Inhibition of long-chain acyl coenzyme A synthetases during fatty acid loading induces lipotoxicity in macrophages

Advances in production of high-value lipids by oleaginous yeasts

The global market for high-value fatty acids production, mainly omega-3/6, hydroxy fatty-acids, waxes and their derivatives, has seen strong development in the last decade. The reason for this growth was the increasing utilization of these lipids as significant ingredients for cosmetics, food and the oleochemical industries. The large demand for these compounds resulted in a greater scientific interest in research focused on alternative sources of oil production - among which microorganisms attracted the most attention. Microbial oil production offers the possibility to engineer the pathways and store lipids enriched with the desired fatty acids. Moreover, costly chemical steps are avoided and direct commercial use of these fatty acids is available. Among all microorganisms, the oleaginous yeasts have become the most promising hosts for lipid production - their efficient lipogenesis, ability to use various (often highly affordable) carbon sources, feasible large-scale cultivations and wide range of available genetic engineering tools turns them into powerful micro-factories. This review is an in-depth description of the recent developments in the engineering of the lipid biosynthetic pathway with oleaginous yeasts. The different classes of valuable lipid compounds with their derivatives are described and their importance for human health and industry is presented. The emphasis is also placed on the optimization of culture conditions in order to improve the yield and titer of these valuable compounds. Furthermore, the important economic aspects of the current microbial oil production are discussed.

HILPDA Uncouples Lipid Droplet Accumulation in Adipose Tissue Macrophages from Inflammation and Metabolic Dysregulation

Obesity leads to a state of chronic, low-grade inflammation that features the accumulation of lipid-laden macrophages in adipose tissue. Here, we determined the role of macrophage lipid-droplet accumulation in the development of obesity-induced adipose-tissue inflammation, using mice with myeloid-specific deficiency of the lipid-inducible HILPDA protein. HILPDA deficiency markedly reduced intracellular lipid levels and accumulation of fluorescently labeled fatty acids. Decreased lipid storage in HILPDA-deficient macrophages can be rescued by inhibition of adipose triglyceride lipase (ATGL) and is associated with increased oxidative metabolism. In diet-induced obese mice, HILPDA deficiency does not alter inflammatory and metabolic parameters, despite markedly reducing lipid accumulation in macrophages. Overall, we find that HILPDA is a lipid-inducible, physiological inhibitor of ATGL-mediated lipolysis in macrophages and uncouples lipid storage in adipose tissue macrophages from inflammation and metabolic dysregulation. Our data question the contribution of lipid droplet accumulation in adipose tissue macrophages in obesity-induced inflammation and metabolic dysregulation.

RXRs control serous macrophage neonatal expansion and identity and contribute to ovarian cancer progression

Tissue-resident macrophages (TRMs) populate all tissues and play key roles in homeostasis, immunity and repair. TRMs express a molecular program that is mostly shaped by tissue cues. However, TRM identity and the mechanisms that maintain TRMs in tissues remain poorly understood. We recently found that serous-cavity TRMs (LPMs) are highly enriched in RXR transcripts and RXR-response elements. Here, we show that RXRs control mouse serous-macrophage identity by regulating chromatin accessibility and the transcriptional regulation of canonical macrophage genes. RXR deficiency impairs neonatal expansion of the LPM pool and reduces the survival of adult LPMs through excess lipid accumulation. We also find that peritoneal LPMs infiltrate early ovarian tumours and that RXR deletion diminishes LPM accumulation in tumours and strongly reduces ovarian tumour progression in mice. Our study reveals that RXR signalling controls the maintenance of the serous macrophage pool and that targeting peritoneal LPMs may improve ovarian cancer outcomes.

Macrophages can differentiate to perform homeostatic tissue-specific functions. Here the authors show that RXR signalling is critical for large peritoneal macrophage (LPM) expansion during neonatal life and LPM lipid metabolism and survival during adult homeostasis, and that ovarian cancer growth relies on RXR-dependent LPMs. 

Long-Chain Acyl-CoA Synthetase 1 Role in Sepsis and Immunity: Perspectives From a Parallel Review of Public Transcriptome Datasets and of the Literature

A potential role for the long-chain acyl-CoA synthetase family member 1 (ACSL1) in the immunobiology of sepsis was explored during a hands-on training workshop. Participants first assessed the robustness of the potential gap in biomedical knowledge identified via an initial screen of public transcriptome data and of the literature associated with ACSL1. Increase in ACSL1 transcript abundance during sepsis was confirmed in several independent datasets. Querying the ACSL1 literature also confirmed the absence of reports associating ACSL1 with sepsis. Inferences drawn from both the literature (via indirect associations) and public transcriptome data (via correlation) point to the likely participation of ACSL1 and ACSL4, another family member, in inflammasome activation in neutrophils during sepsis. Furthermore, available clinical data indicate that levels of ACSL1 and ACSL4 induction was significantly higher in fatal cases of sepsis. This denotes potential translational relevance and is consistent with involvement in pathways driving potentially deleterious systemic inflammation. Finally, while ACSL1 expression was induced in blood in vitro by a wide range of pathogen-derived factors as well as TNF, induction of ACSL4 appeared restricted to flagellated bacteria and pathogen-derived TLR5 agonists and IFNG. Taken together, this joint review of public literature and omics data records points to two members of the acyl-CoA synthetase family potentially playing a role in inflammasome activation in neutrophils. Translational relevance of these observations in the context of sepsis and other inflammatory conditions remain to be investigated.

