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

ENCoRE: an efficient software for CRISPR screens identifies new players in extrinsic apoptosis

BACKGROUND

As CRISPR/Cas9 mediated screens with pooled guide libraries in somatic cells become increasingly established, an unmet need for rapid and accurate companion informatics tools has emerged. We have developed a lightweight and efficient software to easily manipulate large raw next generation sequencing datasets derived from such screens into informative relational context with graphical support. The advantages of the software entitled ENCoRE (Easy NGS-to-Gene CRISPR REsults) include a simple graphical workflow, platform independence, local and fast multithreaded processing, data pre-processing and gene mapping with custom library import.

RESULTS

We demonstrate the capabilities of ENCoRE to interrogate results from a pooled CRISPR cellular viability screen following Tumor Necrosis Factor-alpha challenge. The results not only identified stereotypical players in extrinsic apoptotic signaling but two as yet uncharacterized members of the extrinsic apoptotic cascade, Smg7 and Ces2a. We further validated and characterized cell lines containing mutations in these genes against a panel of cell death stimuli and involvement in p53 signaling.

CONCLUSIONS

In summary, this software enables bench scientists with sensitive data or without access to informatic cores to rapidly interpret results from large scale experiments resulting from pooled CRISPR/Cas9 library screens.

Functions of Intracellular Retinoid Binding-Proteins

Multiple binding and transport proteins facilitate many aspects of retinoid biology through effects on retinoid transport, cellular uptake, metabolism, and nuclear delivery. These include the serum retinol binding protein sRBP (aka Rbp4), the plasma membrane sRBP receptor Stra6, and the intracellular retinoid binding-proteins such as cellular retinol-binding proteins (CRBP) and cellular retinoic acid binding-proteins (CRABP). sRBP transports the highly lipophilic retinol through an aqueous medium. The major intracellular retinol-binding protein, CRBP1, likely enhances efficient retinoid use by providing a sink to facilitate retinol uptake from sRBP through the plasma membrane or via Stra6, delivering retinol or retinal to select enzymes that generate retinyl esters or retinoic acid, and protecting retinol/retinal from excess catabolism or opportunistic metabolism. Intracellular retinoic acid binding-proteins (CRABP1 and 2, and FABP5) seem to have more diverse functions distinctive to each, such as directing retinoic acid to catabolism, delivering retinoic acid to specific nuclear receptors, and generating non-canonical actions. Gene ablation of intracellular retinoid binding-proteins does not cause embryonic lethality or gross morphological defects. Metabolic and functional defects manifested in knockouts of CRBP1, CRBP2 and CRBP3, however, illustrate their essentiality to health, and in the case of CRBP2, to survival during limited dietary vitamin A. Future studies should continue to address the specific molecular interactions that occur between retinoid binding-proteins and their targets and their precise physiologic contributions to retinoid homeostasis and function.

Physiological insights into all-trans-retinoic acid biosynthesis

All-trans-retinoic acid (atRA) provides essential support to diverse biological systems and physiological processes. Epithelial differentiation and its relationship to cancer, and embryogenesis have typified intense areas of interest into atRA function. Recently, however, interest in atRA action in the nervous system, the immune system, energy balance and obesity has increased considerably, especially concerning postnatal function. atRA action depends on atRA biosynthesis: defects in retinoid-dependent processes increasingly relate to defects in atRA biogenesis. Considerable evidence indicates that physiological atRA biosynthesis occurs via a regulated process, consisting of a complex interaction of retinoid binding-proteins and retinoid recognizing enzymes. An accrual of biochemical, physiological and genetic data have identified specific functional outcomes for the retinol dehydrogenases, RDH1, RDH10, and DHRS9, as physiological catalysts of the first step in atRA biosynthesis, and for the retinal dehydrogenases RALDH1, RALDH2, and RALDH3, as catalysts of the second and irreversible step. Each of these enzymes associates with explicit biological processes mediated by atRA. Redundancy occurs, but seems limited. Cumulative data support a model of interactions among these enzymes with retinoid binding-proteins, with feedback regulation and/or control by atRA via modulating gene expression of multiple participants. The ratio apo-CRBP1/holo-CRBP1 participates by influencing retinol flux into and out of storage as retinyl esters, thereby modulating substrate to support atRA biosynthesis. atRA biosynthesis requires the presence of both an RDH and an RALDH: conversely, absence of one isozyme of either step does not indicate lack of atRA biosynthesis at the site. This article is part of a Special Issue entitled: Retinoid and Lipid Metabolism.

