Adipose-specific lipoprotein lipase deficiency more profoundly affects brown than white fat biology

Exploring the Synergistic Action of Medium-Chain Triglycerides and Omega-3 Fatty Acids to Enhance Cellular Uptake and Anti-Inflammatory Responses

Objectives: Omega-3 fatty acids (n-3 FA) exhibit pro-healing and anti-inflammatory properties. Injectable lipid emulsions containing n-3 FA are being explored for the treatment of acute adverse conditions. Our previous studies demonstrated that a triglyceride (TG)-rich emulsion (TGRP) containing medium-chain TG (MCT) and n-3 TG (8:2 ratio) is rapidly cleared from the blood and efficiently taken up by organs. This study systematically examined the impact of varying MCT:n-3 ratios on cellular uptake and metabolic function in inflammatory processes. Methods and results: We measured the uptake of radio-labeled TGRP, comprising pure MCT, n-3, or mixed at selected ratios (8:2, 6:4, 2:8), both in vitro and in vivo. Murine macrophages with MCT:n-3 (6:4 or 2:8) had a 2-fold higher TG uptake. IV-injected mixed TGRP also enhanced blood clearance and organ uptake. n-3 TGRP reduced LPS-induced pro-inflammatory cytokines (TNF-α, IL-1, IL-6) in a dose-dependent manner. The 8:2 ratio enhanced mitochondrial respiration and glycolytic capacity in macrophages. Pro-inflammatory lipids decreased with MCT:n-3 (2:8) and pure n-3 TGRP. Bolus injections of n-3 TGRP with MCT lowered LPS-induced IL-6 in plasma and tissues. Conclusions: MCT and n-3 FA support metabolic activity and exhibit anti-inflammatory effects, suggesting that optimizing their ratio may enhance the therapeutic effects of emulsions for inflammatory conditions.

Single-cell transcriptome reveals three types of adipocytes associated with intramuscular fat content in pigs

Intramuscular fat is an essential component of muscle tissue, and understanding its contribution to skeletal muscle fat infiltration and meat quality, together with the underlying genetic mechanisms, is a major topic in pig husbandry. However, the composition of cell types and gene expression profiles essential for this purpose remain largely unexplored. Here, we performed single-cell transcriptome analysis on muscle tissue from adult pigs and identified 15 cell types, including three previously uncharacterized types of adipocytes: Adipocyte 1, Adipocyte 2, and Aregs. Phenotypic analysis showed their proportions correlated closely with intramuscular fat content. Based on integrated analysis of ATAC-seq with RNA-seq data, Adipocyte 1 and Aregs have gene expression profiles and transcription factor (TF) motif enrichment typical of adipocytes. On the other hand, myogenic TF motifs were enriched in marker gene promoters in Adipocyte 2, suggesting that these cells originate from muscle cells. Moreover, the marker gene promoters and lineage-specific TF expression in these three adipocyte types were conserved between pigs and humans. These findings provide deep insights towards understanding the complexity of mammalian intramuscular adipocyte types and the gene regulation underlying their organization and function.

Downregulation of adipose LPL by PAR2 contributes to the development of hypertriglyceridemia

Lipoprotein lipase (LPL) hydrolyzes circulating triglycerides (TGs), releasing fatty acids (FA) and promoting lipid storage in white adipose tissue (WAT). However, the mechanisms regulating adipose LPL and its relationship with the development of hypertriglyceridemia are largely unknown. WAT from obese humans exhibited high PAR2 expression, which was inversely correlated with the LPL gene. Decreased LPL expression was also inversely correlated with elevated plasma TG levels, suggesting that adipose PAR2 might regulate hypertriglyceridemia by downregulating LPL. In mice, aging and high palmitic acid diet (PD) increased PAR2 expression in WAT, which was associated with a high level of macrophage migration inhibitory factor (MIF). MIF downregulated LPL expression and activity in adipocytes by binding with CXCR2/4 receptors and inhibiting Akt phosphorylation. In a MIF overexpression model, high-circulating MIF levels suppressed adipose LPL, and this suppression was associated with increased plasma TGs but not FA. Following PD feeding, adipose LPL expression and activity were significantly reduced, and this reduction was reversed in Par2-/- mice. Recombinant MIF infusion restored high plasma MIF levels in Par2-/- mice, and the levels decreased LPL and attenuated adipocyte lipid storage, leading to hypertriglyceridemia. These data collectively suggest that downregulation of adipose LPL by PAR2/MIF may contribute to the development of hypertriglyceridemia.

Remodeling of Adipose Tissues by Fatty Acids: Mechanistic Update on Browning and Thermogenesis by n-3 Polyunsaturated Fatty Acids

Enhancing thermogenesis by increasing the amount and activity of brown and brite adipocytes is a potential therapeutic target for obesity and its associated diseases. Diet plays important roles in energy metabolism and a myriad of dietary components including lipids are known to regulate thermogenesis through recruitment and activation of brown and brite adipocytes. Depending on types of fatty acids (FAs), the major constituent in lipids, their health benefits differ. Long-chain polyunsaturated FAs (PUFAs), especially n-3 PUFAs remodel adipose tissues in a healthier manner with reduced inflammation and enhanced thermogenesis, while saturated FAs exhibit contrasting effects. Lipid mediators derived from FAs act as autocrine/paracrine as well as endocrine factors to regulate thermogenesis. We discuss lipid mediators that may contribute to the differential effects of FAs on adipose tissue remodeling and hence, cardiometabolic diseases. We also discuss current understanding of molecular and cellular mechanisms through which n-3 PUFAs enhance thermogenesis. Elucidating molecular details of beneficial effects of n-3 PUFAs on thermogenesis is expected to provide information that can be used for development of novel therapeutics for obesity and its associated diseases.

The Effects of Six Weeks of Fasted Aerobic Exercise on Body Shape and Blood Biochemical Index in Overweight and Obese Young Adult Males

BACKGROUND/OBJECTIVE

The effects of fasted aerobic exercise on body composition and whether it causes adverse effects remain controversial. This study was to compare the effects of fasted and non-fasted aerobic exercise on body shape and blood biochemical indexes in overweight and obese young adult males, and observe whether FAE triggers adverse reactions.

METHODS

Thirty overweight and obese young adult males were randomly divided into fasted aerobic exercise (FAE) group, non-fasted aerobic exercise (NFAE) group, and control group. They were subjected to indoor treadmill intervention five days a week combined with diet control for six weeks. The FAE group had breakfast 0.5 h after exercise, and the NFAE group exercised 1 h after breakfast. Both groups filled out adverse reaction questionnaires during exercise, and the control group did not have any intervention. Height, weight, body mass index (BMI), and body fat percentage of the three groups of subjects before and after the experiment were measured by the GAIA KIKO bio-resistance antibody composition analyzer in Korea; waist circumference (WC), hip circumference (HC), waist-to-hip ratio (WHR), and waist-to-height ratio (WHtR) were measured by the tape measure method; fasting plasma glucose (FPG), fasting insulin (FINs), total cholesterol (TC), triglyceride (TG), high density lipoprotein cholesterol (HDL-C), low density lipoprotein cholesterol (LDL-C), very low density lipoprotein (VLDL), and HDL-C/LDL-C were measured by Roche C8000 automatic biochemical analysis instrument.

RESULTS

Weight, BMI, body fat percentage, WC, HC, WHR, WHtR, TG, TC, LDL-C and VLDL decreased very significantly (P < 0.01) in the FAE and NFAE groups after the 6-week experiment. The decrease in FINS was significant in the FAE group (P < 0.05) and the decrease in HDL-C was very significant in the NFAE group (P < 0.01). There was no significant difference in the frequency of adverse reactions between two groups (P > 0.05).

CONCLUSION

Six-week FAE and NFAE significantly improved body shape in overweight and obese young adult males, while FAE significantly reduced fasting insulin levels and increased tissue cell sensitivity to insulin. And compared to NFAE, 30 min of FAE in the morning did not increase adverse effects.

Anti-adiposity and lipid-lowering effects of schisandrol A in diet-induced obese mice

Lignan schisandrol A (SolA) is known to have antioxidant and anti-inflammatory effects. However, the impact of SolA on obesity is poorly understood. To test the hypothesis that SolA has anti-obesity effects, C57BL/6J mice were fed a high-fat diet with or without SolA (0.006%, w/w) for 16 weeks. SolA decreased visceral fat mass (10%) by increasing energy expenditure and upregulating white adipose tissue thermogenic genes mRNA expression. Furthermore, SolA upregulated adipose Lpl mRNA expression and decreased plasma free fatty acid (FFA), triglyceride (TG), apolipoprotein (apo) B, aspartate aminotransferase levels and TG/HDL-cholesterol and apoB/apoA1 ratios as well as hepatic lipid droplets. Increased hepatic β-oxidation and fecal FFA and TG levels were observed in the SolA-supplemented mice, suggesting an association of its lipid-lowering effect with increased hepatic β-oxidation, fecal fat excretion and adipose Lpl. Conclusionally, this study provides evidence on the protective effects of SolA against adiposity, dyslipidemia and nonalcoholic fatty liver disease in obese mice.

