Objectives: To investigate the influence of interferon-β-1b (INF-β-1b) therapy on blood antioxidants (α-tocopherol and NADPH) in multiple sclerosis (MS). Patients and Methods: Patients with relapsing-remitting MS (n = 14) have been studied during 6 months of INF-β-1b therapy. α-Tocopherol was determined by HPLC and UV or electrochemical detection; NADPH was quantified spectrophotometrically. Results: The erythrocyte α-tocopherol level was reduced (p < 0.001) before treatment, but had regained the control level by 6 months of therapy. The plasma α-tocopherol/lipid ratios were constant during therapy. Plasma triglyceride levels were transiently increased (p = 0.0270) after 1 month of treatment. INF-β-1b had also induced a transient decrease in NADPH after 1 month, but thereafter the level returned to approximately the initial value (p = 0.0248). Conclusion: INF-β-1b seems to exert a sparing effect toward the erythrocyte α-tocopherol content. The fall in NADPH in parallel to the rise in plasma triglycerides suggests stimulation of fatty acid synthesis by INF-β-1b.

Effect of high-carbohydrate feeding on triglyceride and saturated fatty acid synthesis.

It has been known for decades that low-fat, high-carbohydrate diets can increase plasma triglyceride levels, but the mechanism for this effect has been uncertain. Recently, new isotopic and nonisotopic methods have been used to determine in vivo whether low-fat, high-carbohydrate diets increase triglyceride levels by stimulating fatty acid synthesis. The results of a series of studies in lean and obese weight-stable volunteers showed that very-low-fat (10%), high-carbohydrate diets enriched in simple sugars increased the fraction of newly synthesized fatty acids, along with a proportionate increase in the concentration of plasma triglyceride. Furthermore, the concentration of the saturated fatty acid, palmitate, increased and the concentration of the essential polyunsaturated fatty acid, linoleate, decreased in triglyceride and VLDL triglyceride. The magnitude of the increase in triglyceride varied considerably among subjects, was unrelated to sex, body mass index, or insulin levels, and was higher when fatty acid synthesis was constantly elevated rather than having a diurnal variation. It was notable that minimal stimulation of fatty acid synthesis occurred with higher fat diets (>30%) or with 10% fat diets enriched in complex carbohydrate. Public health recommendations to reduce dietary fat must take into account the distinct effects of different types of carbohydrate that may increase plasma triglycerides and fatty acid synthesis in a highly variable manner. The mediators and health consequences of this dietary effect deserve further study.

Effect of High‐Carbohydrate Feeding on Triglyceride and Saturated Fatty Acid Synthesis

Abstract. It has been known for decades that low‐fat, high‐carbohydrate diets can increase plasma triglyceride levels, but the mechanism for this effect has been uncertain. Recently, new isotopic and nonisotopic methods have been used to determine in vivo whether low‐fat, high‐carbohydrate diets increase triglyceride levels by stimulating fatty acid synthesis. The results of a series of studies in lean and obese weight‐stable volunteers showed that very‐low‐fat (10%), high‐carbohydrate diets enriched in simple sugars increased the fraction of newly synthesized fatty acids, along with a proportionate increase in the concentration of plasma triglyceride. Furthermore, the concentration of the saturated fatty acid, palmitate, increased and the concentration of the essential polyunsaturated fatty acid, linoleate, decreased in triglyceride and VLDL triglyceride. The magnitude of the increase in triglyceride varied considerably among subjects, was unrelated to sex, body mass index, or insulin levels, and was higher when fatty acid synthesis was constantly elevated rather than having a diurnal variation. It was notable that minimal stimulation of fatty acid synthesis occurred with higher fat diets (&gt;30%) or with 10% fat diets enriched in complex carbohydrate. Public health recommendations to reduce dietary fat must take into account the distinct effects of different types of carbohydrate that may increase plasma triglycerides and fatty acid synthesis in a highly variable manner. The mediators and health consequences of this dietary effect deserve further study.

