Determination of lipoprotein lipase activity using a novel fluorescent lipase assay

Tirzepatide modulates the regulation of adipocyte nutrient metabolism through long-acting activation of the GIP receptor

Tirzepatide, a glucose-dependent insulinotropic polypeptide/glucagon-like peptide 1 receptor (GIPR/GLP-1R) agonist, has, in clinical trials, demonstrated greater reductions in glucose, body weight, and triglyceride levels compared with selective GLP-1R agonists in people with type 2 diabetes (T2D). However, cellular mechanisms by which GIPR agonism may contribute to these improved efficacy outcomes have not been fully defined. Using human adipocyte and mouse models, we investigated how long-acting GIPR agonists regulate fasted and fed adipocyte functions. In functional assays, GIPR agonism enhanced insulin signaling, augmented glucose uptake, and increased the conversion of glucose to glycerol in a cooperative manner with insulin; however, in the absence of insulin, GIPR agonists increased lipolysis. In diet-induced obese mice treated with a long-acting GIPR agonist, circulating triglyceride levels were reduced during oral lipid challenge, and lipoprotein-derived fatty acid uptake into adipose tissue was increased. Our findings support a model for long-acting GIPR agonists to modulate both fasted and fed adipose tissue function differentially by cooperating with insulin to augment glucose and lipid clearance in the fed state while enhancing lipid release when insulin levels are reduced in the fasted state.

A miniaturised semi-dynamic in-vitro model of human digestion

Reliable in-vitro digestion models that are able to successfully replicate the conditions found in the human gastrointestinal tract are key to assess the fate and efficiency of new formulations aimed for oral consumption. However, current in-vitro models either lack the capability to replicate crucial dynamics of digestion or require large volumes of sample/reagents, which can be scarce when working with nanomaterials under development. Here, we propose a miniaturised digestion system, a digestion-chip, based on incubation chambers integrated on a polymethylmethacrylate device. The digestion-chip incorporates key dynamic features of human digestion, such as gradual acidification and gradual addition of enzymes and simulated fluids in the gastric phase, and controlled gastric emptying, while maintaining low complexity and using small volumes of sample and reagents. In addition, the new approach integrates real-time automated closed-loop control of two key parameters, pH and temperature, during the two main phases of digestion (gastric and intestinal) with an accuracy down to ± 0.1 °C and ± 0.2 pH points. The experimental results demonstrate that the digestion-chip successfully replicates the gold standard static digestion INFOGEST protocol and that the semi-dynamic digestion kinetics can be reliably fitted to a first kinetic order model. These devices can be easily adapted to dynamic features in an automated, sensorised, and inexpensive platform and will enable reliable, low-cost and efficient assessment of the bioaccessibility of new and expensive drugs, bioactive ingredients or nanoengineered materials aimed for oral consumption, thereby avoiding unnecessary animal testing.

The GPIHBP1-LPL complex and its role in plasma triglyceride metabolism: Insights into chylomicronemia

GPIHBP1 is a protein found in the endothelial cells of capillaries that is anchored by glycosylphosphatidylinositol and binds to high-density lipoproteins. GPIHBP1 attaches to lipoprotein lipase (LPL), subsequently carrying the enzyme and anchoring it to the capillary lumen. Enabling lipid metabolism is essential for the marginalization of lipoproteins alongside capillaries. Studies underscore the significance of GPIHBP1 in transporting, stabilizing, and aiding in the marginalization of LPL. The intricate interplay between GPIHBP1 and LPL has provided novel insights into chylomicronemia in recent years. Mutations hindering the formation or reducing the efficiency of the GPIHBP1-LPL complex are central to the onset of chylomicronemia. This review delves into the structural nuances of the GPIHBP1-LPL interaction, the consequences of mutations in the complex leading to chylomicronemia, and cutting-edge advancements in chylomicronemia treatment.

Preclinical Development and Characterization of Novel Adeno-Associated Viral Vectors for the Treatment of Lipoprotein Lipase Deficiency

Lipoprotein lipase deficiency (LPLD) results from mutations within the lipoprotein lipase (LPL) gene that lead to a complete lack of catalytically active LPL protein. Glybera was one of the first adeno-associated virus (AAV) gene replacement therapy to receive European Medicines Agency regulatory approval for the treatment of LPLD. However, Glybera is no longer marketed potentially due to a combination of economical, manufacturing, and vector-related issues. The aim of this study was to develop a more efficacious AAV gene therapy vector for LPLD. Following preclinical biodistribution, efficacy and non-Good Laboratory Practice toxicity studies with novel AAV1 and AAV8-based vectors in mice, we identified AAV8 pVR59. AAV8 pVR59 delivered a codon-optimized, human gain-of-function hLPLS447X transgene driven by a CAG promoter in an AAV8 capsid. AAV8 pVR59 was significantly more efficacious, at 10- to 100-fold lower doses, compared with an AAV1 vector based on Glybera, when delivered intramuscularly or intravenously, respectively, in mice with LPLD. Efficient gene transfer was observed within the injected skeletal muscle and liver following delivery of AAV8 pVR59, with long-term correction of LPLD phenotypes, including normalization of plasma triglycerides and lipid tolerance, for up to 6 months post-treatment. While intramuscular delivery of AAV8 pVR59 was well tolerated, intravenous administration augmented liver pathology. These results highlight the feasibility of developing a superior AAV vector for the treatment of LPLD and provide critical insight for initiating studies in larger animal models. The identification of an AAV gene therapy vector that is more efficacious at lower doses, when paired with recent advances in production and manufacturing technologies, will ultimately translate to increased safety and accessibility for patients.

Role of lipoprotein lipase activity measurement in the diagnosis of familial chylomicronemia syndrome

BACKGROUND

Activity assays for lipoprotein lipase (LPL) are not standardised for use in clinical settings.

OBJECTIVE

This study sought to define and validate a cut-off points based on a ROC curve for the diagnosis of patients with familial chylomicronemia syndrome (FCS). We also evaluated the role of LPL activity in a comprehensive FCS diagnostic workflow.

METHODS

A derivation cohort (including an FCS group (n = 9), a multifactorial chylomicronemia syndrome (MCS) group (n = 11)), and an external validation cohort (including an FCS group (n = 5), a MCS group (n = 23) and a normo-triglyceridemic (NTG) group (n = 14)), were studied. FCS patients were previously diagnosed by the presence of biallelic pathogenic genetic variants in the LPL and GPIHBP1 genes. LPL activity was also measured. Clinical and anthropometric data were recorded, and serum lipids and lipoproteins were measured. Sensitivity, specificity and cut-offs for LPL activity were obtained from a ROC curve and externally validated.

