Lipoprotein 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.
In 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.
We conclude that both LPL and GPIHBP1 are present in mouse pancreas, and that LPL expression in beta cells is dependent on leptin.
Lipoprotein lipase; Diabetes mellitus; Islet cells; Exocrine pancreas; Endothelium; Ob/ob mice; High fat diet; Heparin; qPCR; Immunofluorescence
The triglycerides in chylomicrons are hydrolyzed by lipoprotein lipase (LpL) along the luminal surface of the capillaries. However, the endothelial cell molecule that facilitates chylomicron processing by LpL has not yet been defined. Here, we show that glycosylphosphatidylinositol-anchored high density lipoprotein–binding protein 1 (GPIHBP1) plays a critical role in the lipolytic processing of chylomicrons. Gpihbp1-deficient mice exhibit a striking accumulation of chylomicrons in the plasma, even on a low-fat diet, resulting in milky plasma and plasma triglyceride levels as high as 5,000 mg/dl. Normally, Gpihbp1 is expressed highly in heart and adipose tissue, the same tissues that express high levels of LpL. In these tissues, GPIHBP1 is located on the luminal face of the capillary endothelium. Expression of GPIHBP1 in cultured cells confers the ability to bind both LpL and chylomicrons. These studies strongly suggest that GPIHBP1 is an important platform for the LpL-mediated processing of chylomicrons in capillaries.
Purpose of review
This review summarizes recent data indicating that glycosylphosphatidylinositol-anchored high density lipoprotein–binding protein 1 (GPIHBP1) plays a key role in the lipolytic processing of chylomicrons.
Lipoprotein lipase (LpL) hydrolyzes triglycerides in chylomicrons at the luminal surface of the capillaries in heart, adipose tissue, and skeletal muscle. However, the endothelial cell molecule that facilitates the lipolytic processing of chylomicrons has never been clearly defined. Mice lacking GPIHBP1 manifest chylomicronemia, with plasma triglyceride levels as high as 5,000 mg/dl. In wild-type mice, GPIHBP1 is expressed on the luminal surface of capillaries in heart, adipose tissue, and skeletal muscle. Cells transfected with GPIHBP1 bind both chylomicrons and LpL avidly.
The chylomicronemia in Gpihbp1-deficient mice, the fact that GPIHBP1 is located within the lumen of capillaries, and the fact that GPIHBP1 binds LpL and chylomicrons suggest that GPIHBP1 is a key platform for the lipolytic processing of triglyceride-rich lipoproteins.
Chylomicronemia; lipoprotein lipase; hypertriglyceridemia; GPI-anchored proteins
Purpose of review
This review will provide an update on the structure of GPIHBP1, a 28-kDa glycosylphosphatidylinositol-anchored glycoprotein, and its role in the lipolytic processing of triglyceride-rich lipoproteins.
Gpihbp1 knockout mice on a chow diet have milky plasma and plasma triglyceride levels of more than 3000 mg/dl. GPIHBP1 is located on the luminal surface of endothelial cells in tissues where lipolysis occurs: heart, skeletal muscle, and adipose tissue. The pattern of lipoprotein lipase (LPL) release into the plasma after an intravenous injection of heparin is abnormal in Gpihbp1-deficient mice, suggesting that GPIHBP1 plays a direct role in binding LPL within the tissues of mice. Transfection of CHO cells with a GPIHBP1 expression vector confers on cells the ability to bind both LPL and chylomicrons. Two regions of GPIHBP1 are required for the binding of LPL – an amino-terminal acidic domain and the cysteine-rich Ly6 domain. GPIHBP1 expression in mice changes with fasting and refeeding and is regulated in part by peroxisome proliferator-activated receptor-γ.
GPIHBP1, an endothelial cell-surface glycoprotein, binds LPL and is required for the lipolytic processing of triglyceride-rich lipoproteins.
chylomicrons; endothelial; lipoprotein lipase; PPARγ
Lipoprotein 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.
The 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).
This 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.
