The Angiopoietin-Like Protein 3 and 8 Complex Interacts with Lipoprotein Lipase and Induces LPL Cleavage

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.

Competitive displacement of lipoprotein lipase from heparan sulfate is orchestrated by a disordered acidic cluster in GPIHBP1

Movement of lipoprotein lipase (LPL) from myocytes or adipocytes to the capillary lumen is essential for intravascular lipolysis and plasma triglyceride homeostasis-low LPL activity in the capillary lumen causes hypertriglyceridemia. The trans-endothelial transport of LPL depends on ionic interactions with GPIHBP1's intrinsically disordered N-terminal tail, which harbors two acidic clusters at positions 5-12 and 19-30. This polyanionic tail provides a molecular switch that controls LPL detachment from heparan sulfate proteoglycans (HSPGs) by competitive displacement. When the acidic tail was neutralized in gene-edited mice, LPL remained trapped in the sub-endothelial spaces triggering hypertriglyceridemia. Due to its disordered state, the crystal structure of LPL•GPIHBP1 provided no information on these electrostatic interactions between LPL and GPIHBP1 acidic tail. In the current study, we positioned the acidic tail on LPL using zero-length crosslinking. Acidic residues at positions 19-30 in GPIHBP1 mapped to Lys445, Lys441, Lys414, and Lys407 close to the interface between the C- and N-terminal domains in LPL. Modeling this interface revealed widespread polyelectrolyte interactions spanning both LPL domains, which explains why the acidic tail stabilizes LPL activity and protein conformation. In functional assays, we showed that the acidic cluster at 19-30 also had the greatest impact on preserving LPL activity, mitigating ANGPTL4-catalyzed LPL inactivation, preventing PSCK3-mediated LPL cleavage, and, importantly, displacing LPL from HSPGs. Our current study provides key insights into the biophysical mechanism(s) orchestrating intravascular compartmentalization of LPL activity-an intriguing pathway entailing competitive displacement of HSPG-bound LPL by a disordered acidic tail in GPIHBP1.

Current and Emerging Treatment Options for Hypertriglyceridemia: State-of-the-Art Review

Hypertriglyceridemia (HTG) is associated with a residual risk of atherosclerotic cardiovascular disease. Extremely elevated triglyceride (TG) concentrations, particularly due to familial chylomicronemia syndrome (FCS), pose a risk for acute pancreatitis. Standard therapies with statins, fibrates, omega-3 fatty acids, and niacin may be insufficient to reduce elevated TG levels and improve clinical outcomes in patients with HTG. Novel antisense oligonucleotides and small interfering ribonucleic acids target the key modulators of TG-rich lipoprotein catabolism. Among apolipoprotein C-III (apoC-III) inhibitors, olezarsen and plozasiran appear to be safer alternatives for volanesorsen regarding the risk of drug-induced thrombocytopenia in patients with FCS or severe HTG. After the failure of vupanorsen, a new angiopoietin-like protein 3 (ANGPTL3) inhibitor, zodasiran, demonstrated the potential to decrease TG levels in patients with moderate HTG. Meanwhile, the fibroblast growth factor 21 (FGF21) analog, pegozafermin, became another candidate for the treatment of severe HTG. This comprehensive review outlines pharmacological targets in TG-rich lipoprotein metabolism, discusses international guidelines, and summarizes the latest evidence from clinical trials to provide insight into the current and emerging treatment options for primary HTG.

Leptin-mediated suppression of lipoprotein lipase cleavage enhances lipid uptake and facilitates lymph node metastasis in gastric cancer

BACKGROUND

Lymph node metastasis (LNM) is the primary mode of metastasis in gastric cancer (GC). However, the precise mechanisms underlying this process remain elusive. Tumor cells necessitate lipid metabolic reprogramming to facilitate metastasis, yet the role of lipoprotein lipase (LPL), a pivotal enzyme involved in exogenous lipid uptake, remains uncertain in tumor metastasis. Therefore, the aim of this study was to investigate the presence of lipid metabolic reprogramming during LNM of GC as well as the role of LPL in this process.

