Loss of Stearoyl-CoA Desaturase-1 Attenuates Adipocyte Inflammation
Effects of Adipocyte-Derived Oleate
Background and Purpose— Adipose inflammation is crucial to the pathogenesis of metabolic disorders. This study aimed at identify the effects of stearoyl-CoA desaturase-1 (SCD1) on the inflammatory response of a paracrine network involving adipocytes, macrophages, and endothelial cells.
Methods and Results— Loss of SCD1 in both genetic (Agouti) and diet-induced obesity (high-fat diet) mouse models prevented inflammation in white adipose tissue and improved its basal insulin signaling. In SCD1-deficient mice, white adipose tissue exhibited lower inflammation, with a reduced response to lipopolysaccharide in isolated adipocytes, but not in peritoneal macrophages. Mimicking the in vivo paracrine regulation of white adipose tissue inflammation, SCD1-deficient adipocyte-conditioned medium attenuated the induction of tumor necrosis factor (TNF) α/interleukin 1β gene expression in RAW264.7 macrophages and reduced the adhesion response in endothelial cells. We further demonstrated that the adipocyte-derived oleate (18:1n9), but not palmitoleate (16:1n7), mediated the inflammation in macrophages and adhesion responses in endothelial cells.
Conclusions— Loss of SCD1 attenuates adipocyte inflammation and its paracrine regulation of inflammation in macrophages and endothelial cells. The reduced oleate level is linked to the inflammation-modulating effects of SCD1 deficiency.
Chronic inflammation plays a causative role in the emergence of various metabolic disorders, including type 2 diabetes mellitus, insulin resistance, and atherosclerosis.1 This inflammatory condition is provoked by diverse factors, including reactive oxygen species, endoplasmic reticulum stress, hypoxia, lipotoxicity, and protein kinase C isoforms.2–4 More important, lipids have been implicated in the coordinate regulation of metabolism and inflammatory and immune responses.5 The modulation of inflammation by lipids has been further demonstrated by a recent study that showed that fatty acids are ligands for Toll-like receptor 4 (TLR4) in macrophages.6
In the initiation of chronic inflammation, white adipose tissue (WAT) plays a central role, although other tissues, such as liver, might also be involved.7 The cross talk among adipocytes, macrophages, and endothelial cells in WAT orchestrates the inflammatory response in this tissue. The subsequent increased production of proinflammatory cytokines and chemokines, such as tumor necrosis factor (TNF) α, interleukin (IL) 6, plasminogen activator inhibitor (PAI)-1, and monocyte chemoattractant protein (MCP) 1 in WAT, leads to insulin resistance and increases the risk of cardiovascular disease associated with obesity.8,9 Therefore, the prevention of WAT inflammation is potentially beneficial for controlling chronic inflammation.
Stearoyl-CoA desaturase (SCD) catalyzes the rate-limiting step in the conversion of saturated to monounsaturated fatty acids (MUFAs) (mainly oleate [18:1n9] and palmitoleate [16:1n7]). This enzyme plays a central role in lipogenesis in rodents and humans.10 Several SCD gene isoforms (SCD1–4) have been identified in the mouse, and two SCD isoforms (SCD1 and SCD5) that are highly homologous to the mouse SCDs are well characterized in humans. Despite the high abundance of oleate (18:1n9) as a major MUFA from diet, the expression of SCD is highly regulated by developmental, dietary, hormonal, and environmental factors. Because of the involvement of MUFAs in the regulation of diverse processes, including signal transduction, cell differentiation, and neuronal development,11–13 SCD is regarded as an important enzyme in the regulation of normal and pathophysiological processes. Altered SCD activity has been implicated in a variety of morbidities, such as obesity, diabetes, atherosclerosis, cancer, and immune disorders.13
Although SCD1 has been well established as a key regulator of metabolism, recent studies14,15 have also reported its influence on inflammatory processes. However, the function of SCD1 expression in regulating adipocyte and WAT inflammation is poorly understood. In the current study, we demonstrate that SCD1 deficiency prevents WAT inflammation and improves insulin signaling under the challenge of obesity. For the first time to our knowledge, we demonstrate the effects of SCD1 deficiency on adipocyte inflammation, and reveal unique functions of oleate as a lipid inflammation mediator in linking the network of adipocytes, macrophages, and endothelial cells.
