Linking nutritional regulation of Angptl4, Gpihbp1, and Lmf1 to lipoprotein lipase activity in rodent adipose tissue

Characterization of sexual dimorphism in ANGPTL4 levels and function

ANGPTL4 is an attractive pharmacological target for lowering plasma triglycerides and cardiovascular risk. Since most preclinical studies on ANGPTL4 were performed in male mice, little is known about sexual dimorphism in ANGPTL4 regulation and function. Here, we aimed to study potential sexual dimorphism in ANGPTL4 mRNA and protein levels and ANGPTL4 function. Additionally, we performed exploratory studies on the function of ANGPTL4 in the liver during fasting using Angptl4-transgenic and Angptl4-/- mice. Compared to female mice, male mice showed higher hepatic and adipose ANGPTL4 mRNA and protein levels, as well as a more pronounced effect of genetic ANGPTL4 modulation on plasma lipids. By contrast, very limited sexual dimorphism in ANGPTL4 levels was observed in human liver and adipose tissue. In human and mouse adipose tissue, ANGPTL8 mRNA and/or protein levels were significantly higher in females than males. Adipose LPL protein levels were higher in female than male Angptl4-/- mice, which was abolished by ANGPTL4 (over) expression. At the human genetic level, the ANGPTL4 E40K loss-of-function variant was associated with similar plasma triglyceride reductions in women and men. Finally, ANGPTL4 ablation in fasted mice was associated with changes in hepatic gene expression consistent with PPARα activation. In conclusion, the levels of ANGPTL4 and the magnitude of the effect of ANGPTL4 on plasma lipids exhibit sexual dimorphism. Nonetheless, inactivation of ANGPTL4 should confer a similar metabolic benefit in women and men. Expression levels of ANGPTL8 in human and mouse adipose tissue are highly sexually dimorphic, showing higher levels in females than males.

The response to fasting and refeeding reveals functional regulation of lipoprotein lipase proteoforms

Lipoprotein lipase (LPL) is responsible for the intravascular catabolism of triglyceride-rich lipoproteins and plays a central role in whole-body energy balance and lipid homeostasis. As such, LPL is subject to tissue-specific regulation in different physiological conditions, but the mechanisms of this regulation remain incompletely characterized. Previous work revealed that LPL comprises a set of proteoforms with different isoelectric points, but their regulation and functional significance have not been studied thus far. Here we studied the distribution of LPL proteoforms in different rat tissues and their regulation under physiological conditions. First, analysis by two-dimensional electrophoresis and Western blot showed different patterns of LPL proteoforms (i.e., different pI or relative abundance of LPL proteoforms) in different rat tissues under basal conditions, which could be related to the tissue-specific regulation of the enzyme. Next, the comparison of LPL proteoforms from heart and brown adipose tissue between adults and 15-day-old rat pups, two conditions with minimal regulation of LPL in these tissues, yielded virtually the same tissue-specific patterns of LPL proteoforms. In contrast, the pronounced downregulation of LPL activity observed in white adipose tissue during fasting is accompanied by a prominent reconfiguration of the LPL proteoform pattern. Furthermore, refeeding reverts this downregulation of LPL activity and restores the pattern of LPL proteoforms in this tissue. Importantly, this reversible proteoform-specific regulation during fasting and refeeding indicates that LPL proteoforms are functionally diverse. Further investigation of potential differences in the functional properties of LPL proteoforms showed that all proteoforms exhibit lipolytic activity and have similar heparin-binding affinity, although other functional aspects remain to be investigated. Overall, this study demonstrates the ubiquity, differential distribution and specific regulation of LPL proteoforms in rat tissues and underscores the need to consider the existence of LPL proteoforms for a complete understanding of LPL regulation under physiological conditions.

Genetic Mimicry Analysis Reveals the Specific Lipases Targeted by the ANGPTL3-ANGPTL8 Complex and ANGPTL4

Angiopoietin-like proteins, ANGPTL3, ANGPTL4, and ANGPTL8, are involved in regulating plasma lipids. In vitro and animal-based studies point to LPL and endothelial lipase (EL, LIPG) as key targets of ANGPTLs. To examine the ANGPTL mechanisms for plasma lipid modulation in humans, we pursued a genetic mimicry analysis of enhancing or suppressing variants in the LPL, LIPG, lipase C hepatic type (LIPC), ANGPTL3, ANGPTL4, and ANGPTL8 genes using data on 248 metabolic parameters derived from over 110,000 nonfasted individuals in the UK Biobank and validated in over 13,000 overnight fasted individuals from 11 other European populations. ANGPTL4 suppression was highly concordant with LPL enhancement but not HL or EL, suggesting ANGPTL4 impacts plasma metabolic parameters exclusively via LPL. The LPL-independent effects of ANGPTL3 suppression on plasma metabolic parameters showed a striking inverse resemblance with EL suppression, suggesting ANGPTL3 not only targets LPL but also targets EL. Investigation of the impact of the ANGPTL3-ANGPTL8 complex on plasma metabolite traits via the ANGPTL8 R59W substitution as an instrumental variable showed a much higher concordance between R59W and EL activity than between R59W and LPL activity, suggesting the R59W substitution more strongly affects EL inhibition than LPL inhibition. Meanwhile, when using a rare and deleterious protein-truncating ANGPTL8 variant as an instrumental variable, the ANGPTL3-ANGPTL8 complex was very LPL specific. In conclusion, our analysis provides strong human genetic evidence that the ANGPTL3-ANGPTL8 complex regulates plasma metabolic parameters, which is achieved by impacting LPL and EL. By contrast, ANGPTL4 influences plasma metabolic parameters exclusively via LPL.

Early-life exposure to bisphenol A induces dysregulation of lipid homeostasis by the upregulation of SCD1 in male mice

Exposure of Bisphenol A (BPA) is closely associated with an increased prevalence of obesity-related metabolic syndrome. However, the potential mechanism of BPA-induced adipogenesis remains to be fully elucidated. Herein, potential mechanisms of BPA-induced adipogenesis in 3T3-L1 preadipocytes were evaluated using RNA-Seq. Then, using an early-life BPA exposure model, we further evaluated the effects of BPA exposure on lipid and glucose homeostasis. The results showed that lipid content in 3T3-L1 adipocytes was significantly increased after BPA exposure (p < 0.01) and male C57BL/6 mice with the dose of 500 μg/kg/day BPA by once-a-day oral administration for 8 weeks displayed a NAFLD-like phenotype. RNA-Seq analysis of preadipocytes showed that BPA exposure affected multiple biological processes including glycosphingolipid biosynthesis, regulation of lipolysis in adipocytes, PPAR signaling pathway and fatty acid metabolism. The dysregulation in a series of genes of mice was associated to de novo lipogenesis and lipid transport, which was linked to obesity. Importantly, we also found a significant expression increase of stearoyl-CoA desaturase 1 (SCD1) and a significant decrease of apolipoprotein D (APOD) in both fat (p < 0.01) and livers (p < 0.01) of male mice. Besides, the dysregulation of pro-inflammatory genes (TNF-α,IL-6 and SAA3) showed that BPA exposure promoted progression of hepatic inflammation. In conclusion, this study elucidated a novel mechanism in which obesity associated with BPA exposure by targeting SCD1. Exposure to BPA should be carefully examined in the chronic liver metabolic diseases.

Seasonal and sex-dependent gene expression in emu (Dromaius novaehollandiae) fat tissues

Emu (Dromaius novaehollandiae) farming has been gaining wide interest for fat production. Oil rendered from this large flightless bird’s fat is valued for its anti-inflammatory and antioxidant properties for uses in therapeutics and cosmetics. We analyzed the seasonal and sex-dependent differentially expressed (DE) genes involved in fat metabolism in emus. Samples were taken from back and abdominal fat tissues of a single set of four male and four female emus in April, June, and November for RNA-sequencing. We found 100 DE genes (47 seasonally in males; 34 seasonally in females; 19 between sexes). Seasonally DE genes with significant difference between the sexes in gene ontology terms suggested integrin beta chain-2 (ITGB2) influences fat changes, in concordance with earlier studies. Six seasonally DE genes functioned in more than two enriched pathways (two female: angiopoietin-like 4 (ANGPTL4) and lipoprotein lipase (LPL); four male: lumican (LUM), osteoglycin (OGN), aldolase B (ALDOB), and solute carrier family 37 member 2 (SLC37A2)). Two sexually DE genes, follicle stimulating hormone receptor (FSHR) and perilipin 2 (PLIN2), had functional investigations supporting their influence on fat gain and loss. The results suggested these nine genes influence fat metabolism and deposition in emus.

