Molecular cloning and expression of cDNA for rat pancreatic cholesterol esterase

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.

Infection-responsive electrospun nanofiber mat for antibacterial guided tissue regeneration membrane

The release of anti-infection drugs in a targeted and efficient manner in response to the attack time and degree of severity of infection is a requirement of new generation implants. Herein, we design an infection-responsive guided tissue regeneration (GTR)/guided bone regeneration (GBR) membrane based on electrospun nanofibers. Polycaprolactone (PCL) nanofiber mats are coated with polydopamine to endow hydroxyl groups on the surface and then functionalized with siloxane to introduce amino groups. Metronidazole (MNA), an antibiotic drug, is esterified and then grafted onto the surface of the modified PCL nanofiber mats via ester linkages. The ester bonds can be selectively hydrolyzed by cholesterol esterase (CE), an enzyme secreted by macrophagocytes accumulated at the site of infection, whose concentration is positively related to the severity of the infection. The drug can be triggered to release from the nanofiber membranes in responsive to the CE. With the increase of the CE concentration, a higher amount of MNA is released from the nanofiber mat, resulting in the enhancement of the antibacterial capability of the MNA-grafted nanofiber mat. The nanofiber mat has good cytocompatibility. This CE-responsive drug delivery system based on the electrospun nanofiber mat is promising as an optimal choice for antibacterial GTR/GBR membrane.

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.

Synthesis of fatty acid ethyl esters in mammalian tissues after ethanol exposure: a systematic review of the literature

The ability to undergo non-oxidative metabolism from ethanol to fatty acid ethyl esters (FAEEs) varies greatly among tissues and organs. To gain a greater understanding of non-oxidative ethanol metabolism to FAEE, we aimed to collect all published data on FAEE synthesis in mammalian organs and tissues to identify all tissues, organs, and enzymes that are known to, or likely possess FAEE-synthetic activity. A systematic search for relevant papers was performed and two independent reviewers examined potentially relevant abstracts (articles on FAEEs that pertain to ethanol exposure) to determine whether they met the inclusion criteria. Information on FAEE synthesis was retrieved from papers meeting the inclusion/exclusion criteria and summarized by organ/tissue/matrix examined. The systematic search through four databases yielded 78 articles that investigated FAEE synthesis by tissues, tissue fractions and cell lines, and 29 articles that attempted to purify and/or characterize the enzymes involved in FAEE synthesis. Two enzyme activities have been studied: FAEE synthase (FAEES, which conjugates ethanol and free fatty acid) and acyl-CoA: ethanol O-acyltransferase (AEAT, which conjugates ethanol and fatty acyl-CoA). Both activities are expressed by a variety of different enzymes. FAEES activity is the most widely studied and has been purified from several tissues and shown to be associated with several well-known enzymes, while the identity of enzymes possessing AEAT activity remains unknown. The organs and tissues that have been shown to synthesize FAEEs are discussed, with special emphasis on the studies that attempted to elucidate the enzymology of FAEE synthesis in those tissues.

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.

Macrophage cholesteryl ester mobilization and atherosclerosis

Accumulation of cholesteryl esters (CE) stored as cytoplasmic lipid droplets is the main characteristic of macrophage foam cells that are central to the development of atherosclerotic plaques. Since only unesterified or free cholesterol (FC) can be effluxed from the cells to extracellular cholesterol acceptors, hydrolysis of CE is the obligatory first step in CE mobilization from macrophages. This reaction, catalyzed by neutral cholesteryl ester hydrolase (CEH), is increasingly being recognized as the rate-limiting step in FC efflux. CEH, therefore, regulates the process of reverse cholesterol transport and ultimate elimination of cholesterol from the body. In this review, we summarize the earlier controversies surrounding the identity of CEH in macrophages, discuss the characteristics of the various candidates recognized to date and examine their role in mobilizing cellular CE and thus regulating atherogenesis. In addition, physiological requirements to hydrolyze lipid droplet-associated substrate and complexities of interfacial catalysis are also discussed to emphasize the importance of evaluating the biochemical characteristics of candidate enzymes that may be targeted in the future to attenuate atherosclerosis.

Regulation of cholesteryl ester hydrolases

Recent developments in understanding the biochemical and molecular nature of the CE hydrolases and their impact on cellular cholesterol trafficking have further defined the enzyme's mechanism of action with reasonable clarity. The availability of the cDNA probe for the human lysosomal acid lipase/CE hydrolase and the hormone-sensitive lipase now makes it possible to study CE hydrolase gene regulation and expression in human tissue; and it can now be stated with more assurance that the cytoplasmic CE hydrolase (NCEH) is most likely activated through phosphorylation by the cyclic AMP-dependent protein kinase. Evidence also shows that the NCEH is most likely identical to the hormone-sensitive lipase and that it plays an important role in cholesterol efflux properties of the cell. Recent advances in the discovery of the role of the eicosanoid/cytokine network in the regulation of CE hydrolysis, highlighted in Figure 10, further emphasize the interesting but complex nature of the cholesterol trafficking processes in cells, particularly under pathophysiological conditions such as cell injury, repair, and inflammation. It can be speculated that in several years, when the crystal structure of the CE hydrolase is known, the structure-function properties of this enzyme's catalytic domain, as it relates to the physical state of the CE substrates, should further clarify the precise role of this enzyme in intracellular cholesterol mobilization and trafficking under a variety of cellular conditions.

Purification, characterization, and molecular cloning of organic-solvent-tolerant cholesterol esterase from cyclohexane-tolerant Burkholderia cepacia strain ST-200

Extracellular cholesterol esterase of Burkholderia cepacia strain ST-200 was purified from the culture supernatant. Its molecular mass was 37 kDa. The enzyme was stable at pH 5.5-12 and active at pH 5.5-6, showing optimal activity at pH 7.0 at 45 degrees C. Relative to the commercially available cholesterol esterases, the purified enzyme was highly stable in the presence of various water-miscible organic solvents. The enzyme preferentially hydrolyzed long-chain fatty acid esters of cholesterol, except for that of cholesteryl palmitate. The enzyme exhibited lipolytic activity toward various p-nitrophenyl esters. The hydrolysis rate of p-nitrophenyl caprylate was enhanced 3.5- to 7.2-fold in the presence of 5-20% (vol/vol) water-miscible organic solvents relative to that in the absence of organic solvents. The structural gene encoding the cholesterol esterase was cloned and sequenced. The primary translation product was predicted to be 365 amino acid residues. The mature product is composed of 325 amino acid residues. The amino acid sequence of the product showed the highest similarity to the lipase LipA (87%) from B. cepacia DSM3959.