Retinyl ester hydrolases and their roles in vitamin A homeostasis

In mammals, dietary vitamin A intake is essential for the maintenance of adequate retinoid (vitamin A and metabolites) supply of tissues and organs. Retinoids are taken up from animal or plant sources and subsequently stored in form of hydrophobic, biologically inactive retinyl esters (REs). Accessibility of these REs in the intestine, the circulation, and their mobilization from intracellular lipid droplets depends on the hydrolytic action of RE hydrolases (REHs). In particular, the mobilization of hepatic RE stores requires REHs to maintain steady plasma retinol levels thereby assuring constant vitamin A supply in times of food deprivation or inadequate vitamin A intake. In this review, we focus on the roles of extracellular and intracellular REHs in vitamin A metabolism. Furthermore, we will discuss the tissue-specific function of REHs and highlight major gaps in the understanding of RE catabolism. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

The mPlrp2 and mClps genes are involved in the hydrolysis of retinyl esters in the mouse liver

Retinyl esters are the major chemical forms of vitamin A stored in the liver, and can be delivered to peripheral tissues for conversion into biologically active forms. The function and regulation of the hepatic genes that are potentially involved in catalyzing the hydrolysis of retinyl esters remain unclear. Here we show that two lipid hydrolytic genes, pancreatic-related protein 2 (mPlrp2) and procolipase (mClps), expressed specifically in the mouse pancreas, are associated with the ratio of S-adenosylmethionine (AdoMet) to S-adenosylhomocysteine (AdoHcy). Light illumination deficiency or administration of 5'-AMP elevated the ratio of AdoMet to AdoHcy and induced the expression in the liver of mPlrp2 and mClps, which was blocked by all-trans retinoic acid. Mice fed a vitamin A-free diet exhibited increased activation of hepatic mPlrp2 and mClps expression, which was associated with increased methylation of histone H3K4 residues located near the mPlrp2 and mClps promoters. Inhibition of hepatic mPlrp2 and mClps expression by a methylase inhibitor, methylthioadenosine, markedly decreased plasma retinol levels in these mice. The activated hepatic stellate cell (HSC)-T6 cell line specifically expressed mClps and mPlrp2. Inhibition of mClps gene expressions by short hairpin RNA (shRNA) decreased hydrolysis of retinyl esters in the HSC-T6 cell line. These data suggest that the conditional expression of mPlrp2 and mClps is involved in the hydrolysis of retinyl esters in the mouse liver.

GS2 as a retinol transacylase and as a catalytic dyad independent regulator of retinylester accretion

GS2 (PNPLA4; iPLAeta) is the smallest member of the patatin-like family of phospholipases (PNPLA). It was initially identified by its ability to hydrolyze retinylesters (RE) in cell homogenates, and was later found to esterify retinol using a variety of acyl donors. In the present study we set out to determine its cellular function and examined its impact on RE status in 293T cells transfected with GS2, GS2-M1 (a non-translatable mutant of GS2) and empty vector, in fibroblasts isolated from normal and GS2-null donors and in SCC12b and in a somatic cell knock-out of GS2 (SCC12b-GS2(neo/-)), that we generated by homologous recombination. At 50nM medium retinol, GS2 had no significant impact on RE accumulation. However, at 2muM retinol, GS2 promoted a 1.6- to 5-fold increase in RE accumulation. To verify role of GS2 as a catalyst, RE levels were measured in 293T transfected wild type GS2, catalytic dyad mutants devoid of enzymatic activity, or alanine substitution mutants spanning the entire GS2 sequence. Surprisingly, every GS2 mutant promoted RE accumulation. This activity was also observed in the GS2 paralogues and rat orthologue. The data demonstrate that within the context of the cell GS2 promotes RE accumulation and may do so either as a catalyst or as a regulatory protein that enhances RE formation catalyzed by other acyl transferases.

Homeostasis of retinol in lecithin: retinol acyltransferase gene knockout mice fed a high retinol diet

We analyzed the retinoid levels and gene expression in various tissues after wild-type (Wt) and lecithin:retinol acyltransferase (LRAT-/-) knockout mice were fed a high retinol diet (250 IU/g). As compared to Wt, LRAT-/- mice exhibited a greater and faster increase in serum retinol concentration (mean+/-S.D., Wt, 1.3 +/- 0.2 microM to 1.5 +/- 0.3 microM in 48 h, p > 0.05; LRAT-/-, 1.3 +/- 0.2 microM to 2.2+/-0.3 microM in 48 h, p < 0.01) and a higher level of retinol in adipose tissue (17.2 +/- 2.4 pmol/mg in Wt vs. 34.6 +/- 8.0 pmol/mg in LRAT-/-). In the small intestines of Wt mice higher levels of retinol (96.4 +/- 13.0 pmol/mg in Wt vs. 13.7 +/- 7.6 pmol/mg in LRAT-/- and retinyl esters (2493.4 +/- 544.8 pmol/mg in Wt vs. 8.2 +/- 2.6 pmol/mg in LRAT-/- were detected. More retinol was detected in the feces of LRAT-/- mice (69.3 +/- 32.6 pmol/mg in LRAT-/- vs. 24.1 +/- 8.6 pmol/mg in Wt). LRAT mRNA levels increased in the lungs, small intestines, and livers of Wt mice on the high retinol diet, while CYP26A1 mRNA levels increased greatly only in the LRAT-/- mice. After 4 weeks, no significant differences between Wt mice and LRAT-/- mice were observed in either the serum retinol level or in the prevalence of Goblet cells in jejunal crypts. Our data indicate that the LRAT-/- mice maintain the homeostasis of retinol as the dietary retinol increases by increasing the excretion of retinol from the gastrointestinal tract, increasing the distribution of retinol to adipose tissue, and enhancing the catabolism by CYP26A1. We show that LRAT plays a role in maintaining a stable serum retinol concentration when dietary retinol concentration fluctuates.