ANGPTL4 silencing via antisense oligonucleotides reduces plasma triglycerides and glucose in mice without causing lymphadenopathy

Angiopoietin-like 4 (ANGPTL4) is an important regulator of plasma triglyceride (TG) levels and an attractive pharmacological target for lowering plasma lipids and reducing cardiovascular risk. Here, we aimed to study the efficacy and safety of silencing ANGPTL4 in the livers of mice using hepatocyte-targeting GalNAc-conjugated antisense oligonucleotides (ASOs). Compared with injections with negative control ASO, four injections of two different doses of ANGPTL4 ASO over 2 weeks markedly downregulated ANGPTL4 levels in liver and adipose tissue, which was associated with significantly higher adipose LPL activity and lower plasma TGs in fed and fasted mice, as well as lower plasma glucose levels in fed mice. In separate experiments, injection of two different doses of ANGPTL4 ASO over 20 weeks of high-fat feeding reduced hepatic and adipose ANGPTL4 levels but did not trigger mesenteric lymphadenopathy, an acute phase response, chylous ascites, or any other pathological phenotypes. Compared with mice injected with negative control ASO, mice injected with ANGPTL4 ASO showed reduced food intake, reduced weight gain, and improved glucose tolerance. In addition, they exhibited lower plasma TGs, total cholesterol, LDL-C, glucose, serum amyloid A, and liver TG levels. By contrast, no significant difference in plasma alanine aminotransferase activity was observed. Overall, these data suggest that ASOs targeting ANGPTL4 effectively reduce plasma TG levels in mice without raising major safety concerns.

Angiopoietin-like 4 governs diurnal lipoprotein lipase activity in brown adipose tissue

Objective

Brown adipose tissue (BAT) burns fatty acids (FAs) to produce heat, and shows diurnal oscillation in glucose and triglyceride (TG)-derived FA-uptake, peaking around wakening. Here we aimed to gain insight in the diurnal regulation of metabolic BAT activity.

Methods

RNA-sequencing, chromatin immunoprecipitation (ChIP)-sequencing, and lipidomics analyses were performed on BAT samples of wild type C57BL/6J mice collected at 3-hour intervals throughout the day. Knockout and overexpression models were used to study causal relationships in diurnal lipid handling by BAT.

Results

We identified pronounced enrichment of oscillating genes involved in extracellular lipolysis in BAT, accompanied by oscillations of FA and monoacylglycerol content. This coincided with peak lipoprotein lipase (Lpl) expression, and was predicted to be driven by peroxisome proliferator-activated receptor gamma (PPARγ) activity. ChIP-sequencing for PPARγ confirmed oscillation in binding of PPARγ to Lpl. Of the known LPL-modulators, angiopoietin-like 4 (Angptl4) showed the largest diurnal amplitude opposite to Lpl, and both Angptl4 knockout and overexpression attenuated oscillations of LPL activity and TG-derived FA-uptake by BAT.

Conclusions

Our findings highlight involvement of PPARγ and a crucial role of ANGPTL4 in mediating the diurnal oscillation of TG-derived FA-uptake by BAT, and imply that time of day is essential when targeting LPL activity in BAT to improve metabolic health.

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The transcriptome and lipidome of brown fat show clusters with distinct circadian phases.

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The peak in metabolic brown fat activity is defined by activation of lipolytic processes.

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PPARγ shows oscillating binding to lipolytic genes and may drive diurnal brown fat activity.

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Genetic modulation of the lipoprotein lipase inhibitor Angptl4 flattens rhythmic activity in brown fat.

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Time of day should be considered when studying the metabolic benefits of targeting brown fat.

Characterization of lipoprotein lipase storage vesicles in 3T3-L1 adipocytes

Lipoprotein lipase (LPL) is a secreted triglyceride lipase involved in the clearance of very-low-density lipoproteins and chylomicrons from circulation. LPL is expressed primarily in adipose and muscle tissues and transported to the capillary lumen. LPL secretion is regulated by insulin in adipose tissue; however, few studies have examined the regulatory and trafficking steps involved in secretion. Here, we describe the intracellular localization and insulin-dependent trafficking of LPL in 3T3-L1 adipocytes. We compared LPL trafficking to the better characterized trafficking pathways taken by leptin and GLUT4 (also known as SLC2A4). We show that the LPL trafficking pathway shares some characteristics of these other pathways, but that LPL subcellular localization and trafficking are distinct from those of GLUT4 and leptin. LPL secretion occurs slowly in response to insulin and rapidly in response to the Ca2+ ionophore ionomycin. This regulated trafficking is dependent on Golgi protein kinase D and the ADP-ribosylation factor GTPase ARF1. Together, these data give support to a new trafficking pathway for soluble cargo that is active in adipocytes.

Identification of Potential Candidate Genes From Co-Expression Module Analysis During Preadipocyte Differentiation in Landrace Pig

Preadipocyte differentiation plays an important role in lipid deposition and affects fattening efficiency in pigs. In the present study, preadipocytes isolated from the subcutaneous adipose tissue of three Landrace piglets were induced into mature adipocytes in vitro. Gene clusters associated with fat deposition were investigated using RNA sequencing data at four time points during preadipocyte differentiation. Twenty-seven co-expression modules were subsequently constructed using weighted gene co-expression network analysis. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyses revealed three modules (blue, magenta, and brown) as being the most critical during preadipocyte differentiation. Based on these data and our previous differentially expressed gene analysis, angiopoietin-like 4 (ANGPTL4) was identified as a key regulator of preadipocyte differentiation and lipid metabolism. After inhibition of ANGPTL4, the expression of adipogenesis-related genes was reduced, except for that of lipoprotein lipase (LPL), which was negatively regulated by ANGPTL4 during preadipocyte differentiation. Our findings provide a new perspective to understand the mechanism of fat deposition.

An updated ANGPTL3-4-8 model as a mechanism of triglyceride partitioning between fat and oxidative tissues

In mammals, triglyceride (TG), the main form of lipids for storing and providing energy, is stored in white adipose tissue (WAT) after food intake, while during fasting it is routed to oxidative tissues (heart and skeletal muscle) for energy production, a process referred to as TG partitioning. Lipoprotein lipase (LPL), a rate-limiting enzyme in this fundamental physiological process, hydrolyzes circulating TG to generate free fatty acids that are taken up by peripheral tissues. The postprandial activity of LPL declines in oxidative tissues but rises in WAT, directing TG to WAT; the reverse is true during fasting. However, the molecular mechanism in regulating tissue-specific LPL activity during the fed-fast cycle has not been completely understood. Research on angiopoietin-like (ANGPTL) proteins (A3, A4, and A8) has resulted in an ANGPTL3-4-8 model to explain the TG partitioning between WAT and oxidative tissues. Food intake induces A8 expression in the liver and WAT. Liver A8 activates A3 by forming the A3-8 complex, which is then secreted into the circulation. The A3-8 complex acts in an endocrine manner to inhibit LPL in oxidative tissues. WAT A8 forms the A4-8 complex, which acts locally to block A4's LPL-inhibiting activity. Therefore, the postprandial activity of LPL is low in oxidative tissues but high in WAT, directing circulating TG to WAT. Conversely, during fasting, reduced A8 expression in the liver and WAT disables A3 from inhibiting oxidative-tissue LPL and restores WAT A4's LPL-inhibiting activity, respectively. Thus, the fasting LPL activity is high in oxidative tissues but low in WAT, directing TG to the former. According to the model, we hypothesize that A8 antagonism has the potential to simultaneously reduce TG and increase HDL-cholesterol plasma levels. Future research on A3, A4, and A8 can hopefully provide more insights into human health, disease, and therapeutics.

Role and mechanism of the action of angiopoietin-like protein ANGPTL4 in plasma lipid metabolism

Triglycerides are carried in the bloodstream as the components of very low-density lipoproteins and chylomicrons. These circulating triglycerides are primarily hydrolyzed in muscle and adipose tissue by the enzyme lipoprotein lipase (LPL). The activity of LPL is regulated by numerous mechanisms, including by three members of the angiopoietin-like protein family: ANGPTL3, ANGPTL4, and ANGPTL8. In this review, we discuss the recent literature concerning the role and mechanism of action of ANGPTL4 in lipid metabolism. ANGPTL4 is a fasting- and lipid-induced factor secreted by numerous cells, including adipocytes, hepatocytes, (cardio)myocytes, and macrophages. In adipocytes, ANGPTL4 mediates the fasting-induced repression of LPL activity by promoting the unfolding of LPL, leading to the cleavage and subsequent degradation of LPL. The inhibition of LPL by ANGPTL4 is opposed by ANGPTL8, which keeps the LPL active after feeding. In macrophages and (cardio)myocytes, ANGPTL4 functions as a lipid-inducible feedback regulator of LPL-mediated lipid uptake. In comparison, in hepatocytes, ANGPTL4 functions as a local inhibitor of hepatic lipase and possibly as an endocrine inhibitor of LPL in extra-hepatic tissues. At the genetic level, loss-of-function mutations in ANGPTL4 are associated with lower plasma triglycerides and higher plasma HDL-C levels, and a reduced risk of coronary artery disease, suggesting that ANGPTL4 is a viable pharmacological target for reducing cardiovascular risk. Whole-body targeting of ANGPTL4 is contraindicated because of severe pathological complications, whereas liver-specific inactivation of ANGPTL4, either as monotherapy or coupled to anti-ANGPTL3 therapies might be a suitable strategy for lowering plasma triglycerides in selected patient groups. In conclusion, the tissue-specific targeting of ANGPTL4 appears to be a viable pharmacological approach to reduce circulating triglycerides.

