Lipoprotein lipase: structure, function and mechanism of action

Comparative study on the enzymatic degradation of phenolic esters: The HPLC-UV quantification of tyrosol and gallic acid liberated from tyrosol acyl esters and alkyl gallates by hydrolytic enzymes

HPLC-UV analysis was used to evaluate the enzymatic degradation characteristics of tyrosol acyl esters (TYr-Es) and alkyl gallates (A-GAs). Among various hydrolytic enzymes, TYr-Es can be hydrolyzed by pancrelipase, while A-GAs cannot be hydrolyzed by pancrelipase. Interestingly, carboxylesterase-1b (CES-1b), carboxylesterase-1c (CES-1c) and carboxylesterase-2 (CES-2) are able to hydrolyze TYr-Es and A-GAs, and thus to liberate tyrosol (TYr) and gallic acid (GA). By contrast, the degrees of hydrolysis (DHs) of TYr-Es and A-GAs by CES-1b and CES-1c were significantly higher than those by CES-2. Meanwhile, the DHs of TYr-Es were much higher than those of A-GAs. Especially, the DHs firstly increased and then decreased with the increasing alkyl chain length. Besides, DHs positively correlated with the unsaturation degree at the same chain length. Through regulating carbon length, unsaturation degree and the ester bond structure, controlled-release of phenolic compounds and fatty acids (or fatty alcohols) from phenolic esters will be easily achieved.

Hydrolysis and transport characteristics of tyrosol-SCFA esters in rat intestine and blood: Two-step release of tyrosol and SCFAs to enhance the beneficial effects

The models of rat everted gut sac and hydrolysis by rat plasma were used to clarify the hydrolysis and transport characteristics of tyrosol-SCFA esters (TYr-SEs). HPLC-UV results indicated that TYr-SEs could be hydrolyzed by intestinal lipase, which showed sustained release of SCFAs and TYr. Meanwhile, TYr-SEs and the liberated SCFAs and TYr could cross the membrane and were transported into blood circulation. TYr-SEs were further hydrolyzed by carboxylesterase in plasma. Obviously, the hydrolysis of TYr-SEs in blood also showed sustained release of SCFAs and TYr. Especially, the rates of hydrolysis and transport correlated positively with the acyl chain lengths. Besides, the above rates of the TYr-SE with a straight chain were greater than those of its isomer with a branched chain. Therefore, the above-mentioned two-step release of SCFAs and TYr clearly demonstrated that TYr-SEs would be an effective approach to enhance the beneficial health effects of SCFAs and TYr.

The breast cancer microenvironment and lipoprotein lipase: Another negative notch for a beneficial enzyme?

The energy demand of breast cancers is in part met through the β-oxidation of exogenous fatty acids. Fatty acids may also be used to aid in cell signaling and toward the construction of new membranes for rapidly proliferating tumor cells. A significant quantity of fatty acids comes from the hydrolysis of lipoprotein triacylglycerols and phospholipids by lipoprotein lipase (LPL). The lipid obtained via LPL in the breast tumor microenvironment may thus promote breast tumor growth and development. In this hypothesis article, we introduce LPL, provide a meta-analysis of RNAseq data showing that LPL is associated with poor prognosis, and explain how LPL might play a role in breast cancer prognosis over time.

Gastrointestinal Digestion and Microbial Hydrolysis of Alkyl Gallates: Potential Sustained Release of Gallic Acid

Phenolipids such as alkyl gallates (A-GAs) have been approved by the food industry as non-toxic antioxidant additives, which are also regarded as an emerging source of functional food ingredients. However, comprehensive understanding of their digestive absorption is needed. Thus, the models of live mice and anaerobic fermentation were used to clarify the distribution and microbial hydrolysis characteristics of A-GAs in the gastrointestinal tract. HPLC-UV results demonstrated that A-GAs could be hydrolyzed by intestinal lipases and gut microorganisms including Lactobacillus to produce free gallic acid (GA). Through regulating the chain length of the lipid part in A-GAs, the sustained and controllable release of the GA can be easily achieved. Furthermore, A-GAs were also able to reach the colon and the cecum, which would lead to potential gastrointestinal protective effects. Therefore, A-GAs may be applied as possible ingredient for functional foods.

Identification and Validation of a Novel Six-Gene Expression Signature for Predicting Hepatocellular Carcinoma Prognosis

Background

Hepatocellular carcinoma (HCC) is a highly lethal disease. Effective prognostic tools to guide clinical decision-making for HCC patients are lacking.

Objective

We aimed to establish a robust prognostic model based on differentially expressed genes (DEGs) in HCC.

