Regulation of stearoyl-CoA desaturase by polyunsaturated fatty acids and cholesterol

Peroxisome proliferator-activated receptor α is required for feedback regulation of highly unsaturated fatty acid synthesis

Delta6 desaturase (D6D), the rate-limiting enzyme for highly unsaturated fatty acid (HUFA) synthesis, is induced by essential fatty acid-deficient diets. Sterol regulatory element-binding protein-1c (SREBP-1c) in part mediates this induction. Paradoxically, D6D is also induced by ligands of peroxisome proliferator-activated receptor alpha (PPARalpha). Here, we report a novel physiological role of PPARalpha in the induction of genes specific for HUFA synthesis by essential fatty acid-deficient diets. D6D mRNA induction by essential fatty acid-deficient diets in wild-type mice was diminished in PPARalpha-null mice. This impaired D6D induction in PPARalpha-null mice was not attributable to feedback suppression by tissue HUFAs because PPARalpha-null mice had lower HUFAs in liver phospholipids than did wild-type mice. Furthermore, PPARalpha-responsive genes were induced in wild-type mice under essential fatty acid deficiency, suggesting the generation of endogenous PPARalpha ligand(s). Contrary to genes for HUFA synthesis, the induction of other lipogenic genes under essential fatty acid deficiency was higher in PPARalpha-null mice than in wild-type mice even though mature SREBP-1c protein did not differ between the genotypes. The expression of PPARgamma was markedly increased in PPARalpha-null mice and might have contributed to the induction of genes for de novo lipogenesis. Our study suggests that PPARalpha, together with SREBP-1c, senses HUFA status and confers pathway-specific induction of HUFA synthesis by essential fatty acid-deficient diets.

Effects of fenofibrate and insulin on the biosynthesis of unsaturated fatty acids in streptozotocin diabetic rats

Both insulin and PPAR-alpha up-modulate hepatic Delta9, Delta6 and Delta5 desaturating enzymes involved in the biosynthesis of mono- and polyunsaturated fatty acids. Currently, we have examined for 9 days the independent and simultaneous effects of daily glargine insulin and fenofibrate administration on the insulinemia, glycemia, hepatic acyl-CoA oxidase activity and mRNAs and enzymatic activities of stearoyl-CoA desaturase-1 (SCD-1) and Delta5 desaturase in streptozotocin diabetic rats. Glargine insulin depressed the hyperglycemia of diabetic rats at 4h, but not after 24h of injection. Fenofibrate increased the radioimmunoreactive insulinemia in non-diabetic rats without changing the glycemia. Insulin increased the mRNAs and activities of SCD-1 and Delta5 desaturase depressed in diabetic rats. Fenofibrate increased acyl-CoA oxidase activity, and the mRNAs and activities of both desaturating enzymes in non-diabetic, diabetic and insulin-treated diabetic rats, but was less effective in the mRNAs modification of diabetic animals. Therefore, insulin, and fenofibrate through PPAR-alpha activation, enhance liver mRNAs and activities of SCD-1 and Delta5 desaturases independently and synergistically through different mechanisms. Insulin and fenofibrate independently increased the 18:1/18:0 ratio in liver lipids, increasing the fluidity of the membranes. The 20:4/18:2 ratio was maintained. Fenofibrate increased palmitic acid, but decreased stearic acid percentage in liver lipids.

Expression and regulation of the SCD2 desaturase in the rat ovary

Despite the significant role of the lipid reserve in cell structure and function, very few studies have provided detailed descriptions of unsaturated fatty acid synthesis in the ovary. In the present study, we have shown by RT-PCR, Northern blot, and Western blot analyses the mRNA and protein expression of SCD2 (stearoyl-coenzyme A desaturase 2; also named delta 9 desaturase) in rat ovary. We also have localized Scd2 mRNA by in situ hybridization, mainly in granulosa cells of antral follicles, cumulus oophorus, and corpus luteum. Interestingly, either no or very weak SCD2 expression was observed in primordial follicles and oocytes. After eCG injection for 24 h in immature rats (age, 22 days), the level of SCD2 expression and SCD activity in ovary was increased by approximately fourfold (P < 0.05), and the response was further increased 48 h after hCG treatment. As expected, eCG/hCG treatment increased expression of the steroidogenesis enzymes (CYP11A1 and HSD3B) and STAR. We also found a decrease in the SCD2 expression and SCD activity in the corpus luteum at Days 10 and 15 compared to Day 3 of gestation, paralleled by a decrease in the expression of the steroidogenesis enzymes and STAR. To investigate the molecular mechanisms involved in the regulation of SCD2 expression in ovary, we performed primary culture of rat granulosa cells. We observed that both insulin-like growth factor 1 (IGF1) (7.5 x 10(-8)g/ml) and FSH (350 x 10(-8)g/ml) increased SCD2 expression and SCD activity by approximately threefold. Using specific inhibitors, we demonstrated that the MAPK3/MAP1 and PIK3R1/AKT pathways are involved in the IGF1- and FSH-induced SCD2 expression, respectively. The SCD2 is expressed and active in rat ovary, and it may be involved in the regulation of follicular growth and/or the oocyte maturation.

Intermittent hypoxia induces hyperlipidemia in lean mice

Obstructive sleep apnea, a syndrome leading to recurrent intermittent hypoxia (IH), has been associated previously with hypercholesterolemia, independent of underlying obesity. We examined the effects of experimentally induced IH on serum lipid levels and pathways of lipid metabolism in the absence and presence of obesity. Lean C57BL/6J mice and leptin-deficient obese C57BL/6J-Lep(ob) mice were exposed to IH for five days to determine changes in serum lipid profile, liver lipid content, and expression of key hepatic genes of lipid metabolism. In lean mice, exposure to IH increased fasting serum levels of total cholesterol, high-density lipoprotein (HDL) cholesterol, phospholipids (PLs), and triglycerides (TGs), as well as liver TG content. These changes were not observed in obese mice, which had hyperlipidemia and fatty liver at baseline. In lean mice, IH increased sterol regulatory element binding protein 1 (SREBP-1) levels in the liver, increased mRNA and protein levels of stearoyl-coenzyme A desaturase 1 (SCD-1), an important gene of TG and PL biosynthesis controlled by SREBP-1, and increased monounsaturated fatty acid content in serum, which indicated augmented SCD-1 activity. In addition, in lean mice, IH decreased protein levels of scavenger receptor B1, regulating uptake of cholesterol esters and HDL by the liver. We conclude that exposure to IH for five days increases serum cholesterol and PL levels, upregulates pathways of TG and PL biosynthesis, and inhibits pathways of cholesterol uptake in the liver in the lean state but does not exacerbate the pre-existing hyperlipidemia and metabolic disturbances in leptin-deficient obesity.

Transcriptional profiling of skeletal muscle tissue from two breeds of cattle

We used a 9.6 K cattle muscle/fat cDNA microarray to study gene expression differences between the longuissimus dorsi (LD) muscle of Japanese Black (JB) and Holstein (HOL) cattle. JB cattle exhibit an unusual ability to accumulate intramuscular adipose tissue with fat melting points lower than that in other breeds. The LD biopsies from three JB (Tajima strain) and three HOL animals were used in this breed comparison. Seventeen genes were identified as preferentially expressed in LD samples from JB and seven genes were found to be expressed more highly in HOL. The expression of six selected differentially expressed genes was confirmed by quantitative real-time PCR. The genes more highly expressed in JB are associated with unsaturated fatty acid synthesis, fat deposition, and the thyroid hormone pathway. These results are consistent with the increased amounts and proportions of monounsaturated fatty acids observed in the muscle of JB animals. By discovering as yet uncharacterized genes that are differentially regulated in this comparison, the work may lead us to a better understanding of the regulatory pathways involved in the development of intramuscular adipose tissue.

Chronic intermittent hypoxia upregulates genes of lipid biosynthesis in obese mice

Obstructive sleep apnea (OSA), a condition tightly linked to obesity, leads to chronic intermittent hypoxia (CIH) during sleep. There is emerging evidence that OSA is independently associated with insulin resistance and fatty liver disease, suggesting that OSA may affect hepatic lipid metabolism. To test this hypothesis, leptin-deficient obese (ob/ob) mice were exposed to CIH during the light phase (9 AM-9 PM) for 12 wk. Liver lipid content and gene expression profile in the liver (Affymetrix 430 GeneChip with real-time PCR validation) were determined on completion of the exposure. CIH caused a 30% increase in triglyceride and phospholipid liver content (P < 0.05), whereas liver cholesterol content was unchanged. Gene expression analysis showed that CIH upregulated multiple genes controlling 1) cholesterol and fatty acid biosynthesis [malic enzyme and acetyl coenzyme A (CoA) synthetase], 2) predominantly fatty acid biosynthesis (acetyl-CoA carboxylase and stearoyl-CoA desaturases 1 and 2), and 3) triglyceride and phospholipid biosynthesis (mitochondrial glycerol-3-phosphate acyltransferase). A majority of overexpressed genes were transcriptionally regulated by sterol regulatory element-binding protein (SREBP) 1, a master regulator of lipogenesis. A 2.8-fold increase in SREBP-1 gene expression in CIH was confirmed by real-time PCR (P = 0.001). Expression of major genes of cholesterol biosynthesis, SREBP-2 and 3-hydroxy-3-methylglutaryl-CoA reductase, was unchanged. In conclusion, we have shown that CIH may exacerbate preexisting fatty liver of obesity via upregulation of the pathways of lipid biosynthesis in the liver.