Regulation of lipoprotein metabolism by ANGPTL3, ANGPTL4, and ANGPTL8

Triglyceride-rich lipoproteins deliver fatty acids to tissues for oxidation and for storage. Release of fatty acids from circulating lipoprotein triglycerides is carried out by lipoprotein lipase (LPL), thus LPL serves as a critical gatekeeper of fatty acid uptake into tissues. LPL activity is regulated by a number of extracellular proteins including three members of the angiopoietin-like family of proteins. In this review, we discuss our current understanding of how, where, and when ANGPTL3, ANGPTL4, and ANGPTL8 regulate lipoprotein lipase activity, with a particular emphasis on how these proteins interact with each other to coordinate triglyceride metabolism and fat partitioning.

Oleic and palmitic acids induce hepatic angiopoietin-like 4 expression predominantly via PPAR-γ in Larimichthys crocea

Angiopoietin-like 4 (ANGPTL4) is a potent regulator of TAG metabolism, but knowledge of the mechanisms underlying ANGPTL4 transcription in response to fatty acids is still limited in teleost. In the current study, we explored the molecular characterisation of ANGPTL4 and regulatory mechanisms of ANGPTL4 in response to fatty acids in large yellow croaker (Larimichthys crocea). Here, croaker angptl4 contained a 1416 bp open reading frame encoding a protein of 471 amino acids with highly conserved 12-amino acid consensus motif. Angptl4 was widely expressed in croaker, with the highest expression in the liver. In vitro, oleic and palmitic acids (OA and PA) treatments strongly increased angptl4 mRNA expression in croaker hepatocytes. Moreover, angptl4 expression was positively regulated by PPAR family (PPAR-α, β and γ), and expression of PPARγ was also significantly increased in response to OA and PA. Moreover, inhibition of PPARγ abrogated OA- or PA-induced angptl4 mRNA expression. Beyond that, PA might increase angptl4 expression partly via the insulin signalling. Overall, the expression of ANGPTL4 is strongly upregulated by OA and PA via PPARγ in the liver of croaker, which contributes to improve the understanding of the regulatory mechanisms of ANGPTL4 in fish.

Acute consumption of pecans decreases angiopoietin-like protein-3 in healthy males: a secondary analysis of randomized controlled trials

Angiopoietin-like proteins (ANGPTL)-3 and -4 regulate lipid metabolism, but the effect of tree nuts of varying fatty acid composition on post-meal responses is unknown. The purpose of the study was to conduct a secondary analysis of two studies on ANGPTL3 and -4 responses to meals containing different tree nuts. We hypothesized that the pecan-containing meal would mitigate postprandial rises in ANGPTL3 compared to the traditional meal without nuts in males, but not females. In addition, we hypothesized that there would be no other differences between any other treatments in ANGPTL3 or -4 responses. The two studies were double-blind, randomized crossover trials. Twenty-two adults (10=male, 12=female) completed study 1, which compared meals containing pecans vs. no nuts (control), and thirty adults (14=male, 16=female) completed study 2, which compared meals containing black walnuts, English walnuts (EW), or no nuts (control). Blood was collected at fasting, 30, 60, 120, and 180min postprandially. In study 1, ANGPTL3 was suppressed more in pecan vs. control in males (iAUC: -579.4±219.4 vs. -128.4±87.1pg/mL/3h, P<.05). In study 2, there was no difference in ANGPTL3 between black walnuts vs. EW, but ANGPTL3 was suppressed more in control vs. black walnuts in females only (iAUC: -196.4±138.4 vs. 102.1±90.1pg/mL/3h, P<.05). There were no differences in ANGPTL4 between treatments. In conclusion, adding pecans to a meal decreased ANGPTL3 in males, but not females. These data highlight the importance of investigating the impact of nutrients and sex on postprandial ANGPTL3 ad -4 responses to better understand their ability to reduce cardiovascular disease risk.

Lipoprotein Lipase and Its Delivery of Fatty Acids to the Heart

Ninety percent of plasma fatty acids (FAs) are contained within lipoprotein-triglyceride, and lipoprotein lipase (LPL) is robustly expressed in the heart. Hence, LPL-mediated lipolysis of lipoproteins is suggested to be a key source of FAs for cardiac use. Lipoprotein clearance by LPL occurs at the apical surface of the endothelial cell lining of the coronary lumen. In the heart, the majority of LPL is produced in cardiomyocytes and subsequently is translocated to the apical luminal surface. Here, vascular LPL hydrolyzes lipoprotein-triglyceride to provide the heart with FAs for ATP generation. This article presents an overview of cardiac LPL, explains how the enzyme works, describes key molecules that regulate its activity and outlines how changes in LPL are brought about by physiological and pathological states such as fasting and diabetes, respectively.

The Mechanism of Oral Melatonin Ameliorates Intestinal and Adipose Lipid Dysmetabolism Through Reducing Escherichia Coli-Derived Lipopolysaccharide

BACKGROUND & AIMS

Gut microbiota have been reported to be sensitive to circadian rhythms and host lipometabolism, respectively. Although melatonin-mediated beneficial efforts on many physiological sites have been revealed, the regulatory actions of oral melatonin on the communication between gut microbiota and host are still not clear. Angiopoietin-like 4 (ANGPTL4) has been shown to be strongly responsible for the regulation of systemic lipid metabolism. Herein, we identified that oral melatonin improved lipid dysmetabolism in ileum and epididymal white adipose tissue (eWAT) via gut microbiota and ileac ANGPTL4.

METHODS

Analyses of jet-lag (JL) mice, JL mice with oral melatonin administration (JL+MT), and the control for mRNA and protein expression regarding lipid uptake and accumulation in ileum and eWAT were made. Gut microbiome sequencing and experimental validation of target strains were included. Functional analysis of key factors/pathways in the various rodent models, including the depletion of gut microbiota, mono-colonization of Escherichia coli, and other genetic intervention was made. Analyses of transcriptional regulation and effects of melatonin on E coli-derived lipopolysaccharide (LPS) in vitro were made.

RESULTS

JL mice have a higher level of ileal lipid uptake, fat accumulation in eWAT, and lower level of circulating ANGPTL4 in comparison with the control mice. JL mice also showed a significantly higher abundance of E coli and LPS than the control mice. Conversely, oral melatonin supplementation remarkably reversed these phenotypes. The test of depletion of gut microbiota further demonstrated that oral melatonin-mediated improvements on lipometabolism in JL mice were dependent on the presence of gut microbiota. By mono-colonization of E coli, LPS has been determined to trigger these changes similar to JL. Furthermore, we found that LPS served as a pivotal link that contributed to activating toll-like receptor 4 (TLR4)/signal transducer and activator of transcription 3 (STAT3_/REV-ERBα) signaling to up-regulate nuclear factor interleukin-3-regulated protein (NFIL3) expression, resulting in increased lipid uptake in ileum. In MODE-K cells, the activation of NFIL3 has further been shown to inhibit ANGPTL4 transcription, which is closely associated with lipid uptake and transport in peripheral tissues. Finally, we confirmed that melatonin inhibited LPS via repressing the expression of LpxC in E coli.

CONCLUSIONS

Overall, oral melatonin decreased the quantity of E coli-generated LPS, which alleviated NFIL3-induced transcriptional inhibition of ANGPTL4 through TLR4/IL-22/STAT3 signaling in ileum, thereby resulting in the amelioration of ileal lipid intake and lower fat accumulation in eWAT. These results address a novel regulation of oral melatonin originating from gut microbiota to host distal tissues, suggesting that microbe-generated metabolites are potential therapies for melatonin-mediated improvement of circadian rhythm disruption and related metabolic syndrome.

Regulation of plasma triglyceride partitioning by adipose-derived ANGPTL4 in mice

Elevated plasma triglyceride levels are associated with metabolic disease. Angiopoietin-like protein 4 (ANGPTL4) regulates plasma triglyceride levels by inhibiting lipoprotein lipase (LPL). Our aim was to investigate the role of adipocyte-specific deficiency of ANGPTL4 in mice during high fat diet feeding. Adipocyte-specific ANGPTL4 deficient mice were fed a high fat diet (60% kCal from fat) for either 12 weeks or 6 months. We performed plasma metabolic measurements, triglyceride clearance and uptake assays, LPL activity assays, and assessed glucose homeostasis. Mice lacking adipocyte ANGPTL4 recapitulated the triglyceride phenotypes of whole-body ANGPTL4 deficiency, including increased adipose LPL activity, lower plasma triglyceride levels, and increased uptake of triglycerides into adipose tissue. When fed a high fat diet (HFD), these mice continued to display enhanced adipose LPL activity and initially had improved glucose and insulin sensitivity. However, after 6 months on HFD, the improvements in glucose homeostasis were largely lost. Moreover, despite higher adipose LPL activity levels, mice lacking adipocyte ANGPTL4 no longer had increased triglyceride uptake into adipose compared to littermate controls after chronic high-fat feeding. These observations suggest that after chronic high-fat feeding LPL is no longer rate-limiting for triglyceride delivery to adipocytes. We conclude that while adipocyte-derived ANGPTL4 is an important regulator of plasma triglyceride levels and triglyceride partitioning under normal diet conditions, its role is diminished after chronic high-fat feeding.

