Content of dolichol and retinol in isolated rat non-parenchymal liver cells

The liver sinusoids, that are considered as a functional unit, harbour four types of sinusoidal cells (Ito, Kupffer, endothelial and pit cells). Dolichol content has been determined in many tissues and subcellular compartments, alteration has been reported in many types of liver injury, but until now no data are available on its content in every type of sinusoidal non-parenchymal liver cells. Dolichol and retinol metabolism might intersect in their traffic in biological membranes. Intercellular as well as intracellular exchange of retinoids is an essential element of important processes occurring in liver cells. It has been suggested that the role of dolichol, besides being a carrier of oligosaccharides in the biosynthesis of N-linked glycoproteins, may be to modify membrane fluidity and permeability, and facilitate fusion of membranes. Dolichol in the membrane is intercalated between the two halves of the phospholipid bilayer, but its exact disposition is not known and the movement and distribution of retinoid in membranes may vary with the geometry of the membranes. Therefore the aim of this study is to obtain a global understanding of the sinusoidal system regarding dolichol and retinol content in each type of isolated rat liver sinusoidal cell, in normal conditions and after vitamin A administration. The information that can be drawn from the present results is that with normal vitamin A status of the animal, the dolichol content is almost uniform in all liver cells. After vitamin A supplementation, a great increase of dolichol, together with the known increase of retinol, can be measured only in a subpopulation of the Ito cells, the Ito-1 subfraction. Therefore in the cells that are present in the hepatic sinusoid, different pools of dolichol may have separate functions. Because retinol traffic among cells, membranes and plasma still remains to be fully understood, roles of dolichol in the exchange of vitamin A among sinusoidal liver cells are discussed. © 1998 John Wiley & Sons, Ltd.     

Thioacetamide impairs retinol storage and dolichol content in rat liver cells in vivo

The aim of this paper was to ascertain whether chronic pretreatment with thioacetamide (TAA) might alter the uptake of a load of retinol and dolichol distribution in hepatocytes (HC), hepatic stellate cells (HSC) (Ito-1 and Ito-2 subfractions), Kupffer (KC) and sinusoidal endothelial cells (SEC).The reason why retinol and dolichol content was studied is that their metabolism and transport might be interrelated and that the two isoprenoids might exert different functions in the cells of the hepatic sinusoid.Rats were treated for 2 and 4 months with TAA, a known fibrogenic hepatotoxin, at a low dosage, to produce an early stage of damage. Three days before sacrifice, the rats were given a load of vitamin A, and cells were isolated to investigate its uptake.In HC, the load of retinol was taken up and accumulated, while a decrease in dolichol preceded retinol increase. In HSC, much less of the retinol load was stored than in controls, and dolichol content also decreased. Various minor modifications were seen in KC and SEC.Collectively, the results show that the distribution of these two isoprenoids, which play important roles in cellular differentiation and proliferation, is differently altered in the multiple cell types that line the hepatic sinusoid, and that both isoprenoids seem to participate in the first steps of liver damage.     

Fluid endocytosis in isolated rat parenchymal and non-parenchymal liver cells

Fluid endocytosis in rat liver parenchymal (hepatocytes) and non-parenchymal cells was studied by measuring uptake of [¹²⁵I]polyvinylpyrrolidone (PVP). Radioactive sucrose preparations were also tested but turned out to be unsuitable because of impurities of radioactive glucose and fructose. Fluid endocytosis was temperature dependent without any transition temperature. The rate of endocytosis was inhibited by inhibitors of the glycolytic and the respiratory pathway. Colchicine, but not cytochalasin B, inhibited the uptake of [¹²⁵I]PVP in hepatocytes. Therefore, intact microtubuli, but not microfilaments may be required for normal rate of fluid endocytosis in hepatocytes. Colchicine reduced the rate of fluid endocytosis in the non-parenchymal liver cells. Subcellular fractionation by isopycnic centrifugation in sucrose gradients indicated that [¹²⁵I]PVP taken up by the hepatocytes accumulated in the lysosomes. The rate of uptake expressed as volume of fluid internalized per unit time (endocytic index) was calculated to 0.08 μl/h/10⁶ cells for hepatocytes and 0.07 μl/h/10⁶ cells for non-parenchymal liver cells.     

