Novel site in lipoprotein lipase (LPL415–438) essential for substrate interaction and dimer stability

The Importance of Lipoprotein Lipase Regulation in Atherosclerosis

Lipoprotein lipase (LPL) plays a major role in the lipid homeostasis mainly by mediating the intravascular lipolysis of triglyceride rich lipoproteins. Impaired LPL activity leads to the accumulation of chylomicrons and very low-density lipoproteins (VLDL) in plasma, resulting in hypertriglyceridemia. While low-density lipoprotein cholesterol (LDL-C) is recognized as a primary risk factor for atherosclerosis, hypertriglyceridemia has been shown to be an independent risk factor for cardiovascular disease (CVD) and a residual risk factor in atherosclerosis development. In this review, we focus on the lipolysis machinery and discuss the potential role of triglycerides, remnant particles, and lipolysis mediators in the onset and progression of atherosclerotic cardiovascular disease (ASCVD). This review details a number of important factors involved in the maturation and transportation of LPL to the capillaries, where the triglycerides are hydrolyzed, generating remnant lipoproteins. Moreover, LPL and other factors involved in intravascular lipolysis are also reported to impact the clearance of remnant lipoproteins from plasma and promote lipoprotein retention in capillaries. Apolipoproteins (Apo) and angiopoietin-like proteins (ANGPTLs) play a crucial role in regulating LPL activity and recent insights into LPL regulation may elucidate new pharmacological means to address the challenge of hypertriglyceridemia in atherosclerosis development.

Clinical and functional studies of two novel variants in the LPL gene in subjects with severe hypertriglyceridemia

BACKGROUND

Two novel variants (p.Arg270Gly and p.Asp308Glyfs*3) in the LPL gene have recently been identified in subjects with hypertriglyceridemia (HTG). In this study, we investigated clinical and genetic features of their families and examined the functional significance of these two variants in vitro.

METHODS

Clinical and genetic data were collected. Site-directed mutagenesis and transient expression in cld cells were performed. Lipoprotein lipase (LPL) mass and activity were measured.

RESULTS

In vitro studies showed that LPL mass and activity in the media of cells transfected with the p.Arg270Gly variant were significantly reduced. In the cell lysates, however, LPL mass was preserved but LPL activity was reduced, suggesting that the LPL defect was in the secretion and activity. For the p.Asp308Glyfs*3 variant, LPL mass in the cell lysate was relatively preserved compared to that of the wild-type, while LPL mass in the media was decreased albeit not significantly. LPL activities in the cell lysate and in the media of cells transfected with this variant were significantly reduced, suggesting that the p.Asp308Glyfs*3 variant might affect the activity, and possibly, secretion of LPL.

CONCLUSIONS

These novel variants in the LPL gene were likely pathogenic with the defect in secretion and/or activity.

Emerging strategies of targeting lipoprotein lipase for metabolic and cardiovascular diseases

Although statins and other pharmacological approaches have improved the management of lipid abnormalities, there exists a need for newer treatment modalities especially for the management of hypertriglyceridemia. Lipoprotein lipase (LPL), by promoting hydrolytic cleavage of the triglyceride core of lipoproteins, is a crucial node in the management of plasma lipid levels. Although LPL expression and activity modulation is observed as a pleiotropic action of some the commonly used lipid lowering drugs, the deliberate development of drugs targeting LPL has not occurred yet. In this review, we present the biology of LPL, highlight the LPL modulation property of currently used drugs and review the novel emerging approaches to target 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.

