The role of the plasma membrane in fatty acid uptake by rat liver parenchymal cells.
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ABSTRACT:
SUBMITTER: Wright JD
PROVIDER: S-EPMC1177084 | biostudies-other | 1971 Aug
REPOSITORIES: biostudies-other
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Project description:Rat liver parenchymal cells express Na(+)-dependent and Na(+)- independent nucleoside transport activity. The Na(+)-dependent component shows kinetic properties and substrate specificity similar to those reported for plasma membrane vesicles [Ruiz-Montasell, Casado, Felipe and Pastor-Anglada (1992) J. Membr. Biol. 128, 227-233]. This transport activity shows apparent K(m) values for uridine in the range 8-13 microM and a Vmax of 246 pmol of uridine per 3 min per 10(5) cells. Most nucleosides, including the analogue formycin B, cis-inhibit Na(+)-dependent uridine transport, although thymidine and cytidine are poor inhibitors. Inosine and adenosine inhibit Na(+)-dependent uridine uptake in a dose-dependent manner, reaching total inhibition. Guanosine also inhibits Na(+)-dependent uridine uptake, although there is some residual transport activity (35% of the control values) that is resistant to high concentrations of guanosine but may be inhibited by low concentrations of adenosine. The transport activity that is inhibited by high concentrations of thymidine is similar to the guanosine-resistant fraction. These observations are consistent with the presence of at least two Na(+)-dependent transport systems. Na(+)-dependent uridine uptake is sensitive to N-ethylmaleimide treatment, but Na(+)-independent transport is not. Nitrobenzylthioinosine (NBTI) stimulates Na(+)-dependent uridine uptake. The NBTI effect involves a change in Vmax, it is rapid, dose-dependent, does not need preincubation and can be abolished by depleting the Na+ transmembrane electrochemical gradient. Na(+)-independent uridine transport seems to be insensitive to NBTI. Under the same experimental conditions, NBTI effectively blocks most of the Na(+)-independent uridine uptake in hepatoma cells. Thus the stimulatory effect of NBTI on the concentrative nucleoside transporter of liver parenchymal cells cannot be explained by inhibition of nucleoside efflux.
| S-EPMC1217560 | biostudies-other
Project description:1. Native or partially degraded RNA derived from intact rat liver, or from the parenchymal-cell or the non-parenchymatous fraction of liver, has been shown to be transported into rat parenchymal cells in suspension, without prior degradation to acid-soluble components, when the cell suspension is incubated with the RNA at 37 degrees . The amount of RNA of exogenous origin present in the parenchymal cells in an acid-precipitable form increased rapidly up to 30-60min., after which it gradually decreased, indicating intracellular degradation to acid-soluble components of the RNA taken up by the cells. 2. The RNA taken up by the parenchymal cells from the medium, and the acid-soluble products of its degradation within the cells, could be released back into the medium. 3. The RNA of exogenous origin present in acid-precipitable form in the parenchymal cells represented up to 5% of the RNA of the cells after 60min. of incubation. 4. When the concentration of RNA in the medium was less than 200mug./ml., over 10% of the RNA was transported in an acid-precipitable form in 60min. into the parenchymal cells incubated at a concentration of 2.3x10(6)/ml. 5. Ribonuclease inhibited the uptake of exogenous RNA by the parenchymal cells, whereas 2,4-dinitrophenol, sodium azide, protamine sulphate and polyvinyl sulphate had no significant effect. 6. The uptake of exogenous RNA by liver slices proceeded at a rate which was 4-20% of that obtained in the parenchymal-cell suspensions; the RNA taken up did not appear to become degraded, unlike that taken up by the cell suspensions. 7. It is concluded that dispersion of liver tissue to a suspension of single cells increases the permeability of the parenchymal cells to macromolecular RNA and creates conditions that lead to a rapid degradation of the RNA taken up.