Animals and Diets
All mice used in the study were C57BL6/J males. The homozygous SCD1-deficient mice were generated, genotyped, and maintained as described.16 The breeding of mice was in accordance with the protocols approved by the animal care research committee of the University of Wisconsin, Madison. A standard Purina formula 5008 chow diet was used as regular food. Male chow-fed mice, aged 12 to 15 weeks, were used for primary cell preparation. For studies in genetically obese mice, the SCD1 deficiency was introduced into Agouti mice by crossing Agouti:SCD1-deficient or Agouti:SCD1-nondeficient with SCD1-deficient mice and generating Agouti:SCD1-deficient mice. Wild-type (WT) control mice, Agouti mice, and Agouti:SCD1-deficient mice were fed ad libitum with chow diet until the age of 24 weeks. In diet-induced obesity, 8-week-old WT and SCD1-deficient mice were fed a high-fat diet (HFD) (Research Diets, RD12492) until the age of 24 weeks.
Isolation of Primary Adipocytes, Stromal Vascular Cells, and Resident Peritoneal Macrophages
Primary adipocytes, WAT stromal vascular cells (SVCs), and peritoneal macrophages were isolated as described,17 with minor modifications. Age-matched WT and SCD1-deficient mice were used in the isolation. The WAT used in all studies was from an epididymal fat pad.
Values shown in the study were expressed as mean±SEM. Statistical analysis with three or more groups was done using one-way ANOVA analysis of variance with a Bonferroni Post test, and the difference between the two groups was tested by two-tail, unpaired, Student t test; in both cases, significance was considered as P<0.05.
For additional methods and details, please refer to http://atvb.ahajournals.org for supplemental materials.
Loss of SCD1 Prevents WAT Inflammation in Obesity Mouse Models
WAT is a critical site in the initiation of chronic inflammation in obesity.18 We first examined the effects of SCD1 deficiency on the overall inflammatory status of WAT in both genetic Agouti mutation induced (Agouti) and HFD-induced obese mouse models. The activation of nuclear factor κB (NF-κB), a master proinflammation transcriptional factor, has been linked to adipose tissue inflammation in obesity.7 In the Agouti and HFD-fed mice, SCD1 deficiency substantially reduced WAT inflammation, which was shown by a decrease in the DNA binding activity of NF-κB p65/50, the transcriptional active dimer complex of NF-κB (Figure 1A). The expression of MCP-1, TNF-α, PAI-1, and vascular cell adhesion molecule (VCAM)-1, as well as CC-chemokine receptor 2 and colony-stimulating factor 1 receptor, was also reduced (Figure 1B and supplemental Figure 1). In addition, cell death in WAT, which is closely correlated with WAT inflammation, was also prevented in SCD1-deficient mice, as shown by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining of WAT sections (supplemental Figure 1). These results further indicate that loss of SCD1 suppresses obesity-associated WAT inflammation.
Because inflammation has been linked to insulin resistance,7 we next analyzed basal insulin signaling in WAT from SCD1-deficient mice. The decreased expression of insulin signaling components, such as insulin receptor β subunit, has been observed in obesity-related chronic insulin resistance.19 In SCD1-deficient mice, the decreased protein level of insulin receptorβ subunit in WAT from Agouti and HFD-fed WT mice was improved (Figure 1C). Consistently, the levels of serine 473 phosphorylation (pSer473) of Akt were increased (Figure 1C). Furthermore, the decreased mRNA levels of insulin receptor and insulin receptor substrate (IRS)-1 in WAT from obese mice were also prevented with loss of SCD1 in these mice (supplemental Figure 1). These results suggest that, consistent with the prevention of WAT inflammation, SCD1 deficiency improves basal WAT insulin signaling under the challenges of obesity.