RESULTS

All post-heparin plasma LPL activity in the FCS patients were below 25.1 mU/mL, that was cut-off with best performance. There was no overlap in the LPL activity distributions between the FCS and MCS groups, conversely to the FCS and NTG groups.

CONCLUSION

We conclude that, in addition to genetic testing, LPL activity in subjects with severe hypertriglyceridemia is a reliable criterium in the diagnosis of FCS when using a cut-off of 25.1 mU/mL (25% of the mean LPL activity in the validation MCS group). We do not recommend the NTG patient based cut-off values due to low sensitivity.

Identification of a Gain-of-Function LIPC Variant as a Novel Cause of Familial Combined Hypocholesterolemia

BACKGROUND

Atherosclerotic cardiovascular disease is the main cause of mortality worldwide and is strongly influenced by circulating low-density lipoprotein (LDL) cholesterol levels. Only a few genes causally related to plasma LDL cholesterol levels have been identified so far, and only 1 gene, ANGPTL3, has been causally related to combined hypocholesterolemia. Here, our aim was to elucidate the genetic origin of an unexplained combined hypocholesterolemia inherited in 4 generations of a French family.

METHODS

Using next-generation sequencing, we identified a novel dominant rare variant in the LIPC gene, encoding for hepatic lipase, which cosegregates with the phenotype. We characterized the impact of this LIPC-E97G variant on circulating lipid and lipoprotein levels in family members using nuclear magnetic resonance-based lipoprotein profiling and lipidomics. To uncover the mechanisms underlying the combined hypocholesterolemia, we used protein homology modeling, measured triglyceride lipase and phospholipase activities in cell culture, and studied the phenotype of APOE*3.Leiden.CETP mice after LIPC-E97G overexpression.

RESULTS

Family members carrying the LIPC-E97G variant had very low circulating levels of LDL cholesterol and high-density lipoprotein cholesterol, LDL particle numbers, and phospholipids. The lysophospholipids/phospholipids ratio was increased in plasma of LIPC-E97G carriers, suggestive of an increased lipolytic activity on phospholipids. In vitro and in vivo studies confirmed that the LIPC-E97G variant specifically increases the phospholipase activity of hepatic lipase through modification of an evolutionarily conserved motif that determines substrate access to the hepatic lipase catalytic site. Mice overexpressing human LIPC-E97G recapitulated the combined hypocholesterolemic phenotype of the family and demonstrated that the increased phospholipase activity promotes catabolism of triglyceride-rich lipoproteins by different extrahepatic tissues but not the liver.

CONCLUSIONS

We identified and characterized a novel rare variant in the LIPC gene in a family who presents with dominant familial combined hypocholesterolemia. This gain-of-function variant makes LIPC the second identified gene, after ANGPTL3, causally involved in familial combined hypocholesterolemia. Our mechanistic data highlight the critical role of hepatic lipase phospholipase activity in LDL cholesterol homeostasis and suggest a new LDL clearance mechanism.

A Critical Evaluation of Terpenoid Signaling at Cannabinoid CB1 Receptors in a Neuronal Model

In addition to phytocannabinoids, cannabis contains terpenoids that are claimed to have a myriad of effects on the body. We tested a panel of five common cannabis terpenoids, myrcene, linalool, limonene, α-pinene and nerolidol, in two neuronal models, autaptic hippocampal neurons and dorsal root ganglion (DRG) neurons. Autaptic neurons express a form of cannabinoid CB1 receptor-dependent retrograde plasticity while DRGs express a variety of transient receptor potential (TRP) channels. Most terpenoids had little or no effect on neuronal cannabinoid signaling. The exception was nerolidol, which inhibited endocannabinoid signaling. Notably, this is not via inhibition of CB1 receptors but by inhibiting some aspect of 2-arachidonoylglycerol (2-AG) production/delivery; the mechanism does not involve reducing the activity of the 2-AG-synthesizing diacylglycerol lipases (DAGLs). Nerolidol was also the only terpenoid that activated a sustained calcium response in a small (7%) subpopulation of DRGs. In summary, we found that only one of five terpenoids tested had notable effects on cannabinoid signaling in two neuronal models. Our results suggest that a few terpenoids may indeed interact with some components of the cannabinoid signaling system and may therefore offer interesting insights upon further study.

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.

Adipocyte-specific tribbles pseudokinase 1 regulates plasma adiponectin and plasma lipids in mice

OBJECTIVE

Multiple genome-wide association studies (GWAS) have identified SNPs in the 8q24 locus near TRIB1 that are significantly associated with plasma lipids and other markers of cardiometabolic health, and prior studies have revealed the roles of hepatic and myeloid Trib1 in plasma lipid regulation and atherosclerosis. The same 8q24 SNPs are additionally associated with plasma adiponectin levels in humans, implicating TRIB1 in adipocyte biology. Here, we hypothesize that TRIB1 in adipose tissue regulates plasma adiponectin, lipids, and metabolic health.

METHODS

We investigate the metabolic phenotype of adipocyte-specific Trib1 knockout mice (Trib1_ASKO) fed on chow and high-fat diet (HFD). Through secretomics of adipose tissue explants and RNA-seq of adipocytes and livers from these mice, we further investigate the mechanism of TRIB1 in adipose tissue.

RESULTS

Trib1_ASKO mice have an improved metabolic phenotype with increased plasma adiponectin levels, improved glucose tolerance, and decreased plasma lipids. Trib1_ASKO adipocytes have increased adiponectin production and secretion independent of the known TRIB1 function of regulating proteasomal degradation. RNA-seq analysis of adipocytes and livers from Trib1_ASKO mice indicates that alterations in adipocyte function underlie the observed plasma lipid changes. Adipose tissue explant secretomics further reveals that Trib1_ASKO adipose tissue has decreased ANGPTL4 production, and we demonstrate an accompanying increase in the lipoprotein lipase (LPL) activity that likely underlies the triglyceride phenotype.

CONCLUSIONS

This study shows that adipocyte Trib1 regulates multiple aspects of metabolic health, confirming previously observed genetic associations in humans and shedding light on the further mechanisms by which TRIB1 regulates plasma lipids and metabolic health.

