Angiopoietin-like protein 4 converts lipoprotein lipase to inactive monomers and modulates lipase activity in adipose tissue

Lipoprotein lipase as a target for obesity/diabetes related cardiovascular disease

Worldwide, the prevalence of obesity and diabetes have increased, with heart disease being their leading cause of death. Traditionally, the management of obesity and diabetes has focused mainly on weight reduction and controlling high blood glucose. Unfortunately, despite these efforts, poor medication management predisposes these patients to heart failure. One instigator for the development of heart failure is how cardiac tissue utilizes different sources of fuel for energy. In this regard, the heart switches from using various substrates, to predominantly using fatty acids (FA). This transformation to using FA as an exclusive source of energy is helpful in the initial stages of the disease. However, over the progression of diabetes this has grave end results. This is because toxic by-products are produced by overuse of FA, which weaken heart function (heart disease). Lipoprotein lipase (LPL) is responsible for regulating FA delivery to the heart, and its function during diabetes has not been completely revealed. In this review, the mechanisms by which LPL regulates fuel utilization by the heart in control conditions and following diabetes will be discussed in an attempt to identify new targets for therapeutic intervention. Currently, as treatment options to directly target diabetic heart disease are scarce, research on LPL may assist in drug development that exclusively targets fuel utilization by the heart and lipid accumulation in macrophages to help delay, prevent, or treat cardiac failure, and provide long-term management of this condition during diabetes.

Lipolysis products from triglyceride-rich lipoproteins induce stress protein ATF3 in osteoblasts

High fat diets overwhelm the physiological mechanisms for absorption, storage, and utilization of triglycerides (TG); consequently TG, TG-rich lipoproteins (TGRL), and TGRL remnants accumulate, circulate systemically, producing dyslipidemia. This associates with, or is causative for increased atherosclerotic cardiovascular risk, ischemic stroke, fatty liver disease, and pancreatitis. TGRL hydrolysis by endothelial surface-bound lipoprotein lipase (LPL) generates metabolites like free fatty acids which have proinflammatory properties. While osteoblasts utilize fatty acids as an energy source, dyslipidemia is associated with negative effects on the skeleton. In this study we investigated the effects of TGRL lipolysis products (TGRL-LP) on expression of a stress responsive transcription factor, termed activating transcription factor 3 (ATF3), reactive oxygen species (ROS), ATF3 target genes, and angiopoietin-like 4 (Angptl4) in osteoblasts. As ATF3 negatively associates with osteoblast differentiation, we also investigated the skeletal effects of global ATF3 deletion in mice. TGRL-LP increased expression of Atf3, proinflammatory proteins Ptgs2 and IL-6, and induced ROS in MC3T3-E1 osteoblastic cells. Angptl4 is an endogenous inhibitor of LPL which was transcriptionally induced by TGRL-LP, while recombinant Angptl4 prevented TG-driven Atf3 induction. Atf3 global knockout male mice demonstrated increased trabecular and cortical microarchitectural parameters. In summary, we find that TGRL-LP induce osteoblastic cell stress as evidenced by expression of ATF3, which may contribute to the negative impact of dyslipidemia in the skeleton. Further, concomitant induction of Angptl4 in osteoblasts might play a protective role by reducing local lipolysis.

Endothelial lipase variant T111I does not alter inhibition by angiopoietin-like proteins

High levels of HDL-C are correlated with a decreased risk of cardiovascular disease. HDL-C levels are modulated in part by the secreted phospholipase, endothelial lipase (EL), which hydrolyzes the phospholipids of HDL and decreases circulating HDL-C concentrations. A 584C/T polymorphism in LIPG, the gene which encodes EL, was first identified in individuals with increased HDL levels. This polymorphism results in a T111I point mutation the EL protein. The association between this variant, HDL levels, and the risk of coronary artery disease (CAD) in humans has been extensively studied, but the findings have been inconsistent. In this study, we took a biochemical approach, investigating how the T111I variant affected EL activity, structure, and stability. Moreover, we tested whether the T111I variant altered the inhibition of phospholipase activity by angiopoietin-like 3 (ANGPTL3) and angiopoietin-like 4 (ANGPTL4), two known EL inhibitors. We found that neither the stability nor enzymatic activity of EL was altered by the T111I variant. Moreover, we found no difference between wild-type and T111I EL in their ability to be inhibited by ANGPTL proteins. These data suggest that any effect this variant may have on HDL-C levels or cardiovascular disease are not mediated through alterations in these functions.

ANGPTL4 accelerates ovarian serous cystadenocarcinoma carcinogenesis and angiogenesis in the tumor microenvironment by activating the JAK2/STAT3 pathway and interacting with ESM1

BACKGROUND

Ovarian cancer (OC) is a malignant neoplasm that displays increased vascularization. Angiopoietin-like 4 (ANGPTL4) is a secreted glycoprotein that functions as a regulator of cell metabolism and angiogenesis and plays a critical role in tumorigenesis. However, the precise role of ANGPTL4 in the OC microenvironment, particularly its involvement in angiogenesis, has not been fully elucidated.

METHODS

The expression of ANGPTL4 was confirmed by bioinformatics and IHC in OC. The potential molecular mechanism of ANGPTL4 was measured by RNA-sequence. We used a series of molecular biological experiments to measure the ANGPTL4-JAK2-STAT3 and ANGPTL4-ESM1 axis in OC progression, including MTT, EdU, wound healing, transwell, xenograft model, oil red O staining, chick chorioallantoic membrane assay and zebrafish model. Moreover, the molecular mechanisms were confirmed by Western blot, Co-IP and molecular docking.

RESULTS

Our study demonstrates a significant upregulation of ANGPTL4 in OC specimens and its strong association with unfavorable prognosis. RNA-seq analysis affirms that ANGPTL4 facilitates OC development by driving JAK2-STAT3 signaling pathway activation. The interaction between ANGPTL4 and ESM1 promotes ANGPTL4 binding to lipoprotein lipase (LPL), thereby resulting in reprogrammed lipid metabolism and the promotion of OC cell proliferation, migration, and invasion. In the OC microenvironment, ESM1 may interfere with the binding of ANGPTL4 to integrin and vascular-endothelial cadherin (VE-Cad), which leads to stabilization of vascular integrity and ultimately promotes angiogenesis.

CONCLUSION

Our findings underscore that ANGPTL4 promotes OC development via JAK signaling and induces angiogenesis in the tumor microenvironment through its interaction with ESM1.

ANGPTL4 binds to the leptin receptor to regulate ectopic bone formation

Leptin protein was thought to be unique to leptin receptor (LepR), but the phenotypes of mice with mutation in LepR [db/db (diabetes)] and leptin [ob/ob (obese)] are not identical, and the cause remains unclear. Here, we show that db/db, but not ob/ob, mice had defect in tenotomy-induced heterotopic ossification (HO), implicating alternative ligand(s) for LepR might be involved. Ligand screening revealed that ANGPTL4 (angiopoietin-like protein 4), a stress and fasting-induced factor, was elicited from brown adipose tissue after tenotomy, bound to LepR on PRRX1+ mesenchymal cells at the HO site, thus promotes chondrogenesis and HO development. Disruption of LepR in PRRX1+ cells, or lineage ablation of LepR+ cells, or deletion of ANGPTL4 impeded chondrogenesis and HO in mice. Together, these findings identify ANGPTL4 as a ligand for LepR to regulate the formation of acquired HO.

Macromolecular Interactions of Lipoprotein Lipase (LPL)

Lipoprotein lipase (LPL) is a critical enzyme in humans that provides fuel to peripheral tissues. LPL hydrolyzes triglycerides from the cores of lipoproteins that are circulating in plasma and interacts with receptors to mediate lipoprotein uptake, thus directing lipid distribution via catalytic and non-catalytic functions. Functional losses in LPL or any of its myriad of regulators alter lipid homeostasis and potentially affect the risk of developing cardiovascular disease-either increasing or decreasing the risk depending on the mutated protein. The extensive LPL regulatory network tunes LPL activity to allocate fatty acids according to the energetic needs of the organism and thus is nutritionally responsive and tissue dependent. Multiple pharmaceuticals in development manipulate or mimic these regulators, demonstrating their translational importance. Another facet of LPL biology is that the oligomeric state of the enzyme is also central to its regulation. Recent structural studies have solidified the idea that LPL is regulated not only by interactions with other binding partners but also by self-associations. Here, we review the complexities of the protein-protein and protein-lipid interactions that govern LPL structure and function.