A comparative study of human GS2, its paralogues, and its rat orthologue

We have previously shown that human GS2 (hGS2) catalyzes keratinocyte retinylester and triglyceride hydrolysis. hGS2 and its rat orthologue, rGS2, are 80% homologous and share a proline insertion at residue 56 and a C-terminal truncation compared to the hGS2 paralogues. Both changes are required for hGS2 function. However, the catalytic capabilities of hGS2 are more similar to the paralogue, TTS-2.2, than to rGS2. Only hGS2 and hTTS-2.2 transfer fatty acid from triglyceride to retinol, hydrolyze retinylesters, and generate 1,3-diacylglycerol from triglycerides. Rat-human chimeras containing either the N- or C-terminus of rGS2 are without activity and single substitutions of rat for human residues cause activity loss. The differences between orthologues suggest that GS2 has a unique function in humans or has a function that is fulfilled by other enzymes in rodents. Since retinoid and triglyceride metabolites are transcription factor ligands, we expect that these enzymes will coordinately regulate epidermal homeostasis.

Molecular Screening for GS2 Lipase Regulators: Inhibition of Keratinocyte Retinylester Hydrolysis by TIP47

Retinoic acid at nanomolar concentrations modulates epidermal functions by serving as a transcription factor ligand. Under conditions of retinol sufficiency, it is imperative to limit retinoic acid biosynthesis from serum-derived retinol. In the epidermis, this is accomplished by esterifying retinol with long-chain fatty acids. Retinylester (RE) pools serve as a source of retinol for retinoic acid production under retinol deficiency and when required for proper differentiation. We have recently reported that GS2 lipase is expressed in keratinocytes and has the enzymatic properties of keratinocyte RE hydrolase. As GS2 lipase has a robust activity that can affect the intracellular retinol levels, we postulated that its activity must be regulated. Therefore, we screened keratinocyte cDNA expression libraries for the putative inhibitor. Herein, we report the identity of an inhibitor, TIP47, which prevents RE hydrolysis catalyzed by GS2 lipase and hormone-sensitive lipase. This protein was known to transport mannose-6-phosphate receptors from endosome to trans-Golgi and to be distributed between the cytoplasm and lipid droplets. Using a series of deletion mutants, we found two regions involved in the inhibitory activity. Residues within the carboxyl alpha3-alpha4 helices are essential in the context of the full-length protein. Residues within the amino-terminal also contribute depending on the context.

Species-, sex-, and age-dependent urinary excretion of cauxin, a mammalian carboxylesterase

Domestic cats exhibit physiological proteinuria due to the excretion of cauxin, a carboxylesterase, into the urine. In the present report, we demonstrate that cauxin is excreted in a species-, sex-, and age-dependent manner. Although the cauxin gene is conserved in mammals, including human, mouse, and dog, urinary cauxin was found only in member of the genus Felis and lynx (bobcat, and lynx) and not in other Felidae (genus: Panthera and puma) tested. In mature cats, cauxin excretion was higher in intact males than in castrated males or in intact or spayed females. Daily cauxin excretion decreased immediately after castration. Immunohistochemistry confirmed that cauxin expression in the kidney proximal straight tubules was higher in intact males than in castrated males. Urinary cauxin was detectable by Western blotting in cats older than about 3 months, and its excretion increased with age. In a zymographic esterase assay, urine contained a major cauxin band; by contrast, kidney homogenates contained three major bands, comprising two carboxylesterases and an unidentified esterase, and one minor cauxin band. These results suggest that 1. cauxin excretion is regulated by sex hormones, such as testosterone, 2. cauxin functions as an esterase in the urine rather than in kidney cells, and 3. the decomposition products by cauxin are excreted in a species-, sex-, and age-dependent manner, as is cauxin itself.

An Epididymal Form of Cauxin, a Carboxylesterase-Like Enzyme, Is Present and Active in Mammalian Male Reproductive Fluids1

Mass spectrometric analysis of a prion protein (PrP)-containing complex isolated from ram cauda epididymal fluid revealed a protein that showed homology to a carboxylesterase-like protein previously identified in cat urine (cauxin). Using anti-cauxin antibodies, immunoreactive bands were detected in corpus and cauda epididymal fluid from all mammals tested (ram, boar, mouse, and cat). In the ram, the protein was also present in seminal fluid but not found to be associated with sperm. The bands reacting with the anti-cauxin antibody coincided with those having esterase activity in a zymographic assay and its levels paralleled the esterase activity of native epididymal fluids. A partial nucleotide sequence of 1143 bp, corresponding to 380 amino acids, was obtained by RT-PCR amplification from total RNA from the corpus epididymis (zone 6). The deduced protein sequence shows a high degree of homology (up to 90%) with the different cauxin proteins found in databases but only up to 60% with other known carboxylesterases. By PCR, strong mRNA expression was found in the corpus and cauda epididymis, while the testis, kidney, and caput epididymis had low expression. No mRNA was detected in the lung, heart, or liver. These data demonstrate that an epididymal form of the cauxin enzyme is secreted into mammalian epididymal fluid. In the ram, it is associated with a high molecular-weight PrP-associated complex and may be responsible for the majority of the esterase activity in the cauda epididymal fluid of this species.

