Intracellular transport of endocytosed chylomicron [3H]retinyl ester in rat liver parenchymal cells. Evidence for translocation of a [3H]retinoid from endosomes to endoplasmic reticulum

Genetically modified mouse models to study hepatic neutral lipid mobilization

Excessive accumulation of triacylglycerol is the common denominator of a wide range of clinical pathologies of liver diseases, termed non-alcoholic fatty liver disease. Such excessive triacylglycerol deposition in the liver is also referred to as hepatic steatosis. Although liver steatosis often resolves over time, it eventually progresses to steatohepatitis, liver fibrosis and cirrhosis, with associated complications, including liver failure, hepatocellular carcinoma and ultimately death of affected individuals. From the disease etiology it is obvious that a tight regulation between lipid uptake, triacylglycerol synthesis, hydrolysis, secretion and fatty acid oxidation is required to prevent triacylglycerol deposition in the liver. In addition to triacylglycerol, also a tight control of other neutral lipid ester classes, i.e. cholesteryl esters and retinyl esters, is crucial for the maintenance of a healthy liver. Excessive cholesteryl ester accumulation is a hallmark of cholesteryl ester storage disease or Wolman disease, which is associated with premature death. The loss of hepatic vitamin A stores (retinyl ester stores of hepatic stellate cells) is incidental to the onset of liver fibrosis. Importantly, this more advanced stage of liver disease usually does not resolve but progresses to life threatening stages, i.e. liver cirrhosis and cancer. Therefore, understanding the enzymes and pathways that mobilize hepatic neutral lipid esters is crucial for the development of strategies and therapies to ameliorate pathophysiological conditions associated with derangements of hepatic neutral lipid ester stores, including liver steatosis, steatohepatitis, and fibrosis. This review highlights the physiological roles of enzymes governing the mobilization of neutral lipid esters at different sites in liver cells, including cytosolic lipid droplets, endoplasmic reticulum, and lysosomes. This article is part of a Special Issue entitled Molecular Basis of Disease: Animal models in liver disease.

Hepatocyte-specific deletion of lysosomal acid lipase leads to cholesteryl ester but not triglyceride or retinyl ester accumulation

Lysosomal acid lipase (LAL) hydrolyzes cholesteryl ester (CE) and retinyl ester (RE) and triglyceride (TG). Mice globally lacking LAL accumulate CE most prominently in the liver. The severity of the CE accumulation phenotype progresses with age and is accompanied by hepatomegaly and hepatic cholesterol crystal deposition. In contrast, hepatic TG accumulation is much less pronounced in these mice, and hepatic RE levels are even decreased. To dissect the functional role of LAL for neutral lipid ester mobilization in the liver, we generated mice specifically lacking LAL in hepatocytes (hep-LAL-ko). On a standard chow diet, hep-LAL-ko mice exhibited increased hepatic CE accumulation but unaltered TG and RE levels. Feeding the hep-LAL-ko mice a vitamin A excess/high-fat diet (VitA/HFD) further increased hepatic cholesterol levels, but hepatic TG and RE levels in these mice were lower than in control mice. Performing in vitro activity assays with lysosome-enriched fractions from livers of mice globally lacking LAL, we detected residual acid hydrolytic activities against TG and RE. Interestingly, this non-LAL acid TG hydrolytic activity was elevated in lysosome-enriched fractions from livers of hep-LAL-ko mice upon VitA/HFD feeding. In conclusion, the neutral lipid ester phenotype in livers from hep-LAL-ko mice indicates that LAL is limiting for CE turnover, but not for TG and RE turnovers. Furthermore, in vitro hydrolase activity assays revealed the existence of non-LAL acid hydrolytic activities for TG and RE. The corresponding acid lipase(s) catalyzing these reactions remains to be identified.

