The control of stearoyl-CoA desaturase by dietary linoleic acid

Stearoyl-CoA desaturase 1: A potential target for non-alcoholic fatty liver disease?-perspective on emerging experimental evidence

Non-alcoholic fatty liver disease (NAFLD) is a progressive disease and one of the leading causes of death. An unnamed disease has become a global epidemic disease of public health concern. This spectrum of diseases manifests itself with initial accumulation of excessive triglycerides (due to de novo lipogenesis) in the hepatocytes, leading to simple steatosis. Although its aetiology is multi-factorial, lifestyle changes (diet and physical activity) are considered to be the key thriving factors. In this context, high fructose consumption is associated with an increased risk for developing NAFLD in humans, while high-fructose feeding to experimental animals results in hepatic steatosis and non-alcoholic steatohepatitis, by increasing hepatic lipogenesis. Among several lipogenic genes, the endoplasmic reticulum-bound stearoyl-CoA desaturase 1 (SCD1) is the key determinant of triglycerides biosynthesis pathway, by providing monounsaturated fatty acids, through the incorporation of a double bond at the delta-9 position of saturated fatty acids, specifically, palmitic (C16:0) and stearic (C18:0) acids, yielding palmitoleic (C16:1) and oleic (C18:1) acids, respectively. Various experimental studies involving SCD1 gene knockout and diet-induced rodent models have demonstrated that SCD1 plays a key role in the development of NAFLD, by modulating hepatic lipogenesis and thus triglyceride accumulation in the liver. Several pharmacological and dietary intervention studies have shown the benefits of inhibiting hepatic SCD1 in the pathogenesis of NAFLD. In this review, we give an overview of SCD1 in NAFLD, based on the current experimental evidence and the translational applicability of SCD1 inhibition in human NAFLD conditions, besides discussing the limitations and way-forward.

Interactive effects of maternal and weaning high linoleic acid intake on hepatic lipid metabolism, oxylipins profile and hepatic steatosis in offspring

Non-alcoholic fatty liver disease (NAFLD) has been described as a hepatic manifestation of the metabolic syndrome. When several studies correlated maternal linoleic acid (LA) intake with the development of obesity, only few links have been made between n-6 fatty acid (FA) and NAFLD. Herein, we investigated the influence of both maternal and weaning high LA intake on lipid metabolism and susceptibility to develop later metabolic diseases in offspring. Pregnant rats were fed a control-diet (2% LA) or a LA-rich diet (12% LA) during gestation and lactation. At weaning, offspring was assigned to one of the two diets, i.e., either maintained on the same maternal diet or fed the other diet for 6 months. Physiological, biochemical parameters and hepatic FA metabolism were analyzed. We demonstrated that the interaction between the maternal and weaning LA intake altered metabolism in offspring and could lead to hepatic steatosis. This phenotype was associated with altered hepatic FA content and lipid metabolism. Interaction between maternal and weaning LA intake led to a specific pattern of n-6 and n-3 oxylipins that could participate to the development of hepatic steatosis in offspring. Our findings highlight the significant interaction between maternal and weaning high LA intake to predispose offspring to later metabolic disease and support the predictive adaptive response hypothesis.

Influence of Feeding Linseed on SCD Activity in Grazing Goat Mammary Glands

Simple Summary

The effect of linseed feeding on stearoyl-CoA desaturase (SCD) activity was studied in dairy goats. SCD acts on the synthesis of milk conjugated linoleic acids (CLAs), considered highly important for human health. Linseed feeding significantly changed SCD activity, milk fat, and fatty acid profile; in particular, CLAs were higher in treated animals with potential benefits for human health. Because of the increasing care of consumers for the healthy aspects of foods, results increase the knowledge of beneficial effects of goat milk due to animal nutrition.

Abstract

The effects of linseed feeding on the stearoyl-CoA desaturase (SCD) activity were evaluated on grazing dairy goats divided into two homogeneous groups (C, control, and L, treated) fed the same amount of concentrate which, for group L was supplemented with linseed. Milk yield was unaffected by the treatment. Group L showed significantly higher milk fat (4.10% vs 2.94%, p < 0.01) than group S. Within milk fatty acids, group C showed significantly higher levels of saturated fatty acids and lower values of mono-unsaturated and polyunsaturated fatty acids. In group L, total CLAs were higher than in group S (0.646% vs 0.311%; p < 0.01) mainly because of the differences in CLA cis9 trans 11 (0.623% vs 0.304%; p < 0.01). In treated animals, SCD activity, measured as cis9 C14:1/C14:0, was lower than in the control group, mainly in July and August.

Multi-population GWAS and enrichment analyses reveal novel genomic regions and promising candidate genes underlying bovine milk fatty acid composition

Background

The power of genome-wide association studies (GWAS) is often limited by the sample size available for the analysis. Milk fatty acid (FA) traits are scarcely recorded due to expensive and time-consuming analytical techniques. Combining multi-population datasets can enhance the power of GWAS enabling detection of genomic region explaining medium to low proportions of the genetic variation. GWAS often detect broader genomic regions containing several positional candidate genes making it difficult to untangle the causative candidates. Post-GWAS analyses with data on pathways, ontology and tissue-specific gene expression status might allow prioritization among positional candidate genes.

