Liver parenchymal cells differ from the fat-storing cells in their lipid composition

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.

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.

ATGL and CGI-58 are lipid droplet proteins of the hepatic stellate cell line HSC-T6

Lipid droplets (LDs) of hepatic stellate cells (HSCs) contain large amounts of vitamin A [in the form of retinyl esters (REs)] as well as other neutral lipids such as TGs. During times of insufficient vitamin A availability, RE stores are mobilized to ensure a constant supply to the body. To date, little is known about the enzymes responsible for the hydrolysis of neutral lipid esters, in particular of REs, in HSCs. In this study, we aimed to identify LD-associated neutral lipid hydrolases by a proteomic approach using the rat stellate cell line HSC-T6. First, we loaded cells with retinol and FAs to promote lipid synthesis and deposition within LDs. Then, LDs were isolated and lipid composition and the LD proteome were analyzed. Among other proteins, we found perilipin 2, adipose TG lipase (ATGL), and comparative gene identification-58 (CGI-58), known and established LD proteins. Bioinformatic search of the LD proteome for α/β-hydrolase fold-containing proteins revealed no yet uncharacterized neutral lipid hydrolases. In in vitro activity assays, we show that rat (r)ATGL, coactivated by rat (r)CGI-58, efficiently hydrolyzes TGs and REs. These findings suggest that rATGL and rCGI-58 are LD-resident proteins in HSCs and participate in the mobilization of both REs and TGs.

Adipose triglyceride lipase is involved in the mobilization of triglyceride and retinoid stores of hepatic stellate cells

Hepatic stellate cells (HSCs) store triglycerides (TGs) and retinyl ester (RE) in cytosolic lipid droplets. RE stores are degraded following retinoid starvation or in response to pathogenic stimuli resulting in HSC activation. At present, the major enzymes catalyzing lipid degradation in HSCs are unknown. In this study, we investigated whether adipose triglyceride lipase (ATGL) is involved in RE catabolism of HSCs. Additionally, we compared the effects of ATGL deficiency and hormone-sensitive lipase (HSL) deficiency, a known RE hydrolase (REH), on RE stores in liver and adipose tissue. We show that ATGL degrades RE even in the presence of TGs, implicating that these substrates compete for ATGL binding. REH activity was stimulated and inhibited by comparative gene identification-58 and G0/G1 switch gene-2, respectively, the physiological regulators of ATGL activity. In cultured primary murine HSCs, pharmacological inhibition of ATGL, but not HSL, increased RE accumulation. In mice globally lacking ATGL or HSL, RE contents in white adipose tissue were decreased or increased, respectively, while plasma retinol and liver RE levels remained unchanged. In conclusion, our study shows that ATGL acts as REH in HSCs promoting the degradation of RE stores in addition to its established function as TG lipase. HSL is the predominant REH in adipocytes but does not affect lipid mobilization in HSCs.

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ATGL possesses retinyl ester and triacylglycerol hydrolase activity.

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The lack of ATGL activity causes increased triacylglycerol and retinyl ester storage in hepatic stellate cells.

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ATGL acts as retinyl ester and triacylglycerol lipase in hepatic stellate cells.

Liver fatty acid binding protein (L-Fabp) modulates murine stellate cell activation and diet-induced nonalcoholic fatty liver disease