The role of stearoyl-CoA desaturase in the control of metabolism

Since obesity is becoming increasingly prevalent worldwide, much effort is being devoted to understanding its pathogenesis and treatment. In recent years, several candidate genes have been proposed as therapeutic targets. However, stearoyl-CoA desaturase 1 (SCD1) is of special significance, because it is the major gene target of leptin-a central mediator of energy homeostasis. There is evidence that SCD1 deficiency activates metabolic pathways that promote beta-oxidation and decrease lipogenesis in liver and skeletal muscles. One mechanism is via increased activation of AMP-activated protein kinase. SCD1 mutation results also in global changes in expression of genes involved in lipid metabolism. SCD1 deficient mice have increased energy expenditure, reduced body adiposity, and are resistant to diet-induced obesity. In this review, we examine data from our laboratory and others suggesting that SCD1 is an important component in the regulation of body metabolism.

Regulation of stearoyl-CoA desaturase expression

Stearoyl-CoA desaturase (SCD) is a regulatory enzyme in lipogenesis, catalyzing the rate-limiting step in the overall de novo synthesis of monounsaturated FA, mainly oleate and palmitoleate from stearoyl- and palmitoyl-CoA, respectively. Oleate and palmitoleate are the major monounsaturated FA of membrane phospholipids, TG, wax esters, cholesterol esters, and alkyldiacylglycerol. Several SCD gene isoforms (SCD1, SCD2, SCD3, and SCD4) exist in mice, and two have been characterized in humans. SCD1 gene expression in liver cells is regulated by numerous stimuli including diet and hormones. We are interested in why SCD is such a highly regulated enzyme even though oleate, the major product of this enzyme, is one of the most abundant FA in the diet and is therefore readily available. Dietary oleate is also well known for its TG-lowering effects and, as a major component of olive oil, is expected to have beneficial effects. However, high SCD activity has been implicated in diabetes, obesity, atherosclerosis, and cancer in several animal models; therefore, the role that de novo oleate plays in these disease states has to be carefully evaluated. By using SCD1-/- mice, which are deficient in tissue oleate, we begin to learn more about the physiological role of SCD gene expression and oleate in normal and disease states.

The fatty acid composition of muscle fat and subcutaneous adipose tissue of pasture-fed beef heifers: Influence of the duration of grazing

Our objective was to determine the effect of the duration of grazing before slaughter on the fatty acid composition of muscle fat and s.c. adipose tissue (SAT) of beef heifers. Sixty crossbred Charolais heifers (n = 15 per treatment) were assigned randomly to one of four dietary treatments: 45 animals (Treatments 1, 2, and 3, respectively) were housed at the beginning of the experiment, and 15 (Treatment 4) were fed at pasture. Two groups of 15 heifers were moved to pasture 40 d (Treatment 2) and 99 d (Treatment 3) before slaughter, respectively, resulting in preslaughter grazing periods of 0, 40, 99, or 158 d for Treatments 1, 2, 3, and 4, respectively. Before grazing the predominantly perennial ryegrass pasture, animals were housed and offered grass silage ad libitum and 3 kg of concentrate diet (650 g of grass silage/kg of total DMI). After slaughter, the fatty acid profile of the neutral (NL) and polar lipid (PL) fractions of muscle fat from the LM and the total lipids from SAT were analyzed by gas chromatography. Duration of grazing showed a quadratic tendency on mean carcass weight (P = 0.08), but did not affect growth (P = 0.27) or the lipid content (P = 0.13) of the LM. Increasing the duration of grazing led to a linear increase (P < 0.001) in the concentration (on fresh-tissue basis) of CLA in muscle fat (from 11.80 to 17.75 mg/100 g of muscle in NL, and from 0.52 to 0.82 mg/100 of g muscle in PL) and in SAT (from 3.98 to 10.23 mg/g of SAT; P < 0.001), and increased the concentration of C18:1trans-11 in both muscle fat fractions (P < 0.001) and in SAT (P < 0.001). In the total muscle lipids, the polyunsaturated to saturated fatty acid ratio (P:S) increased from 0.12 to 0.15 with increased duration of grazing following a linear (P < 0.05) and cubic pattern (P < 0.05). Increasing the duration of grazing led to a linear decrease in the n-6:n-3 ratio of muscle fat from 2.00 to 1.32 (P < 0.001), and from 2.64 to 1.65 in the SAT lipids (P < 0.001), mainly as a consequence of the increased concentration of C18:3n-3. It is concluded that muscle fat and SAT fatty acid profile was improved from a human health perspective by pasture feeding, and that this improvement depended on the duration of grazing.

Stearoyl-CoA desaturase as a new drug target for obesity treatment

Stearoyl-CoA desaturase (SCD), the rate-limiting enzyme in monounsaturated fatty acid synthesis, has recently been shown to be the critical control point regulating hepatic lipogenesis and lipid oxidation. As several manifestations of the metabolic syndrome and type 2 diabetes mellitus are associated with alterations in intracellular lipid partitioning, we propose that SCD1 may be a potential therapeutic target in the treatment of obesity and the metabolic syndrome. In support of this notion, we have shown that SCD1-deficient mice have increased energy expenditure, reduced body adiposity, increased insulin sensitivity and are resistant to diet-induced obesity and liver steatosis. Furthermore, SCD1 was found to be specifically repressed during leptin-mediated weight loss, and leptin-deficient ob/ob mice lacking SCD1 showed marked correction of the hypometabolic phenotype and hepatic steatosis. Much evidence indicates that the direct anti-steatotic effect of SCD1 deficiency stems from increased fatty acid oxidation and decreased lipid synthesis. All of these findings reveal that pharmacological manipulation of SCD activity might be of benefit in the treatment of obesity, diabetes, liver steatosis and other diseases of the metabolic syndrome.

Loss of stearoyl-CoA desaturase expression is a frequent event in prostate carcinoma

Prostate carcinogenesis is influenced by genetic alterations resulting in a biochemical condition that favors cell proliferation and survival. Studies of prostate carcinoma using comparative genomic hybridization and cDNA microarray analysis indicate that numerous biochemical processes may be affected during cellular transformation and progression to an invasive phenotype. Among the consistently observed tumor-associated changes are alterations in fatty acid metabolism that influence diverse cellular activities such as signaling, energy utilization, and membrane fluidity. Increases in fatty acid synthase (FAS) levels have been shown to be one of the earliest and most frequent molecular alterations in prostate carcinogenesis. We sought to identify tumor-associated changes in the expression of genes with functional roles associated with lipid metabolism. Defined populations of normal and neoplastic prostate epithelium were acquired by laser capture microdissection and transcript levels were measured by cDNA microarray hybridization. We determined that stearoyl-CoA desaturase (SCD) transcripts were downregulated in cancer relative to normal epithelium. These results were confirmed by quantitative PCR. Further analysis by immunohistochemical evaluation of radical prostatectomy samples employed a quantitative scoring system with a range of 0-300. The median SCD expression levels were 150, 45 and 10 for normal, PIN and carcinoma samples, respectively. Statistically significant differential SCD expression between normal and cancerous epithelium was determined at the p=0.001 level, and between PIN and prostate carcinoma at the p=0.03 level. Of these cases, 92% overexpressed fatty acid synthase (FAS) in cancerous cells and 84.7% exhibited the signature of FAS overexpression and SCD loss in prostate carcinoma as compared to normal prostate epithelium. These results indicate that loss of SCD expression is a frequent event in prostate adenocarcinoma, and further supports a role for altered lipid metabolism as a factor in the process of carcinogenesis.