Metformin and vitamin D modulate adipose-derived stem cell differentiation towards the beige phenotype

Adipose-derived stem cells (ADSCs) represent an ideal stem cell population for regenerative medicine. ADSC adipogenic differentiation is controlled by the activation of a specific transcriptional program, including epigenetic factors and key adipogenic genes. Under certain conditioned media, ADSCs can differentiate into several phenotypes. We previously demonstrated that bioactive molecules could counteract lipid accumulation and regulate adipogenesis, acting on inflammation and vitamin D metabolism. In the present paper, we aimed at evaluating the effect of metformin and vitamin D in targeting ADSC differentiation towards an intermediate phenotype, as beige adipocytes. We exposed ADSCs to different conditioned media and then we evaluated the levels of expression of main markers of adipogenesis, aP2, LPL and ACOT2. We also analysed the gene and protein expression of thermogenic UCP1 protein, and the expression of PARP1 and the beige specific marker TMEM26. Our results showed a novel effect of metformin and vitamin D not only in inhibiting adipogenesis, but also in inducing a specific 'brown-like' phenotype. These findings pave the way for their possible application in the control of de novo lipogenesis useful for the prevention of obesity and its related metabolic disorders.

Loss of myeloid lipoprotein lipase exacerbates adipose tissue fibrosis with collagen VI deposition and hyperlipidemia in leptin-deficient obese mice

During obesity, tissue macrophages increase in number and become proinflammatory, thereby contributing to metabolic dysfunction. Lipoprotein lipase (LPL), which hydrolyzes triglyceride in lipoproteins, is secreted by macrophages. However, the role of macrophage-derived LPL in adipose tissue remodeling and lipoprotein metabolism is largely unknown. To clarify these issues, we crossed leptin-deficient Lep^ob/ob mice with mice lacking the Lpl gene in myeloid cells (Lpl^m−/m−) to generate Lpl^m−/m−;Lep^ob/ob mice. We found the weight of perigonadal white adipose tissue (WAT) was increased in Lpl^m−/m−;Lep^ob/ob mice compared with Lep^ob/ob mice due to substantial accumulation of both adipose tissue macrophages and collagen that surrounded necrotic adipocytes. In the fibrotic epidydimal WAT of Lpl^m−/m−;Lep^ob/ob mice, we observed an increase in collagen VI and high mobility group box 1, while α-smooth muscle cell actin, a marker of myofibroblasts, was almost undetectable, suggesting that the adipocytes were the major source of the collagens. Furthermore, the adipose tissue macrophages from Lpl^m−/m−;Lep^ob/ob mice showed increased expression of genes related to fibrosis and inflammation. In addition, we determined Lpl^m−/m−;Lep^ob/ob mice were more hypertriglyceridemic than Lep^ob/ob mice. Lpl^m−/m−;Lep^ob/ob mice also showed slower weight gain than Lep^ob/ob mice, which was primarily due to reduced food intake. In conclusion, we discovered that the loss of myeloid Lpl led to extensive fibrosis of perigonadal WAT and hypertriglyceridemia. In addition to illustrating an important role of macrophage LPL in regulation of circulating triglyceride levels, these data show that macrophage LPL protects against fibrosis in obese adipose tissues.

Untapped Pharmaceutical Potential of 4,5,4'-Trihydroxy-3,3'-dimethoxybibenzyl for Regulating Obesity: A Cell-Based Study with a Focus on Terminal Differentiation in Adipogenesis

Obesity and its global prevalence has become a threat to human health, while its pharmacotherapy via the application of natural products is still underdeveloped. Here, we probed how 4,5,4'-trihydroxy-3,3'-dimethoxybibenzyl (TDB) derived from an orchid (Dendrobium ellipsophyllum) could exert its roles on the differentiation and function of murine (3T3-L1) and human (PCS-210-010) pre-adipocytes and offer some implications to modulate obesity. Cytotoxic effects of TDB on adipocytes were 2-fold lower than those detected with pre-adipocytes, and no significant difference was detected in cytotoxic profiles between both cell lineages. TDB in a dose-dependent manner decreased cellular lipid accumulation and enhanced lipolysis of both cell lines assessed at early differentiation and during maturation. Underlining molecular mechanisms proved that TBD paused the cell cycle progression by regulating inducers and inhibitors in mitotic clonal expansion, leading to growth arrest of pre-adipocytes at the G0/G1 phase. The compound also governed adipocyte differentiation by repressing expressions of crucial adipogenic regulators and effectors through deactivating the AKT/GSK-3β signaling pathway and activating the AMPK-ACC pathway. To this end, TDB has shown its pharmaceutical potential for modulating adipocyte development and function, and it would be a promising candidate for further assessments as a therapeutic agent to defeat obesity.

Brown adipose TRX2 deficiency activates mtDNA-NLRP3 to impair thermogenesis and protect against diet-induced insulin resistance

Brown adipose tissue (BAT), a crucial heat-generating organ, regulates whole-body energy metabolism by mediating thermogenesis. BAT inflammation is implicated in the pathogenesis of mitochondrial dysfunction and impaired thermogenesis. However, the link between BAT inflammation and systematic metabolism remains unclear. Herein, we use mice with BAT deficiency of thioredoxin-2 (TRX2), a protein that scavenges mitochondrial reactive oxygen species (ROS), to evaluate the impact of BAT inflammation on metabolism and thermogenesis and its underlying mechanism. Our results show that BAT-specific TRX2 ablation improves systematic metabolic performance via enhancing lipid uptake, which protects mice from diet-induced obesity, hypertriglyceridemia, and insulin resistance. TRX2 deficiency impairs adaptive thermogenesis by suppressing fatty acid oxidation. Mechanistically, loss of TRX2 induces excessive mitochondrial ROS, mitochondrial integrity disruption, and cytosolic release of mitochondrial DNA, which in turn activate aberrant innate immune responses in BAT, including the cGAS/STING and the NLRP3 inflammasome pathways. We identify NLRP3 as a key converging point, as its inhibition reverses both the thermogenesis defect and the metabolic benefits seen under nutrient overload in BAT-specific Trx2-deficient mice. In conclusion, we identify TRX2 as a critical hub integrating oxidative stress, inflammation, and lipid metabolism in BAT, uncovering an adaptive mechanism underlying the link between BAT inflammation and systematic metabolism.

Identification and Characterization of Variants in Intron 6 of the LPL Gene Locus among a Sample of the Kuwaiti Population

Lipoprotein lipase (LPL) is responsible for the hydrolysis of lipoproteins; hence defective LPL is associated with metabolic disorders. Here, we identify certain intronic insertions and deletions (InDels) and single nucleotide polymorphisms (SNPs) in intron 6 of the LPL gene and investigate their associations with different phenotypic characteristics in a cohort of the general Kuwaiti population. Two specific regions of intron 6 of the LPL gene, which contain InDels, were amplified via Sanger sequencing in 729 subjects. Genotypic and allelic frequencies were estimated, and genetic modeling was used to investigate genetic associations of the identified variants with lipid profile, body mass index (BMI), and risk of coronary heart disease (CHD). A total of 16 variants were identified, including 2 InDels, 2 novel SNPs, and 12 known SNPs. The most common variants observed among the population were rs293, rs274, rs295, and rs294. The rs293 “A” insertion showed a significant positive correlation with elevated LDL levels, while rs295 was significantly associated with increased BMI. The rs274 and rs294 variants showed a protective effect of the minor allele with decreased CHD prevalence. These findings shed light on the possible role of LPL intronic variants on metabolic disorders.

Pcpe2, a Novel Extracellular Matrix Protein, Regulates Adipocyte SR-BI-Mediated High-Density Lipoprotein Uptake

Objective

To investigate the role of adipocyte Pcpe2 (procollagen C-endopeptidase enhancer 2) in SR-BI (scavenger receptor class BI)-mediated HDL-C (high-density lipoprotein cholesterol) uptake and contributions to adipose lipid storage.