Effect of nanoformulated copper/zinc superoxide dismutase on chronic ethanol-induced alterations in liver and adipose tissue

BACKGROUND

We previously reported that nanoformulated copper/zinc superoxide dismutase (Nano) attenuates non-alcoholic fatty liver disease and adipose tissue (AT) inflammation in obese animals. Here, we sought to determine whether Nano treatment attenuates alcohol-associated liver disease (AALD) and AT inflammation in alcohol-fed mice.

METHODS

We pre-treated E-47 cells (HepG2 cells that over-express CYP2E1) with native- or nano-superoxide dismutase (SOD) for 6 h, followed by treatment with ethanol and/or linoleic acid (LA), a free fatty acid. For in vivo studies, male C57BL/6 mice were fed the Lieber-DeCarli control or ethanol liquid diet for 4 weeks. The mice received Nano once every 2 days during the last 2 weeks of ethanol feeding.

RESULTS

Our in vitro studies revealed that Nano pretreatment reduced LA + ethanol-induced oxidative stress in E-47 cells. Our in vivo experiments showed that ethanol-fed Nano-treated mice had 22% lower hepatic triglyceride levels than mice fed ethanol alone. Nano-treated ethanol-fed mice also had 2-fold lower levels of Cd68 and similarly reduced levels of Ccl2 and Mmp12 mRNAs, than in untreated ethanol-fed mice. We also noted that ethanol feeding caused a remarkable increase in hepatic and/or plasma MCP-1 and CCR2 protein, which was blunted in ethanol + Nano-treated animals. The hepatic content of SREBP-1c, a transcription factor that promotes lipogenesis, was higher in ethanol-fed mice than controls but was attenuated in ethanol + Nano-treated animals. Further, livers of ethanol + Nano-treated mice had significantly higher levels of phosphorylated adenosine monophosphate-activated protein kinase (AMPK) than both control and ethanol-fed mice. In AT, the levels of Il6 mRNA, a hepatoprotective cytokine, and that of Arg1, a marker of anti-inflammatory macrophages, were significantly increased in ethanol + Nano-treated mice compared with control mice.

CONCLUSION

Our data indicate that Nano treatment attenuates ethanol-induced steatohepatitis and that this effect is associated with an apparent activation of AMPK signaling. Our data also suggest that Nano induces Arg1 and Il6 expression in AT, suggesting anti-inflammatory effects in this tissue.

The tumor suppressor protein p53 and the ferroptosis network

Ferroptosis is a form of lipid peroxidation-induced cell death that can be regulated in many ways, from altering the activity of antioxidant enzymes to the level of transcription factors. The p53 tumor suppressor is 'the guardian of the genome' that participates in the control of cell survival and division under various stresses. Beyond its effects on apoptosis, autophagy, and cell cycle, p53 also regulates ferroptosis either through a transcriptional or posttranslational mechanism. On one hand, p53 can enhance ferroptosis by inhibiting the expression of SLC7A11 (solute carrier family 7 member 11) or by enhancing that of SAT1 (spermidine/spermine N1-acetyltransferase 1) and GLS2 (glutaminase 2). On the other hand, p53 suppresses ferroptosis through the direct inhibition of DPP4 (dipeptidyl peptidase 4) activity or by the induction of CDKN1A/p21 (cyclin dependent kinase inhibitor 1 A) expression. Here, we review recent discoveries and emerging trends in the study of the ferroptosis network and highlight the context-dependent impact of p53 on ferroptosis and oxidative stress.

A High-Fat Diet Rich in Saturated and Mono-Unsaturated Fatty Acids Induces Disturbance of Thyroid Lipid Profile and Hypothyroxinemia in Male Rats

SCOPE

Increasing evidence has shown that the disturbance of lipid metabolism might make a possible contribution to the pathogenesis of organ dysfunction, including thyroid, yet it is unknown whether excess intake of dietary fat interferes in thyroid lipid profile. We investigate the effects of dietary fat toward the thyroid lipid profile and thyroid function.

METHODS AND RESULTS

Male Sprague-Dawley rats are fed with high-fat diet (HFD) rich in saturated and mono-unsaturated fatty acids or chow diet for 18 weeks. LC-MS analysis of thyroid shows that total free fatty acids (FFAs) content is significantly higher in HFD rats. The concentration of highly saturated triglycerides significantly increases in HFD rats, whereas the polyunsaturated triglyceride significantly decreases, indicating the decrease in unsaturation in the HFD group. Significant increase of lysophosphatidylcholines (LPC) is observed in HFD rats. Thyroid function tests show hypothyroxinemia (total thyroxine [TT₄ ] and free thyroxine [FT₄ ]) in HFD rats, and elevated thyrotropin (TSH) concentration. The HFD rats also show decreased thyroid uptake of iodine.

CONCLUSION

Excess intake of dietary fat induces disturbance of thyroid lipid profile and hypothyroxinemia, indicating thyroid dysfunction. We speculate that it may provide a new prospect in understanding the pathogenesis of hypothyroidism.