Stearoyl-CoA desaturase-1 deficiency reduces ceramide synthesis by downregulating serine palmitoyltransferase and increasing beta-oxidation in skeletal muscle

Stearoyl-CoA desaturase (SCD) has recently been shown to be a critical control point of lipid partitioning and body weight regulation. Lack of SCD1 function significantly increases insulin sensitivity in skeletal muscles and corrects the hypometabolic phenotype of leptin-deficient ob/ob mice, indicating the direct antilipotoxic action of SCD1 deficiency. The mechanism underlying the metabolic effects of SCD1 mutation is currently unknown. Here we show that SCD1 deficiency reduced the total ceramide content in oxidative skeletal muscles (soleus and red gastrocnemius) by approximately 40%. The mRNA levels and activity of serine palmitoyltransferase (SPT), a key enzyme in ceramide synthesis, as well as the incorporation of [14C]palmitate into ceramide were decreased by approximately 50% in red muscles of SCD1-/- mice. The content of fatty acyl-CoAs, which contribute to de novo ceramide synthesis, was also reduced. The activity and mRNA levels of carnitine palmitoyltransferase I (CPT I) and the rate of beta-oxidation were increased in oxidative muscles of SCD1-/- mice. Furthermore, SCD1 deficiency increased phosphorylation of AMP-activated protein kinase (AMPK), suggesting that AMPK activation may be partially responsible for the increased fatty acid oxidation and decreased ceramide synthesis in red muscles of SCD1-/- mice. SCD1 deficiency also reduced SPT activity and ceramide content and increased AMPK phosphorylation and CPT I activity in muscles of ob/ob mice. Taken together, these results indicate that SCD1 deficiency reduces ceramide synthesis by decreasing SPT expression and increasing the rate of beta-oxidation in oxidative muscles.

Vaccenic acid metabolism in the liver of rat and bovine

Hepatic metabolism of vaccenic acid (VA), especially its conversion into CLA, was studied in the bovine (ruminant species that synthesizes CLA) and in the rat (model for non-ruminant) by using the in vitro technique of liver explants. Liver tissue samples were collected from fed animals (5 male Wistar rats and 5 Charolais steers) and incubated at 37 degrees C for 17 h under an atmosphere of 95% O2/5% CO2 in medium supplemented with 0.75 mM of FA mixture and with 55 microM [1-14C]VA. VA uptake was about sixfold lower in bovine than in rat liver slices (P< 0.01). For both species, VA that was oxidized to partial oxidation products represented about 20% of VA incorporated by cells. The chemical structure of VA was not modified in bovine liver cells, whereas in rat liver cells, 3.2% of VA was converted into 16:0 and only 0.33% into CLA. The extent of esterification of VA was similar for both animal species (70-80% of incorporated VA). Secretion of VA as part of VLDL particles was very low and similar in rat and bovine liver (around 0.07% of incorporated VA). In conclusion, characteristics of the hepatic metabolism of VA were similar for rat and bovine animals, the liver not being involved in tissue VA conversion into CLA in spite of its high capacity for FA desaturation especially in the rat. This indicates that endogenous synthesis of CLA should take place exclusively in peripheral tissues.

Nuclear hormone receptor NHR-49 controls fat consumption and fatty acid composition in C. elegans

Mammalian nuclear hormone receptors (NHRs), such as liver X receptor, farnesoid X receptor, and peroxisome proliferator-activated receptors (PPARs), precisely control energy metabolism. Consequently, these receptors are important targets for the treatment of metabolic diseases, including diabetes and obesity. A thorough understanding of NHR fat regulatory networks has been limited, however, by a lack of genetically tractable experimental systems. Here we show that deletion of the Caenorhabditis elegans NHR gene nhr-49 yielded worms with elevated fat content and shortened life span. Employing a quantitative RT-PCR screen, we found that nhr-49 influenced the expression of 13 genes involved in energy metabolism. Indeed, nhr-49 served as a key regulator of fat usage, modulating pathways that control the consumption of fat and maintain a normal balance of fatty acid saturation. We found that the two phenotypes of the nhr-49 knockout were linked to distinct pathways and were separable: The high-fat phenotype was due to reduced expression of enzymes in fatty acid beta-oxidation, and the shortened adult life span resulted from impaired expression of a stearoyl-CoA desaturase. Despite its sequence relationship with the mammalian hepatocyte nuclear factor 4 receptor, the biological activities of nhr-49 were most similar to those of the mammalian PPARs, implying an evolutionarily conserved role for NHRs in modulating fat consumption and composition. Our findings in C. elegans provide novel insights into how NHR regulatory networks are coordinated to govern fat metabolism.

Effect of unsaturated fat intake from Mediterranean diet on rat liver mRNA expression profile: selective modulation of genes involved in lipid metabolism

BACKGROUND AND AIM

The lipid content of Mediterranean diet is mostly accounted for its disease preventive action. We investigated whether the short term nutritional effect of a fat quota mainly derived from olive and fish oil affects liver mRNA expression profile in rats.

METHODS AND RESULTS

The study was carried out using DNA microarray techniques. The effect was evaluated at liver mRNA expression level to identify genes whose expression was regulated by dietary modifications. Two groups of six rats were alternatively supplied for two weeks with either a control or with an experimental diet. Both diets were semisynthetic and isocaloric, with identical major nutrients composition (protein 20%, carbohydrates 56% and lipids 22% of total energy) being different only in the quality of fats. The lipid quota of the control diet contained exclusively saturated animal fats, derived from butter, while in the experimental diet some unsaturated fats were present, being derived also from olive and fish oil (10% and 6% of total energy, respectively). Out of 26,334 genes analyzed, 11,292 were found expressed in the liver, 72 were induced and 180 were inhibited from the experimental diet. Out of these, 33 of the induced and 59 of the inhibited species have a well known function.

CONCLUSIONS

The diet with olive and fish oil modulates several genes related to lipolysis or lipogenesis and newly identified responders from other metabolisms. Some of these genes are also reported to be similarly modulated by the action of fibrates, but without the complete gene activation typical of these PPARalpha ligands.

High stearoyl-CoA desaturase protein and activity levels in simian virus 40 transformed-human lung fibroblasts

The precise role of monounsaturated fatty acid (MUFA) synthesis in cell proliferation and programmed cell death remains unknown. The strong correlation of high levels of MUFA and neoplastic phenotype suggest that the regulation of stearoyl CoA desaturase (SCD) must play a significant role in cancer development. In this study, the levels of SCD protein and activity were investigated in normal (WI38) and SV40-transformed (SV40-WI38) human lung fibroblasts. Thus, the activity of SCD on exogenous [14C]stearic acid and endogenous [14C]acetate-labeled fatty acids was increased by 2.2- and 2.6-fold, respectively, in SV40-WI38 compared to WI38 fibroblasts. Concomitantly, a 3.3-fold increase in SCD protein content was observed in SV40-transformed cells. Cell transformation also led to high levels of MUFA, which was paralleled by a more fluid membrane environment. Furthermore, the levels of PPAR-gamma, a well-known activator of SCD expression, were highly increased in SV40-transformed fibroblasts. SCD activity appeared linked to the events of programmed cell death, since incubations with 40 microM etoposide induced apoptosis in SV40 cells, and led to a decrease in fatty acid synthesis, SCD activity and in MUFA cellular levels. Taken together, these results suggest that SCD protein and activity levels are associated with the events of neoplastic cell transformation and programmed cell death.

An integrative metabolism approach identifies stearoyl‐CoA desaturase as a target for an arachidonate‐enriched diet

Epidemiological studies have correlated diets containing higher intakes of PUFA with lower rates of chronic metabolic diseases. The molecular mechanisms regulated by the consumption of PUFA were examined by using an integrative metabolism approach assaying the liver transcriptome and lipid-metabolome of mice fed a control diet, an arachidonate (AA)-enriched fungal oil, an eicosapentaenoic (EPA)/docosahexaenoic (DHA)-enriched fish oil, or a combination of the two oils. Hepatic gene transcription and fatty acid (FA) metabolism were significantly altered by diets enriched with AA, as revealed by global error assessment and singular value decomposition (SVD) analysis, respectively. SVD analysis of the lipid data, reinforced with transcriptomics, suggests that the chronic feeding of AA modulates molecular endpoints similar to those previously reported in the obesity-resistant SCD1-/- mouse, namely, genes involved in lipid oxidation/synthesis and the significant changes in FA metabolism stemming from a repressed SCD1 activity. Specifically, the total levels and FA composition of several phospholipid (PL) species were significantly changed, with phosphatidylcholine (PC) demonstrating the greatest alterations. Reduced PC levels were linked to decreased expression of enzymes in PC biosynthesis (choline kinase, -2.2-fold; glycerol-3-phosphate acyltransferase, -2.0-fold). Alterations in PL-FA composition were related to decreased expression of FA biosynthetic genes [fatty acid synthetase, -3.7-fold; stearoyl-CoA desaturase-1 (SCD1), -1.8-fold]. Lower hepatic SCD1 gene expression levels were reflected in various aspects of FA metabolism through increased concentrations of palmitic (fungal oil, +45%; combination, +106%) and stearic acids (fungal oil, +60%; combination, +63%) in PC. Importantly, an integrated approach showed that these effects were not attenuated by the addition of an EPA/DHA-enriched fish oil, thereby identifying a previously unrecognized and distinct role for AA in the regulation of hepatic lipid metabolism.

