Triacylglycerol Metabolism Research Articles

The Dynamic Roles of Phospholipid and Triacylglycerol Metabolism in Regulation and Signaling in Yeast

For several decades, my laboratory has investigated the regulation of lipid metabolism, focusing on how cells sense metabolic activity in the pathways for the synthesis of phospholipids and triacylglycerol (TAG). We isolated mutants with defects in the expression of structural genes of phospholipid biosynthesis, including INO1, encoding inositol 3‐phosphate synthase, the ratelimiting enzyme in the synthesis of the phospholipid precursor, inositol. This led to the discovery of transcription factors and a promoter element responsible for transcriptional regulation in response to inositol. A major breakthrough in the field was the discovery that phosphatidic acid (PA), precursor to both phospholipids and TAG, serves as the signal for derepression of phospholipid biosynthetic genes in the absence of inositol. PA metabolism is influenced by the rates of synthesis and turnover of essentially all lipids that are derived directly or indirectly from it. Because of the interconnectivity of the pathways involving PA, removal or addition of inositol, triggers rapid changes in the levels of a wide spectrum of lipids, including many signaling lipids. We are currently exploiting these attributes of the cellular response to inositol to elucidate the roles of specific lipids, both in regulating lipid metabolism itself, and in activating stress response signaling across multiple membrane compartments.Supported by NIH grant GM‐19629

Influence of apolipoprotein A-V on the metabolic fate of triacylglycerol

Apolipoprotein (apo) A-V functions to modulate intracellular and extracellular triacylglycerol metabolism. The present review addresses molecular mechanisms underlying these effects. The relevance of apoA-V to human disease conditions is illustrated by the strong correlation between single nucleotide polymorphisms in APOA5, elevated plasma triacylglycerol and dyslipidemic disease. Despite undergoing processing for secretion from hepatocytes, a portion of apoA-V escapes this destiny and accumulates as a component of cytosolic lipid droplets. Expression of recombinant apoA-V in hepatocarcinoma cells results in increased lipid droplet size and number at the expense of triacylglycerol secretion.ApoA-V modulates atherosclerosis in hypercholesterolemic apoE null mice. ApoE null/human apoA-V transgenic mice had reduced levels of triacylglycerol and cholesterol in plasma along with decreased aortic lesion size. ApoA-V modulates triacylglycerol metabolic fate. Following its synthesis, apoA-V enters the endoplasmic reticulum and associates with membrane defects created by triacylglycerol accumulation. Association of apoA-V with endoplasmic reticulum membrane defects promotes nascent lipid droplets budding toward the cytosol. Despite its low concentration in plasma (∼150 ng/ml), apoA-V modulates lipoprotein metabolism by binding to glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein 1. This interaction effectively localizes triacylglycerol-rich lipoproteins in the vicinity of glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein1's other ligand, lipoprotein lipase.

Localization of Opi1pFFAT to lipid droplets during inositol starvation in Saccharomyces cerevisiae

Phosphatidic acid (PA) is a signaling lipid that plays an important role in mediating phospholipid biosynthesis. However, the subcellular distribution of PA remains elusive. In this study, PA levels were manipulated by shifting cells from inositol‐containing medium to inositol‐lacking medium. A mutated form of the Opi1p repressor, Opi1pFFAT, which loses the interaction with the endoplasmic reticulum (ER) protein Scs2p but contains a PA‐binding domain, relocalizes to lipid droplets (LDs) after 2‐h of inositol starvation. Opi1pFFAT localization was also monitored in mutant cells with defects in triacylglycerol (TAG) metabolism, including pah1Δ and lro1Δdga1Δ for TAG biosynthesis, and tgl3Δtgl4Δtgl5Δ for TAG mobilization. Opi1pFFAT localized to LDs in the tgl3Δtgl4Δtgl5Δ mutant, but did not localize to LDs in cells unable to synthesize TAG. Another mutated form of Opi1p, Opi1pFFAT/PAc4m, which carries mutations in both FFAT and PA‐binding domains, was also examined. Opi1pFFAT/PAc4m did not localize to LDs, but remained in the nucleus. These results suggest Opi1pFFAT binds to a specific pool of PA that is actively involved in TAG synthesis. Fld1p, a human seipin homolog in yeast that binds to the junction of LDs and ER, was shown to colocalize with Opi1pFFAT followed by 2‐h inositol starvation, suggesting a pool of PA to the sites of LD biogenesis. Supported by NIH grant GM019629.

