The tissue distribution of lipoprotein lipase determines where chylomicrons bind

A nexus of lipid and O-Glcnac metabolism in physiology and disease

Although traditionally considered a glucose metabolism-associated modification, the O-linked β-N-Acetylglucosamine (O-GlcNAc) regulatory system interacts extensively with lipids and is required to maintain lipid homeostasis. The enzymes of O-GlcNAc cycling have molecular properties consistent with those expected of broad-spectrum environmental sensors. By direct protein-protein interactions and catalytic modification, O-GlcNAc cycling enzymes may provide both acute and long-term adaptation to stress and other environmental stimuli such as nutrient availability. Depending on the cell type, hyperlipidemia potentiates or depresses O-GlcNAc levels, sometimes biphasically, through a diversity of unique mechanisms that target UDP-GlcNAc synthesis and the availability, activity and substrate selectivity of the glycosylation enzymes, O-GlcNAc Transferase (OGT) and O-GlcNAcase (OGA). At the same time, OGT activity in multiple tissues has been implicated in the homeostatic regulation of systemic lipid uptake, storage and release. Hyperlipidemic patterns of O-GlcNAcylation in these cells are consistent with both transient physiological adaptation and feedback uninhibited obesogenic and metabolic dysregulation. In this review, we summarize the numerous interconnections between lipid and O-GlcNAc metabolism. These links provide insights into how the O-GlcNAc regulatory system may contribute to lipid-associated diseases including obesity and metabolic syndrome.

Eruptive xanthoma model reveals endothelial cells internalize and metabolize chylomicrons, leading to extravascular triglyceride accumulation

Although tissue uptake of fatty acids from chylomicrons is primarily via lipoprotein lipase (LpL) hydrolysis of triglycerides (TGs), studies of patients with genetic LpL deficiency suggest additional pathways deliver dietary lipids to tissues. Despite an intact endothelial cell (EC) barrier, hyperchylomicronemic patients accumulate chylomicron-derived lipids within skin macrophages, leading to the clinical finding eruptive xanthomas. We explored whether an LpL-independent pathway exists for transfer of circulating lipids across the EC barrier. We found that LpL-deficient mice had a marked increase in aortic EC lipid droplets before and after a fat gavage. Cultured ECs internalized chylomicrons, which were hydrolyzed within lysosomes. The products of this hydrolysis fueled lipid droplet biogenesis in ECs and triggered lipid accumulation in cocultured macrophages. EC chylomicron uptake was inhibited by competition with HDL and knockdown of the scavenger receptor-BI (SR-BI). In vivo, SR-BI knockdown reduced TG accumulation in aortic ECs and skin macrophages of LpL-deficient mice. Thus, ECs internalize chylomicrons, metabolize them in lysosomes, and either store or release their lipids. This latter process may allow accumulation of TGs within skin macrophages and illustrates a pathway that might be responsible for creation of eruptive xanthomas.

THP-1 macrophage cholesterol efflux is impaired by palmitoleate through Akt activation

Lipoprotein lipase (LPL) is upregulated in atherosclerotic lesions and it may promote the progression of atherosclerosis, but the mechanisms behind this process are not completely understood. We previously showed that the phosphorylation of Akt within THP-1 macrophages is increased in response to the lipid hydrolysis products generated by LPL from total lipoproteins. Notably, the free fatty acid (FFA) component was responsible for this effect. In the present study, we aimed to reveal more detail as to how the FFA component may affect Akt signalling. We show that the phosphorylation of Akt within THP-1 macrophages increases with total FFA concentration and that phosphorylation is elevated up to 18 hours. We further show that specifically the palmitoleate component of the total FFA affects Akt phosphorylation. This is tied with changes to the levels of select molecular species of phosphoinositides. We further show that the total FFA component, and specifically palmitoleate, reduces apolipoprotein A-I-mediated cholesterol efflux, and that the reduction can be reversed in the presence of the Akt inhibitor MK-2206. Overall, our data support a negative role for the FFA component of lipoprotein hydrolysis products generated by LPL, by impairing macrophage cholesterol efflux via Akt activation.