Lipoprotein Lipase and Its Regulators: An Unfolding Story

Lipoprotein lipase (LPL) is one of the most important factors in systemic lipid partitioning and metabolism. It mediates intravascular hydrolysis of triglycerides packed in lipoproteins such as chylomicrons and very-low-density lipoprotein (VLDL). Since its initial discovery in the 1940s, its biology and pathophysiological significance have been well characterized. Nonetheless, several studies in the past decade, with recent delineation of LPL crystal structure and the discovery of several new regulators such as angiopoietin-like proteins (ANGPTLs), glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1), lipase maturation factor 1 (LMF1) and Sel-1 suppressor of Lin-12-like 1 (SEL1L), have completely transformed our understanding of LPL biology.

Angiopoietin-Like Protein 4 Overexpression in Visceral Adipose Tissue from Obese Subjects with Impaired Glucose Metabolism and Relationship with Lipoprotein Lipase

Angiopoietin-like protein 4 (ANGPTL4) regulates lipid partitioning by inhibiting circulating and tissue lipoprotein lipase (LPL); ANGPTL4 loss-of-function variants improve insulin sensitivity and reduce type 2 diabetes (T2D) risk with mechanisms partially unknown. This study was designed to explore metabolic implications of differential ANGPTL4 and LPL expression in human adipose tissue (AT). We recruited eighty-eight obese individuals, with and without abnormal glucose metabolism (AGM), undergoing bariatric surgery; visceral AT (VAT) fragments were obtained intra-operatively and analyzed by immunohistochemistry and mRNA by rt-PCR. Data on hepatic ANGPTL4 mRNA were available for 40 participants. VAT ANGPTL4 expression was higher in AGM individuals than in those with normal glucose tolerance (NGT) and associated with VAT inflammation, insulin resistance, and presence of adipocyte size heterogeneity. Increased ANGPTL4 was associated with AGM with OR = 5.1 (95% C.I.: 1.2–23; p = 0.02) and AUROC = 0.76 (95% C.I.: 1.2–23; p < 0.001). High LPL was associated with the detection of homogeneous adipocyte size, reduced microvessel density, and higher HIF-1α levels and inversely correlated to blood transaminases. In conclusion, in obese individuals, VAT ANGPTL4 levels are increased in the presence of local inflammation and AGM. Conversely, higher LPL expression describes a condition of increased lipid storage in adipocytes, which may serve as a protective mechanism against ectopic fat accumulation and related metabolic disease in obesity.

ANGPTL8 has both endocrine and autocrine effects on substrate utilization

The angiopoietin-like protein ANGPTL8 (A8) is one of 3 ANGPTLs (A8, A3, A4) that coordinate changes in triglyceride (TG) delivery to tissues by inhibiting lipoprotein lipase (LPL), an enzyme that hydrolyzes TG. Previously we showed that A8, which is expressed in liver and adipose tissue, is required to redirect dietary TG from oxidative to storage tissues following food intake. Here we show that A8 from liver and adipose tissue have different roles in this process. Mice lacking hepatic A8 have no circulating A8, high intravascular LPL activity, low plasma TG levels, and evidence of decreased delivery of dietary lipids to adipose tissue. In contrast, mice lacking A8 in adipose tissue have higher postprandial TG levels and similar intravascular LPL activity and plasma A8 levels and higher levels of plasma TG. Expression of A8, together with A4, in cultured cells reduced A4 secretion and A4-mediated LPL inhibition. Thus, hepatic A8 (with A3) acts in an endocrine fashion to inhibit intravascular LPL in oxidative tissues, whereas A8 in adipose tissue enhances LPL activity by autocrine/paracrine inhibition of A4. These combined actions of A8 ensure that TG stores are rapidly replenished and sufficient energy is available until the next meal.

Angiopoietin-like protein ANGPTL8 expressed in liver and adipose tissue partner with ANGPTL3 and ANGPTL4 respectively, and replenish adipose tissue triglyceride stores by distinct endocrine and autocrine/paracrine mechanisms.

Danlou Tablet () Improves Chronic Intermittent Hypoxia-induced Dyslipidemia and Arteriosclerosis by HIF-1 α-Angptl4 mRNA Signaling Pathway

OBJECTIVE

To detect whether Danlou Tablet (, DLT) regulates the hypoxia-induced factor (HIF)-1 α-angiopoietin-like 4 (Angptl4) mRNA signaling pathway and explore the role of DLT in treating chronic intermittent hypoxia (CIH)-induced dyslipidemia and arteriosclerosis.

METHODS

The mature adipocytes were obtained from 3T3-L1 cell culturation and allocated into 8 groups including control groups (Groups 1 and 5, 0.1 mL of cell culture grade water); DLT groups (Groups 2 and 6, 0.1 mL of 1,000 µg/mL DLT submicron powder solution); dimethyloxalylglycine (DMOG) groups (Groups 3 and 7, DMOG and 0.1 mL of cell culture grade water); DMOG plus DLT groups (Groups 4 and 8, DMOG and 0.1 mL of 1,000 µg/mL DLT submicron powder solution). Groups 1-4 used mature adipocytes and groups 5-8 used HIF-1 α-siRNA lentivirus-transfected mature adipocytes. After 24-h treatment, real-time polymerase chain reaction and Western blot were employed to determine the mRNA and protein expression levels of HIF-1 α and Angptl4. In animal experiments, the CIH model in ApoE^-/- mice was established. Sixteen mice were complete randomly divided into 4 groups including sham group, CIH model group [intermittent hypoxia and normal saline (2 mL/time) gavage once a day]; Angptl4 Ab group [intermittent hypoxia and Angptl4 antibody (30 mg/kg) intraperitoneally injected every week]; DLT group [intermittent hypoxia and DLT (250 mg/kg) once a day], 4 mice in each group. After 4-week treatment, enzyme linked immunosorbent assay was used to detect the expression levels of serum total cholesterol (TC) and triglyceride (TG). Hematoxylin-eosin and CD68 staining were used to observe the morphological properties of arterial plaques.

RESULTS

Angptl4 expression was dependent on HIF-1 α, with a reduction in mRNA expression and no response in protein level to DMOG or DLT treatment in relation to siHIF-1 α -transfected cells. DLT inhibited HIF-1 α and Angptl4 mRNA expression (P<0.05 or P<0.01) and reduced HIF-1 α and Angptl4 protein expressions with DMOG in mature adipocytes (all P<0.01), as the effect on HIF-1 α protein also existed in the presence of siHIF-1 α (P<0.01). ApoE^-/- mice treated with CIH had increased TG and TC levels (all P<0.01) and atherosclerotic plaque. Angptl4 antibody and DLT both reduce TG and TC levels (all P<0.01), as well as reducing atherosclerotic plaque areas, narrowing arterial wall thickness and alleviating atherosclerotic lesion symptoms to some extent.

CONCLUSION

DLT had positive effects in improving dyslipidemia and arteriosclerosis by inhibiting Angptl4 protein level through HIF-1 α-Angptl4 mRNA signaling pathway.

Fasting induces ANGPTL4 and reduces LPL activity in human adipose tissue

Objective

Studies in mice have shown that the decrease in lipoprotein lipase (LPL) activity in adipose tissue upon fasting is mediated by induction of the inhibitor ANGPTL4. Here, we aimed to validate this concept in humans by determining the effect of a prolonged fast on ANGPTL4 and LPL gene and protein expression in human subcutaneous adipose tissue.

Methods

Twenty-three volunteers ate a standardized meal at 18.00 h and fasted until 20.00 h the next day. Blood was drawn and periumbilical adipose tissue biopsies were collected 2 h and 26 h after the meal.

Results

Consistent with previous mouse data, LPL activity in human adipose tissue was significantly decreased by fasting (−60%), concurrent with increased ANGPTL4 mRNA (+90%) and decreased ANGPTL8 mRNA (−94%). ANGPTL4 protein levels in adipose tissue were also significantly increased by fasting (+46%), whereas LPL mRNA and protein levels remained unchanged. In agreement with the adipose tissue data, plasma ANGPTL4 levels increased upon fasting (+100%), whereas plasma ANGPTL8 decreased (−79%). Insulin, levels of which significantly decreased upon fasting, downregulated ANGPTL4 mRNA and protein in primary human adipocytes. By contrast, cortisol, levels of which significantly increased upon fasting, upregulated ANGPTL4 mRNA and protein in primary human adipocytes as did fatty acids.

Conclusion

ANGPTL4 levels in human adipose tissue are increased by fasting, likely via increased plasma cortisol and free fatty acids and decreased plasma insulin, resulting in decreased LPL activity.

This clinical trial was registered with identifier NCT03757767.

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24-h fast in humans reduces LPL activity in subcutaneous adipose tissue.

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24-h fast in humans increases adipose ANGPTL4 mRNA, protein, and plasma ANGPTL4 levels.

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Cortisol, fatty acids, and insulin regulate ANGPTL4 in vitro.

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ANGPTL4 mediates the reduction in adipose LPL activity during fasting.