Leu452His mutation in lipoprotein lipase gene transfer associated with hypertriglyceridemia in mice in vivo

Mutated mouse lipoprotein lipase (LPL) containing a leucine (L) to histidine (H) substitution at position 452 was transferred into mouse liver by hydrodynamics-based gene delivery (HD). Mutated-LPL (MLPL) gene transfer significantly increased the concentrations of plasma MLPL and triglyceride (TG) but significantly decreased the activity of plasma LPL. Moreover, the gene transfer caused adiposis hepatica and significantly increased TG content in mouse liver. To understand the effects of MLPL gene transfer on energy metabolism, we investigated the expression of key functional genes related to energy metabolism in the liver, epididymal fat, and leg muscles. The mRNA contents of hormone-sensitive lipase (HSL), adipose triglyceride lipase (ATGL), fatty acid-binding protein (FABP), and uncoupling protein (UCP) were found to be significantly reduced. Furthermore, we investigated the mechanism by which MLPL gene transfer affected fat deposition in the liver, fat tissue, and muscle. The gene expression and protein levels of forkhead Box O3 (FOXO3), AMP-activated protein kinase (AMPK), and peroxisome proliferator-activated receptor-gamma coactivator 1 alpha (PGC-1α) were found to be remarkably decreased in the liver, fat and muscle. These results suggest that the Leu452His mutation caused LPL dysfunction and gene transfer of MLPL in vivo produced resistance to the AMPK/PGC-1α signaling pathway in mice.

Biochemistry and pathophysiology of intravascular and intracellular lipolysis

All organisms use fatty acids (FAs) for energy substrates and as precursors for membrane and signaling lipids. The most efficient way to transport and store FAs is in the form of triglycerides (TGs); however, TGs are not capable of traversing biological membranes and therefore need to be cleaved by TG hydrolases ("lipases") before moving in or out of cells. This biochemical process is generally called "lipolysis." Intravascular lipolysis degrades lipoprotein-associated TGs to FAs for their subsequent uptake by parenchymal cells, whereas intracellular lipolysis generates FAs and glycerol for their release (in the case of white adipose tissue) or use by cells (in the case of other tissues). Although the importance of lipolysis has been recognized for decades, many of the key proteins involved in lipolysis have been uncovered only recently. Important new developments include the discovery of glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1), the molecule that moves lipoprotein lipase from the interstitial spaces to the capillary lumen, and the discovery of adipose triglyceride lipase (ATGL) and comparative gene identification-58 (CGI-58) as crucial molecules in the hydrolysis of TGs within cells. This review summarizes current views of lipolysis and highlights the relevance of this process to human disease.

Secretion of triacylglycerol-poor VLDL particles from McA-RH7777 cells expressing human hepatic lipase

Hepatic lipase (HL) plays a role in the catabolism of apolipoprotein (apo)B-containing lipoproteins through its lipolytic and ligand-binding properties. We describe a potential intracellular role of HL in the assembly and secretion of VLDL. Transient or stable expression of HL in McA-RH7777 cells resulted in decreased (by 40%) incorporation of [(3)H]glycerol into cell-associated and secreted triacylglycerol (TAG) relative to control cells. However, incorporation of [(35)S]methionine/cysteine into cell and medium apoB-100 was not decreased by HL expression. The decreased (3)H-TAG synthesis/secretion in HL expressing cells was not attributable to decreased expression of genes involved in lipogenesis. Fractionation of medium revealed that the decreased [(3)H]TAG from HL expressing cells was mainly attributable to decreased VLDL. Expression of catalytically-inactive HL (HL(SG)) (Ser-145 at the catalytic site was substituted with Gly) in the cells also resulted in decreased secretion of VLDL-[(3)H]TAG. Examination of lumenal contents of microsomes showed a 40% decrease in [(3)H]TAG associated with lumenal lipid droplets in HL or HL(SG) expressing cells as compared with control. The microsomal membrane-associated [(3)H]TAG was decreased by 50% in HL expressing cells but not in HL(SG) expressing cells. Thus, expression of HL, irrespective of its lipolytic function, impairs formation of VLDL precursor [(3)H]TAG in the form of lumenal lipid droplets. These results suggest that HL expression in McA-RH7777 cells result in secretion of [(3)H]TAG-poor VLDL.

cDNA sequence and tissues expression analysis of lipoprotein lipase from common carp (Cyprinus carpio Var. Jian)

