Rat high density lipoprotein subfraction (HDL3) uptake and catabolism by isolated rat liver parenchymal cells

Uptake and degradation of rat and human very low density (remnant) apolipoprotein by parenchymal and non-parenchymal rat liver cells

1. The relative contribution of parenchymal and non-parenchymal cells to the in vivo hepatic uptake of serum apolipoproteins was measured 30 min after intravenous injection of radioiodinated rat very low density lipoprotein (VLDL) remnants, rat and human VLDL, low density lipoprotein (LDL) and high density lipoprotein (HDL). Using rat VLDL, VLDL-remnants, LDL and HDL, respectively, the non-parenchymal cells contain 4.7, 4.9, 6.1 and 5.3 times the amount of trichloroacetic acid-precipitable radioactivity per mg cell protein as compared to parenchymal cells. For human VLDL, LDL and HDL these values are 5.1, 12.0 and 5.9 respectively. 2. The abilities of homogenates of human liver, rat liver parenchymal cells and rat liver non-parenchymal cells to hydrolyze human and rat iodinated VLDL apoprotein were determined by measuring the amount of trichloroacetic acid-soluble (non-iodide) radioactivity liberated upon incubation at the optimal pH of 4.2. Non-parenchymal cells possess a 8--21-fold higher maximal capacity to degrade VLDL apoprotein per mg of cell protein than parenchymal cells. This factor is 5--6 for VLDL-remnant apoprotein degradation measured at low (suboptimal) apolipoprotein concentrations. 3. It is concluded that, in addition to parenchymal cells, the non-parenchymal cells play an important role in the hepatic uptake and possibly degradation of VLDL-(remnant) apoprotein.

The effects of dexamethasone on metabolic activity of hepatocytes in primary monolayer culture

The effects of dexamethasone on multiple metabolic functions of adult rat hepatocytes in monolayer culture were studied. Adult rat liver parenchymal cells were isolated by collagenase perfusion and cultured as a primary monolayer in HI/WO/BA, a serum free, completely defined, synthetic culture medium. Cells inoculated into the culture medium formed a monolayer within 24 hr. Electron microscopy showed that the cells in primary culture had a fine structure identical to liver parenchymal cells in vivo, including the observation of desmosomes and bile canaliculi in intercellular space. There was significant gluconeogenesis by the cells 24 hr postinoculation but it had decreased markedly by 48 hr. There was a marked induction of tyrosine aminotransferase (TAT) by dexamethasone, which was maintained for up to 72 hr postinoculation of cells. The transport of alpha-aminoisobutyric acid into the cells in monolayer culture was stimulated by dexamethasone and was dependent on the concentration of dexamethasone. Albumin synthesis and secretion by the cells was measured by a quantitative electroimmunoassay. Albumin production was shown to increase linearly over an incubation period of 24 to 48 hr postinoculation. Dexamethasone depressed the albumin synthesis. The effects of dexamethasone are slow, and at times require more than 6 hr to show variation from the control, indicating that dexamethasone is not a single controlling hormone. Possibly it functions in a cooperative and coordinating role in the regulation of cell metabolism.

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.

Uptake and degradation of 125I-labelled high density lipoproteins in rat liver cells in vivo and in vitro

1. The uptake of 125I-labelled high density lipoproteins (HDL) in various organs of the rat was determined after an intravenous injection. The uptake of 125I-labelled polyvinylpyrrolidone in the same organs was determined in order to assess uptake by fluid endocytosis. The uptake/organ was highest for the liver. The adrenals showed the highest uptake/unit weight of the organs studied. The liver, the kidneys and the spleen showed comparable values for uptake/g of tissue. The uptake of 125I-labelled HDL exceeded by far that of 125I-labelled polyvinylpyrrolidone in the liver, the kidneys, the spleen and the adrenals, indicating that the uptake of 125I-labelled HDL was mediated by adsorptive endocytosis. 2. The in vivo uptake of 125I-labelled HDL was determined in purified hepatocytes and non-parenchymal cells prepared by collagenase perfusion of livers from animals after intravenous injections of 125I-labelled HDL. When expressed per cell, the hepatocytes and the non-parenchymal liver cells took up about the same amount of 125I-labelled HDL. 3. The in vitro uptake and degradation of 125I-labelled HDL in isolated rat hepatocytes was studied. The uptake at increasing concentrations of 125I-labelled HDL was saturable indicating uptake mediated through binding sites. 125I-labelled HDL were easily degraded by contaminating proteases from the perfusate. 4. Subcellular fractionation by isopycnic centrifugation indicated that the accumulation of 125I-labelled HDL did not take place in the lysosomes, but rather on the plasma membrane and possibly in the endosomes (phagosomes). 5. 125I-labelled HDL were internalized into the cells and degraded in the lysosomes. Leupetin and chloroquine, inhibitors of the lysosomal function effectively inhibited the formation of 125I-labelled acid-soluble radioactivity by the cells. Chloroquine, but not the protease inhibitor leupeptin, reduced the hydrolysis of the cholesteryl ester moiety of HDL.