Results

Multi-population GWAS for 16 FA traits quantified using gas chromatography (GC) in sample populations of the Chinese, Danish and Dutch Holstein with high-density (HD) genotypes detects 56 genomic regions significantly associated to at least one of the studied FAs; some of which have not been previously reported. Pathways and gene ontology (GO) analyses suggest promising candidate genes on the novel regions including OSBPL6 and AGPS on Bos taurus autosome (BTA) 2, PRLH on BTA 3, SLC51B on BTA 10, ABCG5/8 on BTA 11 and ALG5 on BTA 12. Novel genes in previously known regions, such as FABP4 on BTA 14, APOA1/5/7 on BTA 15 and MGST2 on BTA 17, are also linked to important FA metabolic processes.

Conclusion

Integration of multi-population GWAS and enrichment analyses enabled detection of several novel genomic regions, explaining relatively smaller fractions of the genetic variation, and revealed highly likely candidate genes underlying the effects. Detection of such regions and candidate genes will be crucial in understanding the complex genetic control of FA metabolism. The findings can also be used to augment genomic prediction models with regions collectively capturing most of the genetic variation in the milk FA traits.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5573-9) contains supplementary material, which is available to authorized users.

Influence of outdoor rearing and oleic acid supplementation on lipid characteristics of muscle and adipose tissues from obese Alentejano pigs

Data evaluating the effect of oleic-acid-enriched diets and outdoor rearing (with access to pasture) on the quality of Alentejano (AL) pork and fat are limited. The aim of this work was to study the effects of feeding low (LO) or high oleic acid (HO) diets to AL pigs reared in individual pens (IND) or outdoors (OUT) with access to pasture. Biceps femoris and dorsal subcutaneous fat (DSF) samples were collected to analyse chemical composition and lipids fatty acid (FA) profile. The expression of seven genes involved in lipogenesis, lipolysis and FA transport, as well as two transcription factors were also evaluated in the muscle. B. femoris and DSF presented lower lipid content in OUT-reared than in IND-reared pigs. Two genes involved in lipogenesis, ACACA and FASN, were under-expressed in B. femoris from OUT-reared pigs, while two involved in lipolysis, LPL and HSL were over-expressed. HO diet also significantly reduced FASN expression. Both rearing system and diet affected the FA profile of tissues, particularly DSF. Overall, OUT-reared pigs presented lower n6 to n3 FA ratios in all tissues, and lower monounsaturated (MUFA) and higher polyunsaturated fatty acids (PUFA) content in DSF. When compared to LO-fed ones, HO-fed pigs presented higher oleic acid proportions in all tissues, and lower SFA, higher MUFA and PUFA levels, as well as lower saturation, atherogenic and thrombogenic indexes in DSF. This study shows that rearing system and oleic acid supplementation have complementary effects and influence the nutritional quality of pork and DSF.

Stearoyl-CoA desaturase indexes and n-6/n-3 fatty acids ratio as biomarkers of cardiometabolic risk factors in normal-weight rabbits fed high fat diets

Background

Biomarkers for cardiometabolic risk (CMR) factors would be important tools to maximize the effectiveness of dietary interventions to prevent cardiovascular diseases. Thus, the aim of this work was to analyze stearoyl-CoA desaturase (SCD) indexes and n-6/n-3 fatty acids (FA) ratio as biomarkers of CMR induced by feeding rabbits on high fat diets (HFDs).

Methods

Rabbits were fed either regular diet or 18 % fat in regular diet (HFD) or 1 % cholesterol diet (HD) or diet containing 1 % cholesterol and 18 % fat (HFD-HD) during 6 weeks. Body weights (BW), blood pressure, visceral abdominal fat (VAF) and glucose tolerance test were determined. Total cholesterol (TC), low density lipoprotein-cholesterol (LDL-C), high density lipoprotein-cholesterol (HDL-C), triglycerides (TG), fasting glucose (FG), and FA levels from plasma were measured. SCD indexes were calculated as product/precursor ratios of individual FA.

Results

BW was similar in all diet groups. HD increased TC, LDL-C, HDL-C, and TG. HFD increased TG, VAF and FG, and decreased HDL-C. The addition of HFD to HD joined to dyslipidemia increased VAF and FG. SCD indexes were increased and n-6/n-3 was unchanged in HD. SCD indexes were reduced and n-6/n-3 FA ratio was increased in HFD and HFD-HD. CMR factors were correlated positively with n-6/n-3 FA ratio. Although VAF had a stronger correlation with n-6/n-3 FA ratio than with SCD indexes, VAF was associated independently to both markers.

Conclusions

HFD simulating lipid composition of the average Western-style diet induced experimental rabbit models of normal-weight metabolic syndrome (MS). SCD indexes and n-6/n-3 were modified according to the type of dietary fat. Considering that VAF and CMR factors appear to be stronger associated to n-6/n-3 FA ratio than to SCD indexes, n-6/n-3 FA ratio may be a better biomarker of MS and CMR in normal-weight subjects than SCD indexes.