Gene expression; Insulin; Gene inactivation; Cycloheximide; Actinomycin D; Transcription; Translation; Posttranslational
Chronic renal failure (CRF) is associated with hypertriglyceridemia and impaired clearance of very low density lipoprotein (VLDL) and chylomicrons which are largely due to lipoprotein lipase (LPL) deficiency/dysfunction. After its release from myocytes and adipocytes, LPL binds to the endothelium in the adjacent capillaries where it catalyzes hydrolysis of triglycerides in VLDL and chylomicrons. The novel endothelium-derived molecule, glycosylphosphatidylinositol-anchored binding protein 1 (GPIHBP1), plays a critical role in LPL metabolism and function by anchoring LPL to the endothelium and binding chylomicrons. GPIHBP1-deficient mice and humans exhibit severe hypertriglyceridemia and diminished heparin-releasable LPL, pointing to the critical role of GPIHBP1 in regulation of LPL activity. Given its central role in regulation of LPL activity and triglyceride metabolism, we explored the effect of chronic kidney disease (CKD) on GPIHBP1 expression.
Expression of GPIHBP1 and LPL were determined by reverse transcriptase-polymerase chain reaction, Western blot and immunohistochemical analyses in the adipose tissue, skeletal muscle and myocardium of rats 12 weeks after 5/6 nephrectomy (CRF) or sham-operation (control).
Compared to the controls, the CRF group exhibited severe hypertriglyceridemia, significant reduction of the skeletal muscle, myocardium and adipose tissue LPL mRNA and protein abundance. This was accompanied by parallel reductions of GPIHBP1 mRNA abundance and immunostaining in the tested tissues.
LPL deficiency in CKD is associated with and compounded by GPIHBP1 deficiency. Together these abnormalities contribute to impaired clearance of triglyceride-rich lipoproteins, diminished availability of lipid fuel for energy storage in adipocytes and energy production in myocytes and consequent hypertriglyceridemia, cachexia, muscle weakness and atherosclerosis.
Lipid metabolism; Triglyceride metabolism; Atherosclerosis; Impaired exercise capacity; Malnutrition syndrome; Cardiovascular disease; End-stage renal disease; Muscle and fat tissues
Macrophage-derived lipoprotein lipase (LPL) has been shown uniformly to promote atherosclerotic lesion formation while the extent to which it affects plasma lipid and lipoprotein levels varies in wild-type and hypercholesterolemic mice. It is known that high levels of LPL in the bulk of adipose tissue and skeletal muscle would certainly mask the contribution of macrophage LPL to metabolism of plasma lipoprotein. Therefore, we chose LPL deficient (LPL-/-) mice with severe hypertriglyceridemia as an alternative model to assess the role of macrophage LPL in plasma lipoprotein metabolism via bone marrow transplant, through which LPL will be produced mainly by hematopoietic cell-derived macrophages.
Methods and Results
Hypertriglyceridemic LPL-/- mice were lethally irradiated, then transplanted with bone marrow from wild-type (LPL+/+) or LPL-/- mice, respectively. Sixteen weeks later, LPL+/+ →LPL-/- mice displayed significant reduction in plasma levels of triglyceride and cholesterol (408±44.9 vs. 2.7±0.5×103 and 82.9±7.1 vs. 229.1±30.6 mg/dl, p<0.05, respectively), while a 2.7-fold increase in plasma high density lipoprotein- cholesterol (p<0.01) was observed, compared with LPL-/-→LPL-/- control mice. The clearance rate for the oral fat load test in LPL+/+ →LPL-/- mice was faster than that in LPL-/-→LPL-/- mice, but slower than that in wild-type mice. Liver triglyceride content in LPL+/+→LPL-/- mice was also significantly increased, compared with LPL-/-→LPL-/- mice (6.8±0.7 vs. 4.6±0.5 mg/g wet tissue, p<0.05, n = 6). However, no significant change was observed in the expression levels of genes involved in hepatic lipid metabolism between the two groups.
Hematopoietic cell-derived LPL could efficiently ameliorate severe hypertriglyceridemia and hypo-alpha-cholesterolemia at the compensation of increased triglyceride content of liver in LPL-/- mice.
GPIHBP1 is an endothelial cell protein that binds lipoprotein lipase (LPL) and chylomicrons. Because GPIHBP1 deficiency causes chylomicronemia in mice, we sought to determine whether some cases of chylomicronemia in humans could be attributable to defective GPIHBP1 proteins.
