ApoA5 lowers triglyceride levels via suppression of ANGPTL3/8-mediated LPL inhibition

Role of liver X receptor in multiple sclerosis: A long furtive life behind a barrier

Liver X receptors (LXRs) are nuclear receptors that function as transcription factors regulating cholesterol metabolism and are implicated in multiple sclerosis (MS) pathogenesis. This mini-review aims to elucidate the potential role of LXRs in MS neuropathology. MS is the most prevalent inflammatory and demyelinating disease of the central nervous system (CNS), impacting both the brain and spinal cord. Furthermore, alterations in brain cholesterol metabolism in MS can modify the functional activity and immune response of LXRs, which are implicated in MS neuropathology. Dysregulation of LXRs and cholesterol homeostasis is associated with the pathogenesis of MS. LXRs play a critical role in regulating the myelination of nerve sheaths, and defects in LXR function may contribute to the progression of MS. LXRs have immunomodulatory effects, including inhibition of the proliferation of lymphocytes, preventing contact of self-antigens to T cells, and regulating the apoptotic process of T cells. LXRs regulate the activity of microglia, which have pro-inflammatory and anti-inflammatory properties involved in immune regulation and clearance of debris as well as the remyelination process. LXRs regulate the functional activity of glial cells and prevent glial cell-mediated neurodegeneration. LXRs have an important role in the regulation of neuroinflammation during MS neuropathology. LXRs may prevent the progression of neuroinflammation in MS by inhibiting the NF-κB and NLRP3 inflammasome signaling pathways. In conclusion, LXRs play a crucial role in MS neuropathology by mitigating neuroinflammation. These findings proposed that LXR agonists, through modulation of cholesterol homeostasis and inflammatory response, could be effective in the management of MS.

Impact of DNA methylation on digestive and metabolic gene expression in red pandas (Ailurus fulgens) during the transition from milk to bamboo diet

BACKGROUND

DNA methylation plays a crucial role in species development and environmental adaptation. In mammals, there are significant dietary changes from infancy to adulthood. Notably, the red panda transitions from milk consumption as juveniles to a bamboo-based diet as adults, with significant alterations in food characteristics and nutritional content. However, the regulatory role of DNA methylation in this process remains unclear. In this study, we investigate the regulatory role of DNA methylation on the expression of digestive and metabolic genes in the liver and pancreas during the red panda's dietary transition from suckling stage to adulthood.

RESULTS

Our findings reveal significant differences in DNA methylation patterns before and after dietary transition, highlighting the specific alterations in the methylation profiles of genes involved in lipid, carbohydrate, and amino acid metabolism. We found that perilipin-4 (PLIN4) is hypomethylated and highly expressed in the liver of adult red pandas, facilitating lipid droplet formation and storage, crucial for adapting to the low-fat content in bamboo. In contrast, genes like lipoprotein lipase (LPL), crucial for lipid breakdown, exhibited hypermethylated with low-expression patterns, reflecting a reduced lipid metabolism capacity in adults. Carbohydrate metabolism-related genes like ADH4 and FAM3C are hypomethylated and highly expressed in adults, enhancing glycogen production and glucose utilization. Genes involved in protein metabolism like CTSZ and GLDC, exhibit hypomethylated with high-expression and hypermethylated with low-expression patterns in the pancreas of adults, respectively, contributing to protein metabolism balance post-weaning.

CONCLUSION

This study reveals the regulatory role of DNA methylation in the dietary transition of red pandas from milk to bamboo and provides methylation evidence for the molecular regulation of adaptive expression of digestive and metabolic genes in red pandas with specialized diets.

Angiopoietin-Like Protein Family-Mediated Functions in Modulating Triglyceride Metabolism and Related Metabolic Diseases

Hypertriglyceridemia, characterized by increased triglyceride (TG) concentrations, is considered the most important risk factor for cardiometabolic disorders, including dyslipidemia, atherosclerotic cardiovascular diseases, and non-alcoholic fatty liver disease (NAFLD). Recently, the angiopoietin-like protein (ANGPTL) family, which comprises ANGPTL1 to ANGPTL8, was confirmed to play an important role in modulating lipoprotein lipase (LPL) activity. However, understanding of the underlying mechanisms remains limited. Importantly, emerging evidence has linked several transcriptional and post-transcriptional factors to the potential alteration of TG metabolism via ANGPTL proteins. This review focused on the similarities and differences in the expression, structural features, and modulatory profile of three ANGPTLs: ANGPTL3, ANGPTL4, and ANGPTL8. In addition, the regulatory functions of those three ANGPTLs in modulating LPL were summarized to provide potential therapeutic and clinical strategies for hypertriglyceridemia and its related cardiometabolic disorders.

ANGPTL3/8 is an atypical unfoldase that regulates intravascular lipolysis by catalyzing unfolding of lipoprotein lipase

Lipoprotein lipase (LPL) carries out the lipolytic processing of triglyceride-rich lipoproteins (TRL) along the luminal surface of capillaries. LPL activity is regulated by the angiopoietin-like proteins (ANGPTL3, ANGPTL4, ANGPTL8), which control the delivery of TRL-derived lipid nutrients to tissues in a temporal and spatial fashion. This regulation of LPL mediates the partitioning of lipid delivery to adipose tissue and striated muscle according to nutritional status. A complex between ANGPTL3 and ANGPTL8 (ANGPTL3/8) inhibits LPL activity in oxidative tissues, but its mode of action has remained unknown. Here, we used biophysical techniques to define how ANGPTL3/8 and ANGPTL3 interact with LPL and how they drive LPL inactivation. We demonstrate, by mass photometry, that ANGPTL3/8 is a heterotrimer with a 2:1 ANGPTL3:ANGPTL8 stoichiometry and that ANGPTL3 is a homotrimer. Hydrogen-deuterium exchange mass spectrometry (HDX-MS) studies revealed that ANGPTL3/8 and ANGPTL3 use the proximal portion of their N-terminal α-helices to interact with sequences surrounding the catalytic pocket in LPL. That binding event triggers unfolding of LPL's α/β-hydrolase domain and irreversible loss of LPL catalytic activity. The binding of LPL to its endothelial transporter protein (GPIHBP1) or to heparan-sulfate proteoglycans protects LPL from unfolding and inactivation, particularly against the unfolding triggered by ANGPTL3. Pulse-labeling HDX-MS studies revealed that ANGPTL3/8 and ANGPTL3 catalyze LPL unfolding in an ATP-independent fashion, which categorizes these LPL inhibitors as atypical unfoldases. The catalytic nature of LPL unfolding by ANGPTL3/8 explains why low plasma concentrations of ANGPTL3/8 are effective in inhibiting a molar excess of LPL in capillaries.

Walking the VLDL tightrope in cardiometabolic diseases

Very-low-density lipoprotein (VLDL), a triglyceride-rich lipoprotein secreted by hepatocytes, is pivotal for supplying peripheral tissues with fatty acids for energy production. As if walking on a tightrope, perturbations in the balance of VLDL metabolism contribute to cardiometabolic dysfunction, promoting pathologies such as cardiovascular disease (CVD) or metabolic dysfunction-associated steatotic liver disease (MASLD). Despite the advent of lipid-lowering therapies, including statins and proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors, risks for cardiovascular events persist. With limitations to currently available CVD therapeutics and no US Food and Drug Administration (FDA)-approved treatment for MASLD, this review summarizes the current understanding of VLDL metabolism that sheds light on novel therapeutic avenues to pursue for cardiometabolic disorders.

