Maturation of lipoprotein lipase in the endoplasmic reticulum. Concurrent formation of functional dimers and inactive aggregates

Genetic and functional studies of the LMF1 gene in Thai patients with severe hypertriglyceridemia

Severe hypertriglyceridemia (HTG) due to chylomicronemia is associated with acute pancreatitis and is related to genetic disturbances in several proteins involved in triglyceride (TG) metabolism. Lipase maturation factor 1 (LMF1) is a protein essential for the maturation of lipoprotein lipase (LPL). In this study, we examined the genetic spectrum of the LMF1 gene among subjects with severe HTG and investigated the functional significance of 6 genetic variants in vitro. All 11 exons of the LMF1 gene were sequenced in 101 Thai subjects with severe HTG. For an in vitro study, we performed site-directed mutagenesis, transient expression in cld cells, and measured LPL protein and LPL activity. We identified 2 common variants [p.(Gly36Asp) and p.(Pro562Arg)] and 12 rare variants [p.(Thr143Met), p.(Asn249Ser), p.(Ala287Val), p.(Met346Val), p.(Thr395Ile), p.(Gly410Arg), p.(Asp433Asn), p.(Asp491Asn), p.(Asn501Tyr), p.(Ala504Val), p.(Arg523His), and p.(Leu563Arg)] in 29 patients. In vitro study of the p.(Gly36Asp), p.(Asn249Ser), p.(Ala287Val), p.(Asn501Tyr), p.(Pro562Arg) and p.(Leu563Arg) variants, however, revealed that both LPL mass and LPL activity in each of the transfected cells were not significantly different from those in the wild type LMF1 transfected cells, suggesting that these variants might not play a significant role in severe HTG phenotype in our subjects.

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Among 101 subjects with severe hypertriglyceridemia (HTG), 2 common and 12 rare variants in the LMF1 gene were identified

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None of the 6 missense variants studied were associated with a reduction in lipoprotein mass or activity

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These rare variants in the LMF1 gene may not play an important role in severe HTG phenotypes in the Thai population

Coexpression of novel furin-resistant LPL variants with lipase maturation factor 1 enhances LPL secretion and activity

LPL is a secreted enzyme that hydrolyzes triglycerides from circulating lipoproteins. Individuals lacking LPL suffer from severe hypertriglyceridemia, a risk factor for acute pancreatitis. One potential treatment is to administer recombinant LPL as a protein therapeutic. However, use of LPL as a protein therapeutic is limited because it is an unstable enzyme that is difficult to produce in large quantities. Furthermore, these considerations also limit structural and biochemical studies that are needed for large-scale drug discovery efforts. We demonstrate that the yield of purified LPL can be dramatically enhanced by coexpressing its maturation factor, LMF1, and by introducing novel mutations into the LPL sequence to render it resistant to proteolytic cleavage by furin. One of these mutations introduces a motif for addition of an N-linked glycan to the furin-recognition site. Furin-resistant LPL has previously been reported, but is not commonly used. We show that our modifications do not adversely alter LPL's enzymatic activity, stability, or in vivo function. Together, these data show that furin-resistant LPL is a useful reagent for both biochemical and biomedical studies.

Lipase maturation factor 1 affects redox homeostasis in the endoplasmic reticulum

Lipoprotein lipase (LPL) is a secreted lipase that clears triglycerides from the blood. Proper LPL folding and exit from the endoplasmic reticulum (ER) require lipase maturation factor 1 (LMF1), an ER-resident transmembrane protein, but the mechanism involved is unknown. We used proteomics to identify LMF1-binding partners necessary for LPL secretion in HEK293 cells and found these to include oxidoreductases and lectin chaperones, suggesting that LMF1 facilitates the formation of LPL's five disulfide bonds. In accordance with this role, we found that LPL aggregates in LMF1-deficient cells due to the formation of incorrect intermolecular disulfide bonds. Cells lacking LMF1 were hypersensitive to depletion of glutathione, but not DTT treatment, suggesting that LMF1 helps reduce the ER Accordingly, we found that loss of LMF1 results in a more oxidized ER Our data show that LMF1 has a broader role than simply folding lipases, and we identified fibronectin and the low-density lipoprotein receptor (LDLR) as novel LMF1 clients that contain multiple, non-sequential disulfide bonds. We conclude that LMF1 is needed for secretion of some ER client proteins that require reduction of non-native disulfides during their folding.

A Compound Heterozygous Mutation of Lipase Maturation Factor 1 is Responsible for Hypertriglyceridemia of a Patient

AIM

Dyslipidemia is the most common lipid metabolism disorder in humans, and its etiology remains elusive. Hypertriglyceridemia (HTG) is a type of dyslipidemia that contributes to atherosclerosis and coronary heart disease. Previous studies have demonstrated that mutations in lipoprotein lipase (LPL), apolipoprotein CII (APOC2), apolipoprotein AV (APOA5), glycosylphosphatidylinositol anchored high-density lipoprotein-binding protein 1 (GPIHBP1), lipase maturation factor 1(LMF1), and glycerol-3 phosphate dehydrogenase 1 (GPD1) are responsible for HTG by using genomic microarrays and next-generation sequencing. The aim of this study was to identify genetic lesions in patients with HTG.

METHOD

Our study included a family of seven members from Jiangsu province across three generations. The proband was diagnosed with severe HTG, with a plasma triglyceride level of 38.70 mmol/L. Polymerase chain reaction (PCR) and Sanger sequencing were performed to explore the possible causative gene mutations for this patient. Furthermore, we measured the post-heparin LPL and hepatic lipase (HL) activities using an antiserum inhibition method.

RESULTS

A compound heterozygous mutation in the LMF1 gene (c.257C＞T/p.P86L and c.1184C＞T/p.T395I) was identified and co-segregated with the affected patient in this family. Both mutations were predicted to be deleterious by three bioinformatics programs (Polymorphism Phenotyping-2, Sorting Intolerant From Tolerant, and MutationTaster). The levels of the plasma post-heparin LPL and HL activities in the proband (57 and 177 mU/mL) were reduced to 24% and 75%, respectively, compared with those assayed in the control subject with normal plasma triglycerides.

CONCLUSION

A compound heterozygous mutation of LMF1 was identified in the presenting patient with severe HTG. These findings expand on the spectrum of LMF1 mutations and contribute to the genetic diagnosis and counseling of families with HTG.

Mutating a conserved cysteine in GPIHBP1 reduces amounts of GPIHBP1 in capillaries and abolishes LPL binding

Mutation of conserved cysteines in proteins of the Ly6 family cause human disease-chylomicronemia in the case of glycosylphosphatidylinositol-anchored HDL binding protein 1 (GPIHBP1) and paroxysmal nocturnal hemoglobinuria in the case of CD59. A mutation in a conserved cysteine in CD59 prevented the protein from reaching the surface of blood cells. In contrast, mutation of conserved cysteines in human GPIHBP1 had little effect on GPIHBP1 trafficking to the surface of cultured CHO cells. The latter findings were somewhat surprising and raised questions about whether CHO cell studies accurately model the fate of mutant GPIHBP1 proteins in vivo. To explore this concern, we created mice harboring a GPIHBP1 cysteine mutation (p.C63Y). The p.C63Y mutation abolished the ability of mouse GPIHBP1 to bind LPL, resulting in severe chylomicronemia. The mutant GPIHBP1 was detectable by immunohistochemistry on the surface of endothelial cells, but the level of expression was ∼70% lower than in WT mice. The mutant GPIHBP1 protein in mouse tissues was predominantly monomeric. We conclude that mutation of a conserved cysteine in GPIHBP1 abolishes the ability of GPIHBP1 to bind LPL, resulting in mislocalization of LPL and severe chylomicronemia. The mutation reduced but did not eliminate GPIHBP1 on the surface of endothelial cells in vivo.

