Pancreatic carboxyl ester lipase: a circulating enzyme that modifies normal and oxidized lipoproteins in vitro

Abnormal exocrine-endocrine cell cross-talk promotes β-cell dysfunction and loss in MODY8

MODY8 (maturity-onset diabetes of the young, type 8) is a dominantly inherited monogenic form of diabetes associated with mutations in the carboxyl ester lipase (CEL) gene expressed by pancreatic acinar cells. MODY8 patients develop childhood-onset exocrine pancreas dysfunction followed by diabetes during adulthood. However, it is unclear how CEL mutations cause diabetes. In the present study, we report the transfer of CEL proteins from acinar cells to β-cells as a form of cross-talk between exocrine and endocrine cells. Human β-cells show a relatively higher propensity for internalizing the mutant versus the wild-type CEL protein. After internalization, the mutant protein forms stable intracellular aggregates leading to β-cell secretory dysfunction. Analysis of pancreas sections from a MODY8 patient reveals the presence of CEL protein in the few extant β-cells. The present study provides compelling evidence for the mechanism by which a mutant gene expressed specifically in acinar cells promotes dysfunction and loss of β-cells to cause diabetes.

Bacterial triacylglycerol lipase is a potential cholesterol esterase: Identification of a key determinant for sterol-binding specificity

Cholesterol esterase (Che) from Burkholderia stabilis (BsChe) is a homolog of well-characterized and industrially relevant bacterial triacylglycerol lipases (Lips). BsChe is a rare bacterial Lip enzyme that exhibits practical Che activity and is currently used in clinical applications to determine total serum cholesterol levels. To investigate the sterol specificity of BsChe, we determined the X-ray structure of BsChe. We discovered a local structural change in the active-site cleft, which might be related to substrate binding and product release. We also performed molecular docking studies by using the X-ray models of BsChe and cholesterol linoleate (CLL), the most favorable substrate for BsChe. The results showed that the sterol moieties of reasonable CLL docking poses localized to a specific active-site cleft surface formed by Leu266 and Ile287, which are unconserved among Burkholderia Lip homologs. Site-directed mutagenesis identified these residues as essential for the Che activity of BsChe, and Leu or Ile substitution conferred marked Che activity to Burkholderia Lips. In particular, Burkholderia cepacia and Burkholderia ubonensis Lips with the V266L/L287I double mutation exhibited ~50-fold and 500-fold higher Che activities than those of the wild-type enzymes, respectively. These results provide new insights into the substrate-binding mechanisms and selectivities of bacterial Lips.

Bile salt-dependent lipase promotes the barrier integrity of Caco-2 cells by activating Wnt/β-catenin signaling via LRP6 receptor

Bile salt-dependent lipase (BSDL) within intestinal lumen can be endocytosed by enterocytes and support the intestinal barrier function. However, the epithelial-supporting effect of this protein has not been verified in a human cell line and neither the direct signaling pathway nor the function of endocytosis in this process has been clearly identified. We sought to investigate the signaling pathway and the membrane receptor through which BSDL might exert these effects using intestinal epithelial cells. Caco-2 cells were treated with recombinant BSDL, and the barrier function, cell proliferation, and activation of the Wnt signaling pathway were assessed. The effect of Wnt signaling activation induced by BSDL and BSDL endocytosis was investigated in LRP6-silenced and non-silenced cells. Moreover, caveolae- and clathrin-dependent endocytosis inhibitors were also applied respectively to analyze their effects on Wnt signaling activation induced by BSDL. BSDL treatment increased the barrier function but not proliferation of Caco-2 cells. It also induced β-catenin nuclear translocation and activated Wnt target gene transcription. Moreover, in the Wnt pathway, BSDL increased the levels of non-phosphorylated-β-catenin (Ser33/37/Thr41) and phosphorylated-β-catenin (Ser552). Notably, the silencing of LRP6 expression impaired BSDL endocytosis and decreased BSDL-induced β-catenin nuclear translocation. The inhibition of BSDL endocytosis induced by caveolae-mediated endocytosis inhibitor was stronger than that by clathrin-mediated endocytosis inhibitor, and the Wnt signaling activation associated with its endocytosis was also most likely caveolae-dependent. Our findings suggested that LRP6, a canonical Wnt pathway co-receptor, can mediate BSDL endocytosis and then activate Wnt signaling in Caco-2 cells.

Association of a new 99-bp indel of the CEL gene promoter region with phenotypic traits in chickens

Carboxyl ester lipase (CEL) encodes a cholesterol ester hydrolase that is secreted into the duodenum as a component of pancreatic juice. The objective of this study was to characterize the CEL gene, investigate the association between the CEL promoter variants and chicken phenotypic traits, and explore the CEL gene regulatory mechanism. An insertion/deletion (indel) caused by a 99-bp insertion fragment was shown for the first time in the chicken CEL promoter, and large differences in allelic frequency were found among commercial breeds, indigenous and feral birds. Association analysis demonstrated that this indel site had significant effects on shank length, shank girth, chest breadth at 8 weeks (p < 0.01), evisceration weight, sebum weight, breast muscle weight, and leg weight (p < 0.05). Tissue expression profiles showed extremely high levels of the CEL gene in pancreatic tissue. Moreover, the expression levels of the genes APOB, MTTP, APOV1 and SREBF1, which are involved in lipid transport, were significantly reduced by adding a 4% oxidized soybean oil diet treatment at the individual level and transfecting the embryonic primary hepatocytes with a CEL-overexpression vector. Interestingly, the results showed that the expression level of the II homozygous genotype was significantly higher than that of the ID and DD genotypes, while individuals with DD genotypes had higher phenotypic values. Therefore, these data suggested that the CEL gene might affect body growth by participating in hepatic lipoprotein metabolism and that the 99-bp indel polymorphism could be a potentially useful genetic marker for improving the economically important traits of chickens.

Pancreatic adenocarcinoma, chronic pancreatitis, and MODY-8 diabetes: is bile salt-dependent lipase (or carboxyl ester lipase) at the crossroads of pancreatic pathologies?

Pancreatic adenocarcinomas and diabetes mellitus are responsible for the deaths of around two million people each year worldwide. Patients with chronic pancreatitis do not die directly of this disease, except where the pathology is hereditary. Much current literature supports the involvement of bile salt-dependent lipase (BSDL), also known as carboxyl ester lipase (CEL), in the pathophysiology of these pancreatic diseases. The purpose of this review is to shed light on connections between chronic pancreatitis, diabetes, and pancreatic adenocarcinomas by gaining an insight into BSDL and its variants. This enzyme is normally secreted by the exocrine pancreas, and is diverted within the intestinal lumen to participate in the hydrolysis of dietary lipids. However, BSDL is also expressed by other cells and tissues, where it participates in lipid homeostasis. Variants of BSDL resulting from germline and/or somatic mutations (nucleotide insertion/deletion or nonallelic homologous recombination) are expressed in the pancreas of patients with pancreatic pathologies such as chronic pancreatitis, MODY-8, and pancreatic adenocarcinomas. We discuss the possible link between the expression of BSDL variants and these dramatic pancreatic pathologies, putting forward the suggestion that BSDL and its variants are implicated in the cell lipid metabolism/reprogramming that leads to the dyslipidemia observed in chronic pancreatitis, MODY-8, and pancreatic adenocarcinomas. We also propose potential strategies for translation to therapeutic applications.