Effect of high versus low doses of fat and vitamin A dietary supplementation on fatty acid composition of phospholipids in mice

Dietary fat and vitamin A provide important precursors for potent bioactive ligands of nuclear hormone receptors, which regulate various enzymes involved in lipid homeostasis, metabolism and inflammation. We determined the effects of dietary fat and dietary vitamin A on hepatic expression of two fatty acid metabolizing enzymes, elongase 6 (ELOVL6) and stearoyl-coenzyme A desaturase 1 (SCD1) and the concentration of saturated fatty acids (SAFA) and monounsaturated fatty acid (MUFA) of phospholipids in serum and liver. Mice (n = 6) were fed 4 weeks with diets containing 2, 5 and 25 % of fat or vitamin A (0, 2,500 and 326,500 RE/kg as retinyl palmitate). MUFAs and SAFAs were measured using GC and ESI-MS/MS. Hepatic expression of metabolizing enzymes was determined using QRT-PCR. ELOVL6 was significantly down-regulated in response to a high-fat diet (p < 0.001) and significantly up-regulated in response to low-fat diet (p < 0.05). SCD1 expression was significantly lower in high- versus low-fat diet (p < 0.05). The vitamin A content in the diet did not influence the hepatic expression of both enzymes. In plasma, the amounts of MUFAs bound to phospholipids significantly decreased in response to a high-fat diet and increased after a low-fat diet. This tendency was also observed in the liver for various phospholipids sub-classes. In summary, this study shows that fat content in the diet has a stronger impact than the content of vitamin A on hepatic gene expression of SCD1 and ELOVL6 and thereby on MUFA and SAFA concentrations in liver and plasma.

Omega-6 fat supplementation alters lipogenic gene expression in bovine subcutaneous adipose tissue

In contrast to rodents, adipose tissue serves as the major site of lipogenesis and storage reservoir for excess dietary energy in cattle. Research in rodents shows that adding corn oil (57% C18:2 n-6) to the diet alters lipogenesis enhancing deposition of omega-6 fatty acids. This study examines changes in lipogenic gene expression of subcutaneous adipose tissue from eighteen steers fed increasing levels of dietary corn oil [0 (NONE), 0.31 kg/d (MED) and 0.62 kg/d (HI)] using two platforms, qPCR and microarray. The results show that MED level of oil supplementation up-regulates gene expression of key lipogenic enzymes but that as oil supplementation reaches HI level mRNA encoding lipogenic enzymes responsible for de novo synthesis and desaturation are down-regulated. Changes in specific lipogenic mRNA levels are correlated with changes in tissue fatty acid composition where de novo and desatured fatty acids were reduced with the highest level of oil supplementation.

Ancient and modern duplication events and the evolution of stearoyl-CoA desaturases in teleost fishes

Stearoyl-CoA desaturases (SCDs) are key enzymes of fatty acid biosynthesis whose regulation underpins responses to dietary, thermal, and hormonal treatment. Although two isoforms are known to exist in the common carp and human and four in mouse, there is no coherent view on how this gene family evolved to generate functionally diverse members. Here we identify numerous new SCD homologs in teleost fishes, using sequence data from expressed sequence tag (EST) and cDNA collections and genomic model species. Phylogenetic analyses of the deduced coding sequences produced only partially resolved molecular trees. The multiple SCD isoforms were, however, consistent with having arisen by an ancient gene duplication event in teleost fishes together with a more recent duplication in the tetraploid carp and possibly also salmonid lineages. Critical support for this interpretation comes from comparison across all vertebrate groups of the gene order in the genomic environments of the SCD isoforms. Using syntenically aligned chromosomal fragments from large-insert clones of common carp and grass carp together with those from genomically sequenced model species, we show that the ancient and modern SCD duplication events in the carp lineage were each associated with large chromosomal segment duplications, both possibly linked to whole genome duplications. By contrast, the four mouse isoforms likely arose by tandem duplications. Each duplication in the carp lineage gave rise to differentially expressed SCD isoforms, either induced by cold or diet as previously shown for the recent duplicated carp isoforms or tissue specific as demonstrated here for the ancient duplicate zebrafish isoforms.

Signalling pathways controlling fatty acid desaturation

Microorganisms, plants and animals regulate the synthesis of unsaturated fatty acids (UFAs) during changing environmental conditions as well as in response to nutrients. Unsaturation of fatty acid chains has important structural roles in cell membranes: a proper ratio of saturated to UFAs contributes to membrane fluidity. Alterations in this ratio have been implicated in various disease states including cardiovascular diseases, immune disorders, cancer and obesity. They are also the major components of triglycerides and intermediates in the synthesis of biologically active molecules such as eicosanoids, which mediates fever, inflammation and neurotransmission. UFAs homeostasis in many organisms is achieved by feedback regulation of fatty acid desaturases gene transcription. Here, we review recently discovered components and mechanisms of the regulatory machinery governing the transcription of fatty acid desaturases in bacteria, yeast and animals.