Endothelial Lipase Variant, T111I, Does Not Alter Inhibition by Angiopoietin-like Proteins

High levels of HDL-C are correlated with a decreased risk of cardiovascular disease. HDL-C levels are modulated in part by the secreted phospholipase, endothelial lipase (EL), which hydrolyzes the phospholipids of HDL and decreases circulating HDL-C concentrations. A 584C/T polymorphism in LIPG, the gene which encodes EL, was first identified in individuals with increased HDL levels. This polymorphism results in a T111I point mutation the EL protein. The association between this variant, HDL levels, and the risk of coronary artery disease (CAD) in humans has been extensively studied, but the findings have been inconsistent. In this study, we took a biochemical approach, investigating how the T111I variant affected EL activity, structure, and stability. Moreover, we tested whether the T111I variant altered the inhibition of phospholipase activity by angiopoietin-like 3 (ANGPTL3) and angiopoietin-like 4 (ANGPTL4), two known EL inhibitors. We found that neither the stability nor enzymatic activity of EL was altered by the T111I variant. Moreover, we found no difference between wild-type and T111I EL in their ability to be inhibited by ANGPTL proteins. These data suggest that any effect this variant may have on HDL-C levels or cardiovascular disease are not mediated through alterations in these functions.

Can blood proteome diversity among fish species help explain perfluoroalkyl acid trophodynamics in aquatic food webs?

Per- and polyfluoroalkyl substances (PFAS) are a diverse family of industrially significant synthetic chemicals infamous for extreme environmental persistence and global environmental distribution. Many PFAS are bioaccumulative and biologically active mainly due to their tendency to bind with various proteins. These protein interactions are important in determining the accumulation potential and tissue distribution of individual PFAS. Trophodynamics studies including aquatic food webs present inconsistent evidence for PFAS biomagnification. This study strives to identify whether the observed variability in PFAS bioaccumulation potential among species could correspond with interspecies protein composition differences. Specifically, this work compares the perfluorooctane sulfonate (PFOS) serum protein binding potential and the tissue distribution of ten perfluoroalkyl acids (PFAAs) detected in alewife (Alosa pseudoharengus), deepwater sculpin (Myoxocephalus thompsonii), and lake trout (Salvelinus namaycush) of the Lake Ontario aquatic piscivorous food web. These three fish sera and fetal bovine reference serum all had unique total serum protein concentrations. Serum protein-PFOS binding experiments showed divergent patterns between fetal bovine serum and fish sera, suggesting potentially two different PFOS binding mechanisms. To identify interspecies differences in PFAS-binding serum proteins, fish sera were pre-equilibrated with PFOS, fractionated by serial molecular weight cut-off filter fractionation, followed by liquid chromatography-tandem mass spectrometry analysis of the tryptic protein digests and the PFOS extracts of each fraction. This workflow identified similar serum proteins for all fish species. However, serum albumin was only identified in lake trout, suggesting apolipoproteins are likely the primary PFAA transporters in alewife and deepwater sculpin sera. PFAA tissue distribution analysis provided supporting evidence for interspecies variations in lipid transport and storage, which may also contribute to the varied PFAA accumulation in these species. Proteomics data are available via ProteomeXchange with identifier PXD039145.

Hepatokines, bile acids and ketone bodies are novel Hormones regulating energy homeostasis

Current views show that an impaired balance partly explains the fat accumulation leading to obesity. Fetal malnutrition and early exposure to endocrine-disrupting compounds also contribute to obesity and impaired insulin secretion and/or sensitivity. The liver plays a major role in systemic glucose homeostasis through hepatokines secreted by hepatocytes. Hepatokines influence metabolism through autocrine, paracrine, and endocrine signaling and mediate the crosstalk between the liver, non-hepatic target tissues, and the brain. The liver also synthetizes bile acids (BAs) from cholesterol and secretes them into the bile. After food consumption, BAs mediate the digestion and absorption of fat-soluble vitamins and lipids in the duodenum. In recent studies, BAs act not simply as fat emulsifiers but represent endocrine molecules regulating key metabolic pathways. The liver is also the main site of the production of ketone bodies (KBs). In prolonged fasting, the brain utilizes KBs as an alternative to CHO. In the last few years, the ketogenic diet (KD) became a promising dietary intervention. Studies on subjects undergoing KD show that KBs are important mediators of inflammation and oxidative stress. The present review will focus on the role played by hepatokines, BAs, and KBs in obesity, and diabetes prevention and management and analyze the positive effects of BAs, KD, and hepatokine receptor analogs, which might justify their use as new therapeutic approaches for metabolic and aging-related diseases.

Inverse effects of APOC2 and ANGPTL4 on the conformational dynamics of lid-anchoring structures in lipoprotein lipase

The lipolytic processing of triglyceride-rich lipoproteins (TRLs) by lipoprotein lipase (LPL) is crucial for the delivery of dietary lipids to the heart, skeletal muscle, and adipose tissue. The processing of TRLs by LPL is regulated in a tissue-specific manner by a complex interplay between activators and inhibitors. Angiopoietin-like protein 4 (ANGPTL4) inhibits LPL by reducing its thermal stability and catalyzing the irreversible unfolding of LPL's α/β-hydrolase domain. We previously mapped the ANGPTL4 binding site on LPL and defined the downstream unfolding events resulting in LPL inactivation. The binding of LPL to glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 protects against LPL unfolding. The binding site on LPL for an activating cofactor, apolipoprotein C2 (APOC2), and the mechanisms by which APOC2 activates LPL have been unclear and controversial. Using hydrogen-deuterium exchange/mass spectrometry, we now show that APOC2's C-terminal α-helix binds to regions of LPL surrounding the catalytic pocket. Remarkably, APOC2's binding site on LPL overlaps with that for ANGPTL4, but their effects on LPL conformation are distinct. In contrast to ANGPTL4, APOC2 increases the thermal stability of LPL and protects it from unfolding. Also, the regions of LPL that anchor the lid are stabilized by APOC2 but destabilized by ANGPTL4, providing a plausible explanation for why APOC2 is an activator of LPL, while ANGPTL4 is an inhibitor. Our studies provide fresh insights into the molecular mechanisms by which APOC2 binds and stabilizes LPL-and properties that we suspect are relevant to the conformational gating of LPL's active site.

Angiopoietin-like 3: An important protein in regulating lipoprotein levels

ANGPTL3 has emerged as a therapeutic target whose inhibition results in profound reductions of plasma lipids, including atherogenic triglyceride-rich lipoproteins and low-density lipoprotein cholesterol. The identification of ANGPTL3 deficiency as a cause of familial combined hypolipidemia in humans hastened the development of anti-ANGPTL3 therapeutic agents, including evinacumab (a monoclonal antibody inhibiting circulating ANGPTL3), vupanorsen (an antisense oligonucleotide [ASO] targeting hepatic ANGPTL3 mRNA for degradation), and others. Advances have also been made in ANGPTL3 vaccination and gene editing strategies, with the former still in preclinical phases and the latter in preparation for Phase 1 trials. Here, we review the discovery of ANGPTL3 as an important regulator of lipoprotein metabolism, molecular characteristics of the protein, mechanisms by which it regulates plasma lipids, and the clinical development of anti-ANGPTL3 agents. The clinical success of therapies inhibiting ANGPTL3 highlights the importance of this target as a novel approach in treating refractory hypertriglyceridemia and hypercholesterolemia.