Pathological cholesterol metabolism fails to modify electrophysiological properties of afflicted neurones in Niemann-Pick disease type C

Niemann-Pick disease type C (NPC) is a recessive inherited neurovisceral lipid storage disease characterized by progressive motor impairment and a loss of neurones including those integrated into the motor system. One of the key neuropathological findings is the intracellular accumulation of lysosomes enriched with free cholesterol. This accumulation is due to impaired transport proteins named NPC1 (approx. 95% of the cases) or NPC2 (approx. 5%) responsible for the transport of endocytosed cholesterol from lysomes to plasma membranes. The perturbed lipid-transport in NPC cells leads to an altered lipid composition of the plasma membrane. Available evidence suggests that the lipid matrix influences the electrophysical properties of ion channels in membranes. We therefore evaluated whether electrophysiological properties of NPC neurones differ from healthy neurones. Both, acute brain slices and primary neuronal cell cultures from wildtype and NPC mice, a well-established mouse model for the Niemann-Pick type C disease, were used for a comparison of electrophysiological properties like resting membrane potential, input resistance, action potential amplitudes and synaptic properties of the neurones. In addition we optically recorded the changes of intraneuronal calcium levels elicited by depolarization. Our results show that the characteristics of ion channels in NPC neurones do not differ significantly from wildtype neurones. We therefore conclude that gross alterations of the electrophysiological properties of neurones will probably not initiate or substantially contribute to the development of the motor impairment or other neurological signs of NPC.

Isolation and molecular characterization of the porcine stearoyl-CoA desaturase gene

Stearoyl-CoA desaturase (SCD) is a rate-limiting enzyme in the biosynthesis of unsaturated fatty acids. So far only a partial porcine SCD sequence is available. Here we described the isolation and molecular characterization of the full-length cDNA and the determination of the genomic DNA sequence of the porcine SCD gene. The 5134-bp cDNA contains a 1080-bp open reading frame (ORF) encoding a protein of 359 amino acids with a calculated molecular mass of 41.3 kDa and a theoretical isoelectric point of 9.4. The porcine SCD protein shares high identity (>80%) with the other mammalian SCD. To further elucidate the genomic structure of the porcine SCD gene, we sequenced 20,985 bp of genomic DNA sequence encompassing the complete pig SCD gene. Similar to the other mammalian orthologs, particularly in term of exon size and exon/intron boundaries, the porcine SCD gene spans a transcription unit of 16,186 bp, consisting of six exons with sizes ranging from 131 to 4048 bp, and five introns varying in size from 518 to 4784 bp. The gene reveals a 176-bp-long 5' UTR and possesses an unusually long 3'UTR of 3848 bp in the last exon. Comparison of different mammalian SCD promoters identified some regulatory domains required for the transcription regulation in the 5' flanking sequence of the porcine SCD gene, such as the conserved polyunsaturated fatty acid response region (PUFA-RE). A total of 21 gene polymorphisms were revealed in the 21-kb DNA sequence, including 19 single nucleotide polymorphisms (SNPs), a 24-bp-long fragment length polymorphism in the fourth intron and a triplet nucleotide insertion in the fifth intron. Reverse transcription (RT)-PCR result indicates that the SCD gene is expressed ubiquitously in pigs.

Microarray gene expression analysis of the Fob3b obesity QTL identifies positional candidate gene Sqle and perturbed cholesterol and glycolysis pathways

Obesity-related diseases are poised to become the primary cause of death in developed nations. While a number of monogenic causes of obesity have recently been identified, these are responsible for only a small proportion of human cases of obesity. Quantitative trait locus (QTL) studies using animal models have revealed hundreds of potential loci that affect obesity; however, few have been further analyzed beyond the original QTL scan. We previously mapped four QTL in an F(2) between divergently selected Fat (F) and Lean (L) lines. A QTL of large effect on chromosome 15 (Fob3) was subsequently mapped to a higher resolution into two smaller-effect QTL (Fob3a and Fob3b) using crosses between the F-line and a congenic line containing L-line alleles at the Fob3 QTL region. Here we report the gene expression characterization of Fob3b. Microarray expression analysis using the NIA-NIH 15K cDNA array set containing 14,938 mouse ESTs was employed to identify candidate genes and pathways that are differentially expressed between the F-line and a congenic line containing only the Fob3b QTL (Fob3b-line). Our study suggests squalene epoxidase (Sqle), a cholesterol biosynthesis enzyme, as a strong positional candidate gene for Fob3b. Several other cholesterol biosynthesis pathway genes unlinked to Fob3b were found to be differentially expressed, suggesting that a perturbation of this pathway could be in part responsible for the phenotypic difference between the F-line and Fob3b-line mice.

Analysis of the contribution to type 2 diabetes susceptibility of sequence variation in the gene encoding stearoyl-CoA desaturase, a key regulator of lipid and carbohydrate metabolism

AIMS/HYPOTHESIS

Stearoyl-CoA desaturase (SCD) is emerging as a key regulator of lipid and carbohydrate metabolism. Scd-null mice display a beneficial metabolic phenotype characterised by resistance to obesity, diabetes and hyperlipidaemia. The human homologue, SCD, maps to a region of chromosome 10 linked to type 2 diabetes, and SCD activity correlates with insulin sensitivity. Given this strong positional and biological candidacy, the present study sought to establish whether sequence variation in SCD influences susceptibility to type 2 diabetes and related traits.

METHODS

The SCD gene was resequenced in 23 diabetic subjects. Six variants within coding and adjacent sequence, including a non-synonymous SNP in exon 5 (M224L), were selected for genotyping in a primary set of 608 diabetic subjects and 600 control subjects.

RESULTS

There was no association (at the allele, genotype or haplotype level) with type 2 diabetes, although genotype frequencies at the +14301 A>C SNP in the 3' untranslated region showed borderline association (p~0.06) when evidence for linkage was taken into account. However, replication studies (350 young-onset diabetic patients; 747 controls) failed to confirm any relationship with diabetes for this variant. No significant associations were seen for diabetes-related traits including BMI and waist-to-hip ratio.

CONCLUSIONS/INTERPRETATION

The present study, the first reported analysis of this gene, indicates that the SCD variants typed do not explain chromosome-10-encoded susceptibility to type 2 diabetes. Although this study provided no evidence that SCD sequence variation influences diabetes susceptibility or related traits, SCD remains a major target for pharmaceutical and/or environmental manipulation.

Mechanisms of regulation of gene expression by fatty acids

Fatty acids (FA) regulate the expression of genes involved in lipid and energy metabolism. In particular, two transcription factors, sterol regulatory element binding protein-1c (SREBP-1c) and peroxisome proliferator activated receptor alpha (PPARalpha), have emerged as key mediators of gene regulation by FA. SREBP-1c induces a set of lipogenic enzymes in liver. Polyunsaturated fatty acids (PUFA), but not saturated or monounsaturated FA, suppress the induction of lipogenic genes by inhibiting the expression and processing of SREBP-1c. This unique effect of PUFA suggests that SREBP-1c may regulate the synthesis of unsaturated FA for incorporation into glycerolipids and cholesteryl esters. PPARalpha plays an essential role in metabolic adaptation to fasting by inducing the genes for mitochondrial and peroxisomal FA oxidation as well as those for ketogenesis in mitochondria. FA released from adipose tissue during fasting are considered as ligands of PPARalpha. Dietary PUFA, except for 18:2 n-6, are likely to induce FA oxidation enzymes via PPARalpha as a "feed-forward " mechanism. PPARalpha is also required for regulating the synthesis of highly unsaturated FA, indicating pleiotropic functions of PPARalpha in the regulation of lipid metabolic pathways. It is yet to be determined whether FA regulate other transcription factors such as liver-X receptor, hepatocyte nuclear factor 4, and carbohydrate response element binding protein.

Lack of stearoyl-CoA desaturase 1 upregulates basal thermogenesis but causes hypothermia in a cold environment

Stearoyl-CoA desaturase (SCD) is a microsomal enzyme involved in the biosynthesis of oleate and palmitoleate. Mice with a targeted disruption of the SCD1 isoform (SCD1-/-) exhibit reduced adiposity and increased energy expenditure. To address whether the energy expenditure is attributable to increased thermogenesis, we investigated the effect of SCD1 deficiency on basal and cold-induced thermogenesis. SCD1-/- mice have increased expression of uncoupling proteins in brown adipose tissue (BAT) relative to controls. The beta3-adrenergic receptor (beta3-AR) expression was increased and the phosphorylation of cAMP response element binding protein and the protein level of peroxisome proliferator-activated receptor-gamma coactivator-1alpha were increased in the SCD1-/- mice. Both lipolysis and fatty acid oxidation were increased in the SCD1-/- mice. When exposed to 4 degrees C, SCD1-/- mice showed hypothermia, hypoglycemia, and depleted liver glycogen. High levels of dietary oleate partially compensated for the hypothermia and rescued plasma glucose and liver glycogen. These results suggest that SCD1 deficiency stimulates basal thermogenesis through the upregulation of the beta3-AR-mediated pathway and a subsequent increase in lipolysis and fatty acid oxidation in BAT. The hypothermia and hypoglycemia in cold-exposed SCD1-/- mice and the compensatory recovery by oleate indicate an important role of SCD1 gene expression in thermoregulation.