Dexamethasone-Mediated Changes in Adipose Triacylglycerol Metabolism Are Exaggerated, Not Diminished, in the Absence of a Functional GR Dimerization Domain

The glucocorticoid (GC) receptor (GR) has multiple effector mechanisms, including dimerization-mediated transactivation of target genes via DNA binding and transcriptional repression mediated by protein-protein interactions. Much attention has been focused on developing selective GR modulators that would dissociate adverse effects from therapeutic anti-inflammatory effects. The GR(dim/dim) mouse has a mutation in the dimerization domain of GR and has been shown to have attenuated transactivation with intact repression. To understand the role of GR dimerization-dependent targets in multiple tissues, we measured metabolic fluxes through several disease-relevant GC target pathways using heavy water labeling and mass spectrometry in wild-type and GR(dim/dim) mice administered the potent GC dexamethasone (DEX). Absolute triglyceride synthesis was increased in both wild-type and GR(dim/dim) mice by DEX in the inguinal and epididymal fat depots. GR(dim/dim) mice showed an exaggerated response to DEX in both depots. De novo lipogenesis was also greatly increased in both depots in response to DEX in GR(dim/dim), but not wild-type mice. In contrast, the inhibitory effect of DEX on bone and skin collagen synthesis rates was greater in wild-type compared with GR(dim/dim) mice. Wild-type mice were more sensitive to DEX-dependent decreases in insulin sensitivity than GR(dim/dim) mice. Wild-type and GR(dim/dim) mice were equally sensitive to DEX-dependent decreases in muscle protein synthesis. Chronic elevation of GCs in GR(dim/dim) mice results in severe runting and lethality. In conclusion, some metabolic effects of GC treatment are exaggerated in adipose tissue of GR(dim/dim) mice, suggesting that selective GR modulators based on dissociating GR transactivation from repression should be evaluated carefully.

Lipid storage in high-altitude Andean Lakes extremophiles and its mobilization under stress conditions in Rhodococcus sp. A5, a UV-resistant actinobacterium

The production of triacylglycerols (TAG) or wax esters (WS) seems to be a widespread feature among extremophile bacteria living in high-altitude Andean Lakes (HAAL), Argentina. Twelve out of twenty bacterial strains isolated from HAAL were able to produce TAG or WS (between 2 and 17% of cellular dry weight) under nitrogen-limiting culture conditions. Among these strains, the extremophile Rhodococcus sp. A5 accumulated significant amounts of TAG during growth on glucose (17%, CDW) and hexadecane (32%, CDW) as sole carbon sources. The role of accumulated TAG in the response to carbon starvation, osmotic stress, UV-radiation and desiccation was investigated in Rhodococcus sp. A5 using an inhibitor of TAG degradation. Cells degraded TAG during these stresses in the absence of the inhibitor. The inhibition of TAG mobilization affected cell survival during osmotic stress only during the initial growth stage. Little or no surviving cells were observed after carbon starvation, UV-treatment and desiccation, when TAG mobilization was inhibited. These results suggested that TAG metabolism is relevant for the adaptation and survival of A5 cells under carbon starvation, osmotic stress and UV irradiation, and essential under desiccation conditions, which prevail in HAAL environments.