Monounsaturated fatty acyl-coenzyme A is predictive of atherosclerosis in human apoB-100 transgenic, LDLr-/- mice

ACAT2, the enzyme responsible for the formation of cholesteryl esters incorporated into apolipoprotein B-containing lipoproteins by the small intestine and liver, forms predominantly cholesteryl oleate from acyl-CoA and free cholesterol. The accumulation of cholesteryl oleate in plasma lipoproteins has been found to be predictive of atherosclerosis. Accordingly, a method was developed in which fatty acyl-CoA subspecies could be extracted from mouse liver and quantified. Analyses were performed on liver tissue from mice fed one of four diets enriched with one particular type of dietary fatty acid: saturated, monounsaturated, n-3 polyunsaturated, or n-6 polyunsaturated. We found that the hepatic fatty acyl-CoA pools reflected the fatty acid composition of the diet fed. The highest percentage of fatty acyl-CoAs across all diet groups was in monoacyl-CoAs, and values were 36% and 46% for the n-3 and n-6 polyunsaturated diet groups and 55% and 62% in the saturated and monounsaturated diet groups, respectively. The percentage of hepatic acyl-CoA as oleoyl-CoA was also highly correlated to liver cholesteryl ester, plasma cholesterol, LDL molecular weight, and atherosclerosis extent. These data suggest that replacing monounsaturated with polyunsaturated fat can benefit coronary heart disease by reducing the availability of oleoyl-CoA in the substrate pool of hepatic ACAT2, thereby reducing cholesteryl oleate secretion and accumulation in plasma lipoproteins.

Obesity - a perspective based on the biochemical interrelationship of lipids and carbohydrates

Many factors affect the onset of obesity including satiety control, reduced levels of physical exercise as well as hormonal and genetic parameters which influence the metabolic pathways leading to the net accumulation of triacylglycerol (TAG). The predominant fatty acid of human adipose tissue TAGs is oleic acid, reflecting primarily the composition of the diet but also the product of de novo lipogenesis. Consequently, both carbohydrates and lipids are potential sources of these stored fats. Many studies have been carried out using a variety of differing experimental protocols on healthy, obese or diabetic humans and animals in positive or neutral energy balance to establish the underlying molecular basis for obesity particularly in humans. This short review discusses the interdependence and control of the metabolism of lipids and carbohydrates as it relates to lipogenesis and proposes a unified hypothesis for obesity which brings together a number of different approaches focusing on (i) the interaction of dietary fat and carbohydrate, which typically represent approximately 80% of the daily caloric intake, and their role in the synthesis of TAGs, (ii) the biochemical pathways which control the amount of TAG produced by controlling the composition of their fatty acids via the action of stearoyl-CoA desaturase (SCD), (iii) the control of lipogenesis and SCD by dietary polyunsaturated fatty acid (PUFA) and (iv) the interaction of PUFAs with the transcription factors, peroxisome proliferator activated receptors (PPAR) alpha and gamma, which maintain the balance between oxidation and storage of lipids. The hypothesis focuses on the central role of stearoyl-CoA desaturase (SCD) and its inhibition by polyunsaturated fatty acids (PUFAs) acting via transcription factors based upon data obtained from both animal and human studies. From these observations it should be possible to determine the relevance of the hypothesis to humans and to speculate how these aspects of metabolism may impact the risk of developing related diseases such as coronary heart disease, Type 2 diabetes and certain forms of cancer.

POLYUNSATURATED FATTY ACID REGULATION OF GENES OF LIPID METABOLISM

Apart from being an important macronutrient, dietary fat has recently gained much prominence for its role in regulating gene expression. Polyunsaturated fatty acids (PUFAs) affect gene expression through various mechanisms including, but not limited to, changes in membrane composition, intracellular calcium levels, and eicosanoid production. Furthermore, PUFAs and their various metabolites can act at the level of the nucleus, in conjunction with nuclear receptors and transcription factors, to affect the transcription of a variety of genes. Several of these transcription mediators have been identified and include the nuclear receptors peroxisome proliferator-activated receptor (PPAR), hepatocyte nuclear factor (HNF)-4alpha, and liver X receptor (LXR) and the transcription factors sterol-regulatory element binding protein (SREBP) and nuclear factor-kappaB (NFkappaB). Their interaction with PUFAs has been shown to be critical to the regulation of several key genes of lipid metabolism. Working out the mechanisms by which these interactions and consequent effects occur is proving to be complicated but is invaluable to our understanding of the role that dietary fat can play in disease management and prevention.

Polyunsaturated fatty acids and gene expression in mammalian systems

Over the last 30 years it has become apparent that specific dietary fatty acids are capable of regulating, either directly or indirectly through various signal pathways, the expression of numerous genes, either positively or negatively. Such nutrient-gene interactions have important effects on cell metabolism, differentiation and growth, and ultimately on disease processes. The present review describes some of the more important fatty acid-gene interactions in relation to health and disease in mammalian species, and focuses on the underlying cell signal mechanisms, including various transcription factors, affected by fatty acids and some of their oxygenated derivatives, e.g. the eicosanoids. The review also attempts to clarify some of the complexities of the effects of fatty acids by suggesting a possible overriding regulation by the redox status of the cell. The latter will at least stimulate controversy in this exciting area of lipid research.