Leptin and the control of metabolism: role for stearoyl-CoA desaturase-1 (SCD-1)

The incidence of obesity has increased sharply in recent years, making it one of the most urgent public health concerns worldwide. The hormone leptin is the central mediator in a negative feedback loop regulating energy homeostasis. Leptin administration leads to reduced food intake, increased energy expenditure, and weight loss. Leptin also mediates unique metabolic effects, specifically depleting lipid from liver and other peripheral tissues. While elucidation of leptin's role has permitted a more detailed view of the biology underlying energy homeostasis, most obese individuals are leptin resistant. A more complete understanding of the molecular components of the leptin pathway is necessary to develop effective treatment for obesity and the Metabolic Syndrome. We review here studies on the identification of one such component, stearoyl-CoA desaturase-1 (SCD-1), as a gene specifically repressed by leptin and discuss the role of this process in mediating the metabolic effects of leptin. Data indicate that pharmacologic manipulation of SCD-1 may be of benefit in the treatment of obesity, diabetes, hepatic steatosis, and other components of the Metabolic Syndrome.

Tissue fatty acid profiles can be used to quantify endogenous rumenic acid synthesis in lambs

Proportions of vaccenic (trans-11 18:1) and rumenic (cis-9, trans-11 18:2) acids in mesenteric adipose, subcutaneous adipose, and longissimus muscle tissue lipids from lambs fed varying proportions of forages and concentrates were used to develop a mathematical model to predict exogenous and endogenous contributions to rumenic acid (RA) in the several tissues. From the model, we were able to estimate the proportion of absorbed RA, the proportion of vaccenic acid (VA) desaturated, the original proportion of VA in the tissue (before desaturation), and finally the proportion of RA synthesized endogenously. Estimates of endogenous RA were in the range of published data estimated by independent procedures. An independent data set of VA and RA in milk fat was used to challenge the model. Predictions were concordant with observations, although estimates of endogenous RA synthesis were lower than previous reports. Changing the amount of exogenous RA through manipulation of the diet influenced desaturation of VA inversely, so that endogenous RA synthesis was decreased when exogenous supply was increased (r = -0.80). The model should be challenged with data from human and nonruminant, as well as ruminant studies.

Lipid metabolism in hepatic steatosis

Hepatic steatosis is a consequence of both obesity and ethanol use. Nonalcoholic steatosis (NASH) resemble alcoholic steatosis and steatohepatitis. Both exhibit increased hepatocellular triglycerides(TG), reflecting an increase in long chain fatty acids (LCFA). LCFA enter cells by both facilitated transport and passive diffusion. A driving force for both is the plasma unbound LCFA concentration ([LCFAu]). In both obese rodents and obese patients, adipocyte LCFA uptake via both facilitated transport and diffusion is increased. However, the LCFA uptake Vmax in hepatocytes is not increased in obese animals. Nevertheless, total LCFA uptake in obese rodents is increased ~3-fold, reflecting increased plasma LCFA concentrations. With advancing obesity, resistance to the antilipolytic effects of insulin results in increased lipolysis within the omental fat depot, a consequent further rise in portal venous LCFA, and an even greater rise in portal [LCFAu]. This causes a further increase in hepatocellular LCFA uptake, increased intracellular generation of reactive oxygen species (ROS), and transition from simple steatosis to NASH. By contrast, in rodent hepatocytes and in human hepatoma cell lines, ethanol up-regulates the LCFA uptake Vmax. Consequently, although plasma LCFA are unaltered, hepatocellular LCFA uptake in ethanol-fed rats is also increased~3-fold, leading to increased ROS generation and evolution of alcoholic hepatitis. Thus, while increased hepatic LCFA uptake contributes to the pathogenesis of both NASH and alcoholic hepatitis,the underlying mechanisms differ. Recognizing these mechanistic differences is important in developing strategies for both prevention and treatment of these conditions.

Effects of dietary fat and oxidized cholesterol on gene expression in rat liver as assessed by cDNA expression array analysis

BACKGROUND

Specific oxysterols acting as ligands for nuclear transcription factors were shown to affect expression of genes involved in lipid metabolism. However, the various biological effects of oxysterols such as cytotoxicity, atherogenicity or mutagenicity suggest that other genes may be also affected by oxysterols than lipid metabolism.

AIM OF THE STUDY

The present study was conducted to investigate the effects of dietary oxidized cholesterol containing significant amounts of oxysterols and its interactions with different dietary fats on gene expression profiles as assessed by DNA array technology in rats.

METHODS

54 male Sprague-Dawley rats were assigned to six groups and were fed six semisynthetic diets, which varied in the type of dietary fat (coconut oil vs. salmon oil) and dietary cholesterol (none cholesterol vs. 5 g unoxidized cholesterol/kg vs. 5 g oxidized cholesterol/kg).

RESULTS

Changes in gene expression as observed in response to dietary oxidized cholesterol were strongly dependent on the type of fat. In the rats fed coconut oil, the expression of 7 genes (5 up- and 2 down-regulated) was altered by dietary oxidized cholesterol, while in the rats fed salmon oil, the expression of 50 genes (16 up- and 34 down-regulated) was altered. 29 genes (22 up- and 7 down-regulated) were identified as possible targets for an altered gene expression by dietary salmon oil as compared to dietary coconut oil.

CONCLUSION

The present study showed that dietary oxidized cholesterol transcriptionally affects many genes involved in xenobiotic metabolism and stress response--an effect that was amplified by the administration of fish oil as dietary fat.

Dwarfism and impaired gut development in insulin-like growth factor II mRNA-binding protein 1-deficient mice

Insulin-like growth factor II mRNA-binding protein 1 (IMP1) belongs to a family of RNA-binding proteins implicated in mRNA localization, turnover, and translational control. Mouse IMP1 is expressed during early development, and an increase in expression occurs around embryonic day 12.5 (E12.5). To characterize the physiological role of IMP1, we generated IMP1-deficient mice carrying a gene trap insertion in the Imp1 gene. Imp1(-/-) mice were on average 40% smaller than wild-type and heterozygous sex-matched littermates. Growth retardation was apparent from E17.5 and remained permanent into adult life. Moreover, Imp1(-/-) mice exhibited high perinatal mortality, and only 50% were alive 3 days after birth. In contrast to most other organs, intestinal epithelial cells continue to express IMP1 postnatally, and Imp1(-/-) mice exhibited impaired development of the intestine, with small and misshapen villi and twisted colon crypts. Analysis of target mRNAs and global expression profiling at E12.5 indicated that Igf2 translation was downregulated, whereas the postnatal intestine showed reduced expression of transcripts encoding extracellular matrix components, such as galectin- 1, lumican, tenascin-C, procollagen transcripts, and the Hsp47 procollagen chaperone. Taken together, the results demonstrate that IMP1 is essential for normal growth and development. Moreover, IMP1 may facilitate intestinal morphogenesis via regulation of extracellular matrix formation.

Structure, function, and dietary regulation of delta6, delta5, and delta9 desaturases

Fatty acid desaturases introduce a double bond in a specific position of long-chain fatty acids, and are conserved across kingdoms. Degree of unsaturation of fatty acids affects physical properties of membrane phospholipids and stored triglycerides. In addition, metabolites of polyunsaturated fatty acids are used as signaling molecules in many organisms. Three desaturases, Delta9, Delta6, and Delta5, are present in humans. Delta-9 catalyzes synthesis of monounsaturated fatty acids. Oleic acid, a main product of Delta9 desaturase, is the major fatty acid in mammalian adipose triglycerides, and is also used for phospholipid and cholesteryl ester synthesis. Delta-6 and Delta5 desaturases are required for the synthesis of highly unsaturated fatty acids (HUFAs), which are mainly esterified into phospholipids and contribute to maintaining membrane fluidity. While HUFAs may be required for cold tolerance in plants and fish, the primary role of HUFAs in mammals is cell signaling. Arachidonic acid is required as substrates for eicosanoid synthesis, while docosahexaenoic acid is required in visual and neuronal functions. Desaturases in mammals are regulated at the transcriptional level. Reflecting overlapping functions, three desaturases share a common mechanism of a feedback regulation to maintain products in membrane phospholipids. At the same time, regulation of Delta9 desaturase differs from Delta6 and Delta5 desaturases because its products are incorporated into more diverse lipid groups. Combinations of multiple transcription factors achieve this sophisticated differential regulation.

