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Biomedical subjects

J M Ntambi

Publications and source records attributed to J M Ntambi.

At least 19 recordsLinked to original sources

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.

Animals↗

Oleoyl-CoA is the major de novo product of stearoyl-CoA desaturase 1 gene isoform and substrate for the biosynthesis of the Harderian gland 1-alkyl-2,3-diacylglycerol.

1-Alkyl-2,3-diacylglycerol (ADG) is a unique neutral lipid found in the eyeball-associated Harderian gland (HG) of the mouse and acts as a lubricant to facilitate eyelid movement. We found that the HG of the mice with a disruption in the gene for stearoyl-CoA desaturase 1 (SCD1) (SCD1-/-) is deficient in ADG. The amount of C20:1n-9, which is a major fatty acid of ADG, was reduced by greater than 90% despite normal elongase enzyme activity proposed to elongate it from C18:1n-9. HG from SCD1-/- mice exhibited high desaturase activity toward C16:0-CoA as substrate but had very low desaturase activity toward C18:0-CoA. Feeding diets containing high levels of oleate to the SCD1-/- mice did not increase the levels of C18:1n-9 or C20:1n-9 in the HG and failed to restore the ADG to the levels found in the HG of the wild-type mouse. De novo ADG synthesis as measured by the incorporation of [(3)H]glycerol and [(14)C]glucose was high in the SCD1+/+ mouse but was reduced by greater than 90% in the HG of SCD1-/- mouse. The deficiencies in the levels of ADG and C20:1n-9 were not compensated for by the expression of SCD2 and SCD3 isoforms in the HG of the SCD1-/- mouse. These observations demonstrate that SCD1-synthesized oleoyl-CoA is a major substrate required for the biosynthesis of normal levels of ADG and that the SCD isoforms present in the HG have different substrate specificity.

Acyl Coenzyme A↗

Cloning and characterization of the human stearoyl-CoA desaturase gene promoter: transcriptional activation by sterol regulatory element binding protein and repression by polyunsaturated fatty acids and cholesterol.

Stearoyl-CoA desaturase (SCD) is a microsomal enzyme required for the biosynthesis of oleate (C18:1) and palmitoleate (C16:1) which are the major monounsaturated fatty acids of membrane phospholipids, triglycerides and cholesterol esters. Previously the full-length human skin cDNA was sequenced and the exon and intron structure of the single functional SCD gene determined. Here we report on the cloning and characterization of the promoter region of the human SCD gene. The human promoter structure is very similar to that of the mouse SCD1 isoform and contains conserved regulatory sequences for the binding of several transcription factors including the sterol regulatory element binding protein (SREBP), CCAAT enhancer binding protein-alpha (C/EBPalpha) and nuclear factor-1 (NF-1) that have been shown to transactivate the transcription of the mouse SCD1 gene. Polyunsaturated fatty acids and cholesterol decreased the SCD promoter-luciferase activity when transiently transfected into HepG2 cells. The decrease in promoter activity correlated with decreases in endogenous SCD mRNA and protein levels. Cotransfection experiment in HepG2 cells showed transactivation of the SCD promoter-luciferase activity by an expression vector containing SREBP-1a and 1c. Our studies indicate that the transcription of the human SCD gene is repressed by polyunsaturated fatty acids and cholesterol and that SREBP plays a role in the transcriptional activation of this gene.

Animals↗

Regulation of stearoyl-CoA desaturase activity by the trans-10,cis-12 isomer of conjugated linoleic acid in HepG2 cells.

Stearoyl-CoA desaturase (SCD) catalyzes the rate-limiting step in the cellular synthesis of monounsaturated fatty acids mainly oleate (C18:1) and palmitoleate (C16:1) which are the major monounsaturated fatty acids of membrane phospholipids, cholesterol esters, waxes, and triglycerides. Several SCD isoforms exist in the mouse whereas the human has one well-characterized SCD gene. The trans-10,cis-12 isomer of conjugated linoleic acid has been previously shown to repress the expression of the mouse SCD1 gene isomer by decreasing SCD gene expression as well as by direct inhibition of SCD enzyme activity. We studied the regulation of human stearoyl-CoA desaturase (SCD) expression by conjugated linoleic acid (CLA) in cultured human hepatoblastoma cell line, HepG2. Treatment of the cells with the trans-10,cis-12 CLA isomer did not cause changes in the SCD gene transcription, mRNA and protein levels. However, this isomer decreased both the SCD activity as well as the levels of monounsaturated fatty acids. The other major CLA isomer, cis-9,trans-11 CLA, had no effect on SCD gene expression and activity. These results suggest that in HepG2 cells the trans-10,cis-12 CLA isomer regulates human SCD activity mainly by a posttranslational mechanism.