The role of the carboxyl ester lipase (CEL) gene in pancreatic disease

The enzyme carboxyl ester lipase (CEL), also known as bile salt-dependent or -stimulated lipase (BSDL, BSSL), hydrolyzes dietary fat, cholesteryl esters and fat-soluble vitamins in the duodenum. CEL is mainly expressed in pancreatic acinar cells and lactating mammary glands. The human CEL gene resides on chromosome 9q34.3 and contains a variable number of tandem repeats (VNTR) region that encodes a mucin-like protein tail. Although the number of normal repeats does not appear to significantly influence the risk for pancreatic disease, single-base pair deletions in the first VNTR repeat cause a syndrome of endocrine and exocrine dysfunction denoted MODY8. Hallmarks are low fecal elastase levels and pancreatic lipomatosis manifesting before the age of twenty, followed by development of diabetes and pancreatic cysts later in life. The mutant protein forms intracellular and extracellular aggregates, suggesting that MODY8 is a protein misfolding disease. Recently, a recombined allele between CEL and its pseudogene CELP was discovered. This allele (CEL-HYB) encodes a chimeric protein with impaired secretion increasing five-fold the risk for chronic pancreatitis. The CEL gene has proven to be exceptionally polymorphic due to copy number variants of the CEL-CELP locus and alterations involving the VNTR. Genome-wide association studies or deep sequencing cannot easily pick up this wealth of genetic variation. CEL is therefore an attractive candidate gene for further exploration of links to pancreatic disease.

Expression of truncated bile salt-dependent lipase variant in pancreatic pre-neoplastic lesions

Pancreatic adenocarcinoma (PDAC) is a dismal disease. The lack of specific symptoms still leads to a delay in diagnosis followed by death within months for most patients. Exon 11 of the bile salt-dependent lipase (BSDL) gene encoding variable number of tandem repeated (VNTR) sequences has been involved in pancreatic pathologies. We hypothesized that BSDL VNTR sequences may be mutated in PDAC. The amplification of BSDL VNTR from RNA extracted from pancreatic SOJ-6 cells allowed us to identify a BSDL amplicon in which a cytosine residue is inserted in a VNTR sequence. This insertion gives rise to a premature stop codon, resulting in a truncated protein and to a modification of the C-terminal amino-acid sequence; that is PRAAHG instead of PAVIRF. We produced antibodies directed against these sequences and examined pancreatic tissues from patients with PDAC and PanIN. Albeit all tissues were positive to anti-PAVIRF antibodies, 72.2% of patient tissues gave positive reaction with anti-PRAAHG antibodies, particularly in dysplastic areas of the tumor. Neoplastic cells with ductal differentiation were not reactive to anti-PRAAHG antibodies. Some 70% of PanIN tissues were also reactive to anti-PRAAHG antibodies, suggesting that the C insertion occurs early during pancreatic carcinogenesis. Data suggest that anti-PRAAHG antibodies were uniquely reactive with a short isoform of BSDL specifically expressed in pre-neoplastic lesions of the pancreas. The detection of truncated BSDL reactive to antibodies against the PRAAHG C-terminal sequence in pancreatic juice or in pancreatic biopsies may be a new tool in the early diagnosis of PDAC.

Farnesoid X Receptor and Its Ligands Inhibit the Function of Platelets

OBJECTIVE

Although initially seemingly paradoxical because of the lack of nucleus, platelets possess many transcription factors that regulate their function through DNA-independent mechanisms. These include the farnesoid X receptor (FXR), a member of the superfamily of ligand-activated transcription factors, that has been identified as a bile acid receptor. In this study, we show that FXR is present in human platelets and FXR ligands, GW4064 and 6α-ethyl-chenodeoxycholic acid, modulate platelet activation nongenomically.

APPROACH AND RESULTS

FXR ligands inhibited the activation of platelets in response to stimulation of collagen or thrombin receptors, resulting in diminished intracellular calcium mobilization, secretion, fibrinogen binding, and aggregation. Exposure to FXR ligands also reduced integrin α_IIbβ₃ outside-in signaling and thereby reduced the ability of platelets to spread and to stimulate clot retraction. FXR function in platelets was found to be associated with the modulation of cyclic guanosine monophosphate levels in platelets and associated downstream inhibitory signaling. Platelets from FXR-deficient mice were refractory to the actions of FXR agonists on platelet function and cyclic nucleotide signaling, firmly linking the nongenomic actions of these ligands to the FXR.

CONCLUSIONS

This study provides support for the ability of FXR ligands to modulate platelet activation. The atheroprotective effects of GW4064, with its novel antiplatelet effects, indicate FXR as a potential target for the prevention of atherothrombotic disease.

Enzymatically Modified Low-Density Lipoprotein Promotes Foam Cell Formation in Smooth Muscle Cells via Macropinocytosis and Enhances Receptor-Mediated Uptake of Oxidized Low-Density Lipoprotein

OBJECTIVE

Enzyme-modified nonoxidized low-density lipoprotein (ELDL) is present in human atherosclerotic lesions. Our objective is to understand the mechanisms of ELDL uptake and its effects on vascular smooth muscle cells (SMC).

APPROACH AND RESULTS

Transformation of murine aortic SMCs into foam cells in response to ELDL was analyzed. ELDL, but not acetylated or oxidized LDL, was potent in inducing SMC foam cell formation. Inhibitors of macropinocytosis (LY294002, wortmannin, amiloride) attenuated ELDL uptake. In contrast, inhibitors of receptor-mediated endocytosis (dynasore, sucrose) and inhibitor of caveolae-/lipid raft-mediated endocytosis (filipin) had no effect on ELDL uptake in SMC, suggesting that macropinocytosis is the main mechanism of ELDL uptake by SMC. Receptor for advanced glycation end products (RAGE) is not obligatory for ELDL-induced SMC foam cell formation, but primes SMC for the uptake of oxidized LDL in a RAGE-dependent manner. ELDL increased intracellular reactive oxygen species, cytosolic calcium, and expression of lectin-like oxidized LDL receptor-1 in wild-type SMC but not in RAGE(-/-) SMC. The macropinocytotic uptake of ELDL is regulated predominantly by intracellular calcium because ELDL uptake was completely inhibited by pretreatment with the calcium channel inhibitor lacidipine in wild-type and RAGE(-/-) SMC. This is in contrast to pretreatment with PI3 kinase inhibitors which completely prevented ELDL uptake in RAGE(-/-) SMC, but only partially in wild-type SMC.

CONCLUSIONS

ELDL is highly potent in inducing foam cells in murine SMC. ELDL endocytosis is mediated by calcium-dependent macropinocytosis. Priming SMC with ELDL enhances the uptake of oxidized LDL.

Aggregation and fusion of low-density lipoproteins in vivo and in vitro

Low-density lipoproteins (LDLs, also known as 'bad cholesterol') are the major carriers of circulating cholesterol and the main causative risk factor of atherosclerosis. Plasma LDLs are 20- to 25-nm nanoparticles containing a core of cholesterol esters surrounded by a phospholipid monolayer and a single copy of apolipoprotein B (550 kDa). An early sign of atherosclerosis is the accumulation of LDL-derived lipid droplets in the arterial wall. According to the widely accepted 'response-to-retention hypothesis', LDL binding to the extracellular matrix proteoglycans in the arterial intima induces hydrolytic and oxidative modifications that promote LDL aggregation and fusion. This enhances LDL uptake by the arterial macrophages and triggers a cascade of pathogenic responses that culminate in the development of atherosclerotic lesions. Hence, LDL aggregation, fusion, and lipid droplet formation are important early steps in atherogenesis. In vitro, a variety of enzymatic and nonenzymatic modifications of LDL can induce these reactions and thereby provide useful models for their detailed analysis. Here, we summarize current knowledge of the in vivo and in vitro modifications of LDLs leading to their aggregation, fusion, and lipid droplet formation; outline the techniques used to study these reactions; and propose a molecular mechanism that underlies these pro-atherogenic processes. Such knowledge is essential in identifying endogenous and exogenous factors that can promote or prevent LDL aggregation and fusion in vivo and to help establish new potential therapeutic targets to decelerate or even block these pathogenic reactions.