Stearoyl-CoA desaturase 1 deficiency increases fatty acid oxidation by activating AMP-activated protein kinase in liver

Stearoyl-CoA desaturase (SCD) catalyzes the rate-limiting step in the biosynthesis of monounsaturated fatty acids. Mice with a targeted disruption of the SCD1 isoform have reduced body adiposity, increased energy expenditure, and up-regulated expression of several genes encoding enzymes of fatty acid beta-oxidation in liver. The mechanisms by which SCD deficiency leads to these metabolic changes are presently unknown. Here we show that the phosphorylation and activity of AMP-activated protein kinase (AMPK), a metabolic sensor that regulates lipid metabolism during increased energy expenditure is significantly increased (approximately 40%, P < 0.01) in liver of SCD1 knockout mice (SCD1-/-). In parallel with the activation of AMPK, the phosphorylation of acetyl-CoA carboxylase at Ser-79 was increased and enzymatic activity was decreased (approximately 35%, P < 0.001), resulting in decreased intracellular levels of malonyl-CoA (approximately 47%, P < 0.001). An SCD1 mutation also increased AMPK phosphorylation and activity and increased acetyl-CoA carboxylase phosphorylation in leptin-deficient ob/ob mice. Lower malonyl-CoA concentrations are known to derepress carnitine palmitoyltransferase 1 (CPT1). In SCD1-/- mice, CPT1 and CPT2 activities were significantly increased (in both cases approximately 60%, P < 0.001) thereby stimulating the oxidation of mitochondrial palmitoyl-CoA. Our results identify AMPK as a mediator of increased fatty acid oxidation in liver of SCD1-deficient mice.

Stearoyl-CoA desaturase 1 gene expression is necessary for fructose-mediated induction of lipogenic gene expression by sterol regulatory element-binding protein-1c-dependent and -independent mechanisms

Stearoyl-CoA desaturase (SCD) synthesizes oleate necessary for the biosynthesis of triglycerides and other lipids. Mice with a targeted disruption of the SCD1 gene are deficient in tissue oleate and have reduced expression of the sterol regulatory element-binding protein (SREBP) and its target genes. The SREBP-1c isoform is a known mediator of insulin action on hepatic gene expression, but its transcriptional effects due to glucose or fructose are still unclear. We found that fructose compared with glucose is a stronger inducer of SREBP-1c and lipogenic gene expression, causing a dramatic increase in hepatic triglyceride levels. However, when fed to the SCD1-/- mice, fructose failed to induce SREBP-1 or lipogenic genes and the triglyceride levels were not increased. Instead fructose feeding caused a decrease in hepatic glycogen and plasma glucose levels. The induction of SREBP-1 and lipogenic gene expression as well as the levels of liver triglycerides, glycogen, and plasma glucose was partially restored when the fructose diet was supplemented with very high levels of oleate (20% by weight) but not with palmitate, stearate, or linoleate. Fructose in a long term feeding induced the expression of SCD1 and that of other lipogenic genes in the liver of SREBP-1c-/- mice, and a further increase in expression of these genes occurred when the fructose diet was supplemented with oleate. Our observations demonstrated that oleate produced by SCD is necessary for fructose-mediated induction of lipogenic gene expression through SREBP-1c-dependent and -independent mechanisms and suggested that SCD1 gene expression is important in lipid and carbohydrate homeostasis.

Evidence that the anti-obesity effect of conjugated linoleic acid is independent of effects on stearoyl-CoA desaturase1 expression and enzyme activity

The trans-10,cis-12 isomer of conjugated linoleic acid (CLA) reduces body fat gain in animals and inhibits stearoyl-CoA desaturase (SCD) activity in 3T3-L1 adipocytes. To test whether CLA's body fat reduction is mediated by SCD1, wild-type and SCD1-null mice were fed diet supplemented with 0.2% trans-10,cis-12 (t10c12) CLA for 4 weeks. The t10c12 CLA-supplemented diet significantly reduced body fat mass in both wild type and SCD1-null mice. Similarly, t10c12 CLA diet decreased blood triglyceride and free fatty acid levels regardless of SCD1 genotypes. Mice fed t10c12 CLA exhibited increased mRNA expression of fatty acid synthase and uncoupling protein 2 in both genotypes. Taken together, the effects of t10c12 CLA on reduction of body fat gain, blood parameters, and mRNA expression in both SCD1-null mice and wild-type mice were similar, indicating that the anti-obesity effect of t10c12 CLA may be independent of the effects of this CLA isomer on SCD1 gene expression and enzyme activity.

Site and mechanism of leptin action in a rodent form of congenital lipodystrophy

Lipodystrophy is characterized by the complete or partial absence of adipose tissue, insulin resistance, hepatic steatosis, and leptin deficiency. Here, we show that low-dose central leptin corrects the insulin resistance and fatty liver of lipodystrophic aP2-nSREBP-1c mice, while the same dose given peripherally does not. Central leptin also repressed stearoyl-CoA desaturase-1 (SCD-1) RNA and enzymatic activity, which were increased in livers of lipodystrophic mice. aP2-nSREBP-1c mice homozygous for an SCD-1 deletion had markedly reduced hepatic steatosis, increased saturated fatty acids, decreased acetyl-CoA carboxylase activity, and decreased malonyl-CoA levels in the liver. Despite the reduction in hepatic steatosis, these mice remained diabetic. A leptin dose-response curve showed that subcutaneous leptin improved hyperglycemia and hyperinsulinemia in aP2-nSREBP-1c mice at doses that did not substantially alter hepatic steatosis or hepatic SCD enzymatic activity. Leptin treatment at this dose improved insulin-stimulated insulin receptor and insulin receptor substrate 2 (IRS-2) phosphorylation, IRS-2-associated PI3K activity, and Akt activity in liver. Together, these data suggest that CNS-mediated repression of SCD-1 contributes to leptin's antisteatotic actions. Intracerebroventricular leptin improves glucose homeostasis by improving insulin signal transduction in liver, but in this case the effect appears to be independent of SCD-1.

Cyp1a2 protects against reactive oxygen production in mouse liver microsomes

H(2)O(2) production was evaluated in liver microsomes prepared from Cyp1a1/1a2(+/+) wild-type and Cyp1a1(-/-) and Cyp1a2(-/-) knockout mice pretreated with 5 microg dioxin (TCDD)/kg body wt or vehicle alone. NADPH-dependent H(2)O(2) production in TCDD-induced microsomes from wild-type mice was about one-third of that in noninduced microsomes. In Cyp1a2(-/-) mice, H(2)O(2) production was the same for induced and noninduced microsomes, with levels significantly higher than those in wild-type mice. Cyp1a1(-/-) microsomes displayed markedly lower levels of H(2)O(2) production in both induced and noninduced microsomes, compared with those in wild-type and Cyp1a2(-/-) microsomes. The CYP1A2 inhibitor furafylline in vitro exacerbated microsomal H(2)O(2) production proportional to the degree of CYP1A2 inhibition, and the CYP2E1 inhibitor diethyldithiocarbamate decreased H(2)O(2) production proportional to the degree of CYP2E1 inhibition. Microsomal H(2)O(2) production was strongly correlated to NADPH-stimulated production of thiobarbituric acid-reactive substances, as well as to decreases in microsomal membrane polarization anisotropy, indicative of peroxidation of unsaturated membrane lipids. Our results suggest that possibly acting as an "electron sink," CYP1A2 might decrease CYP2E1-and CYP1A1-mediated H(2)O(2) production and oxidative stress. In this regard, CYP1A2 may be considered an antioxidant enzyme.

Muscle-type specific intramyocellular and hepatic lipid metabolism during starvation in wistar rats

The physiological dynamics of intramyocellular lipids (IMCLs) in different muscle types and of hepatocellular lipids (HepCLs) are still uncertain. The dynamics of IMCLs in the soleus, tibialis anterior, and extensor digitorum longus (EDL) muscles and HepCL during fed, 12- to 72-h starved, and refed conditions were measured in vivo by (1)H-magnetic resonance spectroscopy (MRS) in Wistar rats. Despite significant elevations of free fatty acids (FFAs) during starvation, HepCLs and IMCLs in soleus remained constant. In tibialis anterior and EDL, however, IMCLs increased significantly by 170 and 450% after 72 h of starvation, respectively. After refeeding, elevated IMCLs dropped immediately in both muscles. Total muscle long-chain acyl-CoAs (LCACoAs) remained constant during the study period. Hepatic palmitoleoyl-CoA (C16:1) decreased significantly during starvation while total hepatic LCACoAs increased significantly. Consistent with constant values for FFAs, HepCLs, IMCLs, and muscle LCACoAs from 12-72 h of starvation, insulin sensitivity did not change. We conclude that during starvation-induced adipocytic lipolysis, oxidative muscles dispose elevated FFAs by oxidation, while nonoxidative ones neutralize FFAs by reesterification. Both mechanisms might prevent impairment of insulin signaling by maintaining low levels of LCACoAs. Hepatic palmitoleoyl-CoA might have a special role in lipid metabolism due to its unique dynamic profile during starvation.