Approach and Results

Pcpe2, a glycoprotein devoid of intrinsic proteolytic activity, is believed to participate in extracellular protein-protein interactions, supporting SR-BI- mediated HDL-C uptake. In published studies, Pcpe2 deficiency increased the development of atherosclerosis by reducing SR-BI-mediated HDL-C catabolism, but the biological impact of this deficiency on adipocyte SR-BI-mediated HDL-C uptake is unknown. Differentiated cells from Ldlr-/-/Pcpe2-/- (Pcpe2-/-) mouse adipose tissue showed elevated SR-BI protein levels, but significantly reduced HDL-C uptake compared to Ldlr-/- (control) adipose tissue. SR-BI-mediated HDL-C uptake was restored by preincubation of cells with exogenous Pcpe2. In diet-fed mice lacking Pcpe2, significant reductions in visceral, subcutaneous, and brown adipose tissue mass were observed, despite elevations in plasma triglyceride and cholesterol concentrations. Significant positive correlations exist between adipose mass and Pcpe2 expression in both mice and humans.

Conclusions

Overall, these findings reveal a novel and unexpected function for Pcpe2 in modulating SR-BI expression and function as it relates to adipose tissue expansion and cholesterol balance in both mice and humans.

Eruptive xanthoma model reveals endothelial cells internalize and metabolize chylomicrons, leading to extravascular triglyceride accumulation

Although tissue uptake of fatty acids from chylomicrons is primarily via lipoprotein lipase (LpL) hydrolysis of triglycerides (TGs), studies of patients with genetic LpL deficiency suggest additional pathways deliver dietary lipids to tissues. Despite an intact endothelial cell (EC) barrier, hyperchylomicronemic patients accumulate chylomicron-derived lipids within skin macrophages, leading to the clinical finding eruptive xanthomas. We explored whether an LpL-independent pathway exists for transfer of circulating lipids across the EC barrier. We found that LpL-deficient mice had a marked increase in aortic EC lipid droplets before and after a fat gavage. Cultured ECs internalized chylomicrons, which were hydrolyzed within lysosomes. The products of this hydrolysis fueled lipid droplet biogenesis in ECs and triggered lipid accumulation in cocultured macrophages. EC chylomicron uptake was inhibited by competition with HDL and knockdown of the scavenger receptor-BI (SR-BI). In vivo, SR-BI knockdown reduced TG accumulation in aortic ECs and skin macrophages of LpL-deficient mice. Thus, ECs internalize chylomicrons, metabolize them in lysosomes, and either store or release their lipids. This latter process may allow accumulation of TGs within skin macrophages and illustrates a pathway that might be responsible for creation of eruptive xanthomas.

Association between the lipoprotein lipase rs1534649 gene polymorphism in intron one with Body Mass Index and High Density Lipoprotein-Cholesterol

Lipoprotein lipase (LPL) is an enzyme involved in lipid metabolism and distribution of fatty acids hence its role in the initiation and development of dyslipidemia and adiposity. Single nucleotide polymorphisms (SNPs) across the LPL gene have been associated with dyslipidemia, however, the association with obesity has been limited towards specific populations. This study examined the association between LPL gene polymorphisms with plasma lipid levels and body mass index (BMI) in the Kuwaiti population. We examined a total of 486 adults (303 and 183 females and males respectively) with plasma lipid levels and BMI. DNA samples were genotyped for two LPL gene polymorphisms (rs1534649 and rs28645722) using TaqMan allelic discrimination. The relationship between the genotypes with both plasma lipid levels and BMI were assessed using linear regression using “SNPassoc” package from R statistical software. Using an additive genetic model, linear regression analysis showed the T-allele of rs1534649 to be associated with increased BMI in a dose-dependent trend β = 2.13 (95% CI 1.33–2.94); p = 1.7 × 10⁻⁷. In addition, a borderline significance was observed between the T-allele and low levels of high density lipoprotein-cholesterol β = −0.04 (95% CI −0.08, −0.006); p = 0.02. There were no associations between rs28645722 and plasma lipid levels (p > 0.05). However, a trend was observed between the A-allele and increased BMI β = 1.75 (95% CI 0.14–3.35); p = 0.03. Our study shows intron one polymorphism rs1534649 to increase the risk of obesity and dyslipidemia. Our findings warrant further investigation of the mechanism of LPL on the development of obesity along with the role of intron one and its impact on LPL gene activity.

Lowering apolipoprotein CIII protects against high-fat diet-induced metabolic derangements

Increased levels of apolipoprotein CIII (apoCIII), a key regulator of lipid metabolism, result in obesity-related metabolic derangements. We investigated mechanistically whether lowering or preventing high-fat diet (HFD)–induced increase in apoCIII protects against the detrimental metabolic consequences. Mice, first fed HFD for 10 weeks and thereafter also given an antisense (ASO) to lower apoCIII, already showed reduced levels of apoCIII and metabolic improvements after 4 weeks, despite maintained obesity. Prolonged ASO treatment reversed the metabolic phenotype due to increased lipase activity and receptor-mediated hepatic uptake of lipids. Fatty acids were transferred to the ketogenic pathway, and ketones were used in brown adipose tissue (BAT). This resulted in no fat accumulation and preserved morphology and function of liver and BAT. If ASO treatment started simultaneously with the HFD, mice remained lean and metabolically healthy. Thus, lowering apoCIII protects against and reverses the HFD-induced metabolic phenotype by promoting physiological insulin sensitivity.

Alveolar lipids in pulmonary disease. A review

Lung lipid metabolism participates both in infant and adult pulmonary disease. The lung is composed by multiple cell types with specialized functions and coordinately acting to meet specific physiologic requirements. The alveoli are the niche of the most active lipid metabolic cell in the lung, the type 2 cell (T2C). T2C synthesize surfactant lipids that are an absolute requirement for respiration, including dipalmitoylphosphatidylcholine. After its synthesis and secretion into the alveoli, surfactant is recycled by the T2C or degraded by the alveolar macrophages (AM). Surfactant biosynthesis and recycling is tightly regulated, and dysregulation of this pathway occurs in many pulmonary disease processes. Alveolar lipids can participate in the development of pulmonary disease from their extracellular location in the lumen of the alveoli, and from their intracellular location in T2C or AM. External insults like smoke and pollution can disturb surfactant homeostasis and result in either surfactant insufficiency or accumulation. But disruption of surfactant homeostasis is also observed in many chronic adult diseases, including chronic obstructive pulmonary disease (COPD), and others. Sustained damage to the T2C is one of the postulated causes of idiopathic pulmonary fibrosis (IPF), and surfactant homeostasis is disrupted during fibrotic conditions. Similarly, surfactant homeostasis is impacted during acute respiratory distress syndrome (ARDS) and infections. Bioactive lipids like eicosanoids and sphingolipids also participate in chronic lung disease and in respiratory infections. We review the most recent knowledge on alveolar lipids and their essential metabolic and signaling functions during homeostasis and during some of the most commonly observed pulmonary diseases.

IDOL regulates systemic energy balance through control of neuronal VLDLR expression

Liver X receptors limit cellular lipid uptake by stimulating the transcription of Inducible Degrader of the LDL Receptor (IDOL), an E3 ubiquitin ligase that targets lipoprotein receptors for degradation. The function of IDOL in systemic metabolism is incompletely understood. Here we show that loss of IDOL in mice protects against the development of diet-induced obesity and metabolic dysfunction by altering food intake and thermogenesis. Unexpectedly, analysis of tissue-specific knockout mice revealed that IDOL affects energy balance, not through its actions in peripheral metabolic tissues (liver, adipose, endothelium, intestine, skeletal muscle), but by controlling lipoprotein receptor abundance in neurons. Single-cell RNA sequencing of the hypothalamus demonstrated that IDOL deletion altered gene expression linked to control of metabolism. Finally, we identify VLDLR rather than LDLR as the primary mediator of IDOL effects on energy balance. These studies identify a role for the neuronal IDOL-VLDLR pathway in metabolic homeostasis and diet-induced obesity.

Role of LpL (Lipoprotein Lipase) in Macrophage Polarization In Vitro and In Vivo

OBJECTIVE

Fatty acid uptake and oxidation characterize the metabolism of alternatively activated macrophage polarization in vitro, but the in vivo biology is less clear. We assessed the roles of LpL (lipoprotein lipase)-mediated lipid uptake in macrophage polarization in vitro and in several important tissues in vivo. Approach and Results: We created mice with both global and myeloid-cell specific LpL deficiency. LpL deficiency in the presence of VLDL (very low-density lipoproteins) altered gene expression of bone marrow-derived macrophages and led to reduced lipid uptake but an increase in some anti- and some proinflammatory markers. However, LpL deficiency did not alter lipid accumulation or gene expression in circulating monocytes nor did it change the ratio of Ly6Chigh/Ly6Clow. In adipose tissue, less macrophage lipid accumulation was found with global but not myeloid-specific LpL deficiency. Neither deletion affected the expression of inflammatory genes. Global LpL deficiency also reduced the numbers of elicited peritoneal macrophages. Finally, we assessed gene expression in macrophages from atherosclerotic lesions during regression; LpL deficiency did not affect the polarity of plaque macrophages.

