Objective

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.

Conclusions

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.

Purpose of review

We summarize recent progress on GPIHBP1, a molecule that transports lipoprotein lipase (LPL) to the capillary lumen, and discuss several newly studied molecules that appear important for the regulation of LPL activity.

Recent findings

LPL, the enzyme responsible for the lipolytic processing of triglyceride-rich lipoproteins, interacts with multiple proteins and is regulated at multiple levels. Several regulators of LPL activity have been known for years and have been investigated thoroughly, but several have been identified only recently, including an endothelial cell protein that transports LPL to the capillary lumen, a microRNA that reduces LPL transcript levels, a sorting protein that targets LPL for uptake and degradation, and a transcription factor that increases the expression of apolipoproteins that regulate LPL activity.

Summary

Proper regulation of LPL is important for controlling the delivery of lipid nutrients to tissues. Recent studies have identified GPIHBP1 as the molecule that transports LPL to the capillary lumen, and have also identified other molecules that are potentially important for regulating LPL activity. These new discoveries open new doors for understanding basic mechanisms of lipolysis and hyperlipidemia.

Summary

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.

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.

Recent findings

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.

Summary

GPIHBP1, an endothelial cell-surface glycoprotein, binds LPL and is required for the lipolytic processing of triglyceride-rich lipoproteins.

Objective

GPIHBP1, a glycosylphosphatidylinositol-anchored Ly6 protein of capillary endothelial cells, binds lipoprotein lipase (LPL) avidly, but its ability to bind related lipase family members has never been evaluated. We sought to define the ability of GPIHBP1 to bind other lipase family members as well as other apolipoproteins and lipoproteins.

Methods and Results

As judged by cell-based and cell-free binding assays, LPL binds to GPIHBP1 but other members of the lipase family do not. We also examined the binding of apoAV–phospholipid disks to GPIHBP1. ApoAV binds avidly to GPIHBP1-transfected cells; this binding requires GPIHBP1’s amino-terminal acidic domain and is independent of its cysteine-rich Ly6 domain (the latter domain is essential for LPL binding). GPIHBP1-transfected cells did not bind HDL. Chylomicrons binds avidly to GPIHBP1-transfected CHO cells, but this binding is dependent on GPIHBP1’s ability to bind LPL within the cell culture medium.

Conclusions

GPIHBP1 binds LPL but does not bind other lipase family members. GPIHBP1 binds apoAV but did not bind apoAI or HDL. The ability of GPIHBP1-transfected CHO cells to bind chylomicrons is mediated by LPL; chylomicron binding does not occur unless GPIHBP1 first captures LPL from the cell culture medium.

Objective

Adult GPIHBP1-deficient mice (Gpihbp1−/−) have severe hypertriglyceridemia; however, the plasma triglyceride levels are only mildly elevated during the suckling phase when lipoprotein lipase (Lpl) is expressed at high levels in the liver. Lpl expression in the liver can be induced in adult mice with dietary cholesterol. We therefore hypothesized that plasma triglyceride levels in adult Gpihbp1−/− mice would be sensitive to cholesterol intake.

Methods and Results

After 4–8 weeks on a western diet containing 0.15% cholesterol, plasma triglyceride levels in Gpihbp1−/− mice were 10,000–12,000 mg/dl. When 0.005% ezetimibe was added to the diet to block cholesterol absorption, Lpl expression in the liver was reduced significantly, and the plasma triglyceride levels were significantly higher (>15,000 mg/dl). We also assessed plasma triglyceride levels in Gpihbp1−/− mice fed western diets containing either high (1.3%) or low (0.05%) amounts of cholesterol. The high-cholesterol diet significantly increased Lpl expression in the liver and lowered plasma triglyceride levels.

Conclusions

Treatment of Gpihbp1−/− mice with ezetimibe lowers Lpl expression in the liver and increases plasma triglyceride levels. A high-cholesterol diet had the opposite effects. Thus, cholesterol intake modulates plasma triglyceride levels in Gpihbp1−/− mice.

In analyzing the sequence tags for mutant mouse embryonic stem (ES) cell lines in BayGenomics (a mouse gene-trapping resource), we identified a novel gene, Agpat6, with sequence similarities to previously characterized glycerolipid acyltransferases. Agpat6’s closest family member is another novel gene that we have provisionally designated Agpat8. Both Agpat6 and Agpat8 are conserved from plants, nematodes, and flies to mammals. AGPAT6, which is predicted to contain multiple membrane-spanning helices, is found exclusively within the endoplasmic reticulum in mammalian cells. To gain insights into the in vivo importance of Agpat6, we used the Agpat6 ES cell line from BayGenomics to create Agpat6-deficient (Agpat6−/−) mice. Agpat6−/− mice lacked full-length Agpat6 transcripts, as judged by northern blots. One of the most striking phenotypes of Agpat6−/− mice was a defect in lactation. Pups nursed by Agpat6−/− mothers die perinatally. Normally, Agpat6 is expressed at high levels in the mammary epithelium of breast tissue, but not in the surrounding adipose tissue. Histological studies revealed that the aveoli and ducts of Agpat6−/− lactating mammary glands were underdeveloped, and there was a dramatic decrease in size and number of lipid droplets within mammary epithelial cells and ducts. Also, the milk from Agpat6−/− mice was markedly depleted in diacylglycerols and triacylglycerols. Thus, we identified a novel glycerolipid acyltransferase of the endoplasmic reticulum, AGPAT6, which is crucial for the production of milk fat by the mammary gland.

SUMMARY

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.

Objective

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.