Distribution of gangliosides in parenchymal and non-parenchymal cells of rat liver

Parenchymal and non-parenchymal cells were isolated from rat liver with purities of more than 90%. Total and ganglioside sialic acid contents were higher in non-parenchymal cells than in parenchymal cells. Thin-layer chromatography of gangliosides showed that the main component in rat liver was ganglioside GM3 and that this was abundant in non-parenchymal cells. Parenchymal cells had ganglioside GD1b as the main component and less GM3 than non-parenchymal cells. These results suggested that the main ganglioside of rat liver, GM3, arises mainly from non-parenchymal cells.     

Comparison of cadmium-metallothionein synthesis in parenchymal and non-parenchymal rat liver cells.

The time course of cadmium-metallothionein synthesis was studied in non-parenchymal and parenchymal cells, isolated by a cell-separation technique from the livers of rats after the simultaneous injection of CdCl2 (0.05 mg of Cd/kg) and a 10-fold molar excess of 2,3-dimercaptopropanol. Under these conditions of dosing, in contrast with the injection of CdCl2 alone, both cell types accumulate similar concentrations of Cd and synthesize equivalent concentrations of metallothionein. It is concluded that both cell types have a similar capacity to synthesize the metalloprotein, and that the limiting factor under normal cadmium exposure is the relatively inefficient metal uptake into the non-parenchymal cells.     

Identification of metallothionein in parenchymal and non-parenchymal liver cells of the adult rat.

Parenchymal and non-parenchymal cells were isolated from the livers of control, starved, Zn2+-injected and Cd2+-injected rats. Parenchymal cells were prepared by differential centrifugation after perfusion of the liver with collagenase. Non-parenchymal cells were separated from parenchymal cells by unit-gravity sedimentation and differential centrifugation. Yields of 2 x 10(8) non-parenchymal cells with greater than 95% viability and less than 0.2% contamination with parenchymal cells were obtained without exposing cells to Pronase. Metallothioneins-I and -II were identified in parenchymal cells and non-parenchymal cells from Zn2+-treated rats. The metallothionein contents of parenchymal cells, non-parenchymal cells and intact liver were quantified by a competitive 203Hg-binding assay. Administration of heavy-metal salts significantly increased the metallothionein content of both cell populations, although the concentration of the protein was approx. 2.5-fold greater in parenchymal cells than in non-parenchymal cells. Overnight starvation increased the metallothionein content of parenchymal cells without altering that of non-parenchymal cells. The potential significance of this differential response by different liver cell types with regard to the influence of Zn2+ on stress-mediated alterations in hepatic metabolism is discussed.     

Chronic ethanol treatment: dolichol and retinol distribution in isolated rat liver cells

The aim of this study was to use chronic ethanol intoxication for 2 and 4 months as a means of studying the distribution of dolichol and retinol in isolated rat liver parenchymal cells, Kupffer cells, sinusoidal endothelial cells, and two subfractions of hepatic stellate cells: Ito 1 and Ito 2. Dolichol and retinol were studied in two batches of rats: on normal nutrition and after a load of vitamin A given 3 d before sacrifice. New observations reported are: (i) on normal nutrition, after 2 months of treatment, dolichol in HC seems to be the first target of chronic ethanol, while retinol is the first target in hepatic stellate cells; (ii) the various types of liver cells are differently affected by chronic ethanol, which highlights the importance of studying each type of sinusoidal cell; (iii) a load of vitamin A given when the damage has already occurred restores dolichol content in HC while retinol decreases; and, (iv) a link between dolichol and vitamin A metabolism might be supposed after the load of vitamin A: the percentage distribution of dolichol with 18 isoprene units (Dolichol -18) increases in all the control cells but decreases after chronic ethanol treatment. A different role of this dolichol and/or a different compartmentalization within the cell need to be further investigated.     

Isoenzyme patterns in parenchymal and non-parenchymal cells isolated from regenerating and regenerated rat liver

Parenchymal and non-parenchymal cells were isolated from adult rat liver that had been fully regenerated after a 70% partial hepatectomy. The characteristics of the parenchymal cell preparations from regenerated rat liver indicated that they were a homogeneous population and comparable with parenchymal cells isolated from intact liver. The parenchymal cells from regenerated adult rat liver contain glucokinase, hexokinase, pyruvate kinase type I and aldolase B. The non-parenchymal cells contain hexokinase, pyruvate kinase type III and aldolase B. When cells were isolated at different times of the day from rats on controlled feeding schedules, variation of tyrosine aminotransferase activity and liver glycogen content were observed in the parenchymal cells in keeping with the reported diurnal oscillations found in whole liver extracts. When parenchymal cells were isolated from rats 48 and 72h after partial hepatectomy, different isoenzyme patterns were observed. These cells appeared to synthesize pyruvate kinase type III, a function that was assigned previously to non-parenchymal cells or to foetal rat liver hepatocytes.     