Methods and Results
Patients with severe hypertriglyceridemia (n=60, with plasma triglycerides above the 95th percentile for age and gender) were screened for mutations in GPIHBP1. A homozygous GPIHBP1 mutation (c.344A>C) that changed a highly conserved glutamine at residue 115 to a proline (p.Q115P) was identified in a 33-year-old male with lifelong chylomicronemia. The patient had failure-to-thrive as a child but had no history of pancreatitis. He had no mutations in LPL, APOA5, or APOC2. The Q115P substitution did not affect the ability of GPIHBP1 to reach the cell surface. However, unlike wild-type GPIHBP1, GPIHBP1-Q115P lacked the ability to bind LPL or chylomicrons (d <1.006 g/mL lipoproteins from Gpihbp1−/− mice). Mouse GPIHBP1 with the corresponding mutation (Q114P) also could not bind LPL.
A homozygous missense mutation in GPIHBP1 (Q115P) was identified in a patient with chylomicronemia. The mutation eliminated the ability of GPIHBP1 to bind LPL and chylomicrons, strongly suggesting that it caused the patient’s chylomicronemia.
lipoprotein; lipase; human; chylomicronemia; hypertriglyceridemia; GPIHBP1
Lipoprotein lipase (LPL) is a hydrolase that cleaves circulating triglycerides to release fatty acids to the surrounding tissues. The enzyme is synthesized in parenchymal cells and is transported to its site of action on the capillary endothelium by glycophosphatidylinositol (GPI)-anchored high-density lipoprotein-binding protein 1 (GPIHBP1). Inactivating mutations in LPL; in its cofactor, apolipoprotein (Apo) C2; or in GPIHBP1 cause severe hypertriglyceridemia. Here we describe an individual with complete deficiency of GPIHBP1. The proband was an Asian Indian boy who had severe chylomicronemia at 2 months of age. Array-based copy-number analysis of his genomic DNA revealed homozygosity for a 17.5-kb deletion that included GPIHBP1. A 44-year-old aunt with a history of hypertriglyceridemia and pancreatitis was also homozygous for the deletion. A bolus of intravenously administered heparin caused a rapid increase in circulating LPL and decreased plasma triglyceride levels in control individuals but not in two GPIHBP1-deficient patients. Thus, short-term treatment with heparin failed to attenuate the hypertriglyceridemia in patients with GPIHBP1 deficiency. The increasing resolution of copy number microarrays and their widespread adoption for routine cytogenetic analysis is likely to reveal a greater role for submicroscopic deletions in Mendelian conditions. We describe the first neonate with complete GPIHBP1 deficiency due to homozygosity for a deletion of GPIHBP1.
Electronic supplementary material
The online version of this article (doi:10.1007/s10545-011-9406-5) contains supplementary material, which is available to authorized users.
Gpihbp1-deficient mice (Gpihbp1−/−) lack the ability to transport lipoprotein lipase to the capillary lumen, resulting in mislocalization of LPL within tissues, defective lipolysis of triglyceride-rich lipoproteins, and chylomicronemia. We asked whether GPIHBP1 deficiency and mislocalization of catalytically active LPL would alter the composition of triglycerides in adipose tissue or perturb the expression of lipid biosynthetic genes. We also asked whether perturbations in adipose tissue composition and gene expression, if they occur, would be accompanied by reciprocal metabolic changes in the liver.
Methods and Results
The chylomicronemia in Gpihbp1−/− mice was associated with reduced levels of essential fatty acids in adipose tissue triglycerides and increased expression of lipid biosynthetic genes. The liver exhibited the opposite changes—increased levels of essential fatty acids in triglycerides and reduced expression of lipid biosynthetic genes.
Defective lipolysis in Gpihbp1−/− mice causes reciprocal metabolic perturbations in adipose tissue and liver. In adipose tissue, the essential fatty acid content of triglycerides is reduced and lipid biosynthetic gene expression is increased, while the opposite changes occur in the liver.
lipoprotein lipase; hypertriglyceridemia; lipolysis; essential fatty acids; lipid biosynthetic genes
The risk of atherosclerosis in the setting of chylomicronemia has been a topic of debate. In this study, we examined susceptibility to atherosclerosis in Gpihbp1-deficient mice (Gpihbp1−/−), which manifest severe chylomicronemia as a result of defective lipolysis.