METHODS

Intracellular lipid levels were quantified using oil red O staining, BODIPY 493/503 staining, and flow cytometry. Lipidomics analysis was employed to identify alterations in intracellular lipid composition following LPL knockdown. Protein expression levels were assessed through immunohistochemistry, Western blotting, and enzyme-linked immunosorbent assays. The mouse popliteal LNM model was utilized to investigate differences in LNM. Immunoprecipitation and mass spectrometry were employed to examine protein associations. In vitro phosphorylation assays and Phos-tag sodium dodecyl-sulfate polyacrylamide gel electrophoresis assays were conducted to detect angiopoietin-like protein 4 (ANGPTL4) phosphorylation.

RESULTS

We identified that an elevated intracellular lipid level represents a crucial characteristic of node-positive (N+) GC and further demonstrated that a high-fat diet can expedite LNM. LPL was found to be significantly overexpressed in N+ GC tissues and shown to facilitate LNM by mediating dietary lipid uptake within GC cells. Leptin, an obesity-related hormone, intercepted the effect exerted by ANGPTL4/Furin on LPL cleavage. Circulating leptin binding to the leptin receptor could induce the activation of inositol-requiring enzyme-1 (IRE1) kinase, leading to the phosphorylation of ANGPTL4 at the serine 30 residue and subsequently reducing its binding affinity with LPL. Moreover, our research revealed that LPL disrupted lipid homeostasis by elevating intracellular levels of arachidonic acid, which then triggered the cyclooxygenase-2/prostaglandin E2 (PGE2) pathway, thereby promoting tumor lymphangiogenesis.

CONCLUSIONS

Leptin-induced phosphorylation of ANGPTL4 facilitates LPL-mediated lipid uptake and consequently stimulates the production of PGE2, ultimately facilitating LNM in GC.

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.

Effects of exercise training on ANGPTL3/8 and ANGPTL4/8 and their associations with cardiometabolic traits

Angiopoietin-like protein (ANGPTL) complexes 3/8 and 4/8 are established inhibitors of LPL and novel therapeutic targets for dyslipidemia. However, the effects of regular exercise on ANGPTL3/8 and ANGPTL4/8 are unknown. We characterized ANGPTL3/8 and ANGPTL4/8 and their relationship with in vivo measurements of lipase activities and cardiometabolic traits before and after a 5-month endurance exercise training intervention in 642 adults from the HERITAGE (HEalth, RIsk factors, exercise Training And GEnetics) Family Study. At baseline, higher levels of both ANGPTL3/8 and ANGPTL4/8 were associated with a worse lipid, lipoprotein, and cardiometabolic profile, with only ANGPTL3/8 associated with postheparin LPL and HL activities. ANGPTL3/8 significantly decreased with exercise training, which corresponded with increases in LPL activity and decreases in HL activity, plasma triglycerides, apoB, visceral fat, and fasting insulin (all P < 5.1 × 10-4). Exercise-induced changes in ANGPTL4/8 were directly correlated to concomitant changes in total cholesterol, LDL-C, apoB, and HDL-triglycerides and inversely related to change in insulin sensitivity index (all P < 7.0 × 10-4). In conclusion, exercise-induced decreases in ANGPTL3/8 and ANGPTL4/8 were related to concomitant improvements in lipase activity, lipid profile, and cardiometabolic risk factors. These findings reveal the ANGPTL3-4-8 model as a potential molecular mechanism contributing to adaptations in lipid metabolism in response to exercise training.

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.

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.

ANGPTL3 (Angiopoietin-Like 3) Preferentially Resides on High-Density Lipoprotein in the Human Circulation, Affecting Its Activity

Background ANGPTL3 (angiopoietin-like protein 3) is an acknowledged crucial regulator of lipid metabolism by virtue of its inhibitory effect on lipoprotein lipase and endothelial lipase. It is currently unknown whether and to which lipoproteins ANGPTL3 is bound and whether the ability of ANGPTL3 to inhibit lipase activity is affected by binding to lipoproteins. Methods and Results Incubation of ultracentrifugation-isolated low-density lipoprotein (LDL) and high-density lipoprotein (HDL) fractions from healthy volunteers with recombinant ANGPTL3 revealed that ANGPTL3 associates with both HDL and LDL particles ex vivo. Plasma from healthy volunteers and a patient deficient in HDL was fractionated by fast protein liquid chromatography, and ANGPTL3 distribution among lipoprotein fractions was measured. In healthy volunteers, ≈75% of lipoprotein-associated ANGPTL3 resides in HDL fractions, whereas ANGPTL3 was largely bound to LDL in the patient deficient in HDL. ANGPTL3 activity was studied by measuring lipolysis and uptake of 3H-trioleate by brown adipocyte T37i cells. Unbound ANGPTL3 did not suppress lipase activity, but when given with HDL or LDL, ANGPTL3 suppressed lipase activity by 21.4±16.4% (P=0.03) and 25.4±8.2% (P=0.006), respectively. Finally, in a subset of the EPIC (European Prospective Investigation into Cancer) Norfolk study, plasma HDL cholesterol and amount of large HDL particles were both positively associated with plasma ANGPTL3 concentrations. Moreover, plasma ANGPTL3 concentrations showed a positive association with incident coronary artery disease (odds ratio, 1.25 [95% CI, 1.01-1.55], P=0.04). Conclusions Although ANGPTL3 preferentially resides on HDL, its activity was highest once bound to LDL particles.