Altered Fatty Acid Profile in the Liver and Serum of Stroke-Prone Spontaneously Hypertensive Rats: Reduced Proportion of cis-Vaccenic Acid

Stroke-prone spontaneously hypertensive rats (SHRSP) are utilized as models for study of the pathogenesis of not only stroke and cardiovascular disorders but also atherosclerosis and metabolic syndrome. Basic information on the profiles of fatty acids and lipid classes in the liver is indispensable to use SHRSP as a model of disorder of lipid metabolism; nevertheless, detailed information on the metabolism of triacylglycerols (TAGs) and fatty acids in the liver of SHRSP is lacking. This study aimed to characterize profiles of lipid classes and fatty acids and to explore the mechanism underlying the characteristic alterations in metabolism of TAGs and fatty acids in the liver of SHRSP, in comparison with spontaneously hypertensive rats (SHR). The characteristic changes observed in SHRSP were (1) markedly lower hepatic TAG contents; (2) altered expressions of genes encoding three enzymes responsible for the control of TAG level, namely, adipose triglyceride lipase (for TAG degradation; up-regulated), carnitine palmitoyltransferase 1a (for fatty acid β-oxidation; up-regulated) and long-chain acyl-CoA synthetase 3 (for glycerolipid synthesis; down-regulated); (3) evidently lower contents and proportions of monounsaturated fatty acids, in particular cis-vaccenic acid (18:1n-7), in the liver and serum; and (4) down-regulation of palmitoleoyl-CoA chain elongase, which is necessary for the biosynthesis of 18:1n-7, in the liver. From the above observations, we concluded that there are significant differences in profiles of lipid classes and fatty acids between SHRSP and SHR, and that altered characteristics in SHRSP are likely responsible for increases in TAG hydrolysis and β-oxidation, and decreases in TAG synthesis and 18:1n-7 synthesis.

The Rhodococcus opacus TadD protein mediates triacylglycerol metabolism by regulating intracellular NAD(P)H pools

BackgroundThe Gram-positive actinomycete Rhodococcus opacus is widely studied for its innate ability to store large amounts of carbon in the form of triacylglycerol (TAG). Several groups have demonstrated that R. opacus PD630 is capable of storing anywhere from 50 to 76% of its cell dry weight as TAG. While numerous studies have focused on phenomenological aspects of this process, few have sought to identify the underlying molecular and biochemical mechanisms responsible for the biosynthesis and storage of this molecule.ResultsHerein we further our previous efforts to illuminate the black box that is lipid metabolism in actinomycetes using a genetic approach. Utilizing a simple, colorimetric genetic screen, we have identified a gene, referred to herein as tadD (triacylglycerol accumulation deficient), which is critical for TAG biosynthesis in R. opacus PD630. Furthermore, we demonstrate that the purified protein product of this gene is capable of oxidizing glyceraldehyde-3-phosphate, while simultaneously reducing NAD(P)+ to NAD(P)H. Supporting this biochemical data, we observed that the ratio of NAD(P)H to NAD(P)+ is elevated in wildtype cultures grown under lipid production conditions as compared to cultures grown under vegetative growth conditions, while the mutant strain demonstrated no change irrespective of growth conditions. Finally, we demonstrate that over-expressing a putative phosphorylative glyceraldehyde-3-phosphate dehydrogenase leads to decreased TAG production during growth on TAG accumulation conditions.ConclusionTaken together, the data support the identification of a key metabolic branch point separating vegetative growth and lipid accumulation lifestyles in Rhodococcus.

Hepatic lipid metabolic pathways modified by resveratrol in rats fed an obesogenic diet