Methods and Results
Gpihbp1−/− mice on a chow diet have plasma triglyceride and cholesterol levels of 2812 ± 209 and 319 ± 27 mg/dl, respectively. Even though nearly all of the lipids were contained in large lipoproteins (50–135 nm), the mice developed progressive aortic atherosclerosis. In other experiments, we found that both Gpihbp1-deficient “apo-B48–only” mice and Gpihbp1-deficient “apo-B100–only” mice manifest severe chylomicronemia. Thus, GPIHBP1 is required for the processing of both apo-B48– and apo-B100–containing lipoproteins.
Chylomicronemia causes atherosclerosis in mice. Also, we found that GPIHBP1 is required for the lipolytic processing of both apo-B48– and apo-B100–containing lipoproteins.
lipoprotein lipase; chylomicronemia; lipolysis; GPIHBP1
Recent studies in mice have established that an endothelial cell protein, GPIHBP1, is essential for the lipolytic processing of triglyceride-rich lipoproteins.
Methods and Results
We report the discovery of a homozygous missense mutation in GPIHBP1 in a young boy with severe chylomicronemia. The mutation, p.C65Y, replaces a conserved cysteine in the GPIHBP1’s Ly6 domain with a tyrosine and is predicted to perturb protein structure by interfering with the formation of a disulfide bond. Studies with transfected CHO cells showed that GPIHBP1-C65Y reaches the cell surface but has lost the ability to bind LPL. When the GPIHBP1-C65Y homozygote was given an intravenous bolus of heparin, only trace amounts of LPL entered the plasma. We also observed very low levels of LPL in the postheparin plasma of a chylomicronemic subject who was homozygous for a different GPIHBP1 mutation (p.Q115P). When the GPIHBP1-Q115P homozygote was given a 6-h infusion of heparin, significant amounts of LPL appeared in the plasma, resulting in a fall in the plasma triglyceride levels from 1780 mg/dl to 120 mg/dl.
We identified a novel GPIHBP1 missense mutation (p.C65Y) associated with defective LPL binding in a young boy with severe chylomicronemia. We also show that homozygosity for the C65Y or Q115P mutations is associated with low levels of LPL in the postheparin plasma, demonstrating that GPIHBP1 is important for plasma triglyceride metabolism in humans.
lipoprotein lipase; GPIHBP1; triglycerides
Lipoprotein lipase (LPL)-deficient mice have been created by gene targeting in embryonic stem cells. At birth, homozygous knockout pups have threefold higher triglycerides and sevenfold higher VLDL cholesterol levels than controls. When permitted to suckle, LPL-deficient mice become pale, then cyanotic, and finally die at approximately 18 h of age. Before death, triglyceride levels are severely elevated (15,087 +/- 3,805 vs 188 +/- 71 mg/dl in controls). Capillaries in tissues of homozygous knockout mice are engorged with chylomicrons. This is especially significant in the lung where marginated chylomicrons prevent red cell contact with the endothelium, a phenomenon which is presumably the cause of cyanosis and death in these mice. Homozygous knockout mice also have diminished adipose tissue stores as well as decreased intracellular fat droplets. By crossbreeding with transgenic mice expressing human LPL driven by a muscle-specific promoter, mouse lines were generated that express LPL exclusively in muscle but not in any other tissue. This tissue-specific LPL expression rescued the LPL knockout mice and normalized their lipoprotein pattern. This supports the contention that hypertriglyceridemia caused the death of these mice and that LPL expression in a single tissue was sufficient for rescue. Heterozygous LPL knockout mice survive to adulthood and have mild hypertriglyceridemia, with 1.5-2-fold elevated triglyceride levels compared with controls in both the fed and fasted states on chow, Western-type, or 10% sucrose diets. In vivo turnover studies revealed that heterozygous knockout mice had impaired VLDL clearance (fractional catabolic rate) but no increase in transport rate. In summary, total LPL deficiency in the mouse prevents triglyceride removal from plasma, causing death in the neonatal period, and expression of LPL in a single tissue alleviates this problem. Furthermore, half-normal levels of LPL cause a decrease in VLDL fractional catabolic rate and mild hypertriglyceridemia, implying that partial LPL deficiency has physiological consequences.
Mice with a deleted Gpihbp1 gene encoding glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPI-HBP1) develop severe chylomicronemia. We screened the coding regions of the human homologue – GPIHBP1 – from the genomic DNA of 160 unrelated adults with fasting chylomicronemia and plasma triglycerides >10 mmol/L, each of whom had normal sequence of the LPL and APOC2 genes.