Design and development of a soluble PDA-Emodin-PVP-MN patch and its anti-obesity effect in rats

Emodin has been proven to have weight-reducing and lipid-lowering effects. In order to make emodin play a better anti-obesity role, we designed and developed an emodin loaded dissolving microneedle patch, in which emodin existed in the form of emodin-polyvinylpyrrolidone co-precipitate (Emodin-PVP). Meanwhile, polydopamine (PDA) was added to the microneedle patch (PDA-Emodin-PVP-MN) for photothermal-enhanced chemotherapy of obesity. The average weight of the patch was 0.1 ± 0.05 g and the drug loading was 0.37 ± 0.031 mg. After 5 min of NIR irradiation (808 nm, 0.6 W/cm2), the rat abdominal temperature could reach 48 ℃, and the cumulative release of emodin reached 96.25%. The diffusion coefficient of emodin in the in vitro agar diffusion experiment was 249.27 mm2 h-1. No obvious toxicity was observed in hemolysis test, CCK-8 assay and microscopic histopathological analysis. The patch significantly reduced the percent of body weight ( P < 0.01), lipid-body ratio ( P < 0.001), serum FFAs ( P < 0.01) and the cell volume of peritesticular adipose tissue in the high-fat diet induced obese rats, indicating the patch had good anti-obesity effect. The mechanism of action may be related to the up-regulation of HSL and LPL protein levels in rat peritesticular adipose tissue.

Associations of Circulating ANGPTL3, C-Terminal Domain-Containing ANGPTL4, and ANGPTL3/8 and ANGPTL4/8 Complexes with LPL Activity, Diabetes, Inflammation, and Cardiovascular Mortality

BACKGROUND

ANGPTL3/4/8 (angiopoietin-like proteins 3, 4, and 8) are important regulators of LPL (lipoprotein lipase). ANGPTL8 forms complexes with ANGPTL3 and ANGPTL4. ANGPTL4/8 complex formation converts ANGPTL4 from a furin substrate to a plasmin substrate, and both cleavages generate similar C-terminal domain-containing (CD)-ANGPTL4 fragments. Whereas several studies have investigated associations of free ANGPTL proteins with cardiovascular risk, there are no data describing associations of the complexes and CD-ANGPTL4 with outcomes or describing the effects of the complexes on LPL bound to GPIHBP1 (glycosylphosphatidylinositol HDL-binding protein 1).

METHODS

Recombinant protein assays were used to study ANGPTL protein and complex effects on GPIHBP1-LPL activity. ANGPTL3/8, ANGPTL3, ANGPTL4/8, and CD-ANGPTL4 were measured with dedicated immunoassays in 2394 LURIC (Ludwigshafen Risk and Cardiovascular Health) study participants undergoing coronary angiography and 6188 getABI study (German Epidemiological Trial on Ankle Brachial Index) participants undergoing ankle brachial index measurement. There was a follow-up for cardiovascular death with a median (interquartile range) duration of 9.80 (8.75-10.40) years in the LURIC study and 7.06 (7.00-7.14) years in the getABI study.

RESULTS

ANGPTL3/8 potently inhibited GPIHBP1-LPL activity and showed positive associations with LDL-C (low-density lipoprotein cholesterol) and triglycerides (both P<0.001). However, in neither study did ANGPTL3/8 correlate with cardiovascular death. Free ANGPTL3 was positively associated with cardiovascular death in the getABI study but not the LURIC study. ANGPTL4/8 and especially CD-ANGPTL4 were positively associated with the prevalence of diabetes, CRP (C-reactive protein; all P<0.001), and cardiovascular death in both studies. In the LURIC and getABI studies, respective hazard ratios for cardiovascular mortality comparing the third with the first ANGPTL4/8 tertile were 1.47 (1.15-1.88) and 1.68 (1.25-2.27) when adjusted for sex, age, body mass index, and diabetes. For CD-ANGPTL4, these hazard ratios were 2.44 (1.86-3.20) and 2.76 (2.00-3.82).

CONCLUSIONS

ANGPTL3/8 potently inhibited GPIHBP1-LPL enzymatic activity, consistent with its positive association with serum lipids. However, ANGPTL3/8, LDL-C, and triglyceride levels were not associated with cardiovascular death in the LURIC and getABI cohorts. In contrast, concentrations of ANGPTL4/8 and particularly CD-ANGPTL4 were positively associated with inflammation, the prevalence of diabetes, and cardiovascular mortality.

Lipoprotein Lipase: Structure, Function, and Genetic Variation

Biallelic rare pathogenic loss-of-function (LOF) variants in lipoprotein lipase (LPL) cause familial chylomicronemia syndrome (FCS). Heterozygosity for these same variants is associated with a highly variable plasma triglyceride (TG) phenotype ranging from normal to severe hypertriglyceridemia (HTG), with longitudinal variation in phenotype severity seen often in a given carrier. Here, we provide an updated overview of genetic variation in LPL in the context of HTG, with a focus on disease-causing and/or disease-associated variants. We provide a curated list of 300 disease-causing variants discovered in LPL, as well as an exon-by-exon breakdown of the LPL gene and protein, highlighting the impact of variants and the various functional residues of domains of the LPL protein. We also provide a curated list of variants of unknown or uncertain significance, many of which may be upgraded to pathogenic/likely pathogenic classification should an additional case and/or segregation data be reported. Finally, we also review the association between benign/likely benign variants in LPL, many of which are common polymorphisms, and the TG phenotype.

Research progress on anti-tumor mechanisms of scutellarin

Scutellarin, one of natural flavonoids from Scutellaria barbata D. Don and Erigeron breviscapus (vant) Hand.-Mazz. Modern pharmacological studies have shown that scutellarin has a good anti-tumor effect. According to the literature review at home and abroad, scutellarin can inhibit the growth and metastasis of tumor cells, block the cell cycle at various stages, induce apoptosis and autophagy, interfere with tumor metabolism, reverse drug resistance of tumor cells and enhance the sensitivity of chemotherapy drugs. In this paper, the anti-tumor mechanism of scutellarin was reviewed, and the shortcomings of current studies and future research directions were analyzed, so as to provide a basis for further exploration of the anti-tumor potential of scutellarin and its further development and utilization.

Angiopoietin-like Proteins and Lipoprotein Lipase: The Waltz Partners That Govern Triglyceride-Rich Lipoprotein Metabolism? Impact on Atherogenesis, Dietary Interventions, and Emerging Therapies

Over 50% of patients who take statins are still at risk of developing atherosclerotic cardiovascular disease (ASCVD) and do not achieve their goal LDL-C levels. This residual risk is largely dependent on triglyceride-rich lipoproteins (TRL) and their remnants. In essence, remnant cholesterol-rich chylomicron (CM) and very-low-density lipoprotein (VLDL) particles play a role in atherogenesis. These remnants increase when lipoprotein lipase (LPL) activity is inhibited. ApoCIII has been thoroughly studied as a chief inhibitor and therapeutic options to curb its effect are available. On top of apoCIII regulation of LPL activity, there is a more precise control of LPL in various tissues, which makes it easier to physiologically divide the TRL burden according to the body's requirements. In general, oxidative tissues such as skeletal and cardiac muscle preferentially take up lipids during fasting. Conversely, LPL activity in adipocytes increases significantly after feeding, while its activity in oxidative tissues decreases concurrently. This perspective addresses the recent improvements in our understanding of circadian LPL regulations and their therapeutic implications. Three major tissue-specific lipolysis regulators have been identified: ANGPTL3, ANGPTL4, and ANGPTL8. Briefly, during the postprandial phase, liver ANGPTL8 acts on ANGPTL3 (which is released continuously from the liver) to inhibit LPL in the heart and muscle through an endocrine mechanism. On the other hand, when fasting, ANGPTL4, which is released by adipocytes, inhibits lipoprotein lipase in adipose tissue in a paracrine manner. ANGPTL3 inhibitors may play a therapeutic role in the treatment of hypertriglyceridemia. Several approaches are under development. We look forward to future studies to clarify (a) the nature of hormonal and nutritional factors that determine ANGPTL3, 4, and 8 activities, along with what long-term impacts may be expected if their regulation is impaired pharmacologically; (b) the understanding of the quantitative hierarchy and interaction of the regulatory actions of apoCIII, apoAV, and ANGPTL on LPL activity; (c) strategies for the safe and proper treatment of postprandial lipemia; and (d) the effect of fructose restriction on ANGPTL3, ANGPTL4, and ANGPTL8.