Association Between Plasma Levels of ANGPTL3, 4, 8 and the Most Common Additional Cardiovascular Risk Factors in Patients with Hypertension

Background

ANGPTL3, 4 and 8 have been reported to be involved in the regulation of lipid and glucose metabolism. The aim of this study was to investigate the expression of ANGPTL3, 4, 8 in hypertensive patients with or without overweight/obesity, T2D, and hyperlipidemia, and the possible association between their expression and the status of the aforementioned comorbidities.

Methods

Plasma levels of ANGPTL3, 4, and 8 in 87 hospitalized patients with hypertension were measured using ELISA kits. Associations between circulating ANGPTLs levels and the most common additional cardiovascular risk factors were assessed using multivariate linear regression analyses. Pearson's correlation analysis was used to examine the association between ANGPTLs and clinical parameters.

Results

In the context of hypertension, (1) although not statistically significant, circulating ANGPTL3 levels were higher in the overweight/obese group than in the normal weight group; (2) circulating levels of ANGPTL3 and ANGPTL8 were significantly lower in patients with T2D than in non-diabetic patients; (3) circulating ANGPTL3 levels were significantly higher in the hyperlipidemic group than in the non-hyperlipidemic group. ANGPTL3 was associated with T2D and hyperlipidemia status, whereas ANGPTL8 was independently associated with T2D status. In addition, circulating ANGPTL3 levels were positively correlated with TC, TG, LDL-C, HCY, and ANGPTL8, and circulating ANGPTL4 levels were positively correlated with UACR and BNP.

Conclusion

Changes in circulating ANGPTL3 and ANGPTL8 levels have been observed in hypertensive patients with the most common additional cardiovascular risk factors, suggesting a role in the common comorbidities of hypertension and cardiovascular disease. Hypertensive patients with overweight/obesity or hyperlipidemia may benefit from therapies targeting ANGPTL3.

Genetically determined deficiency of ANGPTL3 does not alter HDL ability to preserve endothelial homeostasis

Individuals with loss-of-function mutations in the ANGPTL3 gene express a rare lipid phenotype called Familial Combined Hypolipidemia (FHBL2). FHBL2 individuals show reduced plasma concentrations of total cholesterol and triglycerides as well as of lipoprotein particles, including HDL. This feature is particularly remarkable in homozygotes in whom ANGPTL3 in blood is completely absent. ANGPTL3 acts as a circulating inhibitor of LPL and EL and it is thought that EL hyperactivity is the cause of plasma HDL reduction in FHBL2. Nevertheless, the consequences of ANGTPL3 deficiency on HDL functionality have been poorly explored. In this report, HDL isolated from homozygous and heterozygous FHBL2 individuals were evaluated for their ability to preserve endothelial homeostasis as compared to control HDL. It was found that only the complete absence of ANGPTL3 alters HDL subclass distribution, as homozygous, but not heterozygous, carriers have reduced content of large and increased content of small HDL with no alterations in HDL2 and HDL3 size. The plasma content of preβ-HDL was reduced in carriers and showed a positive correlation with plasma ANGPTL3 levels. Changes in composition did not however alter the functionality of FHBL2 HDL, as particles isolated from carriers retained their capacity to promote NO production and to inhibit VCAM-1 expression in endothelial cells. Furthermore, no significant changes in circulating levels of soluble ICAM-1 and E-selectin were detected in carriers. These results indicate that changes in HDL composition associated with the partial or complete absence of ANGPTL3 did not alter some of the potentially anti-atherogenic functions of these lipoproteins.