One patient with severe type 5 hyperlipoproteinemia (MIM 144650), fasting chylomicronemia and relapsing pancreatitis resistant to standard therapy was found to be homozygous for a novel GPIHBP1 missense variant, namely G56R. This mutation was absent from the genomes of 600 control subjects and 610 patients with hyperlipidemia. The GPIHBP1 G56 residue has been conserved throughout evolution and the G56R mutation was predicted to have compromised function. Her homozygous brother also had refractory chylomicronemia and relapsing pancreatitis together with early coronary heart disease. G56R heterozygotes in the family had fasting mild hypertriglyceridemia.
Thus, a very rare GPIHBP1 missense mutation appears to be associated with severe hypertriglyceridemia and chylomicronemia.
Lipoprotein lipase (LPL), the rate-limiting enzyme in triglyceride hydrolysis, is normally not expressed in the liver of adult humans and animals. However, liver LPL is found in the perinatal period, and in adults it can be induced by cytokines. To study the metabolic consequences of liver LPL expression, transgenic mice producing human LPL specifically in the liver were generated and crossed onto the LPL knockout (LPL0) background. LPL expression exclusively in liver rescued LPL0 mice from neonatal death. The mice developed a severe cachexia during high fat suckling, but caught up in weight after switching to a chow diet. At 18 h of age, compared with LPL0 mice, liver-only LPL-expressing mice had equally elevated triglycerides (10,700 vs. 14,800 mg/dl, P = NS), increased plasma ketones (4.3 vs. 1.7 mg/dl, P < 0.05) and glucose (28 vs. 15 mg/dl, P < 0.05), and excessive amounts of intracellular liver lipid droplets. Adult mice expressing LPL exclusively in liver had slower VLDL turnover than wild-type mice, but greater VLDL mass clearance, increased VLDL triglyceride production, and three- to fourfold more plasma ketones. In summary, it appears that liver LPL shunts circulating triglycerides to the liver, which results in a futile cycle of enhanced VLDL production and increased ketone production, and subsequently spares glucose. This may be important to sustain brain and muscle function at times of metabolic stress with limited glucose availability.
GPIHBP1 is a new addition to a group of proteins required for the lipolysis of triglyceride-rich lipoproteins. GPIHBP1 contains an acidic domain and an Ly6 domain with ten cysteines. GPIHBP1 binds lipoprotein lipase (LPL) avidly and likely tethers LPL to the luminal surface of capillaries.
Inactivation of Gpihbp1 in mice is associated with milky plasma and severe chylomicronemia, even on a low-fat chow diet. Recently, four missense mutations in GPIHBP1 were identified in humans with severe chylomicronemia (C65Y, C65S, C68G, and Q115P). All four mutations involve highly conserved residues within GPIHBP1’s Ly6 domain.
This review will provide an update on GPIHBP1’s role in the processing of chylomicrons and the pathogenesis of chylomicronemia.
chylomicronemia; lipoprotein lipase; endothelium; mutation; hypertriglyceridemia
Expression of lipoprotein lipase (LPL) by the macrophage has been proposed to promote foam cell formation and atherosclerosis, primarily on the basis of in vitro studies. LPL-deficient mice might provide a model for testing the role of LPL secretion by the macrophage in an in vivo system. Unfortunately, homozygous deficiency of LPL in the mouse is lethal shortly after birth. Because the fetal liver is the major site of hematopoiesis in the developing fetus, transplantation of C57BL/6 mice with LPL–/– fetal liver cells (FLCs) was used to investigate the physiologic role of macrophage LPL expression in vivo. Thirty-four female C57BL/6 mice were lethally irradiated and reconstituted with FLCs from day 14 LPL+/+, LPL+/–, and LPL–/– donors. No significant differences were detected in plasma levels of post-heparin LPL activity or in serum cholesterol or triglyceride levels between the 3 groups on either a chow diet or an atherogenic diet. After 19 weeks on the atherogenic diet, aortae were collected for quantitative analysis of the extent of aortic atherosclerosis. LPL expression was detected by immunocytochemistry and in situ hybridization in macrophages of aortic atherosclerotic lesions of LPL+/+→C57BL/6 and LPL+/–→C57BL/6 mice, but not in LPL–/–→C57BL/6 mice, whereas myocardial cells expressed LPL in all groups. The mean aortic lesion area was reduced by 55% in LPL–/–→C57BL/6 mice compared with LPL+/+→C57BL/6 mice and by 45% compared with LPL+/–→C57BL/6 mice, respectively. These data demonstrate in vivo that LPL expression by macrophages in the artery wall promotes foam cell formation and atherosclerosis.