Activation of hepatic stellate cells (HSCs) is crucial to the development of fibrosis in nonalcoholic fatty liver disease. Quiescent HSCs contain lipid droplets (LDs), whose depletion upon activation induces a fibrogenic gene program. Here we show that liver fatty acid-binding protein (L-Fabp), an abundant cytosolic protein that modulates fatty acid (FA) metabolism in enterocytes and hepatocytes, also modulates HSC FA utilization and in turn regulates the fibrogenic program. L-Fabp expression decreased 10-fold following HSC activation, concomitant with depletion of LDs. Primary HSCs isolated from L-FABP(-/-) mice contain fewer LDs than wild-type (WT) HSCs, and exhibit up-regulated expression of genes involved in HSC activation. Adenoviral L-Fabp transduction inhibited activation of passaged WT HSCs and increased both the expression of prolipogenic genes and also augmented intracellular lipid accumulation, including triglyceride and FA, predominantly palmitate. Freshly isolated HSCs from L-FABP(-/-) mice correspondingly exhibited decreased palmitate in the free FA pool. To investigate whether L-FABP deletion promotes HSC activation in vivo, we fed L-FABP(-/-) and WT mice a high-fat diet supplemented with trans-fatty acids and fructose (TFF). TFF-fed L-FABP(-/-) mice exhibited reduced hepatic steatosis along with decreased LD abundance and size compared to WT mice. In addition, TFF-fed L-FABP(-/-) mice exhibited decreased hepatic fibrosis, with reduced expression of fibrogenic genes, compared to WT mice.

CONCLUSION

L-FABP deletion attenuates both diet-induced hepatic steatosis and fibrogenesis, despite the observation that L-Fabp paradoxically promotes FA and LD accumulation and inhibits HSC activation in vitro. These findings highlight the importance of cell-specific modulation of hepatic lipid metabolism in promoting fibrogenesis in nonalcoholic fatty liver disease. (Hepatology 2013).

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.

Curcumin attenuates the effects of insulin on stimulating hepatic stellate cell activation by interrupting insulin signaling and attenuating oxidative stress

Hyperinsulinemia associated with type II diabetes mellitus (T2DM) is a risk factor for non-alcoholic steatohepatitis (NASH) and hepatic fibrosis. Hepatic stellate cells (HSCs) are the major effectors in collagen production during hepatic fibrogenesis. Elevated levels of insulin stimulate HSC activation. In addition to its anti-diabetic effects, the antioxidant curcumin, the yellow pigment in curry from turmeric, suppresses HSC activation and protects the liver from fibrogenesis in vitro and in vivo. This study aims at evaluating the effect of curcumin on insulin-induced HSC activation and further elucidating the underlying mechanisms. We report that curcumin dose-dependently eliminates insulin-induced HSC activation by suppressing expression of type I collagen gene and other key genes relevant to HSC activation. Additional experiments indicate that curcumin interrupts insulin signaling in HSCs by reducing the phosphorylation level of insulin receptor (InsR) and suppressing gene expression of InsR. Furthermore, curcumin attenuates insulin-induced oxidative stress in HSCs by inducing gene expression of glutamate-cysteine ligase (GCL), leading to de novo synthesis of glutathione and the suppression of gene expression of InsR. These results support our initial hypothesis that curcumin inhibits the effects of insulin on stimulating HSC activation by interrupting insulin signaling and attenuating oxidative stress. Our results provide novel insights into the mechanisms by which curcumin inhibits the insulin-induced HSC activation.

Hepatic stellate cell lipid droplets: a specialized lipid droplet for retinoid storage

The majority of retinoid (vitamin A and its metabolites) present in the body of a healthy vertebrate is contained within lipid droplets present in the cytoplasm of hepatic stellate cells (HSCs). Two types of lipid droplets have been identified through histological analysis of HSCs within the liver: smaller droplets bounded by a unit membrane and larger membrane-free droplets. Dietary retinoid intake but not triglyceride intake markedly influences the number and size of HSC lipid droplets. The lipids present in rat HSC lipid droplets include retinyl ester, triglyceride, cholesteryl ester, cholesterol, phospholipids and free fatty acids. Retinyl ester and triglyceride are present at similar concentrations, and together these two classes of lipid account for approximately three-quarters of the total lipid in HSC lipid droplets. Both adipocyte-differentiation related protein and TIP47 have been identified by immunohistochemical analysis to be present in HSC lipid droplets. Lecithin:retinol acyltransferase (LRAT), an enzyme responsible for all retinyl ester synthesis within the liver, is required for HSC lipid droplet formation, since Lrat-deficient mice completely lack HSC lipid droplets. When HSCs become activated in response to hepatic injury, the lipid droplets and their retinoid contents are rapidly lost. Although loss of HSC lipid droplets is a hallmark of developing liver disease, it is not known whether this contributes to disease development or occurs simply as a consequence of disease progression. Collectively, the available information suggests that HSC lipid droplets are specialized organelles for hepatic retinoid storage and that loss of HSC lipid droplets may contribute to the development of hepatic disease.