| S-EPMC1264996 | biostudies-other
Project description:Cultured non-parenchymal rat liver cells internalize human urine alpha-N-acetylglucosaminidase, human skin beta-N-acetylglucosaminidase and pig kidney alpha-mannosidase. Different heat-stabilities of endocytosed and endogenous alpha-mannosidase activity provided indirect evidence that the increase in intracellular activity resulted from uptake. The high efficiency and the saturation kinetics of uptake indicated that these enzymes become internalized by adsorptive endocytosis. Competition experiments with glycoproteins bearing known carbohydrates at their non-reducing terminals, with mannans, methyl glycosides and monosaccharides, established that the uptake of these three lysosomal enzymes is mediated by the binding to cell-surface receptors that recognize mannose and N-acetylglucosamine residues. The decreased uptake after treatment of these enzymes with either beta-N-acetylglucosaminidase or alpha-mannosidase was in accordance with the results of the inhibition experiments. Removal of oligosaccharides of the high-mannose type by treatment with endoglucosaminidase H inhibited uptake almost completely, suggesting that the sugars recognized by cell-surface receptors of non-parenchymal liver cells are located in the outer core of these oligosaccharides. A comparison of the uptake of these three lysosomal enzymes by parenchymal and non-parenchymal rat liver cells indicates that infused alpha-N-acetylglucosaminidase is taken up preferentially by hepatocytes, whereas alpha-mannosidase and beta-N-acetylglucosaminidase are localized predominantly in non-parenchymal rat liver cells.
| S-EPMC1161311 | biostudies-other
Project description: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.
| S-EPMC1152892 | biostudies-other
Project description:Alanine disposal by liver parenchymal and haematopoietic cells from 21-day fetuses, newborns and adult rats was studied. Preparations selectively enriched in either haematopoietic cells or hepatocytes were obtained by direct perfusion of fetal- and neonatal-rat livers. L-Alanine transport into liver parenchymal cells was best fitted to two Na(+)-dependent saturable systems. The high-affinity system showed a much higher activity (Vmax.) in hepatocytes from fetuses and newborns than in those from adult rats (2.4, 4.3 and 0.3 nmol/8 min per 10(6) cells for fetuses, newborns and adults respectively). Vmax. for the low-affinity component was slightly lower during the perinatal period than in the adult (about 30 nmol/8 min per 10(6) cells for hepatocytes from fetuses and newborns, versus 48 nmol/8 min per 10(6) cells for adult rat parenchymal cells). Haematopoietic cells from fetal-rat livers showed significant Na(+)-dependent L-alanine uptake which was completely abolished after birth. These results show that the transport systems involved in L-alanine uptake by liver parenchymal cells are fully developed before birth. This probably contributes to fulfilling the high requirement for neutral amino acids for protein synthesis during development. Haematopoietic cells may play an important role in liver amino acid metabolism during fetal life.
| S-EPMC1134441 | biostudies-other
Project description:ObjectiveInsulin control of fatty acid metabolism has long been deemed dominated by suppression of adipose lipolysis. The goal of the present study was to test the hypothesis that this single role of insulin is insufficient to explain observed fatty acid dynamics.Methods and resultsFatty acid kinetics were measured during a meal tolerance test and insulin sensitivity assessed by intravenous glucose tolerance test in overweight human subjects (n=15; body mass index, 35.8 ± 7.1 kg/m(2)). Non-steady state tracer kinetic models were formulated and tested using ProcessDB software. Suppression of adipose fatty acid release, by itself, could not account for postprandial nonesterified fatty acid concentration changes, but adipose suppression combined with insulin activation of fatty acid uptake was consistent with the measured data. The observed insulin K(m) for nonesterified fatty acid uptake was inversely correlated with both insulin sensitivity of glucose uptake (intravenous glucose tolerance test insulin sensitivity; r=-0.626; P=0.01) and whole body fat oxidation after the meal (r=-0.538; P=0.05).ConclusionsThese results support insulin regulation of fatty acid turnover by both release and uptake mechanisms. Activation of fatty acid uptake is consistent with the human data, has mechanistic precedent in cell culture, and highlights a new potential target for therapies aimed at improving the control of fatty acid metabolism in insulin-resistant disease states.
| S-EPMC3417322 | biostudies-literature
Project description:In order to assess the relative importance of the receptor for low-density lipoprotein (LDL) (apo-B,E receptor) in the various liver cell types for the catabolism of lipoproteins in vivo, human LDL was labelled with [14C]sucrose. Up to 4.5h after intravenous injection, [14C]sucrose becomes associated with liver almost linearly with time. During this time the liver is responsible for 70-80% of the removal of LDL from blood. A comparison of the uptake of [14C]sucrose-labelled LDL and reductive-methylated [14C]sucrose-labelled LDL ([14C]sucrose-labelled Me-LDL) by the liver shows that methylation leads to a 65% decrease of the LDL uptake. This indicated that 65% of the LDL uptake by liver is mediated by a specific apo-B,E receptor. Parenchymal and non-parenchymal liver cells were isolated at various times after intravenous injection of [14C]sucrose-labelled LDL and [14C]sucrose-labelled Me-LDL. Non-parenchymal liver cells accumulate at least 60 times as much [14C]sucrose-labelled LDL than do parenchymal cells accumulate at least 60 times as much [14C]sucrose-labelled LDL than do parenchymal cells when expressed per mg of cell protein. This factor is independent of the time after injection of LDL. Taking into account the relative protein contribution of the various liver cell types to the total liver, it can be calculated that non-parenchymal cells are responsible for 71% of the total liver uptake of [14C]sucrose-labelled LDL. A comparison of the cellular uptake of [14C]sucrose-labelled LDL and [14C]sucrose-labelled Me-LDL after 4.5h circulation indicates that 79% of the uptake of LDL by non-parenchymal cells is receptor-dependent. With parenchymal cells no significant difference in uptake between [14C]sucrose-labelled LDL and [14C]sucrose-labelled Me-LDL was found. A further separation of the nonparenchymal cells into Kupffer and endothelial cells by centrifugal elutriation shows that within the non-parenchymal-cell preparation solely the Kupffer cells are responsible for the receptor-dependent uptake of LDL. It is concluded that in rats the Kupffer cell is the main cell type responsible for the receptor-dependent catabolism of lipoproteins containing only apolipoprotein B.