Low density lipoprotein receptors and catabolism in primary cultures of rabbit hepatocytes

Rabbit 125I-labelled low density lipoproteins (LDL) were incubated with primary monolayer cultures of rabbit hepatocytes in studies designed to assess the role of liver in LDL catabolism at the cellular level. After hepatocytes were preincubated for 20 h in lipoprotein-free medium, they exhibited time- and concentration-dependent interaction with 125I-labelled DLD at concentrations to 1 mg LDL protein/ml and times to 24 h. After a 3 h (37 degrees C) incubation with 50 microgram LDL protein/ml, hepatocytes bound 400 ng (LDL protein)/mg (cell protein), internalized 280 ng/mg, and degraded 660 ng/mg. Internalization and degradation may be greater than indicated by these values since pulse studies suggested the presence of a deiodinase which attacks cell associated 125I-labelled LDL. The amounts of LDL bound to hepatocytes after 3 h (37 degrees C) were similar to amounts for fibroblasts, but DLD internalization and degradation were considerably less. Rabbit hyperlipidemic 125I-labelled DLD showed the same amount of binding but 1.39 times more internalization and degradation than normolipidemic 125I-labelled LDL. Binding of both control and hyperlipidemic LDL was 3-fold greater at 24 and 42 h than at O or 3 h but addition of a 50-fold molar excess of high density lipoproteins (HDL) prevented increased LDL binding with time. Induction of specific high affinity receptors for binding LDL was shown to occur by preincubation of hepatocytes for increasing periods in lipoprotein-free medium and then measuring 125I-labelled LDL binding at 4 degrees C in the presence and absence of excess unlabelled LDL. Finally, hepatocytes took up 40 times more LDL than sucrose or dextran over a 24-h period, an indication that the uptake of LDL occurs via some mechanism other than simple bulk fluid endocytosis.

The effects of streptozotocin diabetes on hepatic triglyceride lipase activity in the rat

The function of the hepatic triglyceride lipase (H-TGL) is not yet clear. The purpose of the present study was to investigate the possible hormonal regulation of H-TGL. Postheparin plasma was obtained 3 min after the intravenous injection of 50 U/250 g body weight of heparin into male Wistar rats. The lipase activities were measured using substrate containing [14C] triolein emulsified with gum arabic and were expressed in mumoles of free fatty acid released/ml/hour (mean +/- SD). H-TGL was the lipase activity remaining after inhibition of lipoprotein lipase (LPL) by 1.0 M NaCl. Diabetic rats were prepared by intravenous injection of streptozotocin (STZ), 65 mg/kg body weight. The contributions of H-TGL and LPL to the total plasma triacylglycerol hydrolase (TGH) activity depend on the amount of heparin injected and the time of blood withdrawal after heparin injection. H-TGL was maximally released at higher heparin (50 U/250 g body weight) concentrations, compared to LPL which was maximally released at lower heparin (5 U/250 g body weight) concentrations. H-TGL was significantly higher at 3 min after the injection of 50 U of heparin/250 g body weight than at 20 min. Twenty-four-hour fasting produced a significant fall in H-TGL compared to H-TGL in fed rats. Total TGH was significantly lower in diabetic rats 3 days after STZ injection. In diabetic rats 3, 5, and 7 days after STZ injection, H-TGL were significantly lower than those in control rats. H-TGL and H-TGL/total TGH were 9.49 +/- 0.99 and 0.551 +/- 0.071, respectively, in rats 3 days after STZ injection, compared to H-TGL (13.46 +/- 0.69) and H-TGL/total TGH (0.739 +/- 0.052) in control nondiabetic rats. When diabetic rats were treated with insulin, total TGH (14.37 +/- 3.01) and H-TGL (6.77 +/- 4.12) rose to 25.16 +/- 1.02 (total TGH) and 16.49 +/- 1.13 (H-TGL), that were comparable to activities in control nondiabetic rats. Separation of H-TGL and LPL was performed using heparin-Sepharose affinity chromatography of postheparin plasma. The enzyme activity of peak I from STZ rats, which is eluted by 0.72 M NaCl-Veronal buffer, pH 7.4 and corresponds to H-TGL, was approximately half the activity from control rats. TGH released by heparin from isolated rat liver parenchymal cells was investigated. The enzyme activites released from isolated liver parenchymal cells prepared from STZ rats was approximately half that from control rats. The role of insulin in the regulation of LPL has been well documented. Our findings suggest that H-TGL also is under hormonal regulation by insulin in rats.