ANGPTL4 is a potential driver of HCV-induced peripheral insulin resistance

Chronic hepatitis C (CHC) is associated with the development of metabolic disorders, including both hepatic and extra-hepatic insulin resistance (IR). Here, we aimed at identifying liver-derived factor(s) potentially inducing peripheral IR and uncovering the mechanisms whereby HCV can regulate the action of these factors. We found ANGPTL4 (Angiopoietin Like 4) mRNA expression levels to positively correlate with HCV RNA (r = 0.46, p < 0.03) and HOMA-IR score (r = 0.51, p = 0.01) in liver biopsies of lean CHC patients. Moreover, we observed an upregulation of ANGPTL4 expression in two models recapitulating HCV-induced peripheral IR, i.e. mice expressing core protein of HCV genotype 3a (HCV-3a core) in hepatocytes and hepatoma cells transduced with HCV-3a core. Treatment of differentiated myocytes with recombinant ANGPTL4 reduced insulin-induced Akt-Ser473 phosphorylation. In contrast, conditioned medium from ANGPTL4-KO hepatoma cells prevented muscle cells from HCV-3a core induced IR. Treatment of HCV-3a core expressing HepG2 cells with PPARγ antagonist resulted in a decrease of HCV-core induced ANGPTL4 upregulation. Together, our data identified ANGPTL4 as a potential driver of HCV-induced IR and may provide working hypotheses aimed at understanding the pathogenesis of IR in the setting of other chronic liver disorders.

Combined action of albumin and heparin regulates lipoprotein lipase oligomerization, stability, and ligand interactions

Lipoprotein lipase (LPL), a crucial enzyme in the intravascular hydrolysis of triglyceride-rich lipoproteins, is a potential drug target for the treatment of hypertriglyceridemia. The activity and stability of LPL are influenced by a complex ligand network. Previous studies performed in dilute solutions suggest that LPL can appear in various oligomeric states. However, it was not known how the physiological environment, that is blood plasma, affects the action of LPL. In the current study, we demonstrate that albumin, the major protein component in blood plasma, has a significant impact on LPL stability, oligomerization, and ligand interactions. The effects induced by albumin could not solely be reproduced by the macromolecular crowding effect. Stabilization, isothermal titration calorimetry, and surface plasmon resonance studies revealed that albumin binds to LPL with affinity sufficient to form a complex in both the interstitial space and the capillaries. Negative stain transmission electron microscopy and raster image correlation spectroscopy showed that albumin, like heparin, induced reversible oligomerization of LPL. However, the albumin induced oligomers were structurally different from heparin-induced filament-like LPL oligomers. An intriguing observation was that no oligomers of either type were formed in the simultaneous presence of albumin and heparin. Our data also suggested that the oligomer formation protected LPL from the inactivation by its physiological regulator angiopoietin-like protein 4. The concentration of LPL and its environment could influence whether LPL follows irreversible inactivation and aggregation or reversible LPL oligomer formation, which might affect interactions with various ligands and drugs. In conclusion, the interplay between albumin and heparin could provide a mechanism for ensuring the dissociation of heparan sulfate-bound LPL oligomers into active LPL upon secretion into the interstitial space.

The impact of fasting on adipose tissue metabolism

Fasting and starvation were common occurrences during human evolution and accordingly have been an important environmental factor shaping human energy metabolism. Humans can tolerate fasting reasonably well through adaptative and well-orchestrated time-dependent changes in energy metabolism. Key features of the adaptive response to fasting are the breakdown of liver glycogen and muscle protein to produce glucose for the brain, as well as the gradual depletion of the fat stores, resulting in the release of glycerol and fatty acids into the bloodstream and the production of ketone bodies in the liver. In this paper, an overview is presented of our current understanding of the effects of fasting on adipose tissue metabolism. Fasting leads to reduced uptake of circulating triacylglycerols by adipocytes through inhibition of the activity of the rate-limiting enzyme lipoprotein lipase. In addition, fasting stimulates the degradation of stored triacylglycerols by activating the key enzyme adipose triglyceride lipase. The mechanisms underlying these events are discussed, with a special interest in insights gained from studies on humans. Furthermore, an overview is presented of the effects of fasting on other metabolic pathways in the adipose tissue, including fatty acid synthesis, glucose uptake, glyceroneogenesis, autophagy, and the endocrine function of adipose tissue.

Intracapillary LPL levels in brown adipose tissue, visualized with an antibody-based approach, are regulated by ANGPTL4 at thermoneutral temperatures

Lipoprotein lipase (LPL) is secreted into the interstitial spaces by parenchymal cells and then transported into capillaries by GPIHBP1. LPL carries out the lipolytic processing of triglyceride (TG)-rich lipoproteins (TRLs), but the tissue-specific regulation of LPL is incompletely understood. Plasma levels of TG hydrolase activity after heparin injection are often used to draw inferences about intravascular LPL levels, but the validity of these inferences is unclear. Moreover, plasma TG hydrolase activity levels are not helpful for understanding LPL regulation in specific tissues. Here, we sought to elucidate LPL regulation under thermoneutral conditions (30 °C). To pursue this objective, we developed an antibody-based method to quantify (in a direct fashion) LPL levels inside capillaries. At 30 °C, intracapillary LPL levels fell sharply in brown adipose tissue (BAT) but not heart. The reduced intracapillary LPL levels were accompanied by reduced margination of TRLs along capillaries. ANGPTL4 expression in BAT increased fourfold at 30 °C, suggesting a potential explanation for the lower intracapillary LPL levels. Consistent with that idea, Angptl4 deficiency normalized both LPL levels and TRL margination in BAT at 30 °C. In Gpihbp1-/- mice housed at 30 °C, we observed an ANGPTL4-dependent decrease in LPL levels within the interstitial spaces of BAT, providing in vivo proof that ANGPTL4 regulates LPL levels before LPL transport into capillaries. In conclusion, our studies have illuminated intracapillary LPL regulation under thermoneutral conditions. Our approaches will be useful for defining the impact of genetic variation and metabolic disease on intracapillary LPL levels and TRL processing.

ANGPTL4 suppresses the profibrogenic functions of atrial fibroblasts induced by angiotensin II by up-regulating PPARγ

Objectives

The present study's objective was to investigate the association between angiopoietin-like 4 (ANGPTL4) levels and the prognosis of Atrial fibrillation (AF), the causative effect in angiotensin II- (Ang II) induced AF, and its underlying mechanisms.

Materials and Methods

Baseline serum ANGPTL-4 concentrations were measured in 130 patients with AF. Rat atrial fibroblasts were isolated from 14-day-old SD rats and transfected with Ang II treatment. Transfected cells were divided into: The control group, ANGPTL4-OE group, Ang II group, and Ang II+ANGPTL4-OE group. Transfected cells were used to analyze fibroblasts' proliferation, migration, and collagen production at the cellular level. RT-qPCR and western blotting evaluated the ANGPTL4-targeted gene and PPARγ-Akt pathway.

Results

In patients with AF, serum ANGPTL4 concentrations decreased significantly compared with the healthy group. ANGPTL4 mRNA and protein expressions were significantly down-regulated in Ang II-induced cardiac fibroblasts. ANGPTL4 overexpression potentially attenuated Ang IIinduced fibroblast proliferation, migration, and collagen production in atrial tissue. ANGPTL4 inhibited the signaling proteins, such as PPARγ, α-SMA, and Akt.

Conclusion

Our experimental data speculate that ANGPTL4 is a key factor in regulating AF progression. Therefore, increasing ANGPTL4 expression could be an effective strategy for AF treatment.

Targeting TGF-β1/miR-21 pathway in keratinocytes reveals protective effects of silymarin on imiquimod-induced psoriasis mouse model

Epidermal cells integrate multiple signals that activate the signaling pathways involved in skin homeostasis. TGF-β1 signaling pathway upregulates microRNA (miR)-21-5p in keratinocytes and is often deregulated in skin diseases. To identify the bioactive compounds that enable to modulate the TGF-β1/miR-21-5p signaling pathway, we screened a library of medicinal plant extracts using our miR-ON RILES luciferase reporter system placed under the control of the miR-21-5p in keratinocytes treated with TGF-β1. We identified silymarin, a mixture of flavonolignans extracted from Silybum marianum (L.) Gaertn., as the most potent regulator of miR-21-5p expression. Using Argonaute 2 immunoprecipitation and RT-qPCR, we showed that silymarin regulates the expression of miR-21-5p through a noncanonical TGF-β1 signaling pathway, whereas RNA-sequencing analysis revealed three unexpected transcriptomic signatures associated with keratinocyte differentiation, cell cycle, and lipid metabolism. Mechanistically, we demonstrated that SM blocks cell cycle progression, inhibits keratinocyte differentiation through repression of Notch3 expression, stimulates lipid synthesis via activation of PPARγ signaling and inhibits inflammatory responses by suppressing the transcriptional activity of NF-κB. We finally showed that topical application of silymarin alleviates the development of imiquimod-induced psoriasiform lesions in mice by abrogating the altered expression levels of markers involved in inflammation, proliferation, differentiation, and lipid metabolism.