Retinoid Concentrations in the Mouse during Postnatal Development and after Maternal Vitamin A Supplementation

BACKGROUND

Vitamin A (VA) and its derivates (retinoids) are important nutritional substances, which mediate their biological activity mainly via nuclear retinoid receptors. Maternal VA intake during lactation influences the VA content in milk and the VA status of the progeny. We investigated the effects of maternal supplementation during lactation and direct supplementation to the pups after weaning on the retinoid concentration in serum and liver of neonatal mice using high doses of VA.

METHODS

Dams were fed a basal (4,500 retinol equivalents/kg diet) or a VA-supplemented (324,000 retinol equivalents/kg diet) diet during lactation. Pups kept receiving the same diet after weaning. Serum and liver samples of the pups were collected during lactation at days 1, 3, 5, 7, and 14 and post-weaning at days 21 and 65 after birth. Samples were analysed for retinoids by high-performance liquid chromatography.

RESULTS

Maternal VA supplementation resulted in significantly higher concentrations of retinol, retinyl palmitate and retinyl stearate in serum of mice neonates at days 5, 7, 14, 21 and 65 after birth in comparison to the basal diet, whereas significantly higher concentrations were observed in liver at days 5, 14, 21 and 65 after birth. At day 7 after birth, a decrease in the liver retinoid concentrations occurred in the VA-supplemented diet.

CONCLUSION

Our results show for the first time that supplementation with high doses of VA during the lactation period in mice can affect serum retinol concentrations in the neonates and report that day 7 after birth is a critical time in the tissue distribution of retinoids during postnatal development.

Recent advances in molecular cloning of fatty acid desaturase genes and the regulation of their expression by dietary vitamin A and retinoic acid

Vitamin A, as an essential micronutrient, is involved in higher animals in embryonic development and postnatal growth, reproduction and maintenance of normal skin, immunity and vision. Recently, studies in vivo and in cell lines have shown that vitamin A and its active metabolite, retinoic acid, regulate the expression of fatty acid desaturases including stearoyl-CoA desaturase and delta-5 desaturase. Whereas the former desaturase catalyzes the formation of monounsaturated from saturated fatty acids, the latter enzyme is involved in the desaturation pathway of dietary essential fatty acids for production of polyunsaturated fatty acids. The reaction products of these desaturases serve as critical regulators in a wide range of physiological processes which include fetal growth and development, reproduction, cell differentiation, immune and inflammatory responses.

Biophysical and structural characterization of 1H-NMR-detectable mobile lipid domains in NIH-3T3 fibroblasts

Nature and subcellular localization of 1H-NMR-detectable mobile lipid domains (ML) were investigated by NMR, Nile red fluorescence and electron microscopy, in NIH-3T3 fibroblasts and their H-ras transformants (3T3ras) transfected with a high number of oncogene copies. Substantial ML levels (ratio of (CH2)n/CH3 peak areas R=1. 56+/-0.33) were associated in untransformed fibroblasts with both (a) intramembrane amorphous lipid vesicles, about 60 nm in diameter, distinct from caveolae; and (b) cytoplasmic, osmiophilic lipid bodies surrounded by own membrane, endowed of intramembrane particles. 2D NMR maps demonstrated that ML comprised both mono- and polyunsaturated fatty chains. Lower ML signals were detected in 3T3ras (R=0.76+/-0.37), under various conditions of cell growth. Very few (if any) lipid bodies and vesicles were detected in the cytoplasmic or membrane compartments of 3T3ras cells with R<0.4, while only intramembrane lipid vesicles were associated with moderate R values. Involvement of phosphatidylcholine hydrolysis in ML generation was demonstrated by selective inhibition of endogenous phospholipase C (PC-plc) or by exposure to bacterial PC-plc. This study indicates that: (1) both cytoplasmic lipid bodies and membrane vesicles (possibly in mutual dynamic exchange) may contribute (although to a different extent) to ML signals; and (2) high levels of ras-transfection either inhibit ML formation or facilitate their extrusion from the cell.