Dietary and flight energetic adaptations in a salivary gland transcriptome of an insectivorous bat

We hypothesized that evolution of salivary gland secretory proteome has been important in adaptation to insectivory, the most common dietary strategy among Chiroptera. A submandibular salivary gland (SMG) transcriptome was sequenced for the little brown bat, Myotis lucifugus. The likely secretory proteome of 23 genes included seven (RETNLB, PSAP, CLU, APOE, LCN2, C3, CEL) related to M. lucifugus insectivorous diet and metabolism. Six of the secretory proteins probably are endocrine, whereas one (CEL) most likely is exocrine. The encoded proteins are associated with lipid hydrolysis, regulation of lipid metabolism, lipid transport, and insulin resistance. They are capable of processing exogenous lipids for flight metabolism while foraging. Salivary carboxyl ester lipase (CEL) is thought to hydrolyze insect lipophorins, which probably are absorbed across the gastric mucosa during feeding. The other six proteins are predicted either to maintain these lipids at high blood concentrations or to facilitate transport and uptake by flight muscles. Expression of these seven genes and coordinated secretion from a single organ is novel to this insectivorous bat, and apparently has evolved through instances of gene duplication, gene recruitment, and nucleotide selection. Four of the recruited genes are single-copy in the Myotis genome, whereas three have undergone duplication(s) with two of these genes exhibiting evolutionary 'bursts' of duplication resulting in multiple paralogs. Evidence for episodic directional selection was found for six of seven genes, reinforcing the conclusion that the recruited genes have important roles in adaptation to insectivory and the metabolic demands of flight. Intragenic frequencies of mobile- element-like sequences differed from frequencies in the whole M. lucifugus genome. Differences among recruited genes imply separate evolutionary trajectories and that adaptation was not a single, coordinated event.

Activation of bile salt dependent lipase by (lyso)phosphatidic acid and platelet activating factor

The activity of breast milk BSDL was assayed with or without phospholipids as extra-intestinal effector candidates. Phosphatidic acid, lysophosphatidic acid and platelet activating factor but not phosphatidylcholine and lysophosphatidylcholine stimulated BSDL activity at least as efficiently as taurocholate. The apparent dissociation constants of PA and LPA at saturating concentrations of three different substrates were between 0.1 and 13.4 μM and that of PAF was below or equal to 200 pM. Kinetic data suggested the existence of at least one binding site for each of these effectors. PA, LPA and PAF are likely extra-intestinal modulators of BSDL activity.

Absence of diabetes and pancreatic exocrine dysfunction in a transgenic model of carboxyl-ester lipase-MODY (maturity-onset diabetes of the young)

Background

CEL-MODY is a monogenic form of diabetes with exocrine pancreatic insufficiency caused by mutations in CARBOXYL-ESTER LIPASE (CEL). The pathogenic processes underlying CEL-MODY are poorly understood, and the global knockout mouse model of the CEL gene (CELKO) did not recapitulate the disease. We therefore aimed to create and phenotype a mouse model specifically over-expressing mutated CEL in the pancreas.

Methods

We established a monotransgenic floxed (flanking LOX sequences) mouse line carrying the human CEL mutation c.1686delT and crossed it with an elastase-Cre mouse to derive a bitransgenic mouse line with pancreas-specific over-expression of CEL carrying this disease-associated mutation (TgCEL). Following confirmation of murine pancreatic expression of the human transgene by real-time quantitative PCR, we phenotyped the mouse model fed a normal chow and compared it with mice fed a 60% high fat diet (HFD) as well as the effects of short-term and long-term cerulein exposure.

Results

Pancreatic exocrine function was normal in TgCEL mice on normal chow as assessed by serum lipid and lipid-soluble vitamin levels, fecal elastase and fecal fat absorption, and the normoglycemic mice exhibited normal pancreatic morphology. On 60% HFD, the mice gained weight to the same extent as controls, had normal pancreatic exocrine function and comparable glucose tolerance even after resuming normal diet and follow up up to 22 months of age. The cerulein-exposed TgCEL mice gained weight and remained glucose tolerant, and there were no detectable mutation-specific differences in serum amylase, islet hormones or the extent of pancreatic tissue inflammation.

Conclusions

In this murine model of human CEL-MODY diabetes, we did not detect mutation-specific endocrine or exocrine pancreatic phenotypes, in response to altered diets or exposure to cerulein.

Low-density lipoprotein, its susceptibility to oxidation and the role of lipoprotein-associated phospholipase A2 and carboxyl ester lipase lipases in atherosclerotic plaque formation

An increased level of low-density lipoprotein (LDL) is a very well established risk factor of coronary artery disease (CAD). Unoxidized LDL is an inert transport vehicle of cholesterol and other lipids in the body and is thought to be atherogenic. Recently it has been appreciated that oxidized products of LDL are responsible for plaque formation properties previously attributed to the intact particle. The goal of this article is to review the recent understanding of the LDL oxidation pathway. The role of oxidized products and key enzymes (lipoprotein-associated phospholipase A2 and carboxyl ester lipase) are also extensively discussed in the context of clinical conditions.

Bile salt-stimulated lipase plays an unexpected role in arthritis development in rodents

OBJECTIVE

The present study aimed to explore the hypothesis that bile salt-stimulated lipase (BSSL), in addition to being a key enzyme in dietary fat digestion during early infancy, plays an important role in inflammation, notably arthritis.

METHODS

Collagen-induced arthritis (CIA) and pristane-induced arthritis (PIA) in rodents are commonly used experimental models that reproduce many of the pathogenic mechanisms of human rheumatoid arthritis, i.e. increased cellular infiltration, synovial hyperplasia, pannus formation, and erosion of cartilage and bone in the distal joints. We used the CIA model to compare the response in BSSL wild type (BSSL-WT) mice with BSSL-deficient 'knock-out' (BSSL-KO) and BSSL-heterozygous (BSSL-HET) littermates. We also investigated if intraperitoneal injection of BSSL-neutralizing antibodies affected the development or severity of CIA and PIA in mice and rats, respectively.

RESULTS

In two consecutive studies, we found that BSSL-KO male mice, in contrast to BSSL-WT littermates, were significantly protected from developing arthritis. We also found that BSSL-HET mice were less prone to develop disease compared to BSSL-WT mice, but not as resistant as BSSL-KO mice, suggesting a gene-dose effect. Moreover, we found that BSSL-neutralizing antibody injection reduced both the incidence and severity of CIA and PIA in rodents.

CONCLUSION

Our data strongly support BSSL as a key player in the inflammatory process, at least in rodents. It also suggests the possibility that BSSL-neutralizing agents could serve as a therapeutic model to reduce the inflammatory response in humans.

Comparative Structures and Evolution of Vertebrate Carboxyl Ester Lipase (CEL) Genes and Proteins with a Major Role in Reverse Cholesterol Transport

Bile-salt activated carboxylic ester lipase (CEL) is a major triglyceride, cholesterol ester and vitamin ester hydrolytic enzyme contained within pancreatic and lactating mammary gland secretions. Bioinformatic methods were used to predict the amino acid sequences, secondary and tertiary structures and gene locations for CEL genes, and encoded proteins using data from several vertebrate genome projects. A proline-rich and O-glycosylated 11-amino acid C-terminal repeat sequence (VNTR) previously reported for human and other higher primate CEL proteins was also observed for other eutherian mammalian CEL sequences examined. In contrast, opossum CEL contained a single C-terminal copy of this sequence whereas CEL proteins from platypus, chicken, lizard, frog and several fish species lacked the VNTR sequence. Vertebrate CEL genes contained 11 coding exons. Evidence is presented for tandem duplicated CEL genes for the zebrafish genome. Vertebrate CEL protein subunits shared 53-97% sequence identities; demonstrated sequence alignments and identities for key CEL amino acid residues; and conservation of predicted secondary and tertiary structures with those previously reported for human CEL. Phylogenetic analyses demonstrated the relationships and potential evolutionary origins of the vertebrate CEL family of genes which were related to a nematode carboxylesterase (CES) gene and five mammalian CES gene families.