Genome-wide association study for metabolic syndrome reveals APOA5 single nucleotide polymorphisms with multilayered effects in Koreans

BACKGROUND AND PURPOSE

Genome-wide association studies (GWAS) of metabolic syndrome (MetS) have predominantly focused on non-Asian populations, with limited representation from East Asian cohorts. Moreover, previous GWAS analyses have primarily emphasized the significance of top single nucleotide polymorphisms (SNPs), poorly explaining other SNP signals in linkage disequilibrium. This study aimed to reveal the interaction between rs651821 and rs2266788, the principal variants of apolipoprotein A5 (APOA5), within the most significant loci identified through GWAS on MetS.

METHODS

GWAS on MetS and its components was conducted using the data from the Korean Genome and Epidemiology Study (KoGES) city cohort comprising 58,600 individuals with available biochemical, demographic, lifestyle factors, and the most significant APOA5 locus was analyzed further in depth.

RESULTS

According to GWAS of MetS and its diagnostic components, a significant association between the APOA5 SNPs rs651821/rs2266788 and MetS/triglycerides/high-density lipoprotein phenotypes was revealed. However, a conditional analysis employing rs651821 unveiled a reversal in the odds ratio for rs2266788. Therefore, rs651821 and rs2266788 emerged as independent and opposing signals in the extended GWAS analysis, i.e., the multilayered effects. Further gene-environment interaction analyses regarding lifestyle factors such as smoking, alcohol consumption, and physical activity underscored these multilayered effects.

CONCLUSION

This study unveils the intricate interplay between rs651821 and rs2266788 derived from MetS GWAS. Removing the influence of lead SNP reveals an independent protective signal associated with rs2266788, suggesting a multilayered effect between these SNPs. These findings underline the need for novel perspectives in future MetS GWAS.

APOA5 deficiency causes hypertriglyceridemia by reducing amounts of lipoprotein lipase in capillaries

Apolipoprotein AV (APOA5) deficiency causes hypertriglyceridemia in mice and humans. For years, the cause remained a mystery, but the mechanisms have now come into focus. Here, we review progress in defining APOA5's function in plasma triglyceride metabolism. Biochemical studies revealed that APOA5 binds to the angiopoietin-like protein 3/8 complex (ANGPTL3/8) and suppresses its ability to inhibit the activity of lipoprotein lipase (LPL). Thus, APOA5 deficiency is accompanied by increased ANGPTL3/8 activity and lower levels of LPL activity. APOA5 deficiency also reduces amounts of LPL in capillaries of oxidative tissues (e.g., heart, brown adipose tissue). Cell culture experiments revealed the likely explanation: ANGPTL3/8 detaches LPL from its binding sites on the surface of cells, and that effect is blocked by APOA5. Both the low intracapillary LPL levels and the high plasma triglyceride levels in Apoa5-/- mice are normalized by recombinant APOA5. Carboxyl-terminal sequences in APOA5 are crucial for its function; a mutant APOA5 lacking 40-carboxyl-terminal residues cannot bind to ANGPTL3/8 and lacks the ability to change intracapillary LPL levels or plasma triglyceride levels in Apoa5-/- mice. Also, an antibody against the last 26 amino acids of APOA5 reduces intracapillary LPL levels and increases plasma triglyceride levels in wild-type mice. An inhibitory ANGPTL3/8-specific antibody functions as an APOA5-mimetic reagent, increasing intracapillary LPL levels and lowering plasma triglyceride levels in both Apoa5-/- and wild-type mice. That antibody is a potentially attractive strategy for treating elevated plasma lipid levels in human patients.

A unified model for regulating lipoprotein lipase activity

The regulation of triglyceride (TG) tissue distribution, storage, and utilization, a fundamental process of energy homeostasis, critically depends on lipoprotein lipase (LPL). We review the intricate mechanisms by which LPL activity is regulated by angiopoietin-like proteins (ANGPTL3, 4, 8), apolipoproteins (APOA5, APOC3, APOC2), and the cAMP-responsive element-binding protein H (CREBH). ANGPTL8 functions as a molecular switch, through complex formation, activating ANGPTL3 while deactivating ANGPTL4 in their LPL inhibition. The ANGPTL3-4-8 model integrates the roles of the aforementioned proteins in TG partitioning between white adipose tissue (WAT) and oxidative tissues (heart and skeletal muscles) during the feed/fast cycle. This model offers a unified perspective on LPL regulation, providing insights into TG metabolism, metabolic diseases, and therapeutics.

APOA1/C3/A4/A5 Gene Cluster at 11q23.3 and Lipid Metabolism Disorders: From Epigenetic Mechanisms to Clinical Practices

The APOA1/C3/A4/A5 cluster is an essential component in regulating lipoprotein metabolism and maintaining plasma lipid homeostasis. A genome-wide association analysis and Mendelian randomization have revealed potential associations between genetic variants within this cluster and lipid metabolism disorders, including hyperlipidemia and cardiovascular events. An enhanced understanding of the complexity of gene regulation has led to growing recognition regarding the role of epigenetic variation in modulating APOA1/C3/A4/A5 gene expression. Intensive research into the epigenetic regulatory patterns of the APOA1/C3/A4/A5 cluster will help increase our understanding of the pathogenesis of lipid metabolism disorders and facilitate the development of new therapeutic approaches. This review discusses the biology of how the APOA1/C3/A4/A5 cluster affects circulating lipoproteins and the current progress in the epigenetic regulation of the APOA1/C3/A4/A5 cluster.

Carboxyl-terminal sequences in APOA5 are important for suppressing ANGPTL3/8 activity

Apolipoprotein AV (APOA5) lowers plasma triglyceride (TG) levels by binding to the angiopoietin-like protein 3/8 complex (ANGPTL3/8) and suppressing its capacity to inhibit lipoprotein lipase (LPL) catalytic activity and its ability to detach LPL from binding sites within capillaries. However, the sequences in APOA5 that are required for suppressing ANGPTL3/8 activity have never been defined. A clue to the identity of those sequences was the presence of severe hypertriglyceridemia in two patients harboring an APOA5 mutation that truncates APOA5 by 35 residues ("APOA5Δ35"). We found that wild-type (WT) human APOA5, but not APOA5Δ35, suppressed ANGPTL3/8's ability to inhibit LPL catalytic activity. To pursue that finding, we prepared a mutant mouse APOA5 protein lacking 40 C-terminal amino acids ("APOA5Δ40"). Mouse WT-APOA5, but not APOA5Δ40, suppressed ANGPTL3/8's capacity to inhibit LPL catalytic activity and sharply reduced plasma TG levels in mice. WT-APOA5, but not APOA5Δ40, increased intracapillary LPL levels and reduced plasma TG levels in Apoa5-/- mice (where TG levels are high and intravascular LPL levels are low). Also, WT-APOA5, but not APOA5Δ40, blocked the ability of ANGPTL3/8 to detach LPL from cultured cells. Finally, an antibody against a synthetic peptide corresponding to the last 26 amino acids of mouse APOA5 reduced intracapillary LPL levels and increased plasma TG levels in WT mice. We conclude that C-terminal sequences in APOA5 are crucial for suppressing ANGPTL3/8 activity in vitro and for regulating intracapillary LPL levels and plasma TG levels in vivo.

Angiopoietin-like protein 8: a multifaceted protein instrumental in regulating triglyceride metabolism

PURPOSE OF REVIEW

The angiopoietin-like (ANGPTL) proteins ANGPTL3 and ANGPTL4 are critical lipoprotein lipase (LPL) inhibitors. This review discusses the unique ability of the insulin-responsive protein ANGPTL8 to regulate triglyceride (TG) metabolism by forming ANGPTL3/8 and ANGPTL4/8 complexes that control tissue-specific LPL activities.

RECENT FINDINGS

After feeding, ANGPTL4/8 acts locally in adipose tissue, has decreased LPL-inhibitory activity compared to ANGPTL4, and binds tissue plasminogen activator (tPA) and plasminogen to generate plasmin, which cleaves ANGPTL4/8 and other LPL inhibitors. This enables LPL to be fully active postprandially to promote efficient fatty acid (FA) uptake and minimize ectopic fat deposition. In contrast, liver-derived ANGPTL3/8 acts in an endocrine manner, has markedly increased LPL-inhibitory activity compared to ANGPTL3, and potently inhibits LPL in oxidative tissues to direct TG toward adipose tissue for storage. Circulating ANGPTL3/8 levels are strongly correlated with serum TG, and the ANGPTL3/8 LPL-inhibitory epitope is blocked by the TG-lowering protein apolipoprotein A5 (ApoA5).