Human fatty acid synthesis is reduced after the substitution of dietary starch for sugar

Using new nonisotopic and isotopic methods, we showed previously that fatty acid synthesis was markedly stimulated in weight-stable normal volunteers by a very-low-fat formula diet with 10% of energy as fat and 75% as short glucose polymers. In this study, we determined whether fatty acid synthesis was equally stimulated by a very-low-fat solid diet made with foods consumed typically. Four normal volunteers consumed the same very-low-fat formula diet for 25 d and then an isoenergetic solid food diet with 10% of energy as fat and 75% as starch, simple sugars, and fiber for 25 d. To measure fatty acid synthesis, the fatty acid compositions of the diets were matched to the composition of each subject's adipose tissue and compared with the composition of VLDL-triacylglycerol. In all subjects, the large increases in newly formed palmitate and decreases in linoleate in VLDL-triacylglycerol were quickly reversed by the solid food diet, and the fraction of de novo synthesized fatty acids in fasting VLDL-triacylglycerol decreased from 30-54% to 0-1%. In a second group of subjects, the stimulation of fatty acid synthesis by the formula diet with 75% glucose polymers was similarly reduced by a formula diet with amounts of fat, starch, and sugar chosen to mimic those of the solid food diet, but persisted after the addition of fiber or a diet with 75% sugar. In conclusion, an increase in fatty acid synthesis and palmitaterich, linoleate-poor VLDL-triacylglycerol induced by very-low-fat, high-sugar diets may be reduced by the substitution of dietary starch for sugar with potentially beneficial effects on cardiovascular health.

Signalling pathways involved in the stimulation of fatty acid synthesis by insulin.

Signalling pathways involved in the stimulation of fatty acid synthesis by insulin.

Multiple signalling pathways involved in the stimulation of fatty acid and glycogen synthesis by insulin in rat epididymal fat cells.

We have investigated the signalling pathways involved in the stimulation of glycogen and fatty acid synthesis by insulin in rat fat cells using wortmannin, an inhibitor of phosphatidylinositol 3-kinase, and rapamycin, which blocks activation of p70 ribosomal S6 protein kinase (p70S6K). Insulin produced a decrease in the activity of glycogen synthase kinase-3 which is likely to be important in the observed stimulation of glycogen synthase. Both of these actions were found to be sensitive to inhibition by wortmannin. Activation of three processes is involved in the stimulation of fatty acid synthesis from glucose by insulin, namely glucose uptake, acetyl-CoA carboxylase and pyruvate dehydrogenase. Whereas wortmannin largely abolished the effects of insulin on glucose utilization and acetyl-CoA carboxylase activity, it was without effect on the stimulation of pyruvate dehydrogenase. Although epidermal growth factor stimulated mitogen-activated protein kinase to a greater extent than insulin, it was unable to mimic the effect of insulin on glycogen synthase, glycogen synthase kinase-3, glucose utilization, acetyl-CoA carboxylase or pyruvate dehydrogenase. Rapamycin also failed to have any appreciable effect on stimulation of these parameters by insulin, although it did block the effect of insulin on p70S6K. We conclude that the activity of phosphatidylinositol 3-kinase is required for the effects of insulin on glycogen synthesis, glucose uptake and acetyl-Co-AN carboxylase, but is not involved in signalling to pyruvate dehydrogenase. Activation of mitogen-activated protein kinase or p70S6K, however, does not appear to be sufficient to bring about the stimulation of fatty acid or glycogen synthesis. Altogether is seems likely that at least four distinct signalling pathways are involved in the effects of insulin on rat fat cells.

Medium-chain fatty acids as short-term regulators of hepatic lipogenesis.

Short-term exposure of isolated rat hepatocytes to short- and medium-chain fatty acids led to an activation of acetyl-CoA carboxylase as measured in digitonin-permeabilized hepatocytes. Up to a certain concentration, typical for each of the fatty acids used, fatty acid-dependent activation of acetyl-CoA carboxylase coincided with an increase in the rate of fatty acid synthesis in intact hepatocytes, as determined by the incorporation of 3H from 3H2O water into fatty acids. At higher concentrations loss of stimulation of fatty acid synthesis occurred, but not the enhancement of carboxylase activity. With the fatty acids tested (C8:0-C14:0), the peak in fatty acid synthesis coincided with a peak in the level of malonyl-CoA. The onset of the stimulation of carboxylase activity coincided with the start of the peak in both fatty acid synthesis and malonyl-CoA. The longer the chain length of the fatty acid added, the lower the concentration at which the rate of fatty acid synthesis and the level of malonyl-CoA reached a peak and carboxylase activity started to become elevated. In cell suspensions incubated with increasing concentrations of fatty acids, accumulation of lactate decreased progressively. The latter observation, in combination with the fact that the activity of acetyl-CoA carboxylase is not always related to the rate of fatty acid biosynthesis, suggests that under these conditions not the activity of the carboxylase but the flux through the glycolytic sequence determines, at least in part, the rate of fatty acid synthesis de novo.