Fatty Acids, Monounsaturated↗

Targeted disruption of stearoyl-CoA desaturase1 gene in mice causes atrophy of sebaceous and meibomian glands and depletion of wax esters in the eyelid.

Stearoyl-CoA desaturase (SCD) is a microsomal rate-limiting enzyme in the cellular synthesis of monounsaturated fatty acids (MUFA), mainly oleate (18:1) and palmitoleate (16:1), which are the major MUFA of membrane phospholipids, cholesterol esters and triglycerides. Three well-characterized isoforms of SCD, SCD1, SCD2 and SCD3, exist in mice. To investigate the physiologic functions of SCD1, we generated SCD1 null (SCD1-/-) mice. The skin and eyelid of SCD1-/- mice are deficient in triglycerides and cholesterol esters, and the eyelid also is deficient in wax esters. Furthermore, the eyelid and skin of SCD1-/- mice have higher levels of free cholesterol. SCD1-/- mice develop cutaneous abnormalities and narrow eye fissure with atrophic sebaceous and meibomian glands. Consumption of diets containing high levels of oleate, failed to restore the levels of triglycerides, cholesterol esters and wax esters in SCD1-/- mice to the levels found in the eyelid of wild-type mice. These results reveal a physiologic role of SCD in cholesterol homeostasis as well as in the de novo biosynthesis of cholesterol esters, triglycerides and wax esters required for normal skin and eyelid function.

Animals↗

A lipogenic diet in mice with a disruption of the stearoyl-CoA desaturase 1 gene reveals a stringent requirement of endogenous monounsaturated fatty acids for triglyceride synthesis.

Stearoyl-CoA desaturase (SCD) catalyzes the de novo biosynthesis of oleate and palmitoleate, which are the major fatty acids found in triglycerides, cholesteryl esters, and phospholipids. A high carbohydrate (lipogenic) diet induces lipogenic gene expression by sterol regulatory element binding protein 1 (SREBP-1c)-mediated gene transcription, leading to an increase in the synthesis of triglycerides. The lipogenic diet fed to mice with a null mutation in the SCD1 gene (SCD-/-) fails to induce the synthesis of triglycerides in liver, despite the induction of expression of SREBP-1 and its target genes, fatty acid synthase and glycerol-3-phosphate acyltransferase. The lipogenic diet led to a decrease in the levels of triglyceride, but an increase in the level of cholesteryl esters of saturated fatty acids. Feeding a lipogenic diet supplemented with high levels of oleate to the SCD-/- mice resulted in incorporation of oleate in the liver of SCD-/- mice, but failed to restore triglycerides to the levels in the normal mouse. Triglyceride synthesis, as measured by the incorporation of [(3)H]glycerol, was dramatically reduced in the liver of SCD-/- mouse fed a lipogenic diet compared with the normal mouse. These observations demonstrate that induction of triglyceride synthesis is highly dependent on SCD1 gene expression.

Animals↗

The biosynthesis of hepatic cholesterol esters and triglycerides is impaired in mice with a disruption of the gene for stearoyl-CoA desaturase 1.