J. Clin. Invest. 103:1697–1705 (1999).
The lipolytic processing of triglyceride-rich lipoproteins by lipoprotein lipase (LPL) is the central event in plasma lipid metabolism, providing lipids for storage in adipose tissue and fuel for vital organs such as the heart. LPL is synthesized and secreted by myocytes and adipocytes but then finds its way into the lumen of capillaries, where it hydrolyzes lipoprotein triglycerides. The mechanism by which LPL reaches the lumen of capillaries represents one of the most persistent mysteries of plasma lipid metabolism. Here, we show that GPIHBP1 is responsible for the transport of LPL into capillaries. In Gpihbp1-deficient mice, LPL is mislocalized to the interstitial spaces surrounding myocytes and adipocytes. Also, we show that GPIHBP1 is located at the basolateral surface of capillary endothelial cells and actively transports LPL across endothelial cells. Our experiments define the function of GPIHBP1 in triglyceride metabolism and provide a mechanism for the transport of LPL into capillaries.
Lipoprotein lipase (LPL) is produced by parenchymal cells, mainly adipocytes and myocytes, but its role in hydrolyzing triglycerides in plasma lipoproteins occurs at the capillary lumen. For decades, the mechanism by which LPL reached its site of action in capillaries was unclear, but this mystery was recently solved. GPIHBP1, a GPI-anchored protein of capillary endothelial cells, picks up LPL from the interstitial spaces and shuttles it across endothelial cells to the capillary lumen. When GPIHBP1 is absent, LPL is mislocalized to the interstitial spaces, leading to severe hypertriglyceridemia. Some cases of hypertriglyceridemia in humans are caused by GPIHBP1 mutations that interfere with GPIHBP1's ability to bind LPL, and some are caused by LPL mutations that impair LPL's ability to bind to GPIHBP1. This review will cover recent progress in understanding GPIHBP1's role in health and disease and will discuss some remaining mysteries surrounding the processing of triglyceride-rich lipoproteins.
hypertriglyceridemia; chylomicronemia; GPIHBP1; lipoprotein lipase; endothelial cells; lymphocyte antigen 6 proteins
Lipoprotein lipase (LPL) is a 448-amino-acid head-to-tail dimeric enzyme that hydrolyzes triglycerides within capillaries. LPL is secreted by parenchymal cells into the interstitial spaces; it then binds to GPIHBP1 (glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1) on the basolateral face of endothelial cells and is transported to the capillary lumen. A pair of amino acid substitutions, C418Y and E421K, abolish LPL binding to GPIHBP1, suggesting that the C-terminal portion of LPL is important for GPIHBP1 binding. However, a role for LPL's N terminus has not been excluded, and published evidence has suggested that only full-length homodimers are capable of binding GPIHBP1. Here, we show that LPL's C-terminal domain is sufficient for GPIHBP1 binding. We found, serendipitously, that two LPL missense mutations, G409R and E410V, render LPL susceptible to cleavage at residue 297 (a known furin cleavage site). The C terminus of these mutants (residues 298–448), bound to GPIHBP1 avidly, independent of the N-terminal fragment. We also generated an LPL construct with an in-frame deletion of the N-terminal catalytic domain (residues 50–289); this mutant was secreted but also was cleaved at residue 297. Once again, the C-terminal domain (residues 298–448) bound GPIHBP1 avidly. The binding of the C-terminal fragment to GPIHBP1 was eliminated by C418Y or E421K mutations. After exposure to denaturing conditions, the C-terminal fragment of LPL refolds and binds GPIHBP1 avidly. Thus, the binding of LPL to GPIHBP1 requires only the C-terminal portion of LPL and does not depend on full-length LPL homodimers.
The role of renal lipoprotein lipase (LPL) per se in kidney diseases is still controversial and obscure. The purpose of this study was to observe the preventive effects of Ibrolipim, a LPL activator, on lipid accumulation and LPL expression in the kidneys of minipigs fed a high-sucrose and high-fat diet (HSFD).