SUMMARY

ANGPTL8 plays a crucial role in TG metabolism by forming ANGPTL3/8 and ANGPTL4/8 complexes that differentially modulate LPL activities in oxidative and adipose tissues respectively. Selective ANGPTL8 inhibition in the context of the ANGPTL3/8 complex has the potential to be a promising strategy for treating dyslipidemia.

Genetic variation in apolipoprotein A-V in hypertriglyceridemia

PURPOSE OF REVIEW

While biallelic rare APOA5 pathogenic loss-of-function (LOF) variants cause familial chylomicronemia syndrome, heterozygosity for such variants is associated with highly variable triglyceride phenotypes ranging from normal to severe hypertriglyceridemia, often in the same individual at different time points. Here we provide an updated overview of rare APOA5 variants in hypertriglyceridemia.

RECENT FINDINGS

Currently, most variants in APOA5 that are considered to be pathogenic according to guidelines of the American College of Medical Genetics and Genomics are those resulting in premature termination codons. There are minimal high quality functional data on the impact of most rare APOA5 missense variants; many are considered as variants of unknown or uncertain significance. Furthermore, particular common polymorphisms of APOA5 , such as p.Ser19Trp and p.Gly185Cys in Caucasian and Asian populations, respectively, are statistically overrepresented in hypertriglyceridemia cohorts and are sometimes misattributed as being causal for chylomicronemia, when they are merely risk alleles for hypertriglyceridemia.

SUMMARY

Both biallelic and monoallelic LOF variants in APOA5 are associated with severe hypertriglyceridemia, although the biochemical phenotype in the monoallelic state is highly variable and is often exacerbated by secondary factors. Currently, with few exceptions, the principal definitive mechanism for APOA5 pathogenicity is through premature truncation. The pathogenic mechanisms of most missense variants in APOA5 remain unclear and require additional functional experiments or family studies.

Multifactorial chylomicronemia syndrome

PURPOSE OF REVIEW

The aim of this review was to understand the role of multifactorial chylomicronemia syndrome (MFCS) as a cause of severe hypertriglyceridemia; to distinguish it from other causes of severe hypertriglyceridemia; and to provide a rational approach to treatment.

RECENT FINDINGS

There have been advances in understanding the genetic underpinning of MFCS, and a better appreciation as to how to differentiate it from the much rarer familial chylomicronemia syndrome, in which there are substantial differences in the approach to their treatment. New approaches to triglyceride lowering will help reduce the risk of pancreatitis, the major complication of MFCS.

SUMMARY

MCSF is a condition in which plasma triglyceride levels are severely elevated, usually to due exacerbation of common genetic forms of hypertriglyceridemia by secondary causes of hypertriglyceridemia and/or triglyceride-raising drugs. Triglyceride-induced pancreatitis can be prevented by markedly reducing triglyceride levels by treating secondary causes and/or eliminating of triglyceride-raising drugs, and by using triglyceride-lowering drugs, especially fibrates. MFCS also increases cardiovascular disease risk, for which lifestyle measures and drugs are required.

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.

A novel fully human anti-NT-ANGPTL3 antibody from phage display library exhibits potent ApoB, TG, and LDL-C lowering activities in hyperlipidemia mice

Dyslipidemia is characterized by elevated plasma levels of low-density lipoprotein cholesterol (LDL-C), triglycerides (TG), and TG-rich lipoprotein (TGRLs) in circulation, and is closely associated with the incidence and development of cardiovascular disease. Angiopoietin-like protein 3 (ANGPTL3) deficiency has been identified as a cause of familial combined hypolipidemia in humans, which allows it to be an important therapeutic target for reducing plasma lipids. Here, we report the discovery and characterization of a novel fully human antibody F1519-D95aA against N-terminal ANGPTL3 (NT-ANGPTL3), which potently inhibits NT-ANGPTL3 with a KD as low as 9.21 nM. In hyperlipidemic mice, F1519-D95aA shows higher apolipoprotein B (ApoB) and TG-lowering, and similar LDL-C reducing activity as compared to positive control Evinacumab (56.50% vs 26.01% decrease in serum ApoB levels, 30.84% vs 25.28% decrease in serum TG levels, 23.32% vs 22.52% decrease in serum LDLC levels, relative to vehicle group). Molecular docking and binding energy calculations reveal that the F1519-D95aA-ANGPTL3 complex (10 hydrogen bonds, -65.51 kcal/mol) is more stable than the Evinacumab-ANGPTL3 complex (4 hydrogen bonds, -63.76 kcal/mol). Importantly, F1519-D95aA binds to ANGPTL3 with different residues in ANGPTL3 from Evinacumab, suggesting that F1519-D95aA may be useful for the treatment of patients resistant to Evinacumab. In conclusion, F1519-D95aA is a novel fully human anti-NT-ANGPTL3 antibody with potent plasma ApoB, TG, and LDL-C lowering activities, which can potentially serve as a therapeutic agent for hyperlipidemia and relevant cardiovascular diseases.

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.

Hypertriglyceridemia in Apoa5-/- mice results from reduced amounts of lipoprotein lipase in the capillary lumen

Why apolipoprotein AV (APOA5) deficiency causes hypertriglyceridemia has remained unclear, but we have suspected that the underlying cause is reduced amounts of lipoprotein lipase (LPL) in capillaries. By routine immunohistochemistry, we observed reduced LPL staining of heart and brown adipose tissue (BAT) capillaries in Apoa5-/- mice. Also, after an intravenous injection of LPL-, CD31-, and GPIHBP1-specific mAbs, the binding of LPL Abs to heart and BAT capillaries (relative to CD31 or GPIHBP1 Abs) was reduced in Apoa5-/- mice. LPL levels in the postheparin plasma were also lower in Apoa5-/- mice. We suspected that a recent biochemical observation - that APOA5 binds to the ANGPTL3/8 complex and suppresses its capacity to inhibit LPL catalytic activity - could be related to the low intracapillary LPL levels in Apoa5-/- mice. We showed that an ANGPTL3/8-specific mAb (IBA490) and APOA5 normalized plasma triglyceride (TG) levels and intracapillary LPL levels in Apoa5-/- mice. We also showed that ANGPTL3/8 detached LPL from heparan sulfate proteoglycans and GPIHBP1 on the surface of cells and that the LPL detachment was blocked by IBA490 and APOA5. Our studies explain the hypertriglyceridemia in Apoa5-/- mice and further illuminate the molecular mechanisms that regulate plasma TG metabolism.

Clinical and Prognostic Value of Non-Fasting Lipoproteins and Apolipoproteins in Chinese Patients with Coronary Heart Disease

Background

Lipid profiles differ naturally between individuals and between populations. So far, the data relating to non-fasting lipid profiles has been derived predominantly from studies on Western population. The characteristics and clinical significance of non-fasting lipids in Chinese patients with coronary heart disease (CHD) in response to traditional Chinese diets remain poorly understood.

Methods

A total of 1022 Chinese CHD patients with coronary artery luminal stenosis > 40% as diagnosed by coronary artery angiography were enrolled in the study. All patients received standard treatment for CHD, including statins. They were divided into an intermediate stenosis group (luminal stenosis 40-70%, n = 486) or a severe stenosis group (luminal stenosis > 70%, n = 536). Their blood lipid profiles were measured in the fasting state, and 4 hours after normal breakfast. All participants were followed up for five years. Major adverse cardiovascular events (MACE) including all-cause death, cardiac death, myocardial infarction, unscheduled coronary revascularization and stroke were recorded.