Role of parenchymal and non-parenchymal rat liver cells in the uptake of cholesterolester-labeled serum lipoproteins.

Very low density lipoprotein (VLDL)-remnants, prepared by extrahepatic circulation of VLDL, labeled biosynthetically in the cholesterol (ester) moiety, were injected intravenously into rats in order to determine the relative contribution of parenchymal and non-parenchymal liver cells to the hepatic uptake of VLDL-remnant cholesterol (esters). 82.7% of the injected radioactivity is present in liver, measured 30 min after injection. The non-parenchymal liver cells contain 3.1±0.1 times the amount of radioactivity per mg cell protein as compared to parenchymal cells. The hepatic uptake of biosynthetically labeled (screened) low density lipoprotein (LDL) and high density lipoprotein (HDL) cholesterolesters amounts to 26.8% and 24.4% of the injected dose, measured 6 h after injection. The non-parenchymal cells contain 4.3±0.8 and 4.1±0.7 times the amount of radioactivity per mg cell protein as compared to parenchymal cells for LDL and HDL, respectively. It is concluded that in addition to parenchymal cells, the non-parenchymal cells play an important role in the hepatic uptake of cholesterolesters from VLDL-remnants, LDL and HDL.     

Identity and activities of lysosomal enzymes in parenchymal and non-parenchymal cells from rat liver.

1. Intact parenchymal and non-parenchymal cells were isolated from rat liver. The parenchymal cells were purified by differential centrifugation, while non-parenchymal cells were obtained free of parenchymal cell contamination by preferentially destroying the parenchymal cells with the aid of pronase (0.25%). 2. The ability to isolate pure intact parenchymal and non-parenchymal cells permitted the characterization and measurement of specific activities of various lysosomal enzymes, representing the main functional hydrolytic activities of the lysosomes in these distinct cell types. 3. Lysosomal enzymes catalysing the hydrolysis of the terminal carbohydrate moiety of glycoproteins and glycolipids were not particularly enriched in the non-parenchymal cells as compared to parenchymal cells. The ratio of the specific activities of non-parenchymal cells over parenchymal cells varied between 0.7 for N-acetyl-beta-D-hexoseaminidase to 2.1 for alpha-glucosidase. This suggests no specific role of the non-parenchymal cells in the hydrolysis of terminal carbohydrate moieties of glycoproteins and glycolipids. 4. The enzymes acid phosphatase and aryl sulphatase, representing the phosphate and sulphate hydrolyzing activities, were enriched in the non-paranchymal cells as compared to the parenchymal cells by a factor of 2.5. 5. The most important peptidase cathepsin D, representing protein breakdown capacity, is enriched in the non-parenchymal cells as compared to parenchymal cells by a factor 6.0, suggesting a possible specific function of non-parenchymal cells in protein breakdown. 6. The most enriched lysosomal enzyme, representing lipid hydrolysis, is acid lipase, which is enriched in the non-parenchymal cells with a factor of 10. 7. The distribution of lysosomal enzymes between parenchymal and non-parenchymal cells suggests different functional roles of the lysosomes in these cell types. It can be concluded that the non-parenchymal cells possess a set of lysosomal enzymes which makes them extremely suitable for a phagocytic and antimicrobial function in the liver.     

Dolichol and retinol content of rat liver sinusoidal cells after chronic monensin treatment.

Aim of this study was to ascertain whether an impairment of communication between parenchymal and non-parenchymal liver cells involves vitamin A intercellular transport. The following approach was adopted: liver cells were isolated from rats treated chronically with the hydrophobic ionophore monensin i.p. for 3, 5, and 7 weeks and their retinol and dolichol content was assessed. Monensin, which alters membrane flow, was used because it had previously been reported to induce liver steatosis, cholestasis and glycogenolysis after acute treatment and, by preliminary morphological examination, to impair vitamin A transport between stellate cells and hepatocytes. Dolichol was chosen as a biochemical marker because it is a membrane lipid that modulates the fluidity and permeability of the membranes that retinol must cross. After monensin treatment, a load of vitamin A was given to rats three days before sacrifice, to ascertain whether its uptake by sinusoidal liver cells was altered. The main result was a dolichol decrease in hepatocytes and in the Ito-1 subfraction. In this latter, monensin induced a decrease in dolichol content only after vitamin A load. Moreover, while the hepatocytes were able to take up a load of vitamin A normally, the Ito-1 subfraction was no longer able to store retinol. Therefore the polarised transport of retinol between hepatocytes and stellate cells seemed impaired. The behaviour of sinusoidal endothelial cells and Kupffer cells might be ascribed to the functions of these cells and is not significantly modified by monensin. In conclusion, the altered cross-talk between sinusoidal cells in liver pathology might involve retinol as well as cytokines. Different pools of dolichol might have a role in this membrane process in a hydrophobic environment.     