Effects of Dietary Cholesterol on Hepatic Metabolism of Free Fatty Acid and Secretion of VLDL in the Hamster

We reported previously that dietary cholesterol produces hepatic steatosis, increased secretion of the VLDL, and hypertriglyceridemia in the rat, the result of reduced oxidation of fatty acids, stimulation of fatty acid synthesis, and increased incorporation of fatty acid into hepatic triglyceride. The present study was conducted to determine whether these regulatory actions of dietary cholesterol on fatty acid metabolism also occur in the Golden Syrian hamster. In the hamster, dietary cholesterol (0.5 %) induced hypertriglyceridemia and hypercholesterolemia. Incorporation of [1- 14C] oleate into plasma and hepatic triglyceride was enhanced by dietary cholesterol. Experiments with perfused livers confirmed the stimulatory effect of dietary cholesterol on synthesis and secretion of VLDL-triglyceride and other lipids. These data indicate that increased formation of triglyceride in response to dietary cholesterol is not confined to the rat but may be a more general phenomenon.

Physiological modulation of the uptake and fate of glucose in brown adipose tissue.

Glucose utilization indices (GUI values) and rates of fatty acid synthesis in interscapular brown adipose tissue (IBAT) varied during the diurnal cycle in virgin and late-pregnant rats permitted unrestricted access to food. In virgin rats, peak GUI values and lipogenic rates were observed at the end of the dark (feeding) phase, but were not sustained during the light phase. Whereas peak GUI values were comparable with those observed during re-feeding after 24 h starvation, maximum rates of IBAT fatty acid synthesis in virgin rats during the diurnal cycle were only approx. 25% of those measured during re-feeding after 24 h starvation. Despite hyperphagia, GUI values during the diurnal cycle in late-pregnant rats fed ad libitum were generally lower than those of age-matched virgin controls. The percentage of pyruvate dehydrogenase complex present in the active form (PDHa) was also significantly decreased. Suppression of GUI and PDHa was not parallelled by suppression of fatty acid synthesis. IBAT GUI values in late-pregnant rats during chow re-feeding ad libitum after 24 h starvation were only 25% of those of corresponding virgin controls, and stimulation of fatty acid synthesis was also dramatically attenuated. The suppression of IBAT GUI values after re-feeding in pregnancy was not due to depletion of GLUT 4 protein. The results are discussed in relation to the importance of glucose as a precursor for fatty acid synthesis in IBAT.

Modulation of the activity of acetyl-CoA carboxylase and other lipogenic enzymes by growth hormone, insulin and dexamethasone in sheep adipose tissue and relationship to adaptations to lactation

The mechanisms whereby growth hormone and dexamethasone modulate the stimulation of fatty acid synthesis by insulin in adipose tissue from lactating and non-lactating sheep have been investigated. Maintenance of adipose tissue from wethers (castrated male sheep) in tissue culture for 24 or 48 h with insulin resulted in an increased proportion of acetyl-CoA carboxylase being present in the active state; this effect was enhanced by dexamethasone and was antagonized by growth hormone. Lactation results in a decrease in both the total acetyl-CoA carboxylase of sheep adipose tissue and the proportion of the enzyme in the active state. Maintenance of adipose tissue from lactating sheep in tissue culture for 48 h in the presence of insulin plus dexamethasone increased markedly the proportion of acetyl-CoA carboxylase in the active state and increased slightly the total activity of the enzyme. Both of these effects were prevented by actinomycin D, and the change in activation status was prevented by growth hormone. Tissue culture for 6 days showed that growth hormone could also prevent the ability of insulin plus dexamethasone to increase the total activity of the enzyme. Analogous studies showed that insulin, dexamethasone and growth hormone modulated the activities of other lipogenic enzymes, but the effects were proportionately smaller than for acetyl-CoA carboxylase. Insulin also increased total protein synthesis in adipose tissue, but this was not antagonized by growth hormone. The results suggest that the fall in fatty acid synthesis in sheep adipose tissue during lactation is due to a decrease in both the total acetyl-CoA carboxylase activity and the proportion of the enzyme in the active state; these changes are probably induced by known changes in the serum concentrations of insulin and growth hormone. Lactation appears to result in the loss of a protein that is required for activation of acetyl-CoA carboxylase by insulin; production of this component appears to be prevented by growth hormone.