Monoclonal antibodies that bind to the Ly6 domain of GPIHBP1 abolish the binding of LPL

GPIHBP1, an endothelial cell protein, binds LPL in the interstitial spaces and shuttles it to its site of action inside blood vessels. For years, studies of human GPIHBP1 have been hampered by an absence of useful antibodies. We reasoned that monoclonal antibodies (mAbs) against human GPIHBP1 would be useful for 1) defining the functional relevance of GPIHBP1's Ly6 and acidic domains to the binding of LPL; 2) ascertaining whether human GPIHBP1 is expressed exclusively in capillary endothelial cells; and 3) testing whether GPIHBP1 is detectable in human plasma. Here, we report the development of a panel of human GPIHBP1-specific mAbs. Two mAbs against GPIHBP1's Ly6 domain, RE3 and RG3, abolished LPL binding, whereas an antibody against the acidic domain, RF4, did not. Also, mAbs RE3 and RG3 bound with reduced affinity to a mutant GPIHBP1 containing an Ly6 domain mutation (W109S) that abolishes LPL binding. Immunohistochemistry studies with the GPIHBP1 mAbs revealed that human GPIHBP1 is expressed only in capillary endothelial cells. Finally, we created an ELISA that detects GPIHBP1 in human plasma. That ELISA should make it possible for clinical lipidologists to determine whether plasma GPIHBP1 levels are a useful biomarker of metabolic or vascular disease.

Mobility of "HSPG-bound" LPL explains how LPL is able to reach GPIHBP1 on capillaries

In mice lacking glycosylphosphatidylinositol-anchored high density lipoprotein binding protein 1 (GPIHBP1), the LPL secreted by adipocytes and myocytes remains bound to heparan sulfate proteoglycans (HSPGs) on all cells within tissues. That observation raises a perplexing issue: Why isn't the freshly secreted LPL in wild-type mice captured by the same HSPGs, thereby preventing LPL from reaching GPIHBP1 on capillaries? We hypothesized that LPL-HSPG interactions are transient, allowing the LPL to detach and move to GPIHBP1 on capillaries. Indeed, we found that LPL detaches from HSPGs on cultured cells and moves to: 1) soluble GPIHBP1 in the cell culture medium; 2) GPIHBP1-coated agarose beads; and 3) nearby GPIHBP1-expressing cells. Movement of HSPG-bound LPL to GPIHBP1 did not occur when GPIHBP1 contained a Ly6 domain missense mutation (W109S), but was almost normal when GPIHBP1's acidic domain was mutated. To test the mobility of HSPG-bound LPL in vivo, we injected GPIHBP1-coated agarose beads into the brown adipose tissue of GPIHBP1-deficient mice. LPL moved quickly from HSPGs on adipocytes to GPIHBP1-coated beads, thereby depleting LPL stores on the surface of adipocytes. We conclude that HSPG-bound LPL in the interstitial spaces of tissues is mobile, allowing the LPL to move to GPIHBP1 on endothelial cells.

An LPL-specific monoclonal antibody, 88B8, that abolishes the binding of LPL to GPIHBP1

LPL contains two principal domains: an amino-terminal catalytic domain (residues 1-297) and a carboxyl-terminal domain (residues 298-448) that is important for binding lipids and binding glycosylphosphatidylinositol-anchored high density lipoprotein binding protein 1 (GPIHBP1) (an endothelial cell protein that shuttles LPL to the capillary lumen). The LPL sequences required for GPIHBP1 binding have not been examined in detail, but one study suggested that sequences near LPL's carboxyl terminus (residues ∼403-438) were crucial. Here, we tested the ability of LPL-specific monoclonal antibodies (mAbs) to block the binding of LPL to GPIHBP1. One antibody, 88B8, abolished LPL binding to GPIHBP1. Consistent with those results, antibody 88B8 could not bind to GPIHBP1-bound LPL on cultured cells. Antibody 88B8 bound poorly to LPL proteins with amino acid substitutions that interfered with GPIHBP1 binding (e.g., C418Y, E421K). However, the sequences near LPL's carboxyl terminus (residues ∼403-438) were not sufficient for 88B8 binding; upstream sequences (residues 298-400) were also required. Additional studies showed that these same sequences are required for LPL binding to GPIHBP1. In conclusion, we identified an LPL mAb that binds to LPL's GPIHBP1-binding domain. The binding of both antibody 88B8 and GPIHBP1 to LPL depends on large segments of LPL's carboxyl-terminal domain.

Angiopoietin-like 4 promotes intracellular degradation of lipoprotein lipase in adipocytes

LPL hydrolyzes triglycerides in triglyceride-rich lipoproteins along the capillaries of heart, skeletal muscle, and adipose tissue. The activity of LPL is repressed by angiopoietin-like 4 (ANGPTL4) but the underlying mechanisms have not been fully elucidated. Our objective was to study the cellular location and mechanism for LPL inhibition by ANGPTL4. We performed studies in transfected cells, ex vivo studies, and in vivo studies with Angptl4(-/-) mice. Cotransfection of CHO pgsA-745 cells with ANGPTL4 and LPL reduced intracellular LPL protein levels, suggesting that ANGPTL4 promotes LPL degradation. This conclusion was supported by studies of primary adipocytes and adipose tissue explants from wild-type and Angptl4(-/-) mice. Absence of ANGPTL4 resulted in accumulation of the mature-glycosylated form of LPL and increased secretion of LPL. Blocking endoplasmic reticulum (ER)-Golgi transport abolished differences in LPL abundance between wild-type and Angptl4(-/-) adipocytes, suggesting that ANGPTL4 acts upon LPL after LPL processing in the ER. Finally, physiological changes in adipose tissue ANGPTL4 expression during fasting and cold resulted in inverse changes in the amount of mature-glycosylated LPL in wild-type mice, but not Angptl4(-/-) mice. We conclude that ANGPTL4 promotes loss of intracellular LPL by stimulating LPL degradation after LPL processing in the ER.

Cardiomyocyte-endothelial cell control of lipoprotein lipase

In people with diabetes, inadequate pharmaceutical management predisposes the patient to heart failure, which is the leading cause of diabetes related death. One instigator for this cardiac dysfunction is change in fuel utilization by the heart. Thus, following diabetes, when cardiac glucose utilization is impaired, the heart undergoes metabolic transformation wherein it switches to using fats as an exclusive source of energy. Although this switching is geared to help the heart initially, in the long term, this has detrimental effects on cardiac function. These include the generation of noxious byproducts, which damage the cardiomyocytes, and ultimately result in increased morbidity and mortality. A key perpetrator that may be responsible for organizing this metabolic disequilibrium is lipoprotein lipase (LPL), the enzyme responsible for providing fat to the hearts. Either exaggeration or reduction in its activity following diabetes could lead to heart dysfunction. Given the disturbing news that diabetes is rampant across the globe, gaining more insight into the mechanism(s) by which cardiac LPL is regulated may assist other researchers in devising new therapeutic strategies to restore metabolic equilibrium, to help prevent or delay heart disease seen during diabetes. This article is part of a Special Issue entitled: Heart Lipid Metabolism edited by G.D. Lopaschuk.

Angiopoietin-like 4 Modifies the Interactions between Lipoprotein Lipase and Its Endothelial Cell Transporter GPIHBP1

The release of fatty acids from plasma triglycerides for tissue uptake is critically dependent on the enzyme lipoprotein lipase (LPL). Hydrolysis of plasma triglycerides by LPL can be disrupted by the protein angiopoietin-like 4 (ANGPTL4), and ANGPTL4 has been shown to inactivate LPL in vitro. However, in vivo LPL is often complexed to glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1) on the surface of capillary endothelial cells. GPIHBP1 is responsible for trafficking LPL across capillary endothelial cells and anchors LPL to the capillary wall during lipolysis. How ANGPTL4 interacts with LPL in this context is not known. In this study, we investigated the interactions of ANGPTL4 with LPL-GPIHBP1 complexes on the surface of endothelial cells. We show that ANGPTL4 was capable of binding and inactivating LPL complexed to GPIHBP1 on the surface of endothelial cells. Once inactivated, LPL dissociated from GPIHBP1. We also show that ANGPTL4-inactivated LPL was incapable of binding GPIHBP1. ANGPTL4 was capable of binding, but not inactivating, LPL at 4 °C, suggesting that binding alone was not sufficient for ANGPTL4's inhibitory activity. We observed that although the N-terminal coiled-coil domain of ANGPTL4 by itself and full-length ANGPTL4 both bound with similar affinities to LPL, the N-terminal fragment was more potent in inactivating both free and GPIHBP1-bound LPL. These results led us to conclude that ANGPTL4 can both bind and inactivate LPL complexed to GPIHBP1 and that inactivation of LPL by ANGPTL4 greatly reduces the affinity of LPL for GPIHBP1.