A simple, rapid, and sensitive fluorescence-based method to assess triacylglycerol hydrolase activity

Lipases constitute an important class of water-soluble enzymes that catalyze the hydrolysis of hydrophobic triacylglycerol (TAG). Their enzymatic activity is typically measured using multistep procedures involving isolation and quantification of the hydrolyzed products. We report here a new fluorescence method to measure lipase activity in real time that does not require the separation of substrates from products. We developed this method using adipose triglyceride lipase (ATGL) and lipoprotein lipase (LpL) as model lipases. We first incubated a source of ATGL or LpL with substrate vesicles containing nitrobenzoxadiazole (NBD)-labeled TAG, then measured increases in NBD fluorescence, and calculated enzyme activities. Incorporation of NBD-TAG into phosphatidylcholine (PC) vesicles resulted in some hydrolysis; however, incorporation of phosphatidylinositol into these NBD-TAG/PC vesicles and increasing the ratio of NBD-TAG to PC greatly enhanced substrate hydrolysis. This assay was also useful in measuring the activity of pancreatic lipase and hormone-sensitive lipase. Next, we tested several small-molecule lipase inhibitors and found that orlistat inhibits all lipases, indicating that it is a pan-lipase inhibitor. In short, we describe a simple, rapid, fluorescence-based triacylglycerol hydrolysis assay to assess four major TAG hydrolases: intracellular ATGL and hormone-sensitive lipase, LpL localized at the extracellular endothelium, and pancreatic lipase present in the intestinal lumen. The major advantages of this method are its speed, simplicity, and elimination of product isolation. This assay is potentially applicable to a wide range of lipases, is amenable to high-throughput screening to discover novel modulators of triacylglycerol hydrolases, and can be used for diagnostic purposes.

CREBH normalizes dyslipidemia and halts atherosclerosis in diabetes by decreasing circulating remnant lipoproteins

Loss-of-function mutations in the transcription factor CREB3L3 (CREBH) associate with severe hypertriglyceridemia in humans. CREBH is believed to lower plasma triglycerides by augmenting the action of lipoprotein lipase (LPL). However, by using a mouse model of type 1 diabetes mellitus (T1DM), we found that greater liver expression of active CREBH normalized both elevated plasma triglycerides and cholesterol. Residual triglyceride-rich lipoprotein (TRL) remnants were enriched in apolipoprotein E (APOE) and impoverished in APOC3, an apolipoprotein composition indicative of increased hepatic clearance. The underlying mechanism was independent of LPL as CREBH reduced both triglycerides and cholesterol in LPL-deficient mice. Instead, APOE was critical for CREBH's ability to lower circulating remnant lipoproteins because it failed to reduce TRL cholesterol in Apoe-/- mice. Importantly, humans with CREB3L3 loss-of-function mutations exhibited increased levels of remnant lipoproteins that were deprived of APOE. Recent evidence suggests that impaired clearance of TRL remnants promotes cardiovascular disease in patients with T1DM. Consistently, we found that hepatic expression of CREBH prevented the progression of diabetes-accelerated atherosclerosis. Our results support the proposal that CREBH acts through an APOE-dependent pathway to increase hepatic clearance of remnant lipoproteins. They also implicate elevated levels of remnants in the pathogenesis of atherosclerosis in T1DM.

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.

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.

High-Throughput, Fluorescence-Based Esterase Activity Assay for Assessing Polysorbate Degradation Risk during Biopharmaceutical Development

PURPOSE

Hydrolytic degradation of polysorbate during 2-8°C storage of monoclonal antibody drug products has been attributed to residual enzymes (e.g., esterases) from bioprocessing steps. Robust detection of esterase activity using sensitive, non-polysorbate surrogate substrates can provide an alternate method to assess polysorbate degradation risk, if the correlation between the esterase activity and polysorbate degradation is established.

METHODS

A general esterase activity assay was developed as a monitoring and characterization tool during bioprocess development of monoclonal antibodies.

RESULTS

We report a fluorescence plate-based assay for quantifying esterase activity, utilizing 4-methylumbelliferyl caprylate (MU-C8) as the esterase substrate. The assay was first assessed for substrate, inhibitor and pH specificity using both model enzymes and purified protein samples. The assay was then extensively tested to understand sample matrix effects on activity rates.

CONCLUSIONS

The use of this high-throughput method will allow for rapid characterization of protein samples in under three hours. The esterase activity correlated directly with polysorbate degradation and can provide valuable information on polysorbate degradation risk throughout drug development.

Effects of different oxygen tensions on differentiated human preadipocytes lipid storage and mobilisation

Adipose tissue expansion has been suggested to impair oxygen (O₂) diffusion in the adipose tissue and cause hypoxia. This study aimed at characterising the effects of hypoxia on adipocyte lipid storage and mobilisation functions. Human preadipocytes were exposed to different O₂ tensions (3, 10 and 21%) either acutely for 24 h after differentiation (acute exposure) or during differentiation (14d, chronic hypoxia). Lipoprotein lipase (LPL) activity was decreased dose-dependently by both acute and chronic hypoxia (p < .05). Acute exposure to 3, and 10% O₂ stimulated the expression of lipid storage gene, while chronic exposure to 3% O₂ inhibited the expression of genes involved in lipid storage and mobilisation (p < .05). Acute hypoxia dose-dependently stimulated basal lipolysis. Conversely, chronic hypoxia did not affect basal lipolysis but significantly decreased isoproterenol-stimulated lipolysis (p < .05). In conclusion, the effects of hypoxia on human adipocyte lipid storage and mobilisation functions are complex but could favour ectopic fat deposition.

Identification of Key Excipients for the Solubilization and Structural Characterization of Lipoprotein Lipase, An Enzyme for Hydrolysis of Triglyceride

Lipoprotein lipase (LPL) is an essential enzyme that hydrolyzes triglycerides in chylomicrons and very low-density lipoprotein into glycerol and fatty acids. One major hurdle in using LPL as a therapeutic has been its poor solubility/stability after purification. Solutions used to preserve purified LPL commonly contain either heparin, or concentrated glycerol and sodium chloride, resulting in hypertonic solutions. These solutions are not acceptable as pharmaceutical formulations. This paper describes the identification of a key excipient, sodium laurate, which can solubilize LPL in an isotonic environment without heparin or concentrated glycerol. A follow-up multi-variant study was performed to identify the effect of sodium laurate and its interaction with sodium chloride on the solubility and processing conditions of LPL. The LPL concentration (up to 14 mg/mL) achievable in pharmaceutically relevant and salt-free conditions was identified to be closely correlated to the concentration of sodium laurate, which was co-concentrated with LPL. The result that sodium laurate increases stability of LPL characterized by differential scanning calorimetry and UV absorbance spectra suggests that the mechanism of solubilization of LPL by sodium laurate is related to LPL structural stabilization. The findings indicate that substrates and their enzymatic products can be strong stabilizers for other protein molecules.