Conclusions

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.

Triglyceride synthesis in most mammalian tissues involves the sequential addition of fatty acids to a glycerol backbone, with unique enzymes required to catalyze each acylation step. Acylation at the sn-2 position requires 1-acylglycerol-3-phosphate O-acyltransferase (AGPAT) activity. To date, seven Agpat genes have been identified based on activity and/or sequence similarity, but their physiological functions have not been well established. We have generated a mouse model deficient in AGPAT6, which is normally expressed at high levels in brown adipose tissue (BAT), white adipose tissue (WAT), and liver. Agpat6-deficient mice exhibit a 25% reduction in body weight and resistance to both diet-induced and genetically induced obesity. The reduced body weight is associated with increased energy expenditure, reduced triglyceride accumulation in BAT and WAT, reduced white adipocyte size, and lack of adipose tissue in the subdermal region. In addition, the fatty acid composition of triacylglycerol, diacylglycerol, and phospholipid is altered, with proportionally greater polyunsaturated fatty acids at the expense of monounsaturated fatty acids. Thus, Agpat6 plays a unique role in determining triglyceride content and composition in adipose tissue and liver that cannot be compensated by other members of the Agpat family.

ATP-citrate lyase (Acly) is one of two cytosolic enzymes that synthesize acetyl-coenzyme A (CoA). Because acetyl-CoA is an essential building block for cholesterol and triglycerides, Acly has been considered a therapeutic target for hyperlipidemias and obesity. To define the phenotype of Acly-deficient mice, we created Acly knockout mice in which a β-galactosidase marker is expressed from Acly regulatory sequences. We also sought to define the cell type-specific expression patterns of Acly to further elucidate the in vivo roles of the enzyme. Homozygous Acly knockout mice died early in development. Heterozygous mice were healthy, fertile, and normolipidemic on both chow and high fat diets, despite expressing half-normal amounts of Acly mRNA and protein. Fibroblasts and hepatocytes from heterozygous Acly mice contained half-normal amounts of Acly mRNA and protein, but this did not perturb triglyceride and cholesterol synthesis or the expression of lipid biosynthetic genes regulated by sterol regulatory element-binding proteins. The expression of acetyl-CoA synthetase 1, another cytosolic enzyme for producing acetyl-CoA, was not up-regulated. As judged by β-galactosidase staining, Acly was expressed ubiquitously but was expressed particularly highly in tissues with high levels of lipogenesis, such as in the livers of mice fed a high-carbohydrate diet. β-Galactosidase staining was intense in the developing brain, in keeping with the high levels of de novo lipogenesis of the tissue. In the adult brain, β-galactosidase staining was in general much lower, consistent with reduced levels of lipogenesis; however, β-galactosidase expression remained very high in cholinergic neurons, likely reflecting the importance of Acly in generating acetyl-CoA for acetylcholine synthesis. The Acly knockout allele is useful for identifying cell types with a high demand for acetyl-CoA synthesis.

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.

Recent findings

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.

Summary

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.

Background

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.

Conclusions

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.

Objective

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.

Conclusions

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.

Summary

GPIHBP1 is an endothelial cell protein that serves as a platform for lipoprotein lipase–mediated processing of triglyceride-rich lipoproteins within the capillaries of heart, adipose tissue, and skeletal muscle. The absence of GPIHBP1 causes severe chylomicronemia. A hallmark of GPIHBP1 is the ability to bind lipoprotein lipase, chylomicrons, and apolipoprotein (apo-) AV. A homozygous G56R mutation in GPIHBP1 was recently identified in two brothers with chylomicronemia, and the authors of that study suggested that the G56R substitution was responsible for the hyperlipidemia. In this study, we created a human GPIHBP1 expression vector, introduced the G56R mutation, and tested the ability of the mutant GPIHBP1 to reach the cell surface and bind lipoprotein lipase, chylomicrons, and apo-AV. Our studies revealed that the G56R substitution did not affect the ability of GPIHBP1 to reach the cell surface, nor did the amino acid substitution have any discernible effect on the binding of lipoprotein lipase, chylomicrons, or apo-AV.

Summary

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.

Isoprenylcysteine carboxyl methyltransferase (Icmt) methylates the carboxyl-terminal isoprenylcysteine of CAAX proteins (e.g., Ras and Rho proteins). In the case of the Ras proteins, carboxyl methylation is important for targeting of the proteins to the plasma membrane. We hypothesized that a knockout of Icmt would reduce the ability of cells to be transformed by K-Ras. Fibroblasts harboring a floxed Icmt allele and expressing activated K-Ras (K-Ras-Icmtflx/flx) were treated with Cre-adenovirus, producing K-Ras-IcmtΔ/Δ fibroblasts. Inactivation of Icmt inhibited cell growth and K-Ras–induced oncogenic transformation, both in soft agar assays and in a nude mice model. The inactivation of Icmt did not affect growth factor–stimulated phosphorylation of Erk1/2 or Akt1. However, levels of RhoA were greatly reduced as a consequence of accelerated protein turnover. In addition, there was a large Ras/Erk1/2-dependent increase in p21Cip1, which was probably a consequence of the reduced levels of RhoA. Deletion of p21Cip1 restored the ability of K-Ras-IcmtΔ/Δ fibroblasts to grow in soft agar. The effect of inactivating Icmt was not limited to the inhibition of K-Ras–induced transformation: inactivation of Icmt blocked transformation by an oncogenic form of B-Raf (V599E). These studies identify Icmt as a potential target for reducing the growth of K-Ras– and B-Raf–induced malignancies.