Binding and degradation of human 125I-HDL by rat adrenocortical cells

In order to learn more about the mechanism by which high density lipoprotein (HDL) cholesterol is taken up by the adrenal cortex, binding and degradation of human 125I-HDL by suspensions of intact rat adrenal cortical cells have been examined. Cellular accumulation of 125I-HDL was found to occur in two phases. Our results indicate that the initial phase of association results from reversible binding of 125I-HDL to a specific saturable set of membrane binding sites. Binding site affinity appears equal for both rat and human HDL while affinity for human LDL is approximately one order of magnitude less on the basis of apoprotein weight. In addition, isolated rat adrenal cortical cells were found to degrade human 125I-HDL at a rapid rate. Degradation, like binding, can be prevented by addition of excess unlabeled HDL suggesting that binding and degradation are linked. Thus, one mechanism that could account for adrenal uptake of HDL cholesterol is endocytosis, initiated by lipoprotein binding to the HDL specific membrane binding site.

In vivo uptake of human and rat low density and high density lipoprotein by parenchymal and nonparenchymal cells from rat liver

The relative contribution of the parenchymal and nonparenchymal rat liver cells to the hepatic uptake of human and rat high density lipoprotein (HDL) and low density lipoprotein (LDL) was determined in vivo. Nonparenchymal cells, isolated 6 h after intravenous injection of iodinated human HDL and LDL, contained respectively 4.2 and 6.3 times the amount of trichloroacetic acid-precipitable radioactivity per mg cell protein as compared to parenchymal cells. For rat iodinated HDL and LDL these factors were 3.4 and 4.1, respectively. These results indicate that nonparenchymal liver cells play a substantial role in the hepatic uptake of human and rat HDL and LDL in vivo.

Preferential utilization of free cholesterol from high-density lipoproteins for biliary cholesterol secretion in man

High- and low-density lipoproteins carrying free cholesterol labeled with 3H or 14C were administered to a patient with a bile fistula. The free cholesterol from high-density lipoproteins was more rapidly incorporated into biliary cholesterol than the free cholesterol from low-density lipoproteins. These findings show that the liver in man selectively utilizes and secretes the free cholesterol from a particular lipoprotein.

High density lipoprotein catabolism before and after partial hepatectomy

The serum decay and tissue distribution of iodine-labeled high density lipoprotein (HDL)-apoproteins were measured in rats 2--8 h after partial hepatectomy or sham-operation. A method was developed allowing continuous bloodsampling without using anticoagulantia or anaesthetics. The serum decay of HDL-apoproteins was biexponential. Neither the initial rapid phase (t 1/2 0.3 +/- 0.1 h), nor the slow phase (t 1/2 6.2 +/- 0.3 h) were influenced by the removal of 2/3 of the liver and consequently there was no effect on the fractional catabolic rate (F.C.R.: 2.9 +/- 0.2/day). The level of circulating HDL was decreased by partial hepatectomy but the chemical composition of HDL was unchanged. Total tissue HDL radioactivity in the control rats was 5.7, 2.8, 2.7, 1.0, 0.7, 0.2, 0.4 and 0.1% of the injected dose for skeletal muscle, adipose tissue, liver, jejunum, kidneys, spleen, lungs and heart, respectively. Only the value for liver was affected significantly by partial hepatectomy (0.6%). It is concluded that the in vivo degradation rate of HDL-apoproteins is not influenced by the removal of 2/3 of the liver and that the decrease in serum HDL concentration is due to an impaired rate of hepatic synthesis. These results indicate the possiblity of extrahepatic HDL-apoprotein catabolism or a stimulation of HDL-apoprotein degradation, induced by partial hepatectomy, in the remaining liver lobes.

Uptake and degradation of cholesterol ester-labelled rat plasma lipoproteins in purified rat hepatocytes and nonparenchymal liver cells

1. A new method for isolation and purification of rat liver hepatocytes and nonparenchymal cells by differential centrifugation is described. 2. Cholesterol ester-labelled lipoproteins (prepared by the action of lecithin: cholesterol acyltransferase) intravenously injected were taken up by hepatocytes and nonparenchymal cells. 3. Hepatocytes and nonparenchymal cells in suspension were able to take up and hydrolyse the cholesterol ester portion of lipoproteins. 4. Uptake of cholesterol ester labelled whole rat plasma and high density lipoproteins (HDL) increased with increasing concentrations until a distinct saturation level was reached in hepatocytes. In nonparenchymal cells there was no saturation of lipoprotein uptake. 5. Concanavalin A inhibited cholesterol ester-labelled lipoprotein uptake in hepatocytes, indicating that the uptake at least partially depends on carbohydrate sites on the cell surface. The uptake in nonparenchymal cells was unaffected of concanavalin A. 6. The specific activity of the acid cholesterol ester hydrolase was the same in homogenates from hepatocytes and nonparenchymal cells while acyl-CoA: cholesterol acyltransferase was found almost exclusively in hepatocytes.