Cholesteryl ester hydrolase activity is abolished in HSL-/- macrophages but unchanged in macrophages lacking KIAA1363

Cholesteryl ester (CE) accumulation in macrophages represents a crucial event during foam cell formation, a hallmark of atherogenesis. Here we investigated the role of two previously described CE hydrolases, hormone-sensitive lipase (HSL) and KIAA1363, in macrophage CE hydrolysis. HSL and KIAA1363 exhibited marked differences in their abilities to hydrolyze CE, triacylglycerol (TG), diacylglycerol (DG), and 2-acetyl monoalkylglycerol ether (AcMAGE), a precursor for biosynthesis of platelet-activating factor (PAF). HSL efficiently cleaved all four substrates, whereas KIAA1363 hydrolyzed only AcMAGE. This contradicts previous studies suggesting that KIAA1363 is a neutral CE hydrolase. Macrophages of KIAA1363(-/-) and wild-type mice exhibited identical neutral CE hydrolase activity, which was almost abolished in tissues and macrophages of HSL(-/-) mice. Conversely, AcMAGE hydrolase activity was diminished in macrophages and some tissues of KIAA1363(-/-) but unchanged in HSL(-/-) mice. CE turnover was unaffected in macrophages lacking KIAA1363 and HSL, whereas cAMP-dependent cholesterol efflux was influenced by HSL but not by KIAA1363. Despite decreased CE hydrolase activities, HSL(-/-) macrophages exhibited CE accumulation similar to wild-type (WT) macrophages. We conclude that additional enzymes must exist that cooperate with HSL to regulate CE levels in macrophages. KIAA1363 affects AcMAGE hydrolase activity but is of minor importance as a direct CE hydrolase in macrophages.

HDL as a contrast agent for medical imaging

Contrast-enhanced MRI of atherosclerosis can provide valuable additional information on a patient's disease state. As a result of the interactions of HDL with atherosclerotic plaque and the flexibility of its reconstitution, it is a versatile candidate for the delivery of contrast-generating materials to this pathogenic lesion. We herein discuss the reports of HDL modified with gadolinium to act as an MRI contrast agent for atherosclerosis. Furthermore, HDL has been modified with fluorophores and nanocrystals, allowing it to act as a contrast agent for fluorescent imaging techniques and for computed tomography. Such modified HDL has been found to be macrophage specific, and, therefore, can provide macrophage density information via noninvasive MRI. As such, modified HDL is currently a valuable contrast agent for probing preclinical atherosclerosis. Future developments may allow the application of this particle to further diseases and pathological or physiological processes in both preclinical models as well as in patients.

Plasma carboxyl ester lipase activity modulates apolipoprotein B-containing lipoprotein metabolism in a transgenic mouse model

Pancreatic carboxyl ester lipase (CEL) is in the plasma of many mammals, including humans and rats, but not mice. In vitro, CEL hydrolyzes cholesterol esters of apolipoprotein B-containing lipoproteins (apo B-Lp). To study the effect of CEL on metabolism of apo B-Lp and atherosclerosis in vivo, apo E-knockout (EKO) mice, which have high plasma levels of apo B-Lp and are prone to atherosclerosis, were made to secrete CEL into plasma by introducing a transgene containing a liver-specific promoter and rat CEL complementary DNA. Plasma CEL activity in EKO-CEL mice was comparable with that found in rats. Evidence of modification of apo B-Lp by plasma CEL in vivo was an increase in the free cholesterol to cholesterol ester ratio of apo B-Lp from mice on chow or a Western-type diet. In addition, plasma total cholesterol levels were elevated in EKO-CEL mice, with the elevation found exclusively in the apo B-Lp fraction. Associated with the increase in steady-state apo B-Lp levels was an increase in the plasma half-life of very low-density lipoproteins (VLDL) in EKO-CEL mice, measured by the clearance rate of injected VLDL. Interestingly, despite the increase of apo B-Lp, the atherosclerotic lesion did not differ between EKO and EKO-CEL mice on a Western-type diet. In summary, our results demonstrate that plasma CEL modulates apo B-Lp metabolism in vivo, resulting in reduced VLDL clearance and elevated plasma cholesterol levels.

Transcriptional Profiling of Laser Capture Microdissected Rat Arterial Elements: Fenoldopam-induced Vascular Toxicity as a Model System

Transcriptional profiling of specific elements of vasculature from animal models of vascular toxicity is an approach to gain insight into molecular mechanisms of vascular injury. Feasibility of using laser capture microdissection (LCM) to evaluate differential gene expression in selected elements of mesenteric arteries (MA) from untreated rats and rats given a single vasotoxic dose of 100 mg/kg Fenoldopam and euthanized 1 or 4 hours postdose was assessed. Regions of MA (endothelial cells [EC] and vascular smooth muscle cells [VSMC]) were selectively microdissected from optimal-cutting-temperature (O.C.T.)-embedded-frozen tissue sections. RNA was isolated, linearly amplified (LA), and hybridized to Affymetrix GeneChips®. Enrichment for specific vascular elements was evident by unique gene-expression profiles. Statistical analysis indicated that Fenoldopam treatment resulted in differential expression of 333 versus 458 genes in EC and 371 versus 618 genes in VSMC at the 1-hour or 4-hour time point, respectively. Analysis of regulated EC and VSMC genes common to both time points identified several gene functions or pathways affected by treatment. Several genes were identified in EC and/or VSMC that have not been previously linked to vascular structure or function. These data indicate that tissue–element-enrichment by LCM in conjunction with LA and GeneChip analysis offers a refined approach for assessment of injury-mediated transcriptome changes in distinct elements of the vasculature.

Glycemic control in adolescents with type 1 diabetes mellitus improves lipid serum levels and oxidative stress

INTRODUCTION

Atherosclerosis begins in childhood, and diabetes is a risk factor for coronary heart disease. Dyslipidemia is prevalent in children with type 1 diabetes mellitus (T1DM), with an association between elevated hemoglobin A1c (HbA1c), serum lipid levels, and oxidative stress. Our aim was to examine the effect of metabolic control on serum lipid levels and oxidative stress in adolescents with T1DM.

METHODS

Twenty-six adolescents (13 boys and 13 girls), aged 15.65 +/- 1.5 yr, with disease duration of 5.9 +/- 2.8 yr and average HbA1c 10.8 +/- 1.9% were assigned to intensive insulin therapy for 3 months. Comparisons for HbA1c, low-density lipoprotein (LDL) cholesterol, high-density lipoprotein cholesterol, total triglycerides (TG), total cholesterol (TC), apolipoprotein AI, apolipoprotein AII, apolipoprotein B (ApoB), and thiobarbituric acid reactive substances (TBARS) were done between patients whose HbA1c improved by 0.5% or more (GR1) and the rest of the cohort (patients whose HbA1c improved by <0.5%, did not change, or increased) (GR2).

RESULTS

ApoB (p = 0.047) and TBARS (p = 0.01) were significantly lower at the end of the study in GR1. In GR2, TC (p = 0.01) and LDL (p = 0.03) were significantly higher at study end. Overall, significant beneficial changes in TC (p = 0.006), TG (p = 0.04), LDL (p = 0.02), ApoB (p = 0.015), and oxidative stress (p = 0.001) were found in GR1 compared with GR2.

CONCLUSIONS

We provide direct evidence for the beneficial effect of tight metabolic control on serum lipids and oxidative stress in adolescents with T1DM, indicating that tight metabolic control may reduce cardiovascular risk in these patients.

Bile salt-dependent lipase interacts with platelet CXCR4 and modulates thrombus formation in mice and humans

Bile salt-dependent lipase (BSDL) is an enzyme involved in the duodenal hydrolysis and absorption of cholesteryl esters. Although some BSDL is transported to blood, the role of circulating BSDL is unknown. Here, we demonstrate that BSDL is stored in platelets and released upon platelet activation. Because BSDL contains a region that is structurally homologous to the V3 loop of HIV-1, which binds to CXC chemokine receptor 4 (CXCR4), we hypothesized that BSDL might bind to CXCR4 present on platelets. In human platelets in vitro, both BSDL and a peptide corresponding to its V3-like loop induced calcium mobilization and enhanced thrombin-mediated platelet aggregation, spreading, and activated alpha(IIb)beta(3) levels. These effects were abolished by CXCR4 inhibition. BSDL also increased the production of prostacyclin by human endothelial cells. In a mouse thrombosis model, BSDL accumulated at sites of vessel wall injury. When CXCR4 was antagonized, the accumulation of BSDL was inhibited and thrombus size was reduced. In BSDL(-/-) mice, calcium mobilization in platelets and thrombus formation were attenuated and tail bleeding times were increased in comparison with those of wild-type mice. We conclude that BSDL plays a role in optimal platelet activation and thrombus formation by interacting with CXCR4 on platelets.