CONCLUSIONS

The phenotypic changes observed in macrophages upon deletion of Lpl in vitro is not mimicked in tissue macrophages.

Interrogation of nonconserved human adipose lincRNAs identifies a regulatory role of linc-ADAL in adipocyte metabolism

Long intergenic noncoding RNAs (lincRNAs) have emerged as important modulators of cellular functions. Most lincRNAs are not conserved among mammals, raising the fundamental question of whether nonconserved adipose-expressed lincRNAs are functional. To address this, we performed deep RNA sequencing of gluteal subcutaneous adipose tissue from 25 healthy humans. We identified 1001 putative lincRNAs expressed in all samples through de novo reconstruction of noncoding transcriptomes and integration with existing lincRNA annotations. One hundred twenty lincRNAs had adipose-enriched expression, and 54 of these exhibited peroxisome proliferator-activated receptor γ (PPARγ) or CCAAT/enhancer binding protein α (C/EBPα) binding at their loci. Most of these adipose-enriched lincRNAs (~85%) were not conserved in mice, yet on average, they showed degrees of expression and binding of PPARγ and C/EBPα similar to those displayed by conserved lincRNAs. Most adipose lincRNAs differentially expressed (n = 53) in patients after bariatric surgery were nonconserved. The most abundant adipose-enriched lincRNA in our subcutaneous adipose data set, linc-ADAL, was nonconserved, up-regulated in adipose depots of obese individuals, and markedly induced during in vitro human adipocyte differentiation. We demonstrated that linc-ADAL interacts with heterogeneous nuclear ribonucleoprotein U (hnRNPU) and insulin-like growth factor 2 mRNA binding protein 2 (IGF2BP2) at distinct subcellular locations to regulate adipocyte differentiation and lipogenesis.

Lipoprotein Lipase Inhibitor, Nordihydroguaiaretic Acid, Aggravates Metabolic Phenotypes and Alters HDL Particle Size in the Western Diet-Fed db/db Mice

Lipoprotein lipase (LPL) hydrolyzes triglycerides in lipoprotein to supply fatty acids, and its deficiency leads to hypertriglyceridemia, thereby inducing metabolic syndrome (MetSyn). Nordihydroguaiaretic acid (NDGA) has been recently reported to inhibit LPL secretion by endoplasmic reticulum (ER)-Golgi redistribution. However, the role of NDGA on dyslipidemia and MetSyn remains unclear. To address this question, leptin receptor knock out (KO)-db/db mice were randomly assigned to three different groups: A normal AIN76-A diet (CON), a Western diet (WD) and a Western diet with 0.1% NDGA and an LPL inhibitor, (WD+NDGA). All mice were fed for 12 weeks. The LPL inhibition by NDGA was confirmed by measuring the systemic LPL mass and adipose LPL gene expression. We investigated whether the LPL inhibition by NDGA alters the metabolic phenotypes. NDGA led to hyperglycemia, hypertriglyceridemia, and hypercholesterolemia. More strikingly, the supplementation of NDGA increased the percentage of high density lipoprotein (HDL)_small (HDL_3a+3b+3c) and decreased the percentage of HDL_large (HDL_2a+2b) compared to the WD group, which indicates that LPL inhibition modulates HDL subclasses. was NDGA increased adipose inflammation but had no impact on hepatic stress signals. Taken together, these findings demonstrated that LPL inhibition by NDGA aggravates metabolic parameters and alters HDL particle size.

Inhibition of PRMT3 activity reduces hepatic steatosis without altering atherosclerosis susceptibility in apoE knockout mice

The nuclear receptor liver X receptor (LXR) impacts on cholesterol metabolism as well as hepatic lipogenesis via transcriptional regulation. It is proposed that inhibition of the protein arginine methyltransferase 3 (PRMT3) uncouples these two transcriptional pathways in vivo by acting as a specific lipogenic coactivator of LXR. Here we validated the hypothesis that treatment with the allosteric PRMT3 inhibitor SGC707 will diminish the hepatic steatosis extent, while leaving global cholesterol metabolism, important in cholesterol-driven pathologies like atherosclerosis, untouched. For this purpose, 12-week old hyperlipidemic apolipoprotein E knockout mice were fed a Western-type diet for six weeks to induce both hepatic steatosis and atherosclerosis. The mice received 3 intraperitoneal injections with SGC707 or solvent control per week. Mice chronically treated with SGC707 developed less severe hepatic steatosis as exemplified by the 51% reduced (P < 0.05) liver triglyceride levels. In contrast, the extent of in vivo macrophage foam cell formation and aortic root atherosclerosis was not affected by SGC707 treatment. Interestingly, SGC707-treated mice gained 94% less body weight (P < 0.05), which was paralleled by changes in white adipose tissue morphology, i.e. reduction in adipocyte size and browning. In conclusion, we have shown that through PRMT3 inhibitor treatment specific functions of LXR involved in respectively the development of fatty liver disease and atherosclerosis can be uncoupled, resulting in an overall diminished hepatic steatosis extent without a negative impact on atherosclerosis susceptibility. As such, our studies highlight that PRMT3 inhibition may constitute a novel therapeutic approach to limit the development of fatty liver disease in humans.

Lipoprotein lipase transporter GPIHBP1 and triglyceride-rich lipoprotein metabolism

Increased plasma triglyceride serves as an independent risk factor for cardiovascular disease (CVD). Lipoprotein lipase (LPL), which hydrolyzes circulating triglyceride, plays a crucial role in normal lipid metabolism and energy balance. Hypertriglyceridemia is possibly caused by gene mutations resulting in LPL dysfunction. There are many factors that both positively and negatively interact with LPL thereby impacting TG lipolysis. Glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1), a newly identified factor, appears essential for transporting LPL to the luminal side of the blood vessel and offering a platform for TG hydrolysis. Numerous lines of evidence indicate that GPIHBP1 exerts distinct functions and plays diverse roles in human triglyceride-rich lipoprotein (TRL) metabolism. In this review, we discuss the GPIHBP1 gene, protein, its expression and function and subsequently focus on its regulation and provide critical evidence supporting its role in TRL metabolism. Underlying mechanisms of action are highlighted, additional studies discussed and potential therapeutic targets reviewed.

Impaired thermogenesis and sharp increases in plasma triglyceride levels in GPIHBP1-deficient mice during cold exposure

Glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1), an endothelial cell protein, binds LPL in the subendothelial spaces and transports it to the capillary lumen. In Gpihbp1-/- mice, LPL remains stranded in the subendothelial spaces, causing hypertriglyceridemia, but how Gpihbp1-/- mice respond to metabolic stress (e.g., cold exposure) has never been studied. In wild-type mice, cold exposure increases LPL-mediated processing of triglyceride-rich lipoproteins (TRLs) in brown adipose tissue (BAT), providing fuel for thermogenesis and leading to lower plasma triglyceride levels. We suspected that defective TRL processing in Gpihbp1-/- mice might impair thermogenesis and blunt the fall in plasma triglyceride levels. Indeed, Gpihbp1-/- mice exhibited cold intolerance, but the effects on plasma triglyceride levels were paradoxical. Rather than falling, the plasma triglyceride levels increased sharply (from ∼4,000 to ∼15,000 mg/dl), likely because fatty acid release by peripheral tissues drives hepatic production of TRLs that cannot be processed. We predicted that the sharp increase in plasma triglyceride levels would not occur in Gpihbp1-/-Angptl4-/- mice, where LPL activity is higher and baseline plasma triglyceride levels are lower. Indeed, the plasma triglyceride levels in Gpihbp1-/-Angptl4-/- mice fell during cold exposure. Metabolic studies revealed increased levels of TRL processing in the BAT of Gpihbp1-/-Angptl4-/- mice.