A comparative study on the effects of oral amiodarone and trimeprazine, twoin vitroretinyl ester hydrolase inhibitors, on the metabolic availability of vitamin A in rats

Amiodarone, an antiarrhythmic drug, and trimeprazine, an antipsychotic drug, are bothin vitroinhibitors of retinyl ester hydrolase. To determine whether these agents have deleterious effects on aspects of vitamin A metabolism, Brown Norway rats (n18) were treated at clinically equivalent doses once daily for 26d with either oral drug. On day 27, a tolerance test was used to determine whether these agents interfered with vitamin absorption. During the first 8d, the plasma retinol level declined in all animals. Between days 12 and 27, it rose to near pre-treatment concentrations in the control and trimeprazine groups and remained relatively constant at low levels (P<0·001) in the amiodarone group. The intestinal absorption of vitamin A was reduced (P<0·05) in the amiodarone group compared with the placebo and trimeprazine groups, which did not differ significantly from each other. At the end of the 4-week treatment period, hepatic retinyl ester hydrolase activity was lower in the drug-dosed rats (P=0·06 for amiodarone) than in the controls. With regard to effects on liver reserves, drug treatment resulted in vitamin A depletion (P<0·019), and distinctive patterns of retinol and its esters were seen in response to dosing. In conclusion, amiodarone and trimeprazine have been shown to influence different aspects of retinoid metabolism, namely absorption, storage and transport. In clinical practice, the routine unmonitored use of these drugs and the suggestion that these agents be taken with meals are not recommended.

Mechanisms of digestion and absorption of dietary vitamin A

Mechanisms involved in the digestion and absorption of dietary vitamin A require the participation of several proteins. Dietary retinyl esters are hydrolyzed in the intestine by the pancreatic enzyme, pancreatic triglyceride lipase, and intestinal brush border enzyme, phospholipase B. Unesterified retinol taken up by the enterocyte is complexed with cellular retinol-binding protein type 2 and the complex serves as a substrate for reesterification of the retinol by the enzyme lecithin:retinol acyltransferase (LRAT). The retinyl esters are then incorporated into chylomicrons, intestinal lipoproteins containing other dietary lipids, such as triglycerides, phospholipids, and free and esterified cholesterol, and apolipoprotein B. Chylomicrons containing newly absorbed retinyl esters are then secreted into the lymph. Although under normal dietary conditions much of the dietary vitamin A is absorbed via the chylomicron/lymphatic route, it is also clear that under some circumstances there is substantial absorption of unesterified retinol via the portal route. Evidence supports the idea that the cellular uptake and efflux of unesterified retinol by enterocytes is mediated by lipid transporters, but the exact number, identity, and role of these proteins is not known and is an active area of research.

Identification of a Novel Keratinocyte Retinyl Ester Hydrolase as a Transacylase and Lipase

Retinoic acid influences epidermal morphology and function through its ability to control transcription. Because the circulation presents the epidermis with micromolar amounts of retinol that can be converted to retinoic acid, regulating retinol access is imperative. In keratinocytes the majority of retinol is sequestered as long chain fatty acid esters. Although much has been learned about the major esterifying enzyme, little is known about the hydrolase that accesses retinol from its storage depot. Murine carboxylesterases and hormone sensitive lipase have been shown to have this activity. We found that their in vitro sensitivity to bis-p-nitrophenyl phosphate (BNPP), however, was not shared by the epidermal hydrolase activity. We therefore produced and screened two keratinocyte cDNA expression libraries and identified a previously sequenced gene (GS2) as a keratinocyte retinyl ester (RE) hydrolase insensitive to BNPP. The enzyme also catalyzes fattyacyl CoA-dependent and -independent retinol esterification. The hydrolysis reaction is greater at neutral pH, whereas the esterification reaction is greater at acidic pH. These activities are consistent with the increased RE content that accompanies epidermal maturation. In addition, this enzyme utilizes triolein as substrate and generates diacylglyceride and free fatty acid.