Differential effects of bariatric surgery on plasma levels of ANGPTL3 and ANGPTL4

BACKGROUND AND AIM

Angiopoietin-like 3 (ANGPTL3) and 4 (ANGPTL4) are regulators of triglyceride storage and utilization. Bariatric surgery (BS) leads to profound changes in adipose tissue composition and energy metabolism. We evaluated the impact of BS on plasma levels of ANGPTL3 and ANGPTL4.

METHODS AND RESULTS

Twenty-seven subjects affected by morbid obesity with or without type 2 diabetes (T2D) underwent Roux-en-Y gastric bypass (RYGB) and 18 patients with advanced T2D received Biliopancreatic Diversion (BPD). Fasting ANGPTL proteins levels, insulin sensitivity (evaluated by euglycemic hyperinsulinemic clamp), total bile acids (TBA) and free fatty acids (FFA) were measured at baseline and 1 year after surgery. Both surgical procedures resulted in the loss of fat mass, improved glucose control, and a ∼2-fold increase of insulin sensitivity. ANGPTL4 levels decreased significantly with both RYGB (26.6 ± 0.6 to 24.4 ± 0.3 ng/mL, p = 0.001) and BPD (27.9 ± 1.5 to 24.0 ± 0.5 ng/mL, p = 0.003). In contrast, ANGPTL3 concentrations did not change after RYGB but rose following BPD (225 ± 20 to 300 ± 15 ng/mL, p = 0.003). By multiple regression analysis, changes after BS in ANGPTL4 were independently associated with changes in blood glucose, (p = 0.0169) whereas changes in ANGPTL3 were associated with variations in FFA (p = 0.008) and insulin sensitivity (p = 0.043).

CONCLUSION

Circulating ANGPTL4 is reduced by BS, probably due to the loss of fat mass and improved insulin sensitivity. Conversely, ANGPTL3 levels increased after BPD, but not after RYGB, presumably because of the metabolic changes induced by the malabsorptive effect of BPD.

The Regulation of Triacylglycerol Metabolism and Lipoprotein Lipase Activity

Triacylglycerol (TG) metabolism is tightly regulated to maintain a pool of TG within circulating lipoproteins that can be hydrolyzed in a tissue-specific manner by lipoprotein lipase (LPL) to enable the delivery of fatty acids to adipose or oxidative tissues as needed. Elevated serum TG concentrations, which result from a deficiency of LPL activity or, more commonly, an imbalance in the regulation of tissue-specific LPL activities, have been associated with an increased risk of atherosclerotic cardiovascular disease through multiple studies. Among the most critical LPL regulators are the angiopoietin-like (ANGPTL) proteins ANGPTL3, ANGPTL4, and ANGPTL8, and a number of different apolipoproteins including apolipoprotein A5 (ApoA5), apolipoprotein C2 (ApoC2), and apolipoprotein C3 (ApoC3). These ANGPTLs and apolipoproteins work together to orchestrate LPL activity and therefore play pivotal roles in TG partitioning, hydrolysis, and utilization. This review summarizes the mechanisms of action, epidemiological findings, and genetic data most relevant to these ANGPTLs and apolipoproteins. The interplay between these important regulators of TG metabolism in both fasted and fed states is highlighted with a holistic view toward understanding key concepts and interactions. Strategies for developing safe and effective therapeutics to reduce circulating TG by selectively targeting these ANGPTLs and apolipoproteins are also discussed.

Obesity and cancer-extracellular matrix, angiogenesis, and adrenergic signaling as unusual suspects linking the two diseases

Obesity is an established risk factor for several human cancers. Given the association between excess body weight and cancer, the increasing rates of obesity worldwide are worrisome. A variety of obesity-related factors has been implicated in cancer initiation, progression, and response to therapy. These factors include circulating nutritional factors, hormones, and cytokines, causing hyperinsulinemia, inflammation, and adipose tissue dysfunction. The impact of these conditions on cancer development and progression has been the focus of extensive literature. In this review, we concentrate on processes that can link obesity and cancer, and which provide a novel perspective: extracellular matrix remodeling, angiogenesis, and adrenergic signaling. We describe molecular mechanisms involved in these processes, which represent putative targets for intervention. Liver, pancreas, and breast cancers were chosen as exemplary disease models. In view of the expanding epidemic of obesity, a better understanding of the tumorigenic process in obese individuals might lead to more effective treatments and preventive measures.

A Review of free fatty acid-induced cell signaling, angiopoietin-like protein 4, and skeletal muscle differentiation

Postnatal skeletal muscle differentiation from quiescent satellite cells is a highly regulated process, although our understanding of the contribution of nutritional factors in myogenesis is limited. Free fatty acids (FFAs) are known to cause detrimental effects to differentiated skeletal muscle cells by increasing oxidative stress which leads to muscle wasting and insulin resistance in skeletal muscle. In addition, FFAs are thought to act as inhibitors of skeletal muscle differentiation. However, the precise molecular mechanisms underlying the effects of FFAs on skeletal muscle differentiation remains to be elucidated. There is a clear relationship between dietary FFAs and their ability to suppress myogenesis and we propose the hypothesis that the FFA-mediated increase in angiopoietin-like protein 4 (ANGPTL4) may play a role in the inhibition of differentiation. This review discusses the role of FFAs in skeletal muscle differentiation to-date and proposes potential mechanisms of FFA-induced ANGPTL4 mediated inhibition of skeletal muscle differentiation.

Using Synthetic ApoC-II Peptides and nAngptl4 Fragments to Measure Lipoprotein Lipase Activity in Radiometric and Fluorescent Assays

Lipoprotein lipase (LPL) plays a crucial role in preventing dyslipidemia by hydrolyzing triglycerides (TGs) in packaged lipoproteins. Since hypertriglyceridemia (HTG) is a major risk factor for cardiovascular disease (CVD), the leading cause of death worldwide, methods that accurately quantify the hydrolytic activity of LPL in clinical and pre-clinical samples are much needed. To date, the methods used to determine LPL activity vary considerably in their approach, in the LPL substrates used, and in the source of LPL activators and inhibitors used to quantify LPL-specific activity, rather than other lipases, e.g., hepatic lipase (HL) or endothelial lipase (EL) activity. Here, we describe methods recently optimized in our laboratory, using a synthetic ApoC-II peptide to activate LPL, and an n-terminal Angiopoietin-Like 4 fragment (nAngptl4) to inhibit LPL, presenting a cost-effective and reproducible method to measure LPL activity in human post-heparin plasma (PHP) and in LPL-enriched heparin released (HR) fractions from LPL secreting cells. We also describe a modified version of the triolein-based assay using human serum as a source of endogenous activators and inhibitors and to determine the relative abundance of circulating factors that regulate LPL activity. Finally, we describe how an ApoC-II peptide and nAngptl4 can be applied to high-throughput measurements of LPL activity using the EnzChek™ fluorescent TG analog substrate with PHP, bovine LPL, and HR LPL enriched fractions. In summary, this manuscript assesses the current methods of measuring LPL activity and makes new recommendations for measuring LPL-mediated hydrolysis in pre-clinical and clinical samples.

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.

Cholecystokinin promotes functional expression of the aquaglycerol channel aquaporin 7 in adipocytes

BACKGROUND AND PURPOSE

Cholecystokinin (CCK) promotes triglyceride storage and adiponectin production in white adipose tissue (WAT), suggesting that CCK modulates WAT homeostasis. Our goal was to investigate the role of CCK in regulating the expression and function of the aquaglycerol channel aquaporin 7 (AQP7), a protein that is pivotal for maintaining adipocyte homeostasis and preserving insulin responsiveness.