| S-EPMC1144393 | biostudies-other
Project description:Dietary fat promotes pathological insulin resistance through chronic inflammation. The inactivation of inflammatory proteins produced by macrophages improves diet-induced diabetes, but how nutrient-dense diets induce diabetes is unknown. Membrane lipids affect the innate immune response, which requires domains that influence high-fat-diet-induced chronic inflammation and alter cell function based on phospholipid composition. Endogenous fatty acid synthesis, mediated by fatty acid synthase (FAS), affects membrane composition. Here we show that macrophage FAS is indispensable for diet-induced inflammation. Deleting Fasn in macrophages prevents diet-induced insulin resistance, recruitment of macrophages to adipose tissue and chronic inflammation in mice. We found that FAS deficiency alters membrane order and composition, impairing the retention of plasma membrane cholesterol and disrupting Rho GTPase trafficking-a process required for cell adhesion, migration and activation. Expression of a constitutively active Rho GTPase, however, restored inflammatory signalling. Exogenous palmitate was partitioned to different pools from endogenous lipids and did not rescue inflammatory signalling. However, exogenous cholesterol, as well as other planar sterols, did rescue signalling, with cholesterol restoring FAS-induced perturbations in membrane order. Our results show that the production of endogenous fat in macrophages is necessary for the development of exogenous-fat-induced insulin resistance through the creation of a receptive environment at the plasma membrane for the assembly of cholesterol-dependent signalling networks.
| S-EPMC5671339 | biostudies-literature
Project description:1. Hepatic uptake of low-density lipoprotein (LDL) in parenchymal cells and non-parenchymal cells was studied in control-fed and cholesterol-fed rabbits after intravenous injection of radioiodinated native LDL (125I-TC-LDL) and methylated LDL (131I-TC-MetLDL). 2. LDL was taken up by rabbit liver parenchymal cells, as well as by endothelial and Kupffer cells. Parenchymal cells, however, were responsible for 92% of the hepatic LDL uptake. 3. Of LDL in the hepatocytes, 89% was taken up via the B,E receptor, whereas 16% and 32% of the uptake of LDL in liver endothelial cells and Kupffer cells, respectively, was B,E receptor-dependent. 4. Cholesterol feeding markedly reduced B,E receptor-mediated uptake of LDL in parenchymal liver cells and in Kupffer cells, to 19% and 29% of controls, respectively. Total uptake of LDL in liver endothelial cells was increased about 2-fold. This increased uptake is probably mediated via the scavenger receptor. The B,E receptor-independent association of LDL with parenchymal cells was not affected by the cholesterol feeding. 5. It is concluded that the B,E receptor is located in parenchymal as well as in the non-parenchymal rabbit liver cells, and that this receptor is down-regulated by cholesterol feeding. Parenchymal cells are the main site of hepatic uptake of LDL, both under normal conditions and when the number of B,E receptors is down-regulated by cholesterol feeding. In addition, LDL is taken up by B,E receptor-independent mechanism(s) in rabbit liver parenchymal, endothelial and Kupffer cells. The non-parenchymal liver cells may play a quantitatively important role when the concentration of circulating LDL is maintained at a high level in plasma, being responsible for 26% of hepatic uptake of LDL in cholesterol-fed rabbits as compared with 8% in control-fed rabbits. The proportion of hepatic LDL uptake in endothelial cells was greater than 5-fold higher in the diet-induced hypercholesterolaemic rabbits than in controls.
| S-EPMC1135097 | biostudies-other
Project description: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.
| S-EPMC1154288 | biostudies-other