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

The Role of Vitamin A in Wound Healing

Vitamin A is an essential micronutrient that comes in multiple forms, including retinols, retinals, and retinoic acids. Dietary vitamin A is absorbed as retinol from preformed retinoids or as pro-vitamin A carotenoids that are converted into retinol in the enterocyte. These are then delivered to the liver for storage via chylomicrons and later released into the circulation and to its biologically active tissues bound to retinol-binding protein. Vitamin A is a crucial component of many important and diverse biological functions, including reproduction, embryological development, cellular differentiation, growth, immunity, and vision. Vitamin A functions mostly through nuclear retinoic acid receptors, retinoid X receptors, and peroxisome proliferator-activated receptors. Retinoids regulate the growth and differentiation of many cell types within skin, and its deficiency leads to abnormal epithelial keratinization. In wounded tissue, vitamin A stimulates epidermal turnover, increases the rate of re-epithelialization, and restores epithelial structure. Retinoids have the unique ability to reverse the inhibitory effects of anti-inflammatory steroids on wound healing. In addition to its role in the inflammatory phase of wound healing, retinoic acid has been demonstrated to enhance production of extracellular matrix components such as collagen type I and fibronectin, increase proliferation of keratinocytes and fibroblasts, and decrease levels of degrading matrix metalloproteinases.

The stellate cell system (vitamin A-storing cell system)

Past, present, and future research into hepatic stellate cells (HSCs, also called vitamin A-storing cells, lipocytes, interstitial cells, fat-storing cells, or Ito cells) are summarized and discussed in this review. Kupffer discovered black-stained cells in the liver using the gold chloride method and named them stellate cells (Sternzellen in German) in 1876. Wake rediscovered the cells in 1971 using the same gold chloride method and various modern histological techniques including electron microscopy. Between their discovery and rediscovery, HSCs disappeared from the research history. Their identification, the establishment of cell isolation and culture methods, and the development of cellular and molecular biological techniques promoted HSC research after their rediscovery. In mammals, HSCs exist in the space between liver parenchymal cells (PCs) or hepatocytes and liver sinusoidal endothelial cells (LSECs) of the hepatic lobule, and store 50-80% of all vitamin A in the body as retinyl ester in lipid droplets in the cytoplasm. SCs also exist in extrahepatic organs such as pancreas, lung, and kidney. Hepatic (HSCs) and extrahepatic stellate cells (EHSCs) form the stellate cell (SC) system or SC family; the main storage site of vitamin A in the body is HSCs in the liver. In pathological conditions such as liver fibrosis, HSCs lose vitamin A, and synthesize a large amount of extracellular matrix (ECM) components including collagen, proteoglycan, glycosaminoglycan, and adhesive glycoproteins. The morphology of these cells also changes from the star-shaped HSCs to that of fibroblasts or myofibroblasts.

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.

Vitamin A Absorption, Storage and Mobilization

It is well established that chylomicron remnant (dietary) vitamin A is taken up from the circulation by hepatocytes, but more than 80 % of the vitamin A in the liver is stored in hepatic stellate cells (HSC). It presently is not known how vitamin A is transferred from hepatocytes to HSCs for storage. Since retinol-binding protein 4 (RBP4), a protein that is required for mobilizing stored vitamin A, is synthesized solely by hepatocytes and not HSCs, it similarly is not known how vitamin A is transferred from HSCs to hepatocytes. Although it has long been thought that RBP4 is absolutely essential for delivering vitamin A to tissues, recent research has proven that this notion is incorrect since total RBP4-deficiency is not lethal. In addition to RBP4, vitamin A is also found in the circulation bound to lipoproteins and as retinoic acid bound to albumin. It is not known how these different circulating pools of vitamin A contribute to the vitamin A needs of different tissues. In our view, better insight into these three issues is required to better understand vitamin A absorption, storage and mobilization. Here, we provide an up to date synthesis of current knowledge regarding the intestinal uptake of dietary vitamin A, the storage of vitamin A within the liver, and the mobilization of hepatic vitamin A stores, and summarize areas where our understanding of these processes is incomplete.