Enzymatic chain scission kinetics of poly(epsilon-caprolactone) monolayers

The hydrolytic and enzymatic degradation behavior of poly(epsilon-caprolactone) (PCL) is investigated using the Langmuir monolayer technique, and an improved data acquisition and data reduction procedure is presented. Hydrolytic and enzymatic monolayer degradation experiments of PCL with various molecular weights by Pseudomonas cepacia lipase have been carried out to analyze the influence of subphase pH, subphase temperature, enzyme concentration, and the packing density of polymer chains on the degradation kinetics. The enzymatic monolayer degradation results in an exponential increase in the number of dissolved degradation fragments with increasing degradation time, which confirms random chain scission to be the dominant scission mechanism. The increase in the enzymatic scission rate constant with decreasing initial average molecular weight of the polymers is assigned to the influence of the area density of polar terminal groups on the substrate-enzyme complex formation.

Bile salt-stimulated lipase from human milk binds DC-SIGN and inhibits human immunodeficiency virus type 1 transfer to CD4+ T cells

A wide range of pathogens, including human immunodeficiency virus type 1 (HIV-1), hepatitis C virus, Ebola virus, cytomegalovirus, dengue virus, Mycobacterium, Leishmania, and Helicobacter pylori, can interact with dendritic cell (DC)-specific ICAM3-grabbing nonintegrin (DC-SIGN), expressed on DCs and a subset of B cells. More specifically, the interaction of the gp120 envelope protein of HIV-1 with DC-SIGN can facilitate the transfer of virus to CD4+ T lymphocytes in trans and enhance infection. We have previously demonstrated that a multimeric LeX component in human milk binds to DC-SIGN, preventing HIV-1 from interacting with this receptor. Biochemical analysis reveals that the compound is heat resistant, trypsin sensitive, and larger than 100 kDa, indicating a specific glycoprotein as the inhibitory compound. By testing human milk from three different mothers, we found the levels of DC-SIGN binding and viral inhibition to vary between samples. Using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, Western blotting, and matrix-assisted laser desorption ionization analysis, we identified bile salt-stimulated lipase (BSSL), a Lewis X (LeX)-containing glycoprotein found in human milk, to be the major variant protein between the samples. BSSL isolated from human milk bound to DC-SIGN and inhibited the transfer of HIV-1 to CD4+ T lymphocytes. Two BSSL isoforms isolated from the same human milk sample showed differences in DC-SIGN binding, illustrating that alterations in the BSSL forms explain the differences observed. These results indicate that variations in BSSL lead to alterations in LeX expression by the protein, which subsequently alters the DC-SIGN binding capacity and the inhibitory effect on HIV-1 transfer. Identifying the specific molecular interaction between the different forms may aid in the future design of antimicrobial agents.

Enhanced extracellular lipid accumulation in acidic environments

PURPOSE OF REVIEW

Binding of apolipoprotein B-100-containing lipoproteins (VLDL, IDL, and LDL) to proteoglycans and modifications of the lipoproteins, whether bound or unbound, are key processes in atherogenesis. The complex interplay between binding and modification has been studied at neutral pH conditions. It has been demonstrated that during atherogenesis the extracellular pH of the lesions decreases. We summarize findings suggesting that lipoprotein binding and modification are enhanced at acidic pH.

RECENT FINDINGS

Many enzymes found in the arterial intima, such as secretory sphingomyelinase and cathepsins, are able to hydrolyze lipoproteins in vitro. These enzymes function optimally at slightly acidic pH (pH 5.5-6.5), and are likely to act on lipoproteins optimally in the acidic plaque areas. Also, the ability of human aortic proteoglycans to bind native VLDL, IDL, and LDL is dramatically increased at acidic pH; this binding can be further increased if these apolipoprotein B-100-containing particles are hydrolytically modified.

SUMMARY

Recent in-vitro findings suggest that in areas of atherosclerotic arterial intima where the extracellular pH is decreased, binding of apolipoprotein B-100-containing lipoproteins to proteoglycans and modification of the lipoproteins by acidic enzymes are enhanced. The pH-induced amplification of these processes will lead to enhanced extracellular accumulation of lipoproteins and accelerated progression of the disease.

Detection of bile salt-dependent lipase, a 110 kDa pancreatic protein, in urines of healthy subjects

Bile salt-dependent lipase (BSDL), a 110 kDa glycoprotein secreted by the pancreatic acinar cells, participates in the duodenal hydrolysis of dietary lipid esters. Recent in vitro and in vivo studies demonstrated that the BSDL reaches the blood via a transcytosis motion through enterocytes, suggesting that this enzyme may play a role in vascular biology. Once in the blood, BSDL should be eliminated. We address the hypothesis that BSDL may be filtered by the glomerulus and eliminated in urines. Immunological methods and proteomic were used to detect and to characterize BSDL in urine. The immunoreactive form of BSDL was detected in urines of 36 male subjects devoid of renal failure. Proteomic demonstrated that the immunoreactive protein is BSDL. Experiments using a monoclonal antibody to the oncofetal glycoform of pancreatic BSDL suggested that the protein is not expressed by renal cells but originates from the pancreas via circulation. We demonstrate that under normal physiological conditions, BSDL, a high-molecular weight blood glycoprotein, can be filtered by the renal glomerulus to be eliminated in urines.

Synthesis of tricyclic 1,3-oxazin-4-ones and kinetic analysis of cholesterol esterase and acetylcholinesterase inhibition

A series of thieno[1,3]oxazin-4-ones and thieno[1,3]thiazin-4-ones were synthesized and investigated as inhibitors of the alpha/beta hydrolases cholesterol esterase (CEase) and acetylcholinesterase (AChE). The introduction of a cycloaliphatic five- or six-membered ring fused at the thiophene was favorable for CEase inhibition. Such compounds were analyzed as true alternate substrate inhibitors. 6,7-Dihydro-2-(dimethylamino)-4H,5H-cyclopenta[4,5]thieno[2,3-d][1,3]oxazin-4-one (33) exhibited a K(i) value of 630 nM and excelled in its low susceptibility to CEase-catalyzed degradation. Compound 33 and its analogues did not inhibit AChE. The introduction of a tetrahydropyrido ring with bulky hydrophobic substituents at the basic nitrogen provided inhibitors of AChE which were completely inactive toward CEase. 7-Benzyl-5,6,7,8-tetrahydro-2-(N-3,4-dimethoxybenzyl-N-methylamino)-4H-pyrido[4',3':4,5]thieno[2,3-d][1,3]oxazin-4-one (21) had the IC(50) value of 330 nM for AChE inhibition. A residual enzymatic activity at an infinite inhibitor concentration and thus a catalytically active ternary enzyme-substrate-inhibitor complex was concluded. To specify kinetic parameters of inhibition, a new method was derived to characterize selected thieno[1,3]oxazin-4-ones as hyperbolic mixed-type inhibitors of AChE.