Results

After normal breakfast intake, patients with intermediate or severe stenosis showed an apparent increase in the levels of triglyceride (TG), remnant cholesterol (RC) and Apo (apolipoprotein) A1 compared to the fasting state, but a significant reduction in the levels of total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), non-high-density lipoprotein cholesterol (non-HDL-C), Apo B and Apo E. In addition to the traditional risk factors (older age, male, diabetes and smoking) and coronary artery stenosis, the fasting levels of LDL-C and Apo B, as well as non-fasting levels of HDL-C and Apo A1, were identified as independent predictors of 5-year MACE occurrence by multivariate Cox proportional hazards analysis. Patients in the 1st tertile of the non-fasting HDL-C group ( < 0.86 mmol/L) showed a significantly higher risk of MACE than 3rd tertile ( > 1.07 mmol/L) (1st tertile: 2.786, 95% CI (confidence intervals) [1.808, 4.293], p < 0.001).

Conclusions

This prospective observational study found that lipid profiles in either the fasting or non-fasting states were associated with the long-term risk of MACE in Chinese CHD patients. In addition to the fasting LDL-C level, a low non-fasting HDL-C level may also be an independent risk factors for cardiovascular events. Measurement of lipid profiles during the non-fasting state may be feasible for the management of CHD patients in routine clinical practice in China.

Decoding the role of angiopoietin-like protein 4/8 complex-mediated plasmin generation in the regulation of LPL activity

After feeding, adipose tissue lipoprotein lipase (LPL) activity should be maximized, therefore the potent LPL-inhibitory activity of angiopoietin-like protein 4 (ANGPTL4) must be blocked by ANGPTL8 through formation of ANGPTL4/8 complexes. ANGPTL4/8 tightly binds and protects LPL but also partially inhibits its activity. Recently, we demonstrated ANGPTL4/8 also binds tissue plasminogen activator (tPA) and plasminogen to generate plasmin that cleaves ANGPTL4/8 to restore LPL activity. Although fully active LPL in the fat postprandially is desirable, ANGPTL4/8 removal could subject LPL to profound inhibition by ANGPTL3/8 (the most potent circulating LPL inhibitor), inhibition by other LPL inhibitors like ANGPTL4, ANGPTL3, and ApoC3 or interfere with ApoC2-mediated LPL activation. To understand better these potential paradoxes, we examined LPL inhibition by ANGPTL3/8, ANGPTL4, ANGPTL3, and ApoC3 and LPL stimulation by ApoC2 in the presence of ANGPTL4/8 + tPA + plasminogen. Remarkably, ANGPTL3/8-mediated LPL inhibition was almost completely blocked, with the mechanism being cleavage of fibrinogen-like domain-containing ANGPTL3 present in the ANGPTL3/8 complex. The LPL-inhibitory effects of ANGPTL4, ANGPTL3, and ApoC3 were also largely reduced in the presence of ANGPTL4/8 + tPA + plasminogen. In contrast, the ability of ApoC2 to stimulate LPL activity was unaffected by ANGPTL4/8-mediated plasmin generation. Together, these results explain how plasmin generated by increased postprandial ANGPTL4/8 levels in adipose tissue enables maximal LPL activity by preventing ANGPTL3/8, ANGPTL4, ANGPTL3, and ApoC3 from inhibiting LPL, while permitting ApoC2-mediated LPL activation to occur.

Plasma insulin is required for the increase in plasma angiopoietin-like protein 8 in response to nutrient ingestion

BACKGROUND

Plasma levels of angiopoietin-like protein 8 (ANGPTL8) are regulated by feeding and they increase following glucose ingestion. Because both plasma glucose and insulin increase following food ingestion, we aimed to determine whether the increase in plasma insulin and glucose or both are responsible for the increase in ANGPTL8 levels.

METHODS

ANGPTL8 levels were measured in 30 subjects, 14 with impaired fasting glucose (IFG), and 16 with normal fasting glucose (NFG); the subjects received 75g glucose oral Glucose tolerance test (OGTT), multistep euglycaemic hyperinsulinemic clamp and hyperglycaemic clamp with pancreatic clamp.

RESULTS

Subjects with IFG had significantly higher ANGPTL8 than NGT subjects during the fasting state (p < 0.05). During the OGTT, plasma ANGPTL8 concentration increased by 62% above the fasting level (p < 0.0001), and the increase above fasting in ANGPTL8 levels was similar in NFG and IFG individuals. During the multistep insulin clamp, there was a dose-dependent increase in plasma ANGPTL8 concentration. During the 2-step hyperglycaemic clamp, the rise in plasma glucose concentration failed to cause any change in the plasma ANGPTL8 concentration from baseline.

CONCLUSIONS

In response to nutrient ingestion, ANGPTL8 level increased due to increased plasma insulin concentration, not to the rise in plasma glucose. The incremental increase above baseline in plasma ANGLPTL8 during OGTT was comparable between people with normal glucose tolerance and IFG.

Variability of longitudinal triglyceride phenotype in patients heterozygous for pathogenic APOA5 variants

BACKGROUND

Biallelic pathogenic variants in APOA5 are an infrequent cause of familial chylomicronemia syndrome characterized by severe, refractory hypertriglyceridemia (HTG), and fasting plasma triglyceride (TG) >10 mmol/L (>875 mg/dL). The TG phenotype of heterozygous individuals with one copy of a pathogenic APOA5 variant is less familiar. We evaluated the longitudinal TG phenotype of individuals with a single pathogenic APOA5 variant allele.

METHODS

Medically stable outpatients from Ontario, Canada were selected for study based on having: 1) a rare pathogenic APOA5 variant in a single allele; and 2) at least three serial fasting TG measurements obtained over >1.5 years of follow-up.

RESULTS

Seven patients were followed for a mean of 5.3 ± 3.7 years. Fasting TG levels varied widely both within and between patients. Three patients displayed at least one normal TG measurement (<2.0 mmol/L or <175 mg/dL). All patients displayed mild-to-moderate HTG (2 to 9.9 mmol/L or 175 to 875 mg/dL) at multiple time points. Five patients displayed at least one severe HTG measurement. 10%, 54%, and 36% of all TG measurements were in normal, mild-to-moderate, and severe HTG ranges, respectively.

CONCLUSIONS

Heterozygosity for pathogenic variants in APOA5 is associated with highly variable TG phenotypes both within and between patients. Heterozygosity confers susceptibility to elevated TG levels, with secondary factors likely modulating the phenotypic severity.

Current Data and New Insights into the Genetic Factors of Atherogenic Dyslipidemia Associated with Metabolic Syndrome

Atherogenic dyslipidemia plays a critical role in the development of metabolic syndrome (MetS), being one of its major components, along with central obesity, insulin resistance, and hypertension. In recent years, the development of molecular genetics techniques and extended analysis at the genome or exome level has led to important progress in the identification of genetic factors (heritability) involved in lipid metabolism disorders associated with MetS. In this review, we have proposed to present the current knowledge related to the genetic etiology of atherogenic dyslipidemia, but also possible challenges for future studies. Data from the literature provided by candidate gene-based association studies or extended studies, such as genome-wide association studies (GWAS) and whole exome sequencing (WES,) have revealed that atherogenic dyslipidemia presents a marked genetic heterogeneity (monogenic or complex, multifactorial). Despite sustained efforts, many of the genetic factors still remain unidentified (missing heritability). In the future, the identification of new genes and the molecular mechanisms by which they intervene in lipid disorders will allow the development of innovative therapies that act on specific targets. In addition, the use of polygenic risk scores (PRS) or specific biomarkers to identify individuals at increased risk of atherogenic dyslipidemia and/or other components of MetS will allow effective preventive measures and personalized therapy.

A Review of Progress on Targeting LDL Receptor-Dependent and -Independent Pathways for the Treatment of Hypercholesterolemia, a Major Risk Factor of ASCVD

Since the discovery of the LDL receptor in 1973 by Brown and Goldstein as a causative protein in hypercholesterolemia, tremendous amounts of effort have gone into finding ways to manage high LDL cholesterol in familial hypercholesterolemic (HoFH and HeFH) individuals with loss-of-function mutations in the LDL receptor (LDLR) gene. Statins proved to be the first blockbuster drug, helping both HoFH and HeFH individuals by inhibiting the cholesterol synthesis pathway rate-limiting enzyme HMG-CoA reductase and inducing the LDL receptor. However, statins could not achieve the therapeutic goal of LDL. Other therapies targeting LDLR include PCSK9, which lowers LDLR by promoting LDLR degradation. Inducible degrader of LDLR (IDOL) also controls the LDLR protein, but an IDOL-based therapy is yet to be developed. Among the LDLR-independent pathways, such as angiopoietin-like 3 (ANGPTL3), apolipoprotein (apo) B, apoC-III and CETP, only ANGPTL3 offers the advantage of treating both HoFH and HeFH patients and showing relatively better preclinical and clinical efficacy in animal models and hypercholesterolemic individuals, respectively. While loss-of-LDLR-function mutations have been known for decades, gain-of-LDLR-function mutations have recently been identified in some individuals. The new information on gain of LDLR function, together with CRISPR-Cas9 genome/base editing technology to target LDLR and ANGPTL3, offers promise to HoFH and HeFH individuals who are at a higher risk of developing atherosclerotic cardiovascular disease (ASCVD).