Methods

Using datasets from The Cancer Genome Atlas (TCGA), the Gene Expression Omnibus (GEO), and the International Genome Consortium (ICGC), DEGs between HCC tissues and adjacent normal tissues were identified. Using TCGA dataset as the training cohort, we applied the least absolute shrinkage and selection operator (LASSO) algorithm and multivariate Cox regression analyses to identify a multi-gene expression signature. Proportional hazard assumptions and multicollinearity among covariates were evaluated while building the model. The ICGC cohort was used for validation. The Pearson test was used to evaluate the correlation between tumor mutational burden and risk score. Through single-sample gene set enrichment analysis, we investigated the role of signature genes in the HCC microenvironment.

Results

A total of 274 DEGs were identified, and a six-DEG prognostic model was developed. Patients were stratified into low- or high-risk groups based on risk scoring by the model. Kaplan-Meier analysis revealed significant differences in overall survival and progression-free interval. Through univariate and multivariate Cox analyses, the model proved to be an independent prognostic factor compared to other clinic-pathological parameters. Time-dependent receiver operating characteristic curve analysis revealed satisfactory prediction of overall survival, but not progression-free interval. Functional enrichment analysis showed that cancer-related pathways were enriched, while immune infiltration analyses differed between the two risk groups. The risk score did not correlate with levels of PD-1, PD-L1, CTLA4, or tumor mutational burden.

Conclusions

We propose a six-gene expression signature that could help to determine HCC patient prognosis. These genes may serve as biomarkers in HCC and support personalized disease management.

Effect of carbon chain length on the hydrolysis and transport characteristics of alkyl gallates in rat intestine

Phenolipids such as alkyl gallates (A-GAs) have been approved by food industry as non-toxic antioxidant additives. However, their digestion and absorption mechanisms in the intestine have not yet been clarified. In this research, the hydrolysis and transport characteristics of A-GAs with fatty alcohols of various chain lengths (C1:0, C2:0, C3:0, C4:0, C8:0, C12:0 and C16:0) were estimated by the everted-rat-gut-sac model (ERGSM) for the first time. High-performance liquid chromatography measurements proved that measurable peaks corresponding to methyl gallate (G-C1:0), ethyl gallate (G-C2:0), propyl gallate (G-C3:0) and butyl gallate (G-C4:0) were discovered in the serosal fluids, which showed the short-chain alkyl gallates can cross the membrane in the form of esters. Besides, all A-GAs were hydrolyzed to GA in the mucosal solution, which contributed evidently to the transport of GA across the membrane of the small intestine. Meanwhile, the hydrolysis rate of A-GAs and transport rate of GA initially increased and then decreased with the chain length, exhibiting a maximum for octyl gallate (G-C8:0). In general, all A-GAs have the behavior of sustained-release. In consequence, the production of A-GAs should be an effective method to extend action time and further increases biological activities of GA.

Growth Hormone and Obesity

Growth hormone (GH) exerts IGF-I dependent protein anabolic and direct lipolytic effects. Obesity reversibly suppresses GH secretion driven by elevated FFA levels, whereas serum IGF-I levels remain normal or elevated due to elevated portal insulin levels. Fasting in lean individuals suppresses hepatic IGF-I production and increases pituitary GH release, but this pattern is less pronounced in obesity. Fasting in obesity is associated with increased sensitivity to the insulin-antagonistic effects of GH. GH treatment in obesity induces a moderate reduction in fat mass and an increase in lean body mass but the therapeutic potential is uncertain.

Mapping the sites of the lipoprotein lipase (LPL)-angiopoietin-like protein 4 (ANGPTL4) interaction provides mechanistic insight into LPL inhibition

Cardiovascular disease has been the leading cause of death throughout the world for nearly 2 decades. Hypertriglyceridemia affects more than one-third of the population in the United States and is an independent risk factor for cardiovascular disease. Despite the frequency of hypertriglyceridemia, treatment options are primarily limited to diet and exercise. Lipoprotein lipase (LPL) is an enzyme responsible for clearing triglycerides from circulation, and its activity alone can directly control plasma triglyceride concentrations. Therefore, LPL is a good target for triglyceride-lowering therapeutics. One approach for treating hypertriglyceridemia may be to increase the amount of enzymatically active LPL by preventing its inhibition by angiopoietin-like protein 4 (ANGPTL4). However, little is known about how these two proteins interact. Therefore, we used hydrogen-deuterium exchange MS to identify potential binding sites between LPL and ANGPTL4. We validated sites predicted to be located at the protein-protein interface by using chimeric variants of LPL and an LPL peptide mimetic. We found that ANGPTL4 binds LPL near the active site at the lid domain and a nearby α-helix. Lipase lid domains cover the active site to control both enzyme activation and substrate specificity. Our findings suggest that ANGPTL4 specifically inhibits LPL by binding the lid domain, which could prevent substrate catalysis at the active site. The structural details of the LPL-ANGPTL4 interaction uncovered here may inform the development of therapeutics targeted to disrupt this interaction for the management of hypertriglyceridemia.