Lysosome-mediated degradation of a distinct pool of lipid droplets during hepatic stellate cell activation

Activation of hepatic stellate cells (HSCs) is a critical step in the development of liver fibrosis. During activation, HSCs lose their lipid droplets (LDs) containing triacylglycerols (TAGs), cholesteryl esters, and retinyl esters (REs). We previously provided evidence for the presence of two distinct LD pools, a preexisting and a dynamic LD pool. Here we investigate the mechanisms of neutral lipid metabolism in the preexisting LD pool. To investigate the involvement of lysosomal degradation of neutral lipids, we studied the effect of lalistat, a specific lysosomal acid lipase (LAL/Lipa) inhibitor on LD degradation in HSCs during activation in vitro The LAL inhibitor increased the levels of TAG, cholesteryl ester, and RE in both rat and mouse HSCs. Lalistat was less potent in inhibiting the degradation of newly synthesized TAG species as compared with a more general lipase inhibitor orlistat. Lalistat also induced the presence of RE-containing LDs in an acidic compartment. However, targeted deletion of the Lipa gene in mice decreased the liver levels of RE, most likely as the result of a gradual disappearance of HSCs in livers of Lipa^-/- mice. Lalistat partially inhibited the induction of activation marker α-smooth muscle actin (α-SMA) in rat and mouse HSCs. Our data suggest that LAL/Lipa is involved in the degradation of a specific preexisting pool of LDs and that inhibition of this pathway attenuates HSC activation.

Hepatic Retinyl Ester Hydrolases and the Mobilization of Retinyl Ester Stores

For mammals, vitamin A (retinol and metabolites) is an essential micronutrient that is required for the maintenance of life. Mammals cannot synthesize vitamin A but have to obtain it from their diet. Resorbed dietary vitamin A is stored in large quantities in the form of retinyl esters (REs) in cytosolic lipid droplets of cells to ensure a constant supply of the body. The largest quantities of REs are stored in the liver, comprising around 80% of the body’s total vitamin A content. These hepatic vitamin A stores are known to be mobilized under times of insufficient dietary vitamin A intake but also under pathological conditions such as chronic alcohol consumption and different forms of liver diseases. The mobilization of REs requires the activity of RE hydrolases. It is astounding that despite their physiological significance little is known about their identities as well as about factors or stimuli which lead to their activation and consequently to the mobilization of hepatic RE stores. In this review, we focus on the recent advances for the understanding of hepatic RE hydrolases and discuss pathological conditions which lead to the mobilization of hepatic RE stores.

Lysosomal Acid Lipase Hydrolyzes Retinyl Ester and Affects Retinoid Turnover

Lysosomal acid lipase (LAL) is essential for the clearance of endocytosed cholesteryl ester and triglyceride-rich chylomicron remnants. Humans and mice with defective or absent LAL activity accumulate large amounts of cholesteryl esters and triglycerides in multiple tissues. Although chylomicrons also contain retinyl esters (REs), a role of LAL in the clearance of endocytosed REs has not been reported. In this study, we found that murine LAL exhibits RE hydrolase activity. Pharmacological inhibition of LAL in the human hepatocyte cell line HepG2, incubated with chylomicrons, led to increased accumulation of REs in endosomal/lysosomal fractions. Furthermore, pharmacological inhibition or genetic ablation of LAL in murine liver largely reduced in vitro acid RE hydrolase activity. Interestingly, LAL-deficient mice exhibited increased RE content in the duodenum and jejunum but decreased RE content in the liver. Furthermore, LAL-deficient mice challenged with RE gavage exhibited largely reduced post-prandial circulating RE content, indicating that LAL is required for efficient nutritional vitamin A availability. In summary, our results indicate that LAL is the major acid RE hydrolase and required for functional retinoid homeostasis.

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.

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.

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.

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.