DGAT1-deficiency affects the cellular distribution of hepatic retinoid and attenuates the progression of CCl4-induced liver fibrosis

BACKGROUND

Diacylglycerol O-acyltransferase 1 (DGAT1) catalyzes the final step of triglyceride synthesis, transferring an acyl group from acyl-CoA to diacylglycerol. DGAT1 also catalyzes the acyl-CoA-dependent formation of retinyl esters in vitro and in mouse intestine and skin. Although DGAT1 is expressed in both hepatocytes and hepatic stellate cells (HSCs), we reported genetic and nutritional studies that established that DGAT1 does not contribute to retinyl ester formation in the liver.

METHODS

We now have explored in more depth the role(s) of DGAT1 in hepatic retinoid metabolism and storage.

RESULTS

Our data show that DGAT1 affects the cellular distribution between hepatocytes and HSCs of stored and newly absorbed dietary retinol. For livers of Dgat1-deficient mice, a greater percentage of stored retinyl ester is present in HSCs at the expense of hepatocytes. This is also true for newly absorbed oral [(3)H]retinol. These differences are associated with significantly increased expression, by 2.8-fold, of cellular retinol-binding protein, type I (RBP1) in freshly isolated HSCs from Dgat1-deficient mice, raising the possibility that RBP1, which contributes to retinol uptake into cells and retinyl ester synthesis, accounts for the differences. We further show that the retinyl ester-containing lipid droplets in HSCs are affected in Dgat1-null mice, being fewer in number but, on average, larger than in wild type (WT) HSCs. Finally, we demonstrate that DGAT1 affects experimentally induced HSC activation in vivo but that this effect is independent of altered retinoic acid availability or effects on gene expression.

CONCLUSIONS

Our studies establish that DGAT1 has a role in hepatic retinoid storage and metabolism, but this does not involve direct actions of DGAT1 in retinyl ester synthesis.

Hepatic metabolism of retinoids and disease associations

The liver is the most important tissue site in the body for uptake of postprandial retinoid, as well as for retinoid storage. Within the liver, both hepatocytes and hepatic stellate cells (HSCs) are importantly involved in retinoid metabolism. Hepatocytes play an indispensable role in uptake and processing of dietary retinoid into the liver, and in synthesis and secretion of retinol-binding protein (RBP), which is required for mobilizing hepatic retinoid stores. HSCs are the central cellular site for retinoid storage in the healthy animal, accounting for as much as 50-60% of the total retinoid present in the entire body. The liver is also an important target organ for retinoid actions. Retinoic acid is synthesized in the liver and can interact with retinoid receptors which control expression of a large number of genes involved in hepatic processes. Altered retinoid metabolism and the accompanying dysregulation of retinoid signaling in the liver contribute to hepatic disease. This is related to HSCs, which contribute significantly to the development of hepatic disease when they undergo a process of cellular activation. HSC activation results in the loss of HSC retinoid stores and changes in extracellular matrix deposition leading to the onset of liver fibrosis. An association between hepatic disease progression and decreased hepatic retinoid storage has been demonstrated. In this review article, we summarize the essential role of the liver in retinoid metabolism and consider briefly associations between hepatic retinoid metabolism and disease. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

Distinct populations of hepatic stellate cells in the mouse liver have different capacities for retinoid and lipid storage

Hepatic stellate cell (HSC) lipid droplets are specialized organelles for the storage of retinoid, accounting for 50–60% of all retinoid present in the body. When HSCs activate, retinyl ester levels progressively decrease and the lipid droplets are lost. The objective of this study was to determine if the HSC population in a healthy, uninjured liver demonstrates heterogeneity in its capacity for retinoid and lipid storage in lipid droplets. To this end, we utilized two methods of HSC isolation, which leverage distinct properties of these cells, including their vitamin A content and collagen expression. HSCs were isolated either from wild type (WT) mice in the C57BL/6 genetic background by flotation in a Nycodenz density gradient, followed by fluorescence activated cell sorting (FACS) based on vitamin A autofluorescence, or from collagen-green fluorescent protein (GFP) mice by FACS based on GFP expression from a GFP transgene driven by the collagen I promoter. We show that GFP-HSCs have: (i) increased expression of typical markers of HSC activation; (ii) decreased retinyl ester levels, accompanied by reduced expression of the enzyme needed for hepatic retinyl ester synthesis (LRAT); (iii) decreased triglyceride levels; (iv) increased expression of genes associated with lipid catabolism; and (v) an increase in expression of the retinoid-catabolizing cytochrome, CYP2S1. Conclusion: Our observations suggest that the HSC population in a healthy, uninjured liver is heterogeneous. One subset of the total HSC population, which expresses early markers of HSC activation, may be “primed” and ready for rapid response to acute liver injury.