A novel frameshift mutation in the lipoprotein lipase gene is rescued by alternative messenger RNA splicing

BACKGROUND

Type I hyperlipoproteinemia, manifesting as chylomicronemia and severe hypertriglyceridemia, is a rare autosomal recessive disorder usually caused by mutations in the lipoprotein lipase gene (LPL).

OBJECTIVE

We sought to determine whether mutations in LPL could explain the clinical indications of a patient presenting with pancreatitis and hypertriglyceridemia.

METHODS

Coding regions of LPL were amplified by polymerase chain reaction and analyzed by nucleotide sequencing. The LPL messenger RNA transcript was also analyzed to investigate whether alternative splicing was occurring.

RESULTS

The patient was homozygous for the mutation c.767_768insTAAATATT in exon 5 of the LPL gene. This mutation is predicted to result in either a truncated nonfunctional LPL, or alternatively a new 5' donor splice site may be used, resulting in a full-length LPL with an in-frame deletion of 3 amino acids. Analysis of messenger RNA from the patient showed that the new splice site is used in vivo.

CONCLUSION

Homozygosity for a mutation in the LPL gene was consistent with the clinical findings. Use of the new splice site created by the insertion mutation rescues an otherwise damaging frameshift mutation, resulting in expression of an almost full-length LPL that is predicted to be partially functional. The patient therefore has a less severe form of type I hyperlipoproteinemia than would be expected if she lacked any functional LPL.

Familial lipoprotein lipase deficiency caused by known (G188E) and novel (W394X) LPL gene mutations

Lipoprotein lipase (LPL) is the key enzyme in the catabolism of triglyceride-rich lipoproteins in the circulation. Familial LPL deficiency is characterized by hypertriglyceridaemia and absence of LPL activity. We report a case of LPL deficiency in a 43-year-old woman, who initially presented in childhood with chylomicronaemia syndrome. At that time, her plasma triglyceride concentration was ∼30 mmol/L and post-heparin lipolytic activity was very low. In addition to having the known missense mutation LPL G188E, the patient was also found to have a novel nonsense mutation in exon 8, namely LPL W394X. The novel substitution in exon 8 (c.1262G > A) predicts a truncated protein product of 393 amino acids that lacks the carboxyl-terminal 12% of the mature LPL. Trp394 is part of a cluster of exposed tryptophan residues in the carboxyl-terminal domain of LPL important for binding lipid substrate. Of 11 members from her three-generation family, three were heterozygotes for G188E (mean plasma triglyceride, 3.5 ± 2.0 mmol/L), whereas six were heterozygotes for W394X (triglyceride, 4.3 ± 1.8 mmol/L). In summary, we describe a case of familial LPL deficiency caused by compound heterozygosity for known (G188E) and novel (W394X) LPL gene mutations.

Fasting serum triglyceride and high-density lipoprotein cholesterol levels in patients intended to be treated for dyslipidemia

Objective

The aim of the present investigation was to evaluate the influence of serum triglycerides (TG) on other plasma lipids in patients to be treated for dyslipidemia.

Methodology

Lipid profiles of a cohort of 801 patients (487 males and 314 females) aged 57 ± 9 years (mean ± SD) were evaluated. Patients were stratified according to their plasma lipid levels. They were divided into various groups on the basis of serum TG (≥150 or <150 mg/dL) and high-density lipoprotein cholesterol (HDL-C) (≥40 or <40 mg/dL).

Results

Patients with TG ≥ 150 mg/dL had a higher total cholesterol and lower HDL-C levels compared with those with TG < 150 mg/dL, (p < 0.001). Patients with HDL-C < 40 mg/dL had a lower serum total cholesterol and higher TG compared with those with HDL-C ≥ 40 mg/dL (p = 0.011 and p < 0.0001, respectively). In all patients as well as in the subgroups, an inverse correlation between TG and HDL-C was found (r = −0.377, p < 0.001).

Conclusions

Although, the metabolic pathway for TG and HDL-C is closely linked, an inverse correlation between TG and HDL-C levels seems to exist in the entire sampled population. This correlation also appears to persist in fasting patients with low levels of TG.