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24-h fast in humans decreases adipose ANGPTL8 mRNA and plasma ANGPTL8 levels.

Triglyceride metabolism and angiopoietin-like proteins in lipoprotein lipase regulation

Hypertriglyceridemia is a risk factor for a series of diseases, such as cardiovascular disease (CVD), diabetes and nonalcoholic fatty liver disease (NAFLD). Angiopoietin-like proteins (ANGPTLs) family, especially ANGPTL3, ANGPTL4 and ANGPTL8, which regulate lipoprotein lipase (LPL) activity, play pivotal roles in triglyceride (TG) metabolism and related diseases/complications. There are many transcriptional and post-transcriptional factors that participate in physiological and pathological regulation of ANGPTLs to affect triglyceride metabolism. This review is intended to focus on the similarity and difference in the expression, structural features, regulation profile of the three ANGPTLs and inhibitory models for LPL. Description of the regulatory factors of ANGPTLs and the properties in regulating the lipid metabolism involved in the underlying mechanisms in pathological effects on diseases will provide potential therapeutic approaches for the treatment of dyslipidemia related diseases.

Characterization of ANGPTL4 function in macrophages and adipocytes using Angptl4-knockout and Angptl4-hypomorphic mice

Angiopoietin-like protein (ANGPTL)4 regulates plasma lipids, making it an attractive target for correcting dyslipidemia. However, ANGPTL4 inactivation in mice fed a high fat diet causes chylous ascites, an acute-phase response, and mesenteric lymphadenopathy. Here, we studied the role of ANGPTL4 in lipid uptake in macrophages and in the above-mentioned pathologies using Angptl4-hypomorphic and Angptl4 ^-/- mice. Angptl4 expression in peritoneal and bone marrow-derived macrophages was highly induced by lipids. Recombinant ANGPTL4 decreased lipid uptake in macrophages, whereas deficiency of ANGPTL4 increased lipid uptake, upregulated lipid-induced genes, and increased respiration. ANGPTL4 deficiency did not alter LPL protein levels in macrophages. Angptl4-hypomorphic mice with partial expression of a truncated N-terminal ANGPTL4 exhibited reduced fasting plasma triglyceride, cholesterol, and NEFAs, strongly resembling Angptl4 ^-/- mice. However, during high fat feeding, Angptl4-hypomorphic mice showed markedly delayed and attenuated elevation in plasma serum amyloid A and much milder chylous ascites than Angptl4 ^-/- mice, despite similar abundance of lipid-laden giant cells in mesenteric lymph nodes. In conclusion, ANGPTL4 deficiency increases lipid uptake and respiration in macrophages without affecting LPL protein levels. Compared with the absence of ANGPTL4, low levels of N-terminal ANGPTL4 mitigate the development of chylous ascites and an acute-phase response in mice.

New insights into angiopoietin-like proteins in lipid metabolism and cardiovascular disease risk

PURPOSE OF REVIEW

The angiopoietin-like proteins (ANGPTLs), consisting of ANGPTL3, ANGPTL4, and ANGPTL8, have gained significant interest for their role as inhibitors of lipoprotein lipase (LPL) and for their potential as therapeutic targets for correcting dyslipidemia. This review provides an overview of the most relevant new insights on the connection between ANGPTLs, plasma lipids, and coronary artery disease.

RECENT FINDINGS

Carriers of loss-of-function variants in ANGPTL3 have a reduced risk of coronary artery disease and reduced plasma levels of triglycerides and LDL-C, while carriers of loss-of-function variants in ANGPTL4 have a reduced risk of coronary artery disease and reduced plasma levels of triglycerides and increased HDL-C. There is evidence that carrier status of ANGPTL4 loss-of-function variants may also influence risk of type 2 diabetes. ANGPTL3 is produced in liver and is released as a complex with ANGPTL8 to suppress LPL activity in fat and muscle tissue. ANGPTL4 is produced by numerous tissues and likely mainly functions as a locally released LPL inhibitor. Both proteins inactivate LPL by catalyzing the unfolding of the hydrolase domain in LPL and by promoting the cleavage of LPL. Antisense oligonucleotide and monoclonal antibody-based inactivation of ANGPTL3 reduce plasma triglyceride and LDL-C levels in human volunteers and suppress atherosclerosis in mouse models.

SUMMARY

ANGPTL3/ANGPTL8 and ANGPTL4 together assure the appropriate distribution of plasma triglycerides across tissues during different physiological conditions. Large-scale genetic studies provide strong rationale for continued research efforts to pharmacologically inactivate ANGPTL3 and possibly ANGPTL4 to reduce plasma lipids and coronary artery disease risk.

The Chylomicronemia Syndrome Is Most Often Multifactorial: A Narrative Review of Causes and Treatment

The chylomicronemia syndrome occurs when triglyceride levels are severely elevated (usually >16.95 mmol/L [1500 mg/dL]) and is characterized by such clinical features as abdominal pain, acute pancreatitis, eruptive xanthomas, and lipemia retinalis. It may result from 1 of 3 conditions: the presence of secondary forms of hypertriglyceridemia concurrent with genetic causes of hypertriglyceridemia, termed multifactorial chylomicronemia syndrome (MFCS); a deficiency in the enzyme lipoprotein lipase and some associated proteins, termed familial chylomicronemia syndrome (FCS); or familial partial lipodystrophy. Most chylomicronemia syndrome cases are the result of MFCS; FCS is very rare. In all these conditions, triglyceride-rich lipoproteins accumulate because of impaired plasma clearance. This review describes the 3 major causes of the chylomicronemia syndrome; their consequences; and the approaches to treatment, which differ considerably by group.

Lipoprotein lipase in mouse kidney: effects of nutritional status and high-fat diet

Activity of lipoprotein lipase (LPL) is high in mouse kidney, but the reason is poorly understood. The aim was to characterize localization, regulation, and function of LPL in kidney of C57BL/6J mice. We found LPL mainly in proximal tubules, localized inside the tubular epithelial cells, under all conditions studied. In fed mice, some LPL colocalized with the endothelial markers CD31 and GPIHBP1 and could be removed by perfusion with heparin, indicating a vascular location. The role of angiopoietin-like protein 4 (ANGPTL4) for nutritional modulation of LPL activity was studied in wild-type and Angptl4^-/- mice. In Angptl4^-/- mice, kidney LPL activity remained high in fasted animals, indicating that ANGPTL4 is involved in suppression of LPL activity on fasting, like in adipose tissue. The amount of ANGPTL4 protein in kidney was low, and the protein appeared smaller in size, compared with ANGPTL4 in heart and adipose tissue. To study the influence of obesity, mice were challenged with high-fat diet for 22 wk, and LPL was studied after an overnight fast compared with fasted mice given food for 3 h. High-fat diet caused blunting of the normal adaptation of LPL activity to feeding/fasting in adipose tissue, but in kidneys this adaptation was lost only in male mice. LPL activity increases to high levels in mouse kidney after feeding, but as no difference in uptake of chylomicron triglycerides in kidneys is found between fasted and fed states, our data confirm that LPL appears to have a minor role for lipid uptake in this organ.

Decreased GPIHBP1 protein levels in visceral adipose tissue partly underlie the hypertriglyceridemic phenotype in insulin resistance

GPIHBP1 is a protein localized at the endothelial cell surface that facilitates triglyceride (TG) lipolysis by binding lipoprotein lipase (LPL). Whether Glycosyl Phosphatidyl Inositol high density lipoprotein binding protein 1 (GPIHBP1) function is impaired and may underlie the hyperTG phenotype observed in type 2 diabetes is not yet established. To elucidate the mechanism underlying impaired TG homeostasis in insulin resistance state we studied the effect of insulin on GPIHBP1 protein expression in human microvascular endothelial cells (HMVEC) under flow conditions. Next, we assessed visceral adipose tissue GPIHBP1 protein expression in type 2 diabetes Lepr^db/db mouse model as well as in subjects with ranging levels of insulin resistance. We report that insulin reduces the expression of GPIHBP1 protein in HMVECs. Furthermore, GPIHBP1 protein expression in visceral adipose tissue in Lepr^db/db mice is significantly reduced as is the active monomeric form of GPIHBP1 as compared to Lepr^db/m mice. A similar decrease in GPIHBP1 protein was observed in subjects with increased body weight. GPIHBP1 protein expression was negatively associated with insulin and HOMA-IR.

In conclusion, our data suggest that decreased GPIHBP1 availability in insulin resistant state may hamper peripheral lipolysis capacity.

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.