EXPERIMENTAL APPROACH

The effect of the bioactive fragment of CCK, CCK-8, in regulating adipose AQP7 expression and glycerol efflux was assessed in rats as well as in pre-adipocytes. Moreover, the involvement of insulin receptors in the effects of CCK-8 was characterized in pre-adipocytes lacking insulin receptors.

KEY RESULTS

CCK-8 induced AQP7 gene expression in rat WAT, concomitantly increasing plasma glycerol concentration. In isolated pre-adipocytes, CCK-8 also enhanced both AQP7 expression and glycerol leakage. The effect of CCK-8 was independent of the lipolysis rate, as CCK-8 failed to promote fatty acid release by adipocytes. In addition, CCK-8 did not enhance hormone sensitive lipase phosphorylation, which is the rate-limiting step of lipolysis. Moreover, the effects of CCK-8 were dependent on the activation of protein kinase B and PPARγ. Silencing insulin receptor (IR) expression inhibited CCK-8-induced Aqp7 expression in pre-adipocytes. Furthermore, insulin enhanceded the effect of CCK-8.

CONCLUSIONS AND IMPLICATIONS

CCK regulates AQP7 expression and function, and this effect is dependent on insulin. Accordingly, CCK receptor agonists could be suitable for preserving and improving insulin responsiveness in WAT.

Hepatic PTEN Signaling Regulates Systemic Metabolic Homeostasis through Hepatokines-Mediated Liver-to-Peripheral Organs Crosstalk

Liver-derived circulating factors deeply affect the metabolism of distal organs. Herein, we took advantage of the hepatocyte-specific PTEN knockout mice (LPTENKO), a model of hepatic steatosis associated with increased muscle insulin sensitivity and decreased adiposity, to identify potential secreted hepatic factors improving metabolic homeostasis. Our results indicated that protein factors, rather than specific metabolites, released by PTEN-deficient hepatocytes trigger an improved muscle insulin sensitivity and a decreased adiposity in LPTENKO. In this regard, a proteomic analysis of conditioned media from PTEN-deficient primary hepatocytes identified seven hepatokines whose expression/secretion was deregulated. Distinct expression patterns of these hepatokines were observed in hepatic tissues from human/mouse with NAFLD. The expression of specific factors was regulated by the PTEN/PI3K, PPAR or AMPK signaling pathways and/or modulated by classical antidiabetic drugs. Finally, loss-of-function studies identified FGF21 and the triad AHSG, ANGPTL4 and LECT2 as key regulators of insulin sensitivity in muscle cells and in adipocytes biogenesis, respectively. These data indicate that hepatic PTEN deficiency and steatosis alter the expression/secretion of hepatokines regulating insulin sensitivity in muscles and the lipid metabolism in adipose tissue. These hepatokines could represent potential therapeutic targets to treat obesity and insulin resistance.

ANGPTL4: a new mode in the regulation of intravascular lipolysis

PURPOSE OF REVIEW

Lipoprotein lipase (LPL) is the rate-limiting enzyme for intravascular processing of circulating triglyceride-rich lipoproteins (TRLs). One emerging strategy for therapeutic lowering of plasma triglyceride levels aims at increasing the longevity of LPL activity by attenuating its inhibition from angiopoietin-like proteins (ANGPTL) 3, 4 and 8. This mini-review focuses on recent insights into the molecular mechanisms underpinning the regulation of LPL activity in the intravascular unit by ANGPTLs with special emphasis on ANGPTL4.

RECENT FINDINGS

Our knowledge on the molecular interplays between LPL, its endothelial transporter GPIHBP1, and its inhibitor(s) ANGPTL4, ANGPTL3 and ANGPTL8 have advanced considerably in the last 2 years and provides an outlined on how these proteins regulate the activity and compartmentalization of LPL. A decisive determinant instigating this control is the inherent protein instability of LPL at normal body temperature, a property that is reciprocally impacted by the binding of GPIHBP1 and ANGPTLs. Additional layers in this complex LPL regulation is provided by the different modulation of ANGPTL4 and ANGPTL3 activities by ANGPTL8 and the inhibition of ANGPTL3/8 complexes by apolipoprotein A5 (APOA5).

SUMMARY

Posttranslational regulation of LPL activity in the intravascular space is essential for the differential partitioning of TRLs across tissues and their lipolytic processing in response to nutritional cues.

Alpha1-antitrypsin combined fatty acids induced angiopoietin-like protein 4, expression in breast cancer: A pilot study

INTRODUCTION

The effect of nutrition on inflammation and breast cancer (BC) prognosis is still inconclusive. Mechanism of data suggests that different types of fatty acids (FAs) play an essential role in carcinogenesis, and binding of alpha 1 antitrypsin (A1AT) may modulate carcinogenesis. The increased expression in the bound form of A1AT and release of Angiopoietin-like protein 4 (Angptl4) targets the gene of peroxisome proliferator-activated receptor-gamma (PPAR-γ). Our aim of the study was to compare the effect of FA-free (A1AT-0) and FAs bound forms of A1AT on levels of IL-1β, PPAR-gamma, and Angplt4 in breast cancer and control women.

METHODOLOGY

10 women with breast cancer and ten control women within the age group 25-60 years with normal (Pi) M allele A1AT were recruited. Mononuclear cells were isolated and treated with different A1AT and FAs on the various combinations (linoleic acid, alpha-linolenic acid) for time-dependent study (2,4,18 and 24 h) and analyzed for the interleukin -1 beta(IL-1b), PPAR-gamma, and Angiopoietin-like protein 4 (Angptl4) expression by using ELISA method and gas chromatography for analyzing FAs. One-way ANOVA combined with multiple comparisons is used to compare the means.

RESULTS

100% of the study subjects were homozygous for the normal allele of A1AT. Time-dependent effects of A1AT and A1AT conjugated fatty acids on IL-I b, PPAR-g and Angptl4 showed statistically significant P = 0.07, P = 0.001, and P = 0.02 respectively, compared to those of the former study subjects. But within the groups, PPAR-g levels in case group (F(15,40)1.606, P = 0.003) and Angptl4 in the control group (F(15,32)0.64, P = 0.043) differed significantly.

CONCLUSION

To the best of our knowledge, it's the first kind of study, and we speculate that the A1AT complex with different types of FAs results in a new form of A1AT having a solid capability to regulate the inflammation-induced synthesis, processing, and release of an active form of IL-1β. Our experimental data shows that the anti-inflammatory property of A1AT combined FAs likely to be mediated PPAR γand Angptl4 activation, thereby inhibiting the IL-1b. These findings may be worth assessing BC's biological effects and therapeutic effectiveness.

Angiopoietin-like 4-Induced 3D Capillary Morphogenesis Correlates to Stabilization of Endothelial Adherens Junctions and Restriction of VEGF-Induced Sprouting

Angiopoietin-like 4 (ANGPTL4) is a target of hypoxia that accumulates in the endothelial extracellular matrix. While ANGPTL4 is known to regulate angiogenesis and vascular permeability, its context-dependent role related to vascular endothelial growth factor (VEGF) has been suggested in capillary morphogenesis. We here thus develop in vitro 3D models coupled to imaging and morphometric analysis of capillaries to decipher ANGPTL4 functions either alone or in the presence of VEGF. ANGPTL4 induces the formation of barely branched and thin endothelial capillaries that display linear adherens junctions. However, ANGPTL4 counteracts VEGF-induced formation of abundant ramified capillaries presenting cell–cell junctions characterized by VE-cadherin containing reticular plaques and serrated structures. We further deciphered the early angiogenesis steps regulated by ANGPTL4. During the initial activation of endothelial cells, ANGPTL4 alone induces cell shape changes but limits the VEGF-induced cell elongation and unjamming. In the growing sprout, ANGPTL4 maintains cohesive VE-cadherin pattern and sustains moderate 3D cell migration but restricts VEGF-induced endothelium remodeling and cell migration. This effect is mediated by differential short- and long-term regulation of P-Y1175-VEGFR2 and ERK1-2 signaling by ANGPTL4. Our in vitro 3D models thus provide the first evidence that ANGPTL4 induces a specific capillary morphogenesis but also overcomes VEGF effect.