Angiopoietin-like protein 8 differentially regulates ANGPTL3 and ANGPTL4 during postprandial partitioning of fatty acids

Angiopoietin-like protein (ANGPTL)8 has been implicated in metabolic syndrome and reported to regulate adipose FA uptake through unknown mechanisms. Here, we studied how complex formation of ANGPTL8 with ANGPTL3 or ANGPTL4 varies with feeding to regulate LPL. In human serum, ANGPTL3/8 and ANGPTL4/8 complexes both increased postprandially, correlated negatively with HDL, and correlated positively with all other metabolic syndrome markers. ANGPTL3/8 also correlated positively with LDL-C and blocked LPL-facilitated hepatocyte VLDL-C uptake. LPL-inhibitory activity of ANGPTL3/8 was >100-fold more potent than that of ANGPTL3, and LPL-inhibitory activity of ANGPTL4/8 was >100-fold less potent than that of ANGPTL4. Quantitative analyses of inhibitory activities and competition experiments among the complexes suggested a model in which localized ANGPTL4/8 blocks the LPL-inhibitory activity of both circulating ANGPTL3/8 and localized ANGPTL4, allowing lipid sequestration into fat rather than muscle during the fed state. Supporting this model, insulin increased ANGPTL3/8 secretion from hepatocytes and ANGPTL4/8 secretion from adipocytes. These results suggest that low ANGPTL8 levels during fasting enable ANGPTL4-mediated LPL inhibition in fat tissue to minimize adipose FA uptake. During feeding, increased ANGPTL8 increases ANGPTL3 inhibition of LPL in muscle via circulating ANGPTL3/8, while decreasing ANGPTL4 inhibition of LPL in adipose tissue through localized ANGPTL4/8, thereby increasing FA uptake into adipose tissue. Excessive caloric intake may shift this system toward the latter conditions, possibly predisposing to metabolic syndrome.

Fluorescent substrates for haloalkane dehalogenases: Novel probes for mechanistic studies and protein labeling

Haloalkane dehalogenases are enzymes that catalyze the cleavage of carbon-halogen bonds in halogenated compounds. They serve as model enzymes for studying structure-function relationships of >100.000 members of the α/β-hydrolase superfamily. Detailed kinetic analysis of their reaction is crucial for understanding the reaction mechanism and developing novel concepts in protein engineering. Fluorescent substrates, which change their fluorescence properties during a catalytic cycle, may serve as attractive molecular probes for studying the mechanism of enzyme catalysis. In this work, we present the development of the first fluorescent substrates for this enzyme family based on coumarin and BODIPY chromophores. Steady-state and pre-steady-state kinetics with two of the most active haloalkane dehalogenases, DmmA and LinB, revealed that both fluorescent substrates provided specificity constant two orders of magnitude higher (0.14-12.6 μM^-1 s^-1) than previously reported representative substrates for the haloalkane dehalogenase family (0.00005-0.014 μM^-1 s^-1). Stopped-flow fluorescence/FRET analysis enabled for the first time monitoring of all individual reaction steps within a single experiment: (i) substrate binding, (ii-iii) two subsequent chemical steps and (iv) product release. The newly introduced fluorescent molecules are potent probes for fast steady-state kinetic profiling. In combination with rapid mixing techniques, they provide highly valuable information about individual kinetic steps and mechanism of haloalkane dehalogenases. Additionally, these molecules offer high specificity and efficiency for protein labeling and can serve as probes for studying protein hydration and dynamics as well as potential markers for cell imaging.

Simple and rapid real-time monitoring of LPL activity in vitro

Since elevated plasma triglycerides are an independent risk factor for cardiovascular diseases, lipoprotein lipase (LPL) is an interesting target for drug development. However, investigation of LPL remains challenging, as most of the commercially available assays are limited to the determination of LPL activity. Thus, we focused on the evaluation of a simple in vitro real-time fluorescence assay for the measurement of LPL activity that can be combined with additional cell or molecular biological assays in the same cell sample. Our procedure allows for a more comprehensive characterization of potential regulatory compounds targeting the LPL system.

The presented assay procedure provides several advantages over currently available commercial in vitro LPL activity assays:1.

12-well cell culture plate design for the simultaneous investigation of up to three different compounds of interest (including all assay controls).

2.

24 h real-time acquisition of LPL activity for the identification of the optimal time point for further measurements.

3.

Measurement of LPL activity can be supplemented by additional cell or molecular biological assays in the same cell sample.

A novel NanoBiT-based assay monitors the interaction between lipoprotein lipase and GPIHBP1 in real time

The hydrolysis of triglycerides in triglyceride-rich lipoproteins by LPL is critical for the delivery of triglyceride-derived fatty acids to tissues, including heart, skeletal muscle, and adipose tissues. Physiologically active LPL is normally bound to the endothelial cell protein glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein 1 (GPIHBP1), which transports LPL across endothelial cells, anchors LPL to the vascular wall, and stabilizes LPL activity. Disruption of LPL-GPIHBP1 binding significantly alters triglyceride metabolism and lipid partitioning. In this study, we modified the NanoLuc® Binary Technology split-luciferase system to develop a novel assay that monitors the binding of LPL to GPIHBP1 on endothelial cells in real time. We validated the specificity and sensitivity of the assay using endothelial lipase and a mutant version of LPL and found that this assay reliably and specifically detected the interaction between LPL and GPIHBP1. We then interrogated various endogenous and exogenous inhibitors of LPL-mediated lipolysis for their ability to disrupt the binding of LPL to GPIHBP1. We found that angiopoietin-like (ANGPTL)4 and ANGPTL3-ANGPTL8 complexes disrupted the interactions of LPL and GPIHBP1, whereas the exogenous LPL blockers we tested (tyloxapol, poloxamer-407, and tetrahydrolipstatin) did not. We also found that chylomicrons could dissociate LPL from GPIHBP1 and found evidence that this dissociation was mediated in part by the fatty acids produced by lipolysis. These results demonstrate the ability of this assay to monitor LPL-GPIHBP1 binding and to probe how various agents influence this important complex.