ObjectiveThe scientific community is on the look-out for safe biomolecules useful in the prevention of obesity and related aberrations such as fatty liver. This study analyzed the influence of resveratrol on hepatic triacylglycerol metabolism. MethodsMale Sprague-Dawley rats were divided into control and resveratrol-treated groups (30 mg/kg of body weight per day) and fed a commercial obesogenic diet for 6 wk. Liver triacylglycerol content and the activity of carnitine palmitoyl transferase-Ia (CPT-Ia), acyl-coenzyme A oxydase (ACO), fatty acid synthase (FAS), glucose-6-phosphate dehydrogenase (G6PDH), malic enzyme (ME), acetyl-coenzyme A carboxylase (ACC), adenosine monophosphate-activated protein kinase (AMPK), and peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) activation were measured. Mitochondrial protein cytochrome C oxidase subunit 2 (COXII), mitochondrial transcription factor A (TFAM), sterol regulatory element-binding protein-1c (SREBP-1c), peroxisome proliferator-activated receptor-α (PPAR-α), sirtuin-1 (SIRT1), hepatocyte nuclear factor receptor-4α (HNF-4α), and PGC-1α mRNA levels were also analyzed. Serum insulin was quantified. ResultsResveratrol decreased liver fat accumulation, increased CPT-Ia and ACO, and decreased ACC activities. Other lipogenic enzymes, FAS, ME, and G6PDH were not modified. The polyphenol activated AMPK and PGC-1α. The expression of SRBP-1c, PPAR-α, SIRT1, PGC-1α, HNF-4α, TFAM, and COXII was not modified. No changes in serum insulin levels were observed. ConclusionResveratrol partly prevents the increase in liver fat accumulation induced by high-fat high-sucrose feeding by increasing fatty acid oxidation and decreasing lipogenesis. These effects are mediated by the activation of the AMPK/SIRT1 axis.

Design and synthesis of 1,3‐dicapryloyl‐2‐acetylglycerol as molecular probe for triacylglycerol metabolism study

AbstractThe increasing number of health issues caused by obesity and lack of healthy and efficient fat substitutes require development on new fat substitute. This study proposes a new potential reduced‐calorie structured triacylglycerol (TAG) of “1,3‐medium‐chain, 2‐short‐chain triglycerides (MSM)” configuration, and the fat substitution method is based on the suppression of 2‐monoacylglycerol absorption into intestinal cells, which would decrease the recombination of TAG and reduce lipid absorption in the digestive tract. A molecular probe, 1,3‐dicapryloyl‐2‐acetylglycerol, is offered for nutritionists to conduct further study on its metabolism in human body. The synthesis method for this structured TAG starts from the lipase‐catalyzed esterification between glycerol and caprylic acid, yielding 1,3‐dicapryloylglycerol, and is followed by direct esterification with acetic anhydride to form 1,3‐dicapryloyl‐2‐acetylglycerol. Its structure is characterized by FTIR,1H NMR,13C NMR, and LC‐ESI‐MS.Practical application: This kind of pure structured triglyceride can function as a molecular probe for nutritionists to test its metabolism in human body and conduct further evaluation on their fat substitute potential. The synthesis method can also be easily adopted by food industry since it involves only two simple reaction steps.

Metabolic analysis of the cutaneous fungi Malassezia globosa and M. restricta for insights on scalp condition and dandruff

Dandruff is a global consumer problem, characterized by flaking and scaling of the scalp, accompanied by itch and irritancy. However, the aetiology of the condition remains poorly understood, although there is a strong consensus that the cutaneous fungi Malassezia globosa and M.restricta are a major contributory factor. Although there is a paucity of understanding on how these commensal microorganisms adopt a pathogenic phenotype, a rich source of potential insights now exists in the shape of the recently published whole-genome sequence of M.globosa, a functional annotation and metabolic reconstruction of which is freely accessible via the integrated microbial genomes (IMG) online community resource (http://www.hmpdacc-resources.org/cgi-bin/imgm_hmp/main.cgi). In these studies, we have taken a combined in-silico and in-vitro approach to investigate aspects of lipid and amino acid metabolism by M.globosa and M.restricta that have the potential to impact on scalp condition and dandruff. The IMG platform was employed to analyse the metabolism of triacylglycerols and fatty acids, as well as the aromatic amino acid tryptophan, by M.globosa, to investigate pro-inflammatory pathways linked in the literature to dandruff and pityriasis versicolour, respectively. Results were equivocal, leaving question marks over the ability of M.globosa to fully degrade unsaturated fatty acids and metabolize tryptophan to indole-3-pyruvic acid. In-vitro assay systems were then developed to study the biotransformation of these metabolites by both M.globosa and M.restricta, as well as their effect on human keratinocytes, and the results here indicated that neither unsaturated fatty acids nor indole derivatives are likely to be major aetiological factors in dandruff.