Molecular characterization of gilthead sea bream (Sparus aurata) lipoprotein lipase. Transcriptional regulation by season and nutritional condition in skeletal muscle and fat storage tissues

Lipoprotein lipase (LPL) of gilthead sea bream (Sparus aurata) was cloned and sequenced using a RT-PCR approach completed by 3' and 5'RACE assays. The nucleotide sequence covered 1669 bp with an open reading frame of 525 amino acids, including a putative signal peptide of 23 amino acids long. Sequence alignment and phylogenetic analysis revealed a high degree of conservation among most fish and higher vertebrates, retaining the consensus sequence the polypeptide "lid", the catalytic triad and eight cysteine residues at the N-terminal region. A tissue-specific regulation of LPL was also found on the basis of changes in season and nutritional condition as a result of different dietary protein sources. First, the expression of LPL in mesenteric adipose tissue was several times higher than in liver and skeletal muscle. Secondly, the spring up-regulation of LPL expression in the mesenteric adipose tissue was coincident with a pronounced increase of whole body fat content. Thirdly, the highest expression of LPL in the skeletal muscle was found in summer, which may serve to cover the increased energy demands for muscle growth and protein accretion. Further, in fish fed plant-protein-based diets, hepatic LPL expression was up-regulated whereas an opposite trend was found in the mesenteric adipose tissue, which may contribute to drive dietary lipids towards liver fat storage. Finally, it is of interest that changes in circulating triglyceride (TG) levels support the key role of LPL in the clearance of TG-rich lipoproteins. This study is the first report in fish of a co-regulated expression of LPL in oxidative and fat storage tissues under different physiological conditions.

Biochemical and molecular characterization of two phosphatidic acid-selective phospholipase A1s, mPA-PLA1alpha and mPA-PLA1beta

We have identified a novel phospholipase A1, named mPA-PLA1beta, which is specifically expressed in human testis and characterized it biochemically together with previously identified mPA-PLA1alpha. The sequence of mPAPLA1beta encodes a 460-amino acid protein containing a lipase domain with significant homology to the previously identified phosphatidic acid (PA)-selective PLA1, mPA-PLA1alpha. mPA-PLA1beta contains a short lid and deleted beta9 loop, which are characteristics of PLA1 molecules in the lipase family, and is a member of a subfamily in the lipase family that includes mPA-PLA1alpha and phosphatidylserine-specific PLA1. Both mPA-PLA1beta and mPA-PLA1alpha recombinant proteins exhibited PA-specific PLA1 activity and were vanadate-sensitive. When mPAPLA1beta-expressing cells were treated with bacterial phospholipase D, the cells produced lysophosphatidic acid (LPA). In both mPA-PLA1alpha and beta-expressing cells, most of the PA generated by the phospholipase D (PLD) treatment was converted to LPA, whereas in control cells it was converted to diacylglycerol. When expressed in HeLa cells most mPA-PLA1alpha protein was recovered from the cell supernatant. By contrast, mPA-PLA1beta was recovered almost exclusively from cells. Consistent with this observation, we found that mPA-PLA1beta has higher affinity to heparin than mPA-PLA1alpha. We also found that the membrane-associated mPA-PLA1s were insoluble in solubilization by 1% Triton X-100 and were detected in Triton X-100-insoluble buoyant fractions of sucrose gradients. The present study raises the possibility that production of LPA by mPA-PLA1alpha and -beta occurs on detergent-resistant membrane domains of the cells where they compete with lipid phosphate phosphatase for PA.

VLDL induces adipocyte differentiation in ApoE-dependent manner

OBJECTIVE

To clarify the role of very low density lipoprotein (VLDL) and apolipoprotein E (apoE) in adipogenesis, we studied newly developed hyperlipidemic obese (ob/ob;apoE-/-) mice. Because hydrolysis of VLDL is believed to be the major source of adipogenic free fatty acids, a higher plasma level of VLDL in these mice should exaggerate obesity.

METHODS AND RESULTS

When fed a high-fat, high-cholesterol diet, ob/ob;apoE-/- mice did not show increased body weight or an increased amount of adipose tissue in spite of increased plasma VLDL levels, whereas ob/ob mice showed an increased body weight and amount of adipose tissue, suggesting that there is a novel apoE-dependent pathway for adipogenesis. In vitro experiments using bone marrow stromal cells and 3T3-L1 cells confirmed this notion. ApoE-deficient VLDL did not induce adipogenesis, whereas normal VLDL induced adipogenesis in these cells. The incubation of apoE-deficient VLDL with recombinant human apoE restored its adipogenic activity. Tetrahydrolipstatin, a lipoprotein lipase inhibitor, did not affect the adipogenic activity of VLDL, suggesting that hydrolysis of VLDL did not play a major role in its effects. In fact, lipid components of VLDL or free fatty acids induced only partial adipogenesis.

CONCLUSIONS

Our findings indicate that VLDL induces adipogenesis in an apoE-dependent manner both in vitro and in vivo.