Endocytosis of very low-density lipoproteins: an unexpected mechanism for lipid acquisition by breast cancer cells

We previously described the expression of CD36 and LPL by breast cancer (BC) cells and tissues and the growth-promoting effect of VLDL observed only in the presence of LPL. We now report a model in which LPL is bound to a heparan sulfate proteoglycan motif on the BC cell surface and acts in concert with the VLDL receptor to internalize VLDLs via receptor-mediated endocytosis. We also demonstrate that gene-expression programs for lipid synthesis versus uptake respond robustly to triglyceride-rich lipoprotein availability. The literature emphasizes de novo FA synthesis and exogenous free FA uptake using CD36 as paramount mechanisms for lipid acquisition by cancer cells. We find that the uptake of intact lipoproteins is also an important mechanism for lipid acquisition and that the relative reliance on lipid synthesis versus uptake varies among BC cell lines and in response to VLDL availability. This metabolic plasticity has important implications for the development of therapies aimed at the lipid dependence of many types of cancer, in that the inhibition of FA synthesis may elicit compensatory upregulation of lipid uptake. Moreover, the mechanism that we have elucidated provides a direct connection between dietary fat and tumor biology.-.

Direct observation of Thermomyces lanuginosus lipase diffusional states by Single Particle Tracking and their remodeling by mutations and inhibition

Lipases are interfacially activated enzymes that catalyze the hydrolysis of ester bonds and constitute prime candidates for industrial and biotechnological applications ranging from detergent industry, to chiral organic synthesis. As a result, there is an incentive to understand the mechanisms underlying lipase activity at the molecular level, so as to be able to design new lipase variants with tailor-made functionalities. Our understanding of lipase function primarily relies on bulk assay averaging the behavior of a high number of enzymes masking structural dynamics and functional heterogeneities. Recent advances in single molecule techniques based on fluorogenic substrate analogues revealed the existence of lipase functional states, and furthermore so how they are remodeled by regulatory cues. Single particle studies of lipases on the other hand directly observed diffusional heterogeneities and suggested lipases to operate in two different modes. Here to decipher how mutations in the lid region controls Thermomyces lanuginosus lipase (TLL) diffusion and function we employed a Single Particle Tracking (SPT) assay to directly observe the spatiotemporal localization of TLL and rationally designed mutants on native substrate surfaces. Parallel imaging of thousands of individual TLL enzymes and HMM analysis allowed us to observe and quantify the diffusion, abundance and microscopic transition rates between three linearly interconverting diffusional states for each lipase. We proposed a model that correlate diffusion with function that allowed us to predict that lipase regulation, via mutations in lid region or product inhibition, primarily operates via biasing transitions to the active states.

No interactions between heparin and atacicept, an antagonist of B cell survival cytokines

Background and Purpose

The TNF family ligands, B cell activating factor of the TNF family (BAFF, also known as B lymphocyte stimulator, BLyS) and a proliferation‐inducing ligand (APRIL), share the transmembrane activator and calcium‐modulator and cyclophilin ligand (CAML)‐interactor (TACI) as one of their common receptors. Atacicept, a chimeric recombinant TACI/IgG1‐Fc fusion protein, inhibits both ligands. TACI and APRIL also bind to proteoglycans and to heparin that is structurally related to proteoglycans. It is unknown whether the portion of TACI contained in atacicept can bind directly to proteoglycans, or indirectly via APRIL, and whether this could interfere with the anti‐coagulant properties of heparin.

Experimental Approach

Binding of atacicept and APRIL to proteoglycan‐positive cells was measured by FACS. Activities of heparin and atacicept were measured with activated factor Xa inhibition and cell‐based assays. Effects of heparin on circulating atacicept was monitored in mice.

Key Results

Atacicept did not bind to proteoglycan‐positive cells, but when complexed to APRIL could do so indirectly via APRIL. Multimers of atacicept obtained after exposure to cysteine or BAFF 60‐mer bound directly to proteoglycans. Atacicept alone, or in complex with APRIL, or in a multimeric form did not interfere with heparin activity in vitro. Conversely, heparin did not influence inhibition of BAFF and APRIL by atacicept and did not change circulating levels of atacicept.

Conclusions and Implications

Lack of detectable interference of APRIL‐bound or free atacicept on heparin activity makes it unlikely that atacicept at therapeutic doses will interfere with the function of heparin in vivo.

The Effect of Acute Continuous Hypoxia on Triglyceride Levels in Constantly Fed Healthy Men

Introduction

Elevated plasma triglyceride (TG) concentrations are an important contributor to deleterious metabolic alterations. Evidence in animals suggest that acute exposure to an environment with reduced oxygen inhibits plasma TG clearance and causes important rise in plasma TG, especially in the postprandial state. The objective of this study was to characterize the effects of an acute exposure to normobaric hypoxia on prandial TG levels in 2 distinct lipoprotein subtypes in healthy humans: chylomicrons which are secreted by the intestine and carry dietary lipids, and denser TG carriers (mainly VLDL), which are secreted by the liver and carry endogenous lipids. Plasma lipolytic activity was also assessed. It was hypothesized that hypoxia would reduce prandial plasma lipolytic activity and raise prandial TG levels in both lipoprotein subtypes.

Methods

Using a randomized crossover design, 9 healthy young men were studied for 6 h in a constantly fed state while being exposed to either normobaric hypoxia (FiO₂ = 0.12) and normoxic conditions on two different days. Prandial glucose, TG, non-esterified fatty acid (NEFA), and post-heparin plasma lipolytic activity were measured during each session.

Results

Six hours of exposure to hypoxia marginally increase prandial glycemia (+5%, p = 0.06) while increasing insulinemia by 40% (p = 0.04). Hypoxia induced a 30% rise in prandial NEFA levels and tended to slightly increased total prandial TG levels by 15% (p = 0.11). No difference was observed in TG concentrations and metabolism of chylomicrons between conditions. However, TG in the VLDL containing fraction decreased significantly overtime under normoxia but not under hypoxia (time × condition interaction, p = 0.02). No difference was observed in post-heparin plasmatic lipolytic activity between conditions.