A full-length cDNA coding lipoprotein lipase (LPL) was cloned from liver of adult common carp (Cyprinus carpio Var. Jian) by RT-PCR and rapid amplification of cDNA ends (RACE) approaches. The cDNA obtained was 2,411 bp long with a 1,524 bp open reading frame (ORF) encoding 507 amino acids. This amino acid sequence contains two structural regions: N-terminus (24-354 residues) and C-terminus (355-507 residues). Before N-terminus, 1-23 residues is signal peptide, 6-23 residues is transmembrance helix. At N-terminus, some conversed functional sites were found, including two N-linked glycosylation sites Asn(41) and Asn(88); one catalytic triad Ser(174), Asp(198) and His(283); one conserved heparin-binding site Arg(321) to Arg(324) (RKNR); eight cysteines residues Cys(69) and Cys(82), Cys(258) and Cys(281), Cys(306) and Cys(325), Cys(317) and Cys(320) which are involved in four disulfide bridges; one polypeptide "lid" that participates in substrate specificity. At C-terminus, Asn(401) is another N-linked glycosylation site, and Trp(434) and Trp(435) (WW) is lipid-binding site. The amino acid sequence has a high similarity, and shows similar structural features to LPL of other species. Tissue distribution of LPL mRNA in liver, head kidney, mesenteric adipose tissue, heart and white muscle of common carp was analyzed by semi-quantitative RT-PCR method using beta-actin gene as internal control. The result showed that the expressions of LPL mRNA were detected in all examined tissues of common carp. The expression levels of LPL in the mesenteric adipose tissue was highest among these tissues, following in liver and head kidney, and the lowest expression was found in heart and white muscle.

Identification of the active form of endothelial lipase, a homodimer in a head-to-tail conformation

Endothelial lipase (EL) is a member of a subfamily of lipases that act on triglycerides and phospholipids in plasma lipoproteins, which also includes lipoprotein lipase and hepatic lipase. EL has a tropism for high density lipoprotein, and its level of phospholipase activity is similar to its level of triglyceride lipase activity. Inhibition or loss-of-function of EL in mice results in an increase in high density lipoprotein cholesterol, making it a potential therapeutic target. Although hepatic lipase and lipoprotein lipase have been shown to function as homodimers, the active form of EL is not known. In these studies, the size and conformation of the active form of EL were determined. Immunoprecipitation experiments suggested oligomerization. Ultracentrifugation experiments showed that the active form of EL had a molecular weight higher than the molecular weight of a simple monomer but less than a dimer. A construct encoding a covalent head-to-tail homodimer of EL (EL-EL) was expressed and had similar lipolytic activity to EL. The functional molecular weights determined by radiation inactivation were similar for EL and the covalent homodimer EL-EL. We previously showed that EL could be cleaved by proprotein convertases, such as PC5, resulting in loss of activity. In cells overexpressing PC5, the covalent homodimeric EL-EL appeared to be more stable, with reduced cleavage and conserved lipolytic activity. A comparative model obtained using other lipase structures suggests a structure for the head-to-tail EL homodimer that is consistent with the experimental findings. These data confirm the hypothesis that EL is active as a homodimer in head-to-tail conformation.

Molecular cloning and tissue distribution of lipoprotein lipase full-length cDNA from Pengze crucian carp (Carassius auratus var. Pengze)

A full-length cDNA coding lipoprotein lipase (LPL) was cloned from liver of adult Pengze crucian carp (Carassius auratus var. Pengze) by RT-PCR and rapid amplification of cDNA ends (RACE) approaches. The cDNA obtained was 1877 bp long with a 1524 bp open reading frame (ORF) encoding 507 amino acids, including a putative signal peptide of 23 amino acids long. The deduced amino acid sequence has a high similarity and shows similar structural features to LPL of other species. The LPL protein has a calculated molecular mass of 57.7 kDa and isolectric point of 7.85. Tissue distribution of LPL mRNA in mesenteric adipose tissue, liver, heart, head kidney and white muscle of adult Pengze crucian carp was analyzed by semi-quantitative RT-PCR method using beta-actin gene as internal control, the result showed that this gene was ubiquitously expressed in all tissues tested with the highest abundance in mesenteric adipose tissue, following in head kidney and liver, and the lowest expression was found in heart and white muscle.