Angiopoietin-like 4 promotes the intracellular cleavage of lipoprotein lipase by PCSK3/furin in adipocytes

Lipoprotein lipase (LPL) catalyzes the breakdown of circulating triglycerides in muscle and fat. LPL is inhibited by several proteins, including angiopoietin-like 4 (ANGPTL4), and may be cleaved by members of the proprotein convertase subtilisin/kexin (PCSK) family. Here, we aimed to investigate the cleavage of LPL in adipocytes by PCSKs and study the potential involvement of ANGPTL4. A substantial portion of LPL in mouse and human adipose tissue was cleaved into N- and C-terminal fragments. Treatment of different adipocytes with the PCSK inhibitor decanoyl-RVKR-chloromethyl ketone markedly decreased LPL cleavage, indicating that LPL is cleaved by PCSKs. Silencing of Pcsk3/furin significantly decreased LPL cleavage in cell culture medium and lysates of 3T3-L1 adipocytes. Remarkably, PCSK-mediated cleavage of LPL in adipocytes was diminished by Angptl4 silencing and was decreased in adipocytes and adipose tissue of Angptl4^-/- mice. Differences in LPL cleavage between Angptl4^-/- and WT mice were abrogated by treatment with decanoyl-RVKR-chloromethyl ketone. Induction of ANGPTL4 in adipose tissue during fasting enhanced PCSK-mediated LPL cleavage, concurrent with decreased LPL activity, in WT but not Angptl4^-/- mice. In adipocytes, after removal of cell surface LPL by heparin, levels of N-terminal LPL were still markedly higher in WT compared with Angptl4^-/- adipocytes, suggesting that stimulation of PCSK-mediated LPL cleavage by ANGPTL4 occurs intracellularly. Finally, treating adipocytes with insulin increased full-length LPL and decreased N-terminal LPL in an ANGPTL4-dependent manner. In conclusion, ANGPTL4 promotes PCSK-mediated intracellular cleavage of LPL in adipocytes, likely contributing to regulation of LPL in adipose tissue. Our data provide further support for an intracellular action of ANGPTL4 in adipocytes.

Effect of diet-induced weight loss on angiopoietin-like protein 4 and adipose tissue lipid metabolism in overweight and obese humans

Angiopoietin-like protein 4 (ANGPTL4) plays a role in lipid partitioning by inhibiting lipoprotein lipase (LPL)-dependent plasma clearance of triacylglycerol in adipose tissue. We investigated the effects of diet-induced weight loss on plasma ANGPTL4 concentrations in relation to in vivo adipose tissue LPL activity and lipolysis and adipose tissue ANGPTL4 release in overweight/obese participants. Sixteen individuals (BMI: 28-35 kg/m2 ; 10 women) were randomized to a dietary intervention composed of either a low-calorie diet (1250 kcal/day) for 12 weeks (n = 9) or a very low-calorie diet (500 kcal/day) for 5 weeks, followed by a 4-week weight stable period. Before and after the intervention, we measured arteriovenous concentration differences in combination with adipose tissue blood flow before and after intake of a high-fat mixed meal with [U-13 C]-palmitate to assess in vivo adipose tissue LPL activity and lipolysis. The intervention significantly reduced body weight (-8.6 ± 0.6 kg, P < 0.001). Plasma ANGPTL4 concentrations were unaffected. Significant postprandial adipose tissue ANGPTL4 release into the circulation was observed (P < 0.01). No association was observed between plasma ANGPTL4 and in vivo LPL activity. After intervention, fasting and postprandial plasma ANGPTL4 concentrations were positively associated with adipose tissue nonesterified FA (NEFA) and glycerol release, reflecting in vivo adipose tissue lipolysis (fasting NEFA: P = 0.039 and postprandial NEFA: P = 0.003). In conclusion, plasma ANGPTL4 is unaffected by weight loss and is secreted from human adipose tissue after a high-fat meal in overweight/obese participants. Plasma ANGPTL4 concentrations were not related to in vivo adipose tissue LPL activity, but were positively associated with in vivo adipose tissue lipolysis after weight loss.

Loss of angiopoietin-like 4 (ANGPTL4) in mice with diet-induced obesity uncouples visceral obesity from glucose intolerance partly via the gut microbiota

AIMS/HYPOTHESIS

Angiopoietin-like 4 (ANGPTL4) is an important regulator of triacylglycerol metabolism, carrying out this role by inhibiting the enzymes lipoprotein lipase and pancreatic lipase. ANGPTL4 is a potential target for ameliorating cardiometabolic diseases. Although ANGPTL4 has been implicated in obesity, the study of the direct role of ANGPTL4 in diet-induced obesity and related metabolic dysfunction is hampered by the massive acute-phase response and development of lethal chylous ascites and peritonitis in Angptl4^-/- mice fed a standard high-fat diet. The aim of this study was to better characterise the role of ANGPTL4 in glucose homeostasis and metabolic dysfunction during obesity.

METHODS

We chronically fed wild-type (WT) and Angptl4^-/- mice a diet rich in unsaturated fatty acids and cholesterol, combined with fructose in drinking water, and studied metabolic function. The role of the gut microbiota was investigated by orally administering a mixture of antibiotics (ampicillin, neomycin, metronidazole). Glucose homeostasis was assessed via i.p. glucose and insulin tolerance tests.

RESULTS

Mice lacking ANGPTL4 displayed an increase in body weight gain, visceral adipose tissue mass, visceral adipose tissue lipoprotein lipase activity and visceral adipose tissue inflammation compared with WT mice. However, they also unexpectedly had markedly improved glucose tolerance, which was accompanied by elevated insulin levels. Loss of ANGPTL4 did not affect glucose-stimulated insulin secretion in isolated pancreatic islets. Since the gut microbiota have been suggested to influence insulin secretion, and because ANGPTL4 has been proposed to link the gut microbiota to host metabolism, we hypothesised a potential role of the gut microbiota. Gut microbiota composition was significantly different between Angptl4^-/- mice and WT mice. Interestingly, suppression of the gut microbiota using antibiotics largely abolished the differences in glucose tolerance and insulin levels between WT and Angptl4^-/- mice.

CONCLUSIONS/INTERPRETATION

Despite increasing visceral fat mass, inactivation of ANGPTL4 improves glucose tolerance, at least partly via a gut microbiota-dependent mechanism.

Regulation of angiopoietin-like 4 and lipoprotein lipase in human adipose tissue

BACKGROUND

Elevated plasma triglycerides are increasingly viewed as a causal risk factor for coronary artery disease. One protein that raises plasma triglyceride levels and that has emerged as a modulator of coronary artery disease risk is angiopoietin-like 4 (ANGPTL4). ANGPTL4 raises plasma triglyceride levels by inhibiting lipoprotein lipase (LPL), the enzyme that catalyzes the hydrolysis of circulating triglycerides on the capillary endothelium.

OBJECTIVE

The objective of the present study was to assess the association between ANGPTL4 and LPL in human adipose tissue, and to examine the influence of nutritional status on ANGPTL4 expression.

METHODS

We determined ANGPTL4 and LPL mRNA and protein levels in different adipose tissue depots in a large number of severely obese patients who underwent bariatric surgery. Furthermore, in 72 abdominally obese subjects, we measured ANGPTL4 and LPL mRNA levels in subcutaneous adipose tissue in the fasted and postprandial state.

RESULTS

ANGPTL4 mRNA levels were highest in subcutaneous adipose tissue, whereas LPL mRNA levels were highest in mesenteric adipose tissue. ANGPTL4 and LPL mRNA levels were strongly positively correlated in the omental and subcutaneous adipose tissue depots. In contrast, ANGPTL4 and LPL protein levels were negatively correlated in subcutaneous adipose tissue, suggesting a suppressive effect of ANGPTL4 on LPL protein abundance in subcutaneous adipose tissue. ANGPTL4 mRNA levels were 38% higher in the fasted compared to the postprandial state.

CONCLUSION

Our data provide valuable insights into the relationship between ANGPTL4 and LPL in human adipose tissue, as well as the physiological function and regulation of ANGPTL4 in humans.

VLDL triglyceride accumulation in skeletal muscle and adipose tissue in type 2 diabetes

PURPOSE OF REVIEW

Insulin resistance is closely linked to accumulation of lipid outside adipose tissue (ectopic fat storage). VLDL particles transport lipids from the liver to peripheral tissues. However, whether abnormalities in VLDL-triglyceride storage in muscle and adipose tissue exist in type 2 diabetes has previously been unknown, primarily because of methodological difficulties. Here, we review recent research on VLDL-triglyceride storage.

RECENT FINDINGS

In a recent study, men with type 2 diabetes had increased skeletal muscle VLDL-triglyceride storage compared to weight-matched nondiabetic men, potentially leading to intramyocellular triglyceride accumulation. In contrast, studies of adipose tissue VLDL-triglyceride storage have shown similar storage capacity in men with and without diabetes, both in the postabsorptive and the postprandial period. In the initial submission, studies have failed to show associations between lipoprotein lipase activity, considered the rate-limiting step in storage of lipids from lipoproteins, and VLDL-TG storage in both muscle and adipose tissue.

SUMMARY

Differences in muscle VLDL-triglyceride storage may lead to ectopic fat storage and contribute to the development of type 2 diabetes, whereas the ability to store VLDL-triglyceride in adipose tissue is preserved in type 2 diabetes.