Intrinsic and extrinsic regulation of cardiac lipoprotein lipase following diabetes

Cardiac lipoprotein lipase (LPL) is a pivotal enzyme controlling heart metabolism by providing the majority of fatty acids required by this organ. From activation in cardiomyocytes to secretion to the vascular lumen, cardiac LPL is regulated by multiple pathways, which are altered during diabetes. Hence, dimerization/activation of LPL is modified following diabetes, a process controlled by lipase maturation factor 1. The role of AMP-activated protein kinase, protein kinase D, and heparan sulfate proteoglycans, intrinsic factors that regulate the intracellular transport of LPL is also shifted, and is discussed. More recent studies have identified several exogenous factors released from endothelial cells (EC) and adipose tissue that are required for proper functioning of LPL. In response to hyperglycemia, both active and latent heparanase are released from EC to facilitate LPL secretion. Diabetes also increased the expression of glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1) in EC, which mediates the transport of LPL across EC. Angiopoietin-like protein 4 secreted from the adipose tissue has the potential to reduce coronary LPL activity. Knowledge of these intrinsic and extrinsic factors could be used develop therapeutic targets to normalize LPL function, and maintain cardiac energy homeostasis after diabetes.

GPIHBP1 missense mutations often cause multimerization of GPIHBP1 and thereby prevent lipoprotein lipase binding

RATIONALE

GPIHBP1, a GPI-anchored protein of capillary endothelial cells, binds lipoprotein lipase (LPL) in the subendothelial spaces and shuttles it to the capillary lumen. GPIHBP1 missense mutations that interfere with LPL binding cause familial chylomicronemia.

OBJECTIVE

We sought to understand mechanisms by which GPIHBP1 mutations prevent LPL binding and lead to chylomicronemia.

METHODS AND RESULTS

We expressed mutant forms of GPIHBP1 in Chinese hamster ovary cells, rat and human endothelial cells, and Drosophila S2 cells. In each expression system, mutation of cysteines in GPIHBP1's Ly6 domain (including mutants identified in patients with chylomicronemia) led to the formation of disulfide-linked dimers and multimers. GPIHBP1 dimerization/multimerization was not unique to cysteine mutations; mutations in other amino acid residues, including several associated with chylomicronemia, also led to protein dimerization/multimerization. The loss of GPIHBP1 monomers is relevant to the pathogenesis of chylomicronemia because only GPIHBP1 monomers-and not dimers or multimers-are capable of binding LPL. One GPIHBP1 mutant, GPIHBP1-W109S, had distinctive properties. GPIHBP1-W109S lacked the ability to bind LPL but had a reduced propensity for forming dimers or multimers, suggesting that W109 might play a more direct role in binding LPL. In support of that idea, replacing W109 with any of 8 other amino acids abolished LPL binding-and often did so without promoting the formation of dimers and multimers.

CONCLUSIONS

Many amino acid substitutions in GPIHBP1's Ly6 domain that abolish LPL binding lead to protein dimerization/multimerization. Dimerization/multimerization is relevant to disease pathogenesis, given that only GPIHBP1 monomers are capable of binding LPL.

Lipoprotein lipase: from gene to atherosclerosis

Lipoprotein lipase (LPL) is a key enzyme in lipid metabolism and responsible for catalyzing lipolysis of triglycerides in lipoproteins. LPL is produced mainly in adipose tissue, skeletal and heart muscle, as well as in macrophage and other tissues. After synthesized, it is secreted and translocated to the vascular lumen. LPL expression and activity are regulated by a variety of factors, such as transcription factors, interactive proteins and nutritional state through complicated mechanisms. LPL with different distributions may exert distinct functions and have diverse roles in human health and disease with close association with atherosclerosis. It may pose a pro-atherogenic or an anti-atherogenic effect depending on its locations. In this review, we will discuss its gene, protein, synthesis, transportation and biological functions, and then focus on its regulation and relationship with atherosclerosis and potential underlying mechanisms. The goal of this review is to provide basic information and novel insight for further studies and therapeutic targets.

The ER-associated degradation adaptor protein Sel1L regulates LPL secretion and lipid metabolism

Sel1L is an essential adaptor protein for the E3 ligase Hrd1 in the endoplasmic reticulum (ER)-associated degradation (ERAD), a universal quality-control system in the cell; but its physiological role remains unclear. Here we show that mice with adipocyte-specific Sel1L deficiency are resistant to diet-induced obesity and exhibit postprandial hypertriglyceridemia. Further analyses reveal that Sel1L is indispensable for the secretion of lipoprotein lipase (LPL), independent of its role in Hrd1-mediated ERAD and ER homeostasis. Sel1L physically interacts with and stabilizes the LPL maturation complex consisting of LPL and lipase maturation factor 1 (LMF1). In the absence of Sel1L, LPL is retained in the ER and forms protein aggregates, which are degraded primarily by autophagy. The Sel1L-mediated control of LPL secretion is also seen in other LPL-expressing cell types including cardiac myocytes and macrophages. Thus, our study reports a role of Sel1L in LPL secretion and systemic lipid metabolism.

Lipase maturation factor 1 (lmf1) is induced by endoplasmic reticulum stress through activating transcription factor 6α (Atf6α) signaling

Lipase maturation factor 1 (Lmf1) is a critical determinant of plasma lipid metabolism, as demonstrated by severe hypertriglyceridemia associated with its mutations in mice and human subjects. Lmf1 is a chaperone localized to the endoplasmic reticulum (ER) and required for the post-translational maturation and activation of several vascular lipases. Despite its importance in plasma lipid homeostasis, the regulation of Lmf1 remains unexplored. We report here that Lmf1 expression is induced by ER stress in various cell lines and in tunicamycin (TM)-injected mice. Using genetic deficiencies in mouse embryonic fibroblasts and mouse liver, we identified the Atf6α arm of the unfolded protein response as being responsible for the up-regulation of Lmf1 in ER stress. Experiments with luciferase reporter constructs indicated that ER stress activates the Lmf1 promoter through a GC-rich DNA sequence 264 bp upstream of the transcriptional start site. We demonstrated that Atf6α is sufficient to induce the Lmf1 promoter in the absence of ER stress, and this effect is mediated by the TM-responsive cis-regulatory element. Conversely, Atf6α deficiency induced by genetic ablation or a dominant-negative form of Atf6α abolished TM stimulation of the Lmf1 promoter. In conclusion, our results indicate that Lmf1 is an unfolded protein response target gene, and Atf6α signaling is sufficient and necessary for activation of the Lmf1 promoter. Importantly, the induction of Lmf1 by ER stress appears to be a general phenomenon not restricted to lipase-expressing cells, which suggests a lipase-independent cellular role for this protein in ER homeostasis.

