Agnieszka Dobrzyn

Stearoyl-CoA Desaturase-1 Mediates the Pro-lipogenic Effects of Dietary Saturated Fat

Dietary saturated fats have often been implicated in the promotion of obesity and related disorders. It has been shown recently that saturated fats act through the transcription factor SREBP-1c (sterol regulatory element-binding protein-1c) and its requisite coactivator, peroxisome proliferator-activated receptor-γ coactivator-1β (PGC-1β), to exert their pro-lipogenic effects. We show here that a diet high in the saturated fat stearate induces lipogenic genes in wild-type mice, with the induction of the Scd1 (stearoyl-CoA desaturase-1) gene preceding that of other lipogenic genes. However, in Scd1-/- mice, stearate does not induce lipogenesis, and Srebp-1c and Pgc-1β levels are markedly reduced. Instead, genes of fatty acid oxidation such as Cpt-1 (carnitine palmitoyltransferase-1) as well as Pgc-1α are induced. Mitochondrial fatty acid oxidation is increased, and white adipose tissue and hepatic glycogen stores are depleted in stearate-fed Scd1-/- mice. Furthermore, AMP-activated protein kinase is also induced by stearate feeding in Scd1-/- mice. These results indicate that the desaturation of saturated fats such as stearate by SCD is an essential step mediating their induction of lipogenesis. In the absence of SCD1, stearate promotes oxidation, leading to protection from saturated fat-induced obesity. SCD1 thus serves as a molecular switch in the promotion or prevention of lipid-induced disorders brought on by consumption of excess saturated fat.

Stearoyl-CoA desaturase 1 deficiency elevates insulin-signaling components and down-regulates protein-tyrosine phosphatase 1B in muscle

We have shown previously that mice with a targeted disruption in the stearoyl-CoA desaturase 1 gene (SCD1-/-) have increased insulin sensitivity compared with control mice. Here we show that the SCD1-/- mice have increased insulin signaling in muscle. The basal tyrosine phosphorylation of the insulin receptor and insulin receptor substrates 1 and 2 are elevated. The tyrosine phosphorylation of insulin-like growth factor-1 receptor was similar between SCD1+/+ and SCD1-/- mice. The association of insulin receptor substrates 1 and 2 with αp85 subunit of phosphatidylinositol 3-kinase as well as the phosphorylation of Akt-Ser-473 and Akt-Thr-308 are also elevated in the SCD1-/- mice. Interestingly, the mRNA levels, protein mass, and activity of the protein-tyrosine phosphatase-1B implicated in the attenuation of the insulin signal are reduced in the SCD1-/- mice, whereas the levels of the leukocyte antigen-related protein phosphatase are similar between two groups of mice. The content of glucose transporter 4 in the plasma membrane and basal as well as insulin-mediated glucose uptake are increased in the SCD1-/- mice. In addition, the muscle glycogen content and the activities of glycogen synthase and phosphorylase are increased in the SCD1-/- mice. We hypothesize that loss of SCD1 function induces increased insulin signaling at least in part by a reduction in the expression of protein-tyrosine phosphatase 1B. SCD1 could be a therapeutic target in the treatment of diabetes.

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.

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 monousaturated 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.

Exercise and training effects on ceramide metabolism in human skeletal muscle

In rat skeletal muscle prolonged exercise affects the content and composition of ceramides, but in human skeletal muscle no data are available on this compound. Our aim was to examine the content of ceramide‐ and sphingomyelin fatty acids and neutral, Mg2+‐dependent sphingomyelinase activity in skeletal muscle in untrained and trained subjects before and after prolonged exercise. Healthy male subjects were recruited into an untrained (n= 8, VO2,max 3.8 ± 0.2 l min1) and a trained (n= 8, VO2,max 5.1 ± 0.1 l min1) group. Before and after a 3‐h exercise bout (58 ± 1%VO2,max) a muscle biopsy was excised from the vastus lateralis. Ceramide and sphingomyelin were isolated using thin‐layer chromatography. The content of individual ceramide fatty acids and sphingomyelin fatty acids was measured by means of gas‐liquid chromatography. The activity of neutral, Mg2+‐dependent sphingomyelinase was measured using N‐[14CH3]‐sphingomyelin as a substrate. Prior to exercise, the muscle total ceramide fatty acid content in both groups was similar (201 ± 18 and 197 ± 9 nmol g−1 in the untrained and trained group, respectively) and after exercise a 25% increase in the content was observed in each group. At rest, the muscle total sphingomyelin fatty acid content was higher in untrained than in trained subjects (456 ± 10, 407 ± 7 nmol g−1, respectively; P < 0.05). After exercise a 20% increase (P < 0.05) in total sphingomyelin was observed only in the trained subjects. The muscle neutral, Mg2+‐dependent sphingomyelinase activity was similar in the two groups at rest and a similar reduction was observed after exercise in both groups (untrained from 2.19 ± 0.08 to 1.78 ± 0.08 and trained from 2.31 ± 0.12 to 1.80 ± 0.09 nmol (mg protein)−1 h−1; P < 0.05 in each case). In conclusion, we have reported, for the first time, the values for ceramide fatty acid content and neutral, Mg2+‐dependent sphingomyelinase activity in human skeletal muscle. The results indicate that acute prolonged exercise affects ceramide metabolism in human skeletal muscle both in untrained and in trained subjects and this may influence muscle cell adaptation and metabolism.