Loss of SCD1 Reduces Inflammation in WAT and Primary Adipocytes, But Not in Peritoneal Macrophages
To test the role of SCD1 in WAT inflammation, we next analyzed the inflammatory response in WAT from WT and SCD1-deficient mice, which were fed a standard laboratory chow diet. The DNA binding activity of NF-κB p65/50 was reduced in WAT from SCD1-deficient mice (Figure 2A). To study the inflammation at a cellular level, we further isolated primary adipocytes, which were efficiently separated from SVCs in WAT (supplemental Figure 2). The numbers of isolated adipocytes were comparable between WT and SCD1-deficient mice (supplemental Figure 3). Consistently, the SCD1-deficient adipocytes exhibited lower expression of proinflammatory genes, such as MCP-1 and IL-6 (Figure 2B). On challenge with lipopolysaccharide (LPS), a TLR4 ligand, the induction of these two genes was also significantly reduced in the SCD1-deficient adipocytes (Figure 2B).
We tested the potential contribution of anti-inflammatory factors, such as peroxisome proliferator-activated receptor (PPAR)γ and PPARα, and the production of T-helper 2 anti-inflammatory cytokines. The gene expression of PPARγ and its target genes (PEPCK, Glut4, ADRP, and aP2) was not elevated in SCD1-deficient adipocytes (supplemental Figure 4), and the expression of T-helper 2 cytokines (IL-4 and IL-13) was undetectable in these adipocytes. The expression of PPARα target genes [camitine palmitoyl transferase (CPT) 1 and medium chain acyl CoA dehydrogenase (MCAD)] was not increased in the adipocytes from SCD1-deficient mice (supplemental Figure 4). These data suggest that the anti-inflammatory pathways were not involved in the inflammation-attenuating effects of SCD1 deficiency. Furthermore, the expression of certain marker genes for mitochondrial function, oxidative stress, and endoplasmic reticulum stress response was not substantially different between WT and SCD1-deficient adipocytes (supplemental Figure 4 and 5), suggesting that these processes may not play dominant role in SCD1-deficient adipocytes.
In isolated peritoneal macrophages, the baseline and LPS-stimulated expression of proinflammatory genes (MCP-1, TNF-α, cyclooxygenase (COX)-2, and IL-6) was not significantly different between WT and SCD1-deficient mice (supplemental Figure 2). A similar phenomenon was observed on treatment with lipoteichoic acid, a TLR2 ligand. In addition, no genotypic difference was detected in the expression of TLR4 and TLR2, two key receptors mediating inflammation. These data indicate that the modulation of inflammation by SCD1 deficiency is cell-type selective for adipocytes that express high levels of SCD1.
Furthermore, SCD1-deficient adipocytes displayed significantly lower TLR4, but not TLR2, gene expression (Figure 2C). In parallel, the TLR4 protein level was also lower in WAT from SCD1-deficient mice (Figure 2D). This selective downregulation of TLR4 expression in SCD1-deficient adipocytes may be one of the causative factors in attenuating the inflammatory responses.
SCD1-Deficient Adipocyte-Conditioned Medium Induces Lower Inflammation in RAW264.7 Macrophages
WAT inflammation is attributable to the interaction between adipocytes and resident macrophages in a paracrine manner.20,21 We collected the WT and SCD1-deficient adipocyte-conditioned media (CM) that contain adipocyte-derived soluble factors, and tested their effects on inflammation. The induction of proinflammatory genes TNF-α and IL-1β was significantly lower in RAW264.7 macrophages treated with SCD1-deficient CM, compared with the treatment with WT CM (Figure 3A). As the two adipocyte CMs were further diluted by 2- and 4-fold with basal media, the difference in the induction of TNF-α and IL-1β genes was reduced, and comparable induction was observed at 4-fold dilution (Figure 3A). These data indicate that adipocyte-derived soluble factors are the likely mediators of inflammation in macrophages, and that the levels of these factors are lower in SCD1-deficient CM.