Inverse association between apolipoprotein C-II and cardiovascular mortality: role of lipoprotein lipase activity modulation

AIMS

Apolipoprotein C-II (ApoC-II) is thought to activate lipoprotein lipase (LPL) and is therefore a possible target for treating hypertriglyceridemia. Its relationship with cardiovascular risk has not been investigated in large-scale epidemiologic studies, particularly allowing for apolipoprotein C-III (ApoC-III), an LPL antagonist. Furthermore, the exact mechanism of ApoC-II-mediated LPL activation is unclear.

METHODS AND RESULTS

ApoC-II was measured in 3141 LURIC participants of which 590 died from cardiovascular diseases during a median (inter-quartile range) follow-up of 9.9 (8.7-10.7) years. Apolipoprotein C-II-mediated activation of the glycosylphosphatidylinositol high-density lipoprotein binding protein 1 (GPIHBP1)-LPL complex was studied using enzymatic activity assays with fluorometric lipase and very low-density lipoprotein (VLDL) substrates. The mean ApoC-II concentration was 4.5 (2.4) mg/dL. The relationship of ApoC-II quintiles with cardiovascular mortality exhibited a trend toward an inverse J-shape, with the highest risk in the first (lowest) quintile and lowest risk in the middle quintile. Compared with the first quintile, all other quintiles were associated with decreased cardiovascular mortality after multivariate adjustments including ApoC-III as a covariate (all P < 0.05). In experiments using fluorometric substrate-based lipase assays, there was a bell-shaped relationship for the effect of ApoC-II on GPIHBP1-LPL activity when exogenous ApoC-II was added. In ApoC-II-containing VLDL substrate-based lipase assays, GPIHBP1-LPL enzymatic activity was almost completely blocked by a neutralizing anti-ApoC-II antibody.

CONCLUSION

The present epidemiologic data suggest that increasing low circulating ApoC-II levels may reduce cardiovascular risk. This conclusion is supported by the observation that optimal ApoC-II concentrations are required for maximal GPIHBP1-LPL enzymatic activity.

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.

Broadening the Scope of Dyslipidemia Therapy by Targeting APOC3 (Apolipoprotein C3) and ANGPTL3 (Angiopoietin-Like Protein 3)

The positive relationship between increased levels of circulating triglycerides and cardiovascular events has been observed for decades. Driven by genetic cohort studies, inhibitors of APOC3 (apolipoprotein C3) and ANGPTL (angiopoietin-like protein) 3 that reduce circulating triglycerides are poised to enter clinical practice. We will review the biology of how inhibition of these 2 proteins affects circulating lipoproteins as well as the current state of clinical development of monoclonal antibodies, antisense oligonucleotides, and silencing RNAs targeting APOC3 and ANGPTL3.

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.

Comparison of the structure-function properties of wild-type human apoA-V and a C-terminal truncation associated with elevated plasma triglycerides

Background

Plasma triglycerides (TGs) are causally associated with coronary artery disease and acute pancreatitis. Apolipoprotein A-V (apoA-V, gene APOA5) is a liver-secreted protein that is carried on triglyceride-rich lipoproteins and promotes the enzymatic activity of lipoprotein lipase (LPL), thereby reducing TG levels. Little is known about apoA-V structure-function; naturally occurring human APOA5 variants can provide novel insights.

Methods

We used hydrogen-deuterium exchange mass spectrometry to determine the secondary structure of human apoA-V in lipid-free and lipid-associated conditions and identified a C-terminal hydrophobic face. Then, we used genomic data in the Penn Medicine Biobank to identify a rare variant, Q252X, predicted to specifically eliminate this region. We interrogated the function of apoA-V Q252X using recombinant protein in vitro and in vivo in apoa5 knockout mice.

Results

Human apoA-V Q252X carriers exhibited elevated plasma TG levels consistent with loss of function. Apoa5 knockout mice injected with AAV vectors expressing wildtype and variant APOA5-AAV recapitulated this phenotype. Part of the loss of function is due to reduced mRNA expression. Functionally, recombinant apoA-V Q252X was more readily soluble in aqueous solutions and more exchangeable with lipoproteins than WT apoA-V. Despite lacking the C-terminal hydrophobic region (a putative lipid binding domain) this protein also decreased plasma TG in vivo.

Conclusions

Deletion of apoA-V's C-terminus leads to reduced apoA-V bioavailability in vivo and higher TG levels. However, the C-terminus is not required for lipoprotein binding or enhancement of intravascular lipolytic activity. WT apoA-V is highly prone to aggregation, and this property is markedly reduced in recombinant apoA-V lacking the C-terminus.

Angiopoietin-like protein 4/8 complex-mediated plasmin generation leads to cleavage of the complex and restoration of LPL activity

Triglyceride (TG) metabolism is highly regulated by angiopoietin-like protein (ANGPTL) family members [Y. Q. Chen et al., J. Lipid Res. 61, 1203-1220 (2020)]. During feeding, ANGPTL8 forms complexes with the fibrinogen-like domain-containing protein ANGPTL4 in adipose tissue to decrease ANGPTL3/8- and ANGPTL4-mediated lipoprotein lipase (LPL)-inhibitory activity and promote TG hydrolysis and fatty acid (FA) uptake. The ANGPTL4/8 complex, however, tightly binds LPL and partially inhibits it in vitro. To try to reconcile the in vivo and in vitro data on ANGPTL4/8, we aimed to find novel binding partners of ANGPTL4/8. To that end, we performed pulldown experiments and found that ANGPTL4/8 bound both tissue plasminogen activator (tPA) and plasminogen, the precursor of the fibrinolytic enzyme plasmin. Remarkably, ANGPTL4/8 enhanced tPA activation of plasminogen to generate plasmin in a manner like that observed with fibrin, while minimal plasmin generation was observed with ANGPTL4 alone. The addition of tPA and plasminogen to LPL-bound ANGPTL4/8 caused rapid, complete ANGPTL4/8 cleavage and increased LPL activity. Restoration of LPL activity in the presence of ANGPTL4/8 was also achieved with plasmin but was blocked when catalytically inactive plasminogen (S760A) was added to tPA or when plasminogen activator inhibitor-1 was added to tPA + plasminogen, indicating that conversion of plasminogen to plasmin was essential. Together, these results suggest that LPL-bound ANGPTL4/8 mimics fibrin to recruit tPA and plasminogen to generate plasmin, which then cleaves ANGPTL4/8, enabling LPL activity to be increased. Our observations thus reveal a unique link between the ANGPTL4/8 complex and plasmin generation.

Targeting macrophages in atherosclerosis using nanocarriers loaded with liver X receptor agonists: A narrow review

Macrophages are involved in the whole process of atherosclerosis, which is characterized by accumulation of lipid and inflammation. Presently, clinically used lipid-lowering drugs cannot completely retard the progress of atherosclerosis. Liver X receptor (LXR) plays a key role in regulation of lipid metabolism and inflammation. Accumulating evidence have demonstrated that synthetic LXR agonists can significantly retard the development of atherosclerosis. However, these agonists induce sever hypertriglyceridemia and liver steatosis. These side effects have greatly limited their potential application for therapy of atherosclerosis. The rapid development of drug delivery system makes it possible to delivery interested drugs to special organs or cells using nanocarriers. Macrophages express various receptors which can recognize and ingest specially modified nanocarriers loaded with LXR agonists. In the past decades, a great progress has been made in this field. These macrophage-targeted nanocarriers loaded with LXR agonists are found to decrease atherosclerosis by reducing cholesterol accumulation and inflammatory reactions. Of important, these nanocarriers can alleviate side effects of LXR agonists. In this article, we briefly review the roles of macrophages in atherosclerosis, mechanisms of action of LXR agonists, and focus on the advances of macrophage-targeted nanocarriers loaded with LXR agonists. This work may promote the potential clinical application of these nanocarriers.