Conclusion

Acute hypoxia in healthy men tends to increase prandial VLDL-TG levels. These results lend support to the increased blood lipid levels reported in animals exposed acutely to lower partial pressures of oxygen.

No effect of acute normobaric hypoxia on plasma triglyceride levels in fasting healthy men

Circulating fatty acids are a major systemic energy source in the fasting state as well as a determinant of hepatic triglycerides (TG)-rich very-low-density lipoprotein production. Upon acute hypoxia, sympathetic arousal induces adipose tissue lipolysis, resulting in an increase in circulating nonesterified fatty acids (NEFA). Animal studies suggest that TG clearance may also be strongly reduced under hypoxia, though this effect has been shown to be dependent on temperature. Whether the hypoxia-induced rise in blood fatty acid concentrations affects fasting TG levels in humans under thermoneutral conditions remains unknown. TG, NEFA, and glycerol levels were measured in fasted healthy young men (n = 10) exposed for 6 h to either normoxia (ambient air) or acute hypoxia (fraction of inspired oxygen = 0.12) in a randomized, crossover design. Participants were casually clothed and rested in front of a fan in an environmental chamber maintained at 28 °C during each trial. Under hypoxia, a significantly greater increase in NEFA occurred (condition × time interaction, p = 0.049) and glycerol levels tended to be higher (condition × time, p = 0.104), suggesting an increase in adipose tissue lipolysis. However, plasma TG levels did not change over time and did not differ between the normoxia and hypoxia conditions. In conclusion, acute exposure to normobaric hypoxia under thermoneutral condition in healthy men during fasting state increased lipolysis without affecting circulating TG.

Cell therapy could be a potential way to improve lipoprotein lipase deficiency

BACKGROUND

Lipoprotein lipase (LPL) deficiency is an autosomal recessive genetic disorder characterized by extreme hypertriglyceridemia, with no cure presently available. The purpose of this study was to test the possibility of using cell therapy to alleviate LPL deficiency.

METHODS

The LPL coding sequence was cloned into the MSCV retrovirus vector, after which MSCV-hLPL and MSCV (empty construct without LPL coding sequence) virion suspensions were made using the calcium chloride method. A muscle cell line (C2C12), kidney cell line (HEK293T) and pre-adipocyte cell line (3 T3-L1) were transfected with the virus in order to express recombinant LPL in vitro. Finally, each transfected cell line was injected subcutaneously into nude mice to identify the cell type which could secret recombinant LPL in vivo. Control cells were transfected with the MSCV empty vector. LPL activity was analyzed using a radioimmunoassay.

RESULTS

After virus infection, the LPL activity at the cell surface of each cell type was significantly higher than in the control cells, which indicates that all three cell types can be used to generate functional LPL. The transfected cells were injected subcutaneously into nude mice, and the LPL activity of the nearby muscle tissue at the injection site in mice injected with 3 T3-L1 cells was more than 5 times higher at the injection sites than at non-injected control sites. The other two types of cells did not show this trend.

CONCLUSION

The subcutaneous injection of adipocytes overexpressing LPL can improve the LPL activity of the adjacent tissue of nude mice. This is a ground-breaking preliminary study for the treatment of LPL deficiency, and lays a good foundation for using cell therapy to correct LPL deficiency.

ANGPTL8 promotes the ability of ANGPTL3 to bind and inhibit lipoprotein lipase

OBJECTIVE

Several members of the angiopoietin-like (ANGPTL) family of proteins, including ANGPTL3 and ANGPTL8, regulate lipoprotein lipase (LPL) activity. Deficiency in either ANGPTL3 or ANGPTL8 reduces plasma triglyceride levels and increases LPL activity, whereas overexpression of either protein does the opposite. Recent studies suggest that ANGPTL8 may functionally interact with ANGPTL3 to alter clearance of plasma triglycerides; however, the nature of this interaction has remained elusive. We tested the hypothesis that ANGPTL8 forms a complex with ANGPTL3 and that this complex is necessary for the inhibition of vascular LPL by ANGPTL3.

METHODS

We analyzed the interactions of ANGPTL3 and ANGPTL8 with each other and with LPL using co-immunoprecipitation, western blotting, lipase activity assays, and the NanoBiT split-luciferase system. We also used adenovirus injection to overexpress ANGPTL3 in mice that lacked ANGPTL8.

RESULTS

We found that ANGPTL3 or ANGPTL8 alone could only inhibit LPL at concentrations that far exceeded physiological levels, especially when LPL was bound to its endothelial cell receptor/transporter GPIHBP1 (glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein 1). Physical interaction was observed between ANGPTL3 and ANGPTL8 when the proteins were co-expressed, and co-expression with ANGPTL3 greatly enhanced the secretion of ANGPTL8. Importantly, ANGPTL3-ANGPTL8 complexes had a dramatically increased ability to inhibit LPL compared to either protein alone. Adenovirus experiments showed that 2-fold overexpression of ANGPTL3 significantly increased plasma triglycerides only in the presence of ANGPTL8. Protein interaction assays showed that ANGPTL8 greatly increased the ability of ANGPTL3 to bind LPL.

CONCLUSIONS

Together, these data indicate that ANGPTL8 binds to ANGPTL3 and that this complex is necessary for ANGPTL3 to efficiently bind and inhibit LPL.

Endoplasmic reticulum (ER) stress and cAMP/PKA pathway mediated Zn-induced hepatic lipolysis

The present study was performed to determine the effect of Zn exposure influencing endoplasmic reticulum (ER) stress, explore the underlying molecular mechanism of Zn-induced hepatic lipolysis in a fish species of significance for aquaculture, yellow catfish Pelteobagrus fulvidraco. We found that waterborne Zn exposure evoked ER stress and unfolded protein response (UPR), and activated cAMP/PKA pathway, and up-regulated hepatic lipolysis. The increase in ER stress and lipolysis were associated with activation of cAMP/PKA signaling pathway. Zn also induced an increase in intracellular Ca²⁺ level, which could be partially prevented by dantrolene (RyR receptor inhibitor) and 2-APB (IP3 receptor inhibitor), demonstrating that the disturbed Ca²⁺ homeostasis in ER contributed to ER stress and dysregulation of lipolysis. Inhibition of ER stress by PBA attenuated UPR, inhibited the activation of cAMP/PKA pathway and resulted in down-regulation of lipolysis. Inhibition of protein kinase RNA-activated-like ER kinase (PERK) by GSK2656157 and inositol-requiring enzyme (IRE) by STF-083010 differentially influenced Zn-induced changes of lipid metabolism, indicating that PERK and IRE pathways played different regulatory roles in Zn-induced lipolysis. Inhibition of PKA by H89 blocked the Zn-induced activation of cAMP/PKA pathway with a concomitant inhibition of ER stress-mediated lipolysis. Taken together, our findings highlight the importance of the ER stress-cAMP/PKA axis in Zn-induced lipolysis, which provides new insights into Zn toxicology in fish and probably in other vertebrates.