Saturated Fatty Acids Undergo Intracellular Crystallization and Activate the NLRP3 Inflammasome in Macrophages

OBJECTIVE

Inflammation provoked by the imbalance of fatty acid composition, such as excess saturated fatty acids (SFAs), is implicated in the development of metabolic diseases. Recent investigations suggest the possible role of the NLRP3 (nucleotide-binding oligomerization domain, leucine-rich repeat and pyrin domain containing 3) inflammasome, which regulates IL-1β (interleukin 1β) release and leads to inflammation, in this process. Therefore, we investigated the underlying mechanism by which SFAs trigger NLRP3 inflammasome activation.

APPROACH AND RESULTS

The treatment with SFAs, such as palmitic acid and stearic acid, promoted IL-1β release in murine primary macrophages while treatment with oleic acid inhibited SFA-induced IL-1β release in a dose-dependent manner. Analyses using polarized light microscopy revealed that intracellular crystallization was provoked in SFA-treated macrophages. As well as IL-1β release, the intracellular crystallization and lysosomal dysfunction were inhibited in the presence of oleic acid. These results suggest that SFAs activate NLRP3 inflammasome through intracellular crystallization. Indeed, SFA-derived crystals activated NLRP3 inflammasome and subsequent IL-1β release via lysosomal dysfunction. Excess SFAs also induced crystallization and IL-1β release in vivo. Furthermore, SFA-derived crystals provoked acute inflammation, which was impaired in IL-1β-deficient mice.

CONCLUSIONS

These findings demonstrate that excess SFAs cause intracellular crystallization and subsequent lysosomal dysfunction, leading to the activation of the NLRP3 inflammasome, and provide novel insights into the pathogenesis of metabolic diseases.

Fatty Acids as Useful Serological Markers for Crohn's Disease

BACKGROUND

We have previously reported that patients with Crohn's disease (CD) have a very specific erythrocyte membrane phospholipid fatty acid profile. The findings of this study suggest that the activities of enzymes involved in the metabolism of linoleic acid (LA), that is, delta-6 desaturase, are higher in CD patients than in healthy individuals.

METHODS

We evaluated the utilities of various fatty acid compositions of the plasma (p-) as new serological markers for CD compared to those of erythrocyte membranes (e-).

RESULTS

Fifty CD patients and 50 healthy individuals were enrolled. In both plasma and erythrocyte membranes, the weight percentages of palmitic acid (PA) were significantly higher, while those of LA were significantly lower in CD patients than in controls. Fatty acids with high sensitivity and specificity were p-PA (0.86 and 0.74) and e-PA (0.80 and 0.74). With PA and LA as a CD fatty acid index (CDFAi), that is, CDFAi = (PA/LA), the sensitivity and specificity of plasma CDFAi (p-CDFAi) and e-CDFAi were 0.80 and 0.80; and 0.82 and 0.88 respectively.

CONCLUSION

In CD patients, various fatty acids were specifically altered in both plasma and erythrocytes, and p-PA and p-CDFAi are potentially useful as new serological markers for CD.

Nanoformulated copper/zinc superoxide dismutase exerts differential effects on glucose vs lipid homeostasis depending on the diet composition possibly via altered AMPK signaling

Evidence suggests that superoxide dismutase 1 (SOD1) promotes glucose vs lipid metabolism depending on the diet type. We recently reported that nanoformulated SOD1 (Nano) improved lipid metabolism without altering glucose homeostasis in high-fat (HF) diet-fed mice. Here, we sought to determine the effects and potential mechanisms of Nano in modulating glucose and lipid homeostasis in mice fed a normal chow diet (CD) vs HF diet. Mice were fed a CD or a HF diet (45%) for 10 wk and injected with Nano once every 2 days for 15 days. The fasting glucose level was lower (P < 0.05) in CD + Nano-treated mice compared to control. Conversely, blood glucose was not altered but serum triglycerides were lower in HF + Nano-treated mice. Genes involved in fatty acid synthesis were reduced by Nano in the skeletal muscle of CD but not of HF diet-fed mice. Adenosine monophosphate-activated protein kinase (AMPK), which promotes both glucose and lipid metabolism depending on the fuel availability, is activated by Nano in CD-fed mice. Moreover, Nano increased phosphorylation of ACC, a downstream target of AMPK, in both CD and HF diet-fed mice. Nano increased mitochondrial respiration in C2C12 myocytes in the presence of glucose or fatty acid, and this effect is inhibited by Compound C, an AMPK inhibitor. Our data suggest that Nano promotes glucose and lipid metabolism in CD and HF diet-fed mice, respectively, and this effect is mediated partly via AMPK signaling.

Effects of fermented ginseng root and ginseng berry on obesity and lipid metabolism in mice fed a high-fat diet

Background

Previous studies have shown that both ginseng root and ginseng berry exhibit antiobesity and antidiabetic effects. However, a direct comparison of the efficacy and mechanisms between the root and the berry after oral administration remains to be illuminated.

Methods

In this study, we observed the effects of fermented ginseng root (FGR) and fermented ginseng berry (FGB) on obesity and lipid metabolism in high-fat diet induced obese mice.

Results

FGR and FGB significantly inhibited the activity of pancreatic lipase in vitro. Both FGR and FGB significantly suppressed weight gain and excess food intake and improved hypercholesterolemia and fatty liver, while only FGR significantly attenuated hyperglycemia and insulin resistance. Both FGR and FGB significantly inhibited the mRNA expression of Ldlr and Acsl1 while FGR also significantly inhibited expression of Cebpa and Dgat2 in liver. FGR significantly decreased the epididymal fat weight of mice while FGB significantly inhibited the mRNA expression of genes Cebpa, Fas, Hsl, Il1b, and Il6 in adipose tissue.