Angiopoietin-like 3 in lipoprotein metabolism

Triglycerides and cholesterol circulate in the bloodstream as part of various lipoprotein particles. Three members of the angiopoietin-like (ANGPTL) protein family - ANGPTL3, ANGPTL4 and ANGPTL8 - have emerged as important regulators of plasma lipoprotein levels by inhibiting the enzyme lipoprotein lipase. Here, I review the role of ANGPTL3 in lipoprotein metabolism. In contrast to ANGPTL4 and ANGPTL8, ANGPTL3 is exclusively produced in the liver and can therefore be classified as a true hepatokine. ANGPTL3 cooperates with ANGPTL8 to inhibit lipoprotein lipase and is mostly active after feeding, whereas ANGPTL4 is mostly active after fasting. Inactivation of ANGPTL3 in mice reduces plasma triglyceride and free fatty acid levels and suppresses atherosclerosis. In humans, homozygous loss-of-function mutations in ANGPTL3 lead to low plasma levels of low-density lipoproteins, high-density lipoproteins and triglycerides, a condition referred to as familial combined hypolipidaemia. Heterozygous carriers of loss-of-function mutations in ANGPTL3 have a lower risk of coronary artery disease than non-carriers. At present, researchers are investigating antisense oligonucleotide and monoclonal antibody-based inactivation of ANGPTL3 in human clinical trials for the therapeutic management of dyslipidaemia and atherosclerosis. Thus, ANGPTL3 is an important liver-derived regulator of lipoprotein metabolism that holds considerable promise as a target for atherosclerosis.

The retinol-binding protein receptor STRA6 regulates diurnal insulin responses

It has long been appreciated that insulin action is closely tied to circadian rhythms. However, the mechanisms that dictate diurnal insulin sensitivity in metabolic tissues are not well understood. Retinol-binding protein 4 (RBP4) has been implicated as a driver of insulin resistance in rodents and humans, and it has become an attractive drug target in type II diabetes. RBP4 is synthesized primarily in the liver where it binds retinol and transports it to tissues throughout the body. The retinol-RBP4 complex (holo-RBP) can be recognized by a cell-surface receptor known as stimulated by retinoic acid 6 (STRA6), which transports retinol into cells. Coupled to retinol transport, holo-RBP can activate STRA6-driven Janus kinase (JAK) signaling and downstream induction of signal transducer and activator of transcription (STAT) target genes. STRA6 signaling in white adipose tissue has been shown to inhibit insulin receptor responses. Here, we examined diurnal rhythmicity of the RBP4/STRA6 signaling axis and investigated whether STRA6 is necessary for diurnal variations in insulin sensitivity. We show that adipose tissue STRA6 undergoes circadian patterning driven in part by the nuclear transcription factor REV-ERBα. Furthermore, STRA6 is necessary for diurnal rhythmicity of insulin action and JAK/STAT signaling in adipose tissue. These findings establish that holo-RBP and its receptor STRA6 are potent regulators of diurnal insulin responses and suggest that the holo-RBP/STRA6 signaling axis may represent a novel therapeutic target in type II diabetes.

ANGPTL4 promotes bile acid absorption during taurocholic acid supplementation via a mechanism dependent on the gut microbiota

Angiopoietin-like 4 (ANGPTL4) raises plasma triglyceride levels by inhibiting lipoprotein lipase. A set of compounds that are able to reduce plasma triglyceride levels are bile acids (BA). Because BA have been shown to decrease ANGPTL4 secretion by intestinal cells, we hypothesized that BA lower plasma triglycerides (partly) via ANGPTL4. To test that hypothesis, wild-type and Angptl4^-/- mice were fed chow supplemented with taurocholic acid (TCA) for seven days. TCA supplementation effectively lowered plasma triglycerides in wild-type and Angptl4^-/- mice, indicating that ANGPTL4 is not required for plasma triglyceride-lowering by BA. Intriguingly, however, plasma and hepatic BA concentrations were significantly lower in TCA-supplemented Angptl4^-/- mice than in TCA-supplemented wild-type mice. These changes in the Angptl4^-/- mice were accompanied by lower BA levels in ileal scrapings and decreased expression of FXR-target genes in the ileum, including the BA transporter Slc10a2. By contrast, faecal excretion of specifically primary BA was higher in the Angptl4^-/- mice, suggesting that loss of ANGPTL4 impairs intestinal BA absorption. Since the gut microbiota converts primary BA into secondary BA, elevated excretion of primary BA in Angptl4^-/- mice may reflect differences in gut microbial composition and/or functionality. Indeed, colonic microbial composition was markedly different between Angptl4^-/- and wild-type mice. Suppression of the gut bacteria using antibiotics abolished differences in plasma, hepatic, and faecal BA levels between TCA-supplemented Angptl4^-/- and wild-type mice. In conclusion, 1) ANGPTL4 is not involved in the triglyceride-lowering effect of BA; 2) ANGPTL4 promotes BA absorption during TCA supplementation via a mechanism dependent on the gut microbiota.

Angiopoietin-like 4 directs uptake of dietary fat away from adipose during fasting

Objective

Angiopoietin-like 4 (ANGPTL4) is a fasting-induced inhibitor of lipoprotein lipase (LPL) and a regulator of plasma triglyceride metabolism. Here, we examined the kinetics of Angptl4 induction and tested the hypothesis that ANGPTL4 functions physiologically to reduce triglyceride delivery to adipose tissue during nutrient deprivation.

Methods

Gene expression, LPL activity, and triglyceride uptake were examined in fasted and fed wild-type and Angptl4^−/− mice.

Results

Angptl4 was strongly induced early in fasting, and this induction was suppressed in mice with access to food during the light cycle. Fasted Angptl4^−/− mice manifested increased LPL activity and triglyceride uptake in adipose tissue compared to wild-type mice.

Conclusions

Angptl4 is induced early in fasting to divert uptake of fatty acids and triglycerides away from adipose tissues.

•Angptl4 is induced within the first few hours of fasting.

•Angptl4 expression is driven by fasting rather than circadian rhythms.

•Fasted Angptl4^−/− mice have increased triglyceride uptake in adipose tissue.

•Angptl4^−/− mice also have increased LPL activity specifically in adipose tissue.

•Data support a model where ANGPTL4 acts locally in adipose during fasting.

High Concentrations of Angiopoietin-Like Protein 4 Detected in Serum from Patients with Rheumatoid Arthritis Can Be Explained by Non-Specific Antibody Reactivity

Angiopoietin-like protein 4 (ANGPTL4) is suggested to be a master regulator of plasma triglyceride metabolism. Our aim was to study whether the previously reported high levels of ANGPTL4 detected in serum from patients with rheumatoid arthritis (RA) by ELISA was due to any specific molecular form of this protein (oligomers, monomers or fragments). ANGPTL4 levels were first determined in serum from 68 RA patients and 43 age and sex matched control subjects and the mean values differed by a factor of 5.0. Then, ANGPTL4 was analyzed after size exclusion chromatography (SEC) of serum samples. With serum from one of the RA patients with high levels of ANGPTL4, the dominant reactivity was found in fractions corresponding to high-molecular weight proteins. In addition, a minor peak of reactivity eluting late from the column was found both in the patient and in controls. By the use of HeteroBlock®, and by careful selection of antibodies, we documented non-specific reactions for ANGPTL4 in 39% of samples from the RA patients, most likely due to cross-reactivity of the antibodies with rheumatoid factor (RF). The corresponding figure for control subjects was 6.3%. After corrections for non-specific reactions, the mean level of ANGPTL4 in serum from RA patients was still significantly higher than in control individuals (mean levels were 101±62 and 67±39 ng/ml respectively, P = 0.02). We re-analyzed samples from our previously published studies on ANGPL4 levels in patients on hemodialysis and patients with diabetes type 2. These samples did not show false positive reactions. The levels of ANGPTL4 were comparable to those detected previously.

The ANGPTL3-4-8 model, a molecular mechanism for triglyceride trafficking

Lipoprotein lipase (LPL) is a rate-limiting enzyme for hydrolysing circulating triglycerides (TG) into free fatty acids that are taken up by peripheral tissues. Postprandial LPL activity rises in white adipose tissue (WAT), but declines in the heart and skeletal muscle, thereby directing circulating TG to WAT for storage; the reverse is true during fasting. However, the mechanism for the tissue-specific regulation of LPL activity during the fed–fast cycle has been elusive. Recent identification of lipasin/angiopoietin-like 8 (Angptl8), a feeding-induced hepatokine, together with Angptl3 and Angptl4, provides intriguing, yet puzzling, insights, because all the three Angptl members are LPL inhibitors, and the deficiency (overexpression) of any one causes hypotriglyceridaemia (hypertriglyceridaemia). Then, why does nature need all of the three? Our recent data that Angptl8 negatively regulates LPL activity specifically in cardiac and skeletal muscles suggest an Angptl3-4-8 model: feeding induces Angptl8, activating the Angptl8–Angptl3 pathway, which inhibits LPL in cardiac and skeletal muscles, thereby making circulating TG available for uptake by WAT, in which LPL activity is elevated owing to diminished Angptl4; the reverse is true during fasting, which suppresses Angptl8 but induces Angptl4, thereby directing TG to muscles. The model suggests a general framework for how TG trafficking is regulated.