Hepatic stellate cell (vitamin A-storing cell) and its relative--past, present and future

HSCs (hepatic stellate cells) (also called vitamin A-storing cells, lipocytes, interstitial cells, fat-storing cells or Ito cells) exist in the space between parenchymal cells and liver sinusoidal endothelial cells of the hepatic lobule and store 50-80% of vitamin A in the whole body as retinyl palmitate in lipid droplets in the cytoplasm. In physiological conditions, these cells play pivotal roles in the regulation of vitamin A homoeostasis. In pathological conditions, such as hepatic fibrosis or liver cirrhosis, HSCs lose vitamin A and synthesize a large amount of extracellular matrix components including collagen, proteoglycan, glycosaminoglycan and adhesive glycoproteins. Morphology of these cells also changes from the star-shaped SCs (stellate cells) to that of fibroblasts or myofibroblasts. The hepatic SCs are now considered to be targets of therapy of hepatic fibrosis or liver cirrhosis. HSCs are activated by adhering to the parenchymal cells and lose stored vitamin A during hepatic regeneration. Vitamin A-storing cells exist in extrahepatic organs such as the pancreas, lungs, kidneys and intestines. Vitamin A-storing cells in the liver and extrahepatic organs form a cellular system. The research of the vitamin A-storing cells has developed and expanded vigorously. The past, present and future of the research of the vitamin A-storing cells (SCs) will be summarized and discussed in this review.

Hepatic stellate cells are an important cellular site for β-carotene conversion to retinoid

Hepatic stellate cells (HSCs) are responsible for storing 90-95% of the retinoid present in the liver. These cells have been reported in the literature also to accumulate dietary β-carotene, but the ability of HSCs to metabolize β-carotene in situ has not been explored. To gain understanding of this, we investigated whether β-carotene-15,15'-monooxygenase (Bcmo1) and β-carotene-9',10'-monooxygenase (Bcmo2) are expressed in HSCs. Using primary HSCs and hepatocytes purified from wild type and Bcmo1-deficient mice, we establish that Bcmo1 is highly expressed in HSCs; whereas Bcmo2 is expressed primarily in hepatocytes. We also confirmed that HSCs are an important cellular site within the liver for accumulation of dietary β-carotene. Bcmo2 expression was found to be significantly elevated for livers and hepatocytes isolated from Bcmo1-deficient compared to wild type mice. This elevation in Bcmo2 expression was accompanied by a statistically significant increase in hepatic apo-12'-carotenal levels of Bcmo1-deficient mice. Although apo-10'-carotenal, like apo-12'-carotenal, was readily detectable in livers and serum from both wild type and Bcmo1-deficient mice, we were unable to detect either apo-8'- or apo-14'-carotenals in livers or serum from the two strains. We further observed that hepatic triglyceride levels were significantly elevated in livers of Bcmo1-deficient mice fed a β-carotene-containing diet compared to mice receiving no β-carotene. Collectively, our data establish that HSCs are an important cellular site for β-carotene accumulation and metabolism within the liver.

Expression of carboxylesterase and lipase genes in rat liver cell-types

Approximately 80% of the body vitamin A is stored in liver stellate cells with in the lipid droplets as retinyl esters. In low vitamin A status or after liver injury, stellate cells are depleted of the stored retinyl esters by their hydrolysis to retinol. However, the identity of retinyl ester hydrolase(s) expressed in stellate cells is unknown. The expression of carboxylesterase and lipase genes in purified liver cell-types was investigated by real-time PCR. We found that six carboxylesterase and hepatic lipase genes were expressed in hepatocytes. Adipose triglyceride lipase was expressed in Kupffer cells, stellate cells and endothelial cells. Lipoprotein lipase expression was detected in Kupffer cells and stellate cells. As a function of stellate cell activation, expression of adipose triglyceride lipase decreased by twofold and lipoprotein lipase increased by 32-fold suggesting that it may play a role in retinol ester hydrolysis during stellate cell activation.