Purification, cellular levels, and functional domains of lipase maturation factor 1

Over a third of the US adult population has hypertriglyceridemia, resulting in an increased risk of atherosclerosis, pancreatitis, and metabolic syndrome. Lipoprotein lipase (LPL), a dimeric enzyme, is the main lipase responsible for TG clearance from the blood after food intake. LPL requires an endoplasmic reticulum (ER)-resident, transmembrane protein known as lipase maturation factor 1 (LMF1) for secretion and enzymatic activity. LMF1 is believed to act as a client specific chaperone for dimeric lipases, but the precise mechanism by which LMF1 functions is not understood. Here, we examine which domains of LMF1 contribute to dimeric lipase maturation by assessing the function of truncation variants. N-terminal truncations of LMF1 show that all the domains are necessary for LPL maturation. Fluorescence microscopy and protease protection assays confirmed that these variants were properly oriented in the ER. We measured cellular levels of LMF1 and found that it is expressed at low levels and each molecule of LMF1 promotes the maturation of 50 or more molecules of LPL. Thus we provide evidence for the critical role of the N-terminus of LMF1 for the maturation of LPL and relevant ratio of chaperone to substrate.

Multimerization of glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1) and familial chylomicronemia from a serine-to-cysteine substitution in GPIHBP1 Ly6 domain

GPIHBP1, a glycosylphosphatidylinositol-anchored glycoprotein of microvascular endothelial cells, binds lipoprotein lipase (LPL) within the interstitial spaces and transports it across endothelial cells to the capillary lumen. The ability of GPIHBP1 to bind LPL depends on the Ly6 domain, a three-fingered structure containing 10 cysteines and a conserved pattern of disulfide bond formation. Here, we report a patient with severe hypertriglyceridemia who was homozygous for a GPIHBP1 point mutation that converted a serine in the GPIHBP1 Ly6 domain (Ser-107) to a cysteine. Two hypertriglyceridemic siblings were homozygous for the same mutation. All three homozygotes had very low levels of LPL in the preheparin plasma. We suspected that the extra cysteine in GPIHBP1-S107C might prevent the trafficking of the protein to the cell surface, but this was not the case. However, nearly all of the GPIHBP1-S107C on the cell surface was in the form of disulfide-linked dimers and multimers, whereas wild-type GPIHBP1 was predominantly monomeric. An insect cell GPIHBP1 expression system confirmed the propensity of GPIHBP1-S107C to form disulfide-linked dimers and to form multimers. Functional studies showed that only GPIHBP1 monomers bind LPL. In keeping with that finding, there was no binding of LPL to GPIHBP1-S107C in either cell-based or cell-free binding assays. We conclude that an extra cysteine in the GPIHBP1 Ly6 motif results in multimerization of GPIHBP1, defective LPL binding, and severe hypertriglyceridemia.

Regulation of lipoprotein lipase by Angptl4

Triglyceride (TG)-rich chylomicrons and very low density lipoproteins (VLDL) distribute fatty acids (FA) to various tissues by interacting with the enzyme lipoprotein lipase (LPL). The protein angiopoietin-like 4 (Angptl4) is under sensitive transcriptional control by FA and the FA-activated peroxisome proliferator activated receptors (PPARs), and its tissue expression largely overlaps with that of LPL. Growing evidence indicates that Angptl4 mediates the physiological fluctuations in LPL activity, including the decrease in adipose tissue LPL activity during fasting. This review focuses on the major ambiguities concerning the mechanism of LPL inhibition by Angptl4, as well as on the physiological role of Angptl4 in lipid metabolism, highlighting its function in a variety of tissues, and uses this information to make suggestions for further research.

Beneficial effects of Yerba Mate tea (Ilex paraguariensis) on hyperlipidemia in high-fat-fed hamsters

Yerba Mate tea (Mate), an infusion made from the leaves of the tree Ilex paraguariensis, is a widely consumed beverage in South America. Mate has previously been shown to have hypolipidemic effects. However, its mechanism of action is not well understood. This study was conducted to determine the effect of Mate on hyperlipidemia induced in hamsters by a high-fat diet, as well as its mechanism of action. Fifty male hamsters were randomly assigned to normal control, high-fat control, and high-fat with Mate tea aqueous extract (1%, 2% or 4% w/v) groups. We evaluated the effects of Mate aqueous extract on body weight, serum lipids, antioxidant enzyme activity, lipoprotein metabolism enzyme activity, and gene expression involved in lipid metabolism in hyperlipidemic hamsters. Mate aqueous extract significantly decreased body-weight gain and lowered serum lipid levels in the hyperlipidemic hamster model. Meanwhile, Mate treatment increased antioxidant enzyme activity, improved lipoprotein lipase (LPL) and hepatic lipase (HL) activities in serum and liver, upregulated mRNA expression of peroxisome proliferator-activated receptor α and low density lipoprotein receptor, and downregulated mRNA expression of sterol regulatory element-binding protein 1c and acetyl CoA carboxylase in the liver. The results indicate that Mate tea ameliorates hyperlipidemia partly by reducing lipid peroxidation, improving endothelial function and LPL and HL activities, and modulating the expression levels of genes involved in lipid oxidation and lipogenesis.

Transgenic expression and genetic variation of Lmf1 affect LPL activity in mice and humans

OBJECTIVE

Lipoprotein lipase (LPL) is a principal enzyme in lipoprotein metabolism, tissue lipid utilization, and energy metabolism. LPL is synthesized by parenchymal cells in adipose, heart, and muscle tissues followed by secretion to extracellular sites, where lipolyic function is exerted. The catalytic activity of LPL is attained during posttranslational maturation, which involves glycosylation, folding, and subunit assembly within the endoplasmic reticulum. A lipase-chaperone, lipase maturation factor 1 (Lmf1), has recently emerged as a critical factor in this process. Previous studies demonstrated that loss-of-function mutations of Lmf1 result in diminished lipase activity and severe hypertriglyceridemia in mice and human subjects. The objective of this study is to investigate whether, beyond its role as a required factor in lipase maturation, variation in Lmf1 expression is sufficient to modulate LPL activity in vivo.

METHODS AND RESULTS

To assess the effects of Lmf1 overexpression in adipose and muscle tissues, we generated aP2-Lmf1 and Mck-Lmf1 transgenic mice. Characterization of relevant tissues revealed increased LPL activity in both mouse strains. In the omental and subcutaneous adipose depots, Lmf1 overexpression was associated with increased LPL specific activity without changes in LPL mass. In contrast, increased LPL activity was due to elevated LPL protein level in heart and gonadal adipose tissue. To extend these studies to humans, we detected association between LMF1 gene variants and postheparin LPL activity in a dyslipidemic cohort.

CONCLUSIONS

Our results suggest that variation in Lmf1 expression is a posttranslational determinant of LPL activity.

Lipase maturation factor 1: a lipase chaperone involved in lipid metabolism

Mutations in lipase maturation factor 1 (LMF1) are associated with severe hypertriglyceridemia in mice and human subjects. The underlying cause is impaired lipid clearance due to lipase deficiency. LMF1 is a chaperone of the endoplasmic reticulum (ER) and it is critically required for the post-translational activation of three vascular lipases: lipoprotein lipase (LPL), hepatic lipase (HL) and endothelial lipase (EL). As LMF1 is only required for the maturation of homodimeric, but not monomeric, lipases, it is likely involved in the assembly of inactive lipase subunits into active enzymes and/or the stabilization of active dimers. Herein, we provide an overview of current understanding of LMF1 function and propose that it may play a regulatory role in lipase activation and lipid metabolism. Further studies will be required to test this hypothesis and elucidate the full spectrum of phenotypes in combined lipase deficiency. This article is part of a Special Issue entitled Triglyceride Metabolism and Disease.

Severity of diabetes governs vascular lipoprotein lipase by affecting enzyme dimerization and disassembly

OBJECTIVE

In diabetes, when glucose consumption is restricted, the heart adapts to use fatty acid (FA) exclusively. The majority of FA provided to the heart comes from the breakdown of circulating triglyceride (TG), a process catalyzed by lipoprotein lipase (LPL) located at the vascular lumen. The objective of the current study was to determine the mechanisms behind LPL processing and breakdown after moderate and severe diabetes.

RESEARCH DESIGN AND METHODS

To induce acute hyperglycemia, diazoxide, a selective, ATP-sensitive K(+) channel opener was used. For chronic diabetes, streptozotocin, a β-cell-specific toxin was administered at doses of 55 or 100 mg/kg to generate moderate and severe diabetes, respectively. Cardiac LPL processing into active dimers and breakdown at the vascular lumen was investigated.