Novel cholesterol esterase secreted by Streptomyces persists during aqueous long-term storage

We isolated a moderate thermophilic actinomycete, Streptomyces sp. X9, from soil and purified cholesterol esterase (CHE) from the culture medium to homogeneity. The molecular masses of the purified CHE estimated by sodium dodecyl sulfate polyacrylamide gel electrophoresis and gel filtration chromatography were 23.6 and 163 kDa, respectively, indicating that the enzyme assumes an oligomeric form. Heavy metals such as Hg2+ and Ag+ similarly inhibited the activity of the CHE in the same manner as those of other bacterial CHEs. The activity of Streptomyces sp. X9 CHE was susceptible to dithiothreitol, beta-mercaptoethanol and p-chloromercuribenzoate, but resistant to phenylmethylsulfonyl fluoride, unlike those of other bacterial CHEs. The purified CHE could utilize both cholesteryl and p-nitrophenyl (pNP) esters of fatty acids as substrates. Steady-state kinetics revealed respective Km values for cholesteryl myristate and pNP-myristate of 0.34 and 1.1 mM, indicating that the cholesteryl residue is important for catalysis. We also found that the Km for the pNP esters are dependent on the chain length of the substrate fatty acid residues. These results indicate that the novel CHE specifically hydrolyzes substrates by recognizing both cholesteryl and fatty acid moieties. The enzyme was stable during long-term aqueous storage at room temperature, indicating its potential application as a diagnostic reagent.

Mutation of F417 but not of L418 or L420 in the lipid binding domain decreases the activity of triacylglycerol hydrolase

Human triacylglycerol hydrolase (hTGH) has been shown to play a role in hepatic lipid metabolism. Triacylglycerol hydrolase (TGH) hydrolyzes insoluble carboxylic esters at lipid/water interfaces, although the mechanism by which the enzyme adsorbs to lipid droplets is unclear. Three-dimensional modeling of hTGH predicts that catalytic residues are adjacent to an alpha-helix that may mediate TGH/lipid interaction. The helix contains a putative neutral lipid binding domain consisting of the octapeptide FLDLIADV (amino acid residues 417-424) with the consensus sequence FLXLXXXn (where n is a nonpolar residue and X is any amino acid except proline) identified in several other proteins that bind or metabolize neutral lipids. Deletion of this alpha-helix abolished the lipolytic activity of hTGH. Replacement of F417 with alanine reduced activity by 40% toward both insoluble and soluble esters, whereas replacement of L418 and L420 with alanine did not. Another potential mechanism of increasing TGH affinity for lipid is via reversible acylation. Molecular modeling predicts that C390 is available for covalent acylation. However, neither chemical modification of C390 nor mutation to alanine affected activity. Our findings indicate that F417 but not L418, L420, or C390 participates in substrate hydrolysis by hTGH.

Acidic residues emulate a phosphorylation switch to enhance the activity of rat hepatic neutral cytosolic cholesterol esterase

Site-directed mutagenesis of rat hepatic neutral cytosolic cholesteryl ester hydrolase (rhncCEH) was used to substitute acidic, basic or neutral amino acid residues for Ser506, required for activation by protein kinase A. The substitution of acidic Asp506 resulted in esterase activities with cholesteryl oleate, p-nitrophenylcaprylate (PNPC) and p-nitrophenylacetate (PNPA) equivalent to those of native rhncCEH with Ser506. The substitution of 2 acidic residues (Asp505/506), emulating the 2 negative charges of phosphoserine, resulted in a 10-fold greater cholesterol esterase activity than that of native rhncCEH, similar to the activity of rhncCEH treated with protein kinase A. In contrast to mutants with Ser506, protein kinase A did not increase the specific activities of mutants with Asp505/506. The substitution of basic (Lys506) or neutral (Asn506) residues abolished activity with cholesteryl oleate but not PNPC or PNPA. The substitution of neutral Gln for basic residues Lys496/Arg503 also abolished cholesterol esterase activity but not PNPC- and PNPA-esterase activities. These structure-activity relationships are modeled by homology with a recently reported crystal structure for the homologous human triacylglycerol hydrolase. The results suggest that the cholesterol esterase activity of carboxylesterases is enhanced by interactions between one or more basic residues on helix alpha16 (residues 485-503) and acidic groups at residues 505-506 in the adjacent surface loop.

Mechanism of differential inhibition of hepatic and pancreatic fatty acid ethyl ester synthase by inhibitors of serine-esterases: in vitro and cell culture studies

Earlier, we have shown that rat hepatic and pancreatic fatty acid ethyl ester (FAEE) synthases are structurally and functionally similar to rat liver carboxylesterase (CE) and pancreatic cholesterol esterase (ChE), respectively. We have also reported that only hepatic FAEE synthase is inhibited by tri-o-tolylphosphate (TOTP) in vivo and in human hepatocellular carcinoma (HepG2) cells. The metabolism of TOTP is a prerequisite for the inhibition of hepatic FAEE synthase as well as esterase activity. To further elucidate the mechanism of such differential inhibition by inhibitors of serine esterases, we synthesized two metabolites of TOTP, 2-(o-cresyl)-4H-1:3:2-benzodioxaphosphoran-2-one (CBDP; cyclic saligenin phosphate) and di-o-tolyl-o-( proportional, variant -hydroxy)tolylphosphate (HO-TOTP), and one ChE inhibitor, 3-benzyl-6-chloro-2-pyrone (3-BCP). The inhibitory effect of CBDP, HO-TOTP, and 3-BCP on FAEE synthase and esterase activity was studied using rat hepatic and pancreatic postnuclear (PN) fractions, commercial porcine hepatic CE and pancreatic ChE, and in HepG2 and rat pancreatic tumor (AR42J) cell lines. Only HO-TOTP and CBDP inhibited FAEE synthase as well as esterase activity of hepatic PN fraction and commercial CE and ChE in a concentration-dependent manner, and the inhibition was found to be irreversible. However, no inhibition was found in pancreatic PN fraction by both TOTP metabolites and 3-BCP. Although 3-BCP inhibited only the esterase activity of commercial ChE in a concentration-dependent manner, the activity was reversible within 30 min of incubation. Studies with HepG2 cells also showed a significant inhibition of FAEE synthase-esterase activity by CBDP and HO-TOTP within 15 min of incubation, while no inhibition was observed in AR42J cells. 3-BCP did not inhibit FAEE synthase-esterase activity either in HepG2 or AR42J cells. Such differential inhibitory effect of the TOTP metabolites on hepatic and pancreatic FAEE synthase-esterase is supported by our earlier in vivo and in vitro studies. Further investigations are needed to understand the biochemical mechanism(s) of inactivation of TOTP metabolites and 3-BCP in the pancreas and AR42J cells towards FAEE synthase-esterase activities.