Carboxyl Ester Lipase Expression in Macrophages Increases Cholesteryl Ester Accumulation and Promotes Atherosclerosis

Carboxyl ester lipase (CEL, also called cholesterol esterase or bile salt-dependent lipase) is a lipolytic enzyme capable of hydrolyzing cholesteryl esters, triacylglycerols, and phospholipids in a trihydroxy bile salt-dependent manner but hydrolyzes ceramides and lysophospholipids via bile salt-independent mechanisms. Although CEL is synthesized predominantly in the pancreas, a low level of CEL expression was reported in human macrophages. This study used transgenic mice with macrophage CEL expression at levels comparable with that observed in human macrophages to explore the functional role and physiological significance of macrophage CEL expression. Peritoneal macrophages from CEL transgenic mice displayed a 4-fold increase in [(3)H]oleate incorporation into cholesteryl [(3)H]oleate compared with CEL-negative macrophages when the cells were incubated under basal conditions in vitro. When challenged with acetylated low density lipoprotein, cholesteryl ester accumulation was 2.5-fold higher in macrophages expressing the CEL transgene. The differences in cholesteryl ester accumulation were attributed to the lower levels of ceramide and lysophosphatidylcholine in CEL-expressing cells than in CEL-negative cells. CEL transgenic mice bred to an atherosclerosis susceptible apoE(-/-) background displayed an approximate 4-fold higher atherosclerotic lesion area than apoE(-/-) mice without the CEL transgene when both were fed a high fat/cholesterol diet. Plasma level of the atherogenic lysophosphatidylcholine was lower in the CEL transgenic mice, but plasma cholesterol level and lipoprotein profile were similar between the two groups. These studies documented that CEL expression in macrophages is pro-atherogenic and that the mechanism is because of its hydrolysis of ceramide and lysophosphatidylcholine in promoting cholesterol esterification and decreasing cholesterol efflux.

Cholesterol esterase action on human high density lipoproteins and inhibition studies: detection by MALDI-TOF MS

The modification of lipoproteins by lipolytic enzymes such as cholesterol esterase (CEase) is assumed to play an important role in the pathogenesis of atherosclerosis. However, details of the activation and inhibition of CEase are still unknown. In this study, matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI-TOF MS) was used to investigate the extracts of human lipoproteins after treatment with CEase and to monitor the effects of the inhibitor 2-(diethylamino)-6,7-dihydro-4H,5H-cyclopenta[4,5] thieno[2,3-d][1,3]oxazin-4-one (DOT-3). This approach has the advantage that all lipid classes can be independently detected; therefore, conclusions on the mechanism of the applied enzyme are possible. Besides the expected decrease of cholesteryl esters (CEs) in HDL, a significantly enhanced content of lysophosphatidylcholine (LPC) was also detected, confirming the broad substrate specificity of CEase. It was also demonstrated that DOT-3 significantly inhibited the CEase-catalyzed cleavage of CEs in HDL. Phospholipid (PL) vesicles prepared from phosphatidylcholine (PC) or PC and cholesteryl linoleate were treated with CEase, and the changes in lipid composition were investigated. From the analysis of the generated LPC species in HDL and in the isolated lipid mixtures, it is evident that CEase catalyzes the cleavage of the fatty acid residues in both the sn-1 and sn-2 positions of the PLs. These effects are obvious in the absence as well as in the presence of detergents.

In Vitro Angiogenic Effects of Pancreatic Bile Salt-Dependent Lipase

OBJECTIVE

Bile salt-dependent lipase (BSDL), a lipolytic enzyme secreted in the duodenum by pancreatic acinar cells, has been detected in the serum of all patients and in atheromatous plaque, suggesting its potential implication in vascular pathophysiology.

METHODS AND RESULTS

In vitro pancreatic BSDL evokes human umbilical vein endothelial cell (HUVEC) proliferation and chemotactic migration. BSDL at mitogen concentration is capable to heal wounded HUVEC monolayer and to promote capillary network formation. HUVEC proliferation depends on the displacement of basic fibroblast growth factor and vascular endothelial growth factor from the extracellular matrix and the activation of extracellular signal-regulated kinases (ERK1/2), p38 mitogen-activated protein kinase, and focal adhesion kinase signaling pathways.

CONCLUSIONS

For the first time to our knowledge, it is suggested that circulating BSDL could be involved in pathophysiological angiogenesis. We delineate the in vitro effects of pancreatic BSDL on endothelial cells, and we show that BSDL promotes proliferation, migration, capillary network formation, and wound-healing of HUVECs via the displacement of bFGF and VEGF from the ECM, suggesting that BSDL could be involved in angiogenesis.

Carboxyl Ester Lipase Cofractionates with Scavenger Receptor BI in Hepatocyte Lipid Rafts and Enhances Selective Uptake and Hydrolysis of Cholesteryl Esters from HDL3

Cholesteryl esters are selectively removed from high density lipoproteins by hepatocytes and steroidogenic cells through a process mediated by scavenger receptor BI. In the liver this cholesterol is secreted into bile, primarily as free cholesterol. Previous work showed that carboxyl ester lipase enhanced selective uptake of cholesteryl ether from high density lipoprotein by an unknown mechanism. Experiments were performed to determine whether carboxyl ester lipase plays a role in scavenger receptor BI-mediated selective uptake. When added to cultures of HepG2 cells, carboxyl ester lipase cofractionated with scavenger receptor BI and [(3)H]cholesteryl ether-labeled high density lipoprotein in lipid raft fractions of cell homogenates. Confocal microscopy of immunostained carboxyl ester lipase and scavenger receptor BI showed a close association of these proteins in HepG2 cells. The enzyme and receptor also cofractionated from homogenates of mouse liver using two different fractionation methods. Antibodies that block scavenger receptor BI function prevented carboxyl ester lipase stimulation of selective uptake in primary hepatocytes from carboxyl ester lipase knockout mice. Heparin blockage of cell-surface proteoglycans also prevented carboxyl ester lipase stimulation of cholesteryl ester uptake by HepG2 cells. Inhibition of carboxyl ester lipase activity in HepG2 cells reduced hydrolysis of high density lipoprotein-cholesteryl esters approximately 40%. In vivo, hydrolysis was similarly reduced in lipid rafts from the livers of carboxyl ester lipase-null mice compared with control animals. Primary hepatocytes from these mice yielded similar results. The data suggest that carboxyl ester lipase plays a physiological role in hepatic selective uptake and metabolism of high density lipoprotein cholesteryl esters by direct and indirect interactions with the scavenger receptor BI pathway.

Acamprosate involvement in triacylglycerol hydrolysis and transacylation with cholesterol in chronically ethanol-drinking rats

Acamprosate (AC) is used as a drug for treating alcoholism. We evaluated the effect of AC on serum triacylglycerol hydrolysis (GEH, glycerol ester hydrolysis), triacylglycerol transacylation with cholesterol (GECAT, glycerol ester:cholesterol acyltransferase), and acylcholesterol hydrolysis (Cease, cholesterol ester hydrolysis) in an experimental model of alcoholism. Ethanol-preferring (PRF), non-preferring (NPF), and control (CR) male Wistar rats were treated with AC (500 mg/kg, p.o.) for 21 consecutive days. The beneficial effect of AC on lipid parameters of PRF rats included decreased triacylglycerol, total cholesterol, and LDL-cholesterol, and increased HDL-cholesterol levels. Acamprosate-compensated changes associated with ethanol consumption were observed. Acamprosate treatment decreased GECAT and increased Cease control rats, but increased GECAT and decreased CEase in PRF animals. In all groups of rats, AC treatment did not influence GEH. In conclusion, our results suggest that AC can influence triacylglycerol metabolism by its action on the balance between hydrolysis and transacylation in rats.

Serum levels of bile salt-stimulated lipase and breast feeding

OBJECTIVES

Bile salt-stimulated lipase (BSSL) is present in the sera of healthy humans, may affect lipoprotein structure and composition, and reduce atherogenicity of oxidized LDL-cholesterol. Our aims were to examine serum levels of BSSL in breast- and formula-fed infants, and explore the influence of BSSL on serum lipid profile and oxidative status.

METHODS

Infants (2-8 weeks old) were prospectively enrolled. Blood was drawn for serum levels of BSSL, total antioxidant status (TAS), and lipid profile.

