Regulation of stearoyl-CoA desaturases and role in metabolism

Stearoyl-CoA desaturase (SCD) is the rate-limiting enzyme catalyzing the synthesis of monounsaturated fatty acids, mainly oleate (18:1) and palmitoleate (16:1). These represent the major monounsaturated fatty acids of membrane phospholipids, triglycerides, wax esters and cholesterol esters. The ratio of saturated to monounsaturated fatty acids affects phospholipid composition and alteration in this ratio has been implicated in a variety of disease states including cardiovascular disease, obesity, diabetes, neurological disease, and cancer. For this reason, the expression of SCD is of physiological significance in both normal and disease states. Several SCD gene isoforms (SCD1, SCD2, SCD3) exist in the mouse and one SCD isoform that is highly homologous to the mouse SCD1 is well characterized in human. The physiological role of each SCD isoform and the reason for having three or more SCD gene isoforms in the rodent genome are currently unknown but could be related the substrate specificities of the isomers and their regulation through tissue-specific expression. The recent studies of asebia mouse strains that have a natural mutation in the SCD1 gene and a mouse model with a targeted disruption of the SCD1 gene have provided clues concerning the role that SCD1 and its endogenous products play in the regulation of metabolism.     

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.     

Recent insights into stearoyl-CoA desaturase-1

PURPOSE OF REVIEW: Stearoyl-Coenzyme A (CoA) desaturase is a central lipogenic enzyme catalyzing the synthesis of monounsaturated fatty acids - mainly oleate (C(18:1)). Oleate is the most abundant monounsaturated fatty acid in dietary fat and is therefore readily available. Why, then, is stearoyl-CoA desaturase a highly regulated enzyme? This review summarizes the recent and timely advances concerning the important role of stearoyl-CoA desaturase in metabolism. RECENT FINDINGS: Recent findings using mice that have a naturally occurring mutation in the SCD1 gene isoform as well as a mouse model with a targeted disruption of the stearoyl-CoA desaturase gene-1 (SCD1-/-) have revealed the role of de-novo synthesized oleate and thus the physiological importance of SCD1 expression. In the highlighted references, it is shown that the SCD1-/- mice have reduced body adiposity, increased insulin sensitivity, and are resistant to diet-induced obesity. The expression of several genes of lipid oxidation is upregulated, whereas lipid synthesis genes are downregulated. SCD1 was also found to be a component of the novel metabolic response to the hormone leptin. SUMMARY: SCD1, therefore, appears to be an important metabolic control point, and inhibition of its expression could be of benefit for the treatment of obesity, diabetes and other metabolic diseases.     

Identification of mouse palmitoyl-coenzyme A Delta9-desaturase

Stearoyl-coenzyme A desaturase (SCD) catalyzes the desaturation of saturated fatty acids to monounsaturated fatty acids in mammalian cells. Currently, there are four known enzymatic isoforms (SCD1-SCD4) in the mouse genome. The physiological roles for multiple SCD isoforms and their substrate specificities are unknown at present. We report here distinct substrate specificities for the mouse SCD isoforms. Each SCD isoform was able to complement the ole1 mutation in Saccharomyces cerevisiae through heterologous expression of transgenic SCD. Fatty acid analysis showed that mouse SCD1, SCD2, and SCD4 desaturate both C18:0 and C16:0, whereas mouse SCD3 uses C16:0 but not C18:0. We identify SCD3 as a mammalian palmitotyl-CoA Delta9-desaturase, and its existence in mouse helps explain distinct physiological roles for each SCD isoform.     

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.     