SVCs from WAT contain multiple cell types, including tissue macrophages, endothelial cells, preadipocytes, and others. With SCD1 deficiency, these cells, which are subject to paracrine regulation by adipocytes in WAT in vivo, showed reduced expression of proinflammatory genes IL-6 and MCP-1 under both basal and LPS-stimulated conditions (Figure 3B).
SCD1-Deficient Adipocyte-Conditioned Media Reduces the Adhesion Response of Endothelial Cells
The paracrine regulation of WAT inflammation involving adipocytes also influences adhesion pathways in endothelial cells, which recruit circulating monocytes into WAT.21 We next investigated the effects of SCD1-deficient CM on adhesion of mouse monocytes to mouse aortic endothelial cells. Compared with the endothelial cells incubated with WT CM, SCD1-deficient CM significantly decreased the adhesion of monocytes to endothelial cells, as assessed by the expression of leukocyte-specific gene CD45 after the adhesion assay (Figure 4A). The mRNA levels of adhesion molecules inter-cellular adhesion molecules (ICAM)-1 and P-selectin were also consistently significantly lower in endothelial cells treated with SCD1-deficient CM, as shown in Figure 6C. Furthermore, the SVCs from the WAT of SCD1-deficient mice displayed a lower ratio of CD68 (macrophage marker gene) expression to adipocyte number and reduced expression of Mac1 (another macrophage marker gene) (Figure 4B and supplemental Figure 7), suggesting less abundance of macrophages in the WAT.
SCD1-Deficient Mice Are Resistant to Obesity-Associated Macrophage Infiltration Into the WAT
Given the reduced adhesion response in endothelial cells on treatment with SCD1-deficient CM, we further tested the effects of SCD1 deficiency on macrophage infiltration into WAT in obesity. SCD1-deficient mice, under two different obesity challenges (the genetic model Agouti22 and the HFD model), exhibited significantly decreased macrophage abundance, as shown by Emr1 (F4/80) staining in WAT sections (Figure 4C) and lower expression of Emr1 (F4/80) and CD68 (supplemental Figure 1). In addition, analysis of the cell morphology in WAT from SCD1-deficient mice also revealed diminished multinucleated cell structures, which are typical of the infiltrated macrophages in the WAT of obese mice (supplemental Figure 1).
Loss of SCD1 Leads to Reduced Content of Unsaturated Fatty Acids in Adipocytes and the Adipocyte Conditioned Media
Next, we set out to identify the adipocyte-derived inflammatory mediators in the CM that were affected by loss of SCD1. Adiponectin and leptin are two adipocyte-specific adipokines that are known to exert endocrine and paracrine regulation on inflammation.9 The levels of adiponectin in CMs were not significantly different between WT and SCD1-deficient adipocytes (supplemental Figure 6). Levels of leptin in WT CM tended to be slightly higher, but were not statistically different from that in SCD1-deficient CM.
Free fatty acids (FFAs), which are released by adipocytes, have been well established as active regulators for inflammation.6,20,23 SCD1 is a key lipogenic enzyme responsible for the de novo synthesis of MUFAs, mainly oleate (18:1n9) and palmitoleate (16:1n7). Thus, loss of SCD1 might cause an alteration in the profile of adipocyte-derived FFAs, which in turn modulates inflammation. Indeed, compared with WT CM, SCD1-deficient CM exhibited significantly lower levels of palmitoleic (16:1n7) and oleic (18:1n9) acids, but comparable levels of the corresponding saturated FFAs, palmitic (16:0) and stearic (18:0) acids, respectively (Figure 5A). These FFAs were mainly derived from adipocytes; their levels in plain 10% Fetal Bovine Serum (FBS)/DMEM media were substantially lower (supplemental Figure 7). Consistently, the contents of oleate (18:1n9) and palmitoleate (16:1n7) were also significantly reduced in SCD1-deficient adipocytes (Figure 5B). Furthermore, WAT from Agouti and HFD-fed mice with SCD1 deficiency exhibited decreased oleate (18:1n9) and palmitoleate (16:1n7) levels (Figure 5C). The level of linoleic acid (18:2n6) was also reduced in the CM of SCD1-deficient adipocytes (Figure 4A), but the content of this FA in WAT was comparable in WT and SCD1-deficient mice (data not shown). In parallel with the reduced concentrations of FFAs in adipocyte CM, WAT and adipocytes from SCD1-deficient mice exhibited decreased levels of lipoprotein lipase, the enzyme responsible for fatty acid uptake in various tissues, including adipose tissue (supplemental Figure 7).