A triple combination strategy of UHPLC-MSn, hypolipidemic activity and transcriptome sequencing to unveil the hypolipidemic mechanism of Nelumbo nucifera alkaloids

ETHNOPHARMACOLOGICAL RELEVANCE

Nelumbo nucifera (N. nucifera), a kind of edible Chinese herbal, has been studied in treating hyperlipidemia. However, the hypolipidemic mechanism of N. nucifera remains unknown. Aims of this review: We aimed to screen the effective constituent of N. nucifera alkaloids and elucidated the potential mechanism for treating hyperlipidemia. A triple combination strategy of UHPLC-MSⁿ, hypolipidemic activity and transcriptome sequencing was built to unveil the hypolipidemic mechanism of Nelumbo nucifera alkaloid.

MATERIALS AND METHODS

We comprehensively investigated the characterization of N. nucifera alkaloids by using UHPLC-LTQ-Orbitrap MSⁿ. And the hypolipidemic activity of candidate active ingredients were evaluated on sodium oleate-induced HepG2 cell. Finally, O-nornuciferine and N. nucifera alkaloid extraction were analyzed by RNA sequence (RNA-seq) to decipher the underlying hypolipidemic mechanism and were verified by qRT-PCR.

RESULTS

35 compounds were identified from N. nucifera alkaloid extraction by UHPLC-LTQ-Orbitrap MSⁿ. Among them, O-nornuciferine and N. nucifera alkaloid extraction which showed significant hypolipidemic activity were analyzed by transcriptome sequencing. After the intervention of O-nornuciferine and N. nucifera alkaloid extraction, 1 and 158 differentially expressed genes (DEGs) were identified, severally. The enrichment analysis indicated that the hypolipidemic effect was adjusted by the expression of numerous key DEGs involved in bile secretion, glycerolipid and sphingolipid metabolism, PPAR signaling pathway.

CONCLUSIONS

O-nornuciferine and N. nucifera alkaloids had exibited significant effects in hyperlipidemia. The candidate genes were LDLR, LPL and ANGPTL4, etc. It was most likely that they adjusted lipid metabolism by modulating expression levels of various key factors which were involved in bile secretion, glycerolipid metabolism, sphingolipid metabolism and PPAR signaling pathway, and so on. This study clarified the hypolipidemic mechanism of the alkaloids in N. nucifera, and laid a foundation for the subsequent development of clinical application and better quality of N. nucifera.

Gut Dysbiosis and Immune System in Atherosclerotic Cardiovascular Disease (ACVD)

Atherosclerosis is a leading cause of cardiovascular disease and mortality worldwide. Alterations in the gut microbiota composition, known as gut dysbiosis, have been shown to contribute to atherosclerotic cardiovascular disease (ACVD) development through several pathways. Disruptions in gut homeostasis are associated with activation of immune processes and systemic inflammation. The gut microbiota produces several metabolic products, such as trimethylamine (TMA), which is used to produce the proatherogenic metabolite trimethylamine-N-oxide (TMAO). Short-chain fatty acids (SCFAs), including acetate, butyrate, and propionate, and certain bile acids (BAs) produced by the gut microbiota lead to inflammation resolution and decrease atherogenesis. Chronic low-grade inflammation is associated with common risk factors for atherosclerosis, including metabolic syndrome, type 2 diabetes mellitus (T2DM), and obesity. Novel strategies for reducing ACVD include the use of nutraceuticals such as resveratrol, modification of glucagon-like peptide 1 (GLP-1) levels, supplementation with probiotics, and administration of prebiotic SCFAs and BAs. Investigation into the relationship between the gut microbiota, and its metabolites, and the host immune system could reveal promising insights into ACVD development, prognostic factors, and treatments.

Lipoprotein size is a main determinant for the rate of hydrolysis by exogenous LPL in human plasma

LPL is a key player in plasma triglyceride metabolism. Consequently, LPL is regulated by several proteins during synthesis, folding, secretion, and transport to its site of action at the luminal side of capillaries, as well as during the catalytic reaction. Some proteins are well known, whereas others have been identified but are still not fully understood. We set out to study the effects of the natural variations in the plasma levels of all known LPL regulators on the activity of purified LPL added to samples of fasted plasma taken from 117 individuals. The enzymatic activity was measured at 25°C using isothermal titration calorimetry. This method allows quantification of the ability of an added fixed amount of exogenous LPL to hydrolyze triglyceride-rich lipoproteins in plasma samples by measuring the heat produced. Our results indicate that, under the conditions used, the normal variation in the endogenous levels of apolipoprotein C1, C2, and C3 or the levels of angiopoietin-like proteins 3, 4, and 8 in the fasted plasma samples had no significant effect on the recorded activity of the added LPL. Instead, the key determinant for the LPL activity was a lipid signature strongly correlated to the average size of the VLDL particles. The signature involved not only several lipoprotein and plasma lipid parameters but also apolipoprotein A5 levels. While the measurements cannot fully represent the action of LPL when attached to the capillary wall, our study provides knowledge on the interindividual variation of LPL lipolysis rates in human plasma.

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.

Metabogenomic analysis to functionally annotate the regulatory role of long non-coding RNAs in the liver of cows with different nutrient partitioning phenotype

Long non-coding RNAs (lncRNAs) hold gene regulatory potential, but require substantial further functional annotation in livestock. Applying two metabogenomic approaches by combining transcriptomic and metabolomic analyses, we aimed to identify lncRNAs with potential regulatory function for divergent nutrient partitioning of lactating crossbred cows and to establish metabogenomic interaction networks comprising metabolites, genes and lncRNAs. Through correlation analysis of lncRNA expression with transcriptomic and metabolomic data, we unraveled lncRNAs that have a putative regulatory role in energy and lipid metabolism, the urea and tricarboxylic acid cycles, and gluconeogenesis. Especially FGF21, which correlated with a plentitude of differentially expressed genes, differentially abundant metabolites, as well as lncRNAs, suggested itself as a key metabolic regulator. Notably, lncRNAs in close physical proximity to coding-genes as well as lncRNAs with natural antisense transcripts appear to perform a fine-tuning function in gene expression involved in metabolic pathways associated with different nutrient partitioning phenotypes.

Hepatokines and Non-Alcoholic Fatty Liver Disease: Linking Liver Pathophysiology to Metabolism

The liver plays a key role in maintaining energy homeostasis by sensing and responding to changes in nutrient status under various metabolic conditions. Recently highlighted as a major endocrine organ, the contribution of the liver to systemic glucose and lipid metabolism is primarily attributed to signaling crosstalk between multiple organs via hepatic hormones, cytokines, and hepatokines. Hepatokines are hormone-like proteins secreted by hepatocytes, and a number of these have been associated with extra-hepatic metabolic regulation. Mounting evidence has revealed that the secretory profiles of hepatokines are significantly altered in non-alcoholic fatty liver disease (NAFLD), the most common hepatic manifestation, which frequently precedes other metabolic disorders, including insulin resistance and type 2 diabetes. Therefore, deciphering the mechanism of hepatokine-mediated inter-organ communication is essential for understanding the complex metabolic network between tissues, as well as for the identification of novel diagnostic and/or therapeutic targets in metabolic disease. In this review, we describe the hepatokine-driven inter-organ crosstalk in the context of liver pathophysiology, with a particular focus on NAFLD progression. Moreover, we summarize key hepatokines and their molecular mechanisms of metabolic control in non-hepatic tissues, discussing their potential as novel biomarkers and therapeutic targets in the treatment of metabolic diseases.

Astrocytic lipid metabolism determines susceptibility to diet-induced obesity

Hypothalamic astrocytes play pivotal roles in both nutrient sensing and the modulation of synaptic plasticity of hypothalamic neuronal circuits in control of feeding and systemic glucose and energy metabolism. Here, we show the relevance of astrocytic fatty acid (FA) homeostasis under the opposing control of angiopoietin-like 4 (ANGPTL-4) and peroxisome proliferator–activated receptor gamma (PPARγ) in the cellular adaptations of hypothalamic astrocytes and neurons to the changing metabolic milieu. We observed that increased availability of FA in astrocytes induced by cell- and time-selective knockdown of Angptl4 protected against diet-induced obesity, while cell- and time-selective knockdown of Angptl4-regulated Pparγ lead to elevated susceptibility to obesity. Overall, our results unravel a previously unidentified role for astrocytic FA metabolism in central control of body weight and glucose homeostasis.