Male Chinese Bama minipigs were fed a control diet or HSFD with or without 0.1 g/kg/day Ibrolipim for 5 months. Body weight, plasma glucose, insulin, lipids, LPL activity, and urinary microalbumin were measured. Renal tissue was obtained for detecting LPL activity and contents of triglyceride and cholesterol, observing the renal lipid accumulation by Oil Red O staining, and examining the mRNA and protein expression of LPL by real time PCR, Western Blot and immunohistochemistry.
Feeding HSFD to minipigs caused weight gain, hyperglycemia, hyperinsulinemia, hyperlipidemia and microalbuminuria. HSFD increased plasma LPL activity while it decreased the mRNA and protein expression and activity of LPL in the kidney. The increases in renal triglyceride and cholesterol contents were associated with the decrease in renal LPL activity of HSFD-fed minipigs. In contrast, supplementing Ibrolipim into HSFD lowered body weight, plasma glucose, insulin, triglyceride and urinary albumin concentrations while it increased plasma total cholesterol and HDL-C. Ibrolipim suppressed the renal accumulation of triglyceride and cholesterol, and stimulated the diet-induced down-regulation of LPL expression and activity in the kidney.
Ibrolipim exerts renoprotective and hypolipidemic effects via the increase in renal LPL activity and expression, and thus the increased expression and activity of renal LPL play a vital role in suppressing renal lipid accumulation and ameliorating proteinuria in diet-induced diabetic minipigs.
Lipoprotein lipase; Lipoprotein lipase activator; Lipid accumulation; Diabetic nephropathy; Swine, Miniature
Hyperlipidemia associated with obesity is closely related to the development of atherosclerosis. Both n-3 polyunsaturated fatty acids (PUFAs) and long-chain monounsaturated fatty acids (MUFAs; i.e., C20:1 and C22:1 isomers) supplementation modulate risk factors for metabolic syndrome via multiple mechanisms, including the restoration of impaired lipid metabolism. We therefore examined the effects of pollock oil, which contains a considerable amount of n-3 PUFAs as well as long-chain MUFAs, on plasma hyperlipidemia and hepatic steatosis in diet-induced obese mice.
Male C57BL/6J mice (24-26 g) were divided into two groups (n = 10/group) and were fed a high-fat diet containing 32% lard (control group) or 17% lard plus 15% pollock oil (experimental group) for 6 weeks. For both groups, fat comprised 60% of the total caloric intake.
Although body and liver masses for the two groups did not differ significantly, hepatic lipids concentrations (triglycerides and total cholesterols) were lower (P < 0.05) after pollock oil ingestion. After 2 weeks on the specified diets, plasma lipid levels (total cholesterol, LDL cholesterol, and triglycerides) significantly decreased (P < 0.05) in the experimental group compared with the control group, although plasma HDL cholesterol levels did not differ. At the end of 6 weeks, plasma adiponectin levels increased (P < 0.05), whereas plasma resistin and leptin levels decreased (P < 0.05) in the experimental mice. Increased levels of long-chain MUFAs and n-3 PUFAs in plasma, liver and adipose tissue by ingesting pollock oil were possibly correlated to these favorable changes. Expression of hepatic genes involved in cholesterol metabolism (SREBP2, HMGCR, and ApoB) and lipogenesis (SREPB1c, SCD-1, FAS, and Acacα) was suppressed in the experimental group, and may have favorably affected hyperlipidemia and hepatic steatosis induced by the high-fat diet.
We demonstrated that pollock oil supplementation effectively improved hyperlipidemia, attenuated hepatic steatosis, and downregulated the express of hepatic genes involved in cholesterol and lipid metabolism in mice with diet-induced obesity.
Pollock oil; n-3 PUFA; MUFA; hyperlipidemia; hepatic steatosis; adipokines
We previously reported that saturated fat (SAT) enriched diets increase arterial cholesteryl ester (CE) deposition, especially from LDL-selective uptake (SU), and this was associated with increased arterial lipoprotein lipase (LpL). We now questioned how n-3 fatty acid rich diets influence arterial cholesterol delivery and arterial LpL levels.