Oleate (18:1n9), But Not Palmitoleate (16:1n7), Contributes to the Induction of TNF-α in RAW264.7 Macrophages Treated With Adipocyte CM
Herein, we asked whether the decreased levels of unsaturated FFAs in SCD1-deficient adipocyte CM were linked to the reduced inflammation in macrophages. Treatment of RAW264.7 macrophages with increasing doses (50 and 200 μmol/L) of oleate (18:1n9) induced significantly higher expression of TNF-α, whereas the same doses of palmitoleic (16:1n7) or linoleic (18:2n6) acid were ineffective (Figure 6A). Given this selective effect of oleate (18:1n9), we subsequently supplemented SCD1-deficient CM with 50 μmol/L of oleate, and found that it significantly enhanced the expression of TNF-α in RAW macrophages to a level comparable to the treatment with WT CM (Figure 6B). The observed reduction in LPL activity in SCD1-deficient CM did not alter the induction of TNF-α in RAW cells (supplemental Figure 7). These data indicate a unique role of oleate in modulating macrophage inflammation, which is not shared by palmitoleate, another enzymatic product of SCD1.
Supplementation of Oleate (18:1n9), But Not Palmitoleate (16:1n7), in SCD1-Deficient Adipocyte CM Enhances the Adhesion Response of Endothelial Cells
Next, we examined the effects of decreased oleate and palmitoleate levels on the adhesion response of endothelial cells. Endothelial cells incubated with WT CM exhibited significantly higher expression levels of adhesion molecules, such as ICAM-1, P-selectin, and E-selectin, compared with treatment with basal media, whereas SCD1-deficient CM led to significantly lower expression levels of ICAM-1 and P-selectin and a lower tendency of E-selectin in these cells (Figure 6C). Supplementation of oleate, 50 μmol/L, but not of palmitoleate (50 μmol/L), to SCD1-deficient CM significantly enhanced the expression of these adhesion molecules in endothelial cells. A similar pattern was consistently observed on monocyte/endothelial cell adhesion (Figure 6D). In these experiments, the reduced LPL activity in SCD1-deficient CM did not alter the adhesion responses in endothelial cells (data not shown). These data indicate that differential regulation on endothelial cell adhesion response exists between oleate and palmitoleate.
To further test the in vivo effects of SCD1 deficiency on endothelial inflammation, we analyzed the expression of adhesion molecules in WAT-derived SVC, which contains multiple cell types (including endothelial cells). The expression of ICAM-1, VCAM-1, and P-selectin was significantly lower in SVCs from the WAT of SCD1-deficient mice (supplemental Figure 8). Endothelial inflammation and dysfunction are closely associated with insulin resistance.24 SVCs with SCD1 deficiency exhibited significantly lower expression of endothelial dysfunction markers, Nox4 (NADPH oxidase 4), and NOS3 (nitric oxide synthase), and a lower tendency of endothelin 1 expression (supplemental Figure 8). These data suggest that endothelial cells in the WAT from SCD-deficient mice may have reduced inflammation with enhanced insulin sensitivity.
Ever since the first finding that TNF-α is overproduced by adipocytes in obesity,25 WAT has been regarded as a critical site for promoting chronic and systemic inflammation17,18 and thereby contributing to the development of atherosclerosis. The present investigation has demonstrated that the inflammation in WAT can be attenuated by SCD1 deficiency under the challenge of obesity. Using isolated primary adipocytes, we showed a reduced response to inflammation with SCD1 deficiency directly in the adipocytes and a decreased paracrine regulation on macrophages and endothelial cells. We further showed that the decreased production of oleate (18:1n9), but not of palmitoleate (16:1n7), by SCD1-deficient adipocytes contributed to these effects. Palmitoleate has been recently demonstrated to be an adipocyte-derived lipokine that improves muscle insulin sensitivity and suppresses hepatosteatosis.26 However, the effects of oleate were not measured in the study. Given that both oleate and palmitoleate are the enzymatic products of SCD1, the current study using SCD1-deficient mice affords a unique model to demonstrate the functional difference of these two MUFAs in regulating inflammation.