Cardiac-specific VEGFB overexpression reduces lipoprotein lipase activity and improves insulin action in rat heart

Cardiac muscle uses multiple sources of energy including glucose and fatty acid (FA). The heart cannot synthesize FA and relies on obtaining it from other sources, with lipoprotein lipase (LPL) breakdown of lipoproteins suggested to be a key source of FA for cardiac use. Recent work has indicated that cardiac vascular endothelial growth factor B (VEGFB) overexpression expands the coronary vasculature and facilitates metabolic reprogramming that favors glucose utilization. We wanted to explore whether this influence of VEGFB on cardiac metabolism involves regulation of LPL activity with consequent effects on lipotoxicity and insulin signaling. The transcriptomes of rats with and without cardiomyocyte-specific overexpression of human VEGFB were compared by using RNA sequencing. Isolated perfused hearts or cardiomyocytes incubated with heparin were used to enable measurement of LPL activity. Untargeted metabolomic analysis was performed for quantification of cardiac lipid metabolites. Cardiac insulin sensitivity was evaluated using fast-acting insulin. Isolated heart and cardiomyocytes were used to determine transgene-encoded VEGFB isoform secretion patterns and mitochondrial oxidative capacity using high-resolution respirometry and extracellular flux analysis. In vitro, transgenic cardiomyocytes incubated overnight and thus exposed to abundantly secreted VEGFB isoforms, in the absence of any in vivo confounding regulators of cardiac metabolism, demonstrated higher basal oxygen consumption. In the whole heart, VEGFB overexpression induced an angiogenic response that was accompanied by limited cardiac LPL activity through multiple mechanisms. This was associated with a lowered accumulation of lipid intermediates, diacylglycerols and lysophosphatidylcholine, that are known to influence insulin action. In response to exogenous insulin, transgenic hearts demonstrated increased insulin sensitivity. In conclusion, the interrogation of VEGFB function on cardiac metabolism uncovered an intriguing and previously unappreciated effect to lower LPL activity and prevent lipid metabolite accumulation to improve insulin action. VEGFB could be a potential cardioprotective therapy to treat metabolic disorders, for example, diabetes.NEW & NOTEWORTHY In hearts overexpressing vascular endothelial growth factor B (VEGFB), besides its known angiogenic response, multiple regulatory mechanisms lowered coronary LPL. This was accompanied by limited cardiac lipid metabolite accumulation with an augmentation of cardiac insulin action. Our data for the first time links VEGFB to coronary LPL in regulation of cardiac metabolism. VEGFB may be cardioprotective in metabolic disorders like diabetes.

The Pathogenic Role of Long Non-coding RNA H19 in Atherosclerosis via the miR-146a-5p/ANGPTL4 Pathway

The abnormally expressed long non-coding RNA (lncRNA) H19 has a crucial function in the development and progression of cardiovascular disease; however, its role in atherosclerosis is yet to be known. We aimed to examine the impacts of lncRNA H19 on atherogenesis as well as the involved mechanism. The outcomes from this research illustrated that the expression of lncRNA H19 was elevated in mouse blood and aorta with lipid-loaded macrophages and atherosclerosis. Adeno-associated virus (AAV)-mediated lncRNA H19 overexpression significantly increased the atherosclerotic plaque area in apoE^−/− mice supplied with a Western diet. The upregulation of lncRNA H19 decreased the miR-146a-5p expression but increased the levels of ANGPTL4 in mouse blood and aorta and THP-1 cells. Furthermore, lncRNA H19 overexpression promoted lipid accumulation in oxidized low-density lipoprotein (ox-LDL)-induced THP-1 macrophages. However, the knockdown of lncRNA H19 served as a protection against atherosclerosis in apoE^−/− mice and lowered the accumulation of lipids in ox-LDL-induced THP-1 macrophages. lncRNA H19 promoted the expression of ANGPTL4 via competitively binding to miR-146a-5p, thus promoting lipid accumulation in atherosclerosis. These findings altogether demonstrated that lncRNA H19 facilitated the accumulation of lipid in macrophages and aggravated the progression of atherosclerosis through the miR-146a-5p/ANGPTL4 pathway. Targeting lncRNA H19 might be an auspicious therapeutic approach for preventing and treating atherosclerotic disease.

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.

Angiopoietin-like 3 inhibition of endothelial lipase is not modulated by angiopoietin-like 8

High plasma triglyceride (TG) levels and low HDL-C levels are risk factors for atherosclerosis and cardiovascular disease. Both plasma TG and HDL-C levels are regulated in part by the circulating inhibitor, angiopoietin-like 3 (ANGPTL3). ANGPTL3 inhibits the phospholipase, endothelial lipase (EL), which hydrolyzes the phospholipids of HDL, thus decreasing plasma HDL levels. ANGPTL3 also inhibits LPL, the lipase primarily responsible for the clearance of TGs from the circulation. Previous studies have shown that ANGPTL3 requires complex formation with the related ANGPTL protein, angiopoietin-like 8 (ANGPTL8), to efficiently inhibit LPL, but the role of ANGPTL8 in EL inhibition is not known. In this study, we characterized inhibition and binding of EL by ANGPTL3 and investigated the role of ANGPTL8 in EL inhibition. We found that inhibition of EL by ANGPTL3 was dose dependent and temperature dependent. Interestingly, this inhibition was diminished when EL was bound to endothelial cells or in the presence of heparin. Unlike previous findings with LPL, we found that ANGPTL8 did not significantly alter the binding or the inhibition of EL by ANGPTL3. In addition, we found that a common ANGPTL8 variant, which encodes an R59W mutation, altered the ability of ANGPTL3 to bind and inhibit LPL but not EL. Together, our data indicate that ANGPTL8 is not necessary for EL inhibition. We conclude that ANGPTL8 is specific for the regulation of TG-rich lipoproteins through the LPL pathway and that therapeutically targeting ANGPTL8 for the treatment of hypertriglyceridemia or cardiovascular disease may have different outcomes than targeting ANGPTL3.

Influence of Dietary Chitosan Feeding Duration on Glucose and Lipid Metabolism in a Diabetic Rat Model

This study was designed to investigate the influence of dietary chitosan feeding-duration on glucose and lipid metabolism in diabetic rats induced by streptozotocin and nicotinamide [a non-insulin-dependent diabetes mellitus (NIDDM) model]. Male Sprague-Dawley rats were used as experimental animals and divided into short-term (6 weeks) and long-term (11 weeks) feeding durations, and each duration contained five groups: (1) control, (2) control + 5% chitosan, (3) diabetes, (4) diabetes + 0.8 mg/kg rosiglitazone (a positive control), and (5) diabetes + 5% chitosan. Whether the chitosan feeding was for 6 or 11 weeks, the chitosan supplementation decreased blood glucose and lipids levels and liver lipid accumulation. However, chitosan supplementation decreased plasma tumor necrosis factor (TNF)-α, insulin levels, alanine aminotransferase (ALT) activity, insulin resistance (HOMA-IR), and adipose tissue lipoprotein lipase activity. Meanwhile, it increased plasma high-density lipoproteins (HDL)-cholesterol level, plasma angiopoietin-like-4 protein expression, and plasma triglyceride levels (at 11-week feeding duration only). Taken together, 11-week (long-term) chitosan feeding may help to ameliorate the glucose and lipid metabolism in a NIDDM diabetic rat model.