Conclusion

Saponin from both FGR and FGB had a beneficial effect on high-fat diet-induced obesity. Compared to FGB, FGR exhibited more potent antihyperglycemic and antiobesity effect. However, only FGB significantly inhibited mRNA expression of inflammatory markers such as interleukins 1β and 6 in adipose tissue.

A combination of dietary N-3 fatty acids and a cyclooxygenase-1 inhibitor attenuates nonalcoholic fatty liver disease in mice

We sought to determine whether a combination of purified n-3 fatty acids (n-3) and SC-560 (SC), a cyclooxygenase-1-specific inhibitor, is effective in ameliorating nonalcoholic fatty liver disease in obesity. Female wild-type mice were fed a high-fat and high-cholesterol diet (HF) supplemented with n-3 in the presence or absence of SC. Mice treated with SC alone exhibited no change in liver lipids, whereas n-3-fed mice tended to have lower hepatic lipids. Mice given n-3+SC had significantly lower liver lipids compared with HF controls indicating enhanced lipid clearance. Total and sulfated bile acids were significantly higher only in n-3+SC-treated mice compared with chow diet (CD) controls. Regarding mechanisms, the level of pregnane X receptor (PXR), a nuclear receptor regulating drug/bile detoxification, was significantly higher in mice given n-3 or n-3+SC. Studies in precision-cut liver slices and in cultured hepatoma cells showed that n-3+SC enhanced not only the expression/activation of PXR and its target genes but also the expression of farnesoid X receptor (FXR), another regulator of bile synthesis/clearance, indicating that n-3+SC can induce both PXR and FXR. The mRNA level of FGFR4 which inhibits bile formation showed a significant reduction in Huh 7 cells upon n-3 and n-3+SC treatment. PXR overexpression in hepatoma cells confirmed that n-3 or SC each induced the expression of PXR target genes and in combination had an enhanced effect. Our findings suggest that combining SC with n-3 potentiates its lipid-lowering effect, in part, by enhanced PXR and/or altered FXR/FGFR4 signaling.

Identification of ACSL4 as a biomarker and contributor of ferroptosis

Ferroptosis, a recently identified form of non-apoptotic cell death, is involved in several physiological and pathological processes. Although lipid peroxidation plays a central role in triggering ferroptosis, the essential regulator of lipid metabolism in ferroptosis remains poorly defined. Here, we show that acyl-CoA synthetase long-chain family member 4 (ACSL4) is required for ferroptotic cancer cell death. Compared with ferroptosis-sensitive cells (e.g., HepG2 and HL60), the expression of ACSL4 was remarkably downregulated in ferroptosis-resistant cells (e.g., LNCaP and K562). In contrast, the expression of other ACSLs, including ACSL1, ACSL3, ACSL5, and ACSL6, did not correlate with ferroptosis sensitivity. Moreover, knockdown of ACSL4 by specific shRNA inhibited erastin-induced ferroptosis in HepG2 and HL60 cells, whereas overexpression of ACSL4 by gene transfection restored sensitivity of LNCaP and K562 cells to erastin. Mechanically, ACSL4-mediated production of 5-hydroxyeicosatetraenoic acid (5-HETE) contributed to ferroptosis. Pharmacological inhibition of 5-HETE production by zileuton limited ACSL4 overexpression-induced ferroptosis. Collectively, these results indicate that ACSL4 is not only a sensitive monitor of ferroptosis, but also an important contributor of ferroptosis.

Nanoformulated copper/zinc superoxide dismutase reduces adipose inflammation in obesity

OBJECTIVE

An intimate association exists between oxidative stress and inflammation. Because adipose tissue (AT) inflammation is intricately linked to metabolic disorders, it was hypothesized that reducing oxidative stress would be effective in ameliorating AT inflammation in obesity.

METHODS

Wild-type mice were fed a high-fat diet (HF) for 8 weeks followed by a 2-week treatment with nanoformulated copper/zinc superoxide dismutase (NanoSOD). The mice were divided into: 1) chow diet, 2) HF, and 3) HF + NanoSOD.

RESULTS

The HF + NanoSOD-treated mice showed a significant decrease in plasma and liver triglycerides when compared with HF-fed mice. Interestingly, NanoSOD reduced the expression of macrophage and inflammatory markers in visceral AT (VAT) and stromal cells derived from VAT. Moreover, the activation of proinflammatory signaling pathways, in particular, the extracellular signal-regulated kinases, was blunted in VAT on NanoSOD treatment. However, markers of oxidative stress were not altered significantly in the HF + NanoSOD group in the experimental conditions. Pretreatment of either macrophages or adipocytes significantly reduced the inflammatory response invoked in an in vitro coculture system, further supporting the role of NanoSOD in inhibiting obesity-linked inflammation.

CONCLUSIONS

This data suggest that NanoSOD is effective not only in reducing AT macrophage accumulation and AT inflammation but also in promoting triglyceride metabolism in obesity.

Production of conjugated linoleic acid by heterologous expression of linoleic acid isomerase in oleaginous fungus Mortierella alpina

OBJECTIVE

To increase the commercial value of oleaginous fungus Mortierella alpina by incorporation of trans-10,cis-12 conjugated linoleic acid (CLA) into the polyunsaturated fatty acids (PUFAs) of M. alpina via Propionibacterium acnes isomerase (PAI) conversion.