Modulation of fatty acid methyl esters in rats pretreated with tri-o-tolyl phosphate

Formation and toxicity of fatty acid methyl esters (FAMEs) have been reported both in vitro and in vivo. In previous studies, it was shown that fatty acid ethyl ester synthase (FAEES), which catalyzes the formation of FAMEs, also expresses esterase activity. Therefore, it was hypothesized that inhibitors of esterases such as tri-o-tolyl phosphate (TOTP) can modulate the formation of FAMEs. To test this, four groups of rats were used. Group 1 served as control (vehicle only). Group 2 was treated with methanol only (3 g/kg via gavage), group 3 was given TOTP only (100 mg/kg i.p. in corn oil), and group 4 was administered TOTP as in group 3, followed by methanol after 18 h. Three hours after exposure, animals were sacrificed and FAEES activity and FAME levels were measured in blood, liver, pancreas, and brown fat. About 95% of FAEES activity was inhibited in the liver and whole blood of TOTP-treated rats (group 3) but no inhibition was observed in the pancreas or brown fat. Total hepatic FAMEs were found to be lowest for the TOTP-treated group (3) and highest in the methanol-treated animals (group 2). Total pancreatic FAMEs in different groups were not statistically different, while significant increases were observed in the brown fat in both methanol-treated groups. To verify that the oxidative metabolism of methanol was unaffected by TOTP, alcohol dehydrogenase activity was also measured and found to be unchanged in any group as compared to control. These results demonstrate that the formation of FAMEs can be modulated in the liver and probably in blood, but not in the pancreas or brown fat by the inhibitors of FAEES.

Purification and characterization of rat pancreatic fatty acid ethyl ester synthase and its structural and functional relationship to pancreatic cholesterol esterase

Formation of fatty acid ethyl esters (FAEEs, catalyzed by FAEE synthase) has been implicated in the pathogenesis of chronic pancreatitis. In previous studies, we demonstrated that FAEE synthase, purified from rat liver microsomes, is identical to rat liver carboxylesterase (pI 6.1), and structurally and functionally different than that from pancreas. In this study, we purified and characterized rat pancreatic microsomal FAEE synthase, and determined its relationship with rat pancreatic cholesterol esterase (ChE). Since most of the serine esterases express p-nitrophenyl acetate (PNPA)-hydrolyzing activity as well as synthetic activity to form fatty acid esters or amides with a wide spectrum of alcohols and amines, respectively, we used PNPA-hydrolyzing activity to monitor the purification of FAEE synthase during various chromatographic purification steps. Synthesizing activity towards FAEEs, fatty acid methyl esters, and fatty acid anilides was measured only in the pooled fractions. At each step of purification (ammonium sulfate saturation, Q Sepharose XL, and heparin-agarose column chromatographies, and high performance liquid chromatography (anion exchange and gel filtration)) synthetic as well as hydrolytic activities copurified. Using ethanol, methanol, or aniline as substrates, the ester or anilide synthesizing activity of the purified protein was found to be 8709, 13000, and 2201 nmol/h/mg protein, respectively. The purified protein displayed a single band with an estimated molecular mass of approximately 68 kD upon SDS-PAGE under reduced denaturing conditions, cross-reacted with antisera against rat pancreatic ChE and showed 100% N-terminal sequence homology of the first 15 amino acids to that of rat pancreatic ChE. These results suggest that the purified protein has broad substrate specificity towards the conjugation of endogenous long chain fatty acids with substrates having hydroxyl and amino groups and is identical to ChE.

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.

Pancreatic cholesterol esterase, ES-10, and fatty acid ethyl ester synthase III gene expression are increased in the pancreas and liver but not in the brain or heart with long-term ethanol feeding in rats

INTRODUCTION

Chronic alcohol consumption predisposes susceptible individuals to both acute and chronic pancreatitis.

AIMS

Our hypothesis was that alcohol increases the risk of pancreatitis by disrupting defense mechanisms and/or enhancing injury-associated pathways through altered gene expression. Hence, we studied the expression of pancreatic genes in rats chronically exposed to ethanol.

METHODOLOGY

Male Wistar rats were pair-fed liquid diets without and with ethanol for 4 weeks. Total RNA was extracted from rat pancreas and other organs. The mRNA expression patterns among pancreatic samples from ethanol-fed rats and controls were compared with use of mRNA differential display. The differentially expressed cDNA tags were isolated, cloned, and sequenced.

RESULTS

One cDNA tag that was overexpressed in the pancreas showed 99% sequence homology to a rat pancreatic cholesterol esterase mRNA (CEL; Enzyme Commission number [EC] 3.1.1.13). The differential expression was confirmed by realtime PCR. Gene expression was also increased in the liver but not in the heart or brain of the alcohol-fed rats. Because CEL has fatty acid ethyl ester (FAEE)-generating activity and FAEEs play a major role in acute alcoholic pancreatitis, we determined the expression of other genes encoding for FAEE-generating enzymes and showed similar organ-specific expression patterns.