Simultaneous stimulation of fatty acid synthesis and oxidation in rat hepatocytes by vanadate

When added to the hepatocyte incubation medium, vanadate increased the rate of fatty acid synthesis de novo as well as the activity of acetyl-CoA carboxylase, whereas it had no effect on the activity of fatty acid synthase. On the other hand, and despite elevating the intracellular levels of malonyl-CoA, vanadate diverted exogenous fatty acids into the oxidation pathway at the expense of the esterification route. This was concomitant to an increase in carnitine palmitoyltransferase I activity. All these effects were not significantly different between periportal and perivenous hepatocytes and were also evident in cells incubated in Ca 2+-free medium. Nevertheless, Ca 2+ ions enhanced carnitine palmitoyltransferase I activity in isolated liver mitochondria. In addition, the effects of vanadate on acetyl-CoA carboxylase and carnitine palmitoyltransferase I were only evident in a permeabilized-cell assay, disappearing upon cell disruption and isolation of the corresponding cell subfraction for enzyme assay. Results show that vanadate exerts specific insulin-like and non-insulin-like effects on hepatic fatty acid metabolism, and suggest that the intracellular concentration of malonyl-CoA is not the only factor responsible for the regulation of the fatty-acid-oxidative process in the liver.

Glucocorticoid stimulation of fatty acid synthesis in explants of human fetal lung

We examined the effects of glucocorticoids and thyroid hormone (T 3) on fatty acid synthesis, fatty acid composition and fatty acid synthetase activity in explants of human fetal lung (16–23 wk gestation). Explants were cultured 1–7 days in the absence (control) or presence of dexamethasone (10 nM) and/or T 3 (2 nM). In control explants fatty acid synthesis and fatty acid synthetase activity increased 200% and 455%, respectively, between 1 and 5 days. Dexamethasone (10 nM) stimulated fatty acid synthesis (tritiated water incorporation) 155% and fatty acid synthetase activity 117% after 5 days in culture. T 3 (2 nM) was not stimulatory, either alone or in the presence of dexamethasone. Dexamethasone increased the proportion of newly synthesized fatty acid recovered in phosphatidylcholine from 72% (control) to 90% ( P < 0.02) of total fatty acid. Dexamethasone stimulation of fatty acid synthetase activity was consistent with a receptor-mediated process: (1) stimulation was saturable and dose-dependent ( K d = 1.5 ± 0.3 nM); (2) the potency of glucocorticoid analogs and other steroids reflected their glucocorticoid activity; (3) stimulation was reversible when cortisol was removed from the medium. Stimulation by dexamethasone was apparent within 24 h of hormone exposure, and increased to a maximum between 4 and 6 days. Fatty acid synthetase activity was higher in Type II cells (3.54 ± 0.58 nmol malate/min per mg protein) than in fibroblasts from treated explants. Although both cell types responded to hormone treatment the stimulation was greater for Type II cells (200% vs. 75% increase). The fatty acid composition of PC showed increases in 14:0 and 16:1 with culture alone which were further stimulated by dexamethasone but not T 3. These results indicate glucocorticoid stimulation of fatty acid synthesis and are consistent with a key role for fatty acid synthetase in the hormonal induction of pulmonary surfactant phosphatidylcholine synthesis in cultured fetal lung.

The regulation of lipogenesis by thyroid hormone and its contribution to thermogenesis.

We have used the tritiated water method to quantitate the effects of thyroid hormone on lipogenesis in the rat and then determined the contribution of this process to thyroid hormone-induced thermogenesis. After thyroid hormone administration to hypothyroid animals, fatty acid synthesis rose after a lag time of 12-16 h and reached a plateau after 4-5 days. This is consistent with the kinetics of an increase in oxygen consumption measured by others in similar animals. A diurnal variation was maintained in all thyroid states, with the peak value in the middle of the dark period being 3-fold higher than the nadir. Fatty acid synthesis in the livers of hyperthyroid animals was 3- to 4-fold higher than that in euthyroid rats, which, in turn, was 3- to 5-fold higher than the rate observed in hypothyroid rats. Slightly smaller but similar fold increases were measured in epididymal fat. A stimulation of fatty acid synthesis by thyroid hormone was also measured in the rest of the carcass, with hyperthyroid rates being twice those in hypothyroid animals. The contribution of the liver was much greater in hyperthyroid rats (34% of total fatty acid synthesis) than in hypothyroid animals (5%). The energy costs of this synthesis were calculated and compared to published values for total oxygen consumption in different thyroid states. Thus, 6-10% of the total increment in oxygen consumption between hyperthyroid and hypothyroid animals could be attributed to lipogenesis, depending on which published figures were used. About 3% of this increment was due to the liver alone.