Molecular modeling of the dimeric structure of human lipoprotein lipase and functional studies of the carboxyl-terminal domain

Lipoprotein lipase (LPL) plays a key role in lipid metabolism. Molecular modeling of dimeric LPL was carried out using insight ii based upon the crystal structures of human, porcine, and horse pancreatic lipase. The dimeric model reveals a saddle-shaped structure and the key heparin-binding residues in the amino-terminal domain located on the top of this saddle. The models of two dimeric conformations - a closed, inactive form and an open, active form - differ with respect to how surface-loop positions affect substrate access to the catalytic site. In the closed form, the surface loop covers the catalytic site, which becomes inaccessible to solvent. Large conformational changes in the open form, especially in the loop and carboxyl-terminal domain, allow substrate access to the active site. To dissect the structure-function relationships of the LPL carboxyl-terminal domain, several residues predicted by the model structure to be essential for the functions of heparin binding and substrate recognition were mutagenized. Arg405 plays an important role in heparin binding in the active dimer. Lys413/Lys414 or Lys414 regulates heparin affinity in both monomeric and dimeric forms. To evaluate the prediction that LPL forms a homodimer in a 'head-to-tail' orientation, two inactive LPL mutants - a catalytic site mutant (S132T) and a substrate-recognition mutant (W390A/W393A/W394A) - were cotransfected into COS7 cells. Lipase activity could be recovered only when heterodimerization occurred in a head-to-tail orientation. After cotransfection, 50% of the wild-type lipase activity was recovered, indicating that lipase activity is determined by the interaction between the catalytic site on one subunit and the substrate-recognition site on the other.

Lipoprotein lipase (LPL) deficiency: a new patient homozygote for the preponderant mutation Gly188Glu in the human LPL gene and review of reported mutations: 75 % are clustered in exons 5 and 6

We have investigated the lipoprotein lipase (LPL) gene of a 2-year-old patient presenting classical features of the familial LPL deficiency including undetectable LPL activity. DNA sequence analysis of exon 5 identified the patient as a homozygote for the Gly188Glu mutation, frequently involved in this disease. A review of cases of LPL deficiency with molecular study of the LPL gene showed a total number of 221 reported mutations involved in this disease. Gly188Glu was involved in 23.5 % of cases and 74.6 % of mutations were clustered in exons 5 and 6. Based on these observations, we propose a method of screening for mutations in this gene.

A newly identified lipoprotein lipase (LPL) gene mutation (F270L) in a Japanese patient with familial LPL deficiency

We have systematically investigated the molecular defects resulting in a primary lipoprotein lipase (LPL) deficiency in a Japanese male infant (proband SH) with fasting hyperchylomicronemia. Neither LPL activity nor immunoreactive LPL mass was detected in pre- or postheparin plasma from proband SH. DNA sequence analysis of the LPL gene of proband SH revealed homozygosity for a novel missense mutation of F270L (Phe(270)-->Leu/TTT(1065)-->TTG) in exon 6. The function of the mutant F270L LPL was determined by both biochemical and immunocytochemical studies. In vitro expression experiments on the mutant F270L LPL cDNA in COS-1 cells demonstrated that the mutant LPL protein was synthesized as a catalytically inactive form and its total amount was almost equal to that of the normal LPL. Moreover, the synthesized mutant LPL was non-releasable by heparin because the intracellular transport of the mutant LPL to the cell surface - by which normal LPL becomes heparin-releasable - was impaired due to the abnormal structure of the mutant LPL protein. These findings explain the failure to detect LPL activities and masses in pre- and postheparin plasma of the proband. The mutant F270L allele generated an XcmI restriction enzyme site in exon 6 of the LPL gene. The carrier status of F270L in the proband's family members was examined by digestion with XcmI. The proband was ascertained to be homozygous for the F270L mutation and his parents and sister were all heterozygous. The LPL activities and masses of the parents and the sister (carriers) were half or less than half of the control values. Regarding the phenotype of the carriers, the mother with a sign of hyperinsulinemia manifested hypertriglyceridemia (type IV hyperlipoproteinemia), whereas the healthy father and the sister were normolipidemic. Hyperinsulinemia may be a strong determinant of hypertriglyceridemia in subjects with heterozygous LPL deficiency.