RESULTS

The PAI gene and the codon optimization version were heterologously expressed in M. alpina via Agrobacterium tumefaciens-mediated transformation (ATMT). Coding usage modification significantly improved the translation of PAI transcripts and trans-10,cis-12 CLA was produced up to 1.2 mg l(-1), which corresponds to approx. 0.05% of the total fatty acid (TFA). Since PAI prefers free linoleic acid as a substrate rather than any other forms, 5 μM long-chain acyl CoA synthetase inhibitor was added and the trans-10,cis-12 CLA content increased approx. 24-fold to 29 mg l(-1), reaching up to 1.2% (w/w) of the TFA in M. alpina.

CONCLUSION

Heterologous expression of PAI in M. alpina by ATMT methods is a practicable way in biosynthesis of CLA and this system may be a feasible platform for industrial production of CLA.

A decade of progress in adipose tissue macrophage biology

One decade has passed since seminal publications described macrophage infiltration into adipose tissue (AT) as a key contributor to inflammation and obesity-related insulin resistance. Currently, a PubMed search for 'adipose tissue inflammation' reveals over 3500 entries since these original reports. We now know that resident macrophages in lean AT are alternatively activated, M2-like, and play a role in AT homeostasis. In contrast, the macrophages in obese AT are dramatically increased in number and are predominantly classically activated, M1-like, and promote inflammation and insulin resistance. Mediators of AT macrophage (ATM) phenotype include adipokines and fatty acids secreted from adipocytes as well as cytokines secreted from other immune cells in AT. There are several mechanisms that could explain the large increase in ATMs in obesity. These include recruitment-dependent mechanisms such as adipocyte death, chemokine release, and lipolysis of fatty acids. Newer evidence also points to recruitment-independent mechanisms such as impaired apoptosis, increased proliferation, and decreased egress. Although less is known about the homeostatic function of M2-like resident ATMs, recent evidence suggests roles in AT expansion, thermoregulation, antigen presentation, and iron homeostasis. The field of immunometabolism has come a long way in the past decade, and many exciting new discoveries are bound to be made in the coming years that will expand our understanding of how AT stands at the junction of immune and metabolic co-regulation.

Prolonged triglyceride storage in macrophages: pHo trumps pO2 and TLR4

Lipid-laden macrophages contribute to pathologies as diverse as atherosclerosis and tuberculosis. Three common stimuli are known to promote macrophage lipid storage: low tissue oxygen tension (pO2), low extracellular pH (pHo), and exposure to agonists such as bacterial LPS. Noting that cells responding to low pO2 or agonistic bacterial molecules often decrease pHo by secreting lactic and other carboxylic acids, we studied how pHo influences the stimulation of triacylglycerol (TAG) storage by low pO2 and LPS. We found that TAG retention after incubation for 48-72 h was inversely related to pHo when primary macrophages were cultured in 21% oxygen, 4% oxygen, or with LPS at either oxygen concentration. Maintaining pHo at ~7.4 was sufficient to prevent the increase in prolonged TAG storage induced by either low pO2 or LPS. The strong influence of pHo on TAG retention may explain why lipid-laden macrophages are found in some tissue environments and not in others. It is also possible that other long-term cellular changes currently attributed to low pO2 or bacterial agonists may be promoted, at least in part, by the decrease in pHo that these stimuli induce.

Inhibition of macrophage fatty acid β-oxidation exacerbates palmitate-induced inflammatory and endoplasmic reticulum stress responses

AIMS/HYPOTHESIS

Saturated fatty acids (SFAs) such as palmitate activate inflammatory pathways and elicit an endoplasmic reticulum (ER) stress response in macrophages, thereby contributing to the development of insulin resistance linked to the metabolic syndrome. This study addressed the question of whether or not mitochondrial fatty acid β-oxidation (FAO) affects macrophage responses to SFA.

METHODS

We modulated the activity of carnitine palmitoyl transferase 1A (CPT1A) in macrophage-differentiated THP-1 monocytic cells using genetic or pharmacological approaches, treated the cells with palmitate and analysed the proinflammatory and ER stress signatures.

RESULTS

To inhibit FAO, we created THP-1 cells with a stable knockdown (KD) of CPT1A and differentiated them to macrophages. Consequently, in CPT1A-silenced cells FAO was reduced. CPT1A KD in THP-1 macrophages increased proinflammatory signalling, cytokine expression and ER stress responses after palmitate treatment. In addition, in human primary macrophages CPT1A KD elevated palmitate-induced inflammatory gene expression. Pharmacological inhibition of FAO with etomoxir recapitulated the CPT1A KD phenotype. Conversely, overexpression of a malonyl-CoA-insensitive CPT1A M593S mutant reduced inflammatory and ER stress responses to palmitate in THP-1 macrophages. Macrophages with a CPT1A KD accumulated diacylglycerols and triacylglycerols after palmitate treatment, while ceramide accumulation remained unaltered. Moreover, lipidomic analysis of ER phospholipids revealed increased palmitate incorporation into phosphatidylethanolamine and phosphatidylserine classes associated with the CPT1A KD.

CONCLUSIONS/INTERPRETATION

Our data indicate that FAO attenuates inflammatory and ER stress responses in SFA-exposed macrophages, suggesting an anti-inflammatory impact of drugs that activate FAO.