RESULTS

After acute hyperglycemia and moderate diabetes, more LPL is processed into an active dimeric form, which involves the endoplasmic reticulum chaperone calnexin. Severe diabetes results in increased conversion of LPL into inactive monomers at the vascular lumen, a process mediated by FA-induced expression of angiopoietin-like protein 4 (Angptl-4).

CONCLUSIONS

In acute hyperglycemia and moderate diabetes, exaggerated LPL processing to dimeric, catalytically active enzyme increases coronary LPL, delivering more FA to the heart when glucose utilization is compromised. In severe chronic diabetes, to avoid lipid oversupply, FA-induced expression of Angptl-4 leads to conversion of LPL to inactive monomers at the coronary lumen to impede TG hydrolysis. Results from this study advance our understanding of how diabetes changes coronary LPL, which could contribute to cardiovascular complications seen with this disease.

Effect of pu-erh tea on body fat and lipid profiles in rats with diet-induced obesity

The antiobesity and antihyperlipidaemic effects of pu-erh tea in rats with high fat diet (HFD)-induced obesity were investigated. Male Sprague-Dawley rats were randomly divided into five groups and fed varying diets for an 8-week period: control diet, HFD, and HFD supplemented with low, moderate or high doses of pu-erh tea extract (0.5 g, 2 g and 4 g/kg BW/day, respectively). Pu-erh tea significantly reduced the total body weight and the weight of various adipose pads. Pu-erh tea administration also significantly lowered plasma total cholesterol, triglyceride concentrations and low-density lipoprotein-cholesterol levels in rats with HFD-induced obesity, but did not affect high-density lipoprotein-cholesterol levels. Moreover, pu-erh tea significantly increased lipoprotein lipase, hepatic lipase and hormone-sensitive lipase activities in epididymal fat tissue in rats with HFD-induced obesity. Analysis of real-time reverse transcription-polymerase chain reaction results indicated that pu-erh tea significantly enhanced mRNA levels of hormone-sensitive lipase in rats with HFD-induced obesity. These results suggest that pu-erh tea attenuated visceral fat accumulation and improved hyperlipidemia in a rat model of HFD-induced obesity.

Lipase maturation factor 1 is required for endothelial lipase activity

Lipase maturation factor 1 (Lmf1) is an endoplasmic reticulum (ER) membrane protein involved in the posttranslational folding and/or assembly of lipoprotein lipase (LPL) and hepatic lipase (HL) into active enzymes. Mutations in Lmf1 are associated with diminished LPL and HL activities ("combined lipase deficiency") and result in severe hypertriglyceridemia in mice as well as in human subjects. Here, we investigate whether endothelial lipase (EL) also requires Lmf1 to attain enzymatic activity. We demonstrate that cells harboring a (cld) loss-of-function mutation in the Lmf1 gene are unable to generate active EL, but they regain this capacity after reconstitution with the Lmf1 wild type. Furthermore, we show that cellular EL copurifies with Lmf1, indicating their physical interaction in the ER. Finally, we determined that post-heparin phospholipase activity in a patient with the LMF1(W464X) mutation is reduced by more than 95% compared with that in controls. Thus, our study indicates that EL is critically dependent on Lmf1 for its maturation in the ER and demonstrates that Lmf1 is a required factor for all three vascular lipases, LPL, HL, and EL.

Ndrg1 in development and maintenance of the myelin sheath

CMT4D disease is a severe autosomal recessive demyelinating neuropathy with extensive axonal loss leading to early disability, caused by mutations in the N-myc downstream regulated gene 1 (NDRG1). NDRG1 is expressed at particularly high levels in the Schwann cell (SC), but its physiological function(s) are unknown. To help with their understanding, we characterise the phenotype of a new mouse model, stretcher (str), with total Ndrg1 deficiency, in comparison with the hypomorphic Ndrg1 knock-out (KO) mouse. While both models display normal initial myelination and a transition to overt pathology between weeks 3 and 5, the markedly more severe str phenotype suggests that even low Ndrg1 expression results in significant phenotype rescue. Neither model replicates fully the features of CMT4D: although axon damage is present, regenerative capacity is unimpaired and the mice do not display the early severe axonal loss typical of the human disease. The widespread large fibre demyelination coincides precisely with the period of rapid growth of the animals and the dramatic (160-500-fold) increase in myelin volume and length in large fibres. This is followed by stabilisation after week 10, while small fibres remain unaffected. Gene expression profiling of str peripheral nerve reveals non-specific secondary changes at weeks 5 and 10 and preliminary data point to normal proteasomal function. Our findings do not support the proposed roles of NDRG1 in growth arrest, terminal differentiation, gene expression regulation and proteasomal degradation. Impaired SC trafficking failing to meet the considerable demands of nerve growth, emerges as the likely pathogenetic mechanism in NDRG1 deficiency.

Lipase maturation factor 1: structure and role in lipase folding and assembly

PURPOSE OF REVIEW

Lipase maturation factor 1 (LMF1) is a membrane-bound protein located in the endoplasmic reticulum. It is essential to the folding and assembly (i.e., maturation) of a selected group of lipases that include lipoprotein lipase, hepatic lipase and endothelial lipase. The purpose of this review is to examine recent studies that have begun to elucidate the structure and function of LMF1 and to place it in the context of lipase folding and assembly.

RECENT FINDINGS

Recent studies identified mutations in LMF1 that cause combined lipase deficiency and hypertriglyceridemia in humans. These mutations result in the truncation of a large, evolutionarily conserved domain (DUF1222), which is essential for interaction with lipases and their attainment of enzymatic activity. The structural complexity of LMF1 has been further characterized by solving its topology in the endoplasmic reticulum membrane. Recent studies indicate that in addition to lipoprotein lipase and hepatic lipase, the maturation of endothelial lipase is also dependent on LMF1. Based on its apparent specificity for dimeric lipases, LMF1 is proposed to play an essential role in the assembly and/or stabilization of head-to-tail lipase homodimers.

SUMMARY

LMF1 functions in the maturation of a selected group of secreted lipases that assemble into homodimers in the endoplasmic reticulum. These dimeric lipases include lipoprotein lipase, hepatic lipase and endothelial lipase, all of which contribute significantly to plasma triglyceride and high-density lipoprotein cholesterol levels in humans. Future studies involving genetically engineered mouse models will be required to fully elucidate the role of LMF1 in normal physiology and diseases.

Novel LMF1 nonsense mutation in a patient with severe hypertriglyceridemia

CONTEXT

Lipase maturation factor 1 (LMF1) gene is a novel candidate gene in severe hypertriglyceridemia. Lmf1 is involved in the maturation of lipoprotein lipase (LPL) and hepatic lipase in endoplasmic reticulum. To date only one patient with severe hypertriglyceridemia and related disorders was found to be homozygous for a nonsense mutation in LMF1 gene (Y439X).

OBJECTIVE

The objective of the study was to investigate LMF1 gene in hypertriglyceridemic patients in whom mutations in LPL, APOC2, and APOA5 genes had been excluded.

RESULTS

The resequencing of LMF1 gene led to the discovery of a novel homozygous nonsense mutation in one patient with severe hypertriglyceridemia and recurrent episodes of pancreatitis. The mutation causes a G>A substitution in exon 9 (c.1395G>A), leading to a premature stop codon (W464X). LPL activity and mass were reduced by 76 and 50%, respectively, compared with normolipidemic controls. The proband over the years has shown a good response to treatment. The proband's son, heterozygous for the W464X, shows normal plasma triglyceride levels.

CONCLUSIONS

We identified the second novel pathogenic mutation in LMF1 gene in a patient with severe hypertriglyceridemia. LPL deficiency in our patient was milder than in the carrier of the Y439X previously described.