Familial lipoprotein lipase (LPL) deficiency: a catalogue of LPL gene mutations identified in 20 patients from the UK, Sweden, and Italy

The aim of this study was to identify mutations in the lipoprotein lipase (LPL) gene in 20 unrelated patients with familial lipoprotein deficiency (FLLD) and to investigate the genotype/phenotype relationship. The previously reported G188E mutation (Monsalve et al., J Clin Invest 86:728-734, 1990) was screened for and found to be present in seven individuals (12/40 alleles). In addition, three patients were heterozygous for the 2.0 kb insertion (Langlois et al., Proc Nalt Acad Sci US 86:948-952, 1989). Two approaches were taken for new mutation detection; single-strand conformation polymorphism and sequencing to identify micro-mutations in the proximal promoter and exons 1-9 of the LPL gene and Southern blotting to identify gross mutations. Ten different point mutations were found (W86G, A158T, H183Q, G188E, S193R, P207L, L252X, N291S, M301T, L303P). Additionally, a two nucleotide deletion in exon 6 (delta1006-1007), a six nucleotide deletion in exon 8 (delta1441-1447), and a silent substitution in the wobble position of codon E118 were identified. In vitro mutagenesis and expression in COS-B cells suggested that the A158T and S193R substitutions virtually abolished enzyme activity. In analysing the genotype/phenotype relationship, there was no strong association between age at diagnosis, severity of symptoms, lipid levels, and the nature/position of the mutation. Triglyceride levels, however, were higher in compound heterozygotes compared to true homozygotes, possibly reflecting increased instability of heterodimers. Overall, 29 of 40 (72.5%) mutant alleles were identified. Failure to identify the mutation in 11 alleles might reflect the inadequacy of the method or the possibility that mutations lie within regions of the gene not screened in the study because of lack of availability of sequence.

HDL metabolism in hypertriglyceridemic states: an overview

Reduced plasma high-density lipoprotein (HDL) cholesterol levels have been recognized as a highly significant independent risk factor for atherosclerotic cardiovascular disease. HDL levels are also inversely related to plasma triglyceride levels and there is a dynamic interaction between HDL and triglyceride (TG) rich lipoproteins in vivo. The mechanisms underlying the lowering of HDL in hypertriglyceridemic states have not been fully elucidated, but there is evidence to suggest that triglyceride enrichment of HDL, a common metabolic consequence of hypertriglyceridemia, may play an important role in this process. There is accumulating evidence to suggest that the primary mechanisms leading to reduced plasma HDL cholesterol levels and HDL particle number in hypertriglyceridemic states may be due to any one or a combination of the following possibilities: (1) small HDL particles, which are the product of the intravascular lipolysis of triglyceride-enriched HDL, may be cleared more rapidly from the circulation, (2) triglyceride-enriched HDL may be intrinsically more unstable in the circulation, with apo A-I loosely bound, (3) the lipolytic process itself of triglyceride-enriched HDL may lower HDL particle number by causing apo A-I to be shed from the HDL particles and cleared from the circulation, (4) a dysfunctional lipoprotein lipase or reduced LPL activity may contribute to the lowering of HDL levels by reducing the availability of surface constituents of triglyceride-rich lipoproteins that are necessary for the formation of nascent HDL particles. This review summarizes the evidence that triglyceride-enrichment of HDL is an important factor determining the rate at which HDL is catabolized, a mechanism which could explain, at least in part, the reduced plasma HDL cholesterol levels and particle number frequently observed in hypertriglyceridemic states.

Common mutations of the lipoprotein lipase gene and their clinical significance

The accumulation of triglyceride-rich lipoproteins is an independent factor for an increased risk for premature arteriosclerosis. Common mutations in the lipoprotein lipase (LPL) gene are at least in part inherited susceptibility factors involved in the age- and sex-dependent phenotypic expression of hypertriglyceridemia. It can be estimated that about 20% of patients with hypertriglyceridemia are carriers of common LPL gene mutations (Asp9Asn, Asn291Ser, Trp86Arg, Gly188Glu, Pro207Leu, Asp250Asn) associated with the HLP. Genotyping of these LPL gene mutations is recommended especially in patients with high risk for premature arteriosclerosis. A comparably high number of individuals are carriers of common mutations (Ser447X) or silent mutations (Thr361) associated with low favorable lipids.

An alternative splicing form of phosphatidylserine-specific phospholipase A1 that exhibits lysophosphatidylserine-specific lysophospholipase activity in humans