It is becoming increasingly clear that in WAT inflammation, adipocytes interact with macrophages and endothelial cells in a paracrine manner.20,21 An array of protein factors, which include cytokines (TNF-α and IL-6), chemokines (MCP-1), adipokines (adiponectin and leptin), and others, are released by WAT and are involved in regulating inflammation in obesity.9 However, it was reported that the adipocyte-secreted TNF-α and IL-6 were unlikely to mediate the proinflammatory effects and, therefore, other unknown adipocyte-derived soluble factors may exert these effects.27 Under our experimental settings, we did not detect significant difference in the levels of adiponectin and leptin in WT and SCD1-deficient CMs. However, because these experiments were done in an in vitro context and unable to replicate the hormonal or other physiological regulations in vivo, we cannot rule out the possibilities that the above protein factors released by adipocytes may contribute to the in vivo regulation of inflammation.
In addition to secreting adipokines and cytokines, adipocytes also actively release FFAs through lipolysis. Recent studies6,23 have demonstrated the potent regulation by FFAs on the immune response of macrophages through members of TLR family. We were, therefore, prompted to explore the function of this class of soluble mediators in regulating the inflammation in an SCD1-deficient system. Although FFAs, most notably saturated FAs, such as palmitate (16:0), are important adipocyte-derived mediators in promoting macrophage inflammation,20 the precise contribution of MUFAs, mainly oleate (18:1n9) and palmitoleate (16:1n7), to the cross talk among adipocytes, macrophages, and endothelial cells has remained largely understudied. Given that SCD1 is highly expressed in adipocytes and has direct impact on the FA profile, we hypothesized that the alteration in the profile of secreted FAs from adipocytes lacking SCD1 might contribute to the reduced paracrine inflammatory regulation on the network of adipocytes, macrophages, and endothelial cells.
Our results demonstrated that oleate, but not palmitoleate, promotes macrophage inflammation and endothelial adhesion response in a paracrine fashion. SCD1-deficient adipocytes released similar levels of saturated FFAs as WT adipocytes. However, the levels of unsaturated FFAs, including palmitoleic (16:1n7), oleic (18:1n9), and linoleic (18:2n6) acids, were detected to be lower. In this regard, the SCD1-deficient CM model provides us with a unique opportunity to selectively examine the roles of these unsaturated FFAs in regulating inflammation independent of the effects from saturated FFAs. Oleic, but not palmitoleic or linoleic, acid contributes to the inflammation in both RAW macrophages and endothelial cells with the doses and time frame of our treatments. In another study,23 oleate was reported to consistently and significantly promote inflammation in RAW264.7 macrophages. Furthermore, of particular interest is the fact that both oleate and palmitoleate are enzymatic products of SCD1, and the only chemical structural difference is the two additional carbons present in the fatty acyl chain of the former. Performing an examination of how this subtle structural difference in the FAs leads to differential outcomes of cell signaling may yield important findings.
In parallel with the reduced paracrine inflammation by SCD1-deficient adipocytes, these adipocytes exhibit a reduced inflammatory response to LPS. We showed that the gene expression level of TLR4 was lower in SCD1-deficient than WT adipocytes. The role of TLR4 in mediating FFA-induced inflammation has been well established.6 In adipocytes, another recent study28 further demonstrated that FAs enhance the expression of TLR4 and induce inflammation through the TLR4/NF-κB cascade. In addition to the reduced TLR level, we further observed a substantially lower level of lipids (triglycerides) in SCD1-deficient adipocytes than WT adipocytes (supplemental Figure 3) and decreased NF-κB DNA binding activity in WAT. Taken together, these data suggest that reduced lipid levels in SCD1-deficient adipocytes might lead to reduced expression of TLR4, and subsequently to a decreased TLR4/NF-κB pathway, resulting in lower inflammation. In line with this reduced TLR4/NF-κB signaling, the basal and LPS-stimulated expression of proinflammatory factors (MCP-1 and IL-6) is lower in SCD1-deficient adipocytes than in WT adipocytes.