Characterization of HSCD5, a novel human stearoyl-CoA desaturase unique to primates

Stearoyl-CoA desaturase (SCD) is an integral membrane protein of the endoplasmic reticulum (ER) that catalyzes the formation of monounsaturated fatty acids from saturated fatty acids. Recent studies suggest that SCD is a key regulator of energy metabolism and has implications in dislipidemia and obesity. Four SCD isoforms (SCD1-4) have been identified in mouse. In human, only one SCD isoform has been characterized so far. Here we report that the previously reported human ACOD4 gene encodes a distinct stearoyl-CoA desaturase, hSCD5. GenBank database mining revealed orthologues of hSCD5 in the primates, but not in the rodents. In transiently transfected 293 cells, hSCD5 co-localized with calnexin on ER membrane. Microsome fractions prepared from hSCD1 and hSCD5 transfected cells displayed similar delta 9 desaturase activity. Quantitative real-time RT-PCR analysis suggested that hSCD5 was abundantly expressed in adult brain and pancreas. These data suggested that hSCD5 plays a role distinct from that of hSCD1 during development and in normal physiological conditions.     

Membrane topology of mouse stearoyl-CoA desaturase 1

Stearoyl-CoA desaturase (SCD) is an integral membrane protein anchored in the endoplasmic reticulum. It catalyzes the biosynthesis of monounsaturated fatty acids that are required for the synthesis of triglycerides, cholesteryl esters, and phospholipids. Four mouse isoforms of SCD (SCD1-4) and two human isoforms have been characterized. In the current study, we characterize the topology of the mouse SCD1 isoform. Hydropathy analysis of the 355-amino acid mouse SCD1 protein predicts that the protein contains four transmembrane domains (TMDs) and three loops connecting the membrane-spanning domains. To define the topology of the protein, recombinant SCD1 constructs containing epitope tags were transiently expressed in HeLa cells and analyzed by indirect immunofluorescence and cysteine derivatization. Our data provide evidence that the N and C termini of SCD1 are oriented toward the cytosol with four transmembrane domains separated by two very short hydrophilic loops in the ER lumen and one large hydrophilic loop in the cytosol. In addition, based on the previous observation that SCD is a thiol enzyme, we sought to investigate whether the cysteine residues were essential for enzyme activity through mutagenesis studies, and our data suggest that the cysteines in SCD are not catalytically essential.     

Colocalization of SCD1 and DGAT2: implying preference for endogenous monounsaturated fatty acids in triglyceride synthesis

Stearoyl-coenzyme A desaturase (SCD) is an endoplasmic reticulum (ER) protein that catalyzes the Delta9-cis desaturation of saturated fatty acids. Mice with targeted disruption in SCD1 (Scd1(-/-)) have significant reduction in the tissue content of triglycerides, suggesting that monounsaturated fatty acids endogenously synthesized by SCD1 are important for triglyceride synthesis. Acyl-coenzyme A:diacylglycerol acyltransferase (DGAT) is the enzyme that catalyzes the final reaction in the synthesis of triglycerides. The lack of DGAT2, one of the two DGAT isoforms, results in almost a complete loss of tissue triglycerides. We hypothesize that SCD1 participates in triglyceride synthesis by providing a more accessible pool of monounsaturated fatty acids through substrate channeling. In this study, we test whether SCD1 is proximal to DGAT2 by colocalization study with confocal microscopy, coimmunoprecipitation, and fluorescence resonance energy transfer using HeLa cells as the model of study. All of the results suggest that SCD1 and DGAT2 are located very close to each other in the ER, which is a very important criterion for the channeling of substrate. By performing subcellular fractionation using mouse livers, we also show, for the first time, that SCD is present in the mitochondria-associated membrane.     

Lack of stearoyl-CoA desaturase-1 function induces a palmitoyl-CoA Delta6 desaturase and represses the stearoyl-CoA desaturase-3 gene in the preputial glands of the mouse