Phosphatidylserine-specific phospholipase A1 (PS-PLA1), which acts specifically on phosphatidylserine (PS) and 1-acyl-2-lysophosphatidylserine (lyso-PS) to hydrolyze fatty acids at the sn-1 position of these phospholipids, was first identified in rat platelets (Sato, T., Aoki, J., Nagai, Y., Dohmae, N., Takio, K., Doi, T., Arai, H., and Inoue, K. (1997) J. Biol. Chem. 272, 2192-2198). In this study we isolated and sequenced cDNA clones encoding human PS-PLA1, which showed 80% homology with rat PS-PLA1 at the amino acid level. In addition to an mRNA encoding a 456-amino acid product (PS-PLA1), an mRNA with four extra bases inserted at the boundary of the exon-intron junction was detected in human tissues and various human cell lines. This mRNA is most probably produced via an alternative use of the 5'-splicing site (two consensus sequences for RNA splicing occur at the boundary of the exon-intron junction) and encodes a 376-amino acid product (PS-PLA1DeltaC) that lacks two-thirds of the C-terminal domain of PS-PLA1. Unlike PS-PLA1, PS-PLA1DeltaC hydrolyzed exclusively lyso-PS but not PS appreciably. Any other phospholipids such as phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidic acid (PA), and their lyso derivatives were not hydrolyzed at all. These data demonstrated that PS-PLA1DeltaC exhibits lyso-PS-specific lysophospholipase activity and that the C-terminal domain of PS-PLA1 is responsible for recognizing diacylphospholipids. In addition, human PS-PLA1 gene was mapped to chromosome 3q13.13-13.2 and was unexpectedly identical to the nmd gene, which is highly expressed in nonmetastatic melanoma cell lines but poorly expressed in metastatic cell lines (van Groningen, J. J., Bloemers, H. P., and Swart, G. W. (1995) Cancer Res. 55, 6237-6243).

Comparison of RRR-alpha- and all-rac-alpha-tocopherol uptake by permanent rat skeletal muscle myoblasts (L6 cells): effects of exogenous lipoprotein lipase

The purpose of the present investigation was to test whether permanent skeletal muscle cells (rat L6 cells) could serve as an in vitro model for alpha-tocopherol (alphaTocH) biodiscrimination studies. L6 cells were incubated in the presence of high density lipoprotein (HDL), low density lipoprotein (LDL), and very low density lipoprotein (VLDL) labeled in the lipid moiety with either all-rac- or RRR-[14C]alphaTocH. These incubations were performed either in the absence or in the presence of exogenously added bovine lipoprotein lipase (LPL) since skeletal muscle is one of the major expression sites of LPL in vivo. Time-dependent uptake studies (up to 24 h) in the absence of LPL have shown that equipotent doses of all-rac- and RRR-[14C]alphaTocH (1.36:1) led to almost identical accumulation of the tracer, independent of the lipoprotein class used as alphaTocH carrier. With regard to alphaTocH donor capacity, it appeared that HDL is the most potent alphaTocH donor, followed by LDL and VLDL. In the presence of LPL, all-rac- and RRR-[14C]alphaTocH uptake was significantly enhanced (between two- and tenfold). Biodiscrimination studies using chiral high-performance liquid chromatographic analysis with radiometric detection of the corresponding methyl ether derivatives on a Chiralcel OD column have demonstrated that the 2S-and 2R-isomers of alphaTocH were taken up in a 1:1 ratio by L6 cells independent of the absence or presence of LPL. In addition, we have not observed biodiscrimination between the four 2R-isomers, i.e., there was no preferential accumulation of the RRR-isomer. These data suggest that L6 cells do not discriminate between different alphaTocH isomers and that the addition of endogenous LPL significantly enhances the uptake of RRR- and all-rac-alphaTocH.

A common mutation in the lipoprotein lipase gene promoter, -93T/G, is associated with lower plasma triglyceride levels and increased promoter activity in vitro

Single-strand conformational polymorphism analysis of the lipoprotein lipase promoter identified a T-->G transition at position -93. The frequency in healthy white men was 3.4% (n = 1575). There was an 83% allelic association between -93T-->G and Asp9-->Asn (D9N); all N9 mutations occurred on a -93G allele, but not all -93G mutations occurred on an N9 allele. It was thus possible to assess the effect on plasma triglyceride (Tg) levels of the rare -93G mutation in the presence of the wild-type D9. Carriers of the -93G, with genotype TG/DD, had significantly lower Tg levels than TT/DD individuals (1.36 versus 1.78 mmol/L, P = .01); carriers of both mutations (TG/DN) had the highest Tg levels (1.93 mmol/L). When the group was stratified above and below the sample mean for body mass index (BMI), carriers of the -93G on a D9 allele (TG/DD) were "protected" against the Tg-raising effect of obesity, as assessed by BMI. In Afro-Caribbeans (n = 91), the carrier frequency of -93G was 18-fold higher (63%), with weaker (17%) allelic association between -93G and N9. In vitro, the -93G promoter had 24% higher activity than the -93T in a rat smooth muscle cell line and 18% higher activity in a human adrenal cell line. A protein identified by band-shift assays bound to the -93G but not to the -93T allele, which may explain the lower Tg levels in -93G carriers.