Regulation by carbohydrate and clofibric acid of palmitoyl-CoA chain elongation in the liver of rats

Regulation of palmitoyl-CoA chain elongation (PCE) and its contribution to oleic acid formation were investigated in rat liver in comparison with stearoyl-CoA desaturase (SCD). Hepatic PCE activity was induced by the administration of 20% wt/vol glucose or fructose in the drinking water of normal rats. In streptozotocin-induced diabetic rats, the activities of both PCE and SCD were suppressed, and fructose, but not glucose, feeding caused an increase in the activities of both enzymes. Treatment of normal rats with clofibric acid in combination with carbohydrate further increased PCE, but not SCD, activity. FA analysis of hepatic lipids revealed that the proportion of oleic acid (18:1 n-9) increased upon administration of carbohydrate or clofibric acid. The treatment of rats with clofibric acid in combination with carbohydrate greatly increased the proportion of 18:1 n-9. A significant correlation was observed between PCE activity and the hepatic proportion of 18:1 n-9 (r2 = 0.874, P < 0.01), whereas the relationship between SCD activity and the proportion of 18:1 n-9 was not significant (r2 = 0.552, P > 0.05). Taken together, these results suggest that carbohydrate induces PCE as well as SCD activity to increase the hepatic 18:1 content in rat liver, and the increased PCE activity seems to be responsible for the further increase in 18:1 n-9 when carbohydrate is administered in combination with clofibric acid.

Stearoyl-CoA desaturase, a short-lived protein of endoplasmic reticulum with multiple control mechanisms

Stearoyl-CoA desaturase (SCD) is a short-lived, polytopic membrane-bound non-heme iron enzyme localized primarily in the endoplasmic reticulum. SCD is required for the biosynthesis of monounsaturated fatty acids, and plays a key role in hepatic synthesis of triglycerides and very-low-density lipoproteins. The intracellular concentration of SCD fluctuates in a wide range in response to complex and often competing hormonal and dietary factors. A combination of transcriptional regulation and rapid protein degradation produces transient elevations of SCD enzyme activity in response to physiologic demands. Dysregulation of SCD has been implicated in non-alcoholic fatty liver disease, hyperlipidemia, and obesity.

Dietary cholesterol modulates delta6 and delta9 desaturase mRNAs and enzymatic activity in rats fed a low-eFA diet

The effects of a 1% addition of cholesterol to a diet low in EFA on FA desaturases were examined. The administration of cholesterol markedly increased the esterified cholesterol content in microsomes and total liver lipids from the first day, whereas the proportion of free cholesterol remained unaltered throughout the treatment. An excellent homeostasis in the free cholesterol content was apparently evoked by the acyl-CoA cholesterol acyltransferase. The cholesterol esters were mainly oleate, palmitate, and stearate, and the addition of cholesterol increased the relative proportions of cholesterol palmitoleate and oleate. The addition of cholesterol to a low-EFA diet induced, as in animals fed a high-EFA diet, a marked increase in liver stearoyl-CoA desaturase-1 mRNA and enzyme activity. This increased activity apparently evoked a similar enhancement of palmitoleic and oleic acids in total and microsomal liver lipids. The cholesterol-rich diet depressed the liver A6 and delta5 desaturase activity. However, the abundance of delta6 desaturase mRNA was not modified throughout the treatment. This indicates that the depressive effect is evoked at a step beyond that controlled by the mRNA level. The depression of both enzymatic activities was consistent with the decrease in the percentages of arachidonic acid and DHA in total and microsomal liver lipids. Taken together, these results indicate that through its modulating effect on the desaturases, dietary cholesterol may lead an animal or human fed low-EFA diet to a true deficiency by the decreased synthesis of the highly polyunsaturated acids derived from linoleic and alpha-linolenic acids.

Long term feeding effects of heated and fried oils on lipids and lipoproteins in rats

Long term feeding effects (20 weeks) of heated and fried oils at 5 and 20% level in the diet on growth, plasma and tissue lipids were studied in rats. Three vegetable oils of widespread usage viz., peanut oil, sesame oil and coconut oil with varying saturation and unsaturation were chosen for the study. No significant difference in growth rate, feed efficiency ratio, and liver weights were observed. Higher plasma cholesterol levels were observed in heated oil fed group of rats compared to corresponding fried oil groups. Low levels of HDL-c and increased LDL-c and VLDL-c were noted in heated/fried oil groups. Significantly low levels (p < 0.001) of triglyceride were observed in heated/fried sesame oil group of rats. No significant change in phospholipid was observed in any of the groups. Significantly low levels of liver cholesterol and high triglyceride levels (at 20%) were observed in coconut oil group. The fatty acid composition of plasma and liver reflected the type of diet consumed. Although linoleic acid levels were quite low in some of the heated/fried oil groups the arachidonic acid levels were quite high indicating repair mechanism. The results of the study however do not present any deleterious effect on growth, plasma and tissue lipid profile of rats as the conditions employed for heating/frying were not too drastic and the oils were not heat abused.

Use of deuterium oxide to measure de novo fatty acid synthesis in normal subjects consuming different dietary fatty acid composition1

The effect of dietary linoleic (C18:2n-6) and palmitic acids (C16:0) on rate of hepatic de novo fatty acid synthesis was assessed in normal subjects. The diet was formulated to provide combinations of high and low levels of C18:2n-6 and C16:0. After 21 days of diet treatment, plasma triacylglycerol level and incorporation of deuterium into the plasma very low density lipoprotein triacylglycerol (VLDL-TG) pool over 24 hours was measured. Plasma triacylglycerol levels were within the normal range. Increasing dietary intake of linoleic acid decreased plasma triacylglycerol level when subjects consumed a low level of dietary palmitic acid. The relative and net amount of de novo synthesized fatty acid in the plasma VLDL-TG pool was not influenced by the diet treatments. A relationship between plasma triacylglycerol level and rate of hepatic de novo fatty acid synthesis was observed.