Characteristics of acid lipase and acid cholesteryl esterase activity in parenchymal and non-parenchymal rat liver cells

(1) Parenchymal and non-parenchymal cells were isolated from rat liver. The characteristics of acid lipase activity with 4-methylumbelliferyl oleate as substrate and acid cholesteryl esterase activity with cholesteryl[1-14C]oleate as substrate were investigated. The substrates were incorporated in egg yolk lecithin vesicles and assays for total cell homogenates were developed, which were linear with the amount of protein and time. With 4-methylumbelliferyl oleate as substrate, both parenchymal and non-parechymal cells show maximal activities at acid pH and the maximal activity for non-parenchymal cells is 2.5 times higher than for parenchymal cells. It is concluded that 4-methylumbelliferyl oleate hydrolysis is catalyzed by similar enzyme(s) in both cell types. (2) With cholesteryl[1-14C]oleate as substrate both parenchymal and non-parenchymal cells show maximal activities at acid pH and the maximal activity for non-parenchymal cells is 11.4 times higher than for parenchymal cells. It is further shown that the cholesteryl ester hydrolysis in both cell types show different properties. (3) The high activity and high affinity of acid cholesteryl esterase from non-parenchymal cells for cholesterol oleate hydrolysis as compared to parenchymal cells indicate a relative specialization of non-parenchymal cells in cholesterol ester hydrolysis. It is concluded that non-parenchymal liver cells in cholesterol ester hydrolysis. It is concluded that non-parenchymal liver cells possess the enzymic equipment to hydrolyze very efficiently internalized cholesterol esters, which supports the suggestion that these cell types are an important site for lipoprotein catabolism in liver.     

Isolation of parenchymal cell-derived particles from non-parenchymal rat liver cell preparations

Rat liver was perfused with collagenase and the non-parenchymal cells were isolated by means of differential centrifugation. Low magnification microscopical examination indicated that in this non-parenchymal cell fraction less than 1 % are parenchymal cells, whereas the observed pyruvate kinase kinetics indicated that 50% of the total amount of pyruvate kinase in this fraction is of parenchymal cell origin. The non-parenchymal cell fraction was further purified by metrizamide density cushion centrifugation followed by centrifugal elutriation. A fraction that consisted of small particles, diameter < 5 μm, was collected. The pyruvate kinase activity in this fraction showed characteristics of absolute L-type kinetics and further examination of these particles, called blebs, indicated that they were of parenchymal cell origin. Determination of enzyme markers with regard to the different subcellular structures indicated that the blebs, as compared with parenchymal cells, contained lower specific activities of enzyme markers for the endoplasmic reticulum, mitochondria and especially peroxisomes. Electron micrographs indicated the complete absence of nuclei. It is suggested that the pure isolated blebs form a unique test material to study the involvement of the nucleus and/or peroxisomes in metabolic processes. The identification of these blebs in the non-parenchymal cell preparations might also explain some discrepancies in the literature about the presence of certain metabolic processes in non-parenchymal cells.     

Difference spectra, catalase- and peroxidase activities of isolated parenchymal and non-parenchymal cells from rat liver

The reduced minus oxidized difference spectra from isolated parenchymal and non-parenchymal cells from rat liver indicate that the non-parenchymal cells contain a considerable amount of peroxidase. This interpretation is favoured by the more than 30 times higher specific activity of peroxidase (EC 1.11.1.7) in the non-parenchymal cells as compared to the parenchymal cells. The catalase (EC 1.11.1.6) activity in the non-parenchymal cells is 4 times lower than in the parenchymal cells. These results are consistent with an antimicrobial function of the non-parenchymal cells in liver.     