Stearoyl-CoA desaturase-1 deficiency attenuates obesity and insulin resistance in leptin-resistant obese mice.

Obesity and adiposity greatly increase the risk for secondary conditions such as insulin resistance. Mice deficient in the enzyme stearoyl-CoA desaturase-1 (SCD1) are lean and protected from diet-induced obesity and insulin resistance. In order to determine the effect of SCD1 deficiency on various mouse models of obesity, we introduced a global deletion of the Scd1 gene into leptin-deficient ob/ob mice, leptin-resistant Agouti (A(y)/a) mice, and high-fat diet-fed obese (DIO) mice. SCD1 deficiency lowered body weight, adiposity, hepatic lipid accumulation, and hepatic lipogenic gene expression in all three mouse models. However, glucose tolerance, insulin, and leptin sensitivity were improved by SCD1 deficiency only in A(y)/a and DIO mice, but not ob/ob mice. These data uncouple the effects of SCD1 deficiency on weight loss from those on insulin sensitivity and suggest a beneficial effect of SCD1 inhibition on insulin sensitivity in obese mice that express a functional leptin gene.

Statin Therapy and New-onset Diabetes: Molecular Mechanisms and Clinical Relevance

Despite positive effects on the plasma lipid profile and vascular events, statin use is associated with various side effects. Among these, statins might cause a disruption of a number of regulatory pathways including insulin signaling. This may affect insulin sensitivity, pancreatic beta-cell function and adipokine secretion. The statin-associated risk of new-onset diabetes (NOD) appears to be a dose-dependent class effect. It still remains unclear whether statin treatment is associated with increased risk of NOD in the general population or if there are groups of individuals at particular risk. However, according to the available data it seems that cardiovascular (CV) benefits in high-risk individuals strongly favor statin therapy since it outweighs other risks. Whether statins should be used for primary prevention among patients with a relatively low baseline CV risk is still questionable, however the results of primary prevention trials have shown reductions in mortality in this population. Thus, there is a need for randomized, placebo-controlled statin studies with carefully selected groups of patients and NOD as a key end point in order to resolve queries concerning this issue.

Stearoyl-CoA Desaturase 1 Deficiency Increases CTP:Choline Cytidylyltransferase Translocation into the Membrane and Enhances Phosphatidylcholine Synthesis in Liver

Stearoyl-CoA desaturase (SCD) is the rate-limiting enzyme in monounsaturated fatty acid synthesis. Previously, we showed that Scd1 deficiency reduces liver triglyceride accumulation and considerably decreases synthesis of very low density lipoprotein and its secretion in both lean and obese mice. In the present study, we found that Scd1 deficiency significantly modulates hepatic glycerophospholipid profile. The content of phosphatidylcholine (PC) was increased by 40% and the activities of CTP:choline cytidylyltransferase (CCT), the rate-limiting enzyme in de novo PC synthesis, and choline phosphotransferase were increased by 64 and 53%, respectively, in liver of Scd1-/- mice. In contrast, the protein level of phosphatidylethanolamine N-methyltransferase, an enzyme involved in PC synthesis via methylation of phosphatidylethanolamine, was decreased by 80% in the liver of Scd1-/- mice. Membrane translocation of CCT is required for its activation. Immunoblot analyses demonstrated that twice as much CCTα was associated with plasma membrane in livers of Scd1-/- compared with wild type mice, suggesting that Scd1 mutation leads to an increase in CCT membrane affinity. The incorporation of [3H]glycerol into PC was increased by 2.5-fold in Scd1-/- primary hepatocytes compared with those of wild type mice. Furthermore, mitochondrial glycerol-3-phosphate acyltransferase activity was reduced by 42% in liver of Scd1-/- mice; however, the activities of microsomal glycerol-3-phosphate acyltransferase, diacylglycerol acyltransferase, and ethanolamine phosphotransferase were not affected by Scd1 mutation. Our study revealed that SCD1 deficiency specifically increases CCT activity by promoting its translocation into membrane and enhances PC biosynthesis in liver.

Loss of stearoyl-CoA desaturase 1 rescues cardiac function in obese leptin-deficient mice

The heart of leptin-deficient ob/ob mice is characterized by pathologic left ventricular hypertrophy along with elevated triglyceride (TG) content, increased stearoyl-CoA desaturase (SCD) activity, and increased myocyte apoptosis. In the present study, using an ob/ob;SCD1−/− mouse model, we tested the hypothesis that lack of SCD1 could improve steatosis and left ventricle (LV) function in leptin deficiency. We show that disruption of the SCD1 gene improves cardiac function in ob/ob mice by correcting systolic and diastolic dysfunction without affecting levels of plasma TG and FFA. The improvement is associated with reduced expression of genes involved in FA transport and lipid synthesis in the heart, as well as reduction in cardiac FFA, diacylglycerol, TG, and ceramide levels. The rate of FA β-oxidation is also significantly lower in the heart of ob/ob;SCD1−/− mice compared with ob/ob controls. Moreover, SCD1 deficiency reduces cardiac apoptosis in ob/ob mice due to increased expression of antiapoptotic factor Bcl-2 and inhibition of inducible nitric oxide synthase and caspase-3 activities. Reduction in myocardial lipid accumulation and inhibition of apoptosis appear to be one of the main mechanisms responsible for improved LV function in ob/ob mice caused by SCD1 deficiency.

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.