Triglyceride levels are ethnic-specifically associated with an index of stearoyl-CoA desaturase activity and n-3 PUFA levels in Asians

Accumulated evidence suggests that hypertriglyceridemia (HTG) is independently associated with an increased incidence of cardiovascular disease. The hypotriglyceridemic effects of n-3 PUFAs have been confirmed in Caucasians, but the effect in Asians is less clear. Recent evidence indicates that stearoyl-CoA desaturase (SCD) activity induced with high-carbohydrate diets increases plasma triglyceride levels. We investigated the relationship between triglyceride levels and the ratio of plasma oleic acid to stearic acid (the 18:1/18:0 ratio), a plasma marker of SCD activity, and n-3 PUFAs in 411 Japanese, 418 Korean, and 251 Mongolian adults. The Japanese and Koreans had higher values for triglyceride than their Mongolian counterparts, despite lower body mass index values for the Japanese and Koreans. The Japanese and Koreans ate fish more frequently and had remarkably higher values for n-3 PUFAs than did the Mongolians. Multiple regression analysis showed that triglyceride levels had a great magnitude of correlation with the increases in 18:1/18:0 ratio for the Japanese and Mongolians, and n-3 PUFAs remained significant for the Mongolians. HTG is ethnicity-specifically associated with an increase in the 18:1/18:0 ratio and a decrease in n-3 PUFA in plasma for Japanese, Koreans, and Mongolians.

Putative metabolic effects of the liver X receptor (LXR)

The nuclear receptors liver X receptor (LXR)alpha and LXRbeta are sensors of cholesterol metabolism and lipid biosynthesis. They have recently been found to be regulators of inflammatory cytokines, suppressors of hepatic glucose production, and involved in different cell-signaling pathways. LXRalpha is a target gene of the peroxisome proliferator-activated receptor-gamma, a target of drugs used in treating elevated levels of glucose seen in diabetes. Furthermore, insulin induces LXRalpha in hepatocytes, resulting in increased expression of lipogenic enzymes and suppression of key enzymes in gluconeogenesis, including PEPCK. LXR seems to have an important role in the regulation of glucocorticoid action and a role in the overall energy homeostasis suggested by its putative regulatory effect on leptin and uncoupling protein 1. The physiological roles of LXR indicate that it is an interesting potential target for drug treatment of diabetes.

Lipogenesis in fetal rat lung: importance of C/EBPalpha, SREBP-1c, and stearoyl-CoA desaturase

Alveolar type II cells increase lipogenesis and convert glycogen into the phospholipids of surfactant in the late term fetal lung. Recent studies suggest that CCAAT/enhancing-binding protein (C/EBP) isoforms and sterol regulatory element binding protein (SREBP)-1c regulate fatty acid synthesis in adult type II cells in vitro. To define the temporal relationships and enzymes involved in lipogenesis in fetal rat lung, the mRNA levels of selected transcription factors and enzymes were determined. There was an increase in the mRNA levels of C/EBPalpha, C/EBPbeta, C/EBPdelta, peroxisomal proliferator-activated receptor gamma (PPARgamma), and SREBP-1c, but not SREBP-1a or SREBP-2 from fetal Days 19-21. There was also an increase in the mRNA levels of fatty acid synthase, stearoyl-CoA desaturase 1 (SCD-1), fatty acid translocase, glycerol-3-P acyl transferase, and phosphatidate cytidylyltransferase. By in situ hybridization, there was detectible expression of fatty acid synthase, SCD-1, and C/EBPalpha along the alveolar septae with the same distribution pattern as surfactant protein-C, whereas PPARgamma expression appeared to be restricted to macrophages. Regulation of lipogenesis at the mRNA level is predominately on enzymes of fatty acid synthesis and appears to be regulated by C/EBPalpha and SREBP-1c. SCD-1 and phosphatidate cytidylyltransferase are important components of the lipogenic response in the fetal lung that have not been recognized previously.

Gene and protein kinase expression profiling of reactive oxygen species-associated lipotoxicity in the pancreatic beta-cell line MIN6

Oligonucleotide microarrays were used to define oleic acid (OA)-regulated gene expression and proteomic technology to screen protein kinases in MIN6 insulinoma cells. The effects of oxidative stress caused by OA and potential protective effects of N-acetyl-L-cysteine (NAC), a scavenger of reactive oxygen species (ROS), on global gene expression and beta-cell function were investigated. Long-term exposure of MIN6 cells to OA led to a threefold increase in basal insulin secretion, a 50% decrease in insulin content, an inhibition of glucose-stimulated insulin secretion (GSIS), and a twofold increase in the level of ROS. The addition of NAC normalized both the OA-induced insulin content and ROS elevation, but it failed to restore GSIS. Microarray studies and subsequent quantitative PCR analysis showed that OA consistently regulated the expression of 45 genes involved in metabolism, cell growth, signal transduction, transcription, and protein processing. The addition of NAC largely normalized the expression of the OA-regulated genes involved in cell growth and differentiation but not other functions. A protein kinase screen showed that OA regulated the expression and/or phosphorylation levels of kinases involved in stress-response mitogen-activated protein kinase, phosphatidylinositol 3-kinase, and cell cycle control pathways. Importantly, these findings indicate that chronic OA exposure can impair beta-cell function through ROS-dependent and -independent mechanisms.

Coordinated upregulation of oxidative pathways and downregulation of lipid biosynthesis underlie obesity resistance in perilipin knockout mice: a microarray gene expression profile

Obesity is a major risk factor for diabetes and heart disease. We previously reported that the inactivation of the gene for perilipin (plin), an adipocyte lipid droplet surface protein, produced lean and obesity-resistant mice. To dissect the underlying mechanisms involved, we used oligonucleotide microarrays to analyze the gene-expression profile of white adipose tissue (WAT), liver, heart, skeletal muscle, and kidney of plin(-/-) and plin(+/+) mice. As compared with wild-type littermates, the WAT of plin(-/-) mice had 270 and 543 transcripts that were significantly up- or downregulated. There was a coordinated upregulation of genes involved in beta-oxidation, the Krebs cycle, and the electron transport chain concomitant with a downregulation of genes involved in lipid biosynthesis. There was also a significant downregulation of the stearoyl CoA desaturase-1 gene, which has been associated with obesity resistance. Thus, in response to the constitutive activation of lipolysis associated with absence of perilipin, WAT activated pathways to rid itself of the products of lipolysis and activated pathways of energy expenditure that contribute to the observed obesity resistance. The biochemical pathways involved in obesity resistance in plin(-/-) mice identified in this study may represent potential targets for the treatment of obesity.

A plasminogen-like protein selectively degrades stearoyl-CoA desaturase in liver microsomes

Stearoyl-CoA desaturase (SCD) is an integral membrane protein of the endoplasmic reticulum that is rapidly and selectively degraded when isolated liver microsomes are incubated at 37 degrees C. We previously reported the purification of a 90-kDa microsomal protein with SCD protease activity and characterized the inhibitor sensitivity of the protease. Here we show that the 90-kDa protein is a microsomal form of plasminogen (Pg) and that the purified SCD protease contains a spectrum of plasmin-like derivatives. The 90-kDa protein was identified as Pg by mass spectrometry of its tryptic peptides. The purified SCD protease reacted with Pg antibody, and immunoblotting demonstrated enrichment of Pg by the purification procedure established for the SCD protease. Analysis of microsomes by zymography demonstrated a single band of proteolytic activity at 70-kDa corresponding to the mobility of Pg in nonreduced polyacrylamide gels. When microsomes were incubated at 37 degrees C prior to zymography, an intense band of proteolytic activity developed at 30-kDa. The purified SCD protease displayed a spectrum of proteolytic bands ranging from 70 to 30 kDa. Degradation of SCD by the purified protease and by microsomes was inhibited by bdellin, a plasmin inhibitor from the medicinal leech Hirudo medicinalis. To explore the role of Pg in the degradation of SCD in vivo, we examined SCD expression and degradation in microsomes isolated from Pg-deficient (Pg-/-) mice. Compared with microsomes from wild-type littermate control mice, liver microsomes from Pg-/- mice had significantly higher levels of SCD. Degradation of SCD in microsomes from Pg-/- mice was markedly diminished, whereas liver microsomes from control mice showed rapid SCD degradation similar to that observed in rat liver microsomes. These findings indicate that SCD is degraded by a protease related to Pg and suggest that plasmin moonlights as an intracellular protease.

cDNA nucleotide sequence coding for stearoyl-CoA desaturase and its expression in the zebrafish (Danio rerio) embryo

A cDNA sequence of stearoyl-CoA desaturase (SCD) was determined from zebrafish (Danio rerio) and compared to the corresponding genes in several teleosts. Zebrafish SCD cDNA has a size of 1,061 bp, encodes a polypeptide of 325 amino acids, and shares 88, 85, 84, and 83% similarities with tilapia (Oreochromis mossambicus), grass carp (Ctenopharyngodon idella), common carp (Cyprinus carpio), and milkfish (Chanos chanos), respectively. This 1,061 bp sequence specifies a protein that, in common with other fatty acid desaturases, contains three histidine boxes, believed to be involved in catalysis. These observations suggested that SCD genes are highly conserved. In addition, an oligonucleotide probe complementary to zebrafish SCD mRNA was hybridized to mRNA of approximately 396 bases with Northern blot analysis. The Northern blot and RT-PCR analyses showed that the SCD mRNA was expressed predominantly in the liver, intestine, gill, and muscle, while a lower level was found in the brain. Furthermore, we utilized whole-mount in situ hybridization and real-time quantitative RT-PCR to identify expression of the zebrafish SCD gene at five different stages of development. This revealed that very high levels of transcripts were found in zebrafish at all stages during embryogenesis and early development.