CONCLUSION

Our results demonstrate that chronic ethanol consumption induced expression of FAEE-related genes in the pancreas and liver. This upregulation may be a central mechanism leading to acinar cell injury.

Differential inhibition of hepatic, pancreatic, and plasma fatty acid ethyl ester synthase by tri-o-tolylphosphate in rats

Fatty acid conjugation of alcohols, catalyzed by fatty acid ethyl ester synthase (FAEES), results in the formation of lipophilic esters. Although the activity of FAEES is reported in almost all organs, including plasma, the interrelationship among various proteins expressing FAEES activity in different organs/tissues is not well understood. Earlier, we have reported an inhibition of FAEES activity in human hepatoma cells by tri-o-tolylphosphate (TOTP; serine esterase inhibitor). The present study was undertaken to further characterize the hepatic, plasma, and pancreatic FAEES in rats after ip injection of 10, 25, 50, or 100 mg/kg TOTP in corn oil or vehicle alone. After 18 h, animals were euthanized and FAEES activity in the plasma and postnuclear fractions of hepatic and pancreatic homogenates were assayed by measuring the ester formation following incubation with [1-(14)C]oleic acid and ethanol or methanol as substrates. Significant inhibition of FAEES activity was observed in hepatic postnuclear fraction. The esterase activity also showed a pattern similar to fatty acid ethyl and methyl ester synthesizing activity. A trend similar to hepatic synthesizing and hydrolyzing activities was also found in the plasma of TOTP-treated rats. However, no inhibition of synthetic activity toward formation of fatty acid ethyl or methyl esters or p-nitrophenyl acetate hydrolyzing activity was observed in the pancreas of rats after TOTP exposure. Our results also show that the protein expressing FAEES activity in the pancreas does not cross-react with antibodies to rat adipose tissue FAEES using Western blot analysis, which recognizes approximately 60- and approximately 84-kDa proteins in the liver and plasma, respectively. Furthermore, the inhibition in liver is at the functional level of enzyme as no change was observed between control and treated animals by immunohistochemistry. We conclude that fatty acid ethyl or methyl ester synthesizing enzyme(s) in the liver and plasma, which are inhibited by TOTP, are different from that present in the pancreas.

HIV protease inhibitor induces fatty acid and sterol biosynthesis in liver and adipose tissues due to the accumulation of activated sterol regulatory element-binding proteins in the nucleus

The mechanism by which human immunodeficiency virus (HIV) protease inhibitor therapy adversely induces lipodystrophy and hyperlipidemia has not been defined. This study explored the mechanism associated with the adverse effects of the prototype protease inhibitor ritonavir in mice. Ritonavir treatment increased plasma triglyceride and cholesterol levels through increased fatty acid and cholesterol biosynthesis in adipose and liver. Ritonavir treatment also resulted in hepatic steatosis and hepatomegaly. These abnormalities, which were especially pronounced after feeding a Western type high fat diet, were due to ritonavir-induced accumulation of the activated forms of sterol regulatory binding protein (SREBP)-1 and -2 in the nucleus of liver and adipose, resulting in elevated expression of lipid metabolism genes. Interestingly, protease inhibitor treatment did not alter SREBP mRNA levels in these tissues. Thus, the adverse lipid abnormalities associated with protease inhibitor therapy are caused by the constitutive induction of lipid biosynthesis in liver and adipose tissues due to the accumulation of activated SREBP in the nucleus.

From interaction of lipidic vehicles with intestinal epithelial cell membranes to the formation and secretion of chylomicrons

Lipophilic drugs are carried by chylomicrons that are secreted by the small intestine and transported in lymph. This review discusses the digestion, uptake, and transport of dietary lipids and the impact that these processes have on the absorption of lipophilic drugs by the gastrointestinal tract. This chapter complements Dr. Chris Potter's chapter on the "pre-absorptive" events of drug processing and solubilization. This chapter reviews the digestion of lipids in the gastric and intestinal lumen and the role of bile salts in the solubilization of lipid digestion products for uptake by the gut. Both the passive and active uptake of lipid digestion products is discussed. How intestinal lipid transporters located at the brush border membrane may play a role in the uptake of lipids by the enterocytes is examined, as is the regulation of the absorption of cholesterol by the human ATP-binding cassette transporter-1 (ABC1). The intracellular trafficking and the resynthesis of complex lipids from lipid digestion products are explored, and the formation and secretion of chylomicrons are described.

Mutation of residues 423 (Met/Ile), 444 (Thr/Met), and 506 (Asn/Ser) confer cholesteryl esterase activity on rat lung carboxylesterase. Ser-506 is required for activation by cAMP-dependent protein kinase

Site-directed mutagenesis is used to identify amino acid residues that dictate reported differences in substrate specificity between rat hepatic neutral cytosolic cholesteryl ester hydrolase (hncCEH) and rat lung carboxylesterase (LCE), proteins differing by only 4 residues in their primary sequences. Beginning with LCE, the substitution Met(423) --> Ile(423) alone or in combination with other mutations increased activity with p-nitrophenylcaprylate (PNPC) relative to more hydrophilic p-nitrophenylacetate (PNPA), typical of hncCEH. The substitution Thr(444) --> Met(444) was necessary but not sufficient for expression of cholesteryl esterase activity in COS-7 cells. The substitution Asn(506) --> Ser(506), creating a potential phosphorylation site, uniformly increased activity with both PNPA and PNPC, was necessary but not sufficient for expression of cholesteryl esterase activity and conferred susceptibility to activation by cAMP-dependent protein kinase, a property of hncCEH. The 3 mutations in combination were necessary and sufficient for expression of cholesteryl esterase activity by the mutated LCE. The substitution Gln(186) --> Arg(186) selectively reduced esterase activity with PNPA and PNPC but was not required for cholesteryl esterase activity. Homology modeling from x-ray structures of acetylcholinesterases is used to propose three-dimensional models for hncCEH and LCE that provide insight into the effects of these mutations on substrate specificity.