Identification of the retinol-binding protein (RBP) interaction site and functional state of RBPs for the membrane receptor

This laboratory has advanced a model whereby retinol is transported around the body bound to retinol-binding protein (RBP), is transferred across the membrane of cells by a specific receptor/transporter, and is picked up from the membrane by an intracellular homolog, cellular retinol-binding protein (CRBP). This process involves a number of protein-protein interactions, and we hypothesized that conformational changes were an integral part of the retinol transfer mechanism. Previously we identified the potential interaction site on RBP for its membrane receptor. Here we confirm by the analysis of chimera containing a grafted CD loop from RBP that this is indeed the receptor interaction site and go on to demonstrate that the conformational changes that occur to this region on the apo to holo transition in RBP also take place in a chimera binding a quite different ligand, thus establishing the concept. We have also gone on to support the hypothesis that CRBP may also bind to a receptor in the membrane. Previous evidence has indicated that one such receptor might be lecithin:retinol acyltransferase, an enzyme that catalyzes retinol esterification. Here we provide the first evidence that the plasma membrane receptor for RBP could be the same as that for CRBP. This observation offers support for the intracellular phase of the uptake process for retinol, providing an efficient and highly unique mechanism in eukaryotic biology.

Vitamin A physiology and its application as a biomarker of contaminant-related toxicity in marine mammals: a review

In recent decades, marine mammal populations living in highly polluted areas have experienced incidences of low reproductive success, developmental abnormalities and disease outbreaks. In many of these cases, environmental contaminants were suspected as causal or contributing factors. However, demonstrating a mechanistic link between contaminant exposure and effect in marine mammal populations has proven challenging. Consequently, the development and application of relatively noninvasive biomarkers represents a potentially valuable means of monitoring wildlife populations exposed to elevated levels of contaminants. One touted biomarker is vitamin A (retinol), a "dietary hormone" whose metabolites are required for reproduction, growth, development, immune function, vision and epithelial maintenance. Laboratory studies have shown that many contaminants, including polychlorinated biphenyls (PCBs), polychlorinated dibenzo-para-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), can disrupt vitamin A physiology and alter the distribution of its essential metabolites. Field studies suggest that complex environmental mixtures of these chemicals can also interfere with vitamin A dynamics in free-ranging marine mammals and other fish-eating wildlife. However, circulatory retinol, which is the least invasive measurement of vitamin A status, appears to have variable responses to contaminant exposure. In addition, "normal" circulatory retinol levels have not yet been described for most wildlife species, and not enough is known about the natural physiological events that can alter these concentrations. Confounding factors must therefore be characterized before retinoids can be used as an effective indicator of adverse health effects in marine mammals exposed to elevated levels of environmental contaminants.

Intestinal absorption of dietary cholesteryl ester is decreased but retinyl ester absorption is normal in carboxyl ester lipase knockout mice

Carboxyl ester lipase (CEL; EC 3.1.1.13) hydrolyzes cholesteryl esters and retinyl esters in vitro. In vivo, pancreatic CEL is thought to liberate cholesterol and retinol from their esters prior to absorption in the intestine. CEL is also a major lipase in the breast milk of many mammals, including humans and mice, and is thought to participate in the processing of triglycerides to provide energy for growth and development while the pancreas of the neonate matures. Other suggested roles for CEL include the direct facilitation of the intestinal absorption of free cholesterol and the modification of plasma lipoproteins. Mice with different CEL genotypes [wild type (WT), knockout (CELKO), heterozygote] were generated to study the functions of CEL in a physiological system. Mice grew and developed normally, independent of the CEL genotype of the pup or nursing mother. Consistent with this was the normal absorption of triglyceride in CELKO mice. The absorption of free cholesterol was also not significantly different between CELKO (87 +/- 26%, mean +/- SD) and WT littermates (76 +/- 10%). Compared to WT mice, however, CELKO mice absorbed only about 50% of the cholesterol provided as cholesteryl ester (CE). There was no evidence for the direct intestinal uptake of CE or for intestinal bacterial enzymes that hydrolyze it, suggesting that another enzyme besides CEL can hydrolyze dietary CE in mice. Surprisingly, CELKO and WT mice absorbed similar amounts of retinol provided as retinyl ester (RE). RE hydrolysis, however, was required for absorption, implying that CEL was not the responsible enzyme. The changes in plasma lipid and lipoprotein levels to diets with increasing lipid content were similar in mice of all three CEL genotypes. Overall, the data indicate that in the mouse, other enzymes besides CEL participate in the hydrolysis of dietary cholesteryl esters, retinyl esters, and triglycerides.