Peculiar distribution of fodrin in fat-storing cells

Fat-storing cells (FSCs) show unique morphology containing many lipid droplets in the cytoplasm. In this study, we found that a membrane skeletal protein, fodrin, shows peculiar distribution in FSCs of rat liver. By immunofluorescence microscopy of FSCs in culture, intense labeling for fodrin was seen as coarse filaments in the cytoplasm. Especially in FSCs isolated from vitamin A-treated rats, the labeling was often seen as many small rings in the cytoplasm. In contrast, labeling for fodrin in human fibroblasts or rat adipocytes in culture was seen diffusely in the cell cortex. Distribution of actin, tubulin, vimentin, and desmin in FSCs was also examined, but none of them appeared correlated with fodrin. By immunoelectron microscopy using nanogold labeling with silver enhancement, positive labeling for fodrin was seen around some lipid droplets in FSCs in vivo. We assume that the peculiar distribution of fodrin may be related to the morphological characteristics of FSCs.

The hepatic stellate (Ito) cell: its role in human liver disease

The hepatic stellate (Ito) cell lies within the space of Disse and has a variety of functions. Stellate cells store vitamin A in characteristic lipid droplets. In the normal human liver, the cells can be identified by the presence of these lipid droplets; in addition, many stellate cells in the normal liver express alpha-smooth muscle actin. In acute liver injury, there is an expansion of the stellate cell population with increased alpha-smooth muscle actin expression; stellate cells appear to play a role in extracellular matrix remodelling after recovery from injury. In chronic liver injury, the stellate cell differentiates into a myofibroblast-like cell with marked expression of alpha-smooth muscle actin and occasional expression of desmin. Myofibroblast-like cells have a high fibrogenic capacity in the chronically diseased liver and are also involved in matrix degradation. In vitamin A intoxication, hypertrophy and proliferation of the stellate and myofibroblast-like cells may lead to non-cirrhotic portal hypertension, fibrosis and cirrhosis. In liver tumours, myofibroblast-like cells are involved in the capsule formation around the tumour and in the production of extracellular matrix within it. The transition of stellate cells into myofibroblast-like cells is regulated by an intricate network of intercellular communication between stellate cells and activated Kupffer cells, damaged hepatocytes, platelets, endothelial and inflammatory cells, involving cytokines and nonpeptide mediators such as reactive oxygen species, eicosanoids and acetaldehyde. The findings suggest that the stellate cell plays an active role in a number of human liver diseases, with a particular reactivity pattern in fibrotic liver disorders.

Lecithin:retinol acyltransferase and retinyl ester hydrolase activities are differentially regulated by retinoids and have distinct distributions between hepatocyte and nonparenchymal cell fractions of rat liver

The cellular distribution of enzymes that esterify retinol and hydrolyze retinyl esters (RE) was studied in liver of vitamin A-sufficient, -deficient, and deficient rats treated with retinoic acid or N-(4-hydroxyphenyl)-retinamide. Livers were perfused and cell fractions enriched in hepatocytes, and nonparenchymal cells were obtained for assays of RE and enzyme activity. The specific activity of lecithin:retinol acyltransferase (LRAT) was approximately 10-fold greater in the nonparenchymal cell than the hepatocyte fraction from both vitamin A-sufficient and retinoid-treated rats. Total RE mass, newly synthesized [3H]RE and LRAT activity were positively correlated in liver and isolated cells of both normal (P < 0.0001) and retinoid-treated rats (P < 0.0002). In nonparenchymal cells, these three constituents were nearly equally enriched as evaluated by their relative specific activity values (RSA, defined as the percentage of recovered activity divided by the percentage of recovered protein), which were each significantly greater than 1.0, with values of 4.3 for total RE mass (P < 0.05), 3.6 for newly synthesized [3H]RE (P < 0.01) and 3.8 for LRAT activity (P < 0.01). In contrast, the specific activities of neutral and acid bile salt-independent retinyl ester hydrolases (REH) did not vary with vitamin A status, and their RSA values were close to 1.0 in both hepatocytes and nonparenchymal cells. These data show that LRAT and REH are differentially regulated by retinoids and that these enzymes also differ in their spacial distribution between liver parenchymal and nonparenchymal cells.