Importance of arginines 63 and 423 in modulating the bile salt-dependent and bile salt-independent hydrolytic activities of rat carboxyl ester lipase

Previous studies using chemical modification approach have shown the importance of arginine residues in bile salt activation of carboxyl ester lipase (CEL) activity. However, the x-ray crystal structure of CEL failed to show the involvement of arginine residues in CEL-bile salt interaction. The current study used a site-specific mutagenesis approach to determine the role of arginine residues 63 and 423 in bile salt-dependent and bile salt-independent hydrolytic activities of rat CEL. Mutations of Arg(63) to Ala(63) (R63A) and Arg(423) to Gly(423) (R423G) resulted in enzymes with increased bile salt-independent hydrolytic activity against lysophosphatidylcholine, having 6.5- and 2-fold higher k(cat) values, respectively, in comparison to wild type CEL. In contrast, the R63A and R423A mutant enzymes displayed 5- and 11-fold decreases in k(cat), in comparison with wild type CEL, for bile salt-dependent cholesteryl ester hydrolysis. Although taurocholate induced similar changes in circular dichroism spectra for wild type, R63A, and R423G proteins, this bile salt was less efficient in protecting the mutant enzymes against thermal inactivation in comparison with control CEL. Lipid binding studies revealed less R63A and R423G mutant CEL were bound to 1,2-diolein monolayer at saturation compared with wild type CEL. These results, along with computer modeling of the CEL protein, indicated that Arg(63) and Arg(423) are not involved directly with monomeric bile salt binding. However, these residues participate in micellar bile salt modulation of CEL enzymatic activity through intramolecular hydrogen bonding with the C-terminal domain. These residues are also important, probably through similar intramolecular hydrogen bond formation, in stabilizing the enzyme in solution and at the lipid-water interface.

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.

Lipases and carboxylesterases: possible roles in the hepatic utilization of vitamin A

The formation and hydrolysis of retinyl esters are key processes in the metabolism of the fat-soluble micronutrient vitamin A. Long-chain acyl esters of retinol are the major chemical form of vitamin A (retinoid) stored in the body. Although retinyl esters are found in a variety of tissues and cell types, most of the total body retinoid is accounted for by the retinyl esters stored in the liver. Thus, these esters represent the major endogenous source of retinoid that can be delivered to peripheral tissues for conversion to biologically active forms. This paper summarizes the current state of our knowledge about the identity, function and regulation of the hepatic enzymes that are potentially involved in catalyzing the hydrolysis of retinyl esters. These enzymes include several known and characterized lipases and carboxylesterases.

Effects of ethanol on the expression and secretion of bile salt-dependent lipase by pancreatic AR4-2J cells

The mechanisms by which ethanol administration alters pancreatic function are unknown. We have evaluated the effects of chronic ethanol treatment on secretion of a digestive enzyme: the bile salt-dependent lipase (BSDL), by the rat pancreatic cell line AR4-2J (as a model). We report that ethanol (50-300 mM) in culture medium induced a rise, in secreted and intracellular BSDL, that was a function of the duration of treatment and of the ethanol concentration. This effect was not abolished by pyrazole, which suggests a direct effect of ethanol. We have further established that the increase of BSDL activity was due to an enhanced biosynthesis of the enzyme consecutive to a major steady-state level of mRNA encoding BSDL. Also, the subcellular localization showed a specific accumulation of BSDL in the cytosolic fraction of cells chronically treated with ethanol. Given the enzymatic properties of BSDL, all these data could have some physiological consequences regarding the digestive function, plasma lipid metabolism and intracellular cholesterol homeostasis.

Participation of GRP94-related protein in secretion of pancreatic bile salt-dependent lipase and in its internalization by the intestinal epithelium

In previous studies on the AR4-2J cell line, we have shown that secretion of bile salt-dependent lipase (BSDL) involves a multiprotein complex, including a protein of 94 kDa (p94) that is immunologically related to the chaperone Grp94, which seems to play essential roles in the folding process of BSDL. Combined biochemical and immunocytochemical investigations were carried out to study the secretion of BSDL by normal pancreatic cells and its transport to the small intestine where this enzyme is thought to exert its physiological function. Both BSDL and Grp94 antigenic sites were localized and found to be associated all along the pancreatic acinar cell secretory pathway. Grp94 and BSDL remain associated from leaving the pancreas until arriving at the intestinal lumen. In pancreatic juice, both proteins appear as a complex of high molecular mass (180 kDa) containing at least one each of p94 and BSDL molecules, interacting by hydrophobic forces. At the intestinal level, associated Grp94 and BSDL were detected on microvilli and in the endosomal compartment of enterocytes. The BSDL mRNA, however, was not expressed by the intestinal mucosa. The pancreatic Grp94-BSDL complex was internalized through the endosomal compartment of enterocytes. Finally, the two proteins dissociated in this compartment and BSDL, but not Grp94, was transferred to the basolateral membrane.

An intracellular role for pancreatic bile salt-dependent lipase: evidence for modification of lipid turnover in transfected CHO cells

Pancreatic bile salt-dependent lipase (BSDL) hydrolyzes cholesteryl esters, triglycerides and phospholipids. BSDL is also capable of transferring free fatty acid to cholesterol. BSDL has been detected in many cells including fetal and tumor cells, hepatocytes, macrophages and eosinophils and in tissues such as adrenal glands and testes. The enzyme may be secreted or located within subcellular compartments such as the endoplasmic reticulum or the cytosol. Although the role of the secreted enzyme is well documented, that of the intracellular form(s) is still hypothetical. In the present study, we addressed the effects of BSDL on cell lipid metabolism. For that purpose, the cDNA of rat BSDL was transfected into CHO K1 cells (CHO K1-BSDL clone) which were then loaded with [3H]oleic acid. The results demonstrate that the transfected BSDL is secreted; in spite of that, a large fraction of catalytically active BSDL is found in cell lysate. The lipid metabolism of transfected cells is affected and BSDL induces an enhanced incorporation of [3H]oleic acid in cholesteryl esters whereas fatty acid incorporation in phosphatidylcholine is decreased. These effects were particularly important in the cytosol of transfected cells where transfected BSDL preferentially locates. These data suggested that BSDL could be implicated in the cycle of the cellular homeostasis of cholesterol which is particularly affected in tumoral cells leading to cholesteryl ester storage within cytosolic lipid droplets.