Rapid Triacylglycerol Turnover in Chlamydomonas reinhardtii Requires a Lipase with Broad Substrate Specificity

When deprived of nitrogen (N), the photosynthetic microalga Chlamydomonas reinhardtii accumulates large quantities of triacylglycerols (TAGs), making it a promising source of biofuel. Prominent transcriptional changes associated with the conditions leading to TAG accumulation have been found, suggesting that the key enzymes for TAG metabolism might be among those that fluctuate in their expression during TAG synthesis and breakdown. Using a Saccharomyces cerevisiae lipase null mutant strain for functional complementation, we identified the CrLIP1 gene from Chlamydomonas based on its ability to suppress the lipase deficiency-related phenotypes of the yeast mutant. In Chlamydomonas, an inverse correlation was found between the CrLIP1 transcript level and TAG abundance when Chlamydomonas cultures were reversibly deprived of N. The CrLIP1 protein expressed and purified from Escherichia coli exhibited lipolytic activity against diacylglycerol (DAG) and polar lipids. The lipase domain of CrLIP1 is most similar to two human DAG lipases, DAGLα and DAGLβ. The involvement of CrLIP1 in Chlamydomonas TAG hydrolysis was corroborated by reducing the abundance of the CrLIP1 transcript with an artificial micro-RNA, which resulted in an apparent delay in TAG lipolysis when N was resupplied. Together, these data suggest that CrLIP1 facilitates TAG turnover in Chlamydomonas primarily by degrading the DAG presumably generated from TAG hydrolysis.

Chronic overexpression of PNPLA3I148M in mouse liver causes hepatic steatosis

A genetic variant in PNPLA3 (PNPLA3(I148M)), a triacylglycerol (TAG) hydrolase, is a major risk factor for nonalcoholic fatty liver disease (NAFLD); however, the mechanism underlying this association is not known. To develop an animal model of PNPLA3-induced fatty liver disease, we generated transgenic mice that overexpress similar amounts of wild-type PNPLA3 (PNPLA3(WT)) or mutant PNPLA3 (PNPLA3(I148M)) either in liver or adipose tissue. Overexpression of the transgenes in adipose tissue did not affect liver fat content. Expression of PNPLA3(I148M), but not PNPLA3(WT), in liver recapitulated the fatty liver phenotype as well as other metabolic features associated with this allele in humans. Metabolic studies provided evidence for 3 distinct alterations in hepatic TAG metabolism in PNPLA3(I148M) transgenic mice: increased formation of fatty acids and TAG, impaired hydrolysis of TAG, and relative depletion of TAG long-chain polyunsaturated fatty acids. These findings suggest that PNPLA3 plays a role in remodeling TAG in lipid droplets, as they accumulate in response to food intake, and that the increase in hepatic TAG levels associated with the I148M substitution results from multiple changes in hepatic TAG metabolism. The development of an animal model that recapitulates the metabolic phenotype of the allele in humans provides a new platform in which to elucidate the role of PNLPA3(I148M) in NAFLD.

Abstract 1523: Trafficking of intracytoplasmic lipid droplets in pancreatic cancer cells by pigment epithelium-derived factor alters triacylglycerol metabolism and cellular invasion