Isolation and characterization of a microsomal acid retinyl ester hydrolase

Previous work demonstrated both acid and neutral, bile salt-independent retinyl ester hydrolase activities in rat liver homogenates. Here we present the purification, identification, and characterization of an acid retinyl ester hydrolase activity from solubilized rat liver microsomes. Purification to homogeneity was achieved by sequential chromatography using SP-Sepharose cation exchange, phenyl-Sepharose hydrophobic interaction, concanavalin A-Sepharose affinity and Superose 12 gel filtration chromatography. The isolated protein had a monomer molecular mass of approximately 62 kDa, as measured by mass spectrometry. Gel filtration chromatography of the purified protein revealed a native molecular mass of approximately 176 kDa, indicating that the protein exists as a homotrimeric complex in solution. The purified protein was identified as carboxylesterase ES-10 (EC 3.1.1.1) by N-terminal Edman sequencing and extensive LC-MS/MS sequence analysis and cross-reaction with an anti-ES-10 antibody. Glycosylation analysis revealed that only one of two potential N-linked glycosylation sites is occupied by a high mannose-type carbohydrate structure. Using retinyl palmitate in a micellar assay system the enzyme was active over a broad pH range and displayed Michaelis-Menten kinetics with a K(m) of 86 microm. Substrate specificity studies showed that ES-10 is also able to catalyze hydrolysis of triolein. Cholesteryl oleate was not a substrate for ES-10 under these assay conditions. Real time reverse transcriptase-PCR and Western blot analysis revealed that ES-10 is highly expressed in liver and lung. Lower levels of ES-10 mRNA were also found in kidney, testis, and heart. A comparison of mRNA expression levels in liver demonstrated that ES-10, ES-4, and ES-3 were expressed at significantly higher levels than ES-2, an enzyme previously thought to play a major role in retinyl ester metabolism in liver. Taken together these data indicate that carboxylesterase ES-10 plays a major role in the hydrolysis of newly-endocytosed, chylomicron retinyl esters in both neutral and acidic membrane compartments of liver cells.

Flupenthixol and cefotiam: effects on vitamin A metabolism in rats

We examined the alterations in vitamin A metabolism as a result of flupenthixol or cefotiam administration. The impact of these drugs on indices of vitamin A status was evaluated in Brown Norway and Long-Evans rats. Intramuscular drug administration for 28 d resulted in a decline in systemic retinol. Changes in circulating retinol with time for chronic dosing showed drug treatment (P<0.001) and time (P<0.03) to be significant factors, but rat strain (P=0.33) was not a significant factor. Flupenthixol was the most active retinol-lowering compound (P<0.005). At the end of the 28 d period, hepatic retinyl ester hydrolase activity was greater in drug-treated rats than in controls (P<0.05). With regard to effects on liver reserves: (1) flupenthixol treatment resulted in vitamin A depletion (P<0.05); (2) cefotiam treatment stimulated vitamin A accumulation; (3) distinctive patterns of retinol and its esters were seen in response to treatment. It is reasonable to assume that the drugs interfere with vitamin A in at least two ways: (1) lowering of plasma retinol, an early event in the interaction, may be caused by inhibition of hepatic holo-retinol-binding protein secretion or stimulation of clearance, or both; (2) when plasma retinol levels are persistently low, and as the hepatic deposits of the xenobiotics build up, there are changes in the vitamin A pool size and composition of the liver. Candidate enzymes are retinyl ester hydrolase and cytochrome P450. The relationship between these two events will be studied in further detail.

Effect of a load of Vitamin A after acute thioacetamide intoxication on dolichol, dolichol isoprenoids and retinol content in isolated rat liver cells

This study examines how treatment with a single dose of thioacetamide, a known experimental hepatotoxin, alters the content of dolichol, dolichol isoprene units and retinol in isolated rat parenchymal and non-parenchymal liver cells at different times and when the animals are supplemented with Vitamin A. Thioacetamide (300 mg/kg i.p.) was administered in a single injection to rats, sacrificed at intervals of 0.5, 1, 2, 3, 4, 15 and 30 days. Rats supplemented, following thioacetamide, with Vitamin A, 3 days before sacrifice showed increased mortality and cellular necrosis on the third and fourth days. Parameters indicating tissue necrosis returned to normal values in surviving animals. Dolichol and retinol content showed a variable, reversible decrease, with normal levels being restored in 15-30 days. After Vitamin A, dolichol content only in hepatic stellate cells (HSC) was lower then the controls 3 and 4 days after thioacetamide treatment, in parallel with the decrease of retinol storage. The percentage of dolichol-18 is not modified by thioacetamide alone. When supplemented with Vitamin A the percentage of dolichol-18 always decreased after thioacetamide, showing that damage was still present. Mechanisms that might be operative in liver cells are briefly discussed. This approach would provide an indication to investigate how the length of the dolichol chain is determined.

Penetration and Metabolism of Topical Retinoids in ex vivo Organ-Cultured Full-Thickness Human Skin Explants

The human epidermis contains endogenous retinoids [retinol (vitamin A) and retinyl esters] and carotenoids (mostly beta-carotene). Previous studies in the mouse have shown that the enzymes involved in retinoid metabolism are present in the epidermis. In this study, we wanted to assess the skin penetration and metabolism of topical retinoids in the human. To do this, fresh surgically excised human abdominal skin was mounted on Franz perfusion cells. Topical retinoic acid, retinal, retinol and retinyl palmitate were applied at 2.5 mg/cm(2) in oil-in-water creams containing 0.05% retinoids on the donor compartment, while the receptor compartment was filled with culture medium. The skin was incubated for 24 h at 37 degrees C, then epidermal retinoid concentrations were determined by HPLC. The same experiment was performed with mouse back skin mounted on Franz cells. Finally, topical retinoids were applied on the back of hairless mice for 24 h; then the mice were sacrificed and retinoid concentrations were assayed in the epidermis. In all three models, retinol and its esters were found to be endogenous, as was the case in previous studies in the mouse in vivo. The four applied retinoids penetrated well into the epidermis. Topical retinoic acid did not increase endogenous retinoids, whereas the latter were greatly increased following topical retinal in the mouse. Retinal was also metabolized into retinoic acid, unlike topical retinol and retinyl palmitate, which only increased endogenous retinoids. Topical retinal and retinol did undergo a higher metabolism in both mouse models than in human skin. In summary, the penetration and metabolism patterns of topical retinoids were quite similar in the two mouse models used, indicating that the Franz cells appear to be a good model to predict in vivo metabolism of topical retinoids. When applying this concept to our results obtained in Franz cells with human skin, we conclude that topical retinol and retinal load human skin with both storage and functional vitamin A.

