Background: Apolipoproteins (apo) C-I and C-III regulate plasma triglyceride metabolism by inhibition of lipoprotein lipase (LPL) activity. Results: ApoC-I or apoC-III prevents LPL from binding to lipid droplets. This results in inhibition of LPL. Conclusion: Inhibition of LPL activity by apoC-I and apoC-III is due to displacement of LPL from lipid droplets. Significance: Our proposed mechanism explains several effects of these apolipoproteins on lipoprotein metabolism.

BackgroundLipoprotein lipase (LPL) hydrolyzes triglycerides in lipoproteins and makes fatty acids available for tissue metabolism. The activity of the enzyme is modulated in a tissue specific manner by interaction with other proteins. We have studied how feeding/fasting and some related perturbations affect the expression, in rat adipose tissue, of three such proteins, LMF1, an ER protein necessary for folding of LPL into its active dimeric form, the endogenous LPL inhibitor ANGPTL4, and GPIHBP1, that transfers LPL across the endothelium.ResultsThe system underwent moderate circadian oscillations, for LPL in phase with food intake, for ANGPTL4 and GPIHBP1 in the opposite direction. Studies with cycloheximide showed that whereas LPL protein turns over rapidly, ANGPTL4 protein turns over more slowly. Studies with the transcription blocker Actinomycin D showed that transcripts for ANGPTL4 and GPIHBP1, but not LMF1 or LPL, turn over rapidly. When food was withdrawn the expression of ANGPTL4 and GPIHBP1 increased rapidly, and LPL activity decreased. On re-feeding and after injection of insulin the expression of ANGPTL4 and GPIHBP1 decreased rapidly, and LPL activity increased. In ANGPTL4−/− mice adipose tissue LPL activity did not show these responses. In old, obese rats that showed signs of insulin resistance, the responses of ANGPTL4 and GPIHBP1 mRNA and of LPL activity were severely blunted (at 26 weeks of age) or almost abolished (at 52 weeks of age).ConclusionsThis study demonstrates directly that ANGPTL4 is necessary for rapid modulation of LPL activity in adipose tissue. ANGPTL4 message levels responded very rapidly to changes in the nutritional state. LPL activity always changed in the opposite direction. This did not happen in Angptl4−/− mice. GPIHBP1 message levels also changed rapidly and in the same direction as ANGPTL4, i.e. increased on fasting when LPL activity decreased. This was unexpected because GPIHBP1 is known to stabilize LPL. The plasticity of the LPL system is severely blunted or completely lost in insulin resistant rats.

BackgroundLipoprotein lipase (LPL) hydrolyzes triglycerides in plasma lipoproteins and enables uptake of lipolysis products for energy production or storage in tissues. Our aim was to study the localization of LPL and its endothelial anchoring protein glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1) in mouse pancreas, and effects of diet and leptin deficiency on their expression patterns. For this, immunofluorescence microscopy was used on pancreatic tissue from C57BL/6 mouse embryos (E18), adult mice on normal or high-fat diet, and adult ob/ob-mice treated or not with leptin. The distribution of LPL and GPIHBP1 was compared to insulin, glucagon and CD31. Heparin injections were used to discriminate between intracellular and extracellular LPL.ResultsIn the exocrine pancreas LPL was found in capillaries, and was mostly co-localized with GPIHBP1. LPL was releasable by heparin, indicating localization on cell surfaces. Within the islets, most of the LPL was associated with beta cells and could not be released by heparin, indicating that the enzyme remained mostly within cells. Staining for LPL was found also in the glucagon-producing alpha cells, both in embryos (E18) and in adult mice. Only small amounts of LPL were found together with GPIHBP1 within the capillaries of islets. Neither a high fat diet nor fasting/re-feeding markedly altered the distribution pattern of LPL or GPIHBP1 in mouse pancreas. Islets from ob/ob mice appeared completely deficient of LPL in the beta cells, while LPL-staining was normal in alpha cells and in the exocrine pancreas. Leptin treatment of ob/ob mice for 12 days reversed this pattern, so that most of the islets expressed LPL in beta cells.ConclusionsWe conclude that both LPL and GPIHBP1 are present in mouse pancreas, and that LPL expression in beta cells is dependent on leptin.

This article is available online at http://www.jlr.org triglyceride (TG)-rich lipoproteins (i.e., VLDLs and chylomicrons) (2-4). In normolipidemic individuals, plasma concentrations of apoC-III are 80-100 g/ml, with approximately 60% of apoC-III associated with HDL, 20% with LDL, 20% with the TG-rich lipoproteins, and very little free in plasma (5). In the hypertriglyceridemic state, plasma apoC3 levels increase to >300 g/ml, and the percentage on TG-rich lipoproteins increases from 20% to upward of 60% (5-7). In fact, animal and human studies have established a strong, positive correlation between elevated plasma apoC-III levels and TG levels, which are an independent risk factor for atherosclerotic CVD (1, 8). The overexpression of human apoC-III in mice promoted the development of atherosclerosis and was associated with elevated TG levels, but low HDL cholesterol levels, in plasma (9). In contrast, targeted disruption of apoC-III in mice was associated with the rapid catabolism of TG-rich lipoproteins and a 70% decrease in fasting TG levels (10).Beyond these mouse models, clinical studies established the importance of apoC-III as a predictor of CVD outcomes. Increased serum apoC-III levels were associated with elevated serum TG and, in turn, insulin resistance, CVD, and type II diabetes (11-13). Plasma levels of apoC-III independently predicted risk for coronary heart disease, even after control for blood lipids (12,14). Further indicating a crucial role of apoC-III in regulating lipid metabolism, subjects with gain-of-function mutations in the apoC-III gene exhibited hypertriglyceridemia that was associated with elevated plasma apoC-III protein levels (15, 16). In contrast, subjects with loss-of-function mutations in the apoC-III gene exhibited reduced TG and apoC-III levels (17-21), which had a cardioprotective effect. apoC-III is a small (79 amino acid), O-glycosylated, secretory protein that is synthesized in the liver and intestine (1). ApoC-III is the most abundant apoC in humans and circulates in plasma as a component of HDLs and the Abstract

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Apolipoproteins (apo) C‐I and C‐III are known to inhibit lipoprotein lipase (LPL) activity but the molecular mechanisms for this remain obscure. We present evidence that either apoC‐I or apoC‐III, when bound to triglyceride‐rich lipoproteins, prevent binding of LPL to the lipid/water interface. This results in decreased lipolytic activity of the enzyme. Site‐directed mutagenesis revealed that hydrophobic amino acid residues centrally located in the apoC‐III molecule are critical for attachment to lipid emulsion particles and consequently inhibition of LPL activity. Triglyceride‐rich lipoproteins stabilize LPL and protect the enzyme from inactivating factors such as angiopoietin‐like protein 4 (angptl4). Addition of either apoC‐I or apoC‐III to triglyceride‐rich particles severely diminished their protective effect on LPL and rendered the enzyme more susceptible to inactivation by angptl4. These observations were seen using chylomicrons as well as the synthetic lipid emulsion Intralipid. In the presence of the LPL activator protein apoC‐II more of apoC‐I or apoC‐III was needed for displacement of LPL from the lipid/water interface. In conclusion, we show that apoC‐I and apoC‐III inhibit lipolysis by displacing LPL from lipid emulsion particles. We propose a role for these apolipoproteins also in the irreversible inactivation of LPL by factors such as angptl4. Grant Funding Source: Supported by Swedish Science Council Grant 12203