ANGPTL8 Blockade With a Monoclonal Antibody Promotes Triglyceride Clearance, Energy Expenditure, and Weight Loss in Mice

Angiopoietin-like protein (ANGPTL)8 is a negative regulator of lipoprotein lipase-mediated plasma triglyceride (TG) clearance. In this study, we describe a fully human monoclonal antibody (REGN3776) that binds monkey and human ANGPTL8 with high affinity. Inhibition of ANGPTL8 with REGN3776 in humanized ANGPTL8 mice decreased plasma TGs and increased lipoprotein lipase activity. Additionally, REGN3776 reduced body weight and fat content. The reduction in body weight was secondary to increased energy expenditure. Finally, single administration of REGN3776 normalized plasma TGs in dyslipidemic cynomolgus monkeys. In conclusion, we show that blockade of ANGPTL8 with monoclonal antibody strongly reduced plasma TGs in mice and monkeys. These data suggest that inhibition of ANGPTL8 may provide a new therapeutic avenue for the treatment of dyslipidemia with beneficial effects on body weight.

ANGPTL8 requires ANGPTL3 to inhibit lipoprotein lipase and plasma triglyceride clearance

Angiopoietin-like (ANGPTL)3 and ANGPTL8 are secreted proteins and inhibitors of LPL-mediated plasma triglyceride (TG) clearance. It is unclear how these two ANGPTL proteins interact to regulate LPL activity. ANGPTL3 inhibits LPL activity and increases serum TG independent of ANGPTL8. These effects are reversed with an ANGPTL3 blocking antibody. Here, we show that ANGPTL8, although it possesses a functional inhibitory motif, is inactive by itself and requires ANGPTL3 expression to inhibit LPL and increase plasma TG. Using a mutated form of ANGPTL3 that lacks LPL inhibitory activity, we demonstrate that ANGPTL3 activity is not required for its ability to activate ANGPTL8. Moreover, coexpression of ANGPTL3 and ANGPTL8 leads to a far more efficacious increase in TG in mice than ANGPTL3 alone, suggesting the major inhibitory activity of this complex derives from ANGPTL8. An antibody to the C terminus of ANGPTL8 reversed LPL inhibition by ANGPTL8 in the presence of ANGPTL3. The antibody did not disrupt the ANGPTL8:ANGPTL3 complex, but came in close proximity to the LPL inhibitory motif in the N terminus of ANGPTL8. Collectively, these data show that ANGPTL8 has a functional LPL inhibitory motif, but only inhibits LPL and increases plasma TG levels in mice in the presence of ANGPTL3.

Lipoprotein lipase activity and interactions studied in human plasma by isothermal titration calorimetry

LPL hydrolyzes triglycerides in plasma lipoproteins. Due to the complex regulation mechanism, it has been difficult to mimic the physiological conditions under which LPL acts in vitro. We demonstrate that isothermal titration calorimetry (ITC), using human plasma as substrate, overcomes several limitations of previously used techniques. The high sensitivity of ITC allows continuous recording of the heat released during hydrolysis. Both initial rates and kinetics for complete hydrolysis of plasma lipids can be studied. The heat rate was shown to correspond to the release of fatty acids and was linearly related to the amount of added enzyme, either purified LPL or postheparin plasma. Addition of apoC-III reduced the initial rate of hydrolysis by LPL, but the inhibition became less prominent with time when the lipoproteins were triglyceride poor. Addition of angiopoietin-like protein (ANGPTL)3 or ANGPTL4 caused reduction of the activity of LPL via a two-step mechanism. We conclude that ITC can be used for quantitative measurements of LPL activity and interactions under in vivo-like conditions, for comparisons of the properties of plasma samples from patients and control subjects as substrates for LPL, as well as for testing of drug candidates developed with the aim to affect the LPL system.

Effects of cereal soluble dietary fibres on hydrolysis of p-nitrophenyl laurate by pancreatin

The aim of this study was to investigate the effects of cereal soluble dietary fibres (SDFs), β-glucans (BG) from oat and barley as well as arabinoxylans (AX) from wheat and rye, on the lipolysis of p-nitrophenyl laurate (p-NP laurate). p-NP laurate emulsions were prepared in the presence of increasing concentrations of SDFs (0.1%, 1.0% and 1.5% w/v), and lipolysis of emulsions by pancreatic lipase, particle size distribution of the p-NP laurate droplets, and viscosity of emulsions with soluble dietary fibres were measured. It was found that with increasing viscosity of SDFs, the rate of lipolysis decreased while the initial droplet size of the emulsion increased. Rate coefficients were more consistently correlated with average droplet size than with viscosity, suggesting that SDFs inhibited lipolysis primarily by increasing the size of droplets through flocculation, thereby decreasing the available surface area for lipase action.

Simplified assays of lipolysis enzymes for drug discovery and specificity assessment of known inhibitors

Lipids are used as cellular building blocks and condensed energy stores and also act as signaling molecules. The glycerolipid/ fatty acid cycle, encompassing lipolysis and lipogenesis, generates many lipid signals. Reliable procedures are not available for measuring activities of several lipolytic enzymes for the purposes of drug screening, and this resulted in questionable selectivity of various known lipase inhibitors. We now describe simple assays for lipolytic enzymes, including adipose triglyceride lipase (ATGL), hormone sensitive lipase (HSL), sn-1-diacylglycerol lipase (DAGL), monoacylglycerol lipase, α/β-hydrolase domain 6, and carboxylesterase 1 (CES1) using recombinant human and mouse enzymes either in cell extracts or using purified enzymes. We observed that many of the reported inhibitors lack specificity. Thus, Cay10499 (HSL inhibitor) and RHC20867 (DAGL inhibitor) also inhibit other lipases. Marked differences in the inhibitor sensitivities of human ATGL and HSL compared with the corresponding mouse enzymes was noticed. Thus, ATGListatin inhibited mouse ATGL but not human ATGL, and the HSL inhibitors WWL11 and Compound 13f were effective against mouse enzyme but much less potent against human enzyme. Many of these lipase inhibitors also inhibited human CES1. Results describe reliable assays for measuring lipase activities that are amenable for drug screening and also caution about the specificity of the many earlier described lipase inhibitors.