Detection of retinyl palmitate and retinol in the liver of mice injected with excessive amounts of retinyl acetate

The transport of subcutaneously injected retinyl acetate (RA, 100,000 IU/mouse, 105,470 nM) was investigated in male ICR mice (10-week-old) at 0, 3, 6, 12, 18, 24 and 72 hr after a single injection. The retinol and retinyl palmitate levels of liver homogenates, bile in the gallbladder and serum from peripheral blood were measured by high performance liquid chromatography (HPLC) method. Retinyl palmitate in the lipid droplets of hepatocytes and Ito cells was localized by a modified gold chloride staining method. Accumulation of retinyl palmitate peaked at 12 hr post-injection and decreased thereafter until 24 hr post-injection. Fluorescence microscopy revealed many fluorescent vitamin A-containing lipid droplets in hepatocytes around central veins at 12 hr post-injection, but such droplets were not observed in the vehicle control mice or at in the RA-injected mice after 18 hr of injection. Electron microscopic observation also indicated that many retinyl esters-containing lipid droplets were observed in hepatocytes around the central veins at 12 hr post-injection, but no droplets were seen in the controls or 18 hr post-injection. The retinyl palmitate levels in liver homogenates assessed by HPLC decreased from 12 to 24 hr post-injection and increased significantly in bile, while retinol in liver homogenates and serum markedly increased. One of the morphological alterations was intense vacuolization in hepatocyte cell cords from the portal toward the central vein observed at 24 hr post-injection. Transitional lipid droplets between vacuoles and lipid droplets were identified in those hepatocytes. These results of HPLC analysis of retinol and retinyl palmitate in liver homogenates, serum, and bile, together with the results of gold chloride staining suggested that subcutaneously injected RA was first incorporated in hepatocytes at 12 hr and then partially metabolized through vacuoles, transferred into the blood and secreted into the bile over a 24 hr period. Many retinyl esters-containing lipid droplets were visualized in Ito cells at 72 hr post-injection. Most of vitamin A in the liver homogenates measured by HPLC was retinyl palmitate. Therefore, the contents in those lipid droplets might be retinyl palmitate.

Hydrolysis of retinyl esters in rat liver. Description of a lysosomal activity

When incorporated into liposomes made of phospholipids, retinyl palmitate is an adequate substrate for an acidic REH (aREH). In rat liver, this activity is mainly localized in the lysosomal fraction. Kinetic parameters have been determined for retinyl palmitate (Km = 315 microM; maximal rate = 22.1 nmol retinol/h per mg protein). The aREH activity is different from the lysosomal acidic cholesteryl ester hydrolase (aCEH): cholesteryl oleate does not inhibit aREH activity, neither do some aCEH specific inhibitors, and aREH does not hydrolyse cholesteryl ester. Involvement of aREH in the hydrolysis of lipid droplets retinyl esters in fat storing cells is discussed.

Kinetics of biliary secretion of chylomicron remnant cholesterol (esters) in the rat

Chylomicrons labelled with [3H]cholesterol/[3H]cholesterol esters in a ratio of 25.5: 74.5, were rapidly removed from rat serum in vivo, and taken up predominantly by the parenchymal liver cells (88.2%) of the hepatic uptake at 15 min after injection). Lactoferrin reduced the liver uptake of chylomicron remnants by 72%, at 20 min after injection. It appeared that the free cholesterol which is present in the chylomicrons is not readily exchanged within the used time period with other cholesterol pools in the animal. Between 10-60 min after injection of 3H-labelled chylomicrons, cholesterol esters are hydrolysed in the liver. Appearance of radioactivity in bile was rapid and at 3, 24 and 72 h after injection, 13.4%, 44.0% and 70.0%, respectively, of the injected dose appeared in bile, mainly as bile acids (> 90%). Lactoferrin reduced the biliary secretion of radioactivity, especially during the first hour after injection. The total amount of radioactivity recovered was 58.0% of the injected dose at 72 h after injection. After injection of beta-migrating very low-density lipoprotein labelled with [3H]cholesterol/[3H]cholesterol esters in a ratio of 23.5:76.5, the maximum amount of radioactivity secreted in bile was much lower than with chylomicrons (2.6% cf. 5.2% at 1 h after injection), although the kinetics of the initial liver association and cholesterol ester hydrolysis were even more rapid. Biliary accumulation of radioactivity was also lower with 50.5% of the injected dose recovered at 72 h after injection. It can be concluded from these studies that the processing of chylomicron remnant cholesterol components in the liver and the subsequent secretion in the bile mainly as bile acids is very efficient. The efficient liver uptake of chylomicron remnants by the liver remnant receptor is thereby essential to achieve this high percentage of removal, thus protecting against extrahepatic cholesterol (ester) deposition.