Despite the observed reduced inflammation in WAT and adipocytes of SCD1-deficient mice in our study, a recent study29 using an SCD1-deficient and low density lipoprotein receptor (LDLR)-deficient mouse model showed that SCD1 deficiency resulted in increased atherosclerosis. The mechanism was attributed to the skin inflammation associated with global SCD1 deficiency. Although increased levels of circulating inflammatory factors were detected in the SCD1-deficient and LDLR-deficient mice in that study, the levels of MCP-1 that are highly produced by WAT were actually lower in these mice. Thus, this result is consistent with the decreased inflammation of WAT and adipocytes observed in our study in SCD1-deficient mice. Interestingly, there is another study suggesting that SCD1 knock-down by targeted antisense oligos resulted in reduced atherosclerosis in a mouse model of chronic intermittent hypoxia.30 The apparent discrepancy suggests that there are likely other functions of SCD1 in regulating atherosclerosis independent of skin inflammation. In this regard, the pronounced skin inflammation might override the other beneficial effects of SCD1 deficiency on atherosclerosis, including the reduced WAT inflammation observed in our study. Therefore, better models of SCD1 deficiency (eg, tissue-specific deletion of SCD1) might provide more insights on the exact role of SCD1 in atherosclerosis.
Although adipocytes are sensitive to SCD1 deficiency in inflammation, the inflammatory responses of peritoneal macrophages, one of the major immune cells, are not altered by SCD1 deficiency in our study, which is consistent with the observation made in the SCD1-deficient and LDLR-deficient mouse model.29 Because we detected decreased macrophage infiltration in WAT from SCD1-deficient mice challenged by obesity, adipocytes may likely be the primary cell type regulating WAT inflammation in SCD1-deficient mice. In contrast, a recent study14 showed increased inflammation response in peritoneal macrophages after treating mice with SCD1-targeted antisense oligos, which may contribute to the increased atherosclerosis in those mice. The discrepancy may be partially the result of different specificities in deleting SCD1 expression (genetic knockout vs antisense oligos knock down), or the varying genetic backgrounds of mouse models used in these studies.
In obesity, increased lipolysis in WAT results in elevated plasma circulating levels of FFAs that cause inflammation and insulin resistance.31 Given that the content of oleate was reduced in WAT of obese mouse models with SCD1 deficiency, it is tempting to speculate that this would contribute to the prevention of WAT inflammation in these mice. However, because SCD1 deficiency protects mice from HFD-induced obesity10 or Agouti-induced obesity,32 our current study was not intended to address the precise contributions of oleate to inflammation independent of the obesity-preventing effects.
In summary, this study is the first demonstration on the molecular mechanism of SCD1 deficiency in modulating the levels of adipocyte-derived FAs on inflammation. An understanding of the differential functions of different types of MUFAs (oleate and palmitoleate) on the inflammatory cross talk among adipocytes, macrophages, and endothelial cells will provide a more comprehensive understanding of the relationship between obesity, lipid metabolism, and atherosclerosis.
Sources of Funding
This study was supported by grant RO1-DK62388 from the National Institutes of Health (Dr Ntambi). Dr Liu received an American Heart Association Fellowship (082J804G).
Received December 5, 2008; revision accepted October 21, 2009.
Keaney JF Jr, Larson MG, Vasan RS, Wilson PW, Lipinska I, Corey D, Massaro JM, Sutherland P, Vita JA, Benjamin EJ. Obesity and systemic oxidative stress: clinical correlates of oxidative stress in the Framingham Study. Arterioscler Thromb Vasc Biol. 2003; 23: 434–439.