A new mutation in the human lipoprotein lipase gene causing familial hyperchylomicronaemia

Lipoprotein lipase plays a major role in the regulation of lipid metabolism. The enzyme acts to hydrolyse triglycerides, providing free fatty acids for energy generation or storage, thus affecting the maturation of circulating lipoproteins. Biochemical and molecular analyses were performed on two siblings of consanguineous Pakistani origin, presenting with hyperchylomicronaemia, which revealed that the disorder resulted from lipoprotein lipase deficiency. Molecular analysis of the lipoprotein lipase gene has revealed a novel homozygous mutation, leucine to proline at amino acid residue 303, within the amino terminal domain of the protein.

Therapeutic response to medium-chain triglycerides and omega-3 fatty acids in a patient with the familial chylomicronemia syndrome

We have studied the underlying molecular defect in a patient presenting with recurrent pancreatitis, hypertriglyceridemia, and virtually undetectable postheparin plasma lipoprotein lipase (LPL) mass and activity, who normalized her triglycerides 3 to 6 months after initiation of either medium-chain triglyceride (MCT) oil or omega-3 fatty acid (omega-3-FA) therapy. After treatment, postheparin plasma LPL activity and mass ranged from 24% to 39% of normal and LPL specific activity was normal (1.0 nmol.ng-1.min-1). On discontinuation of MCT oil or omega-3-FA, plasma triglyceride increased to > 2000 mg/dL. Northern blotting revealed both normal size and abundance of LPL mRNA isolated from adipocytes as well as macrophages. Sequence analysis of the LPL gene, which included all 10 exons, intron-exon splice junctions, and 1.7 kb of the 5'-flanking region, and of LPL cDNA failed to identify any mutations. ApoC-II activity and mass assays revealed the presence of normal levels of a fully functional cofactor as well as the absence of circulating plasma inhibitors of lipase function. In summary, we describe a unique patient presenting with classical features of the familial chylomicronemia syndrome who manifests an unusually beneficial therapeutic response to MCT oil and omega-3-FA therapy. Unlike that in most patients with LPL deficiency, the chylomicronemia in this patient is not caused by a mutation in the structural LPL gene but possibly by a posttranscriptional defect. Thus, a subset of LPL-deficient patients with unique genetic defects respond to therapy by normalizing fasting plasma triglycerides; a therapeutic trial with MCT oil should be considered in all patients presenting with the familial chylomicronemia syndrome.

Fat cells

The adipocyte is a metabolically active cell that functions to store energy for times of energy deprivation or enhanced need. Obesity is characterized by increased lipid accumulation and turnover compared with the nonobese state. Both triglyceride synthesis and lipolysis are regulated metabolic processes in the adipocyte. Current research on the metabolic activities of the human adipocyte focus on plasma triglyceride hydrolysis and uptake of fatty acids by LPL, esterification of these fatty acids, and the subsequent triglyceride breakdown by hormone-sensitive lipase in response to stimulation of adrenergic receptors. These topics are discussed in relationship to the development of obesity.

Pharmacodynamic activity of lipoprotein lipase and hepatic lipase, and pharmacokinetic parameters measured in normolipidaemic subjects receiving ciprofibrate (100 or 200 mg/day) or micronised fenofibrate (200 mg/day) therapy for 23 days

The activities of lipoprotein lipase (LPL) and hepatic lipase (HL) were investigated after 23 days of ciprofibrate (100 mg or 200 mg) therapy or fenofibrate (200 mg) therapy. In a double-blind, double-placebo, cross-over study, three groups of six healthy volunteers received either 100 mg ciprofibrate/day followed by 200 mg fenofibrate 'high bioavailability' (HB)/day, or vice versa (group A), 200 mg ciprofibrate HB/day followed by 200 mg fenofibrate HB/day, or vice versa (group B), or 100 mg ciprofibrate/day followed by 200 mg ciprofibrate/day, or vice versa (group C). Fasting plasma lipid levels and safety parameters were evaluated before and after treatment. One hundred milligrams ciprofibrate/day therapy was found to be approximately as effective as 200 mg fenofibrate HB/day therapy in altering the lipid profile. The highest activation of LPL was obtained after treatment with 200 mg ciprofibrate/day. A modest, but statistically significant, increase in HL activity was found after 100 or 200 mg ciprofibrate treatment. Investigation of the pharmacokinetics of ciprofibrate and fenofibric acid revealed a shorter time to reach peak plasma levels, but a longer elimination half life for the ciprofibrate preparations in comparison with fenofibrate. A dose of 200 mg ciprofibrate/day is more effective than 100 mg ciprofibrate/day at increasing LPL and HL activity; however, 200 mg ciprofibrate/day is also associated with a potential detrimental change in safety parameters. Two hundred milligrams fenofibrate HB/day therapy may represent an alternative therapy to 100 mg ciprofibrate/day for hyperlipidaemic patients.