Crystal structure of human carboxylesterase 1 complexed with the Alzheimer's drug tacrine: from binding promiscuity to selective inhibition

Human carboxylesterase 1 (hCE1) is a broad-spectrum bioscavenger that plays important roles in narcotic metabolism, clinical prodrug activation, and the processing of fatty acid and cholesterol derivatives. We determined the 2.4 A crystal structure of hCE1 in complex with tacrine, the first drug approved for treating Alzheimer's disease, and compare this structure to the Torpedo californica acetylcholinesterase (AcChE)-tacrine complex. Tacrine binds in multiple orientations within the catalytic gorge of hCE1, while it stacks in the smaller AcChE active site between aromatic side chains. Our results show that hCE1's promiscuous action on distinct substrates is enhanced by its ability to interact with ligands in multiple orientations at once. Further, we use our structure to identify tacrine derivatives that act as low-micromolar inhibitors of hCE1 and may provide new avenues for treating narcotic abuse and cholesterol-related diseases.

Identification of microsomal rat liver carboxylesterases and their activity with retinyl palmitate

Retinyl esters are a major endogenous storage source of vitamin A in vertebrates and their hydrolysis to retinol is a key step in the regulation of the supply of retinoids to all tissues. Some members of nonspecific carboxylesterase family (EC 3.1.1.1) have been shown to hydrolyze retinyl esters. However, the number of different isoenzymes that are expressed in the liver and their retinyl palmitate hydrolase activity is not known. Six different carboxylesterases were identified and purified from rat liver microsomal extracts. Each isoenzyme was identified by mass spectrometry of its tryptic peptides. In addition to previously characterized rat liver carboxylesterases ES10, ES4, ES3, the protein products for two cloned genes, AB010635 and D50580 (GenBank accession numbers), were also identified. The sixth isoenzyme was a novel carboxylesterase and its complete cDNA was cloned and sequenced (AY034877). Three isoenzymes, ES10, ES4 and ES3, account for more than 95% of rat liver microsomal carboxylesterase activity. They obey Michaelis-Menten kinetics for hydrolysis of retinyl palmitate with Km values of about 1 micro m and specific activities between 3 and 8 nmol.min-1.mg-1 protein. D50580 and AY034877 also hydrolyzed retinyl palmitate. Gene-specific oligonucleotide probing of multiple-tissue Northern blot indicates differential expression in various tissues. Multiple genes are highly expressed in liver and small intestine, important tissues for retinoid metabolism. The level of expression of any one of the six different carboxylesterase isoenzymes will regulate the metabolism of retinyl palmitate in specific rat cells and tissues.

Acyl-CoA: retinol acyltransferase (ARAT) and lecithin:retinol acyltransferase (LRAT) activation during the lipocyte phenotype induction in hepatic stellate cells

We have examined retinol esterification in the established GRX cell line, representative of hepatic stellate cells, and in primary cultures of ex vivo purified murine hepatic stellate cells. The metabolism of [3H]retinol was compared in cells expressing the myofibroblast or the lipocyte phenotype, under the physiological retinol concentrations. Retinyl esters were the major metabolites, whose production was dependent upon both acyl-CoA:retinol acyltransferase (ARAT) and lecithin:retinol acyltransferase (LRAT). Lipocytes had a significantly higher esterification capacity than myofibroblasts. In order to distinguish the intrinsic enzyme activity from modulation of retinol uptake and CRBP-retinol content of the cytosol in the studied cells, we monitored enzyme kinetics in the purified microsomal fraction. We found that both LRAT and ARAT activities were induced during the conversion of myofibroblasts to lipocytes. LRAT induction was dependent upon retinoic acid, while that of ARAT was dependent upon the overall induction of the fat storing phenotype. The fatty acid composition of retinyl-esters suggested a preferential inclusion of exogenous fatty acids into retinyl esters. We conclude that both LRAT and ARAT participate in retinol esterification in hepatic stellate cells: LRAT's activity correlates with the vitamin A status, while ARAT depends upon the availability of fatty acyl-CoA and the overall lipid metabolism in hepatic stellate cells.