Abstract Background: Pancreatic ductal adenocarcinoma (PDAC) is a rapidly progressing disease with high metastatic potential, thus, it requires a robust and renewable source of energy to meet its metabolic demands. High grade PDAC tumors have been shown to lose expression of a potent negative regulator of lipid metabolism, pigment epithelium-derived factor (PEDF). Emerging data support that lipid droplets (LDs) are dynamic organelles in non-adipocytes and they are capable of participating in signaling pathways and as well as providing a source of energy. LDs are composed of a triacylglycerol (TAG) core covered by coat proteins, some of which assist in restricting lipase activity by enzymes such as adipose triglyceride lipase (ATGL). Based on a study of adipose differentiation related protein (ADRP) over expression resulting in lower metalloproteinase (MMP) levels in non-tumor cells, we proposed that ADRP-positive LDs actively participate in cellular invasion. We further postulated that expression of PEDF in PDAC tumors inhibits cellular invasion and reduces lipid energy stores due to an increase in ATGL-mediated TAG catabolism and redistribution of intracytoplasmic LDs. Methods: PANC-1 cells were transfected with the PEDF gene via the pCEP4 vector. PEDF-expressing PANC-1 cells, wild type controls and human pancreatic tumor tissue sections were immunostained for PEDF, ATGL, neutral lipid (Oil-red-O) and the two LD coat proteins, ADRP and tail interacting protein-47 (TIP-47). LD distribution (perinuclear vs. peripheral) was analyzed and matrigel cell invasion assays were performed. To determine the role of LDs in proliferation, TAG synthesis in PANC-1 cells was blocked using a diacylglycerol acyltransferase-1 (DGAT-1) inhibitor. Results: PEDF-expressing PANC-1 cells revealed a 10-fold decrease in cellular invasion and an increase in ATGL expression. Immunostains for LD coat proteins, ADRP and TIP-47, showed a predominantly perinuclear position for LDs in the wild type control cells, whereas, PEDF-expressing cells exhibited more ADRP positive LDs in the peripheral region of the cytoplasm. The LDs located closest to the nucleus had the largest mean diameter. Human PDAC tissue had minimal to no expression of PEDF and exhibited predominantly larger perinuclear LDs with more intense TIP-47 staining as compared to ARDP. Blockade of TAG synthesis with DGAT-1 inhibitor reduced LD size and significantly decreased cellular proliferation. Conclusions: These data suggest that activation of TAG catabolism in PDAC by upregulation of the lipolytic enzyme, ATGL, or blockade of TAG synthesis by a DGAT-1 inhibitor diminishes LD size. Moreover, redistribution of LDs and changes in LD coat protein expression may be one mechanism underlying the anti-invasive function of PEDF in PANC-1 cells. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1523. doi:1538-7445.AM2012-1523

CGI‐58 regulates triacylglycerol metabolism and lipid signaling pathways in plant cells

CGI‐58 (Comparative Gene Identification‐58) encodes an α/β hydrolase‐type protein that is involved in multiple aspects of lipid metabolism in mammals, including the breakdown of triacylglycerols (TAGs) and formation of the hydrophobic barrier of the skin. Mutations in CGI‐58 result not only in aberrant skin formation, but also in accumulation of lipid droplets (LDs) in non‐lipid‐storing tissues such as skin, liver, and blood cells. The activity of CGI‐58 is determined at least in part by its interaction with other proteins including perilipin, which serves to anchor CGI‐58 to the surface of LDs. Phosphorylation of perilipin releases CGI‐58 and allows it to interact with Adipose Triglyceride Lipase (ATGL), thereby stimulating TAG breakdown. Interestingly, disruption of the CGI‐58 homolog in Arabidopsis results also in the accumulation of LDs in tissues that do not normally store lipids (e.g., leaves and stems), but plants lack any apparent homologs to perilipin or ATGL. In an effort to better understand the function of CGI‐58 in plants, we conducted a yeast 2‐hybrid assay using Arabidopsis CGI‐58 as bait. Although we did not identify any obvious LD‐associated proteins or lipases, we did identify several proteins known to be involved in distinct aspects of plant lipid metabolism. Evidence will be presented showing that CGI‐58 modulates the level of linolenic acid as well as lipid signaling pathways in plant cells.