Stearoyl-CoA desaturase (SCD) is a microsomal enzyme required for the biosynthesis of oleate and palmitoleate, which are the major monounsaturated fatty acids of membrane phospholipids, triglycerides, and cholesterol esters. Two well characterized isoforms of SCD, SCD1 and SCD2, exist in the mouse. Most mouse tissues express SCD1 and 2 with the exception of the liver, which expresses mainly the SCD1 isoform. We found that asebia mice homozygous for a natural mutation of the gene for SCD1 (SCD-/-) are deficient in hepatic cholesterol esters and triglycerides despite the presence of normal activities of acyl-CoA:cholesterol acyltransferase and glycerol phosphate acyltransferase, the enzymes responsible for cholesterol ester and triglyceride synthesis, respectively, in the liver of these mice. Feeding diets supplemented with triolein or tripalmitolein to the SCD-/- mice resulted in an increase in the levels of 16:1 and 18:1 in the liver but failed to restore the 18:1 and 16:1 levels of the cholesterol ester and triglycerides to the levels found in normal mice. The SCD-/- mouse had very low levels of triglycerides in the VLDL and LDL lipoprotein fractions compared with the normal animal. Transient transfection of an SCD1 expression vector into Chinese hamster ovary cells resulted in increased SCD activity and esterification of cholesterol to cholesterol esters. Taken together, our observations demonstrate that the oleoyl-CoA and palmitoleyl-CoA produced by SCD1 are necessary to synthesize enough cholesterol esters and triglycerides in the liver and suggest that regulation of SCD1 activity plays an important role in mechanisms of cellular cholesterol homeostasis.

Animals↗

Inhibition of hepatic stearoyl-CoA desaturase activity by trans-10, cis-12 conjugated linoleic acid and its derivatives.

Conjugated linoleic acid (CLA) has been reported to decrease stearoyl-CoA desaturase (SCD) activity by decreasing mRNA expression. This investigation was designed to determine whether structurally related compounds of CLA have a direct inhibitory effect on SCD activity. Trans-10,cis-12 CLA had strong inhibitory activity on SCD while cis-9,trans-11, and trans-9,trans-11 isomers had no effect. Trans-10 octadecenoate was not inhibitory, whereas cis-12 octadecenate was inhibitory, but not as effective as trans-10,cis-12 CLA. Of the oxygenated derivatives, 9-peroxy-cis/trans-10, trans-12 octadecadienoate was a more effective inhibitor than trans-10,cis-12 CLA, whereas 9-hydroxy-trans-10, cis-12 octadecadienoate was less effective. Interestingly, cis-11 octadecadienoate and cis-12 octadecen-10-ynoate were slightly inhibitory. However, trans-9 and trans-11 octadecenoates, and trans-9,cis-12 octadecadienoate were all inactive under test condition, as were linoleate, oleate, and arachidonate. Derivatives of CLA acid modified to alcohol, amide or chloride were all inactive. A cis-12 double bond appears to be a key structural feature for inhibiting SCD activity, especially when coupled with a trans-10 double, whereas a cis-11 double bond is less effective.

Animals↗

Adipocyte differentiation and gene expression.

The major function of adipocytes is to store triacylglycerol in periods of energy excess and to mobilize this energy during times of deprivation. The short-term control of these lipogenic and lipolytic processes is carefully modulated by hormonal signals from the bloodstream, which provide an inventory of the body's metabolic state. Long-term changes in fat storage needs are accomplished by altering both the size and number of fat cells within the body because terminally differentiated adipocytes cannot divide. Alterations in the number of fat cells within the body must be accomplished by the differentiation of preadipocytes, which act as the renewable source of adipocytes. Our understanding of the events that occur during preadipocyte differentiation has advanced considerably in the last few years and has relied mainly on the use of tissue culture models of adipogenesis. This article will discuss the various models used for studying the preadipocyte differentiation process, with the mouse 3T3-L1 cell culture line described in detail. We focus on those genetic events that link effectors to induction of adipocyte gene expression.

3T3 Cells↗

The trans-10,cis-12 isomer of conjugated linoleic acid downregulates stearoyl-CoA desaturase 1 gene expression in 3T3-L1 adipocytes.

Conjugated linoleic acids (CLA) are a group of positional and geometric conjugated dienoic isomers of linoleic acid. The objective of this study was to determine the effects of the cis-9,trans-11 and trans-10,cis-12 isomers of conjugated linoleic acid on lipid composition and gene expression during the differentiation of mouse 3T3-L1 preadipocytes. Treatment of differentiating 3T3-L1 preadipocytes with trans-10,cis-12 conjugated linoleic acid (CLA) resulted in a dose-dependent decrease in the expression of the stearoyl-CoA desaturase 1 gene (SCD1). The expression of other adipocyte genes such as adipose P2 (aP2), fatty acid synthase (FAS), SCD2 and the key adipogenic transcription factors, peroxisome proliferator-activated receptor gamma2 (PPARgamma2) and CCAAT enhancer binding protein alpha (C/EBPalpha), remained elevated. Cells treated with trans-10,cis-12 CLA exhibited smaller lipid droplets, with reduced levels of the major monounsaturated fatty acids, palmitoleate and oleate. By contrast, the cis-9,trans-11 isomer did not alter adipocyte gene expression. Repression of the stearoyl-CoA desaturase gene expression in adipocytes by the trans-10,cis-12 isomer may contribute to the mechanisms by which CLA reduces body fat in mice.