Pleiotropic regulation of mitochondrial function by adipose triglyceride lipase-mediated lipolysis

Lipolysis is defined as the catabolism of triacylglycerols (TGs) stored in cellular lipid droplets. Recent discoveries of essential lipolytic enzymes and characterization of numerous regulatory proteins and mechanisms have fundamentally changed our perception of lipolysis and its impact on cellular metabolism. Adipose triglyceride lipase (ATGL) is the rate-limiting enzyme for TG catabolism in most cells and tissues. This review focuses on recent advances in understanding the (patho)physiological impact due to defective lipolysis by ATGL deficiency on mitochondrial (dys)function. Depending on the type of cells and tissues investigated, absence of ATGL has pleiotropic roles in mitochondrial function.

Differential effects of eicosapentaenoic acid and docosahexaenoic acid in promoting the differentiation of 3T3-L1 preadipocytes

The objective of this study was to determine the effects of enrichment with n-3 fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), on the differentiation of 3T3-L1 preadipocytes. Enrichment with DHA but not EPA significantly increased the differentiation markers compared to control differentiated cells. DHA compared to EPA treatment led to a greater increase in adiponectin secretion and, conditioned media collected from DHA treated cells inhibited monocyte migration. Moreover, DHA treatment resulted in inhibition of pro-inflammatory signaling pathways. DHA treated cells predominantly accumulated DHA in phospholipids whereas EPA treatment led to accumulation of both EPA and its elongation product docosapentaenoic acid (DPA), an n-3 fatty acid. Of note, adding DPA to DHA inhibited DHA-induced differentiation. The differential effects of EPA and DHA on preadipocyte differentiation may be due, in part, to differences in their intracellular modification which could impact the type of n-3 fatty acids incorporated into the cells.

Toll-like receptor agonists promote prolonged triglyceride storage in macrophages

Macrophages in infected tissues may sense microbial molecules that significantly alter their metabolism. In a seeming paradox, these critical host defense cells often respond by increasing glucose catabolism while simultaneously storing fatty acids (FA) as triglycerides (TAG) in lipid droplets. We used a load-chase strategy to study the mechanisms that promote long term retention of TAG in murine and human macrophages. Toll-like receptor (TLR)1/2, TLR3, and TLR4 agonists all induced the cells to retain TAG for ≥3 days. Prolonged TAG retention was accompanied by the following: (a) enhanced FA uptake and FA incorporation into TAG, with long lasting increases in acyl-CoA synthetase long 1 (ACSL1) and diacylglycerol acyltransferase-2 (DGAT2), and (b) decreases in lipolysis and FA β-oxidation that paralleled a prolonged drop in adipose triglyceride lipase (ATGL). TLR agonist-induced TAG storage is a multifaceted process that persists long after most early pro-inflammatory responses have subsided and may contribute to the formation of "lipid-laden" macrophages in infected tissues.

Toll-like receptor 4 deficiency promotes the alternative activation of adipose tissue macrophages

Obesity is characterized by adipose tissue (AT) macrophage (ATM) accumulation, which promotes AT inflammation and dysfunction. Toll-like receptor 4 (TLR4) deficiency attenuates AT inflammation in obesity but does not impede the accumulation of ATMs. The purpose of the current study was to determine whether TLR4 deficiency alters ATM polarization. TLR4(-/-) and wild-type mice were fed a low-fat, high-monounsaturated fat (HF(MUFA)), or a high-saturated fat (HF(SFA)) diet for 16 weeks. Further, we used a bone marrow transplant model to determine the influence of hematopoietic cell TLR4 signaling. The metabolic and inflammatory responses to high-fat feeding and ATM phenotype were assessed. Global and hematopoietic cell TLR4 deficiency, irrespective of recipient genotype, produced a shift in ATM phenotype toward an alternatively activated state, which was accompanied by reduced AT inflammation. Despite the observed shift in ATM phenotype, neither global nor hematopoietic cell TLR4 deficiency influenced systemic insulin sensitivity after high-fat feeding. Results of the current study suggest that TLR4 directly influences ATM polarization but question the relevance of TLR4 signaling to systemic glucose homeostasis in obesity.

Activity of long-chain acyl-CoA synthetase is required for maintaining meiotic arrest in Xenopus laevis

In most vertebrates, fully grown oocytes are arrested in meiotic prophase I and only resume the cell cycle upon external stimuli, such as hormones. The proper arrest and resumption of the meiotic cycle is critical for reproduction. A Galpha(S) signaling pathway essential for the arrest is conserved in organisms from Xenopus to mouse and human. A previous gene association study implicated that mutations of human ACSL6 may be related to premature ovarian failure. However, functional roles of ACSL6 in human infertility have yet to be reported. In the present study, we found that triacsin C, a potent and specific inhibitor for ACSL, triggers maturation in Xenopus and mouse oocytes in the absence of hormone, suggesting ACSL activity is required for the oocyte arrest. In Xenopus, acsl1b may fulfill a major role in the process, because inhibition of acsl1b by knocking down its RNA results in abnormal acceleration of oocyte maturation. Such abnormally matured eggs cannot support early embryonic development. Moreover, direct inhibition of protein palmitoylation, which lies downstream of ACSLs, also causes oocyte maturation. Furthermore, palmitoylation of Galpha(s), which is essential for its function, is inhibited when the ACSL activity is blocked by triacsin C in Xenopus. Thus, disruption of ACSL activity causes inhibition of the Galpha(s) signaling pathway in the oocytes, which may result in premature ovarian failure in human.