Lipases and carboxylesterases: possible roles in the hepatic metabolism of retinol

The formation of hydrolysis of retinyl esters are key processes in the metabolism of the fat-soluble micronutrient vitamin A. Long-chain acyl esters of retinol are the major chemical form of vitamin A (retinoid) stored in the body. Retinyl esters are found in a variety of tissues and cell types, but most of the total body retinoid is accounted for by the retinyl esters stored in the liver. Thus, these esters represent the major endogenous source of retinoid that can be delivered to peripheral tissues for conversion to biologically active forms. This review summarizes current knowledge about the identity, function, and regulation of the hepatic enzymes potentially involved in catalyzing the hydrolysis of retinyl esters. These enzymes include several known and characterized lipases and carboxylesterases. Although there is accumulating evidence that these enzymes function as retinyl ester hydrolases in vitro, it is not clear which play important physiological roles in hepatic retinyl ester metabolism.

Molecular cloning of the bile salt-dependent lipase of ferret lactating mammary gland: an overview of functional residues

Ferret lactating mammary gland bile salt-dependent lipase (BSDL, EC 3.1.1.-) has been cloned by RT-PCR. The open reading frame consists of 1869 nucleotides which encode 623 amino acids of the functional enzyme. When compared to other species, the greatest homology is observed between residues 1 and 484, with little or no homology at the C-terminal end where seven repeated segments of similar sequence are located. Ferret mammary gland BSDL retains residues involved in the active site and the tentative heparin binding site at similar positions in comparison to other milk or pancreatic BSDL. Other important items, such as binding peptide to chaperone molecular, phosphorylation site(s) or bile salt binding sites, were also tentatively located in well conserved regions of seven available BSDL sequences.

Lipid-lowering effects of WAY-121,898, an inhibitor of pancreatic cholesteryl ester hydrolase

WAY-121,898 is an inhibitor of pancreatic cholesteryl ester hydrolase (pCEH). After confirming its in vitro potency and relative lack of a major effect on acyl-CoA:cholesterol acyltransferase (ACAT), it was found that this compound lowers plasma cholesterol in cholesterol-fed, but not chow-fed, rats. Measures of liver cholesteryl ester content and the direct determination of cholesterol absorption (lymph-fistula model) show that inhibition of cholesterol absorption is at least one mechanism for the observed cholesterol lowering. However, WAY-121,898 was also active when administered parenterally to cholesterol-fed rats, and in cholesterol-fed hamsters cholesterol-lowering occurred with oral dosing despite no change in cholesterol absorption, suggesting other modes of action possibly relating to inhibition of liver CEH. Combination treatment in cholesterol-fed rats with the ACAT inhibitor CI-976 resulted in a greater-than-additive reduction in plasma cholesterol, implying that both pCEH and ACAT may play a role in cholesterol absorption in this species. In rabbits, WAY-121,898 prevented the rise in plasma cholesterol due to the feeding of cholesteryl ester but not in rabbits fed (free) cholesterol. In guinea pigs, the compound induced an increase in adrenal cholesteryl ester mass. Taken together, the overall profile in these animal models suggests that WAY-121,898 inhibits more than just the intestinal (lumenal) pCEH, and that the role of this enzyme in cholesterol metabolism may be different within and across species, the former depending upon the dietary cholesterol load.

Synthesis and secretion of the pancreatic-type carboxyl ester lipase by human endothelial cells

Human aortic extracts contain significant cholesteryl ester hydrolytic activity. The enzymic activity was shown to be activated by trihydroxylated bile salt, but not by dihydroxylated bile salt. Monospecific antibodies prepared against rat pancreatic carboxyl ester lipase (CEL, cholesterol esterase) immunoprecipitated cholesteryl ester hydrolytic activity from human aorta, demonstrating that the neutral CEL in aorta is highly similar to and probably identical with the pancreatic enzyme. Reverse transcriptase PCR amplification of mRNA from human aortic endothelial cells revealed de novo synthesis of the pancreatic-type CEL by these cells. Preincubating human aortic endothelial cells with oxidized or native low-density lipoprotein resulted in an 8- and 3-fold increase in CEL activity secreted into the culture medium respectively. A potential physiological role for the endothelial CEL was demonstrated by studies showing its ability to confer partial protection against the cytotoxic effects of lysophosphatidylcholine. The protective effect of CEL is related to its bile-salt-independent lysophospholipase activity. However, CEL hydrolysis of lysophosphatidylcholine can be inhibited by excess cholesterol. Taken together, these results indicate that pancreatic-type CEL is synthesized by cells lining the vessel wall. Moreover, vascular CEL may interact with cholesterol and oxidized lipoproteins to modulate the progression of atherosclerosis.

Modified low density lipoprotein enhances the secretion of bile salt-stimulated cholesterol esterase by human monocyte-macrophages. species-specific difference in macrophage cholesteryl ester hydrolase

Reverse transcriptase-polymerase chain reaction was used to study the biosynthesis of two different cholesteryl ester hydrolases by human and mouse macrophages. Oligonucleotide primers for bile salt-stimulated cholesterol esterase yielded positive reactions with RNA isolated from human peripheral blood monocytes, monocyte-derived macrophages, the human monocytic THP-1 cells, and phorbol ester-induced THP-1 macrophages. In contrast, oligonucleotide primers for hormone-sensitive lipase yielded positive reactions only with RNA isolated from non-differentiated human THP-1 monocytic cells and peripheral blood monocytes, but not those obtained from differentiated THP-1 macrophages or monocyte-derived macrophages. Thus, while human monocytes were capable of synthesizing both enzymes, human macrophages synthesized only bile salt-stimulated cholesterol esterase and not the hormone-sensitive lipase. The synthesis of bile salt-stimulated cholesterol esterase by human macrophages was confirmed by detection of bile salt-stimulated cholesteryl ester hydrolytic activity in conditioned media of differentiated THP-1 cells and human peripheral blood monocyte-derived macrophages. Moreover, incubating human macrophages with oxidized low density lipoprotein (LDL) or acetylated LDL increased bile salt-stimulated cholesterol esterase activity in the conditioned media of these cells. These results with human macrophages were contrasted with results of studies with mouse macrophages, which showed the presence of hormone-sensitive lipase mRNA but not the bile salt-stimulated cholesterol esterase mRNA. Taken together, these results demonstrated species-specific differences in expression of cholesteryl ester hydrolytic enzymes in macrophages. The expression of bile salt-stimulated cholesterol esterase by human macrophages, in a process inducible by modified LDL, suggests a role of this protein in atherogenesis.