Analysis of the putative role of 24-carbon polyunsaturated fatty acids in the biosynthesis of docosapentaenoic (22:5n-6) and docosahexaenoic (22:6n-3) acids

The recent literature on the putative involvement of a single cycle of peroxisomal beta-oxidation of 24:5n-6 and 24:6n-3 polyunsaturated fatty acids in the biosynthesis of the respective docosapentaenoic (22:5n-6) and docosahexaenoic (22:6n-3) fatty acids is critically reviewed. Present evidence suggests that in vitro data in support of the above proposition is an artifact of a low 2,4-dienoyl-CoA reductase activity due to depletion of NADPH resulting from incubation conditions. Kinetic studies with radiolabeled precursors in cell cultures have shown lower initial rates of labeling of 24:6n-3 than that of 22:6n-3, indicating that 24:6n-3 is an elongation product of 22:6n-3 rather than its precursor. Analysis of other literature data supports the proposal that 22:5n-6 and 22:6n-3 are synthesized in mitochondria via channeled carnitine-dependent pathways involving separate n-6- and n-3-specific desaturases. It is proposed that impaired peroxisomal function in some peroxisomal disorders is a secondary consequence of defective mitochondrial synthesis of 22:6n-3; moreover, some disorders of peroxisomal beta-oxidation show normal or increased 22:5n-6 concentrations, indicating that 22:5n-6 is synthesized by independent desaturases without peroxisomal involvement.

Polyunsaturated fatty acid regulation of gene expression

For the past three decades, polyunsaturated fatty acids (PUFA) have been recognized as important energy sources and membrane components. PUFA also play key roles in many cellular events, such as gene regulation. Most recently, research has focused on identifying the mechanisms by which PUFA modulate gene transcription, mRNA stability and cellular differentiation. It is the purpose of this review to examine the effects of PUFA on gene expression in lipogenic as well as other tissues. Because the (n-3) and (n-6) series of PUFA are intimately involved in gene regulation, they will be the focus of review. The effects of other fatty acid families on gene expression are reviewed elsewhere.

Localization of a polyunsaturated fatty acid response region in stearoyl-CoA desaturase gene 1

Polyunsaturated fatty acids (PUFA) repress stearoyl-CoA desaturase gene 1 (SCD1) expression in liver and adipose tissues. We used HepG2 cells to localize genetic regulatory elements for PUFA in the SCD1 5'-flanking region. A chimeric reporter gene construct containing the 4.3 kb SCD1 putative promoter was transiently transfected into HepG2 cells, which were then treated with various fatty acids. We observed greater than 60% repression of transcription with 18:3n - 3 and 75% repression with 20:4n - 6 and 20:5n - 3. No significant change was seen with 18:0. Using smaller SCD1 chimeric constructs, we localized the genetic regulatory region to a 237 bp sequence within the SCD1 proximal promoter. DNA mobility shift analysis with HepG2 and mouse liver nuclear extracts demonstrated specific binding of nuclear proteins to this region. Mobility shift analysis with nuclear extract from 3T3-L1 adipocytes showed a similar pattern of protein binding. Competitive DNA mobility shift analysis identified a 60 bp region containing sites that specifically bind and compete for nuclear proteins. This region conferred responsiveness to PUFA when placed in a heterologous promoter. A homologous region in the stearoyl-CoA desaturase gene 2 (SCD2) promoter also mediated PUFA-specific repression in transfection experiments. These data suggest that a common transcriptional mechanism may exist in liver and adipose tissues for inhibition of lipogenesis by PUFA.

Dietary cholesterol and the activity of stearoyl CoA desaturase in rats: evidence for an indirect regulatory effect

The effect of cholesterol on stearoyl CoA desaturase (SCD) was investigated. Previous work had shown that the addition of cholesterol to the diet of rats produced higher liver SCD activity compared to non-cholesterol-fed controls. We have confirmed this result and investigated the mechanism responsible for this cholesterol-induced higher SCD activity. Rats were fed either a 10% corn oil (CO) or a 10% corn oil/1% cholesterol (CO/CH) diet for 1, 3, or 7 days. SCD mRNA abundance was 3.3, 1.9, and 2.4 times greater in livers from CO/CH-fed animals after 1, 3, and 7 days, respectively. Northern hybridization of RNA from kidney, intestinal mucosa, heart, adipose, and liver demonstrated that cholesterol feeding specifically altered liver SCD mRNA. Liver esterified cholesterol content increased 27-fold with cholesterol feeding. This esterified cholesterol increase was accompanied by a proportionately greater increase in oleic acid compared to other fatty acids. These studies indicate that cholesterol does influence the expression of SCD specifically in the liver and suggest that the product, oleic acid, is preferentially esterified to cholesterol in the liver. Preliminary liver nuclear run-on assays from rats fed CO or CO/CH diets for 1 and 3 days indicate that transcription regulation is not a factor.