Docosahexaenoic acid suppresses the expression of FoxO and its target genes

Docosahexaenoic acid (DHA), an n-3 polyunsaturated fatty acid, has previously been shown to ameliorate obesity-associated metabolic syndrome. To decipher the mechanism responsible for the beneficial effects of DHA on energy/glucose homeostasis and the metabolic syndrome, 30 weaned cross-bred pigs were randomly assigned to three groups and fed ad libitum with a standard diet supplemented with 2% of beef tallow, soybean oil or DHA oil for 30 days, and the gene expression profile of various tissues was evaluated by quantitative real-time polymerase chain reaction. The DHA-supplemented diets reduced the expression of forkhead box O transcription factor (FoxO) 1 and FoxO3 in the liver and adipose tissue. DHA treatments also decreased the expression of FoxO1 and FoxO3 in human hepatoma cells, SK-HEP-1 and human and porcine primary adipocytes. In addition, DHA also down-regulated FoxO target genes, such as microsomal triacylglycerol transfer protein (MTP), glucose-6-phosphatase, apolipoprotein C-III (apoC-III) and insulin-like growth factor binding-protein 1 in the liver, as well as reduced total plasma levels of cholesterol and triacylglycerol in the pig. Transcriptional suppression of FoxO1, FoxO3, apoC-III and MTP by DHA was further confirmed by reporter assays with each promoter construct. Taken together, our study indicates that DHA modulates lipid and glucose homeostasis in part by down-regulating FoxO function. The down-regulation of genes associated with triacylglycerol metabolism and very low density lipoprotein assembly is likely to contribute to the beneficial effects of DHA on the metabolic syndrome.

Kupffer cells ameliorate hepatic insulin resistance induced by high-fat diet rich in monounsaturated fatty acids: the evidence for the involvement of alternatively activated macrophages.

BackgroundResident macrophages (Kupffer cells, KCs) in the liver can undergo both pro- or anti-inflammatory activation pathway and exert either beneficiary or detrimental effects on liver metabolism. Until now, their role in the metabolically dysfunctional state of steatosis remains enigmatic. Aim of our study was to characterize the role of KCs in relation to the onset of hepatic insulin resistance induced by a high-fat (HF) diet rich in monounsaturated fatty acids.MethodsMale Wistar rats were fed either standard (SD) or high-fat (HF) diet for 4 weeks. Half of the animals were subjected to the acute GdCl3 treatment 24 and 72 hrs prior to the end of the experiment in order to induce the reduction of KCs population. We determined the effect of HF diet on activation status of liver macrophages and on the changes in hepatic insulin sensitivity and triacylglycerol metabolism imposed by acute KCs depletion by GdCl3.ResultsWe found that a HF diet rich in MUFA itself triggers an alternative but not the classical activation program in KCs. In a steatotic, but not in normal liver, a reduction of the KCs population was associated with a decrease of alternative activation and with a shift towards the expression of pro-inflammatory activation markers, with the increased autophagy, elevated lysosomal lipolysis, increased formation of DAG, PKCε activation and marked exacerbation of HF diet-induced hepatic insulin resistance.ConclusionsWe propose that in the presence of a high MUFA content the population of alternatively activated resident liver macrophages may mediate beneficial effects on liver insulin sensitivity and alleviate the metabolic disturbances imposed by HF diet feeding and steatosis. Our data indicate that macrophage polarization towards an alternative state might be a useful strategy for treating type 2 diabetes.

Myocardial ATGL Overexpression Decreases the Reliance on Fatty Acid Oxidation and Protects against Pressure Overload-Induced Cardiac Dysfunction

Alterations in myocardial triacylglycerol content have been associated with poor left ventricular function, suggesting that enzymes involved in myocardial triacylglycerol metabolism play an important role in regulating contractile function. Myocardial triacylglycerol catabolism is mediated by adipose triglyceride lipase (ATGL), which is rate limiting for triacylglycerol hydrolysis. To address the influence of triacylglycerol hydrolysis on myocardial energy metabolism and function, we utilized mice with cardiomyocyte-specific ATGL overexpression (MHC-ATGL). Biochemical examination of MHC-ATGL hearts revealed chronically reduced myocardial triacylglycerol content but unchanged levels of long-chain acyl coenzyme A esters, ceramides, and diacylglycerols. Surprisingly, fatty acid oxidation rates were decreased in ex vivo perfused working hearts from MHC-ATGL mice, which was compensated by increased rates of glucose oxidation. Interestingly, reduced myocardial triacylglycerol content was associated with moderately enhanced in vivo systolic function in MHC-ATGL mice and increased isoproterenol-induced cell shortening of isolated primary cardiomyocytes. Most importantly, MHC-ATGL mice were protected from pressure overload-induced systolic dysfunction and detrimental structural remodeling following transverse aortic constriction. Overall, this study shows that ATGL overexpression is sufficient to alter myocardial energy metabolism and improve cardiac function.