Size of the catalytically active unit of rat hepatic carboxylester lipase in the presence and absence of bile salt

Carboxylester lipase (CEL) catalyzes the hydrolysis of cholesteryl esters, retinyl esters, and triacylglycerols. CEL monomer has a MW of approximately 70000. Hydrolysis of these esters is stimulated by millimolar trihydroxy bile salts such as cholate, that also induce aggregation. Liver cytosols from 12 rats were frozen and irradiated at -135 degrees C with high energy electrons. In several experiments, paired samples of cytosol were adjusted to 20 mM cholate before irradiation. All samples were assayed for CEL using cholesteryl oleate as substrate. In untreated cytosols, CEL activity surviving radiation exposure could be fit to a single exponential function, the slope of which yielded a target size of 91 +/- 18 kDa. In a subset of these cytosols irradiated in the presence of cholate the calculated target size was 100 +/- 19 kDa, a value indistinguishable from that obtained for untreated cytosols. Some samples were also assayed using retinyl palmitate and triolein as substrates. With retinyl palmitate the mean target sizes were 96 and 108 kDa in the absence and presence of cholate, respectively, approximately the same as those observed when using cholesteryl oleate. When triolein was used as substrate the target sizes in the absence of cholate were smaller than for the other two esters (67 +/- 18 kDa) and closer to the known monomer molecular weight, but again cholate had no significant effect on this size. The structure responsible for CEL activity contains no more than one 70000 MW monomer and the results show that cholate-induced oligomerization is not required for catalytic activity.

Immunological identification of cholesterol ester hydrolase in the steroidogenic tissues, adrenal glands and testis

Monoclonal antibodies were generated against the purified pancreatic cholesterol ester hydrolase (CEH, EC 3.1.1.13) to examine the expression of CEH in various bovine tissues. The presence of CEH isozyme antigenically indistinguishable from pancreatic enzyme in the steroidogenic tissues, adrenal glands and testis has been first demonstrated here using the immunoprecipitation method. These results suggest that CEH isozyme, similar to pancreatic CEH, might be involved in the cholesterol metabolism in the steroidogenic tissue.

Purification and characterization of rat liver microsomal fatty acid ethyl and 2-chloroethyl ester synthase and their relationship with carboxylesterase (pI 6.1)

Previous studies have shown that fatty acid ethyl ester synthase (FAEES) which catalyzes the formation of ethyl or 2-chloroethyl esters of long-chain fatty acids is localized in the microsomal fraction of rat liver. A recent study suggests that rat adipose tissue FAEES is similar to rat liver microsomal carboxylesterase (CE) [Tsujita and Okuda (1992) J. Biol. Chem. 267, 23489-23494]. Since the interrelationships among FAEES, 2-chloroethyl ester synthase (FACEES), and cholesterol esterase (ChE) are also not clear at present, we purified and characterized FAEES and FACEES from rat hepatic microsomes and studied their functional and structural relationships with CE and ChE. The results of these studies showed that CE, FAEES, and FACEES activities copurified during each step of purification. Although gel-filtration column chromatography of DEAE-Sephacel purified microsomal protein resolved into two peaks with an estimated molecular weight of 180 (major) and 60 kDa (minor, this paper describes characterization of only the 180 kDa protein. CE, FAEES, and FACEES activities associated with homogeneous 180 kDa protein could be inhibited by a beta-esterase inhibitor (diisopropyl fluorophosphate) in an identical manner. This protein, however, showed only the hydrolytic activity, but not the synthetic activity for cholesterol oleate, indicating that it is distinct from ChE. The purified protein could be immunoprecipitated with the antibodies raised against rat adipose tissue FAEES, but not with antibodies against rat pancreatic ChE, demonstrating again that the purified protein is distinct from ChE. A single band corresponding to 60 kDa upon SDS-PAGE, under reduced denaturing conditions, indicates that the purified protein is a trimer. N-terminal amino acid sequence of the first 27 residues were identical to that of rat hepatic microsomal CE [Robbi et al. (1990) Biochem. J., 451-458] which suggests structural similarity of the purified protein with rat hepatic microsomal CE. Therefore, the functional and structural properties of the purified protein demonstrate that FAEES, FACEES, as well as CE activities are expressed by the same protein, purified in this study, which exists as a trimer (180 kDa) and is involved in biosynthesis of long-chain fatty acid esters of xenobiotic alcohols. Further studies on purification and characterization of the enzymes responsible for the esterification of xenobiotic alcohols with endogenous fatty acids from various target organs need to be conducted to determine their functional and structural interrelationships. Inhibition and induction studies of these enzyme(s) and the extent of observed toxicity could be important in understanding their role in etiology of chronic diseases induced by alcohol abuse.

Molecular modeling of the structures of human and rat pancreatic cholesterol esterases

Structural models have been generated for rat and human cholesterol esterases by molecular modeling. For rat cholesterol esterase, three separate models were generated according to the following procedure: (1) the cholesterol esterase sequence was aligned with those of three template enzymes: Torpedo californica acetylcholinesterase, Geotrichum candidum lipase and Candida rugosa lipase; (2) the X-ray structure coordinates of the three template enzymes were used to construct cholesterol esterase models by amino acid replacements of matched sequence positions and by making sequence insertions and deletions as required; (3) bad contracts in each of the cholesterol esterase models were relaxed by molecular dynamics and mechanics; (4) the three cholesterol esterase models were merged into one by arithmetic averaging of atomic coordinates; (5) Ramachandran analysis indicated that the model generated from the AChE template possessed the best set of phi/psi angles. Therefore, this model was subjected to molecular dynamics, with harmonic constraints imposed on the C(alpha) coordinates to drive them toward the coordinates of the averaged model. (6) Subsequent relaxation by molecular mechanics produced the final rat cholesterol esterase model. A model for human cholesterol esterase was produced by repeating steps 1-3 above, albeit with the rat cholesterol esterase model as the template. Hydrophobic and electrostatic analyses of the rat and human cholesterol esterase models suggest the structural origins of molecular recognition of hydrophobic substrates and interfaces, of charged interfaces, and of bile salt activators.