Adipose triglyceride lipase in immune response, inflammation, and atherosclerosis

Consistent with its central importance in lipid and energy homeostasis, lipolysis occurs in essentially all tissues and cell types, including macrophages. The hydrolytic cleavage of triacylglycerol by adipose triglyceride lipase (ATGL) generates non-esterified fatty acids, which are subsequently used as essential precursors for lipid and membrane synthesis, mediators in cell signaling processes or as energy substrate in mitochondria. This review summarizes the current knowledge concerning the consequences of ATGL deficiency in macrophages with particular emphasis on macrophage (dys)-function, apoptosis, and atherosclerosis.

Stearic acid accumulation in macrophages induces toll-like receptor 4/2-independent inflammation leading to endoplasmic reticulum stress-mediated apoptosis

OBJECTIVE

Elevated serum free fatty acid levels are associated with an increased risk of cardiovascular disease and type 2 diabetes mellitus. Macrophages are recruited to atherosclerotic plaques and metabolic tissues during obesity and accumulate lipids, including free fatty acids. We investigated the molecular consequences of intracellular saturated free fatty acid accumulation in macrophages.

METHODS AND RESULTS

Previously, we demonstrated that cotreatment of mouse peritoneal macrophages (MPMs) with stearic acid and triacsin C (an inhibitor of long-chain acyl coenzyme A synthetases) results in intracellular free fatty acid accumulation and apoptosis. Here, we used Western blotting analysis, real-time reverse transcription polymerase chain reaction, and terminal deoxynucleotidyl transferase dUTP nick-end labeling staining to assess endoplasmic reticulum (ER) stress, inflammation, and apoptosis in MPMs. Intracellular stearic acid accumulation induces Toll-like receptor 4/2-independent inflammation that results in ER stress-mediated apoptosis of MPMs. Polarization of MPMs to a proinflammatory M1 phenotype increases their susceptibility to inflammation and ER stress, but not apoptosis, in response to cotreatment with stearic acid and triacsin C.

CONCLUSIONS

Intracellular accumulation of stearic acid in MPMs activates inflammatory signaling, leading to ER stress-mediated apoptosis. M1 macrophages are more prone to stearic acid-induced inflammation and ER stress. These same pathways may be activated in macrophages residing in atherosclerotic plaques and metabolic tissues during conditions of obesity and hyperlipidemia.

Strain specific transcriptional response in Mycobacterium tuberculosis infected macrophages

Background

Tuberculosis (TB), a bacterial infection caused by Mycobacterium tuberculosis (Mtb) remains a significant health problem worldwide with a third of the world population infected and nearly nine million new cases claiming 1.1 million deaths every year. The outcome following infection by Mtb is determined by a complex and dynamic host-pathogen interaction in which the phenotype of the pathogen and the immune status of the host play a role. However, the molecular mechanism by which Mtb strains induce different responses during intracellular infection of the host macrophage is not fully understood. To explore the early molecular events triggered upon Mtb infection of macrophages, we studied the transcriptional responses of murine bone marrow-derived macrophages (BMM) to infection with two clinical Mtb strains, CDC1551 and HN878. These strains have previously been shown to differ in their virulence/immunogenicity in the mouse and rabbit models of pulmonary TB.

Results

In spite of similar intracellular growth rates, we observed that compared to HN878, infection by CDC1551 of BMM was associated with an increased global transcriptome, up-regulation of a specific early (6 hours) immune response network and significantly elevated nitric oxide production. In contrast, at 24 hours post-infection of BMM by HN878, more host genes involved in lipid metabolism, including cholesterol metabolism and prostaglandin synthesis were up-regulated, compared to infection with CDC1551.

In association with the differences in the macrophage responses to infection with the 2 Mtb strains, intracellular CDC1551 expressed higher levels of stress response genes than did HN878.

Conclusions

In association with the early and more robust macrophage activation, intracellular CDC1551 cells were exposed to a higher level of stress leading to increased up-regulation of the bacterial stress response genes. In contrast, sub-optimal activation of macrophages and induction of a dysregulated host cell lipid metabolism favored a less stressful intracellular environment for HN878. Our findings suggest that the ability of CDC1551 and HN878 to differentially activate macrophages during infection probably determines their ability to either resist host cell immunity and progress to active disease or to succumb to the host protective responses and be driven into a non-replicating latent state in rabbit lungs.

Diabetes promotes an inflammatory macrophage phenotype and atherosclerosis through acyl-CoA synthetase 1. Proc Natl Acad Sci U S A. 2012;109:E715-E724. [PMID: 22308341 DOI: 10.1073/pnas.1111600109]

The mechanisms that promote an inflammatory environment and accelerated atherosclerosis in diabetes are poorly understood. We show that macrophages isolated from two different mouse models of type 1 diabetes exhibit an inflammatory phenotype. This inflammatory phenotype associates with increased expression of long-chain acyl-CoA synthetase 1 (ACSL1), an enzyme that catalyzes the thioesterification of fatty acids. Monocytes from humans and mice with type 1 diabetes also exhibit increased ACSL1. Furthermore, myeloid-selective deletion of ACSL1 protects monocytes and macrophages from the inflammatory effects of diabetes. Strikingly, myeloid-selective deletion of ACSL1 also prevents accelerated atherosclerosis in diabetic mice without affecting lesions in nondiabetic mice. Our observations indicate that ACSL1 plays a critical role by promoting the inflammatory phenotype of macrophages associated with type 1 diabetes; they also raise the possibilities that diabetic atherosclerosis has an etiology that is, at least in part, distinct from the etiology of nondiabetic vascular disease and that this difference is because of increased monocyte and macrophage ACSL1 expression.