Cholesterol-receptor-mediated genomics in health and disease

Cross-talk between cell-surface receptor C(k) and intracellular receptors (liver X receptor-alpha and peroxisome-proliferator-activated receptors) controls a set of crucial genes that maintain a finely orchestrated balance between various cellular processes involved in cell growth, differentiation, apoptosis, cholesterol homeostasis and inflammation. Abnormal cross-talk of these receptors can lead to several human diseases, particularly atherosclerosis, cancer and autoimmune diseases. As our understanding of the complex signaling events that link these receptors to human health improves, we are beginning to appreciate the enormous potential of the proposed cross-talk model of cholesterol receptors in the prevention and/or treatment of diseases.

Identification and characterization of murine SCD4, a novel heart-specific stearoyl-CoA desaturase isoform regulated by leptin and dietary factors

Stearoyl-CoA desaturase (SCD) is the rate-limiting enzyme in the biosynthesis of monounsaturated fatty acids. Thus far, three isoforms of SCD (SCD1, SCD2, and SCD3) have been identified and characterized. Regulation of the SCD1 isoform has been shown to be an important component of the metabolic actions of leptin in liver, but the effects of leptin on SCD isoforms in other tissues have not been investigated. We found that although the mRNA levels of SCD1 and SCD2 were not affected by leptin deficiency in the hearts of ob/ob mice, the SCD activity and levels of monounsaturated fatty acids were increased, implying the existence of another SCD isoform. This observation has led to the cDNA cloning and characterization of a fourth SCD isoform (SCD4) that is expressed exclusively in the heart. SCD4 encodes a 352-amino acid protein that shares 79% sequence identity with the SCD1, SCD2, and SCD3 isoforms. Liver X receptor alpha (LXR alpha) agonists and a high carbohydrate fat-free diet induced SCD4 expression, but unlike SCD1, SCD4 expression was not repressed by dietary polyunsaturated fatty acids. SCD4 mRNA levels were elevated 5-fold in the hearts of leptin-deficient ob/ob mice relative to wild type controls. Treatment of ob/ob mice with leptin decreased mRNA levels of SCD4, whereas levels of SCD1 and SCD2 were not affected. Furthermore, in the hearts of SCD1-deficient mice, SCD4 mRNA levels were induced 3-fold, whereas the levels of SCD2 were not altered. The current studies identify a novel heart-specific SCD isoform that demonstrates tissue-specific regulation by leptin and dietary factors.

Peroxisome proliferator-activated receptor α is not the exclusive mediator of the effects of dietary cyclic FA in mice

Cyclic FA monomers (CFAM) formed during heating of alpha-linolenic acid have been reported to interfere in hepatic metabolism in a putatively peroxisome proliferator-activated receptor alpha (PPARalpha)-dependent manner. In the present work, CFAM (0.5% of the diet) were administered for 3 wk to wild-type and PPARalpha-null mice of both genders to elucidate the role of PPARalpha in mediating the effects of CFAM on the activity of acyl-CoA oxidase (ACO) and omega-laurate hydroxylase (CYP4A), the regulation of which is known to be dependent on the PPARalpha. Dietary CFAM enhanced CYP4A activity threefold in male and female wild-type mice. This effect was abolished in PPARalpha-null mice. A twofold induction of ACO activity was found in wild-type female mice fed CFAM; however, no effect was seen in males. In wild-type animals, (omega-1)-laurate hydroxylase (CYP2E1) activity, the expression of which has not been shown to be PPARalpha dependent, was not affected by the CFAM diet. In contrast, stearoyl-CoA desaturase activity was reduced in wild-type mice. CFAM feeding reduced the activities of ACO, CYP2E1, and stearoyl-CoA desaturase and caused accumulation of lipids in the livers of female PPARalpha-null mice. These data show that CFAM apparently activate gene expression via the PPARalpha and have profound effects on lipid homeostasis, exacerbating the disturbances preexisting in mice lacking functional PPARalpha. Although the data emphasize the importance of PPARalpha in the metabolism of the CFAM, these results show that PPARalpha is not the exclusive mediator of the effects of CFAM in lipid metabolism in mice.

Expression of liver X receptor target genes decreases cellular amyloid beta peptide secretion

A hallmark of Alzheimer's disease is the deposition of plaques of amyloid beta peptide (Abeta) in the brain. Abeta is thought to be formed from the amyloid precursor protein (APP) in cholesterol-enriched membrane rafts, and cellular cholesterol depletion decreases Abeta formation. The liver X receptors (LXR) play a key role in regulating genes that control cellular cholesterol efflux and membrane composition and are widely expressed in cells of the central nervous system. We show that treatment of APP-expressing cells with LXR activators reduces the formation of Abeta. LXR activation resulted in increased levels of the ATP-binding cassette transporter A1 (ABCA1) and stearoyl CoA desaturase, and expression of these genes individually decreased formation of Abeta. Expression of ABCA1 led to both decreased beta-cleavage product of APPSw (i.e. C99 peptide) and reduced gamma-secretase-cleavage of C99 peptide. Remarkably, these effects of ABCA1 on APP processing were independent of cellular lipid efflux. LXR and ABCA1-induced changes in membrane lipid organization had favorable effects on processing of APP, suggesting a new approach to the treatment of Alzheimer's disease.

Keratinocyte growth factor and the transcription factors C/EBP alpha, C/EBP delta, and SREBP-1c regulate fatty acid synthesis in alveolar type II cells

Strategies to stimulate endogenous surfactant production require a detailed understanding of the regulation of lipogenesis in alveolar type II cells. We developed culture conditions in which keratinocyte growth factor (KGF) stimulates fatty acid and phospholipid synthesis. KGF stimulated acetate incorporation into phosphatidylcholine, disaturated phosphatidylcholine, and phosphatidylglycerol more than 5% rat serum alone. To determine the mRNA levels of lipogenic enzymes and transport proteins, we analyzed gene expression by oligonucleotide microarrays. KGF increased the mRNA levels for fatty acid synthase, stearoyl-CoA desaturase-1 (SCD-1), and epidermal fatty acid-binding protein more than rat serum alone. In addition, KGF increased the mRNA levels of the transcription factors CCAAT/enhancer-binding protein alpha (C/EBPalpha) and C/EBPdelta as well as SREBP-1c (ADD-1), but not PPARgamma. These changes in C/EBPalpha and C/EBPdelta were confirmed by in situ hybridization. SCD-1 was also found to be highly expressed in alveolar type II cells in vivo. Furthermore, KGF increased protein levels of fatty acid synthase, C/EBPalpha, C/EBPdelta, SREBP-1, epidermal fatty acid-binding protein, and SCD. Finally, the liver X receptor agonist T0901317 increased acetate incorporation and SREBP-1 but not SREBP-2 protein levels. In summary, KGF stimulates lipogenesis in type II cells by a coordinated expression of lipogenic enzymes and transport proteins regulated by C/EBP isoforms and SREBP-1c.

Bacterial DNA evokes epithelial IL-8 production by a MAPK-dependent, NF-kappaB-independent pathway

Recognition of bacterial products by the innate immune system is dependent on pattern-recognition receptors: toll-like receptor 9 (TLR-9) in the case of bacterial DNA. We hypothesized that bacterial DNA can directly affect enteric epithelial cells. RT-PCR revealed constitutive TLR-9 mRNA expression in three human colonic epithelial cell lines (T84, HT-29, Caco-2) and THP-1 monocytes. Epithelial cells, in six-well culture plates or on filter supports, were exposed to E. coli DNA (1-50 microg/ml), synthetic CpG-rich oligonucleotides, or calf thymus DNA for 6-48 h. Exposure to E. coli DNA resulted in an increase in IL-8 mRNA, and a time- and dose-dependent increase in IL-8 secretion. Also, CpG oligonucleotides induced epithelial IL-8 production, whereas calf thymus DNA did not. Exposure to E. coli DNA resulted in phosphorylation of ERK 1/2 MAPK and inhibitors of ERK activity (PD98059, UO126) significantly reduced the evoked IL-8 production. In contrast, inhibitors of NFkappaB activity (PDTC, SN50) did not block E. coli DNA-induced IL-8 production. Electrophoretic mobility shift assays revealed that E. coli DNA stimulated epithelial AP-1 but not NFkappaB activation. The barrier (i.e., transepithelial resistance) and ion transport parameters of epithelial monolayers (assessed in Ussing chambers) were unaltered following E. coli DNA exposure. Thus model gut epithelia express TLR-9 mRNA and, while maintaining their barrier function, can respond to E. coli DNA by increased IL-8 production.