Absence of ANGPTL4 in adipose tissue improves glucose tolerance and attenuates atherogenesis

Alterations in ectopic lipid deposition and circulating lipids are major risk factors for developing cardiometabolic diseases. Angiopoietin-like protein 4 (ANGPTL4), a protein that inhibits lipoprotein lipase (LPL), controls fatty acid (FA) uptake in adipose and oxidative tissues and regulates circulating triacylglycerol-rich (TAG-rich) lipoproteins. Unfortunately, global depletion of ANGPTL4 results in severe metabolic abnormalities, inflammation, and fibrosis when mice are fed a high-fat diet (HFD), limiting our understanding of the contribution of ANGPTL4 in metabolic disorders. Here, we demonstrate that genetic ablation of ANGPTL4 in adipose tissue (AT) results in enhanced LPL activity, rapid clearance of circulating TAGs, increased AT lipolysis and FA oxidation, and decreased FA synthesis in AT. Most importantly, we found that absence of ANGPTL4 in AT prevents excessive ectopic lipid deposition in the liver and muscle, reducing novel PKC (nPKC) membrane translocation and enhancing insulin signaling. As a result, we observed a remarkable improvement in glucose tolerance in short-term HFD-fed AT-specific Angptl4-KO mice. Finally, lack of ANGPTL4 in AT enhances the clearance of proatherogenic lipoproteins, attenuates inflammation, and reduces atherosclerosis. Together, these findings uncovered an essential role of AT ANGPTL4 in regulating peripheral lipid deposition, influencing whole-body lipid and glucose metabolism and the progression of atherosclerosis.

Lipoprotein Lipase Deficiency Impairs Bone Marrow Myelopoiesis and Reduces Circulating Monocyte Levels

OBJECTIVE

Tissue macrophages induce and perpetuate proinflammatory responses, thereby promoting metabolic and cardiovascular disease. Lipoprotein lipase (LpL), the rate-limiting enzyme in blood triglyceride catabolism, is expressed by macrophages in atherosclerotic plaques. We questioned whether LpL, which is also expressed in the bone marrow (BM), affects circulating white blood cells and BM proliferation and modulates macrophage retention within the artery.

APPROACH AND RESULTS

We characterized blood and tissue leukocytes and inflammatory molecules in transgenic LpL knockout mice rescued from lethal hypertriglyceridemia within 18 hours of life by muscle-specific LpL expression (MCKL0 mice). LpL-deficient mice had ≈40% reduction in blood white blood cell, neutrophils, and total and inflammatory monocytes (Ly6C/G^hi). LpL deficiency also significantly decreased expression of BM macrophage-associated markers (F4/80 and TNF-α [tumor necrosis factor α]), master transcription factors (PU.1 and C/EBPα), and colony-stimulating factors (CSFs) and their receptors, which are required for monocyte and monocyte precursor proliferation and differentiation. As a result, differentiation of macrophages from BM-derived monocyte progenitors and monocytes was decreased in MCKL0 mice. Furthermore, although LpL deficiency was associated with reduced BM uptake and accumulation of triglyceride-rich particles and macrophage CSF-macrophage CSF receptor binding, triglyceride lipolysis products (eg, linoleic acid) stimulated expression of macrophage CSF and macrophage CSF receptor in BM-derived macrophage precursor cells. Arterial macrophage numbers decreased after heparin-mediated LpL cell dissociation and by genetic knockout of arterial LpL. Reconstitution of LpL-expressing BM replenished aortic macrophage density.

CONCLUSIONS

LpL regulates peripheral leukocyte levels and affects BM monocyte progenitor differentiation and aortic macrophage accumulation.

Increased thermogenesis by a noncanonical pathway in ANGPTL3/8-deficient mice

Dietary triglyceride (TG) is the most efficient energy substrate. It is processed and stored at substantially lower metabolic cost than is protein or carbohydrate. In fed animals, circulating TGs are preferentially routed for storage to white adipose tissue (WAT) by angiopoietin-like proteins 3 (A3) and 8 (A8). Here, we show that mice lacking A3 and A8 (A3^-/-A8^-/- mice) have decreased fat mass and a striking increase in temperature (+1 °C) in the fed (but not fasted) state, without alterations in food intake or physical activity. Subcutaneous WAT (WAT-SQ) from these animals had morphologic and metabolic changes characteristic of beiging. O₂ consumption rates (OCRs) and expression of genes involved in both fatty acid synthesis and fatty acid oxidation were increased in WAT-SQ of A3^-/-A8^-/- mice, but not in their epididymal or brown adipose tissue (BAT). The hyperthermic response to feeding was blocked by maintaining A3^-/-A8^-/- mice at thermoneutrality or by treating with a β3-adrenergic receptor (AR) antagonist. To determine if sympathetic stimulation was sufficient to increase body temperature in A3^-/-A8^-/- mice, WT and A3^-/-A8^-/- animals were maintained at thermoneutrality and then treated with a β3-AR agonist; treatment induced hyperthermia in A3^-/-A8^-/- , but not WT, mice. Antibody-mediated inactivation of both circulating A3 and A8 induced hyperthermia in WT mice. Together, these data indicate that A3 and A8 are essential for efficient storage of dietary TG and that disruption of these genes increases feeding-induced thermogenesis and energy utilization.

Quantification of Bone Fatty Acid Metabolism and Its Regulation by Adipocyte Lipoprotein Lipase

Adipocytes are master regulators of energy homeostasis. Although the contributions of classical brown and white adipose tissue (BAT and WAT, respectively) to glucose and fatty acid metabolism are well characterized, the metabolic role of adipocytes in bone marrow remains largely unclear. Here, we quantify bone fatty acid metabolism and its contribution to systemic nutrient handling in mice. Whereas in parts of the skeleton the specific amount of nutrients taken-up from the circulation was lower than in other metabolically active tissues such as BAT or liver, the overall contribution of the skeleton as a whole organ was remarkable, placing it among the top organs involved in systemic glucose as well as fatty acid clearance. We show that there are considerable site-specific variations in bone marrow fatty acid composition throughout the skeleton and that, especially in the tibia, marrow fatty acid profiles resemble classical BAT and WAT. Using a mouse model lacking lipoprotein lipase (LPL), a master regulator of plasma lipid turnover specifically in adipocytes, we show that impaired fatty acid flux leads to reduced amounts of dietary essential fatty acids while there was a profound increase in de novo produced fatty acids in both bone marrow and cortical bone. Notably, these changes in fatty acid profiles were not associated with any gross skeletal phenotype. These results identify LPL as an important regulator of fatty acid transport to skeletal compartments and demonstrate an intricate functional link between systemic and skeletal fatty acid and glucose metabolism.

Adipose-specific deletion of Kif5b exacerbates obesity and insulin resistance in a mouse model of diet-induced obesity

Recent studies have shown that KIF5B (conventional kinesin heavy chain) mediates glucose transporter type 4 translocation and adiponectin secretion in 3T3-L1 adipocytes, suggesting an involvement of KIF5B in the homeostasis of metabolism. However, the in vivo physiologic function of KIF5B in adipose tissue remains to be determined. In this study, adipose-specific Kif5b knockout (F-K5bKO) mice were generated using the Cre-LoxP strategy. F-K5bKO mice had similar body weights to controls fed on a standard chow diet. However, F-K5bKO mice had hyperlipidemia and significant glucose intolerance and insulin resistance. Deletion of Kif5b aggravated the deleterious impact of a high-fat diet (HFD) on body weight gain, hepatosteatosis, glucose tolerance, and systematic insulin sensitivity. These changes were accompanied by impaired insulin signaling, decreased secretion of adiponectin, and increased serum levels of leptin and proinflammatory adipokines. F-K5bKO mice fed on an HFD exhibited lower energy expenditure and thermogenic dysfunction as a result of whitening of brown adipose due to decreased mitochondria biogenesis and down-regulation of key thermogenic gene expression. In conclusion, selective deletion of Kif5b in adipose tissue exacerbates HFD-induced obesity and its associated metabolic disorders, partly through a decrease in energy expenditure, dysregulation of adipokine secretion, and insulin signaling.-Cui, J., Pang, J., Lin, Y.-J., Gong, H., Wang, Z.-H., Li, Y.-X., Li, J., Wang, Z., Jiang, P., Dai, D.-P., Li, J., Cai, J.-P., Huang, J.-D., Zhang, T.-M. Adipose-specific deletion of Kif5b exacerbates obesity and insulin resistance in a mouse model of diet-induced obesity.

Triglyceride Treatment in the Age of Cholesterol Reduction

Cholesterol reduction has markedly reduced major cardiovascular disease (CVD) events and shown regression of atherosclerosis in some studies. However, CVD has for decades also been associated with increased levels of circulating triglyceride (TG)-rich lipoproteins. Whether this is due to a direct toxic effect of these lipoproteins on arteries or whether this is merely an association is unresolved. More recent genetic analyses have linked genes that modulate TG metabolism with CVD. Moreover, analyses of subgroups of hypertriglyceridemic (HTG) subjects in clinical trials using fibric acid drugs have been interpreted as evidence that TG reduction reduces CVD events. This review will focus on how HTG might cause CVD, whether TG reduction makes a difference, what pathophysiological defects cause HTG, and what options are available for treatment.