Vitamin A contained in the lipid droplets of rat liver stellate cells is substrate for acid retinyl ester hydrolase

Vitamin A is stored in the lipid droplets of liver stellate cells (LSCs), as retinyl esters whose hydrolysis is necessary for the secretion of retinol into the blood. Here, we isolated these retinyl esters under their physiological form, i.e., in LSC lipid droplets, which had retained their morphological and biochemical characteristics. These retinyl esters are substrate for an hydrolytic enzyme, whose optimum pH is 4.1, and which is kinetically similar to the acidic retinyl ester hydrolase (aREH) we had previously described (Mercier et al., Biochim. Biophys. Acta (1994) 1212, 176-182). The cellular and subcellular localizations of aREH activity in rat liver suggest that this enzyme could be involved in the hydrolysis of the esterified vitamin A stores.

Liver of juvenile Atlantic salmon, Salmo salar L.: a light, transmission, and scanning electron microscopic study, with special reference to the sinusoid

BACKGROUND

This report provides a detailed description of sinusoidal and perisinusoidal structures in the normal liver of the juvenile Atlantic salmon (Salmo salar L.), a teleost species.

METHODS

The liver was studied by light, transmission, and scanning electron microscopy, and organ specimens were sampled after retrograde, whole-body perfusion through the dorsal aorta using 3% glutaraldehyde. Detailed characterization of perisinusoidal stellate cells also included immunohistochemical staining for desmin and evaluation of autofluorescence of the same cells upon excitation in ultraviolet (UV) light.

RESULTS

The sinusoid is lined by one cell type only: the endothelial cell. No intraluminal pit cells or Kupffer cells are present. The space of Disse contains reticulin fibres, visualized by Gomori's silver stain, and perisinusoidal stellate cells (PSC). PSC exhibited autofluorescence in UV light, indicating that these cells store vitamin A in cytoplasmic lipid droplets. Immunohistochemically, PSC were found negative for desmin. The space of Disse, extending deep down between adjacent hepatocytes, receives long, slender microvilli from parenchymal cells. In addition to scattered macrophages, interhepatocytic cells (IHC) are found perisinusoidally. Hepatocytes of Atlantic salmon form branching and anastomosing tubules.

CONCLUSIONS

The sinusoids of Atlantic salmon liver are lined by a fenestrated endothelium, with PSC located in the space of Disse, with macrophages and IHC as inhabitants of the interhepatocytic space. IHC show ultrastructural similarities to mammalian pit cells and teleostean large granular lymphocytes, as well as to piscine monocytes. PSC might be storage cells for vitamin A in Atlantic salmon as shown by autofluorescence in these cells, while immunohistochemical studies indicate that desmin does not seem to be an adequate immunohistochemical marker for PSC in the juvenile Atlantic salmon. Methodologically, fixation for electron microscopy was performed by a new and convenient perfusion method: arterial retrograde whole body perfusion. Liver specimens intended for scanning electron microscopy were fractured at room temperature after prolonged osmium postfixation, leaving hepatocytes intact and producing images well suited to document the three-dimensional structure of cells and tissue.

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.