Alcohol and retinoids

This article represents the proceedings of a symposium at the 2000 ISBRA Meeting in Yokohama, Japan. The chairs were Hirokazu Yokoyama and David Crabb. The presentations were (1) Roles of vitamin A, retinoic acid, and retinoid receptors in the expression of liver ALDH2, by J. Pinaire, R. Hasanadka, M. Fang, and David W. Crabb; (2) Alcohol, vitamin A, and beta-carotene: Adverse interactions, by M. A. Leo and Charles S. Lieber; (3) Retinoic acid, hepatic stellate cells, and Kupffer cells, by Hidekazu Tsukamoto, K. Motomura, T. Miyahara, and M. Ohata; (4) Retinoid storage and metabolism in liver, by William Bosron, S. Sanghani, and N. Kedishvili; (5) Characterization of oxidation pathway from retinol to retinoic acid in esophageal mucosa, by Haruko Shiraishi, Hirokazu Yokoyama, Michiko Miyagi, and Hiromasa Ishii; and (6) Ethanol in an inhibitor of the cytosolic oxidation of retinol in the liver and the large intestine of rats as well as in the human colon mucosa, by Ina Bergheim, Ina Menzl, Alexandr Parlesak, and Christiane Bode.

Alcohol and retinoids

This article represents the proceedings of a symposium at the 2000 ISBRA Meeting in Yokohama, Japan. The chairs were Hirokazu Yokoyama and David Crabb. The presentations were (1) Roles of vitamin A, retinoic acid, and retinoid receptors in the expression of liver ALDH2, by J. Pinaire, R. Hasanadka, M. Fang, and David W. Crabb; (2) Alcohol, vitamin A, and beta-carotene: Adverse interactions, by M. A. Leo and Charles S. Lieber; (3) Retinoic acid, hepatic stellate cells, and Kupffer cells, by Hidekazu Tsukamoto, K. Motomura, T. Miyahara, and M. Ohata; (4) Retinoid storage and metabolism in liver, by William Bosron, S. Sanghani, and N. Kedishvili; (5) Characterization of oxidation pathway from retinol to retinoic acid in esophageal mucosa, by Haruko Shiraishi, Hirokazu Yokoyama, Michiko Miyagi, and Hiromasa Ishii; and (6) Ethanol in an inhibitor of the cytosolic oxidation of retinol in the liver and the large intestine of rats as well as in the human colon mucosa, by Ina Bergheim, Ina Menzl, Alexandr Parlesak, and Christiane Bode.

Inhibition of purified pig and human liver retinyl ester hydrolase by pharmacologic agents

Identification of inhibitors of retinyl ester hydrolase (REH) would help to elucidate its role in vitamin A metabolism in vivo. By using standard incubation conditions, the effects of 215 drugs as potential inhibitors of purified pig and human liver REH when acting on micellar substrate retinyl palmitate were evaluated at 16.7, 167, and 1670 microM. Out of the compounds tested, 103 were inhibitors of the pig liver enzyme. The most potent compounds, in order of decreasing activity, were chloral hydrate, lovastatin, phytomenadione, alimemazine, physostigmine, thioridazine, phenoxybenzamine, probucol, cinnarizine, cyclandelate, amiodarone, flupenthixol, and naftidrofuryl; this order is roughly similar to that of their inhibition of human liver REH. Of the 10 tricyclic ring-containing drugs tested, alimemazine was the most potent enzyme inhibitor. The concentrations necessary for 50% enzyme inhibition ranged from <2.6 up to >540 microM. Moreover, inhibitory kinetic studies showed that at least two pharmaceuticals, chloral hydrate and amiodarone, are potent REH inhibitors at therapeutically achievable serum concentrations. First-pass metabolites were inactive as REH inhibitors compared to that of the parent compounds, in the cases of chloral hydrate, lovastatin, and cyclandelate.

On-line supercritical fluid extraction/enzymatic hydrolysis of vitamin a esters: a new simplified approach for the determination of vitamins a and e in food

An on-line supercritical fluid extraction (SFE)/enzymatic hydrolysis procedure using immobilized lipase has been developed for the determination of vitamin A in dairy and meat products. Several lipases were tried, of which Novozyme 435 (Candida antarctica type B) showed the highest activity toward retinyl palmitate. There was no observed activity with alpha-tocopheryl acetate. When pressure, temperature, modifiers, flow rate, extraction time, and water content were varied, high vitamin A recovery was obtained in milk powder. Collected extracts were analyzed by reversed-phase high-performance liquid chromatography with ultraviolet and fluorescence detection without additional sample cleanup. The procedure gave reliable values of vitamin A as well as of vitamin E in other food items such as infant formula, minced pork and beef meat, and low- and high-fat liver paste. The described method is faster and more automated than conventional methods based on liquid-liquid extraction, or SFE using off-line saponification, for vitamin A and E determination. Results obtained with the new method did not differ significantly from those obtained with the other two methods mentioned above.

Processing of vitamin A and E in the human gastrointestinal tract

We aimed to provide basic data on the processing of vitamin A and E in the human gastrointestinal tract and to assess whether the size of emulsion fat globules affects the bioavailability of these vitamins. Eight healthy men received intragastrically two lipid formulas differing in their fat-globule median diameter (0.7 vs. 10. 1 microm. Formulas provided 28 mg vitamin A as retinyl palmitate and 440 mg vitamin E as all-rac alpha-tocopherol. Vitamins were measured in gastric and duodenal aspirates, as well as in chylomicrons, during the postprandial period. The gastric emptying rate of lipids and vitamin A and E was similar. The free retinol/total vitamin A ratio was not significantly modified in the stomach, whereas it was dramatically increased in the duodenum. The proportion of ingested lipid and vitamins was very similar in the duodenal content. The chylomicron response of lipids and vitamins was not significantly different between the two emulsions. Our main conclusions are as follows: 1) there is no significant metabolism of vitamin A and E in the human stomach, 2) the enzyme(s) present in the duodenal lumen is significantly involved in the hydrolysis of retinyl esters, and 3) the size of emulsion fat globules has no major effect on the overall absorption of vitamin A and E.