Characterization of the chylomicron-remnant-recognition sites on parenchymal and Kupffer cells of rat liver. Selective inhibition of parenchymal cell recognition by lactoferrin

Upon injection of chylomicrons into rats, chylomicron remnants are predominantly taken up by parenchymal cells, with a limited contribution (8.6% of the injected dose) by Kupffer cells. In vitro storage of partially processed chylomicron remnants for only 24 h leads, after in vivo injection, to an avid recognition by Kupffer cells (uptake up to 80% of the total liver-associated radioactivity). Lactoferrin greatly reduces the liver uptake of chylomicron remnants, which appears to be the consequence of a specific inhibition of the uptake by parenchymal cells. Kupffer-cell uptake is not influenced by lactoferrin. In vitro studies with isolated parenchymal and Kupffer cells show that both contain a specific recognition site for chylomicron remnants. The Kupffer-cell recognition site differs in several ways from the recognition site on parenchymal cells as follows. (a) The maximum level of binding is 3.7-fold higher/mg cell protein than with parenchymal cells. (b) Binding of chylomicron remnants is partially dependent on the presence of calcium, while binding to parenchymal cells is not. (c) beta-Migrating very-low-density lipoprotein is a less effective competitor for chylomicron-remnant binding to Kupffer cells compared to parenchymal cells. (d) Lactoferrin leaves Kupffer-cell binding uninfluenced, while it greatly reduces binding of chylomicron remnants to parenchymal cells. The properties of chylomicron-remnant recognition by parenchymal cells are consistent with apolipoprotein E being the determinant for recognition. It can be concluded that the chylomicron-remnant recognition site on Kupffer cells possesses properties which are distinct from the recognition site on parenchymal cells. It might be suggested that partially processed chylomicron remnants are specifically sensitive to a modification, which induces an avid interaction with the Kupffer cells. The recognition site for (modified) chylomicron remnants on Kupffer cells might function as a protection system against the occurrence of these potential atherogenic chylomicron-remnant particles in the blood.

Transport and storage of vitamin A

The requirement of vitamin A (retinoids) for vision has been recognized for decades. In addition, vitamin A is involved in fetal development and in the regulation of proliferation and differentiation of cells throughout life. This fat-soluble organic compound cannot be synthesized endogenously by humans and thus is an essential nutrient; a well-regulated transport and storage system provides tissues with the correct amounts of retinoids in spite of normal fluctuations in daily vitamin A intake. An overview is presented here of current knowledge and hypotheses about the absorption, transport, storage, and metabolism of vitamin A. Some information is also presented about a group of ligand-dependent transcription factors, the retinoic acid receptors, that apparently mediate many of the extravisual effects of retinoids.

Comparative assessment of the toxicology of vitamin A and retinoids in man

As the title implies, any assessment of the toxic effects of vitamin A derivatives must distinguish between vitamin A in the truest sense, i.e. retinol, and retinoic acid and its synthetic derivatives. Just as no single description is universally applicable to the mode of action of vitamin A derivatives, so too do their toxic effects defy generalization. The recommendation made in 1982 by IUPAC [Eur. J. Biochem., 129 (1989) 1] to designate all derivatives with the typical structure of the vitamin as being retinoids may be chemically logical and correct but, when it comes to describing the effects and side-effects of vitamin A derivatives, it leads to misunderstandings. Retinol, which is frequently used as synonym for vitamin A, can eliminate all symptoms of vitamin A deficiency if it is taken in sufficient quantity with the diet. The term retinol will therefore be used here as a synonym for vitamin A whereas retinoic acid and its derivatives--including the synthetic ones--will be referred to as retinoids because they do not cover the whole spectrum of effects exerted by retinol and because they also vary markedly in their side-effects. In contrast to the nomenclature proposed by IUPAC, this system provides a clear and logical distinction for describing biological processes. Other authors have favoured it in recent times [Chytil, F., J. Am. Acad. Dermatol., 15 (1986) 741; Olson, J.A., Semin. Oncol., x (3) (1983) 290; Olson, J.A., Am. J. Clin. Nutr., 45 (1987) 704; Zbinden, G., Acta Dermatovener., 74 (1975) 36]. By vitamin A, therefore, is meant all derivatives that can possibly originate from retinol in the organism. This also covers the small quantities of retinoic acid formed from retinol. On the other hand, by retinoids is meant the natural retinoic acid derivatives and their synthetic forms in their special modes of action. Since retinoic acid cannot be reduced to retinol in the organism, this nomenclature provides a clear demarcation within the biological system. Vitamin A is essential to the growth and development of higher life forms and functions in many different ways within the organism. Although vitamin A was one of the first vitamins to be described, even today there is still some uncertainty as to its mode of action, with the exception of that of retinal (vitamin A aldehyde) in vision.(ABSTRACT TRUNCATED AT 400 WORDS)