ApoC-III inhibits clearance of triglyceride-rich lipoproteins through LDL family receptors

Hypertriglyceridemia is an independent risk factor for cardiovascular disease, and plasma triglycerides (TGs) correlate strongly with plasma apolipoprotein C-III (ApoC-III) levels. Antisense oligonucleotides (ASOs) for ApoC-III reduce plasma TGs in primates and mice, but the underlying mechanism of action remains controversial. We determined that a murine-specific ApoC-III-targeting ASO reduces fasting TG levels through a mechanism that is dependent on low-density lipoprotein receptors (LDLRs) and LDLR-related protein 1 (LRP1). ApoC-III ASO treatment lowered plasma TGs in mice lacking lipoprotein lipase (LPL), hepatic heparan sulfate proteoglycan (HSPG) receptors, LDLR, or LRP1 and in animals with combined deletion of the genes encoding HSPG receptors and LDLRs or LRP1. However, the ApoC-III ASO did not lower TG levels in mice lacking both LDLR and LRP1. LDLR and LRP1 were also required for ApoC-III ASO-induced reduction of plasma TGs in mice fed a high-fat diet, in postprandial clearance studies, and when ApoC-III-rich or ApoC-III-depleted lipoproteins were injected into mice. ASO reduction of ApoC-III had no effect on VLDL secretion, heparin-induced TG reduction, or uptake of lipids into heart and skeletal muscle. Our data indicate that ApoC-III inhibits turnover of TG-rich lipoproteins primarily through a hepatic clearance mechanism mediated by the LDLR/LRP1 axis.

SENP1-mediated NEMO deSUMOylation in adipocytes limits inflammatory responses and type-1 diabetes progression

Adipocyte dysfunction correlates with the development of diabetes. Here we show that mice with a adipocyte-specific deletion of the SUMO-specific protease SENP1 gene develop symptoms of type-1 diabetes mellitus (T1DM), including hyperglycaemia and glucose intolerance with mild insulin resistance. Peri-pancreatic adipocytes from SENP1-deficient mice exhibit heightened NF-κB activity and production of proinflammatory cytokines, which induce CCL5 expression in adjacent pancreatic islets and direct cytotoxic effects on pancreatic islets. Mechanistic studies show that SENP1 deletion in adipocytes enhances SUMOylation of the NF-κB essential molecule, NEMO, at lysine 277/309, leading to increased NF-κB activity, cytokine production and pancreatic inflammation. We further show that NF-κB inhibitors could inhibit pre-diabetic cytokine production, β-cell damages and ameliorate the T1DM phenotype in SENP1-deficient mice. Feeding a high-fat diet augments both type-1 and type-2 diabetes phenotypes in SENP1-deficient mice, consistent with the effects on adipocyte-derived NF-κB and cytokine signalling. Our study reveals previously unrecognized mechanism regulating the onset and progression of T1DM associated with adipocyte dysfunction.

ANGPTL4 mediates shuttling of lipid fuel to brown adipose tissue during sustained cold exposure

Brown adipose tissue (BAT) activation via cold exposure is increasingly scrutinized as a potential approach to ameliorate cardio-metabolic risk. Transition to cold temperatures requires changes in the partitioning of energy substrates, re-routing fatty acids to BAT to fuel non-shivering thermogenesis. However, the mechanisms behind the redistribution of energy substrates to BAT remain largely unknown. Angiopoietin-like 4 (ANGPTL4), a protein that inhibits lipoprotein lipase (LPL) activity, is highly expressed in BAT. Here, we demonstrate that ANGPTL4 is part of a shuttling mechanism that directs fatty acids derived from circulating triglyceride-rich lipoproteins to BAT during cold. Specifically, we show that cold markedly down-regulates ANGPTL4 in BAT, likely via activation of AMPK, enhancing LPL activity and uptake of plasma triglyceride-derived fatty acids. In contrast, cold up-regulates ANGPTL4 in WAT, abolishing a cold-induced increase in LPL activity. Together, our data indicate that ANGPTL4 is an important regulator of plasma lipid partitioning during sustained cold.

DOI:http://dx.doi.org/10.7554/eLife.08428.001

The body stores energy in the form of fat molecules. Most of these molecules are stored in white fat cells. Other fat cells, the so-called brown fat cells, consume fats and produce heat to maintain body temperature in cold conditions. The capacity of brown fat cells to consume fats has led researchers to investigate whether brown fat cells might be a key to combat obesity.

When an organism is cold, fat is shuttled to the brown fat cells. An enzyme called lipoprotein lipase is involved in a process that allows these fat molecules to be taken up by brown fat cells. However, it was not clear exactly how this process works.

A protein called Angiopoietin-like 4 (ANGPTL4) inhibits the activity of lipoprotein lipase in white fat cells and is also found at high levels in brown fat cells. Here, Dijk et al. used genetic and biochemical approaches to study the role of ANGPTL4 in the fat cells of mice. The experiments show that when mice are exposed to cold, the levels of ANGPTL4 decrease in the brown fat cells. This allows the activity of lipoprotein lipase to increase so that these cells are able to take up more fat molecules.

However, the opposite happens in white fat cells during cold exposure. The levels of ANGPTL4 increase, which decreases the activity of lipoprotein lipase in white fat cells to allow fat molecules to be shuttled specifically to the brown fat cells. Further experiments suggest that the opposite regulation of ANGPTL4 in brown and white fat cells could be due to a protein called AMPK. This protein is found at higher levels in brown fat cells than in white fat cells and is produced by brown fat cells during cold exposure.

Taken together, Dijk et al. show that organs and cells work together to ensure that fat molecules are appropriately distributed to cells in need of energy, such as to brown fat cells during cold. How these findings could be used to stimulate fat consumption by brown fat cells in humans remains open for further investigation.

DOI:http://dx.doi.org/10.7554/eLife.08428.002

The Effects of a Single Developmentally Entrained Pulse of Testosterone in Female Neonatal Mice on Reproductive and Metabolic Functions in Adult Life

Early postnatal exposures to sex steroids have been well recognized to modulate predisposition to diseases of adulthood. There is a complex interplay between timing, duration and dose of endocrine exposures through environmental or dietary sources that may alter the sensitivity of target tissues to the exogenous stimuli. In this study, we determined the metabolic and reproductive programming effects of a single developmentally entrained pulse of testosterone (T) given to female mice in early postnatal period. CD-1 female mice pups were injected with either 5 μg of T enanthate (TE) or vehicle (control [CON] group) within 24 hours after birth and followed to adult age. A total of 66% of T-treated mice exhibited irregular cycling, anovulatory phenotype, and significantly higher ovarian weights than vehicle-treated mice. Longitudinal nuclear magnetic resonance measurements revealed that TE group had greater body weight, whole-body lean, and fat mass than the CON group. Adipose tissue cellularity analysis in TE group revealed a trend toward higher size and number than their littermate CONs. The brown adipose tissue of TE mice exhibited white fat infiltration with down-regulation of several markers, including uncoupling protein 1 (UCP-1), cell death-inducing DNA fragmentation factor, α-subunit-like effector A, bone morphogenetic protein 7 as well as brown adipose tissue differentiation-related transcription regulators. T-injected mice were also more insulin resistant than CON mice. These reproductive and metabolic reprogramming effects were not observed in animals exposed to TE at 3 and 6 weeks of age. Collectively, these data suggest that sustained reproductive and metabolic alterations may result in female mice from a transient exposure to T during a narrow postnatal developmental window.

Hepatic ANGPTL3 regulates adipose tissue energy homeostasis

Angiopoietin-like protein 3 (ANGPTL3) is a circulating inhibitor of lipoprotein and endothelial lipase whose physiological function has remained obscure. Here we show that ANGPTL3 plays a major role in promoting uptake of circulating very low density lipoprotein-triglycerides (VLDL-TGs) into white adipose tissue (WAT) rather than oxidative tissues (skeletal muscle, heart brown adipose tissue) in the fed state. This conclusion emerged from studies of Angptl3(-/-) mice. Whereas feeding increased VLDL-TG uptake into WAT eightfold in wild-type mice, no increase occurred in fed Angptl3(-/-) animals. Despite the reduction in delivery to and retention of TG in WAT, fat mass was largely preserved by a compensatory increase in de novo lipogenesis in Angptl3(-/-) mice. Glucose uptake into WAT was increased 10-fold in KO mice, and tracer studies revealed increased conversion of glucose to fatty acids in WAT but not liver. It is likely that the increased uptake of glucose into WAT explains the increased insulin sensitivity associated with inactivation of ANGPTL3. The beneficial effects of ANGPTL3 deficiency on both glucose and lipoprotein metabolism make it an attractive therapeutic target.