A low-carbohydrate high-fat diet decreases lean mass and impairs cardiac function in pair-fed female C57BL/6J mice

Background

Excess body fat is a major health issue and a risk factor for the development of numerous chronic diseases. Low-carbohydrate diets like the Atkins Diet are popular for rapid weight loss, but the long-term consequences remain the subject of debate. The Scandinavian low-carbohydrate high-fat (LCHF) diet, which has been popular in Scandinavian countries for about a decade, has very low carbohydrate content (~5 E %) but is rich in fat and includes a high proportion of saturated fatty acids. Here we investigated the metabolic and physiological consequences of a diet with a macronutrient composition similar to the Scandinavian LCHF diet and its effects on the organs, tissues, and metabolism of weight stable mice.

Methods

Female C57BL/6J mice were iso-energetically pair-fed for 4 weeks with standard chow or a LCHF diet. We measured body composition using echo MRI and the aerobic capacity before and after 2 and 4 weeks on diet. Cardiac function was assessed by echocardiography before and after 4 weeks on diet. The metabolic rate was measured by indirect calorimetry the fourth week of the diet. Mice were sacrificed after 4 weeks and the organ weight, triglyceride levels, and blood chemistry were analyzed, and the expression of key ketogenic, metabolic, hormonal, and inflammation genes were measured in the heart, liver, and adipose tissue depots of the mice using real-time PCR.

Results

The increase in body weight of mice fed a LCHF diet was similar to that in controls. However, while control mice maintained their body composition throughout the study, LCHF mice gained fat mass at the expense of lean mass after 2 weeks. The LCHF diet increased cardiac triglyceride content, impaired cardiac function, and reduced aerobic capacity. It also induced pronounced alterations in gene expression and substrate metabolism, indicating a unique metabolic state.

Conclusions

Pair-fed mice eating LCHF increased their percentage of body fat at the expense of lean mass already after 2 weeks, and after 4 weeks the function of the heart deteriorated. These findings highlight the urgent need to investigate the effects of a LCHF diet on health parameters in humans.

Peroxisome proliferator-activated receptor γ activation favours selective subcutaneous lipid deposition by coordinately regulating lipoprotein lipase modulators, fatty acid transporters and lipogenic enzymes

AIM

Peroxisome proliferator-activated receptor (PPAR) γ activation is associated with preferential lipoprotein lipase (LPL)-mediated fatty acid storage in peripheral subcutaneous fat depots. How PPARγ agonism acts upon the multi-level modulation of depot-specific lipid storage remains incompletely understood.

METHODS

We evaluated herein triglyceride-derived lipid incorporation into adipose tissue depots, LPL mass and activity, mRNA levels and content of proteins involved in the modulation of LPL activity and fatty acid transport, and the expression/activity of enzymes defining adipose tissue lipogenic potential in rats treated with the PPARγ ligand rosiglitazone (30 mg kg(-1)  day(-1) , 23 days) after either a 10-h fasting period or a 17-h fast followed by 6 h of ad libitum refeeding.

RESULTS

Rosiglitazone stimulated lipid accretion in subcutaneous fat (SF) ~twofold and significantly reduced that of visceral fat (VF) to nearly half. PPARγ activation selectively increased LPL mass, activity and the expression of its chaperone LMF1 in SF. In VF, rosiglitazone had no effect on LPL activity and downregulated the mRNA levels of the transendothelial transporter GPIHBP1. Overexpression of lipid uptake and fatty acid transport proteins (FAT/CD36, FATP1 and FABP4) and stimulation of lipogenic enzyme activities (GPAT, AGPAT and DGAT) upon rosiglitazone treatment were of higher magnitude in SF.

CONCLUSIONS

Together these findings demonstrate that the depot-specific transcriptional control of LPL induced by PPARγ activation extends to its key interacting proteins and post-translational modulators to favour subcutaneous lipid storage.

Regulation of lipid metabolism by angiopoietin-like proteins

PURPOSE OF REVIEW

The angiopoietin-like proteins (ANGPTLs) 3, 4 and 8 have emerged as key regulators of plasma lipid metabolism by serving as potent inhibitors of the enzyme lipoprotein lipase (LPL). In this review, we provide an integrated picture of the role of ANGPTL3, ANGPTL4 and ANGPTL8 in lipid metabolism by focusing on their impact on LPL activity and plasma triglyceride clearance during physiological conditions such as fasting, refeeding, exercise and cold exposure.

RECENT FINDINGS

Upon refeeding, circulating ANGPTL3 and ANGPTL8 promote the replenishment of white adipose tissue depots by specifically inhibiting LPL activity in oxidative tissues. During exercise and cold exposure, ANGPTL4 represses local LPL activity to assure that plasma triglycerides are specifically shuttled to exercising muscle and brown adipose tissue, respectively. Overall, ANGPTL4 is the central component of a fatty acid-driven feedback mechanism that regulates plasma triglyceride hydrolysis and subsequent tissue fatty acid uptake in response to changes in lipid availability and cellular fuel demand.

SUMMARY

ANGPTL3, ANGPTL4 and ANGPTL8 together ensure that triglycerides from triglyceride-rich lipoproteins are adequately distributed during different physiological conditions. The impact of the ANGPTLs on plasma lipid levels has led to scrutiny of ANGPTLs as therapeutic targets for dyslipidemia.

Angiopoietin-like 4 promotes intracellular degradation of lipoprotein lipase in adipocytes

LPL hydrolyzes triglycerides in triglyceride-rich lipoproteins along the capillaries of heart, skeletal muscle, and adipose tissue. The activity of LPL is repressed by angiopoietin-like 4 (ANGPTL4) but the underlying mechanisms have not been fully elucidated. Our objective was to study the cellular location and mechanism for LPL inhibition by ANGPTL4. We performed studies in transfected cells, ex vivo studies, and in vivo studies with Angptl4(-/-) mice. Cotransfection of CHO pgsA-745 cells with ANGPTL4 and LPL reduced intracellular LPL protein levels, suggesting that ANGPTL4 promotes LPL degradation. This conclusion was supported by studies of primary adipocytes and adipose tissue explants from wild-type and Angptl4(-/-) mice. Absence of ANGPTL4 resulted in accumulation of the mature-glycosylated form of LPL and increased secretion of LPL. Blocking endoplasmic reticulum (ER)-Golgi transport abolished differences in LPL abundance between wild-type and Angptl4(-/-) adipocytes, suggesting that ANGPTL4 acts upon LPL after LPL processing in the ER. Finally, physiological changes in adipose tissue ANGPTL4 expression during fasting and cold resulted in inverse changes in the amount of mature-glycosylated LPL in wild-type mice, but not Angptl4(-/-) mice. We conclude that ANGPTL4 promotes loss of intracellular LPL by stimulating LPL degradation after LPL processing in the ER.

Cardiomyocyte-endothelial cell control of lipoprotein lipase

In people with diabetes, inadequate pharmaceutical management predisposes the patient to heart failure, which is the leading cause of diabetes related death. One instigator for this cardiac dysfunction is change in fuel utilization by the heart. Thus, following diabetes, when cardiac glucose utilization is impaired, the heart undergoes metabolic transformation wherein it switches to using fats as an exclusive source of energy. Although this switching is geared to help the heart initially, in the long term, this has detrimental effects on cardiac function. These include the generation of noxious byproducts, which damage the cardiomyocytes, and ultimately result in increased morbidity and mortality. A key perpetrator that may be responsible for organizing this metabolic disequilibrium is lipoprotein lipase (LPL), the enzyme responsible for providing fat to the hearts. Either exaggeration or reduction in its activity following diabetes could lead to heart dysfunction. Given the disturbing news that diabetes is rampant across the globe, gaining more insight into the mechanism(s) by which cardiac LPL is regulated may assist other researchers in devising new therapeutic strategies to restore metabolic equilibrium, to help prevent or delay heart disease seen during diabetes. This article is part of a Special Issue entitled: Heart Lipid Metabolism edited by G.D. Lopaschuk.

A lipasin/Angptl8 monoclonal antibody lowers mouse serum triglycerides involving increased postprandial activity of the cardiac lipoprotein lipase

Lipasin/Angptl8 is a feeding-induced hepatokine that regulates triglyceride (TAG) metabolism; its therapeutical potential, mechanism of action, and relation to the lipoprotein lipase (LPL), however, remain elusive. We generated five monoclonal lipasin antibodies, among which one lowered the serum TAG level when injected into mice, and the epitope was determined to be EIQVEE. Lipasin-deficient mice exhibited elevated postprandial activity of LPL in the heart and skeletal muscle, but not in white adipose tissue (WAT), suggesting that lipasin suppresses the activity of LPL specifically in cardiac and skeletal muscles. Consistently, mice injected with the effective antibody or with lipasin deficiency had increased postprandial cardiac LPL activity and lower TAG levels only in the fed state. These results suggest that lipasin acts, at least in part, in an endocrine manner. We propose the following model: feeding induces lipasin, activating the lipasin-Angptl3 pathway, which inhibits LPL in cardiac and skeletal muscles to direct circulating TAG to WAT for storage; conversely, fasting induces Angptl4, which inhibits LPL in WAT to direct circulating TAG to cardiac and skeletal muscles for oxidation. This model suggests a general mechanism by which TAG trafficking is coordinated by lipasin, Angptl3 and Angptl4 at different nutritional statuses.

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