Effect of high fat diets with and without cholesterol on erythrocyte and tissue fatty acids in rats

Male weanling wistar rats were fed synthetic diets containing 20% safflower oil (SFO) or palm oil (PO) with and without cholesterol for a period of six weeks. Erythrocyte membranes were isolated and their fatty acid composition were determined at the end of the experiment. Besides the fatty acid composition of the kidney and spleen lipids were also determined. Erythrocyte membrane, kidney fatty acids of rats fed safflower oil (SFO) had in the majority of variants a higher level of n-6 fatty acids when compared to palm oil group. However, in the spleen, the level of n-6 fatty acids in the palm oil groups were higher than in the safflower oil group. In general the arachidonic acid [20:4 (n-6)] levels were higher in the spleen and erythrocytes particularly in the cholesterol fed groups when compared to the kidney levels. Palm oil fed rats had a higher level of palmitic (16:0) and oleic [18:1 (n-9)] acids. Rats fed diets containing cholesterol significantly reduced the level of stearic (18:0) but increased the level of oleic [18:1 (n-9)], linoleic [18:2 (n-3)], and arachidonic acids [20:4 (n-6)]. The ratios such as 18:1 (n-9)/18:2 (n-6), 18:1 (n-9)/18:0, 18:2 (n-6)/20:4 (n-6) and 20:4 (n-6)/18:2 (n-6) are all indicative of the normal activity of enzymes involved in the desaturation and elongation. Thus these studies indicate that addition of cholesterol can modify the fatty acid composition in erythrocytes, kidney and spleen lipids.

Effect of supplementation of milk fat with peanut oil on blood lipids and lipoproteins in infants

Three different groups of infants were fed with different formulae based on milk fat. Group I received cow's milk fat formulae with 20% butter fat whereas groups II and III received a formulae which was supplemented with 50 and 33% of peanut oil supplementation in 20% milk fat respectively. Anthropometric measurements, cholesterol, triglyceride, lipoproteins and plasma fatty acids were followed up to a period of 6 months. The results indicated that cow's milk-fed infants had higher cholesterol levels (P < 0.01) than the other two groups. No significant differences with respect to high-density lipoproteins (HDLs) were found, whereas low-density lipoproteins (LDLs) and very low-density lipoproteins (VLDls (VLDls) were found to be increasing up to a period of 6 months. No significant differences were observed with respect to saturated fatty acids and oleic acid (18:1) levels whereas linoleic acid (18:2) clearly showed a proportional relationship between the intake and plasma levels, indicating a positive correlation. Arachidonic acid (20:4) did not, however, show a proprotionate relationship with respect to linoleic acid (18:2) intake. The triene/tetraene, oleic/linoleic, linoleic/arachidonic and total n6 fatty acids were all normal indicating normal activity of desaturase and elongase enzymes for the optimal utilisation of linoleic acid. Thus, the present study suggests that a vegetable oil such as peanut oil could be used in milk fat to improve the essential fatty acid (EFA) status of infants.

Effect of long term feeding of rice bran oil upon lipids and lipoproteins in rats

The effects of feeding two levels of rice bran oil (RBO) on the growth, lipid parameters, and fatty acid composition of the plasma and liver of rats (Wistar strain) were compared with those produced on animals which had been fed the same levels of peanut oil (PNO). The control animals were fed synthetic diets containing 5 and 20% peanut oil (PNO) and the experimental groups were fed similar diets, containing the same level of rice bran oil (RBO). There was no significant difference with respect to the organ weights between the control and the experimental groups. In general, groups fed 20% oil gained more weight than groups fed 5% oil. The animals which received rice bran oil in their diet had, in general, comparatively lower levels of cholesterol, triglycerides and phospholipids. On the other hand, animals receiving 20% rice bran oil in their diet, showed an increase of 20% in high density lipoproteins (HDL-C), within 18 weeks (p < 0.05), when compared to the animals fed with peanut oil. Similarly, low density lipoprotein cholesterol (LDL-C) and very low density lipoprotein cholesterol (VLDL-C) were lower in RBO-fed groups, than in the PNO-fed groups. There was, however, no significant differences in the cholesterol/phospholipid (C/P) ratio of the two groups. Analysis of plasma and of liver fatty acids indicated, in a general way, the type of fat consumed. There were no significant difference in the P/S ratio, nor any in the oleic/linoleic, oleic/stearic, palmitoleic/palmitic, oleic/palmitic, and oleic/palmitoleic ratios.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of linoleic acid on desaturation and uptake of deuterium-labeled palmitic and stearic acids in humans

Objectives of this study were to investigate the desaturation of stearic acid (18:0) and palmitic acid (16:0), to determine if differences in their metabolism provide a reasonable explantation for differences in their effect on serum cholesterol levels, and to investigate the affect of linoleic acid on delta 9-desaturase products in man. Deuterium-labeled 16:0 and 18:0 were used to follow the metabolism of these fatty acids in young adult male subjects that were pre-fed diets containing two different levels of linoleic acid. Results indicate that absorption of 16:0 and 18:0 was similar when all components of the mixture used to formulate the deuterated fat mixture were kept above the melting point of tristearin. The percent of 18:0 desaturated to 9c-18:1 was higher than the percent of 16:0 desaturated to 9c-16:1 (9.2% vs. 3.9%). The subject-to-subject variability suggests that differences in ability to desaturate saturated fatty acids may be related to the variability observed in response of serum cholesterol levels to dietary saturated fatty acids. Data for the distribution of 16:0 and 18:0 between triacylglycerol and phosphatidylcholine (PC) was markedly different. Based on PC data, phospholipid acyltransferase selectivity was about 2-fold higher for 18:0 than for 16:0. A 2-fold difference in the linoleic acid content of the pre-fed diets had little influence on desaturation or distribution of 16:0 and 18:0 between plasma lipid classes. A deuterium isotope effect was estimated to reduce delta 9-desaturase enzyme activity by 30-50%.