Apoc2 loss-of-function zebrafish mutant as a genetic model of hyperlipidemia

Apolipoprotein C-II (APOC2) is an obligatory activator of lipoprotein lipase. Human patients with APOC2 deficiency display severe hypertriglyceridemia while consuming a normal diet, often manifesting xanthomas, lipemia retinalis and pancreatitis. Hypertriglyceridemia is also an important risk factor for development of cardiovascular disease. Animal models to study hypertriglyceridemia are limited, with no Apoc2-knockout mouse reported. To develop a genetic model of hypertriglyceridemia, we generated an apoc2 mutant zebrafish characterized by the loss of Apoc2 function. apoc2 mutants show decreased plasma lipase activity and display chylomicronemia and severe hypertriglyceridemia, which closely resemble the phenotype observed in human patients with APOC2 deficiency. The hypertriglyceridemia in apoc2 mutants is rescued by injection of plasma from wild-type zebrafish or by injection of a human APOC2 mimetic peptide. Consistent with a previous report of a transient apoc2 knockdown, apoc2 mutant larvae have a minor delay in yolk consumption and angiogenesis. Furthermore, apoc2 mutants fed a normal diet accumulate lipid and lipid-laden macrophages in the vasculature, which resemble early events in the development of human atherosclerotic lesions. In addition, apoc2 mutant embryos show ectopic overgrowth of pancreas. Taken together, our data suggest that the apoc2 mutant zebrafish is a robust and versatile animal model to study hypertriglyceridemia and the mechanisms involved in the pathogenesis of associated human diseases.

Highlighted Article: Apoc2 loss-of-function zebrafish display severe hypertriglyceridemia, which is characteristic of human patients with defective lipoprotein lipase activity.

Adipocyte SIRT1 knockout promotes PPARγ activity, adipogenesis and insulin sensitivity in chronic-HFD and obesity

OBJECTIVE

Adipose tissue is the primary site for lipid deposition that protects the organisms in cases of nutrient excess during obesogenic diets. The histone deacetylase Sirtuin 1 (SIRT1) inhibits adipocyte differentiation by targeting the transcription factor peroxisome proliferator activated-receptor gamma (PPARγ).

METHODS

To assess the specific role of SIRT1 in adipocytes, we generated Sirt1 adipocyte-specific knockout mice (ATKO) driven by aP2 promoter onto C57BL/6 background. Sirt1 (flx/flx) aP2Cre (+) (ATKO) and Sirt1 (flx/flx) aP2Cre (-) (WT) mice were fed high-fat diet for 5 weeks (short-term) or 15 weeks (chronic-term). Metabolic studies were combined with gene expression analysis and phosphorylation/acetylation patterns in adipose tissue.

RESULTS

On standard chow, ATKO mice exhibit low-grade chronic inflammation in adipose tissue, along with glucose intolerance and insulin resistance compared with control fed mice. On short-term HFD, ATKO mice become more glucose intolerant, hyperinsulinemic, insulin resistant and display increased inflammation. During chronic HFD, WT mice developed a metabolic dysfunction, higher than ATKO mice, and thereby, knockout mice are more glucose tolerant, insulin sensitive and less inflamed relative to control mice. SIRT1 attenuates adipogenesis through PPARγ repressive acetylation and, in the ATKO mice adipocyte PPARγ was hyperacetylated. This high acetylation was associated with a decrease in Ser273-PPARγ phosphorylation. Dephosphorylated PPARγ is constitutively active and results in higher expression of genes associated with increased insulin sensitivity.

CONCLUSION

Together, these data establish that SIRT1 downregulation in adipose tissue plays a previously unknown role in long-term inflammation resolution mediated by PPARγ activation. Therefore, in the context of obesity, the development of new therapeutics that activate PPARγ by targeting SIRT1 may provide novel approaches to the treatment of T2DM.

Lipolysis, and not hepatic lipogenesis, is the primary modulator of triglyceride levels in streptozotocin-induced diabetic mice

OBJECTIVE

Diabetic hypertriglyceridemia is thought to be primarily driven by increased hepatic de novo lipogenesis. However, experiments in animal models indicated that insulin deficiency should decrease hepatic de novo lipogenesis and reduce plasma triglyceride levels.

APPROACH AND RESULTS

To address the discrepancy between human data and genetically altered mouse models, we investigated whether insulin-deficient diabetic mice had triglyceride changes that resemble those in diabetic humans. Streptozotocin-induced insulin deficiency increased plasma triglyceride levels in mice. Contrary to the mouse models with impaired hepatic insulin receptor signaling, insulin deficiency did not reduce hepatic triglyceride secretion and de novo lipogenesis-related gene expression. Diabetic mice had a marked decrease in postprandial triglycerides clearance, which was associated with decreased lipoprotein lipase and peroxisome proliferator-activated receptor α mRNA levels in peripheral tissues and decreased lipoprotein lipase activity in skeletal muscle, heart, and brown adipose tissue. Diabetic heterozygous lipoprotein lipase knockout mice had markedly elevated fasting plasma triglyceride levels and prolonged postprandial triglycerides clearance.

CONCLUSIONS

Insulin deficiency causes hypertriglyceridemia by decreasing peripheral lipolysis and not by an increase in hepatic triglycerides production and secretion.

Coupling of lipolysis and de novo lipogenesis in brown, beige, and white adipose tissues during chronic β3-adrenergic receptor activation

Chronic activation of β3-adrenergic receptors (β3-ARs) expands the catabolic activity of both brown and white adipose tissue by engaging uncoupling protein 1 (UCP1)-dependent and UCP1-independent processes. The present work examined de novo lipogenesis (DNL) and TG/glycerol dynamics in classic brown, subcutaneous "beige," and classic white adipose tissues during sustained β3-AR activation by CL 316,243 (CL) and also addressed the contribution of TG hydrolysis to these dynamics. CL treatment for 7 days dramatically increased DNL and TG turnover similarly in all adipose depots, despite great differences in UCP1 abundance. Increased lipid turnover was accompanied by the simultaneous upregulation of genes involved in FAS, glycerol metabolism, and FA oxidation. Inducible, adipocyte-specific deletion of adipose TG lipase (ATGL), the rate-limiting enzyme for lipolysis, demonstrates that TG hydrolysis is required for CL-induced increases in DNL, TG turnover, and mitochondrial electron transport in all depots. Interestingly, the effect of ATGL deletion on induction of specific genes involved in FA oxidation and synthesis varied among fat depots. Overall, these studies indicate that FAS and FA oxidation are tightly coupled in adipose tissues during chronic adrenergic activation, and this effect critically depends on the activity of adipocyte ATGL.

The ER-associated degradation adaptor protein Sel1L regulates LPL secretion and lipid metabolism

Sel1L is an essential adaptor protein for the E3 ligase Hrd1 in the endoplasmic reticulum (ER)-associated degradation (ERAD), a universal quality-control system in the cell; but its physiological role remains unclear. Here we show that mice with adipocyte-specific Sel1L deficiency are resistant to diet-induced obesity and exhibit postprandial hypertriglyceridemia. Further analyses reveal that Sel1L is indispensable for the secretion of lipoprotein lipase (LPL), independent of its role in Hrd1-mediated ERAD and ER homeostasis. Sel1L physically interacts with and stabilizes the LPL maturation complex consisting of LPL and lipase maturation factor 1 (LMF1). In the absence of Sel1L, LPL is retained in the ER and forms protein aggregates, which are degraded primarily by autophagy. The Sel1L-mediated control of LPL secretion is also seen in other LPL-expressing cell types including cardiac myocytes and macrophages. Thus, our study reports a role of Sel1L in LPL secretion and systemic lipid metabolism.