Lipase maturation factor LMF1, membrane topology and interaction with lipase proteins in the endoplasmic reticulum

Lipase maturation factor 1 (LMF1) is predicted to be a polytopic protein localized to the endoplasmic reticulum (ER) membrane. It functions in the post-translational attainment of enzyme activity for both lipoprotein lipase and hepatic lipase. By using transmembrane prediction methods in mouse and human orthologs, models of LMF1 topology were constructed and tested experimentally. Employing a tagging strategy that used insertion of ectopic glycan attachment sites and terminal fusions of green fluorescent protein, we established a five-transmembrane model, thus dividing LMF1 into six domains. Three domains were found to face the cytoplasm (the amino-terminal domain and loops B and D), and the other half was oriented to the ER lumen (loops A and C and the carboxyl-terminal domain). This representative model shows the arrangement of an evolutionarily conserved domain within LMF1 (DUF1222) that is essential to lipase maturation. DUF1222 comprises four of the six domains, with the two largest ones facing the ER lumen. We showed for the first time, using several naturally occurring variants featuring DUF1222 truncations, that Lmf1 interacts physically with lipoprotein lipase and hepatic lipase and localizes the lipase interaction site to loop C within DUF1222. We discuss the implication of our results with regard to lipase maturation and DUF1222 domain structure.

Lipoprotein lipase: from gene to obesity

Lipoprotein lipase (LPL) is a multifunctional enzyme produced by many tissues, including adipose tissue, cardiac and skeletal muscle, islets, and macrophages. LPL is the rate-limiting enzyme for the hydrolysis of the triglyceride (TG) core of circulating TG-rich lipoproteins, chylomicrons, and very low-density lipoproteins (VLDL). LPL-catalyzed reaction products, fatty acids, and monoacylglycerol are in part taken up by the tissues locally and processed differentially; e.g., they are stored as neutral lipids in adipose tissue, oxidized, or stored in skeletal and cardiac muscle or as cholesteryl ester and TG in macrophages. LPL is regulated at transcriptional, posttranscriptional, and posttranslational levels in a tissue-specific manner. Nutrient states and hormonal levels all have divergent effects on the regulation of LPL, and a variety of proteins that interact with LPL to regulate its tissue-specific activity have also been identified. To examine this divergent regulation further, transgenic and knockout murine models of tissue-specific LPL expression have been developed. Mice with overexpression of LPL in skeletal muscle accumulate TG in muscle, develop insulin resistance, are protected from excessive weight gain, and increase their metabolic rate in the cold. Mice with LPL deletion in skeletal muscle have reduced TG accumulation and increased insulin action on glucose transport in muscle. Ultimately, this leads to increased lipid partitioning to other tissues, insulin resistance, and obesity. Mice with LPL deletion in the heart develop hypertriglyceridemia and cardiac dysfunction. The fact that the heart depends increasingly on glucose implies that free fatty acids are not a sufficient fuel for optimal cardiac function. Overall, LPL is a fascinating enzyme that contributes in a pronounced way to normal lipoprotein metabolism, tissue-specific substrate delivery and utilization, and the many aspects of obesity and other metabolic disorders that relate to energy balance, insulin action, and body weight regulation.

Dimers of the neuropeptide Y (NPY) Y2 receptor show asymmetry in agonist affinity and association with G proteins

In conditions precluding activation of G proteins, the binding of agonists to dimers of the neuropeptide Y (NPY) Y2 receptor shows two components of similar size, but differing in affinity. The dimers of all NPY receptors are solubilized as approximately 180-kDa complexes containing one G protein alpha beta gamma trimer. These heteropentamers are stable to excess agonists, chelators, and alkylators. However, dispersion in the weak surfactant cholate releases approximately 300-kDa complexes. These findings indicate that both protomers in the Y2 dimer are associated with G protein heterotrimers, but the extent of interaction depends on affinity for the agonist peptide. The G protein in contact with the first-liganded, higher-affinity protomer should have a stronger interaction with the receptor and a larger probability of activation.

A quantitative assay measuring the function of lipase maturation factor 1

Newly synthesized lipoprotein lipase (LPL) and related members of the lipase gene family require an endoplasmic reticulum maturation factor for attainment of enzyme activity. This factor has been identified as lipase maturation factor 1 (Lmf1), and mutations affecting its function and/or expression result in combined lipase deficiency (cld) and hypertriglyceridemia. To assess the functional impact of Lmf1 sequence variations, both naturally occurring and induced, we report the development of a cell-based assay using LPL activity as a quantitative reporter of Lmf1 function. The assay uses a cell line homozygous for the cld mutation, which renders endogenous Lmf1 nonfunctional. LPL transfected into the mutant cld cell line fails to attain activity; however, cotransfection of LPL with wild-type Lmf1 restores its ability to support normal lipase maturation. In this report, we describe optimized conditions that ensure the detection of a complete range of Lmf1 function (full, partial, or complete loss of function) using LPL activity as the quantitative reporter. To illustrate the dynamic range of the assay, we tested several novel mutations in mouse Lmf1. Our results demonstrate the ability of the assay to detect and analyze Lmf1 mutations having a wide range of effects on Lmf1 function and protein expression.

Mechanisms of lipase maturation

Lipases are acyl hydrolases that represent a diverse group of enzymes present in organisms ranging from prokaryotes to humans. This article focuses on an evolutionarily related family of extracellular lipases that include lipoprotein lipase, hepatic lipase and endothelial lipase. As newly synthesized proteins, these lipases undergo a series of co- and post-translational maturation steps occurring in the endoplasmic reticulum, including glycosylation and glycan processing, and protein folding and subunit assembly. This article identifies and discusses mechanisms that direct early and late events in lipase folding and assembly. Lipase maturation employs the two general chaperone systems operating in the endoplasmic reticulum, as well as a recently identified lipase-specific chaperone termed lipase maturation factor 1. We propose that the two general chaperone systems act in a coordinated manner early in lipase maturation in order to help create partially folded monomers; lipase maturation factor 1 then facilitates final monomer folding and subunit assembly into fully functional homodimers. Once maturation is complete, the lipases exit the endoplasmic reticulum and are secreted to extracellular sites, where they carry out a number of functions related to lipoprotein and lipid metabolism.

Chylomicronemia with a mutant GPIHBP1 (Q115P) that cannot bind lipoprotein lipase

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.

Hepatic lipase maturation: a partial proteome of interacting factors

Tandem affinity purification (TAP) has been used to isolate proteins that interact with human hepatic lipase (HL) during its maturation in Chinese hamster ovary cells. Using mass spectrometry and Western blotting, we identified 28 proteins in HL-TAP isolated complexes, 16 of which localized to the endoplasmic reticulum (ER), the site of HL folding and assembly. Of the 12 remaining proteins located outside the ER, five function in protein translation or ER-associated degradation (ERAD). Components of the two major ER chaperone systems were identified, the BiP/Grp94 and the calnexin (CNX)/calreticulin (CRT) systems. All factors involved in CNX/CRT chaperone cycling were identified, including UDP-glucose:glycoprotein glucosyltransferase 1 (UGGT), glucosidase II, and the 57 kDa oxidoreductase (ERp57). We also show that CNX, and not CRT, is the lectin chaperone of choice during HL maturation. Along with the 78 kDa glucose-regulated protein (Grp78; BiP) and the 94 kDa glucose-regulated protein (Grp94), an associated peptidyl-prolyl cis-trans isomerase and protein disulfide isomerase were also detected. Finally, several factors in ERAD were identified, and we provide evidence that terminally misfolded HL is degraded by the ubiquitin-mediated proteasomal pathway. We propose that newly synthesized HL emerging from the translocon first associates with CNX, ERp57, and glucosidase II, followed by repeated posttranslational cycles of CNX binding that is mediated by UGGT. BiP/Grp94 may stabilize misfolded HL during its transition between cycles of CNX binding and may help direct its eventual degradation.