CGI-58/ABHD5-Derived Signaling Lipids Regulate Systemic Inflammation and Insulin Action

Mutations of comparative gene identification 58 (CGI-58) in humans cause Chanarin-Dorfman syndrome, a rare autosomal recessive disease in which excess triacylglycerol (TAG) accumulates in multiple tissues. CGI-58 recently has been ascribed two distinct biochemical activities, including coactivation of adipose triglyceride lipase and acylation of lysophosphatidic acid (LPA). It is noteworthy that both the substrate (LPA) and the product (phosphatidic acid) of the LPA acyltransferase reaction are well-known signaling lipids. Therefore, we hypothesized that CGI-58 is involved in generating lipid mediators that regulate TAG metabolism and insulin sensitivity. Here, we show that CGI-58 is required for the generation of signaling lipids in response to inflammatory stimuli and that lipid second messengers generated by CGI-58 play a critical role in maintaining the balance between inflammation and insulin action. Furthermore, we show that CGI-58 is necessary for maximal TH1 cytokine signaling in the liver. This novel role for CGI-58 in cytokine signaling may explain why diminished CGI-58 expression causes severe hepatic lipid accumulation yet paradoxically improves hepatic insulin action. Collectively, these findings establish that CGI-58 provides a novel source of signaling lipids. These findings contribute insight into the basic mechanisms linking TH1 cytokine signaling to nutrient metabolism.

Coordination of Triacylglycerol and Cholesterol Homeostasis by DHR96 and the Drosophila LipA Homolog magro

Although transintestinal cholesterol efflux has been identified as an important means of clearing excess sterols, the mechanisms that underlie this process remain poorly understood. Here, we show that magro, a direct target of the Drosophila DHR96 nuclear receptor, is required in the intestine to maintain cholesterol homeostasis. magro encodes a LipA homolog that is secreted from the anterior gut into the intestinal lumen to digest dietary triacylglycerol. Expression of magro in intestinal cells is required to hydrolyze cholesterol esters and promote cholesterol clearance. Restoring magro expression in the intestine of DHR96 mutants rescues their defects in triacylglycerol and cholesterol metabolism. These studies show that the central role of the intestine in cholesterol efflux has been conserved through evolution, that the ancestral function of LipA is to coordinate triacylglycerol and cholesterol metabolism, and that the region-specific activities of magro correspond to the metabolic functions of its upstream regulator, DHR96.

Lipid metabolism in skeletal muscle: generation of adaptive and maladaptive intracellular signals for cellular function

Fatty acids derived from adipose tissue lipolysis, intramyocellular triacylglycerol lipolysis, or de novo lipogenesis serve a variety of functions in skeletal muscle. The two major fates of fatty acids are mitochondrial oxidation to provide energy for the myocyte and storage within a variety of lipids, where they are stored primarily in discrete lipid droplets or serve as important structural components of membranes. In this review, we provide a brief overview of skeletal muscle fatty acid metabolism and highlight recent notable advances in the field. We then 1) discuss how lipids are stored in and mobilized from various subcellular locations to provide adaptive or maladaptive signals in the myocyte and 2) outline how lipid metabolites or metabolic byproducts derived from the actions of triacylglycerol metabolism or β-oxidation act as positive and negative regulators of insulin action. We have placed an emphasis on recent developments in the lipid biology field with respect to understanding skeletal muscle physiology and discuss unanswered questions and technical limitations for assessing lipid signaling in skeletal muscle.