A hepatokine derived from the ER protein CREBH promotes triglyceride metabolism by stimulating lipoprotein lipase activity

The endoplasmic reticulum (ER)-tethered, liver-enriched stress sensor CREBH is processed in response to increased energy demands or hepatic stress to release an amino-terminal fragment that functions as a transcription factor for hepatic genes encoding lipid and glucose metabolic factors. Here, we discovered that the carboxyl-terminal fragment of CREBH (CREBH-C) derived from membrane-bound, full-length CREBH was secreted as a hepatokine in response to fasting or hepatic stress. Phosphorylation of CREBH-C mediated by the kinase CaMKII was required for efficient secretion of CREBH-C through exocytosis. Lipoprotein lipase (LPL) mediates the lipolysis of circulating triglycerides for tissue uptake and is inhibited by a complex consisting of angiopoietin-like (ANGPTL) 3 and ANGPTL8. Secreted CREBH-C blocked the formation of ANGPTL3-ANGPTL8 complexes, leading to increased LPL activity in plasma and metabolic tissues in mice. CREBH-C administration promoted plasma triglyceride clearance and partitioning into peripheral tissues and mitigated hypertriglyceridemia and hepatic steatosis in mice fed a high-fat diet. Individuals with obesity had higher circulating amounts of CREBH-C than control individuals, and human CREBH loss-of-function variants were associated with dysregulated plasma triglycerides. These results identify a stress-induced, secreted protein fragment derived from CREBH that functions as a hepatokine to stimulate LPL activity and triglyceride homeostasis.

Genetics of Cholesterol-Related Genes in Metabolic Syndrome: A Review of Current Evidence

Metabolic syndrome (MetS) refers to a cluster of metabolic dysregulations, which include insulin resistance, obesity, atherogenic dyslipidemia and hypertension. The complex pathogenesis of MetS encompasses the interplay between environmental and genetic factors. Environmental factors such as excessive nutrients and sedentary lifestyle are modifiable and could be improved by lifestyle modification. However, genetic susceptibility to MetS, a non-modifiable factor, has attracted the attention of researchers, which could act as the basis for future diagnosis, prognosis, and therapy for MetS. Several cholesterol-related genes associated with each characteristic of MetS have been identified, such as apolipoprotein, lipoprotein lipase (LPL), cholesteryl ester transfer protein (CETP) and adiponectin. This review aims to summarize the genetic information of cholesterol-related genes in MetS, which may potentially serve as biomarkers for early prevention and management of MetS.

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.

The Genetic Spectrum of Familial Hypertriglyceridemia in Oman

Familial hypertriglyceridemia (F-HTG) is an autosomal disorder that causes severe elevation of serum triglyceride levels. It is caused by genetic alterations in LPL, APOC2, APOA5, LMF1, and GPIHBP1 genes. The mutation spectrum of F-HTG in Arabic populations is limited. Here, we report the genetic spectrum of six families of F-HTG of Arab ancestry in Oman. Methods: six Omani families affected with triglyceride levels >11.2 mmol/L were included in this study. Ampli-Seq sequencing of the selected gene panels was performed. Whole-exome sequencing and copy number variant analysis were also performed in cases with negative exome results. Three novel pathogenic missense variants in the LPL gene were identified, p.M328T, p.H229L, and p.S286G, along with a novel splice variant c.1322+15T > G. The LPL p.H229L variant existed in double heterozygous mutation with the APOA5 gene p.V153M variant. One family had a homozygous mutation in the LMF1 gene (c.G107A; p.G36D) and a heterozygous mutation in the LPL gene (c.G106A; p.D36N). All affected subjects did not have a serum deficiency of LPL protein. Genetic analysis in one family did not show any pathogenic variants even after whole-exome sequencing. These novel LPL and APOA5 mutations are not reported in other ethnic groups. This suggests that patients with F-HTG in Oman have a founder effect and are genetically unique. This warrants further analysis of patients of F-HTG in the Middle East for preventative and counseling purposes to limit the spread of the disease in a population of high consanguinity.