The mouse preputial gland (PG), a specialized sebaceous structure, is rich in wax esters, triglycerides, and alkyl-2,3-diacylglycerol. We have found that the mouse PG expresses the three gene isoforms (SCD1, SCD2, and SCD3) of the Delta9 stearoyl-CoA desaturase enzyme that catalyzes the biosynthesis of monounsaturated fatty acids mainly, C16:1n-7 and C18:1n-9. However, mice with a targeted disruption in the SCD1 isoform (SCD1(-/-)) have undetectable SCD3 mRNA expression in the PG while the expression of SCD2 isoform was not altered. The levels of C16:1n-7 were reduced by greater than 70% while that of C18:1n-9 were reduced by 28%. The content of the C16:1n-10 (Delta6 hexadecenoic acid) isomer and a major fatty acid of the PG was increased by greater than 2-fold, mainly in the wax ester fraction of the SCD1(-/-) mouse. We demonstrate that the increase in C16:1n-10 is due to induction of a specific palmitoyl-CoA Delta6 desaturase activity. Testosterone administration to the SCD1(-/-) mouse induced SCD3 mRNA expression and resulted in an increase in the Delta9 desaturation of 16:0-CoA, but not of 18:0-CoA. These observations demonstrate that loss of SCD1 function alters the expression of SCD3 and reveal for the first time the presence and regulation of a palmitoyl-CoA Delta6 desaturase enzyme in mammals.     

SCD1 is nutritionally and spatially regulated in the intestine and influences systemic postprandial lipid homeostasis and gut-liver crosstalk

Stearoyl-CoA desaturase-1 is an endoplasmic reticulum (ER)-membrane resident protein that inserts a double bond into saturated fatty acids, converting them into their monounsaturated counterparts. Previous studies have demonstrated an important role for SCD1 in modulating tissue and systemic health. Specifically, lack of hepatic or cutaneous SCD1 results in significant reductions in tissue esterified lipids. While the intestine is an important site of lipid esterification and assimilation into the body, the regulation of intestinal SCD1 or its impact on lipid composition in the intestine and other tissues has not been investigated. Here we report that unlike other lipogenic enzymes, SCD1 is enriched in the distal small intestine and in the colon of chow-fed mice and is robustly upregulated by acute refeeding of a high-sucrose diet. We generated a mouse model lacking SCD1 specifically in the intestine (iKO mice). These mice have significant reductions not only in intestinal lipids, but also in plasma triacylglycerols, diacylglycerols, cholesterol esters, and free cholesterol. Additionally, hepatic accumulation of diacylglycerols is significantly reduced in iKO mice. Comprehensive targeted lipidomic profiling revealed a consistent reduction in the myristoleic (14:1) to myristic (14:0) acid ratios in intestine, liver, and plasma of iKO mice. Consistent with the reduction of the monounsaturated fatty acid myristoleic acid in hepatic lipids of chow fed iKO mice, hepatic expression of Pgc-1α, Sirt1, and related fatty acid oxidation genes were reduced in chow-fed iKO mice. Further, lack of intestinal SCD1 reduced expression of de novo lipogenic genes in distal intestine of chow-fed mice and in the livers of mice fed a lipogenic high-sucrose diet. Taken together, these studies reveal a novel pattern of expression of SCD1 in the intestine. They also demonstrate that intestinal SCD1 modulates lipid content and composition of not only intestinal tissues, but also that of plasma and liver. Further, these data point to intestinal SCD1 as a modulator of gut-liver crosstalk, potentially through the production of novel signaling lipids such as myristoleic acid. These data have important implications to understanding how intestinal SCD1 may modulate risk for post-prandial lipemia, hepatic steatosis, and related pathologies.     

Stearoyl-CoA desaturase and insulin signaling — What is the molecular switch?

Increasing evidence suggests that stearoyl-CoA desaturase (SCD), the rate-limiting enzyme of monounsaturated fatty acid biosynthesis, is an important factor in the pathogenesis of lipid-induced insulin resistance. Mice with a targeted disruption of the SCD1 gene have improved glucose tolerance compared to wild-type mice, despite lower fasting plasma insulin levels. Increased SCD activity has been found in insulin-resistant humans and animals, whereas SCD1 deficiency attenuates both diet- and genetically-induced impairment of insulin action. Phosphorylation of serine and threonine residues on insulin receptor, insulin receptor substrates (IRS1 and IRS2), and on Akt has been shown to be the major step in insulin signaling that is altered due to the lack of SCD1. In this review we discuss perturbations in cell signaling and lipid metabolism cascades in insulin-sensitive tissues due to SCD1 deficiency. In particular, we address the role of cellular signaling molecules including free fatty acids, ceramides, fatty acyl-CoAs, AMP-activated protein kinase, protein tyrosine phosphatase 1B as well as of membrane fluidity. While the precise mechanism of SCD action on insulin signaling remains to be clarified, current findings on SCD point to a very promising novel target for the treatment of insulin resistance.     