Effect of L-triiodothyronine on delta 9 desaturase activity in liver microsomes of male rats

Male rats injected with a single saturating dose of L-triiodothyronine (T3) showed, after a lag time of approximately eight hr, a sharp rise in delta 9 desaturase activity. Desaturase activity reached a plateau which was 1-1.2 times above the base line levels of rats which were not hormone-treated. The plateau was maintained for five days in animals which were kept on daily hormone-treatment. The increase in delta 9 desaturase activity by T3 required ongoing protein synthesis, because the increase in enzymatic activity due to hormone treatment was completely abolished in the presence of cycloheximide. These findings suggest that cycloheximide may block the induction of delta 9 desaturase by T3 and/or inhibit the synthesis of protein(s) essential to the desaturation-response to T3. Modifications observed in liver microsomal fatty acid composition in T3 treated rats were independent of the effect on desaturation. It is suggested that other factors, such as diet, membrane lipid synthesis and degradation, as well as fatty acid turnover and oxidation, could be involved in affecting the fatty acid composition of thyroid hormone-treated rats.

Cheek cell fatty acids as indicators of dietary lipids in humans

Analysis of cheek cell lipids has been suggested as a noninvasive method for monitoring the fatty acid composition of diets in humans. In a pilot study conducted to determine the validity of the method, cheek cell samples were collected from subjects consuming a low fat (20% of calories) diet consisting of fatty acids with either a 1.0 or 0.3 P/S ratio. Neither total lipid nor polar lipid fatty acids in cheek cells consistently reflected the P/S ratio of the diets. However, there were trends, particularly in the nonpolar lipids, suggesting that cheek cell fatty acid ratios might be useful for monitoring the fatty acid composition of the diets. The diet with the higher P/S ratio (1.0 vs 0.3) consistently resulted in cheek cell lipids with lower ratios of 18:1/saturated fatty acids and greater 18:2/20:4, 18:2/18:1 and 18:2/18:0 fatty acid ratios.

Effect of different acids with delta 9,12-dienoic structures on delta 9 desaturation activity in rat liver microsomes

The effect of oral administration, for 24 or 48 hr, of different octadeca fatty acids containing a 9,12-dienoic structure on the fatty acid composition and delta 9 desaturation activity of liver microsomes of rat fed a fat-free diet was studied. The ethyl esters of linoelaidic and gamma-linolenic acids, the methyl ester of linoleic acid and free columbinic acid were administered to rats maintained on a fat-free diet. The supplementation of the fat-free diet with linoelaidate produced no relevant changes in the fatty acid composition pattern of liver microsomes and did not modify the percentage of conversion of palmitic to palmitoleic acid. The addition of linoleate or gamma-linolenate to the fat-free diet returned liver microsome delta 9 desaturation activity toward the control and partially restored the liver microsome fatty acid spectrum found in the fat-free diet. Columbinic acid (5-trans-9-cis,12-cis-18:3), which cannot be transformed into arachidonic acid, also decreased the delta 9 desaturation activity enhanced by the fat-free diet and evoked changes in the microsomal fatty acid composition similar to those produced by the omega 6 fatty acids. These results suggest that the modulation of delta 9 desaturase activity evoked by dietary administration of unsaturated acids of omega 6 series would depend on the cis double bond configuration of these acids.

The regulation of hepatic stearoyl-coenzyme A desaturase in obese-hyperglycaemic (ob/ob) mice by food intake and the fatty acid composition of the diet

1. The effects of food intake and the fatty acid composition of the diet on the hepatic stearoyl-CoA desaturase activity of obese-hyperglycaemic (ob/ob) mice were investigated. 2. Obese mice fed on a commercial mouse diet, ad libitum, had 6.5-fold more activity per liver cell than had lean mice. 3. On a diet containing 14% corn oil the activity was 65% less in obese mice and 62% less in lean mice compared with animals fed on the commercial diet. 4. Feeding with 14% saturated fat in the diet doubled the activity in lean mice compared with those on the commercial diet, but had no effect on the activity in obese mice. 5. Obese mice fed on the corn-oil diet contained a higher proportion of linoleic acid in the liver lipids than did lean mice fed on the commercial diet, but the acyl-CoA desaturase activity was 125% higher than in the lean mice. 6. Limiting the food intake of obese mice by pair-feeding with lean mice decreased their acyl-CoA desaturase activity when the animals were fed on the saturated-fat diet, but the activity remained 75% higher than in lean mice, whereas in obese mice pair-fed on the corn-oil diet the activity was the same as in lean mice. 7. During starvation the acyl-CoA desaturase activity in livers from obese mice decreased more slowly and proportionately less than in livers from lean mice. 8. It is concluded that increased substrate supply as a result of hyperphagia and not low concentration of linoleic acid is the main factor causing high acyl-CoA desaturase activity in obese mice.