Apolipoprotein E synthesized by adipocyte and apolipoprotein E carried on lipoproteins modulate adipocyte triglyceride content

Excessive energy storage of adipose tissue makes contribution to the occurrence and progression of obesity, which accompanies with multiple adverse complications, such as metabolic syndrome, cardiovascular diseases. It is well known that apolipoprotein E, as a component of lipoproteins, performs a key role in maintaining plasma lipoproteins homeostasis. Interestingly, apolipoprotein E is highly expressed in adipocyte and has positive relation with body fat mass. Apolipoprotein E knock-out mice show small fat mass compared to wild type mice. Moreover, adipocyte deficiency in apolipoprotein E shows impaired lipoproeteins internalization and triglyceride accumulation. Apolipopreotein E-deficient lipoproteins can not induce preadipocyte to form round full-lipid adipocyte, whereas apolipoprotein E-containing lipoproteins can. This article mainly reviews the modulation of apolipoprotein E synthesized by adipocyte and apolipoprotein E carried on lipoproteins in adipocyte triglyceride content.

Increased adipocyte O2 consumption triggers HIF-1α, causing inflammation and insulin resistance in obesity

Adipose tissue hypoxia and inflammation have been causally implicated in obesity-induced insulin resistance. Here, we report that, early in the course of high-fat diet (HFD) feeding and obesity, adipocyte respiration becomes uncoupled, leading to increased oxygen consumption and a state of relative adipocyte hypoxia. These events are sufficient to trigger HIF-1α induction, setting off the chronic adipose tissue inflammatory response characteristic of obesity. At the molecular level, these events involve saturated fatty acid stimulation of the adenine nucleotide translocase 2 (ANT2), an inner mitochondrial membrane protein, which leads to the uncoupled respiratory state. Genetic or pharmacologic inhibition of either ANT2 or HIF-1α can prevent or reverse these pathophysiologic events, restoring a state of insulin sensitivity and glucose tolerance. These results reveal the sequential series of events in obesity-induced inflammation and insulin resistance.

The GPIHBP1-LPL complex is responsible for the margination of triglyceride-rich lipoproteins in capillaries

Triglyceride-rich lipoproteins (TRLs) undergo lipolysis by lipoprotein lipase (LPL), an enzyme that is transported to the capillary lumen by an endothelial cell protein, GPIHBP1. For LPL-mediated lipolysis to occur, TRLs must bind to the lumen of capillaries. This process is often assumed to involve heparan sulfate proteoglycans (HSPGs), but we suspected that TRL margination might instead require GPIHBP1. Indeed, TRLs marginate along the heart capillaries of wild-type but not Gpihbp1⁻/⁻ mice, as judged by fluorescence microscopy, quantitative assays with infrared-dye-labeled lipoproteins, and EM tomography. Both cell-culture and in vivo studies showed that TRL margination depends on LPL bound to GPIHBP1. Notably, the expression of LPL by endothelial cells in Gpihbp1⁻/⁻ mice did not restore defective TRL margination, implying that the binding of LPL to HSPGs is ineffective in promoting TRL margination. Our studies show that GPIHBP1-bound LPL is the main determinant of TRL margination.

Physiological regulation of lipoprotein lipase

The enzyme lipoprotein lipase (LPL), originally identified as the clearing factor lipase, hydrolyzes triglycerides present in the triglyceride-rich lipoproteins VLDL and chylomicrons. LPL is primarily expressed in tissues that oxidize or store fatty acids in large quantities such as the heart, skeletal muscle, brown adipose tissue and white adipose tissue. Upon production by the underlying parenchymal cells, LPL is transported and attached to the capillary endothelium by the protein GPIHBP1. Because LPL is rate limiting for plasma triglyceride clearance and tissue uptake of fatty acids, the activity of LPL is carefully controlled to adjust fatty acid uptake to the requirements of the underlying tissue via multiple mechanisms at the transcriptional and post-translational level. Although various stimuli influence LPL gene transcription, it is now evident that most of the physiological variation in LPL activity, such as during fasting and exercise, appears to be driven via post-translational mechanisms by extracellular proteins. These proteins can be divided into two main groups: the liver-derived apolipoproteins APOC1, APOC2, APOC3, APOA5, and APOE, and the angiopoietin-like proteins ANGPTL3, ANGPTL4 and ANGPTL8, which have a broader expression profile. This review will summarize the available literature on the regulation of LPL activity in various tissues, with an emphasis on the response to diverse physiological stimuli.

Fatty acid binding protein 4 and 5 play a crucial role in thermogenesis under the conditions of fasting and cold stress

Hypothermia is rapidly induced during cold exposure when thermoregulatory mechanisms, including fatty acid (FA) utilization, are disturbed. FA binding protein 4 (FABP4) and FABP5, which are abundantly expressed in adipose tissues and macrophages, have been identified as key molecules in the pathogenesis of overnutrition-related diseases, such as insulin resistance and atherosclerosis. We have recently shown that FABP4/5 are prominently expressed in capillary endothelial cells in the heart and skeletal muscle and play a crucial role in FA utilization in these tissues. However, the role of FABP4/5 in thermogenesis remains to be determined. In this study, we showed that thermogenesis is severely impaired in mice lacking both FABP4 and FABP5 (DKO mice), as manifested shortly after cold exposure during fasting. In DKO mice, the storage of both triacylglycerol in brown adipose tissue (BAT) and glycogen in skeletal muscle (SkM) was nearly depleted after fasting, and a biodistribution analysis using 125I-BMIPP revealed that non-esterified FAs (NEFAs) are not efficiently taken up by BAT despite the robustly elevated levels of serum NEFAs. In addition to the severe hypoglycemia observed in DKO mice during fasting, cold exposure did not induce the uptake of glucose analogue 18F-FDG by BAT. These findings strongly suggest that DKO mice exhibit pronounced hypothermia after fasting due to the depletion of energy storage in BAT and SkM and the reduced supply of energy substrates to these tissues. In conclusion, FABP4/5 play an indispensable role in thermogenesis in BAT and SkM. Our study underscores the importance of FABP4/5 for overcoming life-threatening environments, such as cold and starvation.

Apolipoproteins C-I and C-III inhibit lipoprotein lipase activity by displacement of the enzyme from lipid droplets

Apolipoproteins (apo) C-I and C-III are known to inhibit lipoprotein lipase (LPL) activity, but the molecular mechanisms for this remain obscure. We present evidence that either apoC-I or apoC-III, when bound to triglyceride-rich lipoproteins, prevent binding of LPL to the lipid/water interface. This results in decreased lipolytic activity of the enzyme. Site-directed mutagenesis revealed that hydrophobic amino acid residues centrally located in the apoC-III molecule are critical for attachment to lipid emulsion particles and consequently inhibition of LPL activity. Triglyceride-rich lipoproteins stabilize LPL and protect the enzyme from inactivating factors such as angiopoietin-like protein 4 (angptl4). The addition of either apoC-I or apoC-III to triglyceride-rich particles severely diminished their protective effect on LPL and rendered the enzyme more susceptible to inactivation by angptl4. These observations were seen using chylomicrons as well as the synthetic lipid emulsion Intralipid. In the presence of the LPL activator protein apoC-II, more of apoC-I or apoC-III was needed for displacement of LPL from the lipid/water interface. In conclusion, we show that apoC-I and apoC-III inhibit lipolysis by displacing LPL from lipid emulsion particles. We also propose a role for these apolipoproteins in the irreversible inactivation of LPL by factors such as angptl4.

Mice lacking ANGPTL8 (Betatrophin) manifest disrupted triglyceride metabolism without impaired glucose homeostasis

Angiopoietin-like protein (ANGPTL)8 (alternatively called TD26, RIFL, Lipasin, and Betatrophin) is a newly recognized ANGPTL family member that has been implicated in both triglyceride (TG) and glucose metabolism. Hepatic overexpression of ANGPTL8 causes hypertriglyceridemia and increased insulin secretion. Here we examined the effects of inactivating Angptl8 on TG and glucose metabolism in mice. Angptl8 knockout (Angptl8(-/-)) mice gained weight more slowly than wild-type littermates due to a selective reduction in adipose tissue accretion. Plasma levels of TGs of the Angptl8(-/-) mice were similar to wild-type animals in the fasted state but paradoxically decreased after refeeding. The lower TG levels were associated with both a reduction in very low density lipoprotein secretion and an increase in lipoprotein lipase (LPL) activity. Despite the increase in LPL activity, the uptake of very low density lipoprotein-TG is markedly reduced in adipose tissue but preserved in hearts of fed Angptl8(-/-) mice. Taken together, these data indicate that ANGPTL8 plays a key role in the metabolic transition between fasting and refeeding; it is required to direct fatty acids to adipose tissue for storage in the fed state. Finally, glucose and insulin tolerance testing revealed no alterations in glucose homeostasis in mice fed either a chow or high fat diet. Thus, although absence of ANGPTL8 profoundly disrupts TG metabolism, we found no evidence that it is required for maintenance of glucose homeostasis.