Sinusoidal endothelial cells of the liver: fine structure and function in relation to age

Liver endothelial cells form a continuous lining of the liver capillaries, or sinusoids, separating parenchymal cells and fat-storing cells from sinusoidal blood. Liver sinusoidal endothelial cells differ in fine structure from endothelial cells lining larger blood vessels and from other capillary endothelia in that they lack a distinct basement membrane and also contain open pores, or fenestrae, in the thin cytoplasmic projections which constitute the sinusoidal wall. This distinctive morphology supports the protective role played by liver endothelium, the cells forming a general barrier against pathogenic agents and serving as a selective sieve for substances passing from the blood to parenchymal and fat-storing cells, and vice versa. Sinusoidal endothelial cells, furthermore, significantly participate in the metabolic and clearance functions of the liver. They have been shown to be involved in the endocytosis and metabolism of a wide range of macromolecules, including glycoproteins, lipoproteins, extracellular matrix components, and inert colloids, establishing endothelial cells as a vital link in the complex network of cellular interactions and cooperation in the liver. Fine structural studies in combination with the development of cell isolation and culture techniques from both experimental animal and human liver have greatly contributed to the elucidation of these endothelial cell functions. Morphological and biochemical investigations have both revealed little changes with age except for an accumulation of iron ferritin and a decrease in the activities of glucose-6-phosphatase, Mg-ATPase, and in glucagon-stimulated adenylcyclase. Future studies are likely to disclose more fully the role of sinusoidal endothelial cells in the regulation of liver hemodynamics, in liver metabolism and blood clearance, in the maintenance of hepatic structure, in the pathogenesis of various liver diseases, and in the aging process in the liver.

Age-related changes of lipid fractions and total fatty acids in liver lipids and heart lipids of female and male rats aged 37-1200 days (liver) and 331-1200 days (heart)

1. Total lipids and the lipid fractions cholesterol ester, triacylglycerol, free cholesterol, free fatty acids and phospholipids, as well as the fatty acid patterns of total lipids, were measured in liver homogenates of female and male rats (Wistar SPF, strain Hannover) aged 37-1213 days. 2. The same parameters were measured in the apex of the heart in female and male rats aged 331-1213 days. 3. All parameters were monitored every 49th day. Five female and five male animals were used in each experiment. 4. The lipid fractions in liver showed a positive linear regression vs age, whereas all lipids in rat heart showed a negative regression vs age in both sexes. 5. The significance of regression vs age of fatty acids was much less than that in the lipid fractions of liver and heart of these animals.

Incorporation of trans-8- and cis-8-octadecenoic acid isomers in human plasma and lipoprotein lipids

Mixtures of deuterium-labeled trans-8, cis-8 and cis-9-octadecenoic acids (8t-18:1, 8c-18:1, 9c-18:1) were fed as triglycerides (TG) to two adult male subjects. Blood samples were collected sequentially over a 48-hour period. Plasma and lipoprotein lipids were separated by thin layer chromatography and analyzed by gas chromatography-mass spectroscopy. Results indicate (i) absorption of the 8t- and 8c-18:1 isomers were similar to 9c-18:1; (ii) the 8t-18:1 isomer was cleared approximately 30% faster than 9c-18:1 from plasma TG; (iii) cholesterol ester samples contained 8.4 times less 8t-18:1 than 9c-18:1; (iv) incorporation at the 1-acyl phosphatidylcholine (PC) position was higher for 8t-18:1 and 8c-18:1 (2.2 and 1.7 times) than for 9c-18:1; and (v) discrimination at the 2-acyl PC position was 4.6-fold against 8t-18:1 and 1.3-fold against 8c-18:1 compared with 9c-18:1. Discrimination against uptake of the delta-8 isomers in both neutral and phospholipid classes suggests that both 8t- and 8c-18:1 may be preferentially oxidized relative to 9c-18:1. Except for triglycerides, data for each of the lipid classes from total plasma and individual lipoprotein samples were similar. These data indicate that differences for incorporation and turnover of the 8t- and 8c-18:1 isomers relative to 9c-18:1 are not substantially influenced by the lipoprotein classes. The maximum isotopic enrichment detected in the chylomicron triglycerides fractions was 60%, which indicates that a substantial amount of endogenous triglycerides was mobilized during absorption of the deuterated fats.

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.