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.

A protein of capillary endothelial cells, GPIHBP1, is crucial for plasma triglyceride metabolism

GPIHBP1, a protein of capillary endothelial cells (ECs), is a crucial partner for lipoprotein lipase (LPL) in the lipolytic processing of triglyceride-rich lipoproteins. GPIHBP1, which contains a three-fingered cysteine-rich LU (Ly6/uPAR) domain and an intrinsically disordered acidic domain (AD), captures LPL from within the interstitial spaces (where it is secreted by parenchymal cells) and shuttles it across ECs to the capillary lumen. Without GPIHBP1, LPL remains stranded within the interstitial spaces, causing severe hypertriglyceridemia (chylomicronemia). Biophysical studies revealed that GPIHBP1 stabilizes LPL structure and preserves LPL activity. That discovery was the key to crystallizing the GPIHBP1-LPL complex. The crystal structure revealed that GPIHBP1's LU domain binds, largely by hydrophobic contacts, to LPL's C-terminal lipid-binding domain and that the AD is positioned to project across and interact, by electrostatic forces, with a large basic patch spanning LPL's lipid-binding and catalytic domains. We uncovered three functions for GPIHBP1's AD. First, it accelerates the kinetics of LPL binding. Second, it preserves LPL activity by inhibiting unfolding of LPL's catalytic domain. Third, by sheathing LPL's basic patch, the AD makes it possible for LPL to move across ECs to the capillary lumen. Without the AD, GPIHBP1-bound LPL is trapped by persistent interactions between LPL and negatively charged heparan sulfate proteoglycans (HSPGs) on the abluminal surface of ECs. The AD interrupts the HSPG interactions, freeing LPL-GPIHBP1 complexes to move across ECs to the capillary lumen. GPIHBP1 is medically important; GPIHBP1 mutations cause lifelong chylomicronemia, and GPIHBP1 autoantibodies cause some acquired cases of chylomicronemia.

Alterations in sorting and secretion of hepatic apoA5 induce hypertriglyceridemia due to short-term use of olanzapine

Long-term use of olanzapine, an antipsychotic drug, induces hypertriglyceridemia, resulting in a higher risk of cardiovascular disease. However, the effects and underlying mechanisms of short-term use of olanzapine on circulating triglyceride levels remain poorly understood. Here, the role of apolipoprotein A5 (apoA5), a regulator of triglyceride metabolism, was investigated in olanzapine-induced hypertriglyceridemia. Our multi-center clinical study recruited 36 schizophrenia patients who received short-term (8 weeks) of olanzapine. Besides, female C57BL/6J mice were treated with olanzapine (3 mg/kg/day versus 6 mg/kg/day) for 6 weeks. We demonstrated that short-term use of olanzapine increased plasma triglyceride and decreased plasma apoA5 levels in the patients and mice, with a negative correlation between the two factors. However, no obesity was observed in the patients and mice. Interestingly, olanzapine increased hepatic apoA5 protein in the mice, without significant changes in hepatic Apoa5 mRNA. Consistently, in vitro studies indicated that olanzapine increased medium triglyceride levels and decreased medium apoA5 levels in a dose-dependent manner in human HepG2 cells and primary mouse hepatocytes. Whereas the olanzapine treatment increased hepatic apoA5 protein in vitro, without effects on hepatic APOA5 mRNA. Of note, olanzapine increased the co-localization between apoA5 protein and accumulated lipid droplets in hepatocytes, as opposed to at the hepatocellular plasma membrane, in mouse liver as demonstrated by fluorescence staining. Therefore, our study indicated that short-term use of olanzapine induced hypertriglyceridemia due to defects of sorting and secretion of hepatic apoA5.

Mechanism of Lysoforte in Improving Jejuna Morphology and Health in Broiler Chickens

Lysoforte (LFT) plays a vital role in maintaining broilers' health and intestinal morphology. However, the mechanism behind the effects of LFT improving intestinal morphology and health is still unclear. Therefore, this study was implemented to explore the central genes linked to the regulatory effect of LFT. Seventy-five newly hatched Cobb 500 male broilers were randomly divided into three groups: control, LFT500, and LFT1000 groups, with 25 chicks per group. The control chicks were provided with the basal diet, and the birds in LFT500 and LFT1000 groups were offered the same basal diet with 500 g/ton and 1,000 g/ton LFT, respectively. GSE94622 dataset consisted of the control and two LFT-treated groups (LFT500 and LFT1000). Jejuna samples were obtained from Gene Expression Omnibus (GEO). Totally 106–344 DEGs were obtained by comparing LFT500 and LFT1000 vs. control and LFT1000 vs. LFT500. Gene ontology (GO) enrichment suggested that the DEGs are mainly related to the phosphatidylethanolamine biosynthetic process and neuron projection extension. KEGG analysis suggested the DEGs were enriched in AGE-RAGE, fatty acid elongation, ECM-receptor interaction (ECMRI), glycerophospholipid metabolism, focal adhesion, unsaturated fatty acids biosynthesis, and ABC transporters. Moreover, 29 genes, such as REG4, GJB1, KAT2A, APOA5, SERPINE2, ELOVL1, ABCC2, ANKRD9, CYP4V2, and PISD, might be closely related to promoting jejuna morphology in broilers. Taken together, our observation enhances the understanding of LFT in maintaining intestinal architecture and the general health of broiler chickens.

Etiologic Puzzle of Coronary Artery Disease: How Important Is Genetic Component?

In the modern era, coronary artery disease (CAD) has become the most common form of heart disease and, due to the severity of its clinical manifestations and its acute complications, is a major cause of morbidity and mortality worldwide. The phenotypic variability of CAD is correlated with the complex etiology, multifactorial (caused by the interaction of genetic and environmental factors) but also monogenic. The purpose of this review is to present the genetic factors involved in the etiology of CAD and their relationship to the pathogenic mechanisms of the disease. Method: we analyzed data from the literature, starting with candidate gene-based association studies, then continuing with extensive association studies such as Genome-Wide Association Studies (GWAS) and Whole Exome Sequencing (WES). The results of these studies revealed that the number of genetic factors involved in CAD etiology is impressive. The identification of new genetic factors through GWASs offers new perspectives on understanding the complex pathophysiological mechanisms that determine CAD. In conclusion, deciphering the genetic architecture of CAD by extended genomic analysis (GWAS/WES) will establish new therapeutic targets and lead to the development of new treatments. The identification of individuals at high risk for CAD using polygenic risk scores (PRS) will allow early prophylactic measures and personalized therapy to improve their prognosis.

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.

Metabolism of triglyceride-rich lipoproteins in health and dyslipidaemia

Accumulating evidence points to the causal role of triglyceride-rich lipoproteins and their cholesterol-enriched remnants in atherogenesis. Genetic studies in particular have not only revealed a relationship between plasma triglyceride levels and the risk of atherosclerotic cardiovascular disease, but have also identified key proteins responsible for the regulation of triglyceride transport. Kinetic studies in humans using stable isotope tracers have been especially useful in delineating the function of these proteins and revealing the hitherto unappreciated complexity of triglyceride-rich lipoprotein metabolism. Given that triglyceride is an essential energy source for mammals, triglyceride transport is regulated by numerous mechanisms that balance availability with the energy demands of the body. Ongoing investigations are focused on determining the consequences of dysregulation as a result of either dietary imprudence or genetic variation that increases the risk of atherosclerosis and pancreatitis. The identification of molecular control mechanisms involved in triglyceride metabolism has laid the groundwork for a 'precision-medicine' approach to therapy. Novel pharmacological agents under development have specific molecular targets within a regulatory framework, and their deployment heralds a new era in lipid-lowering-mediated prevention of disease. In this Review, we outline what is known about the dysregulation of triglyceride transport in human hypertriglyceridaemia.