3T3 Cells↗

Differential regulation of the stearoyl-CoA desaturase genes by thiazolidinediones in 3T3-L1 adipocytes.

Two stearoyl-CoA desaturase (SCD) isoforms can be expressed during the differentiation of 3T3-L1 preadipocytes into adipocytes. Here we report on the effects of the peroxisome proliferator-activated receptor gamma ligand troglitazone (TRO) on scd1 and scd2 mRNA levels as determined by Northern blotting, on SCD protein expression as determined by Western blotting, and on total lipid composition as determined by GC during differentiation. In preadipocytes, scd1 mRNA and SCD protein were not detected, whereas scd2 mRNA was detected. These cells have high levels of palmitate (16:0), stearate (18:0), and monounsaturated oleate (Delta(9)-18:1) and low levels of monounsaturated palmitoleate (Delta(9)-16:1). In MDI (methylisobutylxanthine, dexamethasone, and insulin)-treated cells, scd1 mRNA and SCD protein were increased approximately 100-fold relative to preadipocyte levels, the scd2 mRNA level was increased 2-fold, Delta(9)-16:1 was increased approximately 20-fold, and 18:0 was decreased approximately 3-fold. In TRO-treated cells, the scd1 mRNA level was lower than that observed in preadipocytes, while the scd2 mRNA level was similar. TRO also decreased scd1 mRNA in primary adipocytes. The TRO-treated cells contained a Delta(9)-18:1 level typical of MDI-treated cells whereas, conversely, these cells also contained a low Delta(9)-16:1 level typical of preadipocytes. The implications of these correlations for the regulatory and enzymatic mechanism(s) used to establish and maintain lipid composition are discussed.

1-Methyl-3-isobutylxanthine↗

Regulation of stearoyl-CoA desaturase genes: role in cellular metabolism and preadipocyte differentiation.

The degree of fatty acid unsaturation in cell membrane lipids determines membrane fluidity, whose alteration has been implicated in a variety of disease states including diabetes, obesity, hypertension, cancer, and neurological and heart diseases. Stearoyl-CoA desaturase (SCD) is a key rate-limiting enzyme in the synthesis of unsaturated fatty acids by insertion of a cis-double bond in the Delta9 position of fatty acid substrates. Palmitate and stearate are the preferred substrates, which are converted to palmitoleate and oleate, respectively. These monounsaturated fatty acids are the major constituents of cellular membrane phospholipids and triacylglycerol stores found in adipose tissue. Two mouse and rat SCD genes (SCD1 and SCD2) have been cloned and characterized. During the differentiation of 3T3-L1 preadipocytes into adipocytes, SCD1 and SCD2 mRNAs are induced concomitant with increased de novo synthesis of palmitoleate and oleate. The physiological significance of expressing the two isoforms in the adipocytes is currently unknown. In this review we discuss the role of the SCD isoforms in metabolism and the recent findings on the differential regulation of mouse SCD genes by the antidiabetic thiazolidinediones (TZDs), during preadipocyte differentiation.

Adipocytes↗

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

The lipid composition of cellular membranes is regulated to maintain membrane fluidity. A key enzyme involved in this process is the membrane-bound stearoyl-CoA desaturase (SCD) which is the rate-limiting enzyme in the cellular synthesis of monounsaturated fatty acids from saturated fatty acids. A proper ratio of saturated to monounsaturated fatty acids contributes to membrane fluidity. Alterations in this ratio have been implicated in various disease states including cardiovascular disease, obesity, non-insulin-dependent diabetes mellitus, hypertension, neurological diseases, immune disorders, and cancer. The regulation of stearoyl-CoA desaturase is therefore of considerable physiological importance and its activity is sensitive to dietary changes, hormonal imbalance, developmental processes, temperature changes, metals, alcohol, peroxisomal proliferators, and phenolic compounds. Two mouse and rat SCD genes (SCD1 and SCD2) and a single human SCD gene have been cloned and characterized. In the past several years we have studied the dietary influences on the genetic expression of the mouse stearoyl-CoA desaturase. The expression of the mouse SCD genes is regulated by polyunsaturated fatty acids and cholesterol at the levels of transcription and mRNA stability. Promoter elements that are responsible for the polyunsaturated fatty acid repression colocalize with the promoter elements for SREBP-mediated regulation of the SCD genes. It is the goal of this review to provide an overview of the genetic regulation of the stearoyl-CoA desaturase in response to dietary polyunsaturated fatty acids and cholesterol.