Lipoprotein lipase releases esterified oxylipins from very low-density lipoproteins

We previously demonstrated that defects in lipoprotein metabolism alter the distribution of oxygenated polyunsaturated fatty acids (PUFAs) in lipoprotein particles. If these oxidation products are released by lipoprotein lipase (LpL), then their delivery to peripheral tissues with bulk lipids could influence cellular function. Using 26-week-old normolipidemic and hyperlipidemic Zucker rats, we measured PUFA alcohols, epoxides, diols, ketones, and triols (i.e. oxylipins) in esterified and non-esterified fractions of whole plasma, VLDL, and LpL-generated VLDL-lipolysates. Whole plasma, VLDL, and lipolysate oxylipin profiles were distinct and altered by hyperlipidemia. While >90% of the whole plasma oxylipins were esterified, the fraction of each oxylipin class in the VLDL varied: 46% of alcohols, 30% of epoxides, 19% of diols, <10% of ketones, and <1% triols. Whole plasma was dominated by arachidonate alcohols, while the linoleate alcohols, epoxides, and ketones showed an increased prevalence in VLDL. LpL-mediated VLDL lipolysis of PUFA alcohols, diols and ketones was detected and the relative abundance of oxygenated linoleates was enhanced in the lipolysates, relative to their corresponding VLDL. In summary esterified oxylipins were seen to be LpL substrates with heterogeneous distributions among lipoprotein classes. Moreover, oxylipin distributions are changes within the context of obesity-associated dyslipidemia. These results support the notion that the VLDL-LpL axis may facilitate the delivery of plasma oxylipins to the periphery. The physiological implications of these findings are yet to be elucidated; however, these molecules are plausible indicators of systemic oxidative stress, and could report this status to the peripheral tissues.

Novel mutations of the lipoprotein lipase gene associated with hypertriglyceridemia in members of type 2 diabetic pedigrees

Increased plasma triglyceride and free fatty acid levels are frequently associated with type 2 diabetes mellitus (T2DM). To test the hypothesis that LPL gene mutations contribute to the hypertriglyceridemia observed in members of T2DM pedigrees, we screened the LPL gene in 53 hypertriglyceridemic members of 26 families. Four known and three novel mutations were identified. All three novel mutations, Lys312insC, Thr361insA, and double mutation Lys312insC + Asn291Ser, are clinically associated with hypertriglyceridemia. In vitro mutagenesis and expression studies confirm that these variants are associated with a significant reduction in LPL activity. The modeled structures displaying the Lys312insC and Thr361insA mutations showed loss of the activity-related C-terminal domain in the LPL protein. Another novel double mutation, Lys312insC + Asn291Ser, resulted in the loss of the catalytic ability of LPL attributable to the complete loss of the C-terminal domain and alteration in the heparin association site. Thus, these novel mutations of the LPL gene contribute to the hypertriglyceridemia observed in members of type 2 diabetic pedigrees.

Substrate specificity of lipoprotein lipase and endothelial lipase: studies of lid chimeras

The triglyceride (TG) lipase gene subfamily, consisting of LPL, HL, and endothelial lipase (EL), plays a central role in plasma lipoprotein metabolism. Compared with LPL and HL, EL is relatively more active as a phospholipase than as a TG lipase. The amino acid loop or "lid" covering the catalytic site has been implicated as the basis for the difference in substrate specificity between HL and LPL. To determine the role of the lid in the substrate specificity of EL, we studied EL in comparison with LPL by mutating specific residues of the EL lid and exchanging their lids. Mutation studies showed that amphipathic properties of the lid contribute to substrate specificity. Exchanging lids between LPL and EL only partially shifted the substrate specificity of the enzymes. Studies of a double chimera possessing both the lid and the C-terminal domain (C-domain) of EL in the LPL backbone showed that the role of the lid in determining substrate specificity does not depend on the nature of the C-domain of the lipase. Using a kinetic assay, we showed an additive effect of the EL lid on the apparent affinity for HDL(3) in the presence of the EL C-domain.