RESULTS

Serum levels of BSSL were similar in breast-fed (0.28 +/- 0.15 microg/l, n = 18) and formula-fed (0.31 +/- 0.09 microg/l, n = 15) infants, and were much lower than reported levels for adults. In breast-fed infants only, BSSL levels were correlated with LDL-cholesterol serum levels (r = -0.53, p = 0.04). Total cholesterol (119.2 +/- 34.3 mg/dl vs 97 +/- 27.2, and p = 0.05) and LDL-cholesterol serum levels (50.5 +/- 26.1 mg/dl vs 33.3 +/- 20.3, p = 0.05), were elevated in breast-fed compared with formula-fed infants, but TAS was similar in both groups (1.02 +/- 0.18 mmol/l and 0.98 +/- 0.12 mmol/l, respectively).

CONCLUSIONS

Lack of difference in BSSL serum levels between formula- and breast-feeding, and lower BSSL levels in infants compared to adults, suggest that human milk does not contribute to BSSL serum levels.

Pancreatic bile salt-dependent lipase induces smooth muscle cells proliferation

BACKGROUND

Because bile salt-dependent lipase (BSDL), an enzyme secreted by the pancreatic acinar cells and associated with LDL in circulating blood, also locates with smooth muscle cells (SMCs) in atherosclerotic lesions, we aimed to investigate its effects on SMCs.

METHODS AND RESULTS

Immunohistochemical experiments allowed us to detect an expression of BSDL in atherosclerotic lesions from hypercholesterolemic monkeys and from human arteries. BSDL was found to be associated with SMCs but not with macrophages. BSDL was significantly mitogenic for cultured SMCs. This effect was inhibited by heparin and anti-BSDL antibodies, whereas heat-denaturated and diisopropylfluorophosphate-treated BSDL were inefficient. The mitogenic effect of BSDL was associated with an activation of the extracellular signal-regulated kinase 1/2 mitogen-activated protein kinase pathway, which was inhibited by heparin, and involved several mechanisms, among them diacylglycerol and oleic acid production as well as a rapid basic fibroblast growth factor release.

CONCLUSIONS

Circulating BSDL is associated with SMCs within the intimal arteria and may trigger SMC proliferation, which could contribute to the development of atherosclerotic lesions.

Influence of oral insulin supplementation on carbohydrate, lipid and protein metabolism in weaned Balb/c mice

BACKGROUND

[corrected] Insulin is substantially present in human milk. Suckling animal models have demonstrated local trophic effects of insulin on the gut mucosa, but the effects of oral insulin in the post-weaning period are controversial.

OBJECTIVE

To examine local and systemic effects of oral insulin supplementation in the post-weaning period.

MATERIALS AND METHODS

Balb/c mice received oral insulin supplementation in their drinking water (1 U/ml) from beyond weaning during 40 days (S group). Plasma glucose, lipid levels, amylase and insulin were measured. The liver and intestine were weighed and thiobarbituric acid-reactive substance (TBARS) levels were examined in liver tissue. These data were compared to a control group (C group).

RESULTS

Fasting glucose levels (mean +/- SD) were lower in the S than in the C group (116. 5 +/- 10.7 vs 143.9 +/- 25.4 mg/dl; p < 0.001), as were prandial glucose levels (123.1 +/- 12.2 vs 145.1 +/- 18.0 mg/dl; p = 0.03). Plasma insulin levels were higher in the S than in the C group (1,110.28 +/- 721.59 vs 308.17 +/- 100.50 pmol/; p = 0.03). Total triglyceride levels (130.4 +/- 32.4 vs 193.6 +/- 48.4 mg/dl; p < 0.02) and total cholesterol levels (83.6 +/- 5.7 vs 93.3 +/- 7.9 mg/dl; p < 0.05) were lower in the S group compared with the C group. Liver weight was significantly higher in the S group compared with C (0.75 +/- 0.1 vs 0. 7 +/- 0.1 g, p = 0.05).

CONCLUSIONS

In the post-weaning period, oral insulin supplementation had a significant effect on plasma glucose levels and lipid profile, as well as on liver weight and protein content in Balb/c mice. The application of these findings in humans should be studied.

Carboxyl ester lipase: structure-function relationship and physiological role in lipoprotein metabolism and atherosclerosis

Carboxyl ester lipase (CEL), previously named cholesterol esterase or bile salt-stimulated (or dependent) lipase, is a lipolytic enzyme capable of hydrolyzing cholesteryl esters, tri-, di-, and mono-acylglycerols, phospholipids, lysophospholipids, and ceramide. The active site catalytic triad of serine-histidine-aspartate is centrally located within the enzyme structure and is partially covered by a surface loop. The carboxyl terminus of the protein regulates enzymatic activity by forming hydrogen bonds with the surface loop to partially shield the active site. Bile salt binding to the loop domain frees the active site for accessibility by water-insoluble substrates. CEL is synthesized primarily in the pancreas and lactating mammary gland, but the enzyme is also expressed in liver, macrophages, and in the vessel wall. In the gastrointestinal tract, CEL serves as a compensatory protein to other lipolytic enzymes for complete digestion and absorption of lipid nutrients. Importantly, CEL also participates in chylomicron assembly and secretion, in a mechanism mediated through its ceramide hydrolytic activity. Cell culture studies suggest a role for CEL in lipoprotein metabolism and oxidized LDL-induced atherosclerosis. Thus, this enzyme, which has a wide substrate reactivity and diffuse anatomic distribution, may have multiple functions in lipid and lipoprotein metabolism, and atherosclerosis.

Site-directed mutagenesis of the basic N-terminal cluster of pancreatic bile salt-dependent lipase. Functional significance

Previous studies have postulated the presence of a heparin-binding site on the bile salt-dependent lipase (BSDL), whereas two bile salt-binding sites regulate the enzyme activity. One of these sites may overlap with the tentative heparin-binding site at the level of an N-terminal basic cluster consisting of positive residues Lys(32), Lys(56), Lys(61), Lys(62), and Arg(63). The present study uses specific site-directed mutagenesis to determine the functional significance of this basic cluster. Mutations in this sequence resulted in recombinant enzymes that were able to bind to immobilized and to cell-associated heparin before moving throughout intestinal cells. Recombinant BSDL was fully active on soluble substrate, but mutants were less active on micellar cholesteryl oleate in comparison with the wild-type enzyme. Activation studies by primary (sodium taurocholate) and by secondary (sodium taurodeoxycholate) bile salts revealed that the activation of BSDL by sodium taurocholate at concentrations below the critical micellar concentration, and not that evoked by micellar bile salts, was affected by substitutions, suggesting that this N-terminal basic cluster likely represents the specific bile salt-binding site of BSDL. Substitutions also affected the activation of the enzyme promoted by anionic phospholipids, extending the function of this site to that of a cationic regulatory site susceptible to accommodate anionic ligands.

Transcriptional regulation of the human carboxyl ester lipase gene in THP-1 monocytes: an E-box required for activation binds upstream stimulatory factors 1 and 2

The bile salt-stimulated carboxyl ester lipase (CEL) is important for the digestion and absorption of dietary lipids, and is expressed at high levels by the exocrine pancreas and the lactating mammary gland. However, the presence of CEL in human plasma suggests that the role of CEL in lipid metabolism may stretch beyond its function in the intestinal lumen, and possibly include interactions with cholesterol and oxidized lipoproteins to modulate the progression of atherosclerosis. We have used the CEL-expressing human monocytic cell line THP-1 to investigate the transcriptional regulation of the human CEL in monocytes. Analyses of the promoter region revealed that an E-box located at -47/-52 is necessary for CEL expression. Point mutations in the E-box almost completely abolish the transcriptional activity. Electrophoretic mobility-shift assay analyses reveal that the E-box binds the upstream stimulatory factors 1 and 2, and the binding of an upstream stimulatory factor-containing complex in THP-1 cells also requires the presence of a putative nuclear receptor-binding site at -60/-66. Furthermore, we demonstrate that the E-box is also necessary for CEL expression in the pancreas and the mammary gland, although there are tissue-specific requirements for additional activating elements.