The acidic domain of GPIHBP1 is important for the binding of lipoprotein lipase and chylomicrons

GPIHBP1, a glycosylphosphatidylinositol-anchored endothelial cell protein of the lymphocyte antigen 6 (Ly6) family, plays a key role in the lipolysis of triglyceride-rich lipoproteins (e.g. chylomicrons). GPIHBP1 is expressed along the luminal surface of endothelial cells of heart, skeletal muscle, and adipose tissue, and GPIHBP1-expressing cells bind lipoprotein lipase (LPL) and chylomicrons avidly. GPIHBP1 contains an amino-terminal acidic domain (amino acids 24-48) that is enriched in aspartate and glutamate residues, and we previously speculated that this domain might be important in binding ligands. To explore the functional importance of the acidic domain, we tested the ability of polyaspartate or polyglutamate peptides to block the binding of ligands to pgsA-745 Chinese hamster ovary cells that overexpress GPIHBP1. Both polyaspartate and polyglutamate blocked LPL and chylomicron binding to GPIHBP1. Also, a rabbit antiserum against the acidic domain of GPIHBP1 blocked LPL and chylomicron binding to GPIHBP1-expressing cells. Replacing the acidic amino acids within GPIHBP1 residues 38-48 with alanine eliminated the ability of GPIHBP1 to bind LPL and chylomicrons. Finally, mutation of the positively charged heparin-binding domains within LPL and apolipoprotein AV abolished the ability of these proteins to bind to GPIHBP1. These studies indicate that the acidic domain of GPIHBP1 is important and that electrostatic interactions play a key role in ligand binding.

Glycosylation of Asn-76 in mouse GPIHBP1 is critical for its appearance on the cell surface and the binding of chylomicrons and lipoprotein lipase

GPIHBP1 is a glycosylphosphatidylinositol-anchored protein in the lymphocyte antigen 6 (Ly-6) family that recently was identified as a platform for the lipolytic processing of triglyceride-rich lipoproteins. GPIHBP1 binds both LPL and chylomicrons and is expressed on the luminal face of microvascular endothelial cells. Here, we show that mouse GPIHBP1 is N-glycosylated at Asn-76 within the Ly-6 domain. Human GPIHBP1 is also glycosylated. The N-linked glycan could be released from mouse GPIHBP1 with N-glycosidase F, endoglycosidase H, or endoglycosidase F1. The glycan was marginally sensitive to endoglycosidase F2 digestion but resistant to endoglycosidase F3 digestion, suggesting that the glycan on GPIHBP1 is of the oligomannose type. Mutating the N-glycosylation site in mouse GPIHBP1 results in an accumulation of GPIHBP1 in the endoplasmic reticulum and a markedly reduced amount of the protein on the cell surface. Consistent with this finding, cells expressing a nonglycosylated GPIHBP1 lack the ability to bind LPL or chylomicrons. Eliminating the N-glycosylation site in a truncated soluble version of GPIHBP1 causes a modest reduction in the secretion of the protein. These studies demonstrate that N-glycosylation of GPIHBP1 is important for the trafficking of GPIHBP1 to the cell surface.

Mutations in LMF1 cause combined lipase deficiency and severe hypertriglyceridemia

Hypertriglyceridemia is a hallmark of many disorders, including metabolic syndrome, diabetes, atherosclerosis and obesity. A well-known cause is the deficiency of lipoprotein lipase (LPL), a key enzyme in plasma triglyceride hydrolysis. Mice carrying the combined lipase deficiency (cld) mutation show severe hypertriglyceridemia owing to a decrease in the activity of LPL and a related enzyme, hepatic lipase (HL), caused by impaired maturation of nascent LPL and hepatic lipase polypeptides in the endoplasmic reticulum (ER). Here we identify the gene containing the cld mutation as Tmem112 and rename it Lmf1 (Lipase maturation factor 1). Lmf1 encodes a transmembrane protein with an evolutionarily conserved domain of unknown function that localizes to the ER. A human subject homozygous for a deleterious mutation in LMF1 also shows combined lipase deficiency with concomitant hypertriglyceridemia and associated disorders. Thus, through its profound effect on lipase activity, LMF1 emerges as an important candidate gene in hypertriglyceridemia.

The cld mutation: narrowing the critical chromosomal region and selecting candidate genes

Combined lipase deficiency (cld) is a recessive, lethal mutation specific to the tw73 haplotype on mouse Chromosome 17. While the cld mutation results in lipase proteins that are inactive, aggregated, and retained in the endoplasmic reticulum (ER), it maps separately from the lipase structural genes. We have narrowed the gene critical region by about 50% using the tw18 haplotype for deletion mapping and a recombinant chromosome used originally to map cld with respect to the phenotypic marker tf. The region now extends from 22 to 25.6 Mbp on the wild-type chromosome, currently containing 149 genes and 50 expressed sequence tags (ESTs). To identify the affected gene, we have selected candidates based on their known role in associated biological processes, cellular components, and molecular functions that best fit with the predicted function of the cld gene. A secondary approach was based on differences in mRNA levels between mutant (cld/cld) and unaffected (+/cld) cells. Using both approaches, we have identified seven functional candidates with an ER localization and/or an involvement in protein maturation and folding that could explain the lipase deficiency, and six expression candidates that exhibit large differences in mRNA levels between mutant and unaffected cells. Significantly, two genes were found to be candidates with regard to both function and expression, thus emerging as the strongest candidates for cld. We discuss the implications of our mapping results and our selection of candidates with respect to other genes, deletions, and mutations occurring in the cld critical region.

Retention of a bean phaseolin/maize gamma-Zein fusion in the endoplasmic reticulum depends on disulfide bond formation

Most seed storage proteins of the prolamin class accumulate in the endoplasmic reticulum (ER) as large insoluble polymers termed protein bodies (PBs), through mechanisms that are still poorly understood. We previously showed that a fusion between the Phaseolus vulgaris vacuolar storage protein phaseolin and the N-terminal half of the Zea mays prolamin gamma-zein forms ER-located PBs. Zeolin has 6 Cys residues and, like gamma-zein with 15 residues, is insoluble unless reduced. The contribution of disulfide bonds to zeolin destiny was determined by studying in vivo the effects of 2-mercaptoethanol (2-ME) and by zeolin mutagenesis. We show that in tobacco (Nicotiana tabacum) protoplasts, 2-ME enhances interactions of newly synthesized proteins with the ER chaperone BiP and inhibits the secretory traffic of soluble proteins with or without disulfide bonds. In spite of this general inhibition, 2-ME enhances the solubility of zeolin and relieves its retention in the ER, resulting in increased zeolin traffic. Consistently, mutated zeolin unable to form disulfide bonds is soluble and efficiently enters the secretory traffic without 2-ME treatment. We conclude that disulfide bonds that lead to insolubilization are a determinant for PB-mediated protein accumulation in the ER.

Mitochondria maintain maturation and secretion of lipoprotein lipase in the endoplasmic reticulum

Considering the physiological Ca2+ dynamics within the ER (endoplasmic reticulum), it remains unclear how efficient protein folding is maintained in living cells. Thus, utilizing the strictly folding-dependent activity and secretion of LPL (lipoprotein lipase), we evaluated the impact of ER Ca2+ content and mitochondrial contribution to Ca2+-dependent protein folding. Exhaustive ER Ca2+ depletion by inhibition of sarcoplasmic/endoplasmic reticulum Ca2+-ATPases caused strong, but reversible, reduction of cell-associated and released activity of constitutive and adenovirus-encoded human LPL in CHO-K1 (Chinese-hamster ovary K1) and endothelial cells respectively, which was not due to decline of mRNA or intracellular protein levels. In contrast, stimulation with the IP3 (inositol 1,4,5-trisphosphate)-generating agonist histamine only moderately and transiently affected LPL maturation in endothelial cells that paralleled a basically preserved ER Ca2+ content. However, in the absence of extracellular Ca2+ or upon prevention of transmitochondrial Ca2+ flux, LPL maturation discontinued upon histamine stimulation. Collectively, these data indicate that Ca2+-dependent protein folding in the ER is predominantly controlled by intraluminal Ca2+ and is largely maintained during physiological cell stimulation owing to efficient ER Ca2+ refilling. Since Ca2+ entry and mitochondrial Ca2+ homoeostasis are crucial for continuous Ca2+-dependent protein maturation in the ER, their pathological alterations may result in dysfunctional protein folding.