Phenylmethanesulfonyl fluoride pretreatment stabilizes plasma lipidome in lipidomic and metabolomic analysis

Though it is standard practice to test the stability of analytes in the matrix for routine bioanalytical method, stability evaluation is always impractical and skipped in untargeted lipidomic and metabolomic analysis because analytes in these studies are enormous, diverse and sometimes unknown. Lipidome represents a major class of plasma metabolome and shows great potential to be diagnostic and prognostic biomarkers. However, lipidome also faces stability problems because plasma contains kinds of lipid degradation enzyme. Here, using liquid chromatography time of flight mass spectrometry based lipidomic methodology, plasma levels of various lipids including triglyceride (TG), diglyceride (DG), free fatty acid (FFA), phosphatidylethanolamine (PE) phosphatidylcholine (PC), lyso-phosphatidylcholine (LPC), lyso-phosphatidylethanolamine (LPE), and sphingomyelin (SM) were dynamically determined within 4 h at ambient temperature. In mouse and rat plasma, the levels of most TG, DG, PC and PE species significantly decreased with respect to time, whereas those of LPC, LPE and FFA significantly increased with respect to time. However, such changes did not occur in human plasma, thus indicating hepatic lipase and esterase might involve in the species-specified degradation of lipid classes in plasma. Phenylmethanesulfonyl fluoride (PMSF) pretreatment prevented such lipidome instability in mouse plasma. The results suggested the instability of plasma lipidome should be highly concerned, and the enhancement of ex vivo stability of plasma lipidome could enable more reliable clinical translation of lipidomic data for biomarker discovery.

Angiopoietin-like 4 Modifies the Interactions between Lipoprotein Lipase and Its Endothelial Cell Transporter GPIHBP1

The release of fatty acids from plasma triglycerides for tissue uptake is critically dependent on the enzyme lipoprotein lipase (LPL). Hydrolysis of plasma triglycerides by LPL can be disrupted by the protein angiopoietin-like 4 (ANGPTL4), and ANGPTL4 has been shown to inactivate LPL in vitro. However, in vivo LPL is often complexed to glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1) on the surface of capillary endothelial cells. GPIHBP1 is responsible for trafficking LPL across capillary endothelial cells and anchors LPL to the capillary wall during lipolysis. How ANGPTL4 interacts with LPL in this context is not known. In this study, we investigated the interactions of ANGPTL4 with LPL-GPIHBP1 complexes on the surface of endothelial cells. We show that ANGPTL4 was capable of binding and inactivating LPL complexed to GPIHBP1 on the surface of endothelial cells. Once inactivated, LPL dissociated from GPIHBP1. We also show that ANGPTL4-inactivated LPL was incapable of binding GPIHBP1. ANGPTL4 was capable of binding, but not inactivating, LPL at 4 °C, suggesting that binding alone was not sufficient for ANGPTL4's inhibitory activity. We observed that although the N-terminal coiled-coil domain of ANGPTL4 by itself and full-length ANGPTL4 both bound with similar affinities to LPL, the N-terminal fragment was more potent in inactivating both free and GPIHBP1-bound LPL. These results led us to conclude that ANGPTL4 can both bind and inactivate LPL complexed to GPIHBP1 and that inactivation of LPL by ANGPTL4 greatly reduces the affinity of LPL for GPIHBP1.

Post-heparin LPL activity measurement using VLDL as a substrate: a new robust method for routine assessment of plasma triglyceride lipolysis defects

Background

Determination of lipoprotein lipase (LPL) activity is important for hyperchylomicronemia diagnosis, but remains both unreliable and cumbersome with current methods. Consequently by using human VLDL as substrate we developed a new LPL assay which does not require sonication, radioactive or fluorescent particles.

Methods

Post-heparin plasma was added to the VLDL substrate prepared by ultracentrifugation of heat inactivated normolipidemic human serums, diluted in buffer, pH 8.15. Following incubation at 37°c, the NEFA (non esterified fatty acids) produced were assayed hourly for 4 hours. LPL activity was expressed as µmol/l/min after subtraction of hepatic lipase (HL) activity, obtained following LPL inhibition with NaCl 1.5 mmol/l. Molecular analysis of LPL, GPIHBP1, APOA5, APOC2, APOE genes was available for 62 patients.

Results

Our method was reproducible (coefficient of variation (CV): intra-assay 5.6%, inter-assay 7.1%), and tightly correlated with the conventional radiolabelled triolein emulsion method (n = 26, r = 0.88). Normal values were established at 34.8±12.8 µmol/l/min (mean±SD) from 20 control subjects. LPL activities obtained from 71 patients with documented history of major hypertriglyceridemia showed a trimodal distribution. Among the 11 patients with a very low LPL activity (<10 µmol/l/min), 5 were homozygous or compound heterozygous for LPL or GPIHBP1 deleterious mutations, 3 were compound heterozygous for APOA5 deleterious mutations and the p.S19W APOA5 susceptibility variant, and 2 were free of any mutations in the usual candidate genes. No homozygous gene alteration in LPL, GPIHBP1 and APOC2 genes was found in any of the patients with LPL activity >10 µmol/l/min.

Conclusion

This new reproducible method is a valuable tool for routine diagnosis and reliably identifies LPL activity defects.

Measurement of the phospholipase activity of endothelial lipase in mouse plasma

Endothelial lipase (EL) is a major negative regulator of plasma HDL levels in mice, rabbits, and most probably, humans. Although this regulatory function is critically dependent on EL's hydrolysis of HDL phospholipids, as yet there is no phospholipase assay specific for EL in plasma. We developed such an assay for the mouse enzyme using a commercially available phospholipid-like fluorescent substrate in combination with an EL neutralizing antibody. The specificity of the assay was established using EL knockout mice and its utility demonstrated by detection of an increase in plasma EL phospholipase activity following exposure of wild-type mice to lipopolysaccharide. The assay revealed that murine pre-heparin plasma does not contain measurable EL activity, indicating that the hydrolysis of HDL phospholipids by EL in vivo likely occurs on the cell surface.