Intracellular transport and degradation of 125I-tyramine cellobiose-labelled low-density lipoprotein endocytosed in vivo in rat liver cells studied by means of subcellular fractionation

The intracellular transport and degradation of in vivo endocytosed 125I-tyramine cellobiose-labelled low density lipoprotein (125I-TC-LDL) in rat liver cells were studied by means of subcellular fractionation in Nycodenz, sucrose and Percoll density gradients, as well as by means of analytical differential centrifugation. Initially, labelled LDL was located in endocytic vesicles of low densities. Subsequently, acid-soluble and acid-precipitable radioactivities were found in organelles with buoyant densities distinctly lower than that of the main peaks of the lysosomal marker enzymes acid phosphatase and N-acetyl-beta-glucosaminidase. These prelysosomal organelles may represent multivesicular bodies (MVBs). Finally, 6 h after injection and onwards, the acid-soluble radioactivity cosegregated completely with the two lysosomal marker enzymes, suggesting that the degradation products were in secondary lysosomes. The rate of intracellular processing of LDL was very slow compared to that of asialoglycoproteins, suggesting that LDL followed a unique intracellular pathway, that may be specific for this type of ligand.

Characteristics of chylomicron remnant uptake into rat liver

We have investigated uptake of 125I-labeled chylomicron remnants into livers of rats in the presence of lactoferrin. This glycoprotein possesses a cluster of four arginines at the N-terminus similar to the arginine rich binding sequence of apoprotein E (apoE) to the LDL-receptor. We found that this protein inhibits uptake of 125I-chylomicron remnant radioactivity by 50% when measured as accumulation of radioactivity into the intact organ, and even more pronounced, over 75%, when measured as uptake into an endosomal fraction prepared therefrom. Provided that the arginine rich sequence is responsible for the inhibition, a similarity in the characteristics of binding of apoE to the LDL (low density lipoprotein)- and chylomicron remnant-receptor is likely. Second, transferrin having sequence homologies with lactoferrin, but lacking the arginine cluster does not interfere with chylomicron remnant uptake. Third, lactoferrin does not inhibit the uptake of chylomicron remnants by the spleen, which is most likely mediated through scavenger cells by a mechanism different from the chylomicron remnant uptake system of the liver. We hypothesize from this that lactoferrin specifically interferes with the physiologically relevant chylomicron remnant uptake system of the liver. Investigation of the mechanism of this inhibition will provide information about the physical characteristics of the remnant receptor system.

Uptake and processing of [3H]retinoids in rat liver studied by electron microscopic autoradiography