Acyl-CoA synthetase 1 is required for oleate and linoleate mediated inhibition of cholesterol efflux through ATP-binding cassette transporter A1 in macrophages

Diabetes and insulin resistance increase the risk of cardiovascular disease caused by atherosclerosis through mechanisms that are poorly understood. Lipid-loaded macrophages are key contributors to all stages of atherosclerosis. We have recently shown that diabetes associated with increased plasma lipids reduces cholesterol efflux and levels of the reverse cholesterol transporter ABCA1 (ATP-binding cassette transporter A1) in mouse macrophages, which likely contributes to macrophage lipid accumulation in diabetes. Furthermore, we and others have shown that unsaturated fatty acids reduce ABCA1-mediated cholesterol efflux, and that this effect is mediated by the acyl-CoA derivatives of the fatty acids. We therefore investigated whether acyl-CoA synthetase 1 (ACSL1), a key enzyme mediating acyl-CoA synthesis in macrophages, could directly influence ABCA1 levels and cholesterol efflux in these cells. Mouse macrophages deficient in ACSL1 exhibited reduced sensitivity to oleate- and linoleate-mediated ABCA1 degradation, which resulted in increased ABCA1 levels and increased apolipoprotein A-I-dependent cholesterol efflux in the presence of these fatty acids, as compared with wildtype mouse macrophages. Conversely, overexpression of ACSL1 resulted in reduced ABCA1 levels and reduced cholesterol efflux in the presence of unsaturated fatty acids. Thus, the reduced ABCA1 and cholesterol efflux in macrophages subjected to conditions of diabetes and elevated fatty load may, at least in part, be mediated by ACSL1. These observations raise the possibility that ABCA1 levels could be increased by inhibition of acyl-CoA synthetase activity in vivo. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Macrophage adipose triglyceride lipase deficiency attenuates atherosclerotic lesion development in low-density lipoprotein receptor knockout mice

OBJECTIVE

The consequences of macrophage triglyceride (TG) accumulation on atherosclerosis have not been studied in detail so far. Adipose triglyceride lipase (ATGL) is the rate-limiting enzyme for the initial step in TG hydrolysis. Because ATGL knockout (KO) mice exhibit massive TG accumulation in macrophages, we used ATGL KO mice to study the effects of macrophage TG accumulation on atherogenesis.

METHODS AND RESULTS

Low-density lipoprotein receptor (LDLr) KO mice were transplanted with bone marrow from ATGL KO (ATGL KO→LDLr KO) or wild-type (WT→LDLr KO) mice and challenged with a Western-type diet for 9 weeks. Despite TG accumulation in ATGL KO macrophages, atherosclerosis in ATGL KO→LDLr KO mice was 43% reduced associated with decreased plasma monocyte chemoattractant protein-1 (MCP-1) and macrophage interleukin-6 concentrations. This coincided with a reduced amount of macrophages, possibly because of a 39% increase in intraplaque apoptosis and a decreased migratory capacity of ATGL KO macrophages. The reduced number of white blood cells might be due to a 36% decreased Lin(-)Sca-1(+)cKit(+) hematopoietic stem cell population.

CONCLUSIONS

We conclude that the attenuation of atherogenesis in ATGL KO→LDLr KO mice is due to decreased infiltration of less inflammatory macrophages into the arterial wall and increased macrophage apoptosis.

Gene-nutrient interactions with dietary fat modulate the association between genetic variation of the ACSL1 gene and metabolic syndrome

Long-chain acyl CoA synthetase 1 (ACSL1) plays an important role in fatty acid metabolism and triacylglycerol (TAG) synthesis. Disturbance of these pathways may result in dyslipidemia and insulin resistance, hallmarks of the metabolic syndrome (MetS). Dietary fat is a key environmental factor that may interact with genetic determinants of lipid metabolism to affect MetS risk. We investigated the relationship between ACSL1 polymorphisms (rs4862417, rs6552828, rs13120078, rs9997745, and rs12503643) and MetS risk and determined potential interactions with dietary fat in the LIPGENE-SU.VI.MAX study of MetS cases and matched controls (n = 1,754). GG homozygotes for rs9997745 had increased MetS risk {odds ratio (OR) 1.90 [confidence interval (CI) 1.15, 3.13]; P = 0.01}, displayed elevated fasting glucose (P = 0.001) and insulin concentrations (P = 0.002) and increased insulin resistance (P = 0.03) relative to the A allele carriers. MetS risk was modulated by dietary fat, whereby the risk conferred by GG homozygosity was abolished among individuals consuming either a low-fat (<35% energy) or a high-PUFA diet (>5.5% energy). In conclusion, ACSL1 rs9997745 influences MetS risk, most likely via disturbances in fatty acid metabolism, which was modulated by dietary fat consumption, particularly PUFA intake, suggesting novel gene-nutrient interactions.