StearoylCoA desaturase-5: a novel regulator of neuronal cell proliferation and differentiation

Recent studies have demonstrated that human stearoylCoA desaturase-1 (SCD1), a Δ9-desaturase that converts saturated fatty acids (SFA) into monounsaturated fatty acids, controls the rate of lipogenesis, cell proliferation and tumorigenic capacity in cancer cells. However, the biological function of stearoylCoA desaturase-5 (SCD5), a second isoform of human SCD that is highly expressed in brain, as well as its potential role in human disease, remains unknown. In this study we report that the constitutive overexpression of human SCD5 in mouse Neuro2a cells, a widely used cell model of neuronal growth and differentiation, displayed a greater n-7 MUFA-to-SFA ratio in cell lipids compared to empty-vector transfected cells (controls). De novo synthesis of phosphatidylcholine and cholesterolesters was increased whereas phosphatidylethanolamine and triacylglycerol formation was reduced in SCD5-expressing cells with respect to their controls, suggesting a differential use of SCD5 products for lipogenic reactions. We also observed that SCD5 expression markedly accelerated the rate of cell proliferation and suppressed the induction of neurite outgrowth, a typical marker of neuronal differentiation, by retinoic acid indicating that the desaturase plays a key role in the mechanisms of cell division and differentiation. Critical signal transduction pathways that are known to modulate these processes, such epidermal growth factor receptor (EGFR)Akt/ERK and Wnt, were affected by SCD5 expression. Epidermal growth factor-induced phosphorylation of EGFR, Akt and ERK was markedly blunted in SCD5-expressing cells. Furthermore, the activity of canonical Wnt was reduced whereas the non-canonical Wnt was increased by the presence of SCD5 activity. Finally, SCD5 expression increased the secretion of recombinant Wnt5a, a non-canonical Wnt, whereas it reduced the cellular and secreted levels of canonical Wnt7b. Our data suggest that, by a coordinated modulation of key lipogenic pathways and transduction signaling cascades, SCD5 participates in the regulation of neuronal cell growth and differentiation.     

Stearoyl-CoA desaturase-1 (SCD1) augments saturated fatty acid-induced lipid accumulation and inhibits apoptosis in cardiac myocytes

Mismatch between the uptake and utilization of long-chain fatty acids in the myocardium leads to abnormally high intracellular fatty acid concentration, which ultimately induces myocardial dysfunction. Stearoyl-Coenzyme A desaturase-1 (SCD1) is a rate-limiting enzyme that converts saturated fatty acids (SFAs) to monounsaturated fatty acids. Previous studies have shown that SCD1-deficinent mice are protected from insulin resistance and diet-induced obesity; however, the role of SCD1 in the heart remains to be determined. We examined the expression of SCD1 in obese rat hearts induced by a sucrose-rich diet for 3 months. We also examined the effect of SCD1 on myocardial energy metabolism and apoptotic cell death in neonatal rat cardiac myocytes in the presence of SFAs. Here we showed that the expression of SCD1 increases 3.6-fold without measurable change in the expression of lipogenic genes in the heart of rats fed a high-sucrose diet. Forced SCD1 expression augmented palmitic acid-induced lipid accumulation, but attenuated excess fatty acid oxidation and restored reduced glucose oxidation. Of importance, SCD1 substantially inhibited SFA-induced caspase 3 activation, ceramide synthesis, diacylglycerol synthesis, apoptotic cell death, and mitochondrial reactive oxygen species (ROS) generation. Experiments using SCD1 siRNA confirmed these observations. Furthermore, we showed that exposure of cardiac myocytes to glucose and insulin induced SCD1 expression. Our results indicate that SCD1 is highly regulated by a metabolic syndrome component in the heart, and such induction of SCD1 serves to alleviate SFA-induced adverse fatty acid catabolism, and eventually to prevent SFAs-induced apoptosis.