The Fibrinogen-like Domain of ANGPTL3 Facilitates Lipolysis in 3T3-L1 Cells by Activating the Intracellular Erk Pathway

Background: ANGPTL3 stimulates lipolysis in adipocytes, but the underlying molecular mechanism is yet unknown. The C-terminal fibrinogen-like domain of ANGPTL3 (ANGPTL3-Fld) activates the AKT pathway in endothelial cells. Hence, we evaluated whether ANGPTL3-Fld stimulates lipolysis in adipocytes through the MAPK kinase pathway. Materials and Methods: 3T3-L1 adipocytes were treated with isoproterenol (ISO), ANGPTL3-Fld, or both. Lipolysis was evaluated through the release of free fatty acids (FFAs) in the culture medium. The activation status of intracellular kinases was evaluated with and without the inhibition of the BRAF–ERK arm of the MAPK pathway. Results: ANGPTL3-Fld alone was not able to activate lipolysis, while the combination of ANGPTL3-Fld and ISO determined a 10-fold enrichment of the FFA concentration in the culture medium with an incremental effect (twofold) when compared with ISO alone. ANGPTL3-Fld alone inhibited hormone-sensitive lipase (HSL), whereas the treatment with ISO induced the activation of HSL. The net balance of ANGPTL3-Fld and ISO cotreatment resulted in HSL activation. The results indicate that ANGPTL3-Fld generated an intracellular activation signal involving the MAPK–ERK pathway, possibly through the PDGFRβ—PLCγ-AMPK axis. Conclusion: ANGPTL3-Fld appears to act as a facilitator of lipolysis in adipocytes, and this effect was driven by a signal mediated by a pathway that is different from the canonical β-adrenergic stimulus.

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.

Correlation between betatrophin/angiogenin-likeprotein3/lipoprotein lipase pathway and severity of coronary artery disease in Kazakh patients with coronary heart disease

BACKGROUND

The results of previous animal experiments and clinical studies have shown that there is a correlation between expression of betatrophin and blood lipid levels. However, there are still differences studies on the correlation and interaction mechanism between betatrophin, angiogenin-likeprotein3 (ANGPTL3) and lipoprotein lipase (LPL). In our previous studies, we found an increase in serum ANGPTL3 Levels in Chinese patients with coronary heart disease (CHD). Therefore, we retrospectively studied Kazakh CHD patients.

AIM

To explore the correlation between the betatrophin/ANGPTL3/LPL pathway and severity of coronary artery disease (CAD) in patients with CHD.

METHODS

Nondiabetic patients diagnosed with CHD were selected as the case group; 79 were of Kazakh descent and 72 were of Han descent. The control groups comprised of 61 Kazakh and 65 Han individuals. The serum levels of betatrophin and LPL were detected by enzyme-linked immunosorbent assay (ELISA), and the double antibody sandwich ELISA was used to detect serum level of ANGPTL3. The levels of triglycerides, total cholesterol, and fasting blood glucose in each group were determined by an automatic biochemical analyzer. At the same time, the clinical baseline data of patients in each group were included.

RESULTS

Betatrophin, ANGPTL3 and LPL levels of Kazakh patients were significantly higher than those of Han patients (P = 0.031, 0.038, 0.021 respectively). There was a positive correlation between the Gensini score and total cholesterol (TC), triglycerides (TG), low- density lipoprotein cholesterol (LDL-C), betatrophin, and LPL in Kazakh patients (r = 0.204, 0.453, 0.352, 0.471, and 0.382 respectively), (P = 0.043, 0.009, 0.048, 0.001, and P < 0.001 respectively). A positive correlation was found between the Gensini score and body mass index (BMI), TC, TG, LDL-C, LPL, betatrophin in Han patients (r = 0.438, 0.195, 0.296, 0.357, 0.328, and 0.446 respectively), (P = 0.044, 0.026, 0.003, 0.20, 0.004, and P < 0.001). TG and betatrophin were the risk factors of coronary artery disease in Kazakh patients, while BMI and betatrophin were the risk factors in Han patients.

CONCLUSION

There was a correlation between the betatrophin/ANGPTL3/LPL pathway and severity of CAD in patients with CHD.

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.

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

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

ANGPTL3 as a Drug Target in Hyperlipidemia and Atherosclerosis

PURPOSE OF REVIEW

Elevated low-density lipoprotein cholesterol (LDL-C) and triglyceride-rich lipoproteins (TRLs) or remnants are important risk factors for the development of atherosclerotic cardiovascular disease (ASCVD). The ongoing challenge of not being able to achieve recommended LDL-C targets despite maximally tolerated lipid-lowering therapy (LLT) has led to the development of novel therapeutic agents including angiopoietin-like 3 (ANGPTL3) inhibitors.