Transfer of retinol-binding protein from HepG2 human hepatoma cells to cocultured rat stellate cells

Rat liver stellate cells were cocultured with HepG2 human hepatoma cells, which are known to synthesize and secrete retinol-binding protein (RBP). Transfer of human RBP from HepG2 cells to stellate cells was studied by cryoimmunoelectron microscopy. In stellate cells, human RBP was found on the cell surface and within endosomes. The transfer of human RBP from HepG2 cells to stellate cells was blocked by addition of RBP antibodies to the culture medium. Very little uptake of RBP was observed when fibroblasts were cocultured with HepG2 cells. In a series of experiments, RBP was bound to its putative cell surface receptor at 4 degrees C, and the stellate cells were washed and then incubated at 37 degrees C in order to allow them to internalize a pulse of RBP. About 50% of the RBP was internalized after 6 min of incubation. The RBP-positive vesicles were initially (after 1-2 min) located close to the cell surface and later were found deeper in the cytoplasm. During the first 10 min, RBP was mainly observed in close association with membranes. After 2 hr, however, most RBP was localized in intracellular vesicles at a distance from the vesicular membranes, suggesting that RBP had been released from its receptor. Saturable binding of RBP to liver cells was demonstrated when cells were incubated with 125I-RBP at 4 degrees C and cell-associated radioactivity was determined. The calculated dissociation constant for the specific binding was 12.7 +/- 3.2 nM. A binding assay was also developed for determination of solubilized RBP receptor. Solubilized proteins from the nonparenchymal liver cells bound about 30 times more 125I-labeled RBP than did parenchymal cells (based on mass of cell protein). These data suggest that RBP mediates the paracrine transfer of retinol from hepatocytes to perisinusoidal stellate cells in liver and that stellate cells bind and internalize RBP by receptor-mediated endocytosis.

Reduced retinoid content in hepatocellular carcinoma with special reference to alcohol consumption

Although alcohol is known to enhance hepatocarcinogenesis, the mechanism of this action remains to be explained. To test the hypothesis that ethanol depletes the liver of antitumor promoters such as retinoid, we measured the retinoid concentration in hepatocellular carcinoma tissues and noncancerous surrounding liver tissues in humans known to have a history of alcohol consumption. By high-performance liquid chromatography, the retinoid contents of 29 surgically resected hepatocellular carcinoma specimens and their noncancerous surrounding tissues were measured. Retinoid contents were decreased in both the cancerous and the surrounding noncancerous liver tissues of patients with a high intake of alcohol. The levels correlated inversely with the estimated cumulative lifetime ethanol consumption. The decrease in the retinoid content of hepatic parenchymal cells paralleled that in stellate cells. When compared with the surrounding liver tissues, the cancerous liver tissues were in the state of retinoid deficiency. In summary, alcohol abuse may help promote the hepatocarcinogenesis in man by depleting the liver of the antitumor promoter, retinoid.

Distribution of lecithin-retinol acyltransferase activity in different types of rat liver cells and subcellular fractions

It is now well documented that lecithin-retinol acyltransferase (LRAT) is the physiologically important enzyme activity involved in the esterification of retinol in the liver. However, no information regarding the cellular distribution of this enzyme in the liver is presently available. This study characterizes the distribution of LRAT activity in the different types of rat liver cells. Purified preparations of isolated parenchymal, fat-storing, and Kupffer + endothelial cells were isolated from rat livers and the LRAT activity present in microsomes prepared from each of these cell fractions was determined. The fat-storing cells were found to contain the highest level of LRAT specific activity (383 +/- 54 pmol retinyl ester formed min-1.mg-1 versus 163 +/- 22 pmol retinyl ester formed min-1.mg-1 for whole liver microsomes). The level of LRAT specific activity in parenchymal cell microsomes (158 +/- 53 pmol retinyl ester formed min-1.mg-1) was very similar to LRAT levels in whole liver microsomes. The Kuppfer + endothelial cell microsome fractions were found to contain LRAT, at low levels of activity. These results indicate that the fat-storing cells are very enriched in LRAT but the parenchymal cells also posses significant levels of LRAT activity.