In vivo studies on the binding sites for lipoprotein (a) on parenchymal and non-parenchymal rat liver cells

The direct correlation between lipoprotein (a) (Lp(a)) concentrations and atherosclerosis stimulated us to investigate the in vivo interaction of Lp(a) with the liver and the various liver cell types. In untreated rats the serum decay of Lp(a) is comparable to that of LDL. By estrogen treatment the interaction of LDL with parenchymal liver cells is increased 17-fold whereas only a 2-fold effect on Lp(a) is found. The decay of Lp(a) in estrogen-treated rats is slower than for LDL. The data indicate that Lp(a) in vivo shows a less efficient interaction than LDL with the estrogen-induced apo-B,E receptor on parenchymal liver cells. It is suggested that the inability of Lp(a) to interact efficiently with the LDL removal system of the liver might be related to its atherogenic action.     

The uptake and metabolism of chylomicron-remnant lipids by rat liver parenchymal and non-parenchymal cells in vitro.

The uptake and metabolism of chylomicron-remnant lipids by individual liver cell types was examined by incubating remnants with monolayer cultures of hepatocytes, Kupffer cells, and endothelial cells from rat liver. Remnants were prepared in vitro from radiolabelled mesenteric-lymph chylomicra, utilizing either purified lipoprotein lipase from bovine milk, or plasma isolated from heparinized rats. The resulting particles contained [3H]phosphatidylcholine and cholesterol, and [14C]oleate in the acylglycerol, phospholipid, fatty-acid and cholesterol-ester fractions. The capacities of the three cell types for uptake of both [3H]lipids and [14C]lipids were determined to be, on a per-cell basis, in the order: Kupffer greater than hepatocytes greater than endothelial. The relative proportions of [3H]phospholipid and total [3H]cholesterol taken up by hepatocytes and non-parenchymal cells remained constant with time. The uptake of [14C]oleoyl lipids by all three cell types was slightly greater than that of the total [3H]cholesterol and [3H]phospholipid components. There was evidence of cholesterol-ester hydrolysis and turnover of [14C]oleate in the phospholipid fraction in hepatocytes and Kupffer cells, but not endothelial cells, over the first 2 h. With both remnant preparations, these observations indicate that significant differences exist between the three major liver cell types with respect to the uptake and metabolism of remnant lipid components.     

Effect of apolipoproteins E and C-III on the interaction of chylomicrons with parenchymal and non-parenchymal cells from rat liver.

[3H]Triacylglycerol-labelled chylomicrons were isolated from intestinal lymph, obtained from rats made hypolipidaemic by treatment with pharmacological amounts of 17 alpha-ethynyloestradiol. Oestrogen treatment results in a large reduction in the content of apolipoproteins (apo) E and C of lymph chylomicrons. Upon incubation in vitro with freshly isolated parenchymal and non-parenchymal cells the apo E-, apo C-poor chylomicrons became readily cell-associated. With increasing chylomicron concentrations this cell-association was saturable and half-maximal cell-association was achieved at about 0.55 mg of triacylglycerol/ml. The cell-association was time- and temperature-dependent. A more than 90% inhibition of the cell-association of the [3H]triacylglycerol moiety was observed with both parenchymal and non-parenchymal cells when pure apo C-III (12.6 micrograms/mg of triacylglycerol) was incorporated into the chylomicrons. These data indicate that apo E-, apo C-poor chylomicrons are bound to both parenchymal and non-parenchymal liver cells at a high-affinity site of limited capacity and that binding to this site is strongly inhibited by apo C-III. With apo C-III-enriched chylomicrons simultaneous determination of the cell-association of the 125I-apo C-III and the [3H]triacylglycerol moiety indicated that more 125I-apo C-III becomes associated to the cells than expected on the basis of [3H]triacylglycerol radioactivity measurements. It is suggested that upon cell-association of apo C-III its binding to the chylomicron particles is lost. Consequently the occupation of the cellular recognition site by apo C-III prevents further chylomicron binding and thus leads to a decrease of the cell-association level of the [3H]triacylglycerol moiety. Apo E enrichment of the chylomicrons led to an increased cell-association rate with parenchymal cells and to a marked increase of the cell-association level with non-parenchymal cells. The cell-association of the apo E radioactivity followed closely the [3H]triacylglycerol radioactivity, indicating that the particle-apo E complex is bound as a unity. The apo E effects were opposed by apo C-III. With apo E-, apo C-III-enriched chylomicrons more 125I-apo E became associated with the cells than could be expected on the basis of the [3H]triacylglycerol measurements. It is concluded that apo C-III can weaken the interaction of apo E with the chylomicrons leading to the cell-association of free apo E. It appears that subtle changes in the apo E and/or apo C-III content of chylomicrons can influence the interaction with both parenchymal and non-parenchymal liver cells.(ABSTRACT TRUNCATED AT 400 WORDS)