Animals↗

Conjugated linoleic acid decreases hepatic stearoyl-CoA desaturase mRNA expression.

Conjugated dienoic derivatives of linoleic acid (CLA) is a collective term for positional and geometric isomers of linoleic acid that occur naturally in foods. The two predominant isomers of CLA are the c9,t11 and t10,c12. One of the effects of CLA is to modify membrane fatty acid composition by decreasing the activity of stearoyl-CoA desaturase enzyme activity. We analyzed the changes of stearoyl-CoA desaturase gene 1 (scd1) mRNA to further define the mechanism for the decrease in Scd enzyme activity by CLA. Mice fed for two weeks with either a fat-free high carbohydrate diet (CHO) or a 5.0% corn oil diet (CO), supplemented with 0.5% CLA had a 45% and 75% decrease respectively, in scd1 mRNA levels in the liver. Consistent with the effects observed in mice, 150 microM CLA suppressed the expression of scd1 mRNA in the H2.35 mouse liver cells by 60%. Further studies with enzymatically prepared c9,t11 isomer showed that the inhibitory effect of CLA on scd1 mRNA expression in H2.35 liver cells was by isomers other than the c9,t11-CLA.

Animals↗

Nuclear factor 1 is essential for the expression of stearoyl-CoA desaturase 1 gene during preadipocyte differentiation.

Stearoyl CoA desaturase gene 1 (SCD1) is a delta 9 desaturase that is transcriptionally activated during the differentiation of 3T3-L1 preadipocytes into adipocytes. We have demonstrated that a SCD1/BP region in SCD1 proximal promoter (-114 to -86 bp) is essential for the transcriptional activation of this gene during differentiation. Mutation in this region abolished the basal activity of the proximal promoter of SCD1, and also failed to induce transcription in response to the differentiation cocktail in transfected cells. The SCD1/BP region contains a TGGCA sequence at -90 bp from the transcription start site. Using competitor oligonucleotides and nuclear factor 1 (NF1)-specific antibodies in gel shift assays, we have shown that in preadipocytes, a NF1 protein binds to this TGGCA sequence. On MDI-induced differentiation of preadipocyte into adipocyte, an additional DNA-protein complex appeared. The appearance of a new NF1 complex is related to the differentiation-specific transcriptional activation of the SCD1 gene. This is the first report to show a differentiation-related change in NF1 protein binding during preadipocyte differentiation.

3T3 Cells↗

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.

Adipocytes↗

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.

Adipocytes↗

Localization of a negative thyroid hormone-response region in hepatic stearoyl-CoA desaturase gene 1.

The effect of thyroid hormone on stearoyl-CoA desaturase gene 1 (SCD1) expression was investigated in mouse liver. Daily injections of 15 micrograms triiodothyronine (T3)/100 g body weight to hypothyroid mice resulted in repression of SCD1 mRNA levels by more than 50% in 48 hours and up to 65% in 6 days. Transient co-transfections were performed with an expression vector for T3 receptor alpha (T3R alpha) in HepG2 cells using chimeric reporter gene constructs of the SCD1 5'-flanking region. Transcriptional repression of the SCD1 putative promoter was observed upon treatment with 100 nM T3 when cotransfected with T3R alpha, but not without cotransfection of receptor. Transient gene expression studies localized a T3 response region to a 70-bp sequence in the SCD1 putative promoter. Eliminating the TATA box and an AP-2 binding site, DNA mobility shift analysis demonstrated specific binding of in vivo nuclear protein from mouse liver nuclear extract to a 43-bp sequence. DNA mobility shift with purified T3R alpha confirmed the presence of a T3 receptor binding site in this thyroid hormone-responsive region. These data indicate that SCD1 contains a negative T3 response region in its proximal promoter.

Animals↗