RECENT FINDINGS

ANGPTL3 is a glycoprotein produced by the liver that inhibits lipoprotein lipase and endothelial lipase. Data from genetic and clinical studies have shown that a lower ANGPTL3 level is associated with lower plasma LDL-C, triglyceride (TG), and other lipoproteins. Pharmacological inactivation of ANGPTL3 with the monoclonal antibody, evinacumab, results in a 50% reduction in LDL-C, even in patients with homozygous familial hypercholesterolemia (HoFH). The safe and effective targeted delivery of nucleic acid-based therapies will shape the future of the lipid arena. ANGPTL3 is a novel target in lipoprotein metabolism, targeting not only LDL-C via an LDL-receptor (LDLR) independent mechanism but also TRLs and carries a significant promise for further ASCVD risk reduction.

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.

Targeting angiopoietin-like 3 in atherosclerosis: From bench to bedside

Atherosclerotic cardiovascular disease (ASCVD) is the largest cause of morbidity and mortality worldwide. Lipid-lowering therapies are the current major cornerstone of ASCVD management. Statins, ezetimibe, fibrates and proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors effectively reduce the plasma low-density lipoprotein cholesterol (LDL-C) level in most individuals at risk of atherosclerosis. Still, some patients (such as those with homozygous familial hypercholesterolaemia), who do not respond to standard therapies, and other patients who cannot take these agents, remain at a high risk of ASCVD. In recent years there has been tremendous progress in understanding the mechanism and efficacy of lipid-lowering strategies. Apart from the recently approved PCSK9 and ATP citrate lyase inhibitors, angiopoietin-like 3 (ANGPTL3) is another potential target for the treatment of dyslipidaemia and its clinical sequalae of atherosclerosis. ANGPTL3 is a pivotal modulator of plasma triglycerides (TG), LDL-C and high-density lipoprotein cholesterol (HDL-C) levels, achieved by inhibiting the activities of lipoprotein lipase and endothelial lipase. Familial combined hypolipidaemia is derived from the Angptl3 loss-of-function mutations, which leads to low levels of LDL-C, HDL-C and TG, and has a 34% decreased risk of ASCVD compared with non-carriers. To date, monoclonal antibodies (evinacumab) and antisense oligonucleotides against ANGPTL3 have been investigated in clinical trials for dyslipidaemia therapy. Herein, we review the biology and function of ANGPTL3, as well as the latest developments of ANGPTL3-targeted therapies. We also summarize evidence from basic research to clinical trials, with the aim of providing novel insights into the biological functions of ANGPTL3 and related targeted therapies.

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

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

Angiopoietin-Like Protein 3 (ANGPTL3) Modulates Lipoprotein Metabolism and Dyslipidemia

Dyslipidemia is characterized by increasing plasma levels of low-density lipoprotein-cholesterol (LDL-C), triglycerides (TGs) and TG-rich lipoproteins (TGRLs) and is a major risk factor for the development of atherosclerotic cardiovascular disorders (ASCVDs). It is important to understand the metabolic mechanisms underlying dyslipidemia to develop effective strategies against ASCVDs. Angiopoietin-like 3 (ANGPTL3), a member of the angiopoietin-like protein family exclusively synthesized in the liver, has been demonstrated to be a critical regulator of lipoprotein metabolism to inhibit lipoprotein lipase (LPL) activity. Genetic, biochemical, and clinical studies in animals and humans have shown that loss of function, inactivation, or downregulated expression of ANGPTL3 is associated with an obvious reduction in plasma levels of TGs, LDL-C, and high-density lipoprotein-cholesterol (HDL-C), atherosclerotic lesions, and the risk of cardiovascular events. Therefore, ANGPTL3 is considered an alternative target for lipid-lowering therapy. Emerging studies have focused on ANGPTL3 inhibition via antisense oligonucleotides (ASOs) and monoclonal antibody-based therapies, which have been carried out in mouse or monkey models and in human clinical studies for the management of dyslipidemia and ASCVDs. This review will summarize the current literature on the important role of ANGPTL3 in controlling lipoprotein metabolism and dyslipidemia, with an emphasis on anti-ANGPTL3 therapies as a potential strategy for the treatment of dyslipidemia and ASCVDs.

The Importance of Lipoprotein Lipase Regulation in Atherosclerosis

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