The role of rat liver cell organelles in retinoid uptake and processing was studied by electron microscopic autoradiography. [3H]Retinoids were administered either orally, to make an inventory of the cell organelles involved, or intravenously as chylomicron remnant constituents to study retinoid processing by the liver with time. No qualitative differences were observed between the two routes of administration. Time-related changes in the distribution of grains were studied using chylomicron remnant [3H]retinoids. The percentages of grains observed over cells and the space of Disse at 5 and 30 min after administration were, respectively: parenchymal cells, 72.6 and 70.4%; fat-storing cells, 5.0 and 18.1%, and the space of Disse, 14.4 and 8.9%. Low numbers of grains were observed over endothelial and Kupffer cells. The percentages of grains observed over parenchymal cell organelles were, respectively: sinusoidal area, 59.6 and 34.4%; smooth endoplasmic reticulum associated with glycogen, 13.8 and 13.4%; mitochondria, 5.4 and 13.6%; rough endoplasmic reticulum, 4.2 and 7.3%, and rough endoplasmic reticulum associated with mitochondria, 3.7 and 6.5%. It is concluded that chylomicron remnant [3H]retinoids in combination with electron microscopic autoradiography provide a good system to study the liver processing of retinoids in vivo. These results, obtained in the intact liver under physiological conditions, further substantiate that retinoids are processed through parenchymal cells before storage occurs in fat-storing cell lipid droplets, that retinoid uptake is not mediated through lysosomes and that the endoplasmic reticulum is a major organelle in retinoid processing.

Distributions of retinoids, retinoid-binding proteins and related parameters in different types of liver cells isolated from young and old rats

The levels of retinoids, retinol-binding protein, cellular retinol-binding protein, cellular retinoic-acid-binding protein, transthyretin and the activities of retinyl palmitate hydrolase and cholesteryl oleate hydrolase were determined in purified parenchymal, fat-storing, endothelial and Kupffer cell preparations, and in liver homogenates from young adult (6-month-old) and old (36-month-old) rats. Retinoid levels were also determined in the plasma from young and old rats. Retinoid contents were determined by HPLC. The binding proteins and transthyretin were measured by specific radioimmunoassays; retinyl palmitate and cholesterol oleate hydrolases were measured by sensitive microassays. The retinoid content of both the liver homogenates and of the fat-storing, and parenchymal cell preparations increased between 6 months and 36 months of age. The cellular distribution of retinoids was similar for the two age groups analyzed with the fat-storing cells being the main retinoid storage sites in the rat liver. Concentrations of retinol-binding protein and transthyretin were high in parenchymal cell preparations. Cellular retinol-binding protein was enriched both in parenchymal and in fat-storing cell preparations; the highest concentrations of cellular retinoic-acid-binding protein were present in fat-storing cell preparations. No major differences were observed between the two age groups in the cellular concentrations and distributions of any of these binding proteins. High activity of cholesterol oleate hydrolase was measured in parenchymal and in Kupffer cell preparations; endothelial cell preparations also contained considerable activities. The distribution of this activity over the various cell types reflects their role in lipoprotein metabolism. Retinyl palmitate hydrolase activity was specifically enriched in parenchymal and in fat-storing cell preparations, consistent with the roles of these cells in retinoid metabolism. No major differences were observed between the two age groups in the cellular distributions of the two hydrolase activities. This study indicates that no major changes occur in the retinoid-related parameters analyzed with age, suggesting that rat liver retinoid metabolism does not change dramatically with age and that retinoid homeostasis is maintained.

Biochemical characteristics of isolated rat liver stellate cells

Hepatic stellate cells play a quantitatively important role in hepatic retinoid metabolism and storage in rats maintained under normal nutritional conditions. Studies were conducted to further explore the biochemical characteristics of hepatic stellate cells. Stellate cells were isolated in high purity and yield from the livers of normal rats. The isolated cells had the morphology expected (on electron micrographs) for stellate cells, and were enriched in retinoids and in the intracellular retinoid-binding proteins. The composition of the stellate cell lipid droplets was examined. These lipid droplets were isolated in high purity and integrity from frozen and thawed stellate cell preparations by differential centrifugation. We estimate that the lipid composition of stellate cell lipid droplets consisted of approximately 42% retinyl ester, 28% triglyceride, 13% cholesterol (total) and 4% phospholipid. Thus, stellate cell lipid droplets contain substantial levels of both cholesterol and triglyceride, in addition to retinyl esters. Stellate cell homogenates were assayed for both retinol-binding protein and transthyretin by specific radioimmunoassays. Within the detection limits of these radioimmunoassays, we were unable to detect the presence of either retinol-binding protein (less than 9 ng per 10(6) cells) or transthyretin (less than 11 ng per 10(6) cells) in the stellate cell preparations. Total RNA, prepared from the isolated stellate cells, was examined by Northern blot analysis for retinol-binding protein mRNA and transthyretin mRNA, using cDNA probes for retinol-binding protein and transthyretin. Within the sensitivity of these assays, retinol-binding protein mRNA and transthyretin mRNA were not detected in stellate cells. These findings suggest that stellate cells do not synthesize or accumulate retinol-binding protein.