Fatty acid, carotenoid and vitamin A composition of tissues of free living gulls

The aim of this study was to investigate fatty acid and carotenoid profile as well as vitamin A (retinol and retinol esters) content in gull (Larus fucus) tissues. Palmitic (16:0) and stearic (18:0) fatty acids were major saturates in all the tissues studied. Oleic acid (18:1n-9) was the major monounsaturate in the tissue phospholipids varying from 11.9% (liver) up to 18.2% (lung). Arachidonic acid (20:4n-6) was the major unsaturate in the phospholipid fraction in all the tissues. Liver contained the highest total carotenoid concentration which was 5 and 7 fold higher compared to kidney and pancreas. In the liver beta-carotene was major carotenoid. In contrast, in all other tissues beta-carotene was minor fraction with lutein being major carotenoid. Zeaxanthin, canthaxanthin, beta-cryptoxanthin and echinenone were also identified in the gull tissues. Liver and kidney were characterised by the highest vitamin A concentrations (1067.5 and 867.5 microg/g, respectively). Retinol comprised from 55.3% (pancreas) down to 8% (kidney) of the total vitamin A but was not detected in the abdominal fat. Retinyl palmitate was the major retinyl ester in the liver, kidney and heart (44.2; 38.1 and 46.0% of total retinyl esters). In muscles and abdominal fat retinyl stearate was the major retinyl ester fraction. Therefore high proportions of beta-carotene were found in gull liver and peripheral tissues were enriched by lutein and zeaxanthin compared to the liver, a very high concentration of retinyl esters in the kidney was observed and tissue-specificity in retinyl ester proportions in peripheral tissues was found.

Hydrolysis of retinyl esters by pancreatic triglyceride lipase

Previously [van Bennekum, A. M., et al. (1999) Biochemistry 38, 4150-4156] we showed that carboxyl ester lipase (CEL)-deficient (CELKO) mice have normal levels of pancreatic, bile salt-dependent retinyl ester hydrolase (REH) activity. In the present study, we further investigated this non-CEL REH activity in pancreas homogenates of CELKO and wild-type (WT) mice, and rats. REH activity was detected in both the presence and absence of tri- and dihydroxy bile salts in rats, WT mice, and CELKO mice. In contrast, pancreatic cholesteryl ester hydrolase (CEH) activity was only detected in the presence of trihydroxy bile salts and only in rats and WT mice, consistent with CEL-mediated cholesteryl ester hydrolysis. Enzyme assays of pancreatic triglyceride lipase (PTL) showed that there was a colipase-stimulated REH activity in rat and mouse (WT and CELKO) pancreas, consistent with hydrolysis of retinyl ester (RE) by PTL. Pancreatic enzyme activities related to either CEL or PTL were separated using DEAE-chromatography. In both rats and mice (WT and CELKO), REH activity could be attributed mainly to PTL, and to a much smaller extent to CEL. Finally, purified human PTL exhibited similar enzymatic characteristics for triglyceride hydrolysis as well as for retinyl ester hydrolysis, indicating that RE is a substrate for PTL in vivo. Altogether, these studies clearly show that PTL is the major pancreatic REH activity in mice, as well as in rats.

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.

Interactions of retinoid binding proteins and enzymes in retinoid metabolism

Naturally occurring retinoids (vitamin A or retinol and its active metabolites) are vital for vision, controlling the differentiation program of epithelial cells in the digestive tract and respiratory system, skin, bone, the nervous system, the immune system, and for hematopoiesis. Retinoids are essential for growth, reproduction (conception and embryonic development), and resistance to and recovery from infection. The functions of retinoids in the embryo begin soon after conception and continue throughout the lifespan of all vertebrates. Both naturally occurring and synthetic retinoids are used in the therapy of various skin diseases, especially acne, for augmenting the treatment of diabetes, and as cancer chemopreventive agents. Retinol metabolites serve as ligands that activate specific transcription factors in the superfamily of steroid/retinoid/thyroid/vitamin D/orphan receptors and thereby control gene expression. Additionally, retinoids may also function through non-genomic actions. Various retinoid binding proteins serve as partners in retinoid function. These binding proteins show high specificity and affinity for specific retinoids and seem to control retinoid metabolism in vivo qualitatively and quantitatively by reducing 'free' retinoid concentrations, protecting retinoids from non-specific interactions, and chaperoning access of metabolic enzymes to retinoids. Implementation of the physiological effects of retinoids depends on the spatial-temporal expressions of binding proteins, receptors and metabolic enzymes. This review will discuss current understanding of the enzymes that catalyze retinol and retinoic acid metabolism and their unique and integral relationship to retinoid binding proteins.