Human bile salt-stimulated lipase has a high frequency of size variation due to a hypervariable region in exon 11

The apparent molecular mass of human milk bile salt-stimulated lipase (BSSL) varies between mothers. The molecular basis for this is unknown, but indirect evidence has suggested the differences to reside in a region of repeats located in the C-terminal part of the protein. We here report that a polymorphism within exon 11 of the BSSL gene is the explanation for the molecular variants of BSSL found in milk. By Southern blot hybridization we analyzed the BSSL gene from mothers known to have BSSL of different molecular masses in their milk. A polymorphism was found within exon 11, previously shown to consist of 16 near identical repeats of 33 bp each. We detected deletions or, in one case, an insertion corresponding to the variation in molecular mass of the BSSL protein found in milk from the respective woman. Furthermore, we found that 56%, out of 295 individuals studied, carry deletions or insertions within exon 11 in one or both alleles of the BSSL gene. Hence, this is a hypervariable region and the current understanding that exon 11 in the human BSSL gene encodes 16 repeats is an oversimplification and needs to be revisited. Natural variation in the molecular mass of BSSL may have clinical implications.

Transcytosis of pancreatic bile salt-dependent lipase through human Int407 intestinal cells

In previous studies, we have shown that the bile-salt-dependent-lipase (BSDL), secreted by pancreatic acinar cells and secreted into the duodenal lumen, can be transcytosed through intestinal cells up to the lamina propria. In this study, we used an in vitro system to provide insights into the apical to basolateral transport of BSDL, across the intestinal barrier. The Int407 human epithelial cell line, grown under conditions that optimize polarity, was used as a tight epithelium model. We attempted to delineate uptake mechanisms and the transcytotic pathway followed by this pancreatic enzyme within the intestinal Int407 cells, which do not produce BSDL. When added to the apical reservoir of Transwell-grown Int407 cells, BSDL was shown to first interact with the apical membrane. Further, BSDL forms clusters that are internalized via clathrin-coated pits. Following endocytosis, BSDL is directed to a nocodazole- and colchicin-sensitive multivesicular compartment. Interestingly, this protein transits through the Golgi apparatus, where it was found to colocalize with the KDEL retrieval-receptor. Finally, enzymatically active intact BSDL was released at the basolateral membrane level. This is the first demonstration for an apical-to-basolateral transcytotic pathway of a secreted pancreatic digestive enzyme through polarized intestinal cells.

Do human bile salt stimulated lipase and colipase-dependent pancreatic lipase share a common heparin-containing receptor?

Bile salt stimulated lipase (BSSL), a lipolytic enzyme secreted with pancreatic juice and with human milk, is in concert with colipase-dependent pancreatic lipase, important for the intestinal digestion of dietary lipids. BSSL may also facilitate uptake of free cholesterol from the intestinal lumen, while colipase-dependent lipase has a similar role for fatty acids. According to this theory, the two lipases bind to the intestinal mucosa via a common heparin-involving receptor. In the present study, binding of the two lipases to heparin was explored in vitro using purified human lipases and heparin molecules varying in both chain length and charge density. Native, but not denatured, BSSL bound avidly to heparin and several of the heparin variants. In contrast, at physiologic salt concentration, colipase-dependent lipase did not bind to heparin. Thus, our data do not support the view that the two lipases share a common intestinal heparin-like receptor. Hence, it seems unlikely that such binding could be of physiologic relevance for colipase-dependent lipase, although for BSSL the data are supportive.

Structure of low density lipoprotein (LDL) particles: basis for understanding molecular changes in modified LDL

Low density lipoprotein (LDL) particles are the major cholesterol carriers in circulation and their physiological function is to carry cholesterol to the cells. In the process of atherogenesis these particles are modified and they accumulate in the arterial wall. Although the composition and overall structure of the LDL particles is well known, the fundamental molecular interactions and their impact on the structure of LDL particles are not well understood. Here, the existing pieces of structural information on LDL particles are combined with computer models of the individual molecular components to give a detailed structural model and visualization of the particles. Strong evidence is presented in favor of interactions between LDL lipid constituents that lead to specific domain formation in the particles. A new three-layer model, which divides the LDL particle into outer surface, interfacial layer, and core, and which is capable of explaining some seemingly contradictory interpretations of molecular interactions in LDL particles, is also presented. A new molecular interaction model for the beta-sheet structure and phosphatidylcholine headgroups is introduced and an overall view of the tertiary structure of apolipoprotein B-100 in the LDL particles is presented. This structural information is also utilized to understand and explain the molecular characteristics and interactions of modified, atherogenic LDL particles.

Roles of molecular chaperones in pancreatic secretion and their involvement in intestinal absorption

This review focuses on the contribution of molecular chaperones in the secretory process of digestive enzymes and their interaction with enterocytes. By using biochemistry and immunocytochemistry, we have shown that Grp94, Cpn10, Cpn60, and protein disulfide isomerase (PDI) are present all along the rough endoplasmic reticulum-Golgi-granule secretory pathway of the pancreatic acinar cells and are secreted into the acinar lumen. Two other molecular chaperones, Grp78 and the Hsp70, appear to be restricted to the rough endoplasmic reticulum and the trans-Golgi apparatus, respectively. We have found that chaperones can be associated with pancreatic enzymes along the secretory pathway. Indeed, double immunogold and immunocoprecipitation revealed an association between Cpn60 and the colipase-dependent lipase (CDL) and between Grp94 and the bile salt-dependent lipase (BSDL). These complexes are secreted into the acinar lumen and diverted to the duodenal lumen. These findings led us to investigate these enzyme-chaperone complexes in intestinal tissue. Grp94, Cpn60, and PDI are present on microvilli and on the endosomal compartment of enterocytes. Furthermore, we have shown that the Grp94-BSDL complexes are internalized by enterocytes through classical endocytosis. Upon dissociation of the BSDL-Grp94 complex in the late endosome, BSDL is transferred to the basolateral membrane. We propose that Grp94 interacts with specific receptors and/or could force the associated protein to adopt a specific conformation that allows its binding to corresponding membrane receptors and its internalization by enterocytes. These two hypotheses need not to be exclusive. The existence of such a pancreatic secretion-intestinal absorption link speaks in favor of a coordinated functional connection between these two entities, through molecular chaperones, in order to optimize intestinal activities.

Expression of a 70-kDa immunoreactive form of bile salt-dependent lipase by human pancreatic tumoral mia PaCa-2 cells

This work describes the characterization of an immunoreactive form of bile salt-dependent lipase (BSDL) expressed by the human pancreatic tumoral Mia PaCa-2 cell line. This BSDL-related protein, which has an M(r) of 70 kDa, is enzymatically active and poorly secreted. Furthermore, a protein with the same electrophoretic migration can also be immunoprecipitated with polyclonal antibodies specific for the human pancreatic BSDL after in vitro translation of RNA isolated from Mia PaCa-2 cells. These data indicated that this BSDL-related protein might be poorly, or not, glycosylated. Reverse transcription and amplification of RNA extracted from Mia PaCa-2 cells using primers able to specifically amplify the full-length mRNA of the human BSDL resulted in a detectable 1.8-kb cDNA product, shorter than that of BSDL (2.2 kb). The sequence of this transcript corresponds to the mRNA sequence that codes for the mature human pancreatic BSDL. However, a deletion of 330 bp is located within the 3'-domain of this cDNA. Therefore data allowed us to conclude that the 70-kDa BSDL-related protein is a 612 amino acid length protein and represents a truncated form of BSDL. The deletion of 110 amino acids occurs in the C-terminal region of the protein, which encompasses 6 tandemly repeated sequences instead of the 16 normally present in the sequence of BSDL. Because feto-acinar pancreatic protein (FAPP), which is the oncofetal counterpart of BSDL, is a C-terminally truncated isoform of BSDL, it is suggested that the 70-kDa BSDL-related protein expressed in MiaPaCa-2 cells could be representative of the protein moiety of FAPP.