Intracellular transport and degradation of chylomicron remnants in rat liver cells after in vivo endocytosis

The intracellular transport and degradation of in vivo endocytosed chylomicron remnants labelled with 125I in the protein moiety was studied in rat liver cells by means of subcellular fractionation in Nycodenz and sucrose density gradients. Initially, the radioactivity was located in low-density endosomes and was sequentially transferred to light and dense lysosomes. Data from gel filtration of the light and dense lysosomal fractions showed radioactive material with a molecular weight of about 1000-2000, representing short peptide fragments or amino acids which remain attached to iodinated tyramine cellobiose. In addition, undegraded apoproteins accumulated in both types of lysosome. Our data suggest that endocytosed chylomicron remnant apoproteins are first located in low-density endosomes and are sequentially transferred to light and dense lysosomes. Furthermore, the degradation process starts in the light lysosomes.

Studies on the in vivo transfer of retinoids from parenchymal to stellate cells in rat liver

Studies were conducted to examine the in vivo transfer of chylomicron (dietary) retinoid from rat liver parenchymal to stellate cells. We specifically addressed the question of whether chylomicron retinyl ester is transferred directly from hepatic parenchymal to stellate cells without first undergoing hydrolysis. [14C]Retinyl palmitate and its non-hydrolyzable ether analog, retinyl [3H]hexadecyl ether, were utilized to answer this question. Chylomicrons labeled with these retinoids were injected intravenously into rats. Liver cell fractions, highly enriched in parenchymal or in stellate cells, were isolated 0.5 h, 4.5 h and 24 h after chylomicron injection. The ratio of 3H: 14C found in parenchymal cell preparations 4.5 h after injection was 1.8 times the ratio for the injected chylomicrons, and 24 h postinjection the ratio had increased to 2.5 times that of the chylomicrons. In the stellate-cell-enriched preparations the 3H: 14C ratio was found to be 0.39, 0.29, and 0.23 times the ratio found in the injected labeled chylomicrons at 0.5 h, 4.5 h and 24 h after injection respectively. From the levels of 14C observed in the isolated stellate cells, it is estimated that 0.5 h postinjection the stellate cells contained approximately 34% of the 14C (i.e. the retinol injected as chylomicron retinyl ester) present in the liver. By 4.5 h the 14C present in isolated stellate cells had risen to approximately 41% of that present in the total liver, and 24 h after injection approximately 55% of hepatic total 14C was found in the stellate cells. These findings suggest that chylomicron retinyl ester is not transferred directly from the parenchymal to stellate cells without first undergoing hydrolysis to retinol.

Liver levels of vitamin A and cellular retinol-binding protein for patients with biliary atresia

We have examined whether the amount of cellular retinol-binding protein in human liver is related to the amount of vitamin A stored in the liver. Levels of vitamin A, as retinol and retinol esters, and of cellular retinol-binding protein have been determined in liver samples from 6 normal adults and 11 children with biliary atresia, with and without vitamin A treatment. The level of cellular retinol-binding protein in the liver was not related to the liver vitamin A concentration examined over a 300-fold range of vitamin A levels. Also, biliary atresia did not appear to interfere with storage of vitamin A, and the level of cellular retinol-binding protein was comparable to that observed in the liver of normal adults. The demonstration of proper vitamin A storage in treated children as well as normal levels of cellular retinol-binding protein suggest the vitamin A deficiency frequently observed in children with biliary atresia may be due primarily to faulty absorption rather than a combination of poor absorption and impaired hepatic vitamin A metabolism.