A lipoprotein lipase-GPI-anchored high-density lipoprotein-binding protein 1 fusion lowers triglycerides in mice: Implications for managing familial chylomicronemia syndrome

Lipoprotein lipase (LPL) is central to triglyceride metabolism. Severely compromised LPL activity causes familial chylomicronemia syndrome (FCS), which is associated with very high plasma triglyceride levels and increased risk of life-threatening pancreatitis. Currently, no approved pharmacological intervention can acutely lower plasma triglycerides in FCS. Low yield, high aggregation, and poor stability of recombinant LPL have thus far prevented development of enzyme replacement therapy. Recently, we showed that LPL monomers form 1:1 complexes with the LPL transporter glycosylphosphatidylinositol-anchored high-density lipoprotein–binding protein 1 (GPIHBP1) and solved the structure of the complex. In the present work, we further characterized the monomeric LPL/GPIHBP1 complex and its derivative, the LPL–GPIHBP1 fusion protein, with the goal of contributing to the development of an LPL enzyme replacement therapy. Fusion of LPL to GPIHBP1 increased yields of recombinant LPL, prevented LPL aggregation, stabilized LPL against spontaneous inactivation, and made it resistant to inactivation by the LPL antagonists angiopoietin-like protein 3 (ANGPTL3) or ANGPTL4. The high stability of the fusion protein enabled us to identify LPL amino acids that interact with ANGPTL4. Additionally, the LPL–GPIHBP1 fusion protein exhibited high enzyme activity in in vitro assays. Importantly, both intravenous and subcutaneous administrations of the fusion protein lowered triglycerides in several mouse strains without causing adverse effects. These results indicate that the LPL–GPIHBP1 fusion protein has potential for use as a therapeutic for managing FCS.

Characterizing familial chylomicronemia syndrome: Baseline data of the APPROACH study

BACKGROUND

Familial chylomicronemia syndrome (FCS) is a rare metabolic disorder caused by mutations in lipoprotein lipase (LPL) or genes required for LPL functionality and is characterized by hyperchylomicronemia that results in recurrent episodes of acute pancreatitis. Owing to the rarity of FCS, there are few case series describing the phenotypic variability in FCS patients in detail.

OBJECTIVE

To provide baseline characteristics in the largest study population to date of patients with FCS.

METHODS

We analyzed baseline demographic and clinical characteristics of adult FCS patients in the phase 3 APPROACH study of volanesorsen sodium (antisense inhibitor of apolipoprotein C-III).

RESULTS

Sixty-six patients were included in the analysis. Mean (SD) age was 46 (13) years; and mean body mass index was 24.9 (5.7) kg/m². We identified causal mutations in 79% (52) of patients, with LPL mutations accounting for 62% (41) of cases. Median age at diagnosis was 24 years, 54% were females, and 81% were Caucasian. All patients followed a low-fat diet, 43% received fibrates, 27% fish oils, and 21% statins. Median fasting triglyceride levels (P25, P75) were 1985 (1179, 3047 mg/dL). Overall, 76% of patients reported ≥1 lifetime episode of acute pancreatitis; 23 patients reported a total of 53 pancreatitis events in the 5 years before enrollment.

CONCLUSIONS

Our data emphasize the severe hypertriglyceridemia characteristic of FCS patients despite restrictive low-fat diets and frequent use of existing hypolipemic therapies. Acute pancreatitis and recurrent acute pancreatitis are frequent complications of FCS. Diagnosis at an older age suggests likely underdiagnosis and underappreciation of this rare disorder.

The heparan sulfate proteoglycan grip on hyperlipidemia and atherosclerosis

Heparan sulfate proteoglycans are found at the cell surface and in the extracellular matrix, where they interact with a plethora of proteins involved in lipid homeostasis and inflammation. Over the last decade, new insights have emerged regarding the mechanism and biological significance of these interactions in the context of cardiovascular disease. The majority of cardiovascular disease-related deaths are caused by complications of atherosclerosis, a disease that results in narrowing of the arterial lumen, thereby reducing blood flow to critical levels in vital organs, such as the heart and brain. Here, we discuss novel insights into how heparan sulfate proteoglycans modulate risk factors such as hyperlipidemia and inflammation that drive the initiation and progression of atherosclerotic plaques to their clinical critical endpoint.

Role of lipoprotein lipase in lipid metabolism

PURPOSE OF REVIEW

A major step in energy metabolism is hydrolysis of triacylglycerol-rich lipoproteins (TRLs) to release fatty acids that can be used or stored. This is accomplished by lipoprotein lipase (LPL) at 'binding lipolysis sites' at the vascular endothelium. A multitude of interactions are involved in this seemingly simple reaction. Recent advances in the understanding of some of these factors will be discussed in an attempt to build a comprehensive picture.

RECENT FINDINGS

The first event in catabolism of TRLs is that they dock at the vascular endothelium. This requires LPL and GPIHBP1, the endothelial transporter of LPL.Kinetic studies in rats with labeled chylomicrons showed that once a chylomicron has docked in the heart it stays for minutes and a large number of triacylglycerol molecules are split. The distribution of binding between tissues reflects the amount of LPL, as evident from studies with mutant mice.Clearance of TRLs is often slowed down in metabolic disease, as was demonstrated both in mice and men. In mice, this was directly connected to decreased amounts of endothelial LPL.

SUMMARY

The LPL system is central in energy metabolism and results from interplay between several factors. Rapid and exciting progress is being made.