Dietary free and esterified cholesterol absorption in cholesterol esterase (bile salt-stimulated lipase) gene-targeted mice

The involvement of pancreatic cholesterol esterase (bile salt-stimulated lipase) in cholesterol absorption through the intestine has been controversial. We have addressed this issue by using homologous recombination in embryonic stem cells to produce mice lacking a functional cholesterol esterase gene. Cholesterol esterase knockout mice and their wild type counterparts were fed a bolus dose of [3H]cholesterol and a trace amount of [beta-14C]sitosterol by gavage. The ratio of the two radiolabels excreted in the feces over a 24-h period was found to be similar in the control and cholesterol esterase-null mice. Similar results were observed when the radiolabeled sterols were supplied in an emulsion with phospholipid and triolein or in lipid vesicles with phosphatidylcholine. Cholesterol absorption results were similar between the control and cholesterol esterase-null mice regardless of whether the animals were fed a low fat diet or a high fat/high cholesterol diet. The rate of [3H]cholesterol appearance in the serum of the gene-targeted mice paralleled that observed in control animals. In contrast to these results, when experiments were performed with [3H]cholesteryl oleate instead of [3H]cholesterol, a higher amount of the 3H radiolabel was found excreted in feces and dramatically less of the radiolabel was detected in the serum of the cholesterol esterase-null mice in comparison with that detected in control animals. Serum cholesterol levels were not significantly different between control and cholesterol esterase-null mice fed either control or an atherogenic diet. These results indicate that cholesterol esterase is responsible for mediating intestinal absorption of cholesteryl esters but does not play a primary role in free cholesterol absorption.

Coenzyme A-independent monoacylglycerol acyltransferase from rat intestinal mucosa

Rat intestinal mucosa contains high diacylglycerol-synthesizing activity (monoacylglycerol acyltransferase (MGAT) activity) due to monoacylglycerol and fatty acid, independently of coenzyme A and ATP. MGAT activity was purified from rat intestinal mucosa by successive chromatography separations on DEAE-cellulose, CM- Sephadex, and anti-IgG-Sepharose against rat pancreatic lipase. The enzyme was electrophoretically homogeneous, and its molecular weight was 49,000, which is identical with that of rat pancreatic lipase. Immunoblotting analysis with antibody against rat pancreatic lipase showed one immunoreactive protein with an estimated molecular weight of 49,000. The activity of the purified enzyme was completely inhibited by addition of the antibody. Using immunocytochemical techniques, it was found that immunoreactive protein against rat pancreatic lipase was uniformly distributed within the absorptive cells of the intestine but was absent from the microvillar membrane. The MGAT activity of intestinal mucosal homogenate was inhibited by about 65% by addition of antibody against rat pancreatic lipase. Trioleoylglycerol- and dioleoylglycerol-hydrolyzing activities of the purified enzyme and pancreatic lipase were inhibited by addition of intestinal mucosa extract. These results suggest that pancreatic lipase is present in intestinal absorptive cells and that it may contribute to resynthesis of diacylglycerol from monoacylglycerol and fatty acids in these cells.

Dietary induction of pancreatic cholesterol esterase: a regulatory cycle for the intestinal absorption of cholesterol

Atherosclerosis has a strong dietary basis without a proven molecular mechanism for cholesterol absorption. To investigate the potential role of pancreas in this process and its interaction with the two dietary forms of cholesterol (free and esterified), we undertook to study the role of pancreatic cholesterol esterase in cholesterol absorption. The results showed that (i) cholesterol esters contribute a disproportionately high fraction of absorbed dietary cholesterol, (ii) rates of intestinal cholesterol absorption are related to pancreatic cholesterol esterase activity, (iii) mRNA specific for pancreatic cholesterol esterase is induced 15-fold by dietary sterol esters and 10-fold by free sterol, (iv) the induction of cholesterol esterase mRNA is reversible, and (v) free cholesterol transport into cultured human intestinal cells is enhanced 300% by pancreatic cholesterol esterase. These data implicate pancreatic cholesterol esterase as pivotal in a metabolic loop under positive feedback control for the absorption of dietary cholesterol, whether free or esterified.

Molecular cloning and expression of rat hepatic neutral cholesteryl ester hydrolase

The 1923 bp cDNA for rat hepatic cholesteryl ester hydrolase (CEH) was cloned by screening a lambda gt11 expression library with an oligonucleotide containing the consensus active site sequence for cholesteryl esterases. Expression of a fusion protein, cross-reacting with antibody to the purified liver CEH, was demonstrated by Western blot analysis. The cDNA was sequenced and found to have only 44% homology with pancreatic CEH. Although unique, the cDNA sequence exhibited much greater overall homology with liver carboxylesterases, in both coding and 5'/3' non-coding regions. In Northern blot analysis, the cDNA hybridized with a single band from liver mRNA but not with pancreatic mRNA. The 1.7 kb coding sequence, predicting a 62 kDa protein, was cloned into an Escherichia coli expression system with an inducible promoter and into COS-7 cells. Both expression systems produced a protein which comigrated with liver CEH (66 kDa) on SDS-PAGE and immunoreacted with antibodies to liver CEH on Western blots. Whereas the prokaryotic system produced an inactive protein, expression in COS-7 cells was accompanied by a 5-fold increase in CEH activity and a corresponding increase in immunoreactive protein.

Characterization of the mouse pancreatic/mammary gland cholesterol esterase-encoding cDNA and gene

Pancreatic cholesterol esterase (CEase) is involved in cholesterol ester hydrolysis and transport of cholesterol into intestinal cells. Isolation and sequencing of the mouse CEase cDNA indicates that the murine enzyme is very similar to the human enzyme, with the exception of the highly repeated C-terminal Pro-rich repeats. Reverse transcription-polymerase chain reaction analysis reveals that the message is synthesized in the pancreas of mice, but not in the liver.