Inhibition in fat-storing cell multiplication by a factor produced by normal cultured murine hepatocytes

In this study we demonstrate that hepatocytes isolated from normal mice may efficiently inhibit the multiplication of fat-storing cells (FSC) in culture, either in a coculture system where both cell types are separated by a filter of 0.45 microns pore size or via their conditioned medium. The inhibition may be completely reversed when the hepatocytes are removed and the culture medium is renewed. The inhibitory factor appears as early as 8 h in the medium with an almost maximum effect being reached after 24 h, as long as protein synthesis is allowed. It rapidly loses its efficiency through dilution. The inhibitory capacity of the conditioned medium is maintained after heating at 56 degrees C, dialysis of 100,000 x g centrifugation, but reduced after trypsin treatment. The infection of the hepatocytes by ectromelia virus causes an almost total suppression of the synthesis of the inhibitory factor. This latter result suggests that the multiplication of FSC, which may be inhibited by normal hepatocytes, would no longer be hindered in case of disregulation.

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.

Production of leukotriene B4 in parenchymal and sinusoidal cells of the liver in rats treated simultaneously with D-galactosamine and endotoxin

A study was conducted to investigate production rate of leukotriene B4 (LTB4) in parenchymal and sinusoidal liver cells of rats with acute hepatic failure (AHF). AHF was induced by simultaneous administration of D-galactosamine (GalN) and endotoxin (LPS), and parenchymal as well as sinusoidal liver cells were isolated by collagenase perfusion method. Following preincubation for 15 min, isolated cellular fractions were incubated with Ca-ionophore (2 microM) for 5 min, and levels of LTB4 in culture media before and 5 min after addition of Ca-ionophore were analyzed by HPLC. Following results were obtained: The production rate of LTB4 was found to be the highest in Kupffer cells (7.2ng/10(6) cells/5 min), followed by endothelial cells (1.1), stellate cells (0.2) and parenchymal cells (not detectable). The production rate of LTB4 in both Kupffer cells and endothelial cells was found to reach a maximum in the fraction isolated 60 min after administration of GalN and LPS. Treatment with AA861, one of the selective inhibitors of 5-lipoxygenase, was shown to reduce the production of LTB4 in Kupffer cells to 53% at 10(-7)M and above 99% at higher than 10(-5)M. In conclusion, the majority of LTB4 generated in the liver of rats with AHF was found to be synthesized in Kupffer cells and, to a lesser extent, in endothelial cells, and the enhanced production of LTB4 was found to be greatly inhibited by treatment with AA861.

Retinoid-mediated induction of the fat-storing phenotype in a liver connective tissue cell line (GRX)

The GRX cell line is derived from murine liver connective tissue cells. It has myofibroblastic characteristics and can be induced to display a phenotype analogous to fat-storing (Ito) cells. Retinol-mediated induction of the fat-storing phenotype was studied in vitro. Based on the incorporation of radiolabelled acetate into cell lipids, cholesterol synthesis increased and phospholipid synthesis was modified shortly after the beginning of the induction, indicating an activation of pre-existing metabolic pathways. Triacylglycerol synthesis was increased only after a delay of 4 d, indicating the de novo induction of enzymes necessary for triacylglycerol metabolism. Retinol incorporation and conversion into retinyl esters were also considerably increased by previous incubation with retinoids. Retinoid-induced changes in GRX cells provide a model for studying in vitro the interconversion of liver connective tissue cells between the myofibroblastic and fat-storing phenotypes. This interconversion is considered to be one of the major control points of normal homeostasis and of pathological modifications of liver connective tissue.