An anti-ANGPTL3/8 antibody decreases circulating triglycerides by binding to a LPL-inhibitory leucine zipper-like motif

Triglycerides (TG) are required for fatty acid transport and storage and are essential for human health. Angiopoietin-like-protein 8 (ANGPTL8) has previously been shown to form a complex with ANGPTL3 that increases circulating TG by potently inhibiting LPL. We also recently showed that the TG-lowering apolipoprotein A5 (ApoA5) decreases TG levels by suppressing ANGPTL3/8-mediated LPL inhibition. To understand how LPL binds ANGPTL3/8 and ApoA5 blocks this interaction, we used hydrogen-deuterium exchange mass-spectrometry and molecular modeling to map binding sites of LPL and ApoA5 on ANGPTL3/8. Remarkably, we found that LPL and ApoA5 both bound a unique ANGPTL3/8 epitope consisting of N-terminal regions of ANGPTL3 and ANGPTL8 that are unmasked upon formation of the ANGPTL3/8 complex. We further used ANGPTL3/8 as an immunogen to develop an antibody targeting this same epitope. After refocusing on antibodies that bound ANGPTL3/8, as opposed to ANGPTL3 or ANGPTL8 alone, we utilized bio-layer interferometry to select an antibody exhibiting high-affinity binding to the desired epitope. We revealed an ANGPTL3/8 leucine zipper-like motif within the anti-ANGPTL3/8 epitope, the LPL-inhibitory region, and the ApoA5-interacting region, suggesting the mechanism by which ApoA5 lowers TG is via competition with LPL for the same ANGPTL3/8-binding site. Supporting this hypothesis, we demonstrate that the anti-ANGPTL3/8 antibody potently blocked ANGPTL3/8-mediated LPL inhibition in vitro and dramatically lowered TG levels in vivo. Together, these data show that an anti-ANGPTL3/8 antibody targeting the same leucine zipper-containing epitope recognized by LPL and ApoA5 markedly decreases TG by suppressing ANGPTL3/8-mediated LPL inhibition.

Angiopoietin-like proteins and postprandial partitioning of fatty acids

PURPOSE OF REVIEW

Over the last two decades, evolving discoveries around angiopoietin-like (ANGPTL) proteins, particularly ANGPTL3, ANGPTL4, and ANGPTL8, have generated significant interest in understanding their roles in fatty acid (FA) metabolism. Until recently, exactly how this protein family regulates lipoprotein lipase (LPL) in a tissue-specific manner to control FA partitioning has remained elusive. This review summarizes the latest insights into mechanisms by which ANGPTL3/4/8 proteins regulate postprandial FA partitioning.

RECENT FINDINGS

Accumulating evidence suggests that ANGPTL8 is an insulin-responsive protein that regulates ANGPTL3 and ANGPTL4 by forming complexes with them to increase or decrease markedly their respective LPL-inhibitory activities. After feeding, when insulin levels are high, ANGPTL3/8 secreted by hepatocytes acts in an endocrine manner to inhibit LPL in skeletal muscle, whereas ANGPTL4/8 secreted by adipocytes acts locally to preserve adipose tissue LPL activity, thus shifting FA toward the fat for storage. Insulin also decreases hepatic secretion of the endogenous ANGPTL3/8 inhibitor, apolipoprotein A5 (ApoA5), to accentuate ANGPTL3/8-mediated LPL inhibition in skeletal muscle.

SUMMARY

The ANGPTL3/4/8 protein family and ApoA5 play critical roles in directing FA toward adipose tissue postprandially. Selective targeting of these proteins holds significant promise for the treatment of dyslipidemias, metabolic syndrome, and their related comorbidities.

Addressing dyslipidemic risk beyond LDL-cholesterol

Despite the success of LDL-lowering drugs in reducing cardiovascular disease (CVD), there remains a large burden of residual disease due in part to persistent dyslipidemia characterized by elevated levels of triglyceride-rich lipoproteins (TRLs) and reduced levels of HDL. This form of dyslipidemia is increasing globally as a result of the rising prevalence of obesity and metabolic syndrome. Accumulating evidence suggests that impaired hepatic clearance of cholesterol-rich TRL remnants leads to their accumulation in arteries, promoting foam cell formation and inflammation. Low levels of HDL may associate with reduced cholesterol efflux from foam cells, aggravating atherosclerosis. While fibrates and fish oils reduce TRL, they have not been uniformly successful in reducing CVD, and there is a large unmet need for new approaches to reduce remnants and CVD. Rare genetic variants that lower triglyceride levels via activation of lipolysis and associate with reduced CVD suggest new approaches to treating dyslipidemia. Apolipoprotein C3 (APOC3) and angiopoietin-like 3 (ANGPTL3) have emerged as targets for inhibition by antibody, antisense, or RNAi approaches. Inhibition of either molecule lowers TRL but respectively raises or lowers HDL levels. Large clinical trials of such agents in patients with high CVD risk and elevated levels of TRL will be required to demonstrate efficacy of these approaches.

Apolipoprotein E content of VLDL limits LPL-mediated triglyceride hydrolysis

High levels of circulating triglycerides (TGs), or hypertriglyceridemia, are key components of metabolic diseases, such as type 2 diabetes, metabolic syndrome, and CVD. As TGs are carried by lipoproteins in plasma, hypertriglyceridemia can result from overproduction or lack of clearance of TG-rich lipoproteins (TRLs) such as VLDLs. The primary driver of TRL clearance is TG hydrolysis mediated by LPL. LPL is regulated by numerous TRL protein components, including the cofactor apolipoprotein C-II, but it is not clear how their effects combine to impact TRL hydrolysis across individuals. Using a novel assay designed to mimic human plasma conditions in vitro, we tested the ability of VLDL from 15 normolipidemic donors to act as substrates for human LPL. We found a striking 10-fold difference in hydrolysis rates across individuals when the particles were compared on a protein or a TG basis. While VLDL TG contents moderately correlated with hydrolysis rate, we noticed substantial variations in non-apoB proteins within these particles by MS. The ability of LPL to hydrolyze VLDL TGs did not correlate with apolipoprotein C-II content, but it was strongly inversely correlated with apolipoprotein E (APOE) and, to a lesser extent, apolipoprotein A-II. Addition of exogenous APOE inhibited LPL lipolysis in a dose-dependent manner. The APOE3 and (particularly) APOE4 isoforms were effective at limiting LPL hydrolysis, whereas APOE2 was not. We conclude that APOE on VLDL modulates LPL activity and could be a relevant factor in the pathogenesis of metabolic disease.

ANGPTL3 as therapeutic target

PURPOSE OF REVIEW

Elevated LDL-C and triglycerides are important risk factors for the development of atherosclerotic cardiovascular disease. Although effective therapies for lipid lowering exist, many people do not reach their treatment targets. In the last two decades, ANGPTL3 has emerged as a novel therapeutic target for lowering plasma LDL-C and triglycerides. Here, an overview of the recent literature on ANGPTL3 is provided, focusing on the therapeutic benefits of inactivation of ANGPTL3 via monoclonal antibodies, antisense oligonucleotides, and other more nascent approaches. In addition, the potential mechanisms by which ANGPTL3 inactivation lowers plasma LDL-C are discussed.

RECENT FINDINGS

ANGPTL3 is a factor secreted by the liver that inhibits lipoprotein lipase and other lipases via the formation of a complex with the related protein ANGPTL8. Large-scale genetic studies in humans have shown that carriers of loss-of-function variants in ANGPTL3 have lower plasma LDL-C and triglyceride levels, and are at reduced risk of atherosclerotic cardiovascular disease. Clinical studies in patients with different forms of dyslipidemia have demonstrated that inactivation of ANGPTL3 using monoclonal antibodies or antisense oligonucleotides markedly lowers plasma LDL-C and triglyceride levels.

SUMMARY

Anti-ANGPTL3 therapies hold considerable promise for reducing plasma LDL-C and triglycerides in selected patient groups.

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.