Interaction of Lipoprotein Lipase and Receptor-associated Protein

Receptor-associated protein (RAP) is a recognized chaperone/escort protein for members of the low density lipoprotein receptor family. In this report, we show that RAP binds to lipoprotein lipase (LPL) and may play a role in the maturation of LPL. Binding of highly purified RAP to LPL was demonstrated in vitro by solid phase assays, surface plasmon resonance, and rate zonal centrifugation. The dissociation constant for this interaction measured by the first two techniques ranged between 2.4 and 13 nM, values similar to those reported for the binding of RAP to LRP or gp330. The specificity of the interaction was demonstrated by competition with a panel of LPL monoclonal antibodies. Rate zonal centrifugation demonstrated the presence of a stable complex with an apparent Mr consistent with the formation of a complex between monomeric LPL and RAP. RAP x LPL complexes were co-immunoprecipitated in adipocyte lysates or from solutions of purified LPL and RAP. The interaction was also demonstrated in whole cells by cross-linking experiments. RAP-deficient adipocytes secreted LPL with a specific activity 2.5-fold lower than the lipase secreted by control cells. Heparin addition to cultured RAP-deficient adipocytes failed to stimulate LPL secretion in the medium, suggesting defective binding of the lipase to the plasma membrane. These studies demonstrate that RAP binds to LPL with high affinity both in purified systems and cell extracts and that RAP-deficient adipocytes secrete poorly assembled LPL. A function of RAP may be to prevent premature interaction of LPL with binding partners in the secretory pathway, namely LRP and heparan sulfate proteoglycan.

Calcium triggers folding of lipoprotein lipase into active dimers

The active form of lipoprotein lipase (LPL) is a noncovalent homodimer of 55-kDa subunits. The dimer is unstable and tends to undergo irreversible dissociation into inactive monomers. We noted that a preparation of such monomers slowly regained traces of activity under assay conditions with substrate, heparin, and serum or in cell culture medium containing serum. We therefore studied the refolding pathway of LPL after full denaturation in 6 M guanidinium chloride or after dissociation into monomers in 1 M guanidinium chloride. In crude systems, we identified serum as the factor promoting reactivation. Further investigations demonstrated that Ca2+ was the crucial component in serum for reactivation of LPL and that refolding involved at least two steps. Studies of far-UV circular dichroism, fluorescence, and proteolytic cleavage patterns showed that LPL started to refold from the C-terminal domain, independent of calcium. The first step was rapid and resulted in formation of an inactive monomer with a completely folded C-terminal domain, whereas the N-terminal domain was in the molten globule state. The second step was promoted by Ca2+ and converted LPL monomers from the molten globule state to dimerization-competent and more tightly folded monomers that rapidly formed active LPL dimers. The second step was slow, and it appears that proline isomerization (rather than dimerization as such) is rate-limiting. Inactive monomers isolated from human tissue recovered activity under the influence of Ca2+. We speculate that Ca2+-dependent control of LPL dimerization might be involved in the normal post-translational regulation of LPL activity.

The metabolic "switch" AMPK regulates cardiac heparin-releasable lipoprotein lipase

The "fuel gauge" AMP-activated protein kinase (AMPK) facilitates ATP production to meet energy demands during metabolic stress. Given the importance of lipoprotein lipase (LPL) in providing hearts with fatty acids (FA), the preferred substrate consumed by the heart, the objective of the present study was to investigate whether activation of AMPK influences LPL at its functionally relevant location, the coronary lumen. Hearts from overnight-fasted rats were first perfused with heparin to release LPL, and homogenates from these hearts were then used to measure total and phospho-AMPK-alpha by Western blotting. Manipulation of AMPK activity [with drugs like adenine 9-beta-D-arabinofuranoside (Ara-A) and insulin (that inhibit) or perhexiline and oligomycin (that stimulate)] and its influence on LPL was also determined. Fasting augmented the activity of both AMPK and luminal LPL on immediate removal of hearts, effects that still remained even after in vitro perfusion of hearts for 1 h. Inhibition of AMPK in fasted hearts using an inhibitor like Ara-A or through provision of insulin markedly lowered the enhanced luminal LPL activity. In contrast, AMPK activators, like perhexiline and oligomycin, produced a significant elevation in heparin-releasable LPL activity. Thus, with fasting or drugs that influence AMPK, a strong correlation between this metabolic switch and cardiac LPL activity was established. Our data suggest that, in addition to its direct role in promoting FA oxidation, AMPK-mediated recruitment of LPL to the coronary lumen could represent an immediate compensatory response by the heart to guarantee FA supply.

Isoproterenol increases active lipoprotein lipase in adipocyte medium and in rat plasma

White adipose tissue (WAT) lipoprotein lipase (LPL) activity channels diet fat towards storage in adipocytes. Adrenaline (ADR) is accepted to reduce WAT or adipocyte LPL activity (LPLa), but available data are not clear-cut regarding long exposure to ADR in vitro or in vivo. We studied the effects of long exposures to ADR or beta-adrenergic agonist on LPL: in isolated rat adipocytes (3 h) and in rats (>1 day). Isoproterenol (ISO) (1 microM) did not alter LPLmRNA nor LPLa in adipocytes, but increased LPLa in medium more than twofold (3.58 +/- 0.35 vs. 1.32 +/- 0.35 mU/10(6) adipocytes, P < 0.001). Effect was time (not present at 1 h, clear at 2 h) and concentration dependent (high sensitivity from 10 to 100 nM, max at 1 microM). Adenylate cyclase activator or cyclic AMP (cAMP) analogue produced a similar increase. Thus in adipocytes ISO produced an increase in LPLa release and/or a decrease in extracellular LPLa degradation. ADR or ISO treated rats had a two to fourfold decrease in WAT LPLa vs. unchanged LPLmRNA. This decrease was 10-fold in WAT heparin-releasable LPLa (5.7 +/- 0.6 vs. 57.3 +/- 10.2 mU/g, P < 0.001), which represents peri/extracellular LPLa. Plasma LPLa was increased 11-fold by ADR (3.30 +/- 0.58 vs. 0.32 +/- 0.08 mU/ml, P < 0.001) whereas only threefold by ISO (P > 0.01). We suggest that in vivo ADR increased release of active LPL to plasma from endothelial cells of LPL-rich tissue(s)-WAT was probably one of these tissues releasing LPL since it lost 90% of its peri/extracellular LPLa-and/or decreased degradation of plasma active LPL. Since liver LPLa was not increased, plasma active LPL might be kept away from hepatic degradation by binding to stabilising entities in plasma (fatty acids (FA), lipoproteins or soluble heparan sulphates (HS)). In conclusion, we believe this is the first report stating that: (a) ISO increases LPLa in isolated adipocyte medium, and (b) ADR administration to rats decreases WAT extracellular active LPL and increases preheparin plasma active LPL.

Endosomal ricin transport: involvement of Rab4- and Rab5-positive compartments

Transport of the ribosome-inactivating protein ricin through endosomes was studied in A431 cells expressing Rab5-, Rab4-, and Rab11-GFP. It was shown that Rab5- and Rab4-positive functional domains of early endosomes are involved in ricin transport. Ricin enters cells by both clathrin-dependent and clathrin-independent mechanisms. The main pool of internalized toxin accumulates in early endosomes and remains associated with them for a long time. In contrast to earlier observations, current observations indicate that the majority of ricin avoids transport to lysosomes. The low level of ricin association with Rab11 as well as with transferrin accumulated in the pericentriolar recycling compartment shows that the compartment is not responsible for keeping ricin away from degradation in lysosomes. Escape from degradation in lysosomes is assumed to result from the potentiality of ricin to form assemblies within compartments.