Lipoprotein profile and lipid metabolism of PXB-cells®, human primary hepatocytes from liver-humanized mice: proposal of novel in vitro system for screening anti-lipidemic drugs

We investigated lipid metabolism in PXB-cells, which are human primary hepatocytes isolated from liver-humanized mice, and HepG2 and HuH-7 human hepatoma cell lines. Lipoprotein levels were higher in PXB-cells than in the 2 other cell lines, and PXB-cells mainly released triglycerides and cholesterol as very low density lipoprotein (VLDL), similar to actual liver tissue, whereas the major lipoprotein released from the 2 hepatoma cell lines was LDL. RT-PCR analysis demonstrated that the gene expression levels of apolipoprotein B100 (ApoB100), the apolipoprotein of VLDL/LDL, were similar in PXB-cells and HepG2 cells, while the overexpression of ApoC2, ApoC3, and ApoE, which are components of VLDL, but not LDL, was observed in PXBcells. A protein immunoassay revealed that ApoB100 levels secreted from PXB-cells and HuH-7 cells were similar; however, ApoC3 levels were higher in PXB-cells than in the two other cell lines. We also examined the anti-lipidemic activities of fenofibrate using this assay system. Fenofibrate suppressed lipoprotein production from PXB-cells in a dose-dependent manner mainly by activating the β-oxidation pathway. These results suggest that PXB-cells produce high levels of lipoproteins and are suitable for screening anti-lipidemic agents.

Differential Expression of the Host Lipid Regulators ANGPTL-3 and ANGPTL-4 in HCV Infection and Treatment

Host lipid metabolism reprogramming is essential for hepatitis C virus (HCV) infection and progression to severe liver disease. Direct-acting antivirals (DAAs) achieve a sustained virological response (SVR) in most patients, but virus eradication does not always protect against hepatocellular carcinoma (HCC). Angiopoietin-like protein-3 (ANGPTL-3) and angiopoietin-like protein-4 (ANGPTL-4) regulate the clearance of plasma lipids by inhibiting cellular lipase activity and possess emerging roles in tumourigenesis. We used ELISA and RT-qPCR to investigate ANGPTL-3 and ANGPTL-4 expression in HCV patients with characterised fibrosis throughout the natural history of hepatitis C and in long-term HCV infection in vitro, before and after DAA treatment. ANGPTL-3 was decreased in patients with advanced fibrosis compared to other disease stages, while ANGPTL-4 was progressively increased from acute infection to cirrhosis and HCC, peaking at the advanced fibrosis stage. Only ANGPTL-3 mRNA was down-regulated during early infection in vitro, although both ANGPTLs were increased later. DAA treatment did not alter ANGPTL-3 levels in advanced fibrosis/cirrhosis and in HCV infection in vitro, in contrast to ANGPTL-4. The association between ANGPTLs and fibrosis in HCV infection was underlined by an inverse correlation between the levels of ANGPTLs and serum transforming growth factor- β (TGF-β). Collectively, we demonstrate the pivotal role of advanced fibrosis in defining the expression fate of ANGPTLs in HCV infection and after treatment and propose a role for ANGPTL-3 as a contributor to post-treatment deregulation of lipid metabolism that could predispose certain individuals to HCC development.

Emerging roles of angiopoietin-like proteins in inflammation: Mechanisms and potential as pharmacological targets

Angiopoietin-like proteins (ANGPTLs), a family of eight secreted glycoproteins termed ANGTPL1-8, are involved in angiogenesis, lipid metabolism, cancer progression, and inflammation. Their roles in regulating lipid metabolism have been intensively studied, as some ANGPTLs are promising pharmacological targets for hypertriglyceridemia and associated cardiovascular disease. Recently, the emerging roles of ANGPTLs in inflammation have attracted great attention. First, elevated levels of multiple circulating ANGPTLs in inflammatory diseases make them potential disease biomarkers. Second, multiple ANGPTLs regulate acute or chronic inflammation via various mechanisms, including triggering inflammatory signaling through their action as ligands for integrin or forming homo- /hetero-oligomers to regulate signal transduction via extra- or intracellular mechanisms. As dysregulation of the inflammatory response is a critical trigger in many diseases, understanding the roles of ANGPTLs in inflammation will aid in drug/therapy development. Here, we summarize the roles, mechanisms, and potential therapeutic values for ANGPTLs in inflammation and inflammatory diseases.

Looking into key bacterial proteins involved in gut dysbiosis

The gastrointestinal microbiota plays a pivotal role in health and has been linked to many diseases. With the rapid accumulation of pyrosequencing data of the bacterial composition, the causal-effect relationship between specific dysbiosis features and diseases is now being explored. The aim of this review is to describe the key functional bacterial proteins and antigens in the context of dysbiosis related-diseases. We subjectively classify the key functional proteins into two categories: Primary key proteins and secondary key proteins. The primary key proteins mainly act by themselves and include biofilm inhibitors, toxin degraders, oncogene degraders, adipose metabolism modulators, anti-inflammatory peptides, bacteriocins, host cell regulators, adhesion and invasion molecules, and intestinal barrier regulators. The secondary key proteins mainly act by eliciting host immune responses and include flagellin, outer membrane proteins, and other autoantibody-related antigens. Knowledge of key bacterial proteins is limited compared to the rich microbiome data. Understanding and focusing on these key proteins will pave the way for future mechanistic level cause-effect studies of gut dysbiosis and diseases.

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.

GPIHBP1 and ANGPTL4 Utilize Protein Disorder to Orchestrate Order in Plasma Triglyceride Metabolism and Regulate Compartmentalization of LPL Activity

Intravascular processing of triglyceride-rich lipoproteins (TRLs) is crucial for delivery of dietary lipids fueling energy metabolism in heart and skeletal muscle and for storage in white adipose tissue. During the last decade, mechanisms underlying focal lipolytic processing of TRLs along the luminal surface of capillaries have been clarified by fresh insights into the functions of lipoprotein lipase (LPL); LPL's dedicated transporter protein, glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1); and its endogenous inhibitors, angiopoietin-like (ANGPTL) proteins 3, 4, and 8. Key discoveries in LPL biology include solving the crystal structure of LPL, showing LPL is catalytically active as a monomer rather than as a homodimer, and that the borderline stability of LPL's hydrolase domain is crucial for the regulation of LPL activity. Another key discovery was understanding how ANGPTL4 regulates LPL activity. The binding of ANGPTL4 to LPL sequences adjacent to the catalytic cavity triggers cooperative and sequential unfolding of LPL's hydrolase domain resulting in irreversible collapse of the catalytic cavity and loss of LPL activity. Recent studies have highlighted the importance of the ANGPTL3-ANGPTL8 complex for endocrine regulation of LPL activity in oxidative organs (e.g., heart, skeletal muscle, brown adipose tissue), but the molecular mechanisms have not been fully defined. New insights have also been gained into LPL-GPIHBP1 interactions and how GPIHBP1 moves LPL to its site of action in the capillary lumen. GPIHBP1 is an atypical member of the LU (Ly6/uPAR) domain protein superfamily, containing an intrinsically disordered and highly acidic N-terminal extension and a disulfide bond-rich three-fingered LU domain. Both the disordered acidic domain and the folded LU domain are crucial for the stability and transport of LPL, and for modulating its susceptibility to ANGPTL4-mediated unfolding. This review focuses on recent advances in the biology and biochemistry of crucial proteins for intravascular lipolysis.

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 Importance of Lipoprotein Lipase Regulation in Atherosclerosis

Lipoprotein lipase (LPL) plays a major role in the lipid homeostasis mainly by mediating the intravascular lipolysis of triglyceride rich lipoproteins. Impaired LPL activity leads to the accumulation of chylomicrons and very low-density lipoproteins (VLDL) in plasma, resulting in hypertriglyceridemia. While low-density lipoprotein cholesterol (LDL-C) is recognized as a primary risk factor for atherosclerosis, hypertriglyceridemia has been shown to be an independent risk factor for cardiovascular disease (CVD) and a residual risk factor in atherosclerosis development. In this review, we focus on the lipolysis machinery and discuss the potential role of triglycerides, remnant particles, and lipolysis mediators in the onset and progression of atherosclerotic cardiovascular disease (ASCVD). This review details a number of important factors involved in the maturation and transportation of LPL to the capillaries, where the triglycerides are hydrolyzed, generating remnant lipoproteins. Moreover, LPL and other factors involved in intravascular lipolysis are also reported to impact the clearance of remnant lipoproteins from plasma and promote lipoprotein retention in capillaries. Apolipoproteins (Apo) and angiopoietin-like proteins (ANGPTLs) play a crucial role in regulating LPL activity and recent insights into LPL regulation may elucidate new pharmacological means to address the challenge of hypertriglyceridemia in atherosclerosis development.