Rapid subunit exchange in dimeric lipoprotein lipase and properties of the inactive monomer

Lipoprotein lipase (LPL), a key enzyme in the metabolism of triglyceride-rich plasma lipoproteins, is a homodimer. Dissociation to monomers leads to loss of activity. Evidence that LPL dimers rapidly exchange subunits was demonstrated by fluorescence resonance energy transfer between lipase subunits labeled with Oregon Green and tetrametylrhodamine, respectively, and also by formation of heterodimers composed of radiolabeled and biotinylated lipase subunits captured on streptavidine-agarose. Compartmental modeling of the inactivation kinetics confirmed that rapid subunit exchange must occur. Studies of activity loss indicated the existence of a monomer that can form catalytically active dimers, but this intermediate state has not been possible to isolate and remains hypothetical. Differences in solution properties and conformation between the stable but catalytically inactive monomeric form of LPL and the active dimers were studied by static light scattering, intrinsic fluorescence, and probing with 4,4'-dianilino-1,1'-binaphtyl-5,5'-disulfonic acid and acrylamide. The catalytically inactive monomer appeared to have a more flexible and exposed structure than the dimers and to be more prone to aggregation. By limited proteolysis the conformational changes accompanying dissociation of the dimers to inactive monomers were localized mainly to the central part of the subunit, probably corresponding to the region for subunit interaction.

The lipase gene family

Development of the lipase gene family spans the change in science that witnessed the birth of contemporary techniques of molecular biology. Amino acid sequencing of enzymes gave way to cDNA cloning and gene organization, augmented by in vitro expression systems and crystallization. This review traces the origins and highlights the functional significance of the lipase gene family, overlaid on the background of this technical revolution. The gene family initially consisted of three mammalian lipases [pancreatic lipase (PL), lipoprotein lipase, and hepatic lipase] based on amino acid sequence similarity and gene organization. Family size increased when several proteins were subsequently added based on amino acid homology, including PL-related proteins 1 and 2, phosphatidylserine phospholipase A1, and endothelial lipase. The physiological function of each of the members is discussed as well as the region responsible for lipase properties such as enzymatic activity, substrate binding, heparin binding, and cofactor interaction. Crystallization of several lipase gene family members established that the family belongs to a superfamily of enzymes, which includes esterases and thioesterases. This superfamily is related by tertiary structure, rather than amino acid sequence, and represents one of the most populous families found in nature.

Lipoprotein lipase from rainbow trout differs in several respects from the enzyme in mammals

Previously we found lipase activity with characteristics similar to lipoprotein lipase (LPL) in tissues from rainbow trout [Biochim. Biophys. Acta 1255 (1995) 205], whereas no equivalent to the related hepatic lipase could be found. An equivalent to apolipoprotein CII was also identified and characterized [Gene 254 (2000) 189]. We present here the full nucleotide sequence for LPL from rainbow trout (Oncorhynchus mykiss) and have investigated some properties of the enzyme. In contrast to what has been found in mammals, LPL mRNA was expressed in livers of adult trout. This indicates that trout LPL carries out functions that hepatic lipase has evolved to take over in mammals. Trout LPL was unstable at 37 degrees C compared with bovine and human LPL. Two sequence differences that may relate to the instability are that trout LPL lacks the disulfide bridge in the C-terminal domain and lacks Pro(258). This residue is conserved in LPL from all mammals and has been shown to be critical for enzyme stability at 37 degrees C. On chromatography on heparin-Sepharose trout and chicken LPL eluted at higher salt concentration than bovine (or other mammalian) LPL. The C-terminal end of LPL has been implied in heparin binding and the higher heparin affinity of the trout and chicken enzymes may be because they have 17 and 15 extra amino acid residues at the C-terminal end, of which three residues are positively charged.