Marciniak SJ, Ron D. Endoplasmic reticulum stress signaling in disease. Physiol Rev. 2006; 86: 1133–1149.
Lau DC, Dhillon B, Yan H, Szmitko PE, Verma S. Adipokines: molecular links between obesity and atheroslcerosis. Am J Physiol Heart Circ Physiol. 2005; 288: H2031–H2041.
Ntambi JM. Regulation of stearoyl-CoA desaturase by polyunsaturated fatty acids and cholesterol. J Lipid Res. 1999; 40: 1549–1558.
Brown JM, Chung S, Sawyer JK, Degirolamo C, Alger HM, Nguyen T, Zhu X, Duong MN, Wibley AL, Shah R, Davis MA, Kelley K, Wilson MD, Kent C, Parks JS, Rudel LL. Inhibition of stearoyl-coenzyme A desaturase 1 dissociates insulin resistance and obesity from atherosclerosis. Circulation. 2008; 118: 1467–1475.
Ntambi JM, Miyazaki M, Stoehr JP, Lan H, Kendziorski CM, Yandell BS, Song Y, Cohen P, Friedman JM, Attie AD. Loss of stearoyl-CoA desaturase-1 function protects mice against adiposity. Proc Natl Acad Sci U S A. 2002; 99: 11482–11486.
Suganami T, Nishida J, Ogawa Y. A paracrine loop between adipocytes and macrophages aggravates inflammatory changes: role of free fatty acids and tumor necrosis factor α. Arterioscler Thromb Vasc Biol. 2005; 25: 2062–2068.
Curat CA, Miranville A, Sengenes C, Diehl M, Tonus C, Busse R, Bouloumie A. From blood monocytes to adipose tissue-resident macrophages: induction of diapedesis by human mature adipocytes. Diabetes. 2004; 53: 1285–1292.
Yen TT, Gill AM, Frigeri LG, Barsh GS, Wolff GL. Obesity, diabetes, and neoplasia in yellow A(vy)/- mice: ectopic expression of the agouti gene. Faseb J. 1994; 8: 479–488.
Nguyen MT, Favelyukis S, Nguyen AK, Reichart D, Scott PA, Jenn A, Liu-Bryan R, Glass CK, Neels JG, Olefsky JM. A subpopulation of macrophages infiltrates hypertrophic adipose tissue and is activated by free fatty acids via Toll-like receptors 2 and 4 and JNK-dependent pathways. J Biol Chem. 2007; 282: 35279–35292.
Hotamisligil GS, Shargill NS, Spiegelman BM. Adipose expression of tumor necrosis factor-alpha: direct role in obesity-linked insulin resistance. Science. 1993; 259: 87–91.
Berg AH, Lin Y, Lisanti MP, Scherer PE. Adipocyte differentiation induces dynamic changes in NF-κB expression and activity. Am J Physiol Endocrinol Metab. 2004; 287: E1178–E1188.
Schaeffler A, Gross P, Buettner R, Bollheimer C, Buechler C, Neumeier M, Kopp A, Schoelmerich J, Falk W. Fatty acid-induced induction of Toll-like receptor-4/nuclear factor-κB pathway in adipocytes links nutritional signalling with innate immunity. Immunology. 2009; 126: 233–245.
MacDonald ML, van Eck M, Hildebrand RB, Wong BW, Bissada N, Ruddle P, Kontush A, Hussein H, Pouladi MA, Chapman MJ, Fievet C, van Berkel TJ, Staels B, McManus BM, Hayden MR. Despite antiatherogenic metabolic characteristics, SCD1-deficient mice have increased inflammation and atherosclerosis. Arterioscler Thromb Vasc Biol. 2009; 29: 341–347.
Savransky V, Jun J, Li J, Nanayakkara A, Fonti S, Moser AB, Steele KE, Schweitzer MA, Patil SP, Bhanot S, Schwartz AR, Polotsky VY. Dyslipidemia and atherosclerosis induced by chronic intermittent hypoxia are attenuated by deficiency of stearoyl coenzyme A desaturase. Circ Res. 2008; 103: 1173–1180.