Isolation, culture and main characteristics of mouse fat-storing cells: interaction with viruses

Fat-storing cells were isolated from 15-day-old mouse sinusoidal cell cultures (Kupffer or endothelial cells), where they had multiplied abundantly; they were then purified by a negative selection method based on the fact that they do not possess Fc receptors, as do both other types of cells. The fat-storing cells, which could be subcultured for at least 10 passages, have the main morphological characteristics already described in vivo, in particular, the rough endoplasmic reticulum and the lipid droplets, which become less and less apparent as the number of passages increases. Subcultured fat-storing cells, almost devoid of lipid droplets and vitamin A, were able to take up retinol, as the appearance of a typical autofluorescence indicated; the number of lipid droplets increased concomitantly. Furthermore, the cultured fat-storing cells were able to internalize one-micron-sized latex beads by phagocytosis. Infection of fat-storing cells with mouse hepatitis virus 3, ectromelia or Sindbis virus led to multiplication of the virus particles. There was a direct relation between the multiplication of mouse hepatitis virus 3 in cultured fat-storing cells and the susceptibility of the animals to the virus. In the case of Sindbis virus, interferon is produced, its production being independent of the presence of vitamin A.

Regulation of hyaluronate synthesis in rat liver fat storing cell cultures by Kupffer cells

During fibrogenesis in chronically inflamed liver the concentration of extracellular matrix hyaluronate increases several-fold, but the mechanism of hyaluronate accumulation has hitherto been unknown. We studied the effect of stimulated Kupffer cells on the synthesis and secretion of hyaluronic acid by rat liver fat storing cells, the main hyaluronate-producing cell type in liver. Conditioned medium was harvested from monolayers of Kupffer cells activated by exposure for 24 h to zymosan and lipopolysaccharide. Addition of these Kupffer cell media to monolayers of fat storing cells stimulated more than 2-fold the incorporation of [3H]glucosamine into both total glycosaminoglycans and hyaluronic acid in the medium. The synthesis rate of hyaluronic acid was enhanced more strongly than that of sulfated glycosaminoglycans, resulting in a significant fractional increase of hyaluronate. The concentration of hyaluronate measured with a radiometric assay in the medium of fat storing cells exposed to Kupffer cell media was raised 2.6-fold within 24 h in comparison to untreated cultures. The synthesis rate of hyaluronate in untreated fat storing cells of 4.2 +/- 0.8 micrograms/mg DNA per h increased up to 8.2 +/- 0.9 micrograms/mg DNA per h in the presence of Kupffer cell conditioned medium. The results demonstrate an activation of hyaluronate synthesis in fat storing cells by Kupffer cell factor(s), a mechanism which might be of relevance for the strong absolute and fractional increase of hyaluronate in the extracellular matrix of fibrotic livers.

Retinol transfer across and between phospholipid bilayer membranes

The transfer of retinol across and between bilayer membranes was studied in vitro using unilamellar liposomes and erythrocytes. Transmembrane movement of retinol in phospholipid bilayer membranes was a spontaneous and rapid process with a halflife of less than 30 s. Retinol transfer between liposomes and between liposomes and erythrocytes was also a spontaneous and rapid process with a halflife of less than 10 min. The results suggest that retinol transport in the cell might not need the participation of specific transfer proteins.

Transfer of retinol from parenchymal to stellate cells in liver is mediated by retinol-binding protein

Newly absorbed chylomicron remnant retinyl ester is endocytosed by parenchymal liver cells, and retinol is subsequently transferred to perisinusoidal stellate cells in liver. In the present study we have used several approaches to elucidate the mechanism for the paracrine transfer of retinol between liver parenchymal and stellate cells. In one series of experiments, chylomicrons labeled with [3H]retinyl palmitate or with retinyl [3H]palmitate were injected intravenously into rats. It was shown that the retinol as well as the palmitate moiety were initially taken up in parenchymal liver cells. However, only the retinol moiety was detected in stellate cells, indicating that the retinyl ester is hydrolyzed before retinol is transferred to stellate cells. It is well known that parenchymal liver cells secrete retinol bound to retinol-binding protein (RBP), and we have recently found that stellate cells do have RBP receptors. Here we report that antibodies against RBP completely block the transfer of retinol from parenchymal to stellate cells. These findings indicate that following uptake of chylomicron remnant retinyl ester in parenchymal cells, the retinyl ester is hydrolyzed, and retinol secreted from parenchymal cells on RBP is taken up by stellate cells by means of RBP receptors.