Methods and Results:
C57BL/6 mice were fed chow or eucaloric high fat diets enriched in SAT or fish oil (n-3) for 12 weeks, and then injected with double radiolabeled or fluorescent-labeled human LDL to separately trace LDL-CE and LDL-apoB uptake. SAT and n-3 diets increased plasma cholesterol levels similarly; n-3 diets lowered plasma triglyceride concentrations. SAT increased arterial LDL-SU with significantly higher CE infiltration into aortic media. In contrast, n-3 markedly reduced total LDL uptake and CE deposition and abolished SU with LDL localized only in aortic intima. Disparate patterns of CE deposition between diets were consistent with distribution of arterial LpL - SAT diets induced higher LpL levels throughout the aorta; n-3 diets decreased LpL levels and limited LpL expression to the aortic intima.
n-3 rich diets decrease arterial total LDL delivery and abrogate LDL-SU in parallel with changing arterial wall LpL expression and distribution.
fatty acids; fish oil; lipoprotein lipase; LDL; atherosclerosis
Apolipoprotein A-V (apoA-V), a minor protein associated with lipoproteins, has a major effect on triacylglycerol (TG) metabolism. We investigated whether apoA-V complexed with phospholipid in the form of a reconstituted HDL (rHDL) has potential utility as a therapeutic agent for treatment of hypertriglyceridemia when delivered intravenously.
Methods and Results
Intravenous injection studies were performed in genetically engineered mouse models of severe hypertriglyceridemia including apoav-/- and gpihbp1-/- mice. Administration of apoA-V rHDL to hypertriglyceridemic apoav−/− mice resulted in a 60% reduction in plasma TG concentration after 4 h. This decline can be attributed to enhanced catabolism/clearance of VLDL where VLDL TG and cholesterol were reduced ∼60%. ApoA-V which associated with VLDL after injection was also rapidly cleared. Site-specific mutations in the heparin-binding region of apoA-V (amino acids 186-227) attenuated apoA-V rHDL TG-lowering activity by 50% suggesting this sequence element is required for optimal TG-lowering activity in vivo. Unlike apoav-/- mice, injection of apoA-V rHDL into gpihbp1-/- mice had no effect on plasma TG levels and apoA-V remained associated with plasma VLDL.
Intravenously injected apoA-V rHDL significantly lowers plasma TG in an apoA-V deficient mouse model. Its intravenous administration may have therapeutic benefit in human subjects with severe HTG, especially in cases involving apoA-V variants associated with HTG.
apoav-/- mice; gpihbp1-/- mice; very low density lipoproteins; apoA-V heparin binding mutant; lipoprotein lipase
Determination of lipoprotein lipase (LPL) activity is important for hyperchylomicronemia diagnosis, but remains both unreliable and cumbersome with current methods. Consequently by using human VLDL as substrate we developed a new LPL assay which does not require sonication, radioactive or fluorescent particles.
Post-heparin plasma was added to the VLDL substrate prepared by ultracentrifugation of heat inactivated normolipidemic human serums, diluted in buffer, pH 8.15. Following incubation at 37°c, the NEFA (non esterified fatty acids) produced were assayed hourly for 4 hours. LPL activity was expressed as µmol/l/min after subtraction of hepatic lipase (HL) activity, obtained following LPL inhibition with NaCl 1.5 mmol/l. Molecular analysis of LPL, GPIHBP1, APOA5, APOC2, APOE genes was available for 62 patients.
Our method was reproducible (coefficient of variation (CV): intra-assay 5.6%, inter-assay 7.1%), and tightly correlated with the conventional radiolabelled triolein emulsion method (n = 26, r = 0.88). Normal values were established at 34.8±12.8 µmol/l/min (mean±SD) from 20 control subjects. LPL activities obtained from 71 patients with documented history of major hypertriglyceridemia showed a trimodal distribution. Among the 11 patients with a very low LPL activity (<10 µmol/l/min), 5 were homozygous or compound heterozygous for LPL or GPIHBP1 deleterious mutations, 3 were compound heterozygous for APOA5 deleterious mutations and the p.S19W APOA5 susceptibility variant, and 2 were free of any mutations in the usual candidate genes. No homozygous gene alteration in LPL, GPIHBP1 and APOC2 genes was found in any of the patients with LPL activity >10 µmol/l/min.
This new reproducible method is